Seasonal and solar cycle variations of the vertical distribution of the occurrence probability of auroral kilometric radiation sources and of upflowing ion events

Abstract

The seasonal and solar cycle variations of the vertical distribution of the occurrence probability of auroral kilometric radiation (AKR) sources and of upflowing ion (UFI) events have been investigated based on 13 years of plasma wave data and 8 years of energetic ion data observed by the Akebono satellite. In the summer polar region, the peak of the vertical distribution of the occurrence probability of AKR sources, identified by an intensity larger than −150 dB W/m2 Hz, is at an altitude higher than 5000 km with a value of 10% in the solar maximum period and at an altitude of 5000–6000 km with a value of 40% in the solar minimum period. In the winter polar region, the peak altitudes and values are 4000–5000 km and 50% in the solar maximum period, and 4000 km and 60% in the solar minimum period, respectively. Based on the analysis of wave polarization data, intense R‐X mode AKR is responsible for these variations. Furthermore, it has been also revealed that the vertical distribution of the occurrence probability of UFI events, identified by an ion velocity distribution function with a peak above 1 keV, also shows similar seasonal and solar cycle variations as the AKR sources. In both seasonal and solar cycle variations, the AKR and UFI occurrence probabilities decrease with increasing solar EUV ionization in the ionosphere. Therefore, ionospheric seasonal and solar cycle variations are inferred to be responsible for the AKR and UFI variations. As for the control mechanism, upwelling plasma from the ionosphere to the exosphere, up to an altitude of about 5000 km, are expected to change the temperature and density of the background plasma and control unstable conditions of plasma waves like ion acoustic waves and electrostatic ion cyclotron (EIC) waves, which generate the auroral particle acceleration regions.