Cluster observations of non–time continuous magnetosonic waves

Abstract

Equatorial magnetosonic waves are normally observed as temporally continuous sets of emissions lasting from minutes to hours. Recent observations, however, have shown that this is not always the case. Using Cluster data, this study identifies two distinct forms of these non–temporally continuous emissions. The first, referred to as rising tone emissions, are characterized by the systematic onset of wave activity at increasing proton gyroharmonic frequencies. Sets of harmonic emissions (emission elements) are observed to occur periodically in the region ±10° off the geomagnetic equator. The sweep rate of these emissions maximizes at the geomagnetic equator. In addition, the ellipticity and propagation direction also change systematically as Cluster crosses the geomagnetic equator. It is shown that the observed frequency sweep rate is unlikely to result from the sideband instability related to nonlinear trapping of suprathermal protons in the wave field. The second form of emissions is characterized by the simultaneous onset of activity across a range of harmonic frequencies. These waves are observed at irregular intervals. Their occurrence correlates with changes in the spacecraft potential, a measurement that is used as a proxy for electron density. Thus, these waves appear to be trapped within regions of localized enhancement of the electron density.

Quasiperiodic Whistler Mode Emissions Observed by the Van Allen Probes Spacecraft

“…Although they have been studied already for a few decades, their generation mechanism still has not been fully understood. We note that QP modulations of the equatorial noise (fast magnetosonic mode; e.g., Němec et al, ; Němec, Santolík, Boardsen, et al, ; Walker et al, ), which is a whistler mode emission below the lower hybrid frequency with wave normals nearly perpendicular to the ambient magnetic field, are different from the QP emissions in this study.…”

Equatorial noise emissions with a quasiperiodic modulation observed by DEMETER at harmonics of the O+ ion gyrofrequency

“…Figure shows four frequency‐time spectrograms of the magnetic field recorded on 13 September 2005 which correspond to the data recorded by the four satellites (see also Walker et al . [] who studied the same event). One can see an emission around 100 Hz when the satellites successively cross the magnetic equator.…”

Fast Magnetosonic Waves Observed by Van Allen Probes: Testing Local Wave Excitation Mechanism

“…Besides, radial and azimuthal propagation, combined with wave trapping by density gradients, may account for the origin of some of the wave occurrence inside the plasmapause, which otherwise could not be explained by the local excitation mechanism (Ma, Li, Chen, Thorne, & Angelopoulos, 2014;Ma, Li, Chen, Thorne, Kletzing, et al, 2014). In addition, recent observations show a short-timescale (∼minute) quasi-periodicity of wave amplitude with a frequencytime dispersion (similar to chorus chirps) (Boardsen et al, 2014;Fu, Cao, et al, 2014;Li et al, 2017;Němec, Santolík, Hrbáčková, Pickett, & Cornilleau-Wehrlin, 2015;Walker et al, 2016), whose cause is still unclear. Fast magnetosonic waves can interact with the local radiation belt electron population resonantly and nonresonantly, efficiently accelerating some particles to high energies while scattering others into the loss cone (e.g., Albert et al, 2016;Artemyev et al, 2015;Bortnik et al, 2015;Chen et al, 2015;Horne et al, 2007;Li et al, 2014Li et al, , 2015Li et al, , 2016Maldonado et al, 2016;Ma et al, 2016;Mourenas et al, 2013;Ni et al, 2017;Shprits, 2016;Shprits et al, 2013;Tao & Li, 2016;Yang et al, 2014;.…”