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Related investigations of pi 2 micropulsations 1972

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c i RELATED INVESTIGATIONS OF Pi 2 MICROPULSATIONS by BRIAN PAUL SMITH B.Sc, University of British Columbia, 1970 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of GEOPHYSICS We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA August, 1972 In presenting this thesis in partial fulfilment of the requirements for an advanced degree.at the University of British Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Geophysics The University of British Columbia Vancouver 8 , Canada Date August 29r1972 i i ABSTRACT Related investigations of irregular, nighttime, type Pi 2 micropulsations were undertaken with regards to the source and occurrence of these geomagnetic fluctuations. In particular, the local times of Pi 2's recorded by a global network of stations, during 1964, were determined. From this, the Pi 2 daily occurrence maximum was observed near 2230 LMT. For this same year (1964), rapid-run mag- netograms from Memambetsu, Japan and Wingst, Germany were analyzed. The i n i t i a l orientation of the impulsive Pi 2 disturbance vectors was observed to be primarily north- east (north-west) before (after) 2230 LMT. These results suggest that Pi 2 source f i e l d lines l i e near the 2230 LMT meridian. Further investigations of the globally observed Pi 2's were made regarding variations i n morphology with magnetic activity. The daily occurrence maximum of Pi 2 was found earlier (later) at 2030 (2330) LMT during inter- vals of high (low) magnetic activity. In this manner, the longitudinal shift of the Pi 2 source i s revealed. A st a t i s t i c a l study of solar wind protons observed by Explorer 34 sa t e l l i t e was made during the intervals of high, and of low, magnetic activity. This study showed that the Pi 2 source shift may be due to a change i n the solar wind flow i i i d i r e c t i o n and/or processes associated with changes i n the solar wind proton pressure. Pi 2 fs are a t r a i n of pulsations having quasi- periods ranging from 40 to 150 seconds and each series l a s t s about 10 minutes. The periods of Pi 2 micropulsations recorded at Ralston, Canada during 196? were correlated with simultaneous, Alouette 2 s a t e l l i t e received, VLF radio signals. Some of these VLF emission phenomena, known as 'whistler cutoff* and 'lower hybrid resonance noise band breakup', indicated the location of the magnetospheric plasmapause. Other emissions, known as ELF, were believed to indicate the plasma sheet inner boundary. The v a r i - ation of the period of the P i 2's with the indicated mag- netospheric subregion locations showed that P i 2 period varies systematically with positions of the plasma sheet inner boundary during i n t e r v a l s of magnetic quiescence. The r e s u l t s imply a l a t i t u d i n a l (radial) movement of the P i 2 source i n a region near the plasma sheet inner bound- ary. Lastly, the rate of Pi 2 occurrence with magnetic a c t i v i t y , during 1964 and 1967, was found to be maximum when the planetary index of magnetic a c t i v i t y , Kp, was 1+ to 2-. The mean Kp index most clos e l y approaches t h i s optimum l e ^ e l during the years of sunspot minimum* Thus, the rate of occurrence r e s u l t i s consistent with the inverse iv relationship of Pi 2 yearly occurrence with the solar cycle. In summation, the source studies revealed a •dynamic* Pi 2 source, in the sense that i t varies both latitudinally and longitudinally. An association was shown between Pi 2 and nightside magnetospheric processes and subregions. The occurrence study indicated that processes generating Pi 2*s are not clear but approach optimum when the Kp level i s between 1+ and 2-. V TABLE OF CONTENTS Page ABSTRACT i i LIST OF TABLES v i LIST OF FIGURES v i ACKNOWLEDGEMENTS v i i i CHAPTER I A BACKGROUND TO INVESTIGATIONS? OF Pi 2 1 a. Introduction 1 b. Pi 2 Morphology 4 c. Subregions of the Magnetosphere 7 d. Theories of Pi 2 12 CHAPTER II , RELATED INVESTIGATIONS OF Pi 2 MICROPULSATIONS 20 a. VLF Emissions and Pi 2 Period 20 b. Pi 2 Diurnal Variations 2g c. Pi 2, Solar Wind and Geomagnetic Activity 30; d« Pi 2 Rate of Occurrence 39 CHAPTER III DISCUSSION OF THE RESULTS 43 a. Pi 2 and Magnetospheric Subregions 43 b. Pi 2 Source 46 c. Pi 2 Source Variations 4# d. On the Pi 2 Rate of Occurrence 49 CHAPTER IV SUMMARY AND CONCLUDING REMARKS 53 BIBLIOGRAPHY 57 v i LIST OF TABLES Page Date, UT, type of radio si g n a l received by, and pos i t i o n of Alouette 2 correlated with Pi 2 period at Ralston 25 LIST OF FIGURES 1 Magnetospheric subregion L v a l u e v a r i - ations of P i 2 period 26 2 Magnetograms at i n i t i a l l y northwest (X+, Y-I Pi 2 31 3 Magnetograms of i n i t i a l l y northeast (X+, Y+7 Pi 2 32 4 Diurnal v a r i a t i o n of i n i t i a l l y northwest and northeast P i 2 occurrence frequencies 33 5 Diurnal v a r i a t i o n of the i n i t i a l l y north- east P i 2 occurrence p r o b a b i l i t y and the t o t a l P i 2 occurrence frequency 34 6 Diurnal v a r i a t i o n of Pi 2 occurrence frequency during high and low magnetic a c t i v i t y 36 7 Solar wind streaming angle v a r i a t i o n of p a r t i c l e pressure during high and low magnetic a c t i v i t y 3# & Kp v a r i a t i o n of the P i 2 occurrence rate during 1964 and 1967 41 9 Nightside equatorial magnetospheric con- vective system, near the plasmapause, during low magnetic a c t i v i t y 50 v i i Nightside equatorial magnetospheric cpn- vective system, near the plasmapause, during high magnetic activity Motion of magnetospheric plasmas due to an implosion (Smith and Watanabe, 1972) v i i i ACKNOWLEDGEMENTS I wish to thank Professor Tomiya Watanabe of the University of British Columbia for suggesting and encour- aging much of the work undertaken in this thesis. His many discussions and helpful comments oh the thesis were greatly appreciated. The Alouette 2 data was prepared by Drs. R. E. Barrington and F. H. Palmer of the Communications Research Centre, Ottawa. I would like to thank them for providing this information, and also Dr. R. E. Horita of the Uni- versity of Victoria for his helpful comments regarding this study. The 1967 micropulsation data recorded at Ralston was kindly provided by Sir Charles S. Wright, Dr. J. E. Lokken and his staff at the Defense Research Establishment Pacific, Esquimalt. I also wish to thank Mr. H. Ueda for his assistance i n reproducing this micropulsation data. The Explorer 34 solar wind data were supplied by the World Data Centre A", Rockets and Satellites, Goddard Space Flight Centre, NASA, Greenbelt, Maryland. The Memambetsu and Wingst rapid-run magnetograms were supplied by the World Data Centre A, for Solar-Terrestrial Physics, National Oceanic and Atmospheric Administration, Boulder, Colorado. I wish to thank these centres for providing this ix service. This thesis was supported by National Research Council operating grant A3564, major equipment grant E2032 and Defense Research Board grant 9511-112 to Dr. T. Watanabe. CHAPTER I A BACKGROUND TO INVESTIGATIONS OF Pi 2 a. Introduction Pi 2 type geomagnetic micropulsations are irregular fluctuations, having a quasi-period between 40 and 150 sec- onds. They are primarily nighttime phenomena, appearing as a series of impulsive oscillations which usually last 5 to 10 minutes. They are often associated with geomagnetic bay disturbances (Angenheister, 1913; Terada, 1917), f l u c - tuations in auroral luminosity (Fukunishi and Hirasawa, 1970) and intensity fluctuations of charged particles pre- cipitating upon the ionosphere (Milton, McPherron, Anderson and Ward, 1967). The frequency of occurrence of this type of micropulsation i s maximum during the years of minimum sunspot activity (Yanagihara, 1956). It is observed that Pi 2 tends to occur with a quasi-period similar to the sun- spot rotation period, 26 to 29 days (Saito and Matsushita, 196S). The interaction of the solar wind and the terres- t r i a l magnetic f i e l d can possibly give rise to geomagnetic micropulsations. Much evidence of this interaction has been reported. The magnetopause, or boundary of this solar- t e r r e s t r i a l interaction, has been observed with the aid of sate l l i t e experiments (Cahill and Amazeen, 1963; Ness, 1965). 