AKR disappearance during magnetic storms

[1] It is well known that auroral kilometric radiation (AKR) is intensified during substorms and has a good correlation with AE index. In the case of the magnetic storms, however, AKR characteristics have not been investigated. In this paper we report unexpected behavior of the storm-time AKR and its related particle precipitation: (1) AKR often disappears in the initial and main phases of the magnetic storms in spite of the large enhancement of AE index and field-aligned current; (2) At that time, the energy spectra of precipitating electrons do not show signature of the field-aligned acceleration but hot electron injection; (3) The radiation activates strongly in the recovery phase; and (4) AKR tends to disappear in larger storms. These results suggest that the field-aligned electric field which accelerates precipitating electrons and drives field-aligned currents is not formed in the initial and main phases of magnetic storms.