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

Abstract

[1] 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 À150 dB W/m 2 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.

Seasonal and solar cycle dynamics of the auroral kilometric radiation source region

“…As shown in Figure 7b, the lowest altitude of the density cavity ranges from ∼4300 km where the solar zenith angle at the foot of the field line is below 90° (in sunlight) and quickly moves to ∼2500 where it levels off (in darkness). The above analysis is consistent with Kumamoto et al [2003, Figure 2]. These authors presented the vertical profiles of occurrence probability of AKR sources as observed by the Akebono plasma wave instrument and showed that the highest occurrence probably drop in altitude by nearly 2000 km between summer (∼5500 km) and winter (∼3500 km).…”

“…Changes in the size of the emission cone, due to refraction, are not a factor in our analysis. Unlike the Kumamoto et al [2003] paper this study is based on the structure of the observed AKR spectrum and not on the location of the spacecraft when AKR is observed. Although our approach is very different than Kumamoto et al [2003] we are finding similar results.…”

“…In that study, ionospheric density variations were believed to be mainly responsible for the changes in the AKR source altitude. All the AKR observations reported by Kumamoto et al [2003] were remote from the AKR source region. It is important to note that the Kumamoto et al [2003] study was not able to determine what portion of the drop in the frequency of occurrence of AKR emissions, at low altitudes, during the solar maximum and during the summer season was due to vertical motion of the AKR source up the auroral field line or to a propagation effect (more refraction) with the expected increase in the height of the ionosphere.…”

“…All the AKR observations reported by Kumamoto et al [2003] were remote from the AKR source region. It is important to note that the Kumamoto et al [2003] study was not able to determine what portion of the drop in the frequency of occurrence of AKR emissions, at low altitudes, during the solar maximum and during the summer season was due to vertical motion of the AKR source up the auroral field line or to a propagation effect (more refraction) with the expected increase in the height of the ionosphere. In addition, Kumamoto and Oya [1998] also noted a seasonal difference in the AKR intensity from Akebono plasma wave data with increases in intensity in the winter polar region over the summer polar region.…”

Spatial and temporal properties of AKR burst emission derived from Cluster WBD VLBI studies

“…The emission mechanism is thought to be an electron-cyclotron maser mechanism (Wu and Lee, 1979) which radiates at the local (relativistic) electron cyclotron frequency and whose free energy is likely derived from a unstable electron "shell" instability in regions of depleted electron density (Ergun et al, 1998(Ergun et al, , 2000. AKR emission is well correlated in intensity and frequency of occurrence with magnetospheric activity (Kurth and Gurnett, 1998;Liou et al, 2000), and with season (Kumamoto et al, 2003). Although a statistical associCorrespondence to: R. L. Mutel (robert-mutel@uiowa.edu) ation with auroral activity and especially with discrete auroral arcs has been reported (Gurnett, 1974;Benson and Akasofu, 1984), the exact locations of individual AKR bursts have been difficult to measure except by in situ observations of isolated regions (e.g., Hilgers et al, 1991;Louarn et al, 1990;Roux et al, 1993;Ergun et al, 1998).…”