2 Hydromagnetic interaction between solar wind discontinuities and the t e r r e s t r i a l magnetic f i e l d have also been reported (Oglivie and Burgala, 1970). Yet the solar wind stimulates not only geomagnetic activity, such as large scale magnetic storms but also causes variations in the structure of the region inside the magnetopause, viz. the magnetosphere. Carpenter (1970), Hones, Asbridge and Bame (1971) have observed changes in magnetospheric subregions with varying magnetic activity, viz. the extraterrestrial ring current, the plasmapause bulge and the magnetotail plasmasheet, re- spectively. These observations imply that associations be- tween micropulsations and magnetospheric subregions may also exist. One of the investigations reported in this thesis shows a systematic variation in Pi 2 period with the location of a subregion of the magnetosphere. The morphology of geomagnetic micropulsations should, in some manner, reflect the nature of the source responsible for these fluctuations of the earth's f i e l d . A review of the subject of micropulsations (Jacobs, 1967) reveals that Pi 2 morphology has been considerable well i n - vestigated. Yet studies of variations of Pi 2 morphology with magnetic activity have been less consistent. Such studies may provide more information on the source mech- anism of Pi 2. In this thesis, a correlated investigation of Pi 2 morphology is reported. Also reported i s a morpho- 3 logic variation study of the change in the most probable time of Pi 2 occurrence, with magnetic activity and with the solar wind. As mentioned, morphologic variation studies have been less consistent and some investigators have reported contradicting observations. To settle some of these contradictions, the variation in the rate of Pi 2 occurrence with the three hour planetary index of magnetic activity, K p , has been investigated and reported in the last part of this thesis. The purpose of this thesis is to establish the existence of relationships among Pi 2 micropulsations^; the magnetosphere, and the solar wind. This is achieved through the investigations into Pi 2 which were previewed in the above. In some of these studies, correlations were made with s a t e l l i t e observations of VLF radio signals in the mag- netosphere and also of protons in the solar wind. The implications of these studies are discussed with regards to the source and generation of Pi 2 micropulsations. However, interpretations of experimental results often d i f f e r . For this reason i t is necessary to outline the established morphology of Pi 2. In order to understand the implied association of Pi 2 micropulsations with the magnetosphere, a review of the observed dynamics of magnetospheric sub- regions is also presented. Lastly, various theories sug- gesting the mechanism of Pi 2 generation are developed. 4 Some of these theories, based on certain investigational results, require different locations for the Pi 2 source. A discussion of these theories i s then useful, to more fu l l y comprehend the generation and source of Pi 2 micro- pulsations. b. Pi 2 Morphology In general, Pi 2 amplitude increases with latitude, reaching maximum near the auroral zones (Saito and Sakur8i, 197Q). Although a few investigators have found an equa- t o r i a l enhancement of Pi 2, the maximum amplitude is found at regions higher than 50° geomagnetic latitude (Jacobs and Sinno, I960). The amplitude of Pi 2 is shown to be maximum near midnight (Saito et a l . , 1968). Yet, Pi 2's are not sinusoidal continuous oscillations, but rather of an impul- sive and irregular nature. Thus a discussion of the occur- rence of such micropulsations is more enlightening than discussing amplitude features. As mentioned, Pi 2*s are often associated with geomagnetic bays or baylike phenomena (Saito, 1961). Pi 2 occurrence has also been connected with impulsive geomagnetic H component variations, during magnetic storms (Fukunishi et a l . , 1970). The local time at which Pi 2 occur most frequently is near 2230 LMT, in low and middle latitudes 5 (Yanagihara, 1957a; Rostoker, 1967a). This occurrence frequency maximum tends to shift from midnight towards dusk longitudes as the Kp index increases (Yanagihara, I960), though this variation i s not well established (Kannangara and Fernando, 1969). Also, some investigations have found the r8te of Pi 2 occurrence to increase linearly with the Kp index (Kannangara et a l . , 1969; Channon and Orr, 1970). Observations by Yanagihara (1956) show that the yearly occurrence frequency of Tnight pulsations* i s inversely proportional to the solar activity. Reports by Afanasieva (1961) and Saito et a l . (1968) suggest that the peak time of Pi 2 occurrence i s found near midnight (2300 to 0100) during sunspot maximum and at earlier times (2000 to 2300) during sunspot minimum. As shown in Chapter II, these occurrence observations imply apparent contradictions. Saito et a l . (1970) gave two examples showing that Pi 2 period i s common from middle latitudes through polar regions. On a random noise type background, Fukunishi et a l . (1970) observed an 80 second, Pi 2 spectral peak at the auroral zone. This peak was also observed at low 8nd middle latitudes. The evidence suggests that the period of Pi 2 micropulsations does not change with latitude, presum- ably even up to the latitudes which divide the magnetotail from the region of closed magnetic lines of force. A diurnal variation of Pi 2 period has been shown, with the 6 period increasing towards midnight and decreasing towards noon (Hirasawa and Nagata, 1966; Troitskaya, 1967). Saito et a l . (196£) showed that the period of Pi 2 associated with geomagnetic bays, i s dependent upon the bay intensity. However Fukunishi et a l . (1970) showed the Pi 2 spectral peak decreases in period with magnetic storm development. Troitskaya (1967) showed that Pi 2 period decreases with increasing Kp index. Yet, Rostoker (1967b) observed that two or more prominent Pi 2 spectral peaks may appear, especially for those events occurring during higher Kp values. The relationships among bay intensity, storm devel- opment and Kp are not clearly understood. This combined with the above facts, suggests that there is not always a direct association of Pi 2 with geomagnetic bays and/or with the Kp index, although such associations are often observed. At low and middle latitudes, the f i r s t pulse of a Pi 2 series on the north-south geomagnetic component i s always northward (Billuad, 1953; Grenet, Kato, Ossaka and Okuda, 1954; Saito, 1961). The movement of the Pi 2 dis- turbance vector i s usually north-east (north-west) prior to (after) midnight (Kato, Ossaka, Watanabe, Okuda and Tamao, 1956; Yanagihara, I960; Saito et a l . , 1968). This i n i t i a l disturbance vector has been shown to converge to the northern auroral zone on the midnight meridian (Saito et a l . , 1968). In middle latitudes, the polarization of 7 the Pi 2 magnetic disturbance f i e l d tends to be counter- clockwise (clockwise) in the northern (southern) hemisphere (Christoffel and Linford, 1966; Sakurai, 1970). Sakurai also observed that at College, located at a lower auroral zone latitude, Pi 2 polarization was counterclockwise in the pre-midnight hours and clockwise in the post-midnight hours. Yet at Point Barrow, located at a higher auroral zone latitude, Pi 2 polarization was either clockwise or indeterminate. As shown in Chapter III of this thesis, polarization morphology can yield information about the origin of Pi 2 micropuisations. c. Subregions of the Magnetosphere The magnetospheric plasmapause is a boundary which marks an abrupt change in plasma density at an average equatorial radius of about 4 Re, where 1 Re i s a length equal to the radius of the earth. At the plasma- pause, the equatorial electron density changes by a factor of 10 to 100 (or from approximately 100/crn^ to 1/cv?) within a distance of about 0.15 Re (Angerami and Carpenter, 1966). The plasmapause i s asymmetric and has an outward bulge near the dusk meridian. The bulge region has been associated with an interaction between the plasma moving towards the sun from the magnetotail and the flow due to 8 the earth's rotation (Brice, 1967). The plasma in this region is characterized by a R-/f density profile. For this reason, Chappell, Harris and Sharp (1970) suggested that the bulge i s slowly f i l l e d by particles from the ionosphere. During times of low magnetic activity, the plasmapause bulge may be located at distances out to 9 Re and near local mid- night. However, when a large surge of substorm activity occurs the bulge moves inward to distances of about 4 Re and to an earlier local time near dusk. The radial speed of this nightside plasmapause motion is of the order of 0.3 Re/hour (Carpenter and Stone, 1967). Thus the bulge region is lost during the main phase of a magnetic storm. Yet the dayside plasmasphere appears shielded from storms, as there is very seldom any radial movement reported there (Carpenter, 1970). During the recovery phase, the bulge begins to be f i l l e d again. The plasmasphere bulge i s in an everchanging state towards dynamic equilibrium, constantly undergoing a supply and loss process by the modulating t a i l plasma flow. One possible mechanism suggested by Carpenter (1970) attributes the loss process to the westward convection drift s of the plasmasphere bulge by the increasing t a i l f i e l d . The described motion and asymmetry of the plasma- pause is of particular importance to certain Pi 2 theories. The equatorial magnetic f i e l d at a distance of 6.6 Re was observed with the ATS 1 s a t e l l i t e . During 9 magnetic substorms, the f i e l d at ATS 1 was observed to be depressed i n the dusk to midnight quadrant. P a r t i a l r i n g currents, formed by the inward convection of charged par- t i c l e s near the midnight meridian, was believed responsible f o r the observed depression (Cummings, 1966). Computations by Schield (1969) showed that an e x t r a - t e r r e s t r i a l r i n g current weakens (enhances) the geomagnetic f i e l d inside (outside) the current s i t e . During magnetic storms, an asymmetric enhancement of the increasing r i n g current proton i n t e n s i t i e s occurs. The increases were observed to be greatest i n the dusk to midnight quadrant (Frank, 1967 and 1970). At the same time, no increases were observed near l o c a l noon. The incessant r i n g current of protons, usually found at values of L (an equatorial distance i n units of earth r a d i i ) greater than, or about 6, penetrates to smaller L values during magnetic storms (Frank and Owens, 1970). The intense asymmetric part of the storm time r i n g currents decays much more rapidl y than the symmetric part (Cummings, 1966). The r i n g current has been considered asymmetric from the ssc to the main phase minimum of a storm, and symmetric during the recovery phase (Hoffman and C a h i l l , 1968). In conjunction with the rapid asymmetric r i n g current decay, sudden recoveries of the f i e l d at ATS 1 have been attributed to abrupt disruptions of the p a r t i a l r i n g currents (Cummings, B a r f i e l d , and Coleman, 1968). This 10 thesis w i l l show that the unique features of this magneto- spheric subregion provide information relating to Pi 2 micropulsations. The plasma sheet is a region of magnetic f i e l d reversal, or neutral sheet, between the northern and south- ern halves of the magnetotail. In this equatorial sheet, a low intensity magnetic f i e l d having a non-zero vertical component, is observed. Since a pressure balance must be maintained, a decrease i n the f i e l d pressure i s balanced by an increase in particle pressure in the plasma sheet. An enhanced particle energy density i s one of the sharp boundaries existing between the plasma sheet and the higher latitude magnetotail region. Comparisons of the average particle energy,-density and energy density have been made (Bame, 1967). The greater energy of the plasma sheet resides in the protons, luring the main phase of a negative bay, a decrease in the electron average energy density occurs in the:plasma sheet (Hones et a l . , 1971). This was shown to be due to a decrease in the thickness of the plasma sheet by a net loss of t a i l particles. Ah increase in the t a i l current i s also observed during this phase of the bay (Coleman and Cummings, 1971). As the bay recovers, the sheet thickens again and the t a i l current decreases. The inner boundary of the plasma sheet (characterized by an expo- nential decrease of electron energy density with decreasing 11 radial distance) was reported at positions out to 11.2 Re equatorial distance at times of low magnetic activity (Vasyliunas, 1968). During the main phase decrease of a storm, this inner boundary moves close to 6 Re distance. The plasma sheet inner boundary returns to i t ' s quiet time position immediately following the main phase minimum of a storm (Goleman et a l . , 1971). One of the investigations of this thesis correlates the period of Pi 2's with the observed position of the plasma sheet inner boundary. The trapping boundary of energetic electrons (for E greater than &0 kev) was found to be located within the proton ring current, near i t ' s outward edge (Frank, 1971). This trapping boundary was either coincident with the plasma sheet inner boundary or in the plasma sheet i t s e l f . The plasma sheet "inner boundary was found coincident with the plasmapause in the post midnight quadrant, and 1 to 3 Re beyond the plasmapause in the pre-midnight quadrant. A low energy density electron trough lies between. The ring current was observed to penetrate 0.5 to 1 Re distance into the plasmasphere. The plasma sheet inner boundary and electron trough are within the proton ring current. During the main phase of a storm, the trough disappears and the entire structure moves inward, towards the earth. The inner edge of the symmetric part of the storm time ring current continues to move inward into the plasmasphere, 12 during the recovery phase of a storm (Coleman et a l . , 1971). With the abrupt disruption of the partial ring currents during storm recovery, the plasma sheet and ring currents disconnect. The ring currents l i k e l y move inward and the plasma sheet retires outward. The close and complicated interaction of the magnetospheric subregions makes i t d i f f i c u l t to ascertain the location of the Pi 2 source. d. Theories of Pi 2 The i n i t i a l kick of either component of a Pi 2 event often has the same sign as the corresponding com- ponent of an accompanying bay. Jacobs et a l . (I960) suggested this kick may be represented by an equivalent overhead current system in the dynamo region of the iono- sphere. Such a current system suggests a nightside auroral zone source for Pi 2. Jacobs (1970) stated that solar wind-geomagnetic f i e l d interaction in the t a i l region gives rise to a broadband hydromagnetic (abbreviated here- after with hm) impulse which travels to the auroral zones. The impulse contains periods ranging from Pi to bay. Near the auroral zone source, a l l frequencies arrive at the same time and the leading edge of the bay i s very steep. The ionosphere disperses the waves travelling to lower latitudes. This ionospheric dispersion causes high frequencies to 13 propagate faster than lower frequencies and consequently, the leading edge of the bay is less steep at low and middle latitudes than at the auroral zone. Thus Jacobs regarded the bay and i n i t i a l Pi 2 pulse as a result of modification by the ionosphere, of solar plasma generated hm distur- bances. An objection to the idea of Pi 2's as fluctuations of an ionospheric current system was voiced by Saito et a l . (1968) who noted the latitudinal variation of bay magnitude is not always the same as that of the amplitude of con- current Pi 2. Furthermore, observations of occassional clockwise polarization in the northern hemisphere led Jacobs (1970) to believe that Pi 2 could not be described by a single current system. He then suggested that Pi 2 may be caused by a disturbance propagating in the ionospheric E-region. The close relationship between auroral lumin- osity fluctuations and micropulsations (Campbell and lees, 1961) and observed Pi 2 phase shifts between two separate stations (Herron, 1966) were cited by Jacobs as further evidence of ionospheric propagation effects. Rostoker (1967a), suggested that Pi 2*s originate as eigen-oscil- lations of auroral f i e l d lines which then propagate through the ionospheric E-region. Thus, the polarization of the f i e l d would be effected by ionospheric screening, dependent on the location