T. Terasawa1, N. Shimada1, K. Tsubouchi1 M. Hoshino2, T. Mukai2, Y. Saito2, T. Yamamoto2, A. Nishida2 S. Machida3, S. Kokubun4, H. Matsumoto5, H. Kojima5 T. R. Sanderson6, A. J. Lazarus7, J. T. Steinberg7, and R. P. Lepping8
1 Dept. of Earth and Planetary Physics,
University of Tokyo, Bunkyo-ku, Tokyo, 113 Japan,
2 ISAS, Sagamihara, Kanagawa, 229 Japan
3 Dept. of Geophysics, Kyoto University, Kyoto, 606 Japan
4 STEL, Nagoya University, Toyokawa, 442 JAPAN
5 RASC, Kyoto University, Uji, 611 JAPAN
6 Space Science Dept., ESA/ESTEC, 2200 AG Noordwijk, Netherland
7 Center for Space Res., MIT, Cambridge, MA 02139, USA
8 Lab. for Extraterr. Phys., NASA/GSFC, Greenbelt, MD 20771, USA
It has been known that an efficient re-acceleration of energetic storm ions can occur between a propagating interplanetary shock and the bow shock. However, it has not been known whether a similar event could occur for electrons. In this paper, we shall report the first observational evidence of re-acceleration of energetic electrons at the front of an interplanetary shock.
Acceleration of energetic particles at shocks is one of the most important topics in space plasma physics [e.g., Scholer, 1985; Tsurutani and Lin, 1985; Wenzel et al., 1985]. In this paper, we report a case study of acceleration phenomenon occurred between a propagating interplanetary shock (IPS) and the earth bow shock. Scholer and Ipavich (1983) reported an acceleration event between an IPS and the bow shock from a comparative study of ISEE-1/3 proton observations: There was a gradual ion increase (time scale of several hours) around the arrival time of the IPS at ISEE-3 position ( 235 Re upstream from the earth). Such a gradual increase is called as an ESP (energetic storm particle) event, and has been interpreted as a result of diffusive shock acceleration of particles injected at solar flares. At the position of ISEE-1, which was a few Re upstream of the nominal bow shock position, spiky increases of proton intensity were found to overlap on the gradual increase. These authors interpreted these spiky increases of protons in terms of the re-acceleration of ESP particles diffusively trapped between the IPS and the bow shock. What we will show in the next section is quite similar to this ISEE-1/3 event. A new finding is a spiky increase of electrons at the passage of an IPS, which seems also explicable in terms of the re-acceleration of electrons between the IPS and the bow shock. Since there has been no such reports for electrons to the authors' knowledge, this paper would be the first report of a re-acceleration event of electrons.
The WIND and GEOTAIL spacecraft recorded passage of a large magnetic cloud (or CME) during 18-20 October 1995, part of an IACG campaign period. This cloud was preceded by an IPS, which arrived at the positions of WIND and GEOTAIL at 10:40 UT and 11:25 UT on 18 October, respectively. Figure 1 shows the position of GEOTAIL as well as nominal surface shapes of the bow shock and magnetopause. WIND was at (175,-4,-3)GSE Re.
Figure 1: The GEOTAIL orbit on the GSE-XY plane during the peri- ods between 18 and 21 October 1995. An interplanetary shock came to GEOTAIL at 11:25 UT on 18 October 1995, when GEOTAIL was 10 Re upstream of the nominal bow shock surface.
In Figure 2, WIND and GEOTAIL observations are compared: The top panel shows low energy protons observed at WIND. A step-like increase at 10:40 UT was due to the heating and compression at the shock front. (Note that exponential increases of proton fluxes with time scales of several hours, which are characteristic to the ESP events, were seen only above several tens of keV, and did not appear in this figure.) The second panel from the top shows the corresponding observation at GEOTAIL. As seen, the particle environment around GEOTAIL was much noisier than around WIND. Several bursty increases of ions in the all energy range (up to the instrumental limit 40 keV) between 0 UT and 10 UT were due to the appearance of diffuse ions, which are commonly observed in the upstream region of the quasi-parallel bow shock. From about 10:35 UT, protons at GEOTAIL showed gradual increases toward the arrival of the IPS at 11:25 UT. Within 5 min of the arrival of the IPS, spiky increases of ions were seen in all three energy ranges shown in the figure. These increases were much the same as those reported by Scholer and Ipavich (1983). After the passage of the IPS, spiky increases of ion flux continued till 19:50 UT, when a rapid drop of the flux was observed at the forward edge of the CME cloud. These spiky increases seem to represent a effect of reflection of these ions at the bow shock (Scholer and Ipavich, 1983).
Figure 2: Comparison of WIND and GEOTAIL particle observations. a (top): Proton fluxes at three representative energy ranges observed at WIND. b (second): Ion (proton) counts observed at GEOTAIL for the energy ranges corresponding to the WIND data sets. c (third): Electron fluxes at four representative energy ranges observed at WIND. d (bottom): Electron counts observed at GEOTAIL for the energy ranges corresponding to the WIND data sets.
We now turn to the electron observation. The third panel from the top in Fig. 2 shows WIND observation of electrons. A small spiky increase was seen at the time of shock passage for electrons of 0.4-1.8 keV, which is likely to be the result of heating at the shock front. Higher energy electrons (1.9-8 and 9-30 keV) showed step-like increases at the IPS, which were similar to the behavior of protons shown at the top panel. The bottom panel shows corresponding GEOTAIL observation of electrons. At the passage of the IPS, we see large spiky increases of electrons not only in lower energy channels (0.25-1.1 and 1.3-6.5 keV) but also in a higher energy channel (7.8-18.8 keV).
Figure 3a shows E-t plots for electrons (top) and ions (bottom) during the period of 11:10-11:30 UT around the arrival of the IPS at 11:25 UT. The angular distribution of electrons (Figure 3b) showed a bidirectional anisotropy along the interplanetary magnetic field (IMF). This bidirectional anisotropy started from 10:35 UT, but became weak and almost isotropic after 11:22 UT. Our interpretation of these changes in the anisotropy is as follows: At 10:35 the IMF rotation (not shown) made the IPS and the bow shock connected, and the acceleration of electrons trapped between them was initiated. Near the IPS, the enhanced pitch angle scattering of electrons made the angular distribution of accelerated electrons nearly isotropic.
Figure 3: (a) E-t plots for omnidirectional count rates of electrons (0.06- 40 keV, top) and ions (5-40 keV, bottom). (b) 2D angular distribution of electron count rate in the ecliptic plane in the energy range of 0.06-5.5 keV (observation was made during the interval of 11:20:09-11:21:11).
We have shown that electron re-acceleration occurred during an IPS acceleration event observed 18 October 1995. The bidirectional anisotropy of accelerated electrons was a key to conclude that shock re-acceleration of these electrons. (During an ESP event in 21 February 1994, where only an IPS was working to accelerate electrons and ions, we observed a strong unidirectional anisotropy of electrons in the upstream region of the IPS (Terasawa et al., 1995).) To get more physical information about the acceleration process, we need a quantitative modeling of the event, which is now under way.
Discussions with GEOTAIL and WIND team members are gratefully acknowledged. TT also thanks M. Scholer for his variable comments.
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