Bibliography

Fine Harmonic Structure of Equatorial Noise with a Quasiperiodic Modulation

Abstract Equatorial noise emissions (fast magnetosonic waves) are electromagnetic waves observed routinely in the equatorial region of the inner magnetosphere. They propagate with wave vectors nearly perpendicular to the ambient magnetic field; that is, they are limited to frequencies below the lower hybrid frequency. The waves are generated by instabilities of ring-like proton distribution functions, which result in their fine harmonic structure with intensity maxima close to harmonics of the proton cyclotron frequency in the source region. Although most equatorial noise emissions are continuous in time, some events exhibit a clear quasiperiodic time modulation of the wave intensity, with typical modulation periods on the order of minutes. We analyze 72 such events (17 observed by the Cluster spacecraft, 55 observed by the Van Allen Probes spacecraft) for which high-resolution data were available. The analysis of the observed harmonic structure allows us to determine source radial distances of the events. It is found that the calculated source radial distances are generally close to the radial distances where the events were observed. The harmonic numbers where the events are generated range between about 12 and 30. Two events for which the spacecraft passed through the generation region were identified and analyzed. No simultaneous ultra-low-frequency magnetic field pulsations and no periodic plasma number density variations were observed. Although the in situ measured proton distribution functions were shown to be responsible for the wave growth, an insufficient resolution of the particle instruments prevented us from detecting a quasiperiodic modulation possibly present in the particle data.

Němec, F.; Tomori, A.; Santolik, O.; Boardsen, S.; Hospodarsky, G.; Kurth, W.; Pickett, J.; Kletzing, C.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 03/2020

YEAR: 2020     DOI: 10.1029/2019JA027509

equatorial noise; Fast Magnetosonic Waves; quasiperiodic modulation; Van Allen Probes

Equatorial noise with quasiperiodic modulation: Multipoint observations by the Van Allen Probes spacecraft

Electromagnetic wave measurements performed by the two Van Allen Probes spacecraft are used to analyze equatorial noise emissions with a quasiperiodic modulation of the wave intensity. These waves are confined to the vicinity of the geomagnetic equator, and they occur primarily on the dayside. In situ plasma number density measurements are used to evaluate density variations related to the wave occurrence. It is shown that the events are sometimes effectively confined to low density regions, being observed at successive satellite passes over a time duration as long as one hour. The events typically occur outside the plasmasphere, and they are often cease to exist just at the plasmapause. The analysis of the spatial separations of the spacecraft at the times when the events were observed simultaneously by both of them allows us to estimate the event spatial dimensions. It is found that the event spatial extent is typically lower than about 0.25RE in radial distance and within about one hour in magnetic local time. Modulation periods of the events decrease with increasing plasma number density up to about 100cm-3. Principally no dependence is observed at larger densities, possibly indicating a propagation from other locations.

emec, F.; ik, O.; Boardsen, S.; Hospodarsky, G.; Kurth, W.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 05/2018

YEAR: 2018     DOI: 10.1029/2018JA025482

equatorial noise; quasiperiodic modulation; RBSP; Van Allen Probes

Equatorial noise emissions with quasiperiodic modulation of wave intensity

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.

emec, F.; Santolik, O.; a, Hrb\; Pickett, J.; Cornilleau-Wehrlin, N.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 04/2015

YEAR: 2015     DOI: 10.1002/2014JA020816

equatorial noise; magnetosonic waves; quasiperiodic modulation