of the station relative to the Pi 2 source. There are certain reported dissimilarities between 14 Pi 2's observed in the auroral zone and those observed at low latitudes (Fukunishi et a l . , 1970). Observed auroral latitude Pi 2 is more irregular in waveform than the damped, mid-latitude Pi 2 . Likely, this i s because the effect of injected, energetic particles is more prevalent in the auroral zone. As mentioned already, the power spectrum of auroral latitude Pi 2 during a magnetospheric substorm was found to have a background random noise type of dependence with frequency. This random noise is less prevalent at lower latitudes and this might imply that auroral latitude Pi 2's may be generated by a mechanism different from that for low and middle latitude Pi 2 r s . Also, as the Kp index increased, the fundamental mode of the auroral latitude Pi 2 became obscured and the amplitude of the higher frequency Pi 1 riders increased. A funda- mental mode of period greater than 40 seconds was not clearly observable for Kp greater than 3 . Yet the fundamental mode was shown to decrease in period as Kp increased for middle latitude Pi 2 . According to Hirasawa et a l . (1966), the similar diurnal variations of Pi 2 period and Pc 4 period (a continuous micropulsation of period from 45 to 150 seconds) imply a similar generating mechanism at low and middle latitudes. Hirasawa et a l . had attributed Pc 4 to hm oscillations at the plasmapause. Fukunishi et a l . (1970) calculated the Kp dependence of Pi 2 period from formulae 15 expressing the latitude variation in the eigen-period of hm oscillations (Obayashi and Jacobs, 1958) and the Kp variation of the plasmapause (latitude) position (Blnsack, 1967). The Kp versus Pi 2 period relation obtained by them agrees well with the Kp-Pi 2 period observations of Troitskaya (1967). They also observed a decrease in Pi 2 period corresponding to the suggested inward motion of the plasmapause, from 4.6 to 4.2 Re, during the expansive phase of a magnetospheric substorm. Pukunishi et a l . (1970) thus concluded that lower latitude Pi 2 is a transient surface oscillation on the plasmapause, excited by hm disturbances. The auroral latitude Pi 2's are considered to be f l u c - tuations of ionospheric current produced by the influx of energetic particles to the upper atmosphere. Yet i t may be that only Pi fluctuations of period less than 40 seconds, or Pi 1, are caused by ionospheric current disturbances. Heacock, Mullen and Hessler (1971) showed that when a strong Pi (Pi 1 and 2) occurs at College, in the auroral zone, a faint Pi also occurs at Anchorage, a lower latitude station. However, at Anchorage, Pi 1 at a period about 2.5 seconds was enhanced compared to other periods. This implies that Pi 1 may. propagate as hm waves in the ionospheric horizontal waveguide. Also, the variation in Pi 2 period with the suggested plasmapause position does not prove that Pi 2 occur there. As previously mentioned, other magnetospheric 16 subregions, such as the ring currents and the plasma sheet inner boundary, also vary with magnetic activity. Kato (1971) analyzed geomagnetic micropulsations associated with sudden storm commencements (abbreviated hereafter with ssc) by dividing the signal into several frequency channels. He showed that the i n i t i a l predominant Pi 2 had periods of about 200 seconds. These 200 second fluctuations clearly exhibited a damped type of oscillation and were of a global scale. Periods of about 100 seconds were shown to be of a more impulsive or resonant nature, occurring on a semi-global scale. Kato showed the f i l t e r e d waveforms recorded simultaneously at College and Onagawa during a ssc are similar. From such a frequency analysis, i t was concluded that Pi 2 is f i r s t excited by an hm burst. This i s due to a solar wind shock front impacting the magnetopause, on the dayside. The damped 200 second flu c - tuations which are thus excited, travel along f i e l d lines to high latitudes and also across f i e l d lines to the night- side of the magnetosphere. Yet, i f this were the case, these 200 second fluctuations having a f i n i t e propagation velocity, would not arrive simultaneously at two stations separated by 50 degrees of longitude and 36 degrees of latitude, as they were observed. Following the burst, Kato suggested Pi 2's are excited in the magnetosphere night- side and occur with the growth of the substorm. At this 17 time, the particle flow pressure increases against the magnetic f i e l d pressure near the ring current s i t e . The mechanism proposed by Kato (1971) calls for a hm burst impacting the f i e l d in the vicini t y of the inner boundary of the partial ring current. Such an inward burst requires a sudden disruption of partial ring currents. As w i l l be discussed in Chapter III, a sudden partial ring current disruption gives rise to a plasma implosion. Another theory of Pi 2 generation by an implosion of plasmas in the night- time magnetosphere was proposed by Atkinson (1966). This idea is based on the theory of reconnection of open t a i l f i e l d lines proposed by Dungey (1961, 1962, 1967) and also by Axford, Petschek and Siscoe (1965). Atkinson suggested that intermittent f i e l d line reconnection gives rise to the implosion of t a i l plasmas towards closed magnetic lines of force. Heacock et a l . (1971) reported that 95$ of the time when Pi events occur at College or at Sodankyla, the plasmapause is located inside the f i e l d line connected to these auroral zone stations. This implies that most Pi 2 fs occur outside the plasmapause, since Pi 2 amplitude in i t ' s latitudinal variation becomes maximum in the auroral zones. Evidence has been offered by Saito et a l . (1970) to show that Pi 2 micropulsations associated with a small substorm are due to oscillations of the f i e l d lines through the IB plasma sheet inner boundary. The f i e l d lines connected to the maximum amplitude Pi 2 latitudes, in two examples, were shown to pass through the equatorial plane of the night- side magnetosphere at distances of 9.5 and 6.5 Re, respec- tively. These distances were near the predicted plasma sheet inner boundary. V/asyliunas (1968) found that the plasma sheet inner boundary on the nightside moves inward, towards the earth, as the Kp index increases from G to 3. Saito et a l . (1970) gave an empirical formula for the relation of Kp to the geocentric distance of the plasma sheet inner boundary, Rps, A formula was given by Rycroft and Thomas (1970) for the relation between Kp and the equatorial distance of the nightside plasmapause, Rpp, From the two formulae, Saito et a l . (1970) found that Pi 2 peak amplitude latitudes mapped closer to the •inferred* plasma sheet inner boundary than to the 'inferred* plasma- pause position, for Kp indices less than 4* At the i n i t i a l stage of a magnetospheric substorm, Pi 2's are observed to (1) (2) 19 have a period common from middle latitudes to polar regions. At auroral latitudes however, this period may be observed overlapping the noise-like background. The prominent spectral peak of this Pi 2 is frequently seen in the spec- trum of auroral zenith intensity fluctuations. Saito et a l . (1970) f e l t that i f Pi 2 at plasma sheet inner boundary latitudes (auroral zone) were due to the auroral electrojet fluctuations, then the usual coincident set of Pi 2 patterns at conjugate stations would not be observed. They consider auroral electrojet fluctuations to be a secondary pheno- menon produced by a possible Alfven wave modulation of the precipitation of auroral particles. Pi 2 are considered to be primarily due to a transient torsional oscillation of the f i e l d lines through the inner boundary of the plasma sheet. CHAPTER II RELATED INVESTIGATIONS OF Pi 2 MICROPULSATIONS a. VLF Emissions and Pi 2 Period This research is aimed at locating the region of generation of Pi 2 micropulsations. Previous investigators have reported the variation of the plasmapause (Rycroft et a l . , 1970) and the plasma sheet inner boundary (Saito et a l . , 1979) positions with the three hour planetary K index. The period of Pi 2 has been observed to decrease with increasing Kp (Troitskaya, 1967). Hence i t is reasonable to assume the dependence of Pi 2 period upon the nightside position of a magnetospheric subregion. However, there