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

Abstract

The analysis of ionospheric equatorial noise (EN) with a quasiperiodic (QP) modulation observed by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) spacecraft is presented. These EN emissions, also called whistler mode or fast magnetosonic waves, play an important role in acceleration of radiation belt electrons. A statistical analysis with 103 events shows that they occur just after intense magnetic storms. Usually, they are generated by unstable proton ring distribution close to the magnetic equator at harmonics of the proton gyrofrequency in the inner magnetosphere (2 < L < 8). But at lower L values down in the ionosphere three events have been analyzed and it appears that the EN waves are at harmonics of—or very close to—a O+ ion gyrofrequency which can be found close to or slightly above the satellite. The wave propagation analysis indicates that these emissions are coming from an area above the satellite. Concerning one event, the EN emissions are observed on several consecutive orbits and there is a temporal coincidence with observations performed by the Cluster satellites at higher altitudes in the magnetosphere. EN emissions at lower frequencies have been also observed by the Cluster satellites in the same longitudinal sector as DEMETER but at ~5 RE. The analysis of the Spatio Temporal Analysis of Field Fluctuations data on board C1 reveals that the magnetic field spectrogram has peaks close to harmonics of the local proton gyrofrequency as usually reported. It is shown that the DEMETER and Cluster EN waves have a similar QP modulation but with slightly different period and frequency.

Detailed Properties of Equatorial Noise With Quasiperiodic Modulation

“…A possible generation of a small fraction of the events inside the plasmasphere thus cannot be ruled out. We note that sporadic event observations well inside the plasmasphere are consistent with an analogous type of events observed at altitudes as low as 700 km (Parrot et al, ).…”

“…Němec, Santolík, Hrbáčková, et al () further investigated high‐resolution Cluster Wideband Data for all 17 events where these were available, and they revealed the presence of a harmonic line structure in all of them. The emissions may occasionally consist of two frequency bands of interleaved QP elements (Li et al, ) and were also observed at altitudes as low as 700 km (Parrot et al, ).…”

Local Heating of Oxygen Ions in the Presence of Magnetosonic Waves: Possible Source for the Warm Plasma Cloak?

“…Through electric field observations of equatorial noise (magnetosonic waves) from IMP 6 and Hawkeye 1 satellites, Gurnett (1976) reported emissions of dominant spectral peaks at various proton gyrofrequency harmonics with minor peaks at one fourth and one eighth the proton gyrofrequency harmonics, that is, heavier ion harmonics. Parrot et al (2016) reported observations of equatorial noise emissions in the ionosphere with O+ harmonics. Recent observations of magnetosonic waves with CLUSTER (Walker et al, 2016), DEMETER (Santolík et al, 2016) and the Van Allen Probes (Min et al, 2018) reveal multiple spectral peaks in between the proton gyrofrequency harmonics, which could indicate resonance with heavier ions.…”

Dependence of Properties of Magnetospheric Line Radiation and Quasiperiodic Emissions on Solar Wind Parameters and Geomagnetic Activity

“…Out of them, 2,181 events were observed in the daytime half orbits and just 83 events were detected in the nighttime half orbits. Given that the events observed at low latitudes should be classified as QP modulated whistler mode equatorial noise (also known as fast magnetosonic waves) rather than QP emissions (Němec et al, , ; Parrot et al, ), we consider only QP events which at least partially spanned to geomagnetic latitudes larger than 40°. This leaves us with 2,172 events, out of which 79 were observed on the nightside.…”