Oblique lower band chorus waves: Time shifts between discrete elements observed by the Cluster spacecraft

Abstract

We report observations of remarkable frequency differences and time shifts between the corresponding elements of lower band chorus recorded by the four Cluster spacecraft at low magnetic latitudes on 20 January 2004. The Poynting flux measurements made by the Spatio-Temporal Analysis of Field Fluctuations (STAFF) instrument confirm that the chorus source is located in the magnetic equatorial plane. Surprisingly, the spacecraft located closer to the equator systematically received the corresponding chorus elements later than the spacecraft located at higher magnetic latitudes. Owing to the orbit, the spacecraft located closer to the equator were at lower L shells. The time shifts and frequency differences depended almost linearly on the perpendicular distance between the various spacecraft. Analysis of data from both the wideband data and STAFF instruments shows that chorus emissions were generated with highly oblique angles of propagation. We discuss several properties of the chorus source that could lead to these observations. We show that the sources moving across the magnetic field lines are reasonably consistent with these observations. We propose that the transverse motion of the chorus sources is a consequence of a feedback between the oblique waves and counter streaming electrons during nonlinear cyclotron interactions, and we provide an estimate of the transverse velocity of the source.