is a considerable degree of uncertainty as to which subregion generates Pi 2. Some authors (Saito et a l . , 1970) indicate the plasma sheet inner boundary while others (Hirasawa et a l . , 1966) suggest the plasmapause as the generating subregion. The hypoth- esis of Saito and Sakurai i s based on a result of their analysis, showing that the latitude of maximum Pi 2 ampli- tude maps along the f i e l d line to the equatorial plane, near the predicted plasma sheet inner boundary. Hirasawa and Nagata;,s hypothesis i s founded on the comparison of their observations of the diurnal variation of the equa- t o r i a l distance of the plasmapause position (Carpenter, 1966) with the diurnal variation of Pi 2 period. Further- 21 more, Fukunishi et a l . (1970) calculated the Kp variation of Pi 2 period from formulae expressing the latitude vari- ation in the eigen-period of hm oscillations (ObayaShi et a l . , 1958) and the Kp variation of the plasmapause position (Binsack, 1967). The Kp versus Pi 2 period relation obtained by them agrees reasonably well with observations by Troitskaya (1967). In the investigation reported here, the variations of Pi 2 period with changes in equatorial distances of the nightside magnetospheric subregion boundaries, were directly examined. These boundaries, v i z . the plasmapause and plasma sheet inner boundary, were determined by observing sudden changes in the characteristics of certain types of VLF radio signals received by the Alouette 2 s a t e l l i t e . The polar orbit of this s a t e l l i t e was almost circular and i t scanned a wide latitude range. The VLF receiver on board often observed a sudden change i n the characteristics of VLF radio signals.. As the s a t e l l i t e moved to higher latitudes, a common type of change observed, most frequently at invariant latitudes of about 60°, i s a cutoff i n whistler activity. 'Breakups* in the lower hybrid resonance (abbre- viated hereafter with LHR) noise band were also observed, often concurrent with a cutoff in whistler a c t i v i t y , LHR 'breakup* involves abrupt frequency and bandwidth changes as well as a transition from a smooth to an irregular 22 appearance on frequency-time records. The whistler cutoff and LHR 'breakup1 events take place when the sat e l l i t e crosses the plasmapause latitude (Carpenter, Walter, Barrington and McEwen, 1968). In addition to these types of events, a sudden change in ELF emission strength was detected by the Alouette 2 receiver. It i s tentatively assumed that such a change takes place when the sa t e l l i t e crosses the plasma sheet inner boundary. This assumption w i l l be discussed further in Chapter III. During 1967, the magnetospheric subregion bound- ary crossing events were collected with respect to the sate l l i t e orbits within ± 2 hours of longitudinal distance from Ottawa. Approximately sixty examples were obtained in t o t a l . The geomagnetic micropulsation data were obtained at Ralston, southeast of Calgary, Alberta. In this research, the, following conditions were met with regards to the selection of Pi 2. i) Pi were considered only i f they were observed within an interval of ± 2 hours about the time of a sa t e l l i t e 'boundary crossing' event. A shorter interval would yield an insufficient number of Pi 2 events and a longer interval may make a comparison between Pi 2 and subregion boundary positions meaningless. i i ) Selected Pi 2 must have occurred within the local time interval of 1430 to 0530 at Ralston (or approx- 23 imately 2200 to 1300 UT). Pi 2 i s primarily a nighttime phenomena with optimum occurrence being near 2230 LMT. During the daytime, Pi 2 are generally smaller in amplitude and often contaminated with other types of micropulsations (mostly Pc3 and Pc4). i i i ) The average Kp index, during a 9 hour interval about the event, must not be greater than 3 . During times of greater magnetic activity, the magnetospheric subregions often become coincident in location (Frank, 1971). Thus intervals of magnetic quiescence suit this research purpose and since Pi 2 occur most frequently when 1+ ^ Kp £ 2- (as w i l l be shown in section II.d), few examples are eliminated by this criterion. The period of each of the selected Pi 2 series was scaled from the north-south component. If more than one series of coherent Pi 2 occurred in the time interval, then the mean value of a l l the Pi 2 was considered to indicate the period corresponding to the sa t e l l i t e event. The Pi 2 periods were correlated with the boundary crossings. During most crossing events, any possible events due to wide band VLF intensity changes were suppressed with the use of an automatic gain control (abbreviated hereafter with AGC). It i s again pertinent to mention that Pi 2 period does not change with latitude. Saito et a l . (1970) gave two examples showing that Pi 2 period i s common from middle 24 latitudes through polar regions. On the other hand, Fuku- nishi et a l . (1970) mentioned that the wave forms of • i. ' • - . auroral zone Pi 2 have a nearly random noise type of power spectrum, whereas the Pi 2 spectrum in low and middle latitudes is characterized by an outstanding peak near 80 seconds. Yet, they showed that auroral zone Pi 2 spectrum has three peaks overlapping the noise-like background. One of the peaks i s centered at about 80 seconds as in low and middle latitudes. The other two spectral peaks exist near 20 and 10 seconds. The common peak at about 80 seconds seems to indicate that hm oscillations of that period are of the fundamental mode prevailing i n a latitudinal range from low to auroral zone, presumably even up to the latitudes which divide the magnetotail from the region of closed mag- netic lines of force. The periods of the selected Pi 2 series are listed in Table 1. Also liste d are the time, nature and equatorial distance (an L. value in units of earth radii) of each of the correlated subregion boundary crossings. Based on this table, Figure 1 shows a plot of the Pi 2 period change with the: equatorial distance of the plasmapause and/or the plasma sheet inner boundary. An open circ l e represents a plasma- pause event and a solid (dark) c i r c l e , a plasma sheet inner boundary event. A triangle represents a coincidence in location of these two subregions. 25 Table I Alouette 2 events and the correlated Pi 2 periods at Ralston during 1967. Date U.T. Events L value Pi 2 period Feb. 28 0937 LHR, ELF, AGC 4;88 66 (sec.) March 4 0837 LHR, ELF, AGO 2.90 48 March 11 0836 ELF 2.86 43 April 7 0602 LHR, ELF, AGC 2.82 65 May 12 0959 LHR, AGC 3.26 55 June 8 0715 ELF, AGC 5.06 80 June 8 0510 ELF, AGC 6.28 76 June 8 0916 ELF, AGC 4.80 59 June 13 0433 LHR, ELF, AGC ,4.83 48 July 11 0205 ELF, AGC 8.50 92 July 26 0215 ELF, AGC 4.38 58 Sept. 4 1236 ELF, AGC 5.52 66 Sept. 9 1152 ELF, AGC 3.45 72 Sept. 10 0807 Whsl ., ELF, AGC 4.18 69 Sept. 12 1041 Whsl., LHR 4.57 69 Sept. 25 0603 Whsl ., ELF 3.99 56 Oct. 5 0849 LHR, ELF, AGC 7.64 95 Nov. 23 0949 LHR, AGC 3.62 47 Nov. 28 0910 LHR, ELF 4.33 72 Dec. 6 0913 ELF, AGC 6.87 81 Dec. 8 1151 ELF, AGC 3.43 61 Dec. 9 1206 LHR, ELF 3.56 67 26 r r~~— i 1 1 • INNER PLASMA SHEET BOUNDARY EVENT O PLASMAPAUSE EVENT A COMBINED EVENT 0 L _ 1 _J !_ I I 40 50 60 70 80 90 . 100 P E R I O D ( S E C . ) Fig. 1. Magnetospheric subregion L value variations of Pi 2 period 27 The curve marked with 'plasmapause1 is a theo- r e t i c a l one to show how Pi 2 period is expected to change with the location of the plasmapause i f Pi 2's are due to plasmapause fluctuations. The curve is based on two empir- i c a l formulae. One relates Pi 2 period to Kp index: T - 109 - 14 Kp ( ± 5 ) , (for Kp £ 5 0 ) (1) where T i s the Pi 2 period (in seconds). The formula i s a least squares f i t to a scatter plot showing the Kp variation of Pi 2 period (Troitskaya, 1967, Fig. VII. g). The other formula represents the Kp variation of the nightside plasma- pause position: Rpp•- 5.64 - 0.7&jKp~, (for Kp ̂  5 0 ) (2) given by Rycroft et a l . (1970), where Rpp i s the equatorial distance of the plasmapause. The 'plasma sheet inner bound- ary' curve i s also a theoretical one to show how Pi 2 period is expected to change with the location of the inner bound- ary, i f Pi 2 are generated at this subregion boundary. It was obtained by combining (1) and the following empirical relation: Rps - 15 - 6/Kp , (for Kp £ 3 o ) . (3) 28 This equation relates Kp to Rps, the equatorial distance of the plasma sheet inner boundary in units of earth r a d i i . It was derived by Saito et a l . (1970) from six data points about the position of the plasma sheet inner boundary which were obtained by Vasyliunas (1968). For Pi 2 periods less than 70 seconds, the two curves approach one another. The closeness of these curves is consistent with the observed coincidence of the plasma- pause and inner boundary during periods of greater magnetic activity (Frank, 197!)