SC-triggered plasma waves observed by the Akebono satellite in the polar regions and the plasmasphere

Plasma wave phenomena associated with sudden commencements (SCs) were analyzed based on observations conducted with the Akebono satellite, which has been collecting data for more than 13 years (since March 1989). Simultaneous plasma wave observation data for 257 SCs reveal that enhanced plasma waves are observed with an exact one-to-one correspondence with the SCs throughout the entire observation region, including the polar and plasmasphere regions. Electromagnetic whistler mode and ion cyclotron waves are enhanced in the low latitude plasmasphere, while electrostatic whistler mode and electromagnetic ion cyclotron waves are generated in the polar region. The onset times of the SC-triggered plasma waves exhibit a delay or lead time characteristic, compared with the onset times of SCs identified by the Kakioka Magnetic Observatory, with a time resolution of 1 s. By comparing the difference in SCs and enhanced electron plasma waves onset times, the propagation route of the SC disturbances can be identified in the plasmasphere

Seasonal and solar cycle dependences of the correlation between auroral kilometric radiation and the AE index

Seasonal and solar cycle dependences of the correlation between auroral kilometric radiation (AKR) and the auroral electrojet (AE) index have been investigated based on the plasma wave data obtained by the Akebono satellite. Under any seasonal and solar activity conditions, a clear correlation has been found between the AKR power flux and the AE index. The properties of the correlation, however, vary depending on season and solar activity. AKR power flux increases as about the 1.2 power of AE index in all seasonal and solar activity conditions. However, even for the same AE index, AKR power flux during solar minimum is 5dB larger than that during solar maximum. As for the seasonal variations, the AKR power flux in winter is 22dB larger than that in summer even for the same AE index. The results suggest that long-term variations of AKR depend not only on auroral current variations but also on factors associated with the total energy flux of auroral electrons and the generation process of AKR

Solar zenith angle and solar activity dependences of vertical profile of electron number density in the nightside auroral region

Solar zenith angle and solar activity dependences of electron number density in the nightside auroral region from the topside ionosphere to the magnetosphere within a geocentric radial distance of 2.6 R_E were statistically investigated based on analysis of 7-years of plasma wave data measured by the plasma wave instrument onboard the Akebono (EXOS-D) satellite. The results are summarized as follows: (1) Electron number density N_e changes depending on solar zenith angle and solar activity: N_e in sunlight is about 3 times larger than that in darkness, and N_e during solar maximum is about 10 times larger than that during solar minimum. (2) During solar maximum, geopotential scale height is almost constant within a range from 250km to 400km. During solar minimum, geopotential scale height is drastically changes at a geopotential height around 2000-2500km, or an actual height of 3000-4000km: Geopotential scale height is 250-400km below the transition height and larger than 500km above the transition height. In order to discuss the auroral phenomena in various seasonal and solar activity conditions, the variations of ambient electron number density, as obviously shown in this study, should be taken into consideration in future studies

Seasonal dependence of the vertical distributions of auroral kilometric radiation sources and auroral particle acceleration regions observed by the Akebono satellite

Seasonal variations in the vertical distributions of occurrence probabilities of auroral kilometnc radiation (AKR) sources and auroral acceleration regions indicated by upward-flowing ion (UFI) events were compared based on statistical analyses of plasma waves and energetic particles data observed by the Akebono satellite. The peak altitude in the vertical distribution of occurrence probability of AKR sources whose emission intensities were larger than -150dBW/m^2-Hz occurred at 5000-6000 km in the summer polar region and at 3000-4000 km in the winter polar region. The analyses have also clarified that the vertical distributions of occurrence probabilities of auroral acceleration regions also show seasonal variations that are quite similar to those of the AKR sources. Based on the observation that intense AKR emissions can be generated even in high-density plasma, we suggest that processes other than cyclotron maser instability (CMI), such as mode conversions, may be dominant under conditions of dense ambient plasma in the summer polar regions. The contri-bution of the emissions generated by these mechanisms to the seasonal variations in the vertical distributions of AKR sources is thought to be significant

Second harmonics of auroral kilometric radiation observed by the Akebono satellite

The second harmonic wave properties of auroral kilometric radiation(AKR) were examined using data obtained by the Akebono satellite. The results of our statistical analysis indicate that the probability of a harmonic event occurrence is more than 60% of all AKR events. The relationship between the frequencies of the fundamentals and the second harmonics is exactly two times for the upper and lower cut-off frequencies of the spectra as well as the fine structures, within the resolution of the Akebono observations. The intensity ratio of the second harmonics to the fundamentals exhibits a two-fold nature, with both a linear and a quadratic relationship. Further data analyses also revealed that the second harmonic waves of AKR, which propagate in the X-mode, are generated from a source that is identical to that of the fundamental waves that propagate in the O-mode. These results suggest that the mechanism of AKR harmonic structure generation should allow the coexistence of different AKR emission processes

Control factor of solar cycle variation of auroral kilometric radiation

Solar cycle variations of auroral kilometric radiation (AKR) observed by the Akebono satellite have been compared with the variations of F10.7 and solar wind dynamic pressure. F10.7 and solar wind dynamic pressure show different solar cycle variations: F10.7 increases during solar maximum and decreases during solar minimum. Solar wind dynamic pressure suddenly increases in the declining phase of solar activity and gradually decreases. The pressure minimum occurs during solar maximum. Statistical analysis of the Akebono data has shown that AKR occurrence minimum occurs during solar maximum, however AKR occurrence maximum coincides not with solar wind dynamic pressure peak but with F10.7 minimum. Up-flowing ion (UFI) events and ambient plasma density, which are associated with generation conditions of AKR, also show similar behavior. They are dependent not on solar wind dynamic pressure but on F10.7. These results suggest the anti-correlation between discrete aurora and solar activities, which has been never recognized through the studies on secular variations of auroral phenomena mainly based on old auroral records obtained in mid-latitude regions