Multiple harmonic ULF waves in the plasma sheet boundary layer observed by Cluster

The passage of the Cluster satellites in a polar orbit through Earth's magnetotail has provided numerous observations of harmonically related Pc 1-2 ULF wave events, with the fundamental near the local proton cyclotron frequency Omega(cp). Broughton et al. (2008) reported observations by Cluster of three such events in the plasma sheet boundary layer, and used the wave telescope technique to determine that their wave vectors k were nearly perpendicular to B. This paper reports the results of a search for such waves throughout the 2003 Cluster tail passage. During the 4 month period of July-October 2003, 35 multiple-harmonic wave events were observed, all in the plasma sheet boundary layer (PSBL). From the first observed event (22 July) to the last (28 October), 13 of Cluster's 42 tail passes had at least one event. The wave events were rather evenly distributed from X-GSE = -7 R-E out to the Cluster apogee distance of -18 R-E, with one event observed at -4 R-E. Z(GSE) for these events ranged from -10 to -3 R-E and +3 to +7 R-E (i.e., there were no events for vertical bar Z vertical bar < 3 R-E). The wave events, with durations from similar to 1 to 50 min, were consistently associated with signatures of the PSBL: elevated fluxes of counterstreaming ions with energies ranging from similar to 3 to 30 keV, and elevated fluxes of electrons with energies ranging from 0.25 to similar to 5 keV. Analysis of plasma parameters suggests that although waves occurred only when the ion beta exceeded 0.1 (somewhat larger than typical for the PSBL), ion particle pressure may be of more physical importance in controlling wave occurrence. Electron distributions were more isotropic in pitch angles than the ion distributions, but some evidence of counterstreaming electrons was detected in 83% of the events. The ions also showed clear signatures of shell-like or ring-like distributions; i.e., with reduced fluxes below the energy of maximum flux. The suprathermal ion fluxes were asymmetric in all events studied, with more ions streaming earthward (for events both north and south of the central plasma sheet). Good agreement between the observed frequency of the fundamental harmonic and the local Omega(cp) suggests that the waves were observed near the region of their origin and did not propagate along B, consistent with the wave telescope analysis.