The Rapid Responses of Magnetosonic Waves to the Compression and Expansion of Earth's Magnetosphere

Abstract

Under different solar wind dynamic pressures, we observed the magnetosonic (MS) wave amplification and attenuation associated with the compression and expansion of the Earth's magnetosphere. By analyzing the wave and particle variations recorded by the twin Van Allen Probes, we found that the magnetospheric compression or expansion can alter the keV proton phase space density distribution in velocity space and thus affects the MS wave intensity in upper band (f > 50 Hz) in the dawnside magnetosphere (magnetic local time ~ 4.0–7.9 and L ~ 5.7–3.0). During the magnetospheric compression period, the reduction of the 0.1 to 2 keV protons and the enhancement of the 3 to 7 keV protons form a positive phase space density gradient in their velocity space (i.e., the proton ring distribution), and meanwhile, the upper band MS waves are significantly amplified in the proton ring distribution region. During the subsequent magnetospheric expansion period, the enhancement of the 0.1 to 2 keV protons and the reduction of the protons above 3 keV produce a negative phase space density gradient in their velocity space, and meanwhile, the upper band MS waves are rapidly damped. These observations demonstrate that the intensity of the upper band MS waves in the dawnside magnetosphere is highly variable during the change in solar wind pressure.

Prompt Disappearance and Emergence of Radiation Belt Magnetosonic Waves Induced by Solar Wind Dynamic Pressure Variations

“…The physical processes acting in the two events could be roughly considered the inverse of each other. These phenomena are essentially different from the observations in a recent study of “amplification and attenuation of magnetosonic waves associated with the compression and expansion of the Earth's magnetosphere” (Li, ). In their study (Li, ), there were step‐like variations in the background plasma density but gradual variations in the solar wind dynamic pressure for the amplification and attenuation of magnetosonic waves.…”

“…These phenomena are essentially different from the observations in a recent study of “amplification and attenuation of magnetosonic waves associated with the compression and expansion of the Earth's magnetosphere” (Li, ). In their study (Li, ), there were step‐like variations in the background plasma density but gradual variations in the solar wind dynamic pressure for the amplification and attenuation of magnetosonic waves. As a result, it was difficult to differentiate between the contributions of the solar wind dynamic pressure variation and the local plasma density modulation (Chen & Thorne, ; Ma et al, ; Yuan et al, ) to the evolution of magnetosonic waves.…”

Effect of Low‐Harmonic Magnetosonic Waves on the Radiation Belt Electrons Inside the Plasmasphere

“…In the energy spectrum of hot protons (>0.1 keV) at 90° pitch angle, the phase space density of the 5‐ to 30‐keV protons is obviously larger than those at other energies, indicating that there are significant proton ring distributions near the magnetic equator on the duskside (MLAT ~ 8.4°–0.8° and MLT ~ 15.5–18.6 hr). The strong proton ring distributions can excite the MS waves (Chen et al, ; Horne et al, ; Li, Liu, et al, ; Liu, Chen, et al, ).…”

“…Fast magnetosonic (MS) waves are intense electromagnetic emissions between the proton cyclotron frequency (f cp ) and the lower hybrid resonance frequency (f LHR ) in the Earth's magnetosphere (Boardsen et al, 1992;Perraut et al, 1982). Although there are some reports that the MS waves in the solar wind can penetrate into the magnetosphere (Leonovich et al, 2003), the MS waves in the magnetosphere are mainly generated by the proton ring distributions (Chen et al, 2010(Chen et al, , 2011Horne et al, 2000;Li, Liu, et al, 2017;Liu et al, 2011;Liu, Li, et al, 2018;Xiao et al, 2013). These emissions are also called "equatorial noise" since they are often observed near the magnetic equator (Russell et al, 1970;Santolík et al, 2002).…”

Ring current proton scattering by low-frequency magnetosonic waves

“…Magnetosonic (MS) waves, also known as equatorial noise, are predominantly observed near the magnetic equator at frequencies between the proton cyclotron frequency ( f cp ) and the lower hybrid resonance frequency ( f LHR ) both inside and outside the plasmasphere (Russell et al, ; Perraut et al, ; Santolík et al, ; Meredith et al, ; Fu HS et al, ; Posch et al, ; Li LY et al, , b; Yuan ZG et al, ; Liu X et al, ; Liu B et al, ). These waves are mostly linearly polarized; they propagate nearly perpendicular to the background magnetic field (Zhima et al, ; Yu J et al, ; Su ZP et al, ), and they are believed to be excited by proton ring distributions (Horne et al, ; Chen LJ et al, , ; Liu KJ et al, ; Xiao FL et al, ).…”