The Jovian polar magnetosphere has relativistic particle accelerations with quasiperiodicity (QP accelerations), which are accompanied by periodic auroral emissions and low-frequency radio bursts called QP bursts. Although there have been some observations, the generation process of QP bursts by relativistic electrons from QP accelerations has not been revealed yet. This paper presents calculated wave growth rates for the discussion of the QP radio burst generation processes based on wave generation theories. Linear growth rates were computed for free space mode waves and plasma waves in cold plasma dispersion relations, assuming that these waves are generated by relativistic electron beams in two kinds of polar source regions, as suggested by wave observations and by the ray-tracing results reported in our previous studies. One of the source regions is at high altitudes where emission frequency f is close to local right-handed extraordinary (RX) mode cutoff frequency fRX and the other is at low altitudes where f is close to local plasma frequency fp. We found that ordinary (O) mode free space waves are sufficiently amplified, with broad beaming at both of the sources in the duration of the relativistic electron populations when they have an unstable velocity distribution like a ring beam structure. This means that O mode free space waves can be generated directly from energetic electrons via the "cyclotron maser instability" (CMI) process. We also confirmed that extraordinary (X) mode free space waves are not sufficiently amplified at both of the sources in the beam duration but Z mode waves propagating along field lines from the sources toward the Jovian polar ionosphere are significantly excited. Z mode waves propagating toward the planet could be converted to free space O mode waves at a steep plasma density gradient via the "mode conversion" (MC) process. We conclude that both direct (CMI) and indirect (MC) process can generate O mode QP radio bursts with radiation characteristics consistent with those observed by spacecraft. This suggests that relativistic electrons with unstable velocity distributions are generated by the QP acceleration and that Z mode and O mode QP radio bursts are excited by these particles.
ASJC Scopus subject areas
- Space and Planetary Science