Equatorial noise emissions with quasiperiodic modulation of wave intensity

Abstract

Equatorial noise (EN) emissions are electromagnetic wave events at frequencies between the proton cyclotron frequency and the lower hybrid frequency observed in the equatorial region of the inner magnetosphere. They propagate nearly perpendicular to the ambient magnetic field, and they exhibit a harmonic line structure characteristic of the proton cyclotron frequency in the source region. However, they were generally believed to be continuous in time. We investigate more than 2000 EN events observed by the Spatio‐Temporal Analysis of Field Fluctuations and Wide‐Band Data Plasma Wave investigation instruments on board the Cluster spacecraft, and we show that this is not always the case. A clear quasiperiodic (QP) time modulation of the wave intensity is present in more than 5% of events. We perform a systematic analysis of these EN events with QP modulation of the wave intensity. Such events occur usually in the noon‐to‐dawn magnetic local time sector. Their occurrence seems to be related to the increased geomagnetic activity, and it is associated with the time intervals of enhanced solar wind flow speeds. The modulation period of these events is on the order of minutes. Compressional ULF magnetic field pulsations with periods about double the modulation periods of EN wave intensity and magnitudes on the order of a few tenths of nanotesla were identified in about 46% of events. We suggest that these compressional magnetic field pulsations might be responsible for the observed QP modulation of EN wave intensity, in analogy to formerly reported VLF whistler mode QP events.

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

“…The fast magnetosonic wave mode is one of the dominant wave modes in the Earth's inner magnetosphere [Russell and Holzer, 1970;Gurnett, 1976;Korth et al, 1984;Kasahara et al, 1994;Santolík et al, 2004;Meredith et al, 2008;Ma et al, 2013;Hrbáčková et al, 2015;Němec et al, 2015;Posch et al, 2015]. These waves have been observed around the magnetic equator at radial distances from the topside ionosphere out to at least 8 R E .…”

“…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].…”

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

“…On the same day as Figure 3, there were also signatures of the toroidal ULF modulation of exohiss and magnetosonic emissions from 02:50 to 03:05 UT ( Figure S5) and from 05:00 to 05:20 UT ( Figure S6). These magnetosonic frequency sweep structures might be composed of the non-time-continuous harmonic emission lines along the proton gyrofrequency harmonics in the high-resolution frequency-time spectrogram (Němec et al, 2015;Walker et al, 2016). 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).…”

“…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.…”