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

Abstract

Equatorial noise (often phenomenologically described as magnetosonic waves in the literature) is a natural electromagnetic emission, which is generated by instability of ion distributions and which can interact with electrons in the Van Allen radiation belts. We use multicomponent electromagnetic measurements of the DEMETER spacecraft to investigate if equatorial noise propagates inward down to the Earth. Analysis of a selected event recorded under disturbed geomagnetic conditions shows that equatorial noise can be observed at an altitude of 700 km, while propagating radially downward as a superposition of spectral lines from different distant sources observed at frequencies both below and above the local proton cyclotron frequency. Changes in the local ion composition encountered by the waves during their inward propagation disconnect the identified wave mode from the low‐frequency magnetosonic mode. The local ion composition also induces a cutoff which prevents the waves from propagating down to the ground.

Detailed Properties of Equatorial Noise With Quasiperiodic Modulation

“…This is in agreement with the former results on all EN emissions (Němec et al, , ). The observed larger latitudinal spread and lower intensity of the events at higher frequencies appear to be in agreement with the ray tracing calculation of event propagation, which shows that the emissions with lower frequencies tend to stay confined closer to the equatorial plane (Santolík et al, ). The calculated intensities of individual QP elements can be compared with former results obtained for normal continuous EN emissions.…”

“…They propagate with wave vectors oriented nearly perpendicular to the ambient magnetic field (Santolík et al, ; Walker, Balikhin, Shklyar, et al, ), that is, they are limited to frequencies below the lower hybrid frequency. They may be occasionally detected at altitudes as low as 700 km (Němec, Parrot, & Santolík, ; Santolík et al, ) and at radial distances as large as 10 R E (Hrbáčková et al, ). Similar emissions off the geomagnetic equator were also reported (Tsurutani et al, ; Zhima et al, ).…”

Particle‐in‐Cell Simulations of the Fast Magnetosonic Mode in a Dipole Magnetic Field: 1‐D Along the Radial Direction

“…Němec et al () analyzed 10 years of Cluster observations and showed statistically that azimuthal propagation is dominant where the (total) plasma density is low ( cm −3 ), as occurs outside the plasmapause, but no preferential propagation direction is found where the density is high ( 100 cm −3 ), as occurs inside the plasmapause. While the azimuthal direction may be the preferential direction of propagation outside the plasmapause where free energy source is typically located (e.g., Chen et al, ), the presence of the radial component in the propagation direction is believed to explain many observational features including, but not limited to, the occurrence of fast magnetosonic waves deep in the plasmasphere and off‐harmonic frequency spectrum of the electric and magnetic field fluctuations (Horne et al, ; Perraut et al, ; Posch et al, ; Santolík et al, , ; Zhima et al, ). Santolík et al () analyzed the fluctuating electric field to observationally confirm a radial component of the wave normal vector.…”

Direct observation of generation and propagation of magnetosonic waves following substorm injection

“…The corresponding Bernstein mode instability tends to generate magnetosonic waves at frequencies close to the harmonics of the proton gyrofrequency with wave vectors nearly perpendicular to the background magnetic field [ Curtis and Wu , ; Boardsen et al , ; Horne et al , ; Gary et al , ; Liu et al , ]. Previous observations have shown that the magnetosonic emission lines can considerably deviate from the harmonics of the local proton gyrofrequency [ Santolík et al , , ] and even extend below the local proton gyrofrequency in the radiation belt slot region [e.g., Li et al , ; Yang et al , ]. These discrepancies may be interpreted as a result of the magnetosonic wave propagation [ Kasahara et al , ; Horne et al , ; Chen and Thorne , ; Ma et al , ; Santolík et al , ; Horne and Miyoshi , ].…”