Ion-Dispersion in the Cusp: A case study investigated with satellite and radar

Lunde, J.1, Buchert, S. C.2, Ogawa, Y.3, Hirahara, M.4, Seki, K.5, Ebihara, Y.6, Sakanoi, T.7 and Asamura, K.8

1 University of Tromsø, Tromsø, Norway
2 Swedish Institute of Space Physics, Uppsala, Sweden
3 National Institute of Polar Research (NIPR), Itabashi, Japan
4 Rikkyo University, Tokyo, Japan
5 STEL, Nagoya University, Nagoya, Japan
6 IAR, Nagoya University, Nagoya, Japan
7 PPARC, Tohoku University, Sendai, Japan
8 ISAS/JAXA, Japan

We present results from a joint Norwegian-Japanese-Swedish campaign, ESR/REIMEI, which was carried out in the winter 2006 on Svalbard. The aim was to conduct coordinated temporally and spatially highly resolving observations of structured aurora, electron precipitation and ion-outflow with the ESR (EISCAT Svalbard Radar) and the low altitude REIMEI satellite. The results are mainly from the daytime cusp. Clear signatures of so-called ion-dispersion are seen in the satellite data, meaning that the energy of precipitating ions decreases over time continuously or in steps. In particular fast variations of the electron flux and energy in these events have been found and studied.

In one event, on the 1st February of 2006, the REIMEI satellite flew at ~ 610 km height close past the ESR facilty (75.27˚ N and 111.65 E CGM). The ESR 32m antenna was pointed 233.8 in azimuth and 78.7˚ in elevation, corresponding to the satellite shortest range direction (~ 628 km at ~ 09:27 UT). In the satellite data, a continuously decreasing ion energy was clearly seen, followed by a step-like decrease. The downward ion-precipitation was in the range between 100 – 1000 eV. Additionally, during the ion-dispersion, the REIMEI data show quasi-periodic fluctuations in the electron energy and flux while the interplanetary magnetic field (IMF) had been relatively steady the 2 hours before. Features of the ion dispersion agree relatively well with previously published observations of satellites at various altitudes. These had been interpreted as effects of the magnetic reconnection process occurring at the dayside magnetopause. The much faster electron variations seen here might be caused by fine spatial structures in the reconnection region that map down onto the ionosphere, or by temporal oscillations arising in the ionosphere-magnetosphere coupling.