Wave particle interactions in the high-altitude polar cusp: a Cluster case study

摘要

On 23 March 2002, the four Cluster spacecraft crossed in closeconfiguration (~100 km separation) the high-altitude(10 R_E) cusp region. During a large part of the crossing, the STAFFand EFW instruments have detected strong electromagnetic wave activity at lowfrequencies, especially when intense field-aligned proton fluxes were detectedby the CIS/HIA instrument. In all likelihood, such fluxes correspond tonewly-reconnected field lines. A focus on one of these ion injection periodshighlights the interaction between waves and protons. The wave activity has beeninvestigated using the k-filtering technique. Experimental dispersionrelations have been built in the plasma frame for the two most energetic wavemodes. Results show that kinetic Alfvén waves dominate the electromagneticwave spectrum up to 1 Hz (in the spacecraft frame). Above0.8 Hz, intense Bernstein waves are also observed. The closesimultaneity observed between the wave and particle events is discussed as anevidence for local wave generation. A mechanism based on currentinstabilities is consistent with the observations of the kinetic Alfvénwaves. A weak ion heating along the recently-opened field lines is alsosuggested from the examination of the ion distribution functions. During aninjection event, a large plasma convection motion, indicative of a reconnectionsite location, is shown to be consistent with the velocity perturbationinduced by the large-scale Alfvén wave simultaneously detected.