Geotail Observations of Solar Wind and Interplanetary Phenomena

GEOTAIL OBSERVATIONS INTERPLANETARY

OF SOLAR WIND PHENOMENA

AND

T. Terasawa 1

1Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hon9o, Bunkyo-ku, Tokyo 113-0033, JA PAN ABSTRACT

In this review, based on the recent GEOTAIL observation I will cover three broad topics, bow shock and foreshock phenomena, interplanetary phenomena, interstellar medium, and solar flare effects. (1) I will concentrate on the physics of energetic diffuse ions found in the bow shock and foreshock regions, and discuss three subtopics, namely, the origin of these ions, their nonlinear reaction to the shock structure, and their transport process from the region upstream of the nose bow shock to the predawn foreshock region. (2) I will discuss two subtopics about propagating interplanetary shocks ahead of coronal mass ejecta: It is shown that in addition to the well-known diffusive ion acceleration at these shocks there sometimes occurs diffusive electron acceleration. The nonlinear reaction of the accelerated particles, the same phenomenon as found in the bow shock case, is identified at least once in an moderately strong interplanetary shock. (3) In addition to the above 'normal' bow shock/interplanetary phenomena, GEOTAIL has provided unique 3D phase space information on a 'stranger', namely pickup He + ions of local interstellar medium origin. (4) I will then describe unexpected direct observations of solar flare signals on GEOTAIL: the spiky enhancements (duration -.~ several minutes) of the sunward electric fields and background counts of plasma detectors, both of which occur concurrently with the peaks of large solar X-ray flares.

INTRODUCTION The number of GEOTAIL papers published in years 1991-2003 treating topics of solar wind and interplanetary phenomena is found to be ,-- 50", which is slightly less than 10 % of all GEOTAII~ papers published in tile same interval (~-600). This relatively small percentage reflects the fact that GEOTAII~ is basically designed for the studies of the magnetosphere especially the magnetotail , to which the main interest of the GEOTAII~ scientific community is directed. Nevertheless, as briefly reviewed in this article, GEOTAIL has been providing unique and important data.sets for the study of the regions surrounding the earth's magnetosphere. To get a measure how tim interplanetary phenomena are covered by tile GEOTAIL observations, let us start counting the number of observed interplanetary shocks (IPSs). During years 1999-2001 GEOTAIL recorded ,-- 40 IPSs in the solar wind region upstream of the earth's bow shock. This number of IPSs amounts to 36% of all IPSs detected by SOIIO during the same interval. This percentage, being consistent with the fact, that GEOTAIL has been exposed to the solar wind for ,.~ 40% of time during its orbital motion (Figure l a). Figure l a, at the same time, shows how the bow shock and its upstream region are covered by the GEOTAIL observation. Earlier in its mission phase, (]EOTAIL covered more distant-tail part of tile magnetosphere (Figure I b). During this orbital phase the orbital percentage for the solar wind region was much limited, but data for several important events were obtained (see the following discussion). In this review, I will choose a limited number of topics from the solar w i n d / i n t e r p l a n e t a r y / u p s t r e a m / f l a r e studies based on GEOTAIL observations consisting of the low energy plasma experiment (LEP; Mukai et

* We have not included papers simply using GEOTAIL as a solar wind monitor.

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al., 1994), the comprehensive plasma instrument (CPI; Frank et al., 1994), the energetic particles and ion composition instrument (EPIC; Williams et al., 1994), the high energy particle experiment (HEP; Doke et al., 1994), the plasma wave instrument (PWI; Matsumoto et al., 1994), the electric field experiment (EFD; Tsuruda et al., 1994), and the magnetic field experiment (MGF; Kokubun et al., 1994).

Fig. 1. (a) Sample orbits of GEOTAIL for the near-earth phase (1 March - 10 November 1995). (b) The orbit of GEOTAIL during the distant-tail phase (1 January- 31 December 1994). The nominal positions of the bow shock (a and b) and the magnetopause (a) are also shown. The unit of spatial scale is the earth radius (RE).

BOW SHOCK AND FORESHOCK PHENOMENA In this section I concentrate on the topics of upstream energetic ions (of several tens of keV). | refer the reader to the original papers for other topics such as motions/positions of the bow shock (Tsubouchi et ai., 2000; Dmitriev et al., 2002), whistler wave observations (Hayashi et al., 1994; Matsui et al., 1997; Zhang ct al., 1998, 1999), 2fp~ radio emissions (Reiner ct al., 1997; Kasaba et al., 2000a), the hot flow anomaly (Sibeck et al., 1999), waves/particles and disturbance in the magnetosheath (Petrincc et al., 1997a, 1997b; Sibeck et al., 1997, 2000; Nemecek ct al., 1998; Zong et al., 1998, 1999; Seon et al., 1999; Matsuoka et al., 2000; Safrankova et al., 2000; Zastenkcr ct al., 2002; Nagano et al., 2003), the observations of anomalously low density solar wind in the magnetosheath (Kasaba et al., 2000b; Terasawa et al., 2000), and UI.F waves associated with lunar wake (Nakagawa et al., 2003). It is also noted that Matsumoto et a]. (1997) presented an excellent review of the PWI measurement of waves (frequency ~> 10 Hz) in the upstream and bow shock regions.