•• However, the 'plasma sheet inner boundary* curve was derived from data which occurred prima- r i l y during times of low Kp. In this range (periods> 70 seconds), the curves separate sufficiently to substantiate an agreement between the •plasma sheet inner boundary' curve and the data from direct observations. The implication of this result i s discussed in Chapter III. b. Pi 2 Diurnal Variations The purpose of this investigation is to more clearly establish the source f i e l d lines of Pi 2 micropul- sations. As previously mentioned (Chapter I), the mor- phology of micropulsations should reflect the nature of the source responsible for these fluctuations. Various charac- t e r i s t i c s of Pi 2 have been observed. F i r s t l y the maximum 29 amplitude of Pi 2 tends to occur near midnight and at geo- magnetic latitudes in the auroral zone (Saito et a l . , 1970), or at least higher than 50° (Jacobs et a l . , I960). Secondly, the direction of the i n i t i a l movement of the Pi 2 distur- bance vector at low and middle latitudes is usually north- east (north-west) prior to (after) midnight (Kato et a l . , 1956; Yanagihara, I960). This disturbance vector has been shown to converge to the northern auroral zone on the mid- night meridian (Saito, 1961). Thirdly, the local time at which Pi 2 occur most frequently i s near 2230 (Yanagihara, 1957a; Rostoker, 1967a). When these three factors are considered, i t is reasonable to expect the source of Pi 2 to be found on auroral zone f i e l d lines near the midnight meridian. The investigation reported here compares the diurnal variation in the occurrence frequency of i n i t i a l l y north-east oriented Pi 2 fs with that of north-west Pi 2. From these observations, the diurnal variation in the probability of occurrence of north-east Pi 2 is found. This- distribution i s compared with the diurnal variation in the occurrence frequency of a l l the observed Pi 2 events. Pi 2 events occurring from January to May 1964 were selected from the rapid run magnetograms at Wingst, Germany and Memambetsu, Japan. The i n i t i a l impulsive movements of the horizontal disturbance vector were observed. In particular, the i n i t i a l north-south component of each 30 Pi 2 event was compared with the east-west component. As Pi 2's are primarily nightside, dusk to dawn, phenomena, only events occurring from 1400 to 08.00. LMT were used. The i n i t i a l movement of a l l of these events was usually either north-east or north-west. Examples of these events are shown in Figure 2 and 3. For each of these two groups, the diurnal variation in the number of Pi 2 (occurring in hourly intervals) is plotted in Figure 4. The diurnal variation in the occurrence probability of north-east Pi 2 is compared with the variation in the occurrence frequency of a l l observed Pi 2 events, in Figure 5. The most probable local time of occurrence of i n i t i a l l y north-east (north-west) Pi 2 is at 1930 (2330) as is shown in Figure 4. From Figure 5, i t can be seen that Pi 2 occurrence is centered about 2205 LMT. This i s near 2300 LMT after which the probability of i n i t i a l north-east orientation becomes less than 50$ (also shown in Figure 5). Thus, Pi 2 i s found to occur about 2205 LMT, prior to (after) which the i n i t i a l Pi 2 orientation i s primarily north-east (north-west). c. Pi 2. Solar Wind and Geomagnetic Activity Studies of morphologic variations in micropulsations may provide information on variations in the source of these 31 ^^AyvvvN/vv\/V\y^—/V* AA/̂ /̂̂  j^/\Ayvv^v\ V\/\A/\/\A/ 05-.4-1 LT MARCH 2 X •"1 05'-51 LT MARCH 11 0Z-4L LT JANUARY 2,6 • — • V W v y v y \ A / v \ / \ / ^ y v v V A A 09-11 LT MARCH 2 NAARCU 1 0 Fig. 2 . Magnetograms of i n i t i a l l y north-west (X+, Y-T Pi 2 X y 19-Z 6 LT W\NGST MARCH 10 X Y 20-01 LT WINGST MARCH 16 X 1 9 - 3 6 LT Y WINGST APRIL 2.8 Fig. 3. Magnetograms of i n i t i a l l y north-east (X-*v)Y+) Pi 2 33 17 20 23 LOCAL TIME 02 05 Fig. 4 . Diurnal variation of i n i t i a l l y north-west and north-east Pi 2 occurrence frequencies 34 23 LOCAL TIME Fig. 5. Diurnal variation of the i n i t i a l l y norjbh-east Pi 2 occurrence proba- b i l i t y and the total Pi 2 occurrence frequency 35 fluctuations. Yanagihara (I960) noted a tendency for the most probable time of Pi 2 occurrence to shift from midnight to about 2200 LMT with increasing magnetic activity. In this investigation the tendency is further substantiated by comparing the most probable local time of Pi 2 occur- rence during periods of low magnetic activity with the most probable time during high activity. In this manner, the •dynamic* behaviour of the Pi 2 source is demonstrated. In order to account for this behaviour, the direction and pressure of the solar wind protons is examined during periods of high, magnetic activity. The local time distribution of Pi 2 events observed by a global network of stations during 1964 (Romana and Veldkamp, 1968) was determined for periods of magnetic quiescence (Kp - 3+) and also for periods of high magnetic activity (Kp - 4-). The diurnal variation in the Pi 2 occurrence was thus determined for each group (one of nigh Kp, the other low). These variations are shown in Figure 6. Next, a study of the solar wind proton streaming angle and pressure was undertaken, in the belief that the most probable time of Pi 2 occurrence may be conditioned by solar wind. The streaming angle, fl, is measured positive eastward from the sun-earth line to the direction of the solar wind flow. Data was obtained, from the World Data Center A, Greenbelt, Md., for the Explorer 34 experiment, 36 UJ 03 IE z 400 300 200 100 \— 0 17 Fig. 6. Kp = 0 o ~ 3+ 20 23 02 LOCAL TIME 05 Diurnal variation of Pi 2 occurrence frequency during high and low magnetic activity 37 of which technicalities have been described elsewhere (Oglivie, 1968). The proton flow direction, velocity and density data were observed at three minute intervals. Approximately 2,000 such datum points, selected from June to November, 1967 when the s a t e l l i t e was outside the bow shock, were selected during times of high Kp (£4.-). The number of points in each flow direction (in a 22.5 degree interval) were recorded and the average proton pressure in each of these directional flow intervals was calculated. This procedure was repeated during times of low Kp (- 3+). Thus the probability of occurrence of each directional interval, P(jrf), was found and the product of P(jz0 and the 2 average proton pressure (NmV ) in that directional interval o was calculated. This value, P(^)«(NmV ), represents the 'expected' solar wind proton pressure, i n that directional flow interval during the prescribed magnetic activity. The solar wind streaming angle variation of this solar wind proton pressure during times of high and of low Kp is shown in Figure 7. The combined results (for low plus high Kp) of the Pi 2 occurrence frequencies shown in Figure 6 yield a peak time near 2230. The most probable local time of occurrence of Pi 2 during low (high) Kp i s after (prior to) this time. In particular, Pi 2 occurs most frequently at 2030 (2330) LMT during high;(low) Kp. This westward shift 38 SOLAR WIND STREAMING ANGLE, * Fig. 7. Solar wind streaming angle variation of particle pressure during high and low magnetic activity 39 is in agreement with Yanagihara's observation. The streaming angle variation in the solar wind proton pressure has a pressure peak directed at an angle of 188° (165°) during low (high) Kp. Thus, during periods of low magnetic activity, Pi 2's occur most frequently near 2330 LMT, when the solar wind protons at 1 AU are streaming at an angle of 188°. During periods of high magnetic activity, this streaming direction is 165° and Pi 2's occur most frequently at an earlier time of 2030. As expected, the proton stream- ing pressure is greater during high Kp than during low Kp. d. Pi 2 Rate of Occurrence Observations of changes i n Pi 2 morphology with magnetic activity and the solar cycle have been less con- sistent and even appear contradictory at times. As men- tioned in section I.b, the rate of Pi 2 occurrence was reported to increase with Kp (Yanagihara 1957b, Kannangara et'al., 1969; Shannon et a l . , 1970). This might imply that the occurrence of Pi 2 should increase during sunspot maxi- mum (when the Kp index i s higher). Yet, Yanagihara (1956) reported that the yearly occurrence frequency of 'night pulsations* i s inversely proportional to the solar activity. In section II.c, the peak time of Pi 2 occurrence was shown to shift westward with increasing magnetic activity. 