Cluster observations of non–time continuous magnetosonic waves. Issue 10 (18th October 2016)

• Record Type: Journal Article
• Authors: Walker, Simon N.; Demekhov, Andrei G.; Boardsen, Scott A.; Ganushkina, Natalia Y.; Sibeck, David G.; Balikhin, Michael A.
• Is Part Of: Journal of geophysical research. Volume 121:Issue 10(2016:Oct.)
• Page Start: 9701
• Page End: 9716
• Publication Date: 2016-10-18
• Subjects: magnetosonic waves -- rising tone; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1002/2016JA023287
• Languages: English
• ISSNs: 2169-9380

Abstract

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. Key Points: The rate of change of frequency of rising tone EMW is greatest in the vicinity of the geomagnetic equator It is highly unlikely that the modulation results from the sideband instability Propagation of EMW may be spatially restricted by narrow density irregularities