Meso-Scale Structures of Radiation Belt / Ring Current Detected by Low-Energy Ions M. Yamauchi, R. Lundin, L. Eliasson, and O. Norberg Swedish Institute of Space Physics, Box 812, S-98128 Kiruna, Sweden ABSTRACT We show examples of newly found fine structures in low-energy (< 1 keV) ion distribution inside the ring current / radiation belt region. They are, from equatorward to poleward, (1) wedge-like energy-latitude dispersion of high intensity oxygen, (2) irregular field-aligned low-energy (<200 eV) ion flow, and (3) wavy structures of field-aligned ions. They are primarily observed in the morning sector. The "wedge-like" structure continuously exists for several hours whereas the wavy structure is found when the total background ion intensity is high, i.e., during the main phase of magnetic storms. However, the precise conditions for these phenomena to occur are still unknown. -------------------------------------------- Yamauchi, M., R. Lundin, L. Eliasson, and O. Norberg (1996), Meso-scale structures of radiation belt/ring current detected by low-energy ions, Adv. Space Res., 17(2), 171-174. doi:10.1016/0273-1177(95)00531-I (accepted manuscript) Copyright: 1995 COSPAR. INTRODUCTION The energetic particles (keV range and MeV range) of the ring current or radiation belt have been studied with stresses on intensities, global morphology, and behaviour during magnetic storms and substorms /1/2/3/. However, the internal fine structures of the ring current or radiation belt lack sufficient investigations. Only the SAMPEX satellite showed a fine structure of the radiation belt in the MeV range /4/. The SAMPEX observation suggests the existence of similar structures for lower-energies, but no report has appeared on this topic. Studies of low-energy (< 1 keV) component particles have in general been very poor. Recently, Collin et al. /5/ showed some statistical study, but not yet the details of the fine structures. The Viking (altitude about 12000 km) and Freja satellites (altitude about 1600 km) detected some fine structures for low-energy (< 1 keV) ions. There are some similarities between these and the SAMPEX's finding, but the structures in the low-energy ions manifest more complicated forms. The purpose of this brief report is to present these new findings. Further studies of these structures will be published elsewhere. OBSERVATIONS Figures 1 and 2 show Viking energy-time spectrogram for 50 eV - 40 keV ions for orbit number 900 and orbit number 1114. Details of the Viking particle instrument can be found in /6/. The high energy part (> 10 keV) shows intense trapped ions in outer radiation belt or plasma sheet. The particle flux decreases toward lower energies in the keV range, but it again increases in the 100 eV range. The minimum particle flux around 1 keV could be artificial due to the sensitivity change between two different instruments which cover the high energy (keV) range and the low energy (100 eV) range, respectively. However, we have adjusted the different sensitivities, and more importantly, the same result was obtained by /5/. Therefore, we believe that the low-energy ion population is not a simple extension of high-energy part. The third reason to support this conclusion is that the low-energy ions are very structured whereas the high-energy ions are rather uniform. Fig. 1. Viking ion energy-time spectrogram of orbit 900 (4 Aug 1986). Invariant latitude (I) is used to describe the magnetic latitude. L value is obtained from the relation L = cos-2I; i-e-. I=60¡ corresponds to L=4. Fig. 2. Viking ion energy-time spectrogram of orbit 1114 (12 Sep 1986). The structured low-energy (< 100 eV) ions are also independent from auroral zone phenomena. In Figure 1, they disappear at the right (poleward) end of the figure at 69.5¡ Inv (L=8.2), and no phenomenon other than the trapped energetic (>10 keV) particles of the radiation belt or the plasma sheet is detected between 69.5¡ Inv and 73.5¡ Inv (L=12.4, not shown in the figure), beyond which the auroral oval starts. Thus, they are inherent to the radiation belt but not to the auroral zone. One may recognise three different structures in Figure 1. From the left (equatorward) to right (poleward), they are (1) wedge-like energy-latitude dispersion of high intensity ions up to 61.5¡ Inv (L=4.4), (2) irregular field-aligned low-energy ion flow between 61¡ Inv and 64¡ Inv (L=5.2), and (3) intense and confined ion region with loss cone and at the upper energy side between 64¡ Inv and 69.5¡ Inv. The high-energy cut-off of the last phenomenon is very clear and wavy. Such wavy structure is better recognized in Figure 2, which is obtained during a large magnetic storm (Dst = -157 nT, several hours after the onset). The first phenomenon is also characterized by a clear cut-off in the energy domain, which forms a wedge-like energy-space dispersion of trapped ions in the 100 eV range. Hence, we call this "wedge-like" hereafter. The dispersion is found to be spatial, not temporal, if one compare the dispersion direction between descending and ascending satellite traversals. A clear loss-cone structure is seen in both the upward and the downward direction. The spatial scale-size of each dispersion is nearly one degree. The phenomenon is not observed very frequently, but intense ones are normally found for a few consecutive orbits. Since the orbital period of Viking is a little more than 4 hours, the lifetime of this structure is probably several hours for intense events. Correlation with the geomagnetic activity (AE and Dst indices) is not straightforward, and we cannot at present conclude on any relations between the "wedge-like" phenomenon and magnetic storm or substorm activities. The phenomenon is observed primarily in the morning sector according to a survey of all Viking data, but we have to leave conclusions on the nightside because the satellite coverage over the nightside is rather poor. Freja observed a similar phenomenon at