40 Together, this shift and the increasing rate of Pi 2 with Kp may imply that the peak time of Pi 2 occurrence should be observed earlier during sunspot maximum. Yet, Afanasieva (1961) and Saito et a l . (1968) show that the Pi 2 peak time of occurrence is found near midnight (2300 to 0100) during sunspot maximum and at earlier times (2000 to 2300s) during sunspot minimum. In an attempt to explain these two apparent contradictions, an investigation was made of the Kp variation in the rate of Pi 2 occurrence near sunspot minimum (1964) and at a more active year (1967). The Pi 2 events used in this investigation were those observed by a global network of stations during 1964 (Romana et a l . , 1968) and 1967 (Romana and Van Sabben, 1970). For each of these years the total number of Pi 2 events was found for each three hour Kp level. The rates of occurrence for different Kp levels were obtained by dividing the total number at each level by .the total number of times that the particular level occurred during that year. The rates of Pi 2 occurrence, for both 1964 and 1967 were calculated using over 3,000 reported Pi 2 events from each year. The rates, showing a s t a t i s t i c a l deviation about each point, are plotted in Figure 8. The Kp variation in the Pi 2 occurrence rate, -shown in Figure 8, i s similar for both years despite the fact that the average Kp index was higher during 1967. In 41 1964 0o 10  2 o  3 o  4 o 5Q 6C K p INDEX Fig. £. Kp variation of the Pi 2 occurrence rate during 1964 and 1967 42 both years, i t can be seen that the Pi 2 occurrence rate increases with Kp for Kp values between Oo and 1+. The occurrence rate does not keep increasing beyond Kp = 2o, contrary to results presented by the investigators men- tioned earlier. Thus, during years of both sunspot minimum (1964) and of higher magnetic activity (1967), Pi 2 occurs most frequently during intervals when 1+ - Kp - 2-. CHAPTER III DISCUSSION OF THE RESULTS a. Pi 2 and Magnetospheric Subregions The results of the investigation presented in section II.a and depicted in Figure 1 show an agreement between the 'plasma sheet inner boundary * curve and the data from direct observations. A l l of the five subregion boundary crossings corresponding to Pi 2 periods longer than about 75 seconds were detected with a sudden cutoff in ELF emission strength and accordingly assumed to identify the plasma sheet inner boundary. In the case corresponding to the Pi 2 period of 95 seconds (December 6th event), a LHR breakup was observed simultaneously. A l l of these five crossings were identified when the sa t e l l i t e was on the nightside of the earth and at invariant latitudes not lower than 63.6°, viz., L s- 5.06. The local times as well as the larger L values indicate that ELF emissions in these cases may be a type of auroral zone VLF hiss. ELF hiss and chorus, which are often concurrent, take place mostly on the dayside, and at latitudes lower than those at which auroral zone VLF hiss i s observed. Although dayside VLF hiss occurs i n the polar cusp region, nightside auroral zone VLF hiss i s also observed. VLF hiss was observed at the auroral zone, near 44 and on the poleward side of the 'trapping boundary' for energetic electrons (E 5c 45 kev), in latitude range of about 7° (Gurnett and Frank, 1972). Frank (1971) found that the 'trapping boundary' i s beyond the outer boundary of the earthward edge of the plasma sheet. In case that the 'trapping boundary' is beyond the outer boundary of the edge, i t is within a range of about 1 Re from the outer boundary of the edge. The earthward edge is a 'micro- scopic' view of the inner boundary of the plasma sheet. Frank found that the plasma sheet inner boundary, or edge, has a f i n i t e width, 1 to 2 Re, within which the average energy of electrons in the range 80 ev - E ̂  46 kev decreases exponentially with decreasing distance. These pieces of information relate auroral zone VLF hiss, the 'trapping boundary* and the plasma sheet inner boundary to one another. They indicate that the 'trapping boundary' as well as the plasma sheet inner boundary can be approxi- mately located through observation of a sudden,cutoff in auroral zone VLF hiss activity on the lower latitude side. The evidence that Saito et a l . (1970) presented to prove their plasma sheet theory was that the latitude of maximum Pi 2 amplitude maps along the f i e l d line to the equatorial plane near the plasma sheet inner boundary. The mapping was done based on a certain model of the average magnetic f i e l d configuration of the outer magnetosphere 45 (Fairfield, 1968). The latitude of maximum Pi 2 amplitude was determined from observations at eight stations d i s t r i - buted over a range of geomagnetic latitude from 40.4° to 73.8°. Two Pi 2 events for time intervals having different Kp values were chosen and for each event the latitude of maximum amplitude was determined. It was then mapped to a point on the equatorial plane which was found to be near the position of the plasma sheet inner boundary, inferred from the equation II.a(3). The approach of Saito et a l . was indirect, since i t depends on the inferred inner boundary location. In this respect, the investigation presented here i s a direct observation of the expected Pi 2 variation in period with a magnetospheric subregion, viz., the plasma sheet inner boundary. The results presented in this thesis indirectly suggest that the region of Pi 2 generation, during times of magnetic quiescence, is found near the plasma sheet inner boundary. During times of higher Kp, the Pi 2 period variation with subregion bound- aries is less discernable. These ambiguous results are in agreement with the observed coincidence in location of magnetospheric subregions during times of higher magnetic activity (Frank, 1971). One method of establishing a more direct association of Pi 2 with the plasma sheet inner boundary might be to determine the latitude of maximum Pi 2 amplitude while simultaneously observing the location of 46 the inner boundary. b. Pi 2 Source From sections II.b and c, i t was shown that Pi 2 occur most frequently around 2230 LMT, east (west) of which their i n i t i a l orientation is primarily north-west (north- east). These results and also those of section II.a imply that the Pi 2 source must l i e oh auroral zone f i e l d lines centered about the 2230 LMT meridian. A theory of night- time plasma implosion may account for the i n i t i a l movements (Smith and Watanabe, 1972). Two theories of implosion processes have been suggested (Atkinson, 1966; Kato, 1971). Atkinson's theory assumes that intermittent f i e l d line reconnection takes place. This gives rise to an implosion of t a i l plasmas towards the region of closed magnetic lines of force. The implosion should cause the observed i n i t i a l northward impulse. The i n i t i a l eastward and westward move- ments may be aided by the motion of the imploding convecting t a i l plasmas as they separate to flow about the closed f i e l d lines. This convective 'flow separation' has been suggested by superposing the magnetospheric convective and co-rotating potential fie l d s (Axford and Hines, 1961; Nishida, 1966; Brice, 1967). The most effective impact of the imploding plasmas might be at this nightside 'flow separation' point 47 which could explain the diurnal variation in the Pi 2 occurrence frequency. As mentioned in section I.d, the mechanism pro- posed by Kato calls for an hm burst impacting the f i e l d in the vicini t y of the inner boundary of the partial ring currents. Such an inward burst requires a sudden disruption of partial ring currents in the dusk to midnight quadrant, as observed by the abrupt f i e l d recovery at ATS 1 (Cummings et a l . , 1968). The ring current weakens the magnetic f i e l d inside the current site and enhances i t outside. From a point of hydromagnetics, the ring current carries away some magnetospheric plasmas and frozen-in f i e l d lines from the inner region to the outer region across the current si t e . A sudden partial ring current disruption i s expected to let the carried away f i e l d lines move back to their normal positions, giving rise to a plasma implosion. These f i e l d lines are concentrated about the 2230 LMT meridian. When implosion occurs, the i n i t i a l movement of the f i e l d lines immediately east (west) of this meridian must move i n - ward and veer eastward (westward). This east and west veering i s likely determined by Maxwell stress which the implosion exerts upon the f i e l d lines, spreading away from 2230 LMT meridian. Field lines far away from the activity centre should move almost radially inward. 