Origin of diffuse ions Energetic ions up to ,.-100 keV (and sometimes ,.~several hundreds keV) have been observed both upstream and downstream from the earth's bow shock (e.g., Lin et al., 1974; Fuselier, 1995; Anagnostopoulos, 1994). These ions have a wide angular distribution, and are called 'diffuse ions'. It is commonly observed that large amplitude low frequency electromagnetic waves (0.01-0.1 Hz) accompany these diffuse ions. There are at least two possible origins for these diffuse ions" One is the diffusive shock acceleration (DSA, hereafter) process at the bow shock, and the other is the leakage of magnetospheric ions. One evidence of the DSA process, for example, is a good correlation between the diffuse ion density and the solar wind density (Trattner et al., 1994). There is, on the other hand, evidence of magnctospheric leakage" During the upstream diffuse ion events magentospheric ions such a~s O +, N +l, and 0 +2 have Been identified (e.g. M/Sbius et al., 1986; Sarris and Krimigis, 1988; Desai et al., 1999; Christon et al., 2000; Keika et al., 2003). Therefore it is quite important to determine to what extent each of these mechanisms contributes to form the upstream population of energetic ions. In the nominal foreshock region, namely the region of [YGSEI ~< 20 -- 30 RE (as covered by the previous spacecraft, such as IMP-6, ISEE-1/2 and AMPTE), it turns out to be a difficult task to discriminate the above two possibilities" When the observer is connected to the quasi-parallel part of the bow shock by

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Fig. 2. (a) The GEOTAIL position around 08:30 on 19 February 1994 relative to the bow shock and the magnetopause. The dashed and solid arrows show the IMF of nearly Parker's spiral direction and nearly radial direction, respectively. (b) The E-t (energy versus time) plot of count rates of energetic ions (5-40 keV//q) in the directions sunward -1-90~ (c) The magnetic field intensity. (d) The IMF longitudinal angle (~B). (e) The IMF azimuthal angle (#B)-

the IMF (interplanetary magnetic field), where the efficiency of DSA process is expected to be maximized, he/she is also connected to the magnetosheath region adjacent to the magnctopausc, where the leakage of the magnetospheric particles is taking place. If we can observe diffuse ions in the foreshock region with a large cross-field separation from the magnetopause, we could circumvent this difficulty. Figure 2 shows such an event (Sugiyama et al., 1995b)" Before ,,-08:28 the IMF was nearly in the Parker's spiral direction except, for two short intervals around ,,~08:16 and ,-~08"19. Under such a spiral geometry (a dashed arrow in Figure 2a), GEOTAll, was magnetically disconnected from the bow shock and the magnetopause. At ,,-08:28 the field lines passing the GEOTAIL position began to intersect tile quasi-parallel bow shock (a solid arrow in Figure 2a), but was separated from the nearest magnetopause surface by ,-~20 RE. In the F-t (energy versus time) plot, (Figure 2b), diffuse ions > 20 keV/q appeared within a few minutes of the beginning of the field intersection with the bow shock. If these ions should have come from the magnetosphere, they need to have crossed the field lines by > 20 RE within a few minutes. However this is impossible since the cross-field diffusion over 20 RE needs at least ,,-3 hours even in the Bohm limit. Therefore, these diffuse ions are most naturally explained in terms of the DSA process at the bow shock connected to the observer by tile IMF. Recently, an important progress has been made through the charge-state measurement of energetic ions _> 50 keV in the foreshock and magnetosheath regions by the EPIC aboard GEOTAII, (Keika et al., 2003). Firstly these authors classified energetic ions into two categories, namely, those with low charge state (LCS, e.g. O + and N +) presumably of the magnetospheric origin, and those with high charge state (tlCS, e.g. 0 +6 and 0 +7) presumably of the solar wind origin. Secondly they have shown that the LCS ions are found preferentially in the duskside magnctosheath while the tICS ions are found preferentially in the dawnside foreshock regions. This spatial feature of the HCS ions provides a strong evidence that they are accelerated at the bow shock and its vicinity. N o n l i n e a r effect It has been known that well ahead of the main shock ramp the upstream solar wind flow is decelerated by a few k m / s ,-, few tens of km/s in the foreshock region where the diffuse suprathermal ions (10-102

