Toru Obara

Magnetic field dipolarization in the deep inner magnetosphere and its role in development of O+-rich ring current

[1]xa0We studied magnetic field dipolarization and associated ion acceleration in the deep inner magnetosphere, using magnetic field data obtained by the magnetometer on board the Mission Demonstration Satellite 1 (MDS-1) and the energetic neutral atom (ENA) flux data obtained by the high-energy neutral analyzer imager on board the Imager for Magnetopause-to-Aurora Global Exploration satellite. Because the MDS-1 satellite has a geosynchronous transfer orbit, we could survey magnetic field variations at L = 3.0–6.5. Analyzing data in the period from February to July 2002, we found that (1) dipolarization can be detected over a wide range of L (i.e., L = 3.5–6.5, which is far inside the geosynchronous altitude); (2) when the MDS-1 satellite was located close to auroral breakup longitude, the occurrence probability of dipolarization was about 50% just inside the geosynchronous altitude and about 16% at L = 3.5–5.0, suggesting that dipolarization in the deep inner magnetosphere is not unusual; (3) magnetic storms were developing whenever dipolarization was found at L = 3.5–5.0; (4) dipolarization was accompanied by magnetic field fluctuations having a characteristic timescale of 3–5 s, which is comparable to the local gyroperiod of O+ ions; and (5) after dipolarization, the oxygen ENA flux in the nightside ring current region was predominantly enhanced by a factor of 2–5 and stayed at an enhanced level for more than 1 h, while clear enhancement was scarcely seen in the hydrogen ENA flux. From these results, we conjectured a scenario for the generation of an O+-rich ring current, in which preexisting thermal O+ ions in the outer plasmasphere (i.e., an oxygen torus known from satellite observations) experience local and nonadiabatic acceleration by magnetic field fluctuations that accompany dipolarization in the deep inner magnetosphere (L = 3.5–5.0).

Mass‐dependent evolution of energetic neutral atoms energy spectra during storm time substorms: Implication for O+ nonadiabatic acceleration

[1]xa0We examine the temporal variations of energy spectra of energetic neutral atoms (ENAs) detected by the High Energy Neutral Atom imager (HENA) onboard the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite during three substorms on 21 October 2001 and three substorms on 19 March 2002; the substorms occurred during the storm main phase. The ENA energy used in the present study ranges from 10 to 198 keV for hydrogen and from 29 to 222 keV for oxygen. We use ENA data obtained from two independent areas of a HENA image, for which HENA lines of sight pass through the inner magnetosphere (∼−6 RE −8 RE). Although the influence of the nonadiabatic acceleration appears in the inner magnetosphere (∼−6 RE < X < ∼−3 RE) as well as the outer magnetosphere (∼−8 RE < X < ∼−6 RE), it is not clear from the present results whether O+ energization in the inner magnetosphere is due to nonadiabatic acceleration in the inner magnetosphere or adiabatic transport of O+ nonadiabatically accelerated in the outer magnetosphere. It is likely, for at least the two substorms, that the nonadiabatic acceleration makes a more significant contribution to O+ energization than increase of the O+ density in the plasma sheet.

Ionospheric Pc5 plasma oscillations observed by the King Salmon HF radar and their comparison with geomagnetic pulsations on the ground and in geostationary orbit

[1]xa0We analyzed Pc5 (1.7–6.7 mHz) oscillations of ionospheric Doppler plasma velocity observed on a westward pointing beam 3 of the SuperDARN King Salmon HF radar in Alaska during the solar maximum in 2002 and the minimum in 2007. Local time distributions of the ionospheric Pc5 oscillations showed peculiar asymmetric characteristics in both years; that is, the occurrence probability had a maximum around the magnetic midnight, whereas backscatter echoes exhibited almost no oscillation on the dayside. We compared these ionospheric Pc5 events with magnetic field variations on the ground under the radar beam at Pebek and King Salmon and the geostationary ETS-8 satellite at almost conjugate longitude. We found only a few nightside events where both the radar and magnetometers detected similar sinusoidal oscillations. On the other hand, from statistical spectral analyses we found that there were positive correlations between the integrated Pc5 range spectral power of velocity oscillations and the geomagnetic pulsations both on the ground and in geostationary orbit although the pulsation powers were quite low. For these ionospheric Pc5 events, we found that both solar wind bulk flow speed and dynamic pressure showed no correlation with the spectral power and more than half of the Pc5 events were observed when the geomagnetic activities were low as inferred from the AE and Dst indices. These results indicate that the azimuthal Pc5 oscillation in the ionospheric plasma flow does not represent well-known characteristics of Pc5 pulsations driven by solar wind changes. We consider that the nightside occurrence peak of the ionospheric Pc5 oscillation might be related to diurnal changes in the ionospheric conductivity, which controls the amplitude of wave electric fields in the ionosphere. Therefore, the Pc5 wave power distributions obtained by radar observations provide features different from those obtained from magnetic field observations.

Recent topics of radiation belt science

Highly energetic electrons in the outer radiation belt disappear during the main phase of the magnetic storm, and rebuilding of the highly energetic electrons is made during the recovery phase of the magnetic storm. The distribution of the new peak of highly energetic electron flux from the Earth is inversely proportional to the magnitude of the magnetic storm. In case of the super storm, the outer electron belt is pushed toward the Earth, filling so‐called slot region. A long‐term variation of the outer radiation belt was identified. A location of the outer radiation belt was found closer to the Earth during the solar maximum periods and far from the Earth during the solar minimum periods. A variation is due to the evidence that the large magnetic storms occur during the solar maximum periods and small magnetic storms take place during the solar minimum periods, resulting in a long‐term variation with respect to the distance from the Earth. Highly energetic electrons at geosynchronous orbit altitude are ...