[1] We 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).Citation: Nosé, M., H. Koshiishi, H. Matsumoto, P. C:son Brandt, K. Keika, K. Koga, T. Goka, and T. Obara (2010), Magnetic field dipolarization in the deep inner magnetosphere and its role in development of O + -rich ring current,

We study magnetic fluctuations embedded in dipolarizations in the inner magnetosphere (a geocentric distance of ≤6.6 R E ) and their associated ion flux changes, using the Engineering Test Satellite VIII and Active Magnetospheric Particle Tracer Explorers/CCE satellites. We select seven events of dipolarization that occur during the main phase of magnetic storms having a minimum value of the Dst index less than −40 nT. It is found that (1) all of the dipolarization events are accompanied by strong magnetic fluctuations with the major frequency close to the local O + gyrofrequency; (2) the magnetic fluctuations appear with significant amplitude in the component nearly parallel to the local magnetic field; (3) the strong flux enhancement is seen in the energy range of 1-10 keV only for O + ions. In terms of frequency and dominant components of the magnetic fluctuations, they are considered to be excited by the drift-driven electromagnetic ion cyclotron (EMIC) instability that is recently identified with the linear theory. We perform particle tracing for H + and O + ions in the electromagnetic fields modeled by the linear dispersion relation of the drift-driven EMIC instability. Results show that the O + ions are accelerated to the energy range of 0.5-5 keV and undergo a significant modification of the spectral shape, while the H + ions have no clear change of spectral shape, being consistent with the observations. We therefore suggest that the electromagnetic fluctuations associated with the dipolarizations can accelerate O + ions locally and nonadiabatically in the inner magnetosphere. This selective acceleration of O + ions may play a role in enhancing the O + energy density in the storm time ring current.

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