Van Allen Probe observations of periodic rising frequencies of the fast magnetosonic mode

Abstract

Near simultaneous periodic dispersive features of fast magnetosonic mode emissions are observed by both Van Allen Probes spacecraft while separated in magnetic local time by~5 h: Probe A at 15 and Probe B at 9-11 h. Both spacecraft see similar frequency features, characterized by a periodic repetition at~180 s. Each repetition is characterized by a rising frequency. Since no modulation is observed in the proton shell distribution, the plasma density, or in the background magnetic field at either spacecraft we conclude that these waves are not generated near the spacecraft but external to both spacecraft locations. Probe A while outside the plasmapause sees the start of each repetition~40 s before probe B while deep inside the plasmasphere. We can qualitatively reproduce the dispersive features but not the quantitative details. The cause for this phenomena remains to be identified.

Magnetospheric Chorus, Exohiss, and Magnetosonic Emissions Simultaneously Modulated by Fundamental Toroidal Standing Alfvén Waves Following Solar Wind Dynamic Pressure Fluctuations

“…Such a modulation phenomenon appears to be not rare in the dayside magnetosphere following the solar wind dynamic pressure fluctuations. Another interesting feature of Figure 3 is the falling frequency magnetosonic emissions in each cycle, different from the previously reported rising frequency magnetosonic emissions (Boardsen et al, 2014;Fu et al, 2014). Compared to the interval of Figure 3, the intervals of Figures S5 and S6 with the weaker ULF pulsations allowed the less obvious quasiperiodic variations in the whistler-mode power.…”

“…The other involves the modulation of wave instabilities by the accompanied ultralow-frequency (ULF) waves (Helliwell, 1965;McPherron et al, 1968;Sato & Fukunishi, 1981;Sato et al, 1974): (1) the ULF pulsations modulate the background parameters and the resonant particle distributions; (2) the wave instabilities are cyclically turned on and off. Although the ULF pulsations were found to substantially modulate the linear growth rates of whistler-mode emissions in several events (W. Li et al, 2011;Manninen et al, 2010), there are still the quasiperiodic whistler-mode emissions occurring without the observable ULF pulsations (Boardsen et al, 2014;Kitamura et al, 1969;Němec et al, 2014;Sato et al, 1974;Titova et al, 2015;Tsurutani & Smith, 1974) or with the ULF pulsations of the essentially different periods (Němec et al, 2015). In contrast, there is evidence both for and against the latter scenario.…”

Survey of the frequency dependent latitudinal distribution of the fast magnetosonic wave mode from Van Allen Probes Electric and Magnetic Field Instrument and Integrated Science waveform receiver plasma wave analysis

“…Boardsen et al [2014] argued, as we did in the introduction, that the most favorable wave growth will be in the azimuthal direction, because this configuration allows a ray to remain in resonance with the source for a much longer time period. The rays are launched only with the perpendicular component of their wave vector in the azimuthal direction.…”

“…Inside the plasmapause they are observed at all magnetic local times (MLT), while outside the plasmapause they occur more frequently on the day side. Studies show that the power in these waves increases with AE* [Meredith et al, 2008], where AE* is the peak AE index over the previous 3 h. Recently quasi-periodic modulation of these waves has been reported [Fu et al, 2014;Boardsen et al, 2014;Němec et al, 2015]. They typically have been observed within magnetic latitudes (MLAT) of ±15° [Kasahara et al, 1994] but have been reported outside this range [e.g., Tsurutani et al, 2014;Posch et al, 2015].…”

Propagation of equatorial noise to low altitudes: Decoupling from the magnetosonic mode

“…These emissions are composed of a complex system of many spectral lines with spacing ranging from a few hertz to a few tens of hertz, linked to ion cyclotron frequencies in local or distant sources [Gurnett, 1976;Santolík et al, 2002a;Ma et al, 2014]. Superposed rising tone quasiperiodic structures can also occasionally occur [Fu et al, 2014;Boardsen et al, 2014;Němec et al, 2015b].…”