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keV) have non-negligible pressure (Formisano and Amata, 1976; Diodato and Moreno, 1977; Gosling et al., 1978; Bonifazi et al, 1980a, 1980b, 1983; Zhang, Schwingenschuh, and Russell, 1995). In the general context of DSA process, shocks affected by this nonlinearity have been studied in terms of 'cosmic ray mediated' shocks (CRMSs, hereafter). The CRMS effect is considered essential in source regions of cosmic rays, such as supernova shocks and shocks in gamma ray bursters, for which only remote astronomical observations are possible. Therefore, the studies of CRMS are done mainly from the theoretical side (e.g., Drury and VSlk, 1981; Ko, 1995; Ellison et al., 2000), and the bow shock provides a unique opportunity for in situ observations of CRMS. However, detailed studies of the CRMS nature of the bow shock have been prevented by the transient variations of the solar wind: It has not been clear how the deceleration of the solar wind flow correlates with the diffuse ion energy density. Terasawa et al. (2001), on the other hand, has presented a case study of an exceptionally clear example of the bow shock observation as a CRMS (Figure 3).

Fig. 3. Data are shown for the period from from 22:30 UT on 8 October (t -- 22.5 h) till 02:12 UT on 9 October (t - 26.2 h): (a) Energy-versus-time plots of energetic ions (5-40 keY/q) and (b) for solar wind ions (0.3-8 keY/q) where counts per 12-sec sample are shown in a pseudo-color scale. (White stripes at t ~ 24 in both panels were due to a data gap.) (c) Solar wind velocities observed by GEOTAIL (red curve) and WIND (black dotted line). (d) Energy density of diffuse ions (Eo/: red) and magnetic and thermal proton subpressures (PB: blue, and Psw: black). To avoid overlaps, Psw is shifted clown by one decade. (e) The amount of the solar wind deceleration (AVs~) is plotted against the energy density of diffuse ions (ED/), where AVs~, is defined as Vsw,GEOTAIL- Vsw,WIND. (f) The GEOTAIL orbit from 0 UT on 8 October 1995 to 0 UT on 10 October 1995. Nominal shapes of the bow shock and magnetopause are also drawn. The GEOTAIL crossing of the bow shock at 01:34 UT on 9 October 1995 was near the subsolar point at (X, Y, Z)aSE=(13.8, 1.0, 1.1) RE. Figure 3 (f) shows the orbit of GEOTAIL for 48 hours: From 22:00 UT on 8 October 1995, the GEOTAIL spacecraft traversed the foreshock region of the nose bow shock over 3 hours. During this interval the solar -270-

wind was more or less steady and continuously monitored by the WIND spacecraft cruising about 130 RE upstream of the GEOTAIL position (The estimated convection delay time of the solar wind from WIND to GEOTAIL was ,,,22 minutes.) Figure 3 (a) and (b) respectively show the energy-versus-time (E-t) plots of diffuse ions (propagating sunward) and solar wind ions observed aboard GEOTAIL. Figure 3 (c) shows changes of the solar wind velocity Vsw,GEOTAILa t GEOTAIL (red curve) and Ysw,WINDa t WIND (black dotted line). The GEOTAIL crossing of the bow shock at t = 25.57 h (01:34 UT) is evidenced by a sudden widening of the solar wind E-t plot (Figure 3 (b)) as well as a sharp drop in the solar wind velocity (Figure 3 (c)). Toward the bow shock, the upstream diffuse ions showed a gradual increase of their intensity, which is seen as the change of the color from yellow to dark red in Figure 3 (a). Figure 3 (c) shows that Vsw,GEOTAIL decreased by 10-150 km/s from the Ysw,WINDalong with the increase of the diffuse ion intensity. To see the change of the diffuse ion intensity quantitatively, their energy density EDt in the shock rest frame is calculated as,

l m ( v - OBs)2f(~dff

(1)

