2:45 PM - 3:00 PM
[PEM13-14] A study of drift kinetic simulation of ULF wave excitation based on observational data of multi-point spacecraft in the ionosphere and magnetosphere
Keywords:ULF wave, ring current, numerical simulation, drift bounce resonance
In the terrestrial magnetosphere, ultra-low frequency (ULF) waves are excited by a variety of external and internal drivers. High-speed solar wind and perturbation of the solar wind dynamic pressure are external energy sources of ULF waves (e.g., Kepko et al., 2002; Zhang et al., 2010), which can cause toroidal oscillations of a field line. Internal energy sources of ULF waves can excite compressional and poloidal mode waves through drift mirror instability (Hasegawa et al., 1969) and drift/drift-bounce resonance (Southwood et al., 1969) of energetic ions in the ring current. During a geomagnetically disturbed period, the solar wind can excite the externally driven waves, while injected ions can excite internally driven waves. In addition, recent studies of feedback instability show that ULF waves near the plasmapause can be generated at the ionosphere (e.g., Streltsov and Mishin, 2018). The coupling of the solar wind-magnetosphere-ionosphere makes it difficult to distinguish the excitation mechanism(s) of ULF waves in satellite observations.
In this presentation, we discuss the possible excitation mechanisms of the ULF waves observed on 29 October 2013 using a global ring current model. We conducted a global drift-kinetic simulation of the ring current (Amano et al., 2011) coupled with an ionospheric potential solver (Nakamizo et al., 2012). During the event, the solar wind dynamic pressure showed a step-like variation, and a moderate substorm (AL ~ -470 nT) was triggered. Both toroidal and poloidal ULF waves were detected by Van Allen Probes on the duskside. From the observations of Iridium satellites, we found that the Region-1 field-aligned current (R1FAC) gradually reached greater than 1.0 uA/m2 in the expansion phase. The spatial variations of the R1FAC intensity and locations were fitted with the Gaussian function to calculate the electric field potential in the ionosphere by the potential solver. The temporal variations of F1FAC were also fitted with the Gaussian and tangent hyperbolic functions. Poloidal magnetic field oscillations were detected at 12:40 UT around midnight during the recovery phase of a substorm. From the field-aligned current data, we consider that the sunward and poleward shift of the current sheet of R1FAC triggered the poloidal oscillations. Their phase speed along a field line is close to Alfvén speed, which indicates these oscillations are Alfvén mode waves. We also found toroidal electromagnetic oscillations at 11:35 UT. In the observation by Van Allen Probes, poloidal ULF waves were detected from 13:15 to 14:30 UT. In our simulation, however, there were no clear poloidal oscillations in that interval. In a previous study that used the ring current model of Amano et al. (2011) coupled with the potential solver of Nakamizo et al. (2012), toroidal oscillations of the magnetic field were not reported (Yamakawa et al., 2022). Our study suggests that the variation of the ionospheric potential caused by R1FAC is important for the excitation of toroidal mode waves.
In this presentation, we discuss the possible excitation mechanisms of the ULF waves observed on 29 October 2013 using a global ring current model. We conducted a global drift-kinetic simulation of the ring current (Amano et al., 2011) coupled with an ionospheric potential solver (Nakamizo et al., 2012). During the event, the solar wind dynamic pressure showed a step-like variation, and a moderate substorm (AL ~ -470 nT) was triggered. Both toroidal and poloidal ULF waves were detected by Van Allen Probes on the duskside. From the observations of Iridium satellites, we found that the Region-1 field-aligned current (R1FAC) gradually reached greater than 1.0 uA/m2 in the expansion phase. The spatial variations of the R1FAC intensity and locations were fitted with the Gaussian function to calculate the electric field potential in the ionosphere by the potential solver. The temporal variations of F1FAC were also fitted with the Gaussian and tangent hyperbolic functions. Poloidal magnetic field oscillations were detected at 12:40 UT around midnight during the recovery phase of a substorm. From the field-aligned current data, we consider that the sunward and poleward shift of the current sheet of R1FAC triggered the poloidal oscillations. Their phase speed along a field line is close to Alfvén speed, which indicates these oscillations are Alfvén mode waves. We also found toroidal electromagnetic oscillations at 11:35 UT. In the observation by Van Allen Probes, poloidal ULF waves were detected from 13:15 to 14:30 UT. In our simulation, however, there were no clear poloidal oscillations in that interval. In a previous study that used the ring current model of Amano et al. (2011) coupled with the potential solver of Nakamizo et al. (2012), toroidal oscillations of the magnetic field were not reported (Yamakawa et al., 2022). Our study suggests that the variation of the ionospheric potential caused by R1FAC is important for the excitation of toroidal mode waves.