48 c. Pi 2 Source Variations From the results presented in the Pi 2 source variation study of section II.c, i t is seen that the flow direction and pressure of solar wind protons and the most probable local time of Pi 2 occurrence during low magnetic activity are different from those, during high activity. It i s possible that the local time at which Pi 2 occurs most frequently is dependent upon the solar wind direction. That i s , the solar wind streaming direction may determine the direction of the earthward convecting, t a i l plasma flow which could possibly affect the meridians of peak intensity partial ring currents. Indeed, Cummings (1966) suggested the partial ring currents are formed by the i n - ward convection of charged particles near the midnight meridian. Yet, from low to high magnetic activity, the peak streaming angle changes by only 23°. This corres- ponds to only about 1 hour thirty minutes while the west- ward shift in peak Pi 2 occurrence is about 3 hours. Thus the streaming angle change may not f u l l y account for the peak Pi 2 time s h i f t . Processes associated with the increased solar wind streaming pressure during high mag- netic activity might give rise to the intense, partial ring currents. This increased plasma density, in the dusk to midnight quadrant, could also account for the westward 49 shift in the peak time of Pi 2 occurrence. Thus, the results of this study display the *dynamic' behaviour of the Pi 2 source which appears dependent upon the solar wind. The meridian of Pi 2 generation then, i s closely associated with the earthward convecting plasmas in the magnetotail. This meridian has been considered to indi- cate the 'flow separation' point of the inward convecting t a i l plasmas and/or the peak intensity of the partial ring currents. In view of this discussion (and the one in section I l l . b ) , sketches are presented of the streamline pattern in the equatorial plane of the magnetosphere during low and high magnetic a c t i v i t i e s . Figures 9 and 10 depict the results of this investigation, showing the solar wind direction and peak Pi 2 occurrence time ('flow separation* point) during low and during high magnetic a c t i v i t i e s , respectively. The plasmapause behaviour was reported by Carpenter (1970) and Chappell et a l . (1970). The partial ring current belts were drawn from the information sug- gested by Frank (1967) and Coleman et a l . (1971). d. On the Pi 2 Rate of Occurrence The finding (section II.d) that Pi 2 occur most frequently during intervals when the Kp is between 1+ and   52 2-, is unexpected. Although this result contradicts the observations of previous investigators, i t is f e l t to be • s t a t i s t i c a l l y ' significant, due to the large amount of data used. A direct consequence of this finding is that the yearly occurrence frequency of Pi 2 should reach maxi- mum when the average Kp index most closely approaches optimum level (1+ - Kp i 2-). This would be during sunspot minimum, in agreement with the observations by Yanagihara (1956). A further consequence of this result is that the peak time of Pi 2 occurrence should not shift during the sunspot cycle. Yet such a shift has been observed (Afana- sieva, 1961; Saito et a l . , 1968). It may be that further investigation is necessary in order to establish whether a systematic change exists (or should be expected) in the Pi 2 peak time of occurrence during the solar cycle. CHAPTER IV SUMMARY AND CONCLUDING REMARKS The investigations presented in this thesis have shown that Pi 2 micropulsations do reflect the nature of their source. This source i s in the auroral zone centered near the 2230 LMT meridian, as indicated by Pi 2 occurrence and i n i t i a l orientation studies. The actual source merid- ian i s 'dynamic* in the sense that i t shifts with changes in magnetic activity. Furthermore, the Pi 2 period variation with the location of the plasma sheet inner boundary implies the association of the Pi 2 source with a magnetospheric subregion, and also with the earthward convecting plasmas in the magnetotail. The 'dynamic* source i s also dependent upon the solar wind, as evidenced by the correlated studies of solar wind and source variations with magnetic activity. Lastly, as suggested in sections I.b and d regarding period morphology and nonsimilar latitudinal variations of Pi 2 and bay amplitude, Pi 2 are not always directly associated with magnetic activity (Kp) and/or magnetic storms (ssc and bays). The relationship i s more complicated, as i s seen iri the Pi 2 rate of occurrence study. In summation, the Pi 2 phenomenon appears as a complex problem involving the solar wind, magnetospheric subregions and ionospheric con- ditions. The author feels that this thesis presents some evidence that Pi 2 micropulsations can provide information % on the states of the solar wind, magnetosphere and iono- sphere. It was suggested that Pi 2 may be explained by a nightside plasma implosion process near the closed f i e l d line boundary and/or the ring current. Two theories regarding implosion have been presented (Atkinson, 1966; Kato, 1971), as to which theory is the more l i k e l y , i t i s d i f f i c u l t to decide. Indeed, i t i s possible that the two mechanisms each generate Pi 2, and in this sense, they may be regarded as of a complementary nature. Atkinson's theory associated Pi 2 with geomagnetic bay disturbances. Yet many Pi 2's not related to this magnetic activity have been observed (Romana et a l . , 1968). These non-related Pi 2's could be due to partial ring current disruptions required by Kato's theory. The Pi 2 plasma implosion theory and also the theory of Pi 2's as propagating ionospheric disturbances, discussed by Jacobs (1970), each explain much of the observed morphology. An objection to Pi 2 as the effect of a current system was that occassional clockwise rota- tions are observed when the polarization is usually counter-clockwise. Yet, Pi 2 polarization may be due to inward moving f i e l d lines which are veered eastward by the skewed position of the plasmasphere bulge (Smith and Watanabe, 1972). This motion i s depicted in Figure 11. 55 Fig. 11. Motion of magnetospheric plasmas due to an implosion (Smith and Watanabe, 1972) 56 In such a situation, the polarization at meridians near the abrupt westward edge of this bulge may occassionally be observed as clockwise (in the northern hemisphere). Another objection to Pi 2 fs due to a current system was that latitudinal variation of bay magnitude is not always the same as that of the amplitude of Pi 2 (Saito et a l . , 1968). Yet Akasofu, Chapman and Meng (1965) proposed a model current system for an intense polar magnetic storm which showed peak intensities in the midnight to dawn quad- rant. Thus, observation of occassional dissimilarities between Pi 2 (occurring near 2230 LMT) and the magnetic bay should be expected on the basis that the activity centres may not be coincident. 57 BIBLIOGRAPHY Afanasieva, V. I., Short period oscillations of the geo- magnetic f i e l d , IAGU Bull., 16G, 48, 1961. Akasofu, S. - I . S . Chapman and C. - I . Meng, The polar electrojet, J. Atmos. 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