where f(ff) is the observed velocity space distribution function, and OBS is the bow shock velocity in the observers' frame. An assumption here is that the bow shock rest frame is nearly identical to the observers' rest frame, namely, that vss is negligibly small and can be set to zero. That no multiple bow shock crossing was observed during this event is consistent with this assumption. The subpressure exerted by the diffuse ions, PDI is calculated as ( T D ! - 1)EDt assuming 7DI=5/3. Figure 3 (d) shows the change of EDI (a solid line) as well as changes of the magnetic subpressure (PB, a dashed line), and the thermal proton subpressure of the solar wind (Psi, dots). EDt and PB (and Psw weakly) showed gradual increases before their final jump at the bow shock. Figure 3 (e) shows the scatter plot of AVs,, plotted against EDr, which shows a clear negative correlation with the correlation coefficient 0.74. From the observed solar wind parameters, the change of the ram pressure of the solar wind, A(p~,,,V~2,~) in the bow shock rest frame, is calculated to be ,.- -(1-2) • 10 -1~ Pa just before the bow shock crossing. On the other hand, subpressure increases, APD#, APs, and AP,~, are obtained as +0.8• 10 -l~ Pa, +0.2• 10- l ~ Pa, and +0.1 x l0 -l~ Pa, respectively. (To calculate these increases we took values of PD#, PB, and P ~ at t = 22.5 h a.~ their 'base' values.) Thus their summation, APDt+APB+AP~,~ ,', 1.1 x l0 -l~ Pa, roughly compensated the ram pressure change. This is what is expected for CRMSs. P r e d a w n f o r e s h o e k region Extensive studies of the nature of diffuse ions and related low frequency waves have been made mainly in the upstream region ahead of the terminator (X > 0 RE), and there have been only a few studies of upstream and bow shock regions well behind the terminator (X << 0 RE) (e.g., Terasawa et hi, 1985; Greenstadt et at., 1990; Sugiyama et hi., 1995a; Bennett et al., 1997). Utilizing the GEOTAIL observations during the interval shown in Figure 1 (b), one can make comprehensive studies of this 'last frontier' of the upstream region. Figure 4 (a) shows the GEOTAIL orbit during the interval of 25 June - 2 July 1994 covering the predawn region (X, Y)CSE "~ (--60,--70) RE to the nose upstream region (IYgr < l0 RE, say). Figure 4 (b) shows the variation of the X component of the solar wind velocity throughout the interval. Figure 4 (c) shows the longitudinal angle of the IMF (~B) which was mainly within +30 ~ of the spiral direction (-45 ~ or 135~ Figure 4 (d) shows that the intensity of protons (,.~ 36 keV) was maximized around the nose upstream region and decreased toward the predawn direction (Terasawa et hi., 2003). Figures 5 (a)-(c) shows the ecliptic projection of the phase space distributions of diffuse protons. The corresponding anisotropy plots are shown in Figure 5 (d)-(f). These protons are more or less isotropic in the nose upstream region ((a) and (d)), but they have anisotropic pancake distributions around the IMF (perpendicular > parallel) in the predawn upstream region ((b)-(c) and (e)-(f)). In the predawn upstream region the proton intensity was maximized when the IMF was close to the spiral direction (not shown). These observations are consistent with the conclusions obtained from an ISEE-3 crossing of this region in 1983 (Terasawa et al., 1985), which had remained unconfirmed because of the incomplete energy and angular coverage of ISEE-3 observations. The new GEOTAIL observation gives a support for the early interpretation that the diffuse ions are predominantly produced in the nose upstream region and transported by the solar wind flow mainly

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drift motion).

PHENOMENA

From the GEOTAIL observations in the solar wind, 1 discuss selected topics about interplanetary (;ME shocks and pickup lie + ions of the LISM (local interstellar matter) origin. For ol, her topics such ms galactic/anomalous cosmic ray particles and solar/corotaing energetic particles (e.g., Ilasebe et al., 1994, 1995, 1997, 2001; Takashima el, al., 1997; Kobayashi et al., 1998; Doke et al., 1999), type Ill solar radio bursts (Kasahara et ai., 2001), and kinks or discontinuities (Whang el, al., 1998; Kessel, Quintana, and Peredo, 1999; Nakagawa el, al., 2000), I refer the reader to the original papers. D i f f u s i v e a c c e l e r a t i o n o f e l e c t r o n s at i n t e r p l a n e t a r y

CME shocks

During the period from late '70s to early '80s, when the I)SA model was being established as the 'standard theory' of cosmic ray origin, there were close collaborations between astrophysical theorists anti space experimentalists working on data from heliospheric shock environments (see reviews, e.g., Tsurutani et al., 1985a, 1985b; Terasawa and Scholer, 1989; Terasawa, 2001). However, not all aspects of DSA were covered by these early studies. One topic not well explored was tile difference between behaviors of ions and electrons: While ion events have been extensively studied observationally and theoretically since the middle '80s (e.g., I,ee, 198a; Kennel et a1.,1986; Gordon et al., 1999; Igarashi et al., 2003), detailed analyses for electron events have become available only recently (e.g., Shimada et al., 1999). It is noted that, shock-accelerated ions can excite right-handed polarized MIlD waves via cyclotron resonant condition and be self-scattered. (These right-handed waves belong to the lowest frequency branch of the whistler mode.) While it might seem possible for shock-accelerated electrons to excite left-handed polarized MHD waves and be self-scattered similarly, severe cyclotron damping by thermal ions for these left-handed waves practically prevents these excitation and scattering processes. To circumvent this difficulty, Levinson (1992) proposed to consider a cyclotron resonant interaction between electrons and obliquely propagating whistler waves. However, since obliqueness reduces the interaction efficiency, high Alfv6n Mach numbers (>-,,40) are needed for the Levinson's process to work. Therefore, if this Levinson's process is the only mechanism by which whistler waves are excited, electron DSA could not be effective for typical heliospheric shocks whose Alfv~n Mach numbers are less than 10-20. -272-

