Cluster observations of trapped ions interacting with magnetosheath mirror modes

Mirror modes are among the most intense low frequency plasma wave phenomena observed in the magnetosheaths of magnetized planets. They appear as large amplitude non-propagating fluctuations in the magnetic field magnitude and plasma density. These structures are widely accepted to represent a non-linear stage of the mirror instability, dominant in plasmas with large ion beta and a significant ion temperature anisotropy <I>T</I>_&perp;/<I>T</I>_&parallel;&gt;1. It has long been recognized that the mirror instability both in the linear and non-linear stage is a kinetic process and that the behavior of resonant particles at small parallel velocities is crucial for its development and saturation. While the dynamics of the instability and the effect of trapped particles have been studied extensively in theoretical models and numerical simulations, only spurious observations of the trapped ions were published to date. In this work we used data from the Cluster spacecraft to perform the first detailed experimental study of ion velocity distribution associated with mirror mode oscillations. We show a conclusive evidence for the predicted cooling of resonant ions at small parallel velocities and heating of trapped ions at intermediate pitch angles

Variability of the Magnetic Field Power Spectrum in the Solar Wind at Electron Scales

At electron scales, the power spectrum of solar-wind magnetic fluctuations can be highly variable and the dissipation mechanisms of the magnetic energy into the various particle species is under debate. In this paper, we investigate data from the Cluster mission's STAFF Search Coil magnetometer when the level of turbulence is sufficiently high that the morphology of the power spectrum at electron scales can be investigated. The Cluster spacecraft sample a disturbed interval of plasma where two streams of solar wind interact. Meanwhile, several discontinuities (coherent structures) are seen in the large-scale magnetic field, while at small scales several intermittent bursts of wave activity (whistler waves) are present. Several different morphologies of the power spectrum can be identified: (1) two power laws separated by a break, (2) an exponential cutoff near the Taylor shifted electron scales, and (3) strong spectral knees at the Taylor shifted electron scales. These different morphologies are investigated by using wavelet coherence, showing that, in this interval, a clear break and strong spectral knees are features that are associated with sporadic quasi parallel propagating whistler waves, even for short times. On the other hand, when no signatures of whistler waves at similar to 0.1-0.2f(ce) are present, a clear break is difficult to find and the spectrum is often more characteristic of a power law with an exponential cutoff.Peer reviewe

Statistical study of the alteration of the magnetic structure of magnetic clouds in the Earth’s magnetosheath

The magnetosheath plays a central role in the solar wind-magnetospheric coupling. Yet the effects of its crossing on solar wind structures such as magnetic clouds (MCs) are generally overlooked when assessing their geoeffectivity. Using 82 MCs observed simultaneously in the solar wind and the magnetosheath, we carry out the first statistical study of the alteration of their magnetic structure in the magnetosheath. For each event, the bow shock properties are obtained from a magnetosheath model. The comparison between the model results and observations shows that in 80% of cases, the MHD-based model captures well the magnetosheath transition; the other events are discussed separately. We find that just downstream of the bow shock the variation of the magnetic field direction shows a very good anticorrelation (r = −0.91) with the angle between the upstream magnetic field and the shock normal. We then focus on the magnetic field north-south component Bz because of its importance for geoeffectivity. Although the sign of Bz is generally preserved in the magnetosheath, we also find evidence of long-lasting intervals of opposite Bz signs in the solar wind and the magnetosheath during some events, with a |Bz| reversal > 10 nT at the magnetopause. We find that these reversals are due to the draping of the field lines and are associated with predominant upstream By. In those cases, the estimated position of the regions of antiparallel fields along the magnetopause is independent of the sign of the upstream Bz . This may have strong implications in terms of reconnection.Peer reviewe

South‐North Hemispheric Asymmetry of the FAE Distribution Around the Cusp Region: Cluster Observation

Cluster data from late July to early October were used to study the distribution of field‐aligned electron (FAE) events around the two cusps. An FAE event was defined as electron parallel flux >3 × 108 (cm2 s)−1. The total number of FAE events around the two cusps was basically identical, but downward FAE events prevailed in the south and upward FAE events in the north. In the southern cusp, the peak of the FAE events distribution versus altitude was about 1.3 RE higher and the peak of the FAE events distribution versus invariant latitude (ILAT) was about 4° ILAT lower. Only the downward FAEs around the southern cusp had a second ILAT peak, which was located about 11° higher than the main peak. The normalized number of FAEs showed nearly the same features as the unnormalized number of the FAEs events. These results indicated a north‐south asymmetry of the FAE distribution around the two cusps. Some causes for the asymmetry are discussed, the main ones being the asymmetry of the magnetospheric configuration resulting from geomagnetic dipolar tilt and solar wind flows, the interplanetary magnetic field asymmetry related to the magnetosphere, and the difference of ionospheric conductivity in the two hemispheres. Various solar wind‐magnetosphere interaction processes, such as quasi‐viscous interaction and reconnection, are responsible for the asymmetry, too. The second distribution peak (at higher ILAT) of the downward FAE events around the southern cusp corresponded to high solar wind speed and may be associated with the northward interplanetary magnetic field Bz field‐aligned current at low altitude. This requires further studies, however

Global manifestations of a substorm onset observed by a multi-satellite and ground station network

With a favorable constellation of spacecraft and ground stations, a study is made on the global manifestations of a substorm onset. The onset occurred simultaneously and conjugately in both hemispheres, confirmed by observations of the auroral breakup from IMAGE FUV-WIC and a sudden intensification of a westward electrojet from ground-based magnetometers. Concurrently with the onset, field-aligned and Hall currents in the auroral ionosphere are observed by CHAMP, which are consistent with the signature of a Harang discontinuity. Immediately after the onset a magnetic field dipolarization is clearly observed by Double Star TC-1, located near the central magnetotail and subsequently, by the Cluster quartet. The observations can be explained by a dawnward propagation of the substorm current wedge at a speed of about 300 km/s

Extended magnetic reconnection across the dayside magnetopause

The extent of where magnetic reconnection (MR), the dominant process responsible for energy and plasma transport into the magnetosphere, operates across Earth’s dayside magnetopause has previously been only indirectly shown by observations. We report the first direct evidence of X-line structure resulting from the operation of MR at each of two widely separated locations along the tilted, subsolar line of maximum current on Earth’s magnetopause, confirming the operation of MR at two or more sites across the extended region where MR is expected to occur. The evidence results from in-situ observations of the associated ion and electron plasma distributions, present within each magnetic X-line structure, taken by two spacecraft passing through the active MR regions simultaneously