Spacecraft observations of turbulence within a magnetic reconnection (guide field ∼ 0) ion diffusion region are presented. In the inertial subrange, electric and magnetic fluctuations both followed a -5/3 power law; at higher frequencies, the spectral indices were -1 and -8/3 respectively. The dispersion relation was found to be consistent with fast mode/whistler waves rather than kinetic Alfvén/ion cyclotron waves. Lower hybrid waves, which could be enhanced by whistler mode conversion, were observed but the associated anomalous resistivity was not found to significantly modify the reconnection rate.
[1] The k-filtering technique is a method to characterize stationary fluctuations in space plasmas in terms of the wave energy distribution in the frequency and wave vector space. It has the ability to distinguish between wave modes of the same frequency in the spacecraft frame of reference, but with different wave vectors. This method is based on simultaneous multi-point measurements of the wave field components, where a filter bank is used to enhance the spatial resolution. We have for the first time combined electric field data from the Electric Field and Wave (EFW) instrument and magnetic field data from the Spatio-Temporal Analysis of Field Fluctuation (STAFF) instrument on the four Cluster spacecraft in order to determine the wave energy distribution. The k-filtering technique has previously been performed with only the magnetic field measurements. The reason to include the electric field measurements is that it is important to include as much data as possible in order to get the best possible estimation of the wave energy distribution. Another reason is that it also enables comparisons between the electric and the magnetic part of the wave energy distribution to make it possible to differentiate the observed waves according to their polarization. The k-filtering method has been extended in order to allow for two measured components of the electric field, and also for the possibility that the electric field measurements from one or more satellites cannot be used for k-filtering. The technique has been applied on satellite data from the magnetosheath and the foreshock, and from these examples it is clear that k-filtering using both electric and magnetic field measurements is a good tool for characterizing the waves that are observed in space.Citation: Tjulin, A., J.-L. Pinçon, F. Sahraoui, M. André, and N. Cornilleau-Wehrlin (2005), The k-filtering technique applied to wave electric and magnetic field measurements from the Cluster satellites,
We present observations of lower hybrid cavities from altitudes above 10,000 km, using data from Viking and Cluster satellites. Lower hybrid cavities (LHCs) are narrow (ion gyroradius scale) density depletions with enhanced amplitude of waves in the lower hybrid frequency range, previously reported below 1750 km altitude by the Freja satellite and several sounding rockets. Detailed analysis of a Cluster event shows the same kind of rotating wave structure as previously observed on sounding rockets, with a change of sense of rotation at the lower hybrid frequency. The scale size of the structures are of the same order as the ion gyro radius, in similarity with the ionospheric observations. There are too few Cluster events yet to enable any occurrence statistics, but the distribution of the Viking LHCs are consistent with Freja results from the topside ionosphere.
[1] We have analyzed Cluster magnetic field and plasma data during high-altitude cusp crossing on 14 February 2003. Cluster encountered a diamagnetic cavity (DMC) during northward interplanetary magnetic field (IMF) conditions, and as IMF rotated southward, the spacecraft reencountered the cavity more at the sunward side. The DMC is characterized by a high level of magnetic field fluctuations and high-energy electrons and protons. Ultralow-frequency turbulence has been suggested as a mechanism to accelerate particles in DMC. We demonstrate in this paper for the first time that many of the low-frequency fluctuations in the cavity are back and forth motion of the DMC boundaries over the spacecraft and transient reconnection signatures. We also find examples of some isolated high-amplitude waves that could possibly be nonlinear kinetic magnetosonic modes. The lack of strong wave power at the vicinity of local ion cyclotron frequency in the DMC suggests that perhaps a mechanism other than wave-particle heating is a dominant source for ion heating in DMCs.
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