Fig. 5. Panels (a)-(c) show the ecliptic cuts of the phase space distributions (PSD) of diffuse protons, f(V), in the velocity range of 1000-2869 km/s (5.2-43 keV) for (a) 00:51-00:54 UT on 2 July 1994 at (X,Y,Z)GsE ---- (+16.6, -t-9.4, +0.3) RE, (b) 20:14-20:17 UT on 28 June 1994 (at (X,Y,Z)GsE -- (-14.2,-52.6,-0.2) RE, and (c) 19:35-19:38 UT on 25 June 1994 at (X,Y,Z)GsE -- (-52.2,-68.7, +2.0) RE, The pseudo-color scale and directions are given in the left of the figure. Directions of the IMF are shown by straight lines (marked 'B'), which are drawn to pass through l~sw (solid circles). Panels (d)-(f) show the traditional anisotropy plots of 36 key protons for the same intervals as (a)-(c). A dark red spot (or tongue) below the center of panel (a) represents a contamination of solar wind heavy ions. Figure 6 shows intensity variations of nonthermal (a) protons and (b) electrons around the passage of a m o d e r a t e l y - s t r o n g (the Alfv6n M a t h number M A ,'~ 6) i n t e r p l a n e t a r y shock on 21 February 1994 (Baker et al., 1995; S h i m a d a et al., 1999). This i n t e r p l a n e t a r y shock was created in a C M E event associating with a near-disk-center (N09 W02) flare at 0138 UT on 20 February 1994, a b o u t 32 hours before the shock arrival. The arrival of the shock front, at G E O T A I L was identified at ~09:03 UT from the observations of t h e r m a l plasmas and magnetic field. Both nonthermal protons and electrons showed gradual increases between 06 UT and 09 UT, and reached their m a x i m a at the shock arrival. These behaviors are consistent with w h a t the s t a n d a r d DSA theory describes. As seen Figure 6 (a), ions showed a text-book like behavior for the diffusive shock acceleration events: exponential increases in the upstream region (before 0903 UT) and flat tops in the downstream region (after 0903 UT). In Figure 6 (b), electrons also had exponentially increasing upstream profilest.The spatial diffusion coefficients corresponding to the observed upstream slopes were ,,~ several• 10 is cm 2 sec -1 both for electrons and protons. Since this value of the diffusion coefficient is by 2-3 orders of magnitude smaller than the typical i n t e r p l a n e t a r y values (Palmer, 1982), local excitations of waves resonating with these particles are needed. Shimada et al. (1999) have confirmed t h a t there was an e n h a n c e m e n t level of low frequency ( < 0.01 Hz) M H D waves which are responsible for tile scattering of ions. For electrons, they have also identified whistler wave e n h a n c e m e n t in the region around the i n t e r p l a n e t a r y shock. Figure 6 (c) shows the power s p e c t r u m of right-hand c o m p o n e n t of the upstream magnetic turbulence in the frequency range of 0.01-8 Hz. Above ,,,1 Hz, there were i n t e r m i t t e n t bursts of whistler waves which were created at the shock front or in the shock downstream region, and propagated upstream ( S h i m a d a et t Note that the behaviors of protons and electrons at tile shock front and in tile downstream region were different: Below several keV there were jumps of the electron phase space densities at the shock. Further electrons showed general decreasing trends till 1400 UT. Shimada et at. (1999) explained these differences in terms of heating of suprathermal electrons at the shock front as well as the adiabatic cooling accompanying with the downstream plasma expansion. -273-

Fig. 6. The GEOTAIL observation in the interplanetary space during the interval of 5-14 UT on 21 February 1994 including an interplanetary shock passage at 09:03 UT. (a): variations of nontherrnal proton flux intensities (arbitrary unit), at the energies of 0.055-0.11, 0.11-0.65, 0.65-1.5, and 1.5-4.0 MeV. (b): variations of nontherrnal electron flux intensities (arbitrary unit), at the energies of 0.42-0.49, 0.85-1.0, 1.7-2.1, 4.2-5.0, and 8.5-10.2keY. (c): the right-hand component of the upstream magnetic turbulence in the frequency range of 0.01-8 Hz (6-gUT).

al., 1999). In the lower frequency range (<1 Hz), on the other hand, there was a more-or-less continuous component whose intensity showed a gradual increase toward the shock (the right edge of the figure). Recently, Nakata el, al. (2003a, 2003b) has pointed out the importance of this gradually increasing component as the scatterer of low energy electrons, and argued that they were created in the nonlinear ca~scading process from the low frequency (<0.01 Hz) MHD waves excited by the shock-accelerated protons. This observation suggests that there is a 'proton-assisted' DSA process for non relativistic electrons by which we can avoid the [~evinson's limitation.

