Ultra‐low‐frequency waves and associated wave vectors observed in the plasma sheet boundary layer by Cluster

Abstract

[1] Waves in the Pc1-Pc2 frequency range (0.1-5 Hz) are studied using Cluster magnetic field data. In the plasma sheet boundary layer, Cluster observed harmonically related waves with the fundamental near the local proton cyclotron frequency (W p ). These waves had components both parallel and perpendicular to the local magnetic field (B 0 ). Application of the wave telescope yielded the full wave vector (k) of the waves and showed that they propagated nearly perpendicular to B 0 . Associated with the waves were two counterstreaming ion beams with a relative velocity of approximately 4000 km/s. During wave activity, the tailward streaming beam increased in energy flux, and subsequently, energy flux increased in directions perpendicular to B 0 . We suggest two possible sources for the waves: inverse Landau resonance with an accelerated electron beam and an electromagnetic ion/ion cyclotron instability. The change in the ion distribution suggests these waves could contribute to the formation of the hot protons of the central plasma sheet.

Multiple harmonic ULF waves in the plasma sheet boundary layer: Instability analysis

“…Wave activity has often been suggested as the agent responsible for part or all of this transition [e.g., Gary and Winske, 1990] (and others reviewed by Broughton et al [2008]), and the harmonic waves reported here show a very close observational link to the PSBL, and, in particular, to its ring-shaped particle distributions. These waves may thus play an important role in energizing the plasma sheet.…”

“…Broughton et al [2008] reviewed many earlier studies investigating possible instabilities that might drive them, and suggested that the two most likely sources of these multiple-harmonic waves were (1) inverse Landau resonance with an accelerated electron beam as described by Chaston et al [2002], or (2) the electromagnetic ion/ion instability caused by coupling between a slow wave on the ion beam and the background ions, as described by Winske and Omidi [1992].…”

“…[39] Broughton et al [2008] compared the occurrence of waves only with properties of suprathermal ions, whereas in this study and in the companion study by Engebretson et al [2010] we have included electron distributions as well. Although there was a strong correlation between the occurrence of harmonic waves and streaming populations of both energetic ions and energetic electrons, Engebretson et al [2010] noted that the correlation was stronger with increased fluxes of energetic ions, and in every case the ion beta exceeded the electron beta by a factor of ∼9.…”

“…[24] Using the model distribution functions, either Model Ion Distribution Function 1 (Table 2), or Model Ion Distribution Function 2 (Table 3) with the model electron distribution function Table 1), we investigated the waves near harmonics of the ion cyclotron frequency and with wave vector k close to 90°from the background magnetic field (based on results by Broughton et al [2008]) to look for instability using jWhamp, Dartmouth College's version of the well known electromagnetic plasma wave dispersion code WHAMP [Rönnmark, 1982[Rönnmark, , 1983.…”

“…Observations by the Cluster spacecraft of multiple-harmonic electromagnetic waves with fundamental frequency near the local proton cyclotron frequency, W cp , were recently reported in the plasma sheet boundary layer by Broughton et al (2008). A companion paper surveys the entire magnetotail passage of Cluster during 2003, and reports 35 such events, all in the PSBL, and all associated with elevated fluxes of counterstreaming ions and electrons.…”