the same and at lower magnetic latitudes. Figure 3 (orbit number 959) shows one such example detected by the ion mass spectrometer on board Freja /7/. Due to a different sensitivity for heavy ions compared to Viking, Freja detected only the strongest ones. But again the phenomenon is often detected for a few consecutive orbits in the midnight-morning sector. Shape of the dispersion (wedge-like) is also very similar to what Viking observed. It is thus most likely that we are observing the same phenomenon. The structured trapped ions observed with Freja are oxygen ions of ionospheric origin, as is clear from Figure 3. Probably, ejected ionospheric ions become trapped for several hours. Since it is inside MeV range radiation belt, and since SAMPEX observed quite similar fine structures in the MeV particle /4/, these ions could be secondary ions produced by MeV particles precipitating into the ionosphere although this interpretation has some difficulties. There is one problem before concluding that the wedge-like dispersion events in Figure 1 and that in Figure 3 are the same phenomenon: the correlation to the geomagnetic activities is different. In Viking observation, we could not find any solid correlation, whereas the wedge-like structure detected by Freja always takes place during major magnetic storms. A further study is necessary to clarify this matter. Fig. 3. Freja energy-time spectrograms for positive ions (0.001-4.5 eV) and electrons (1-100 keV) for orbit 959 (17 Sep 1992). The radiation belt can be identified by 30-100 keV electrons. The second phenomenon is the irregular field-aligned low-energy (<200 eV) ion flow. These ions show different forms within a short interval: upward beams, upward flow with loss cone (similar to upward ion conics), and even downward ion beams with/without loss cones. Upward beams are stronger than the downward beams. Detailed studies are planned for the future. The last phenomenon, the intense and confined low-energy ion region with wavy structure, is composed of field-aligned ions embedded in the trapped background population (see Figure 2), indicating that the electric potential is oscillating up and down. We have not yet found a similar phenomenon in the Freja data. Since a low-altitude satellite (Freja) traverses fast enough to provide a snapshot of such phenomenon, this is most likely a temporal variation with a period of a few minutes. In fact, the magnetic and electric fields (not shown here) show a clear pulsation corresponding to this wavy structure. One good candidate for this is standing inertia AlfvŽn wave because the density also fluctuates whereas the pressure is constant (not shown here). The phenomenon seems to be more related to magnetic storms than the previous phenomena, but further statistical investigations are necessary to clarify this. Note that the field-aligned beams are not well recognized in many cases whereas the wavy structure is still clear for those, and we are not completely sure that the wavy structure in Figure 2 is the same as the wavy structure in Figure 1. SUMMARY The Viking and Freja satellites have revealed internal structures of the low-energy (< 1 keV) ions in the ring current / radiation belt. They could be related to fine structures detected in the MeV range by SAMPEX satellite. They are, from equatorward to poleward, summarized as follows: (1) Spatial structures of trapped ions with clear cut-off in both energy and space are found inside the radiation belt by both Viking and Freja. They contain mostly oxygen ions, indicating that these trapped particles ultimately originated from the ionosphere. (2) Irregular field-aligned low energy (<200 eV) ion flow with/without loss cones. (3) Wavy structure of field-aligned ions of several hundred eV is found but only by the mid-altitude satellite (Viking). It is caused by oscillating (a few minutes' period) field-aligned potential drops. Acknowledgments. The Viking project is sponsored by the Swedish National Space Board. The Freja project is supported by the Swedish National Space Board and the Deutsche Agentur fŸr Raumfahrtangelegenheiten (DARA). The AE index is provided by WDC-C2 for geomagnetism at Kyoto University. REFERENCES 1. D. C. Hamilton, G. Gloeckler, F. M. Ipavich, W. StŸdemann, B. Wilken, and G. Kremser, Ring current development during the great geomagnetic storm of February 1986, J. Geophys. Res., 93, 14343-14355 (1988). 2. I. B. McDiarmid, J. R. Burrows, and E. E. Budzinski, Average characteristics of magnetospheric electrons (150 eV to 200 keV) at 1400 km, J. Geophys. Res., 80, 73-79 (1975). 3. S. Takahashi, M. Takeda, and Y. Yamada, Simulation of storm-time partial ring current system and the dawn-dusk asymmetry of geomagnetic variation, Planet. Space Sci., 39, 821-832 (1991). 4. D. N. Baker, J. B. Blake, L. B. Callis, J. R. Cummings, D. Hovestadt, S. Kanekal, B. Klecker, R. A. Mewaldt, and R. D. Zwickl, Relativistic electron acceleration and decay time scales in the inner and outer radiation belts: SAMPEX, Geophys. Res. Lett., 21, 409-412 (1994). 5. H. L. Collin, J. M. Quinn, and J. B. Cladis, An empirical static model of low-energy ring current ions, Geophys. Res. Lett., 20, 141-144 (1993). 6. Viking special issue, Geophys. Res. Lett., 14, 379-482 (1987). 7. L. Eliasson, O. Norberg, R. Lundin, K. Lundin, S. Olsen, H. Borg, M. AndrŽ, H. Koskinen, P. RiehelŠ, M. Boehm, and B. Whalen, The Freja hot plasma experiment: Instrument and first results, Space Sci. Rev., in press. Fig. 1. Viking ion energy-time spectrogram of orbit 900 (4 Aug 1986). Invariant latitude (I) is used to describe the magnetic latitude. L value is obtained from the relation L = cos-2I; i-e-. I=60¡ corresponds to L=4. Fig. 2. Viking ion energy-time spectrogram of orbit 1114 (12 Sep 1986). Fig. 3. Freja energy-time spectrograms for positive ions (0.001-4.5 eV) and electrons (1-100 keV) for orbit 959 (17 Sep 1992). The radiation belt can be identified by 30-100 keV electrons.