'Cosmic-ray-mediated' interplanetary shock In the previous subsection, we have discussed the nonlinear effect of the DSA process at the bow shock (production of upstream diffuse ions) in terms of the 'cosmic-ray-mediated' shock (CRMS) property. It is of interest how we can find the similar property in interplanetary CME shocks. However, after the search of the CRMS property among several tens of interplanetary shocks observed in the interval of 1994-2001 by GEOTAIL, only one example has been so far identified (Terasawa el, al., 1999). This rarity of the CRMS property makes a marked contrast with the bow shock observation: Essentially in all the qua~si-parallel bow shock crossings where diffusive ions appear, the CRMS property, namely the partial deceleration of the incoming solar wind flow, can be |dent|tied. This difference between the bow shock and interplanetary shocks certainly comes from the fact that interplanetary shocks usually have smaller Alf6n Mach numbers (MA <2-3, see e.g., Berdichevsky et al., 2000) than the bow shock (MA ~ 5-10). Only in some exceptionally large IPSs, MA can exceed 5-10. The unique example of the interplanetary CRMS was obtained on 21 February 1994, during which the evidence of electron DSA was obtained (previous subsection, Figure 6). Figure 7 shows an enlarged time profile over 4 hours: From 8 UT to 9 UT, just before the shock arrival (09:03 UT), gradual increases were seen in the magnetic field magnitude, Babs (the top panel), in the solar wind density, Nsw (the second panel), in the thermal proton temperature, Tsw,p (the third panel), and in the solar wind velocity, Vsw (the fourth panel). For example, Vsw showed a gradual increase of ,,-50 km/s ahead of the main velocity jump of-,~400 km/s. (Note that the velocity increases in the observers' rest frame. In the shock rest frame, the gradual velocity change corresponds to the gradual deceleration of the upstream plasma flow.) The changes in these quantities are what are expected if this shock was a 'cosmic-ray-mediated' shock (CRMS). For quantitative discussion, Figure 7 (e) shows subpressures exerted by nonthermal electrons (250 eV-d0 keV; blue), by nonthermal ions (70 keV-10 MeV; red), by magnetic field (black), and by thermal protons (blue,

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Fig. 7. Observational evidence of the CRMS nature at the interplanetary shock arrived at GEOTAIL on 21 February 1994; (a) the magnetic field magnitude, Babs, (b) the solar wind density Nsw, (c) the proton temperature Tsw,p, (d) the solar wind velocity Vsw,x, and (e) subpressures (magnetic Pb, electron e, thermal proton pressure Psw,p, energetic protons HEP, and their sum). dotted) as well as their sum (thick black). Summing up the subpressure increases from 8 UT to 9 [:IT, one obtains the value of ,,-1.0 • 10 -1~ Pa. On the other hand, the ram pressure of the solar flow (in the shock rest frame) was decreased by ,,,1.3 • - l ~ Pa (luring the same interval, which is fairly close to the above sum of subpressures. Taking into account of the uncertainty of the subpressure determination (~30 %), one can conclude that this interplanetary shock belonged to tile category of CRMS. (Of course, this conclusion is based on the presumption that the observed changes of Babs, Nsw, Tsw,p and Vsw between 8 UT and 9 UT were not accidental, and the change of the shock propagation speed during the same interval was less than ,,~ 50 km/s. We need more CRMS samples of interplanetary shocks to make further quantitative discussion.) LOCAL INTERSTELLAR MEDIUM It is widely known that the heliosphere is filled with the pickup ions (PUIs) of the local interstellar medium (LISM) origin, which have been believed to be well pitch-angle scattered in the solar wind and have a spherical distribution function in the velocity space (e.g., Vasyliunas and Siscoe, 1976). MSbius et al. (1985) first reported the detection of He + PUIs of the LISM origin using information from the A M P T E

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spacecraft (for the recent review, see, e.g., Gloeckler and Geiss, 2001). While previous observations have been made by mass-spectrometer-type sensors which resolve ion species (M/q), Noda et al. (2001) showed that simple electrostatic analyzers only with energy-per-charge separation, such as the LEP on GEOTAIL and the ion analyzer on the NOZOMI spacecraft, can be utilized for the study of He + PUIs. A weak point of the observation by electrostatic analyzers, of course, is that there are contaminating solar wind ions (protons, alphas, and heavier ions) which often make unambiguous identification of He + PUIs not easy. However, while the d a t a from the mass spectrometers so far are limited to the 2D plane in the velocity space, the data from the electrostatic analyzers can provide information of 3D distribution function of He + PUIs.

