Ion Bernstein waves in the magnetic reconnection region

Four-dimensional energy spectra and a diagram for dispersion relations are determined for the first time in a magnetic reconnection region in the magnetotail using data from four-spacecraft measurements by the Cluster mission on a spatial scale of 200 km, about 0.1 ion inertial lengths. The energy spectra are anisotropic with an extension in the perpendicular direction and axially asymmetric with respect to the mean magnetic field. The dispersion diagram in the plasma rest frame is in reasonably good agreement with the ion Bernstein waves at the second and higher harmonics of the proton gyrofrequency. Perpendicular-propagating ion Bernstein waves likely exist in an outflow region of magnetic reconnection, which may contribute to bifurcation of the current sheet in the outflow region.

Multi-spacecraft determination of wave characteristics near the proton gyrofrequency in high-altitude cusp

We present a detailed study of waves with frequencies near the proton gyrofrequency in the high-altitude cusp for northward IMF as observed by the Cluster spacecraft. Waves in this regime can be important for energization of ions and electrons and for energy transfer between different plasma populations. These waves are present in the entire cusp with the highest amplitudes being associated with localized regions of downward precipitating ions, most probably originating from the reconnection site at the magnetopause. The Poynting flux carried by these waves is downward/upward at frequencies below/above the proton gyrofrequency, which is consistent with the waves being generated near the local proton gyrofrequency in an extended region along the flux tube. We suggest that the waves can be generated by the precipitating ions that show shell-like distributions. There is no clear polarization of the perpendicular wave components with respect to the background magnetic field, while the waves are polarized in a parallel-perpendicular plane. The coherence length is of the order of one ion-gyroradius in the direction perpendicular to the ambient magnetic field and a few times larger or more in the parallel direction. The perpendicular phase velocity was found to be of the order of 100km/s, an order of magnitude lower than the local Alfvén speed. The perpendicular wavelength is of the order of a few proton gyroradius or less. Based on our multi-spacecraft observations we conclude that the waves cannot be ion-whistlers, while we suggest that the waves can belong to the kinetic Alfvén branch below the proton gyrofrequency fcp and be described as non-potential ion-cyclotron waves (electromagnetic ion-Bernstein waves) above. Linear wave growth calculations using kinetic code show considerable wave growth of non-potential ion cyclotron waves at wavelengths agreeing with observations. Inhomogeneities in the plasma on the order of the ion-gyroradius suggests that inhomogeneous (drift) or nonlinear effects or both of these should be taken into account.

Low-frequency waves in the Earth's magnetosheath: present status

The terrestrial magnetosheath contains a rich variety of low-frequency (≲ proton gyrofrequency) fluctuations. Kinetic and fluid-like processes at the bow shock, within the magnetosheath plasma, and at the magnetopause all provide sources of wave energy. The dominance of kinetic features such as temperature anisotropies, coupled with the high-β conditions, complicates the wave dispersion and variety of instabilities to the point where mode identification is difficult. We review here the observed fluctuations and attempts to identify the dominant modes, along with the identification tools. Alfvén/ion-cyclotron and mirror modes are generated by T^/T∥>1 temperature anisotropies and dominate when the plasma β is low or high, respectively. Slow modes may also be present within a transition layer close to the subsolar magnetopause, although they are expected to suffer strong damping. All mode identifications are based on linearized theory in a homogeneous plasma and there are clear indications, in both the data and in numerical simulations, that nonlinearity and/or inhomogeneity modify even the most basic aspects of some modes. Additionally, the determination of the wave vector remains an outstanding observational issue which, perhaps, the Cluster mission will overcome.

Super–Alfvénic Beam–Plasma Instabilities in Solar Flares

Special type III radio bursts with significantly lower drifts have been attributed to proton beams at the flare site. These low energy beams (some MeV) are nevertheless super-Alfvénic. Recent reports deal with the stability of such beams against resonant scattering by waves below the proton gyrofrequency. Here a different mechanism is proposed, namely nonresonant Alfvén instabilities, which may have higher growth rates and can thus dominate. A selfconsistent multispecies plasma description of such instabilities, developed for solar wind and cometary plasmas, is applied to slowly drifting type III radio bursts. The instability growth rate is fairly insensitive to the beam velocity but decreases with the ratio of beam to ambient plasma densities. Even low beam densities can trigger these nonresonant and hence generic instabilities. Moreover, growth rates are sizeable fractions of the wave frequency, whereas resonant instabilities would give much lower values at low beam densities.

Waves with harmonic structure below and above the lower hybrid resonance observed on the CENTAUR 35.001 and 35.002 rockets

Measurements obtained from the 0- to 10-kHz electric-wave experiment and the 0- to 4.2-kHz electron-density fluctuation experiment on the CENTAUR 35.001 and 35.002 rockets launched in December 1981 are presented. The observations include (i) spectacular narrow-banded signals with harmonic structure related to the proton gyrofrequency. The bands are observed both below and above the lower hybrid frequency, from 2 to 10 kHz, and are modulated in frequency during a rocket spin, typically by 2 kHz. The character of the signals indicate that they are generated by the presence of the payload in the plasma. The source of free energy remains unidentified; we suggest, however, a perpendicular ion beam emitted by or created by the presence of the payload, generating flute-mode ion cyclotron harmonic waves. (ii) Bursts of harmonic waves are received primarily below 2 kHz. The fundamental frequency of these emissions varies over a large range, from 47 to 700 Hz, and is rarely at any of the characteristic frequencies of the plasma. (iii) Waves with a clear lower cutoff at the lower hybrid frequency are also observed. The lower hybrid frequency estimated from magnetic field and density measurements and from the wave measurements agree within 6% (500 Hz).