Quasiperiodic emissions observed by the Cluster spacecraft and their association with ULF magnetic pulsations

Abstract

Quasiperiodic (QP) emissions are electromagnetic waves at frequencies of about 0.5–4 kHz characterized by a periodic time modulation of the wave intensity, with a typical modulation period on the order of minutes. We present results of a survey of QP emissions observed by the Wide‐Band Data (WBD) instruments on board the Cluster spacecraft. All WBD data measured in the appropriate frequency range during the first 10 years of operation (2001–2010) at radial distances lower than 10 RE were visually inspected for the presence of QP emissions, resulting in 21 positively identified events. These are systematically analyzed, and their frequency ranges and modulation periods are determined. Moreover, a detailed wave analysis has been done for the events that were strong enough to be seen in low‐resolution Spatio‐Temporal Analysis of Field Fluctuations‐Spectrum Analyzer data. Wave vectors are found to be nearly field‐aligned in the equatorial region, but they become oblique at larger geomagnetic latitudes. This is consistent with a hypothesis of unducted propagation. ULF magnetic field pulsations were detected at the same time as QP emissions in 4 out of the 21 events. They were polarized in the plane perpendicular to the ambient magnetic field, and their frequencies roughly corresponded to the modulation period of the QP events.