Fig. 8. (a) A schematic illustration of the torus distribution of He + PUIs of the interstellar origin, which is being viewed from the ( + V x, + V y, +Vz) direction. In the part inside the torus there are contaminating solar wind ions (protons, alphas, and heavier ions). (b) An ecliptic projection of the observed distribution function, which is a cut along the conical surface +22.5 ~ above the ecliptic plane. The observation of these torus He + PUIs was made 16:01-16:47 UT on 4 December 2000 at (X,Y,Z)GsE ~ ( 4 , - 3 0 , 3) RE. The green arrows show the average magnetic field direction during the observation interval, ([BI, ~ B , OB) ~ (7.3 nT, -14.4 ~ 62.4~ (c) A 2D cut of the distribution function along the meridional plane including the field direction. Following the Noda's works, Oka et al. (2002a) have presented a detailed study on the shape of the 3D distribution function of He + PUIs with the GEOTAIL LEP dataset: In the solar wind intervals, which were selected as being free from the contamination of foreshock diffuse ions, unambiguous identification of Hc + PUIs of the LISM origin was possible for ,.~ 57 % of the observation time. In these 57 %, the 3D spherical (isotropic) distribution functions are found for ,.~34 %, and 'torus-like' distribution functions (gyrotropic but not isotropic) are identified for ,.-23 % (Figure 8). Oka et al. (2002a) have further shown the status of the He + PUIs, spherical or 'torus-like', depends on thc level of the magnetic turbulence of the solar wind, high or low. Oka and Terasawa (2003) have further investigated the origin of these turbulence, and concluded that they are intrinsic to the solar wind and excluded the possibility of self-excitation by the PUIs themselves. This observation that the tie + PUIs have a 'torus-like' shape in the velocity space for significant amount of times indicates that the pitch angle scattering process for these ions is not as much effective as previously thought, but is consistent with the recent observations of long scattering mean free path of the He + PUIs as large as ~ 1 AU (Gloeckler et al., 1995; MSbius et al, 1998; Fisk et al., 1997; Schwadron et al., 1999). The PUIs have been considered as an effective source of the accelerated particles produced at various heliospheric shocks, such as the termination shock, CIR shocks, and planetary bow shocks. The direct evidence of the acceleration of He + PUIs at the earth's bow shock has been obtained by Wang et al. (from A M P T E observations, 1995) and Oka et al. (from GEOTAIL observations, 2002b). -276-

Fig. 9. (a)-(d) show the observation from 11:40 to 12:10 UT on 6 November 1997: (a), (b), and (c) are E-t plots for the electron sensor, the ion sensor, and the solar wind ion sensor of the LEP instrument, respectively. (d) shows the soft X-ray intensities at two wavelengths (1-8 A and 0.5-4 A) monitored by the GOES spacecraft (one minute average). (e) shows the Yokhok hard X-ray counts at three energy bands, 23-33 keV (black), 33-53 keV (green), and 53-93 keV (red) with arbitrarily scaled background counts of LEP ion sensor (blue). (f) shows the Yohkoh gamma ray counts at two energy bands, 2.1-2.4 MeV (red) and 4.0-7.2 MeV (dashed red) with arbitrarily scaled background counts of LEP ion sensor (blue). Note that the LEP 'data' are affected by the plasma sheet ions when the scaled counts became less than ,,-300 (panel (e)) or ~40 (panel (f)).

SOLAR FLARE EFFECTS During solar flares photons in various wavelength are emitted. Unexpectedly it is found that the electric field measurement (EFD) of GEOTAIL is affected most likely by extreme ultraviolet (E(JV) photons enhanced during solar flares (Takei et al., 2003a). It is further found that the particle counters (microchannel plates for ions and channeltrons for electrons) of tile LEP instrument are sensitive to the hard X-ray photons (Takei et al., 2003b) as shown below. On 6 November 1997 GEOTAIL was in tile tail plasma sheet and counting rates of particles were moreor-less steady for the interval plotted in Figure 9 (a)-(c) (ll:40-12:10 UT), except during the ,--2 minute interval around 11:53-54 UT, when energy independent count increases were seen in the LEP d a t a from the ion sensor (Figure 9 (b)) and the solar wind ion sensor (Figure 9 (c)). Figure 9 (d) shows the simultaneous GOES soft, X-ray observation, from which we have found that this event occurred during the initial phase of a large flare (X9.4/2B) at S18 W63 on the solar disk. The soft X-ray intensities reached their maxima around 11:54 UT. It is known that photon emission profiles at solar flares are energy dependent and are sharper for higher energy. In panels (e) and (f) of Figure 9 the L E e data (blue) are shown in the subinterval 11:50-12:00 UT, during which the Yohkoh hard X-ray and gamma-ray detectors showed bursty increases starting from ,,~11:52 UT with durations of several minutes (Yoshimori et al., 1999). In these panels it is seen that tile LEP 'data' closely followed the hard X-ray light curve at 53-93 keV. Since the particle counters of the LEP are put inside the GEOTAIL satellite structure and separated from external radiations by Aluminum walls of thickness ,,- several mm, the solar photons penetrating to these counters should have energy above several tens of keV. Yoshikawa (2003)experimentally confirmed that the particle counters of the same type as the LEP's are sensitive to hard X-ray to g a m m a ray photons with a quantum efficiency of ,,, several %. Thus the detection of solar hard X-rays by the LEP sensors is not unreasonable happening. In addition to a case study for the 6 November 1997 event, Takei et al. (2003b) have made a statistical survey of solar flare events to see whether similar count enhancements are seen in the LEP d a t a at other

