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

Abstract

Abstract. On 23 March 2002, the four Cluster spacecraft crossed in close configuration (∼100 km separation) the high-altitude (10 R E ) cusp region. During a large part of the crossing, the STAFF and EFW instruments have detected strong electromagnetic wave activity at low frequencies, especially when intense field-aligned proton fluxes were detected by the CIS/HIA instrument. In all likelihood, such fluxes correspond to newly-reconnected field lines. A focus on one of these ion injection periods highlights the interaction between waves and protons. The wave activity has been investigated using the k-filtering technique. Experimental dispersion relations have been built in the plasma frame for the two most energetic wave modes. Results show that kinetic Alfvén waves dominate the electromagnetic wave spectrum up to 1 Hz (in the spacecraft frame). Above 0.8 Hz, intense Bernstein waves are also observed. The close simultaneity observed between the wave and particle events is discussed as an evidence for local wave generation. A mechanism based on current instabilities is consistent with the observations of the kinetic Alfvén waves. A weak ion heating along the recently-opened field lines is also suggested from the examination of the ion distribution functions. During an injection event, a large plasma convection motion, indicative of a reconnection site location, is shown to be consistent with the velocity perturbation induced by the large-scale Alfvén wave simultaneously detected.

Origin of the turbulent spectra in the high-altitude cusp: Cluster spacecraft observations

“…The latter possibility has also been discussed by Sundkvist et al (2005), who suggest that the cusp waves belong to the kinetic Alfvén branch below the proton cyclotron frequency and are electromagnetic ion-Bernstein waves above that frequency. This is also confirmed by Grison et al (2005) who used the k-filtering technique, and showed that kinetic Alfvén waves dominate the electromagnetic spectrum up to 1Hz and above 0.8 Hz intense Bernstein waves are observed.…”

Wave properties in the magnetic reconnection diffusion region with high β: Application of the k‐filtering method to Cluster multispacecraft data

“…The intermediate mode is only slightly damped at higher frequencies since the average ratio |g|/w r is about 0.1, with a maximum of less than 0.25 (g is the damping rate and w r is the real part of the frequency), which suggests that this wave mode is likely to be observed in the diffusion region. We note that, under the present condition of a high b and high oblique propagations, the fast magnetosonic mode splits into Bernstein modes [Grison et al, 2005;Howes, 2009].…”

Cluster observations of a structured magnetospheric cusp

“…strong magnetic and electric waves are generated in the cusp region as noticed in case studies (e.g. Grison et al, 2005) and statistical studies (e.g. Cargill et al, 2005).…”