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intense flares. It turns out that among 30 X-class flares occurring in the 1997-2000 season 11 flares gave detectable effects on LEP. Note that these 11 flares except the 6 November 1997 flare occurred when GEOTAIL was in the solar wind or magnetosheath. In these regions the ion temperature was relatively low so that there is an energy 'window' in which only solar hard X-ray photons could give detectable counts. For other 19 flares GEOTAIL was in the plasma sheet where the high temperature ions mask the 'signals', if any, of solar photons. (The solar photons on the 6 November 1997 event had quite high intensities so that they gave significant count enhancements to LEP even in the plasma sheet condition.) The lessons from the above study of the solar flare effect on the particle sensors are two-fold. One is experimental: For future missions coming closer to the sun, like the Bepi-Colombo mission to Mercury, solar hard X-rays have intensities inversely proportional to the distance from the sun, and can give significant noise levels to plasma measurements. Second is scientific: Since the 'duty cycle' for the solar E U V / h a r d X ray photon 'detection' by the E F D / L E P sensors or by similar plasma sensors is nearly 100 % without interruption by, e.g., shadowing by the earth, there arises possibility to utilize these solar 'signals' for scientific purpose. Recently, we have obtained an illustrative example: For three homologous flares on 24 November 2000 the d a t a coverage of the Yohkoh hard X ray detector was incomplete because of the earth shadow. Since the three flares were continuously 'monitored' by GEOTAIL, the EFD 'data' of the solar EUV photons was found useful in analyzing these events (Takasaki et al., 2003). SUMMARY AND COMMENT While the GEOTAIL spacecraft project was originally planned for the detailed study of the magnetosphere and magnetotail physics, it has observed many interesting phenomena in the solar wind and the magnctosheath. In this review, we have focused on several selected topics. Firstly, we have reviewed GEOTAIL contributions to the understanding of the physical process in the foreshock and bow shock regions: bow shock origin of diffuse ions, quantitative assessment of the nonlinear back reaction of diffuse ion production ('Cosmic-Ray-Mediated' shock (CRMS) effect), and the transport process of diffuse ions toward the predawn foreshock region. Secondly, we have seen GEOTAIL observations of propagating interplanetary shocks (IPSs) ahead of CMEs. GEOTA[L have provided the first clear evidence of diffusive electron acceleration at, an IPS, as well as the first observation of the CRMS effect at the same IPS. Thirdly, we have described observations of pickup interstellar He + ions by GETAIL. Unique contribution of GEOTAIL is the first identification of phase space torus of these ions. Such a torus shape has bccn theoretically expected at the initial stage of thc assimilation process of pickup ions to the solar wind, but never been identified observationally. Finally, we have seen 'direct observations' of solar flare extreme ultraviolet (EUV) and hard X-ray photons by GEOTAIL sensors. The electric field sensor identifies transient increases (duration several minutes) of the sunward electric field near the peaks of solar flare X ray emission. It has been further shown that since the particle counters of the plasma instrument arc sensitive to ]lard X-ray photons some of big solar events have been 'observed' as significant increases of the background counts of the plasma particle observations. It is hoped that the GEOTAIL mission continues to cover the declining phase of the current solar cycle 23 (May 1996- 2006?) and further into the next solar cycle 24. Particularly, further detection of 'cosmic ray mediated' interplanetary shocks is highly desired. In this respect I note that a quite strong interplanetary shocks detcced on 29 October 2003 (average propagation speed ,-~> 2000 kin/s) was found to have 'cosmic ray mediated' shock features. Detailed studies of this interesting shock are now under way. A CKN OWLED GEM ENT S I am grateful to all the members of the GEOTAIL project for their contributions to the detailed studies of solar wind and interplanetary phenomena. I thank Drs. T. Kosugi, K. Shibata, and K. Watanabe for their assistance in comparing the LEP data with Yohkoh observations. Special thanks are also due to Drs. A. Nishida, T. Mukai, and M. Hoshino for various discussions and comments. This work is partially supported by Grant-in-Aid for Scientific Research, No. 13874055 and 14340066, from the MEXT (Ministry of Education, Culture, Sports, Science, and Technology of Japan).

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