[1] We present a statistical study on reconnection occurrence at the dayside magnetopause performed using the Double Star TC1 plasma and magnetic field data. We examined the magnetopause crossings that occurred during the first year of the mission in the 0600-1800 LT interval and we identified plasma flows, at the magnetopause or in the boundary layer, with a different velocity with respect to the adjacent magnetosheath. We used the Walén relation to test which of these flows could be generated by magnetic reconnection. For some event we observed opposite-directed reconnection jets, which could be associated with the passage of the X-line near the satellite. We analyzed the occurrence of the reconnection jets and reconnection jet reversals in relation to the magnetosheath parameters, in particular the local Alfvèn Mach number, the plasma b, and the magnetic shear angle. We also studied the positions and velocities of the reconnection jets and jet reversals in relation to the magnetosheath magnetic field clock angle. We found that the observations indicate the presence of a reconnection line hinged near the subsolar point and tilted according to the observed magnetosheath clock angle, consistently with the component merging model.

[1] We use Cluster spacecraft observations to study in detail the structure of a magnetic reconnection separatrix region on the magnetospheric side of the magnetopause about 50 ion inertial lengths away from the X-line. The separatrix region is the region between the magnetic separatrix and the reconnection jet. It is several ion inertial lengths wide and it contains a few subregions showing different features in particle and wave data. One subregion, a density cavity adjacent to the separatrix, has strong electric fields, electron beams and intense wave turbulence. The separatrix region shows structures even at smaller scales, for example, solitary waves at Debye length scale. We describe in detail electron distribution functions and electric field spectra in the separatrix region and we compare them to a numerical simulation. Our observations show that while reconnection is ongoing the separatrix region is highly structured and dynamic in the electric field even if the X-line is up to 50 ion inertial lengths away.

Abstract. We report multispacecraft Cluster observations of magnetic reconnection at the high-latitude magnetopause/magnetospheric boundary layer (MP/BL) under mainly northward interplanetary magnetic field (IMF) conditions. The event we study is on 3 December 2001 when the Cluster spacecraft were skimming the high-latitude duskside MP/BL during a period of about four hours. The orbit and configuration of the spacecraft were such that at least one satellite was present in the MP/BL during most of that period. We present the evidence of reconnection in the form of tangential stress balance between the magnetosheath and the MP/BL (Walén test) and in several cases in the form of transmitted magnetosheath ions in the MP/BL and incident/reflected magnetosheath ions in the magnetosheath boundary layer (MSBL) . The observations are consistent with magnetic reconnection occurring tailward of the cusp and going on continuously for a period of about four hours. The observed directions of the reconnection flows are consistent with the IMF orientation, thus indicating that reconnection is globally controlled by the IMF. Observations of a few flow reversals suggest passages of the spacecraft close to the X-line. The observation of low magnetic shear across the magnetopause during a flow reversal is consistent with component merging at least in one case. The observation of reconnection flows on the duskside magnetopause irrespective of the change in the sign of the IMF B Y also suggests aCorrespondence to: A. Retinò (alessandro.retino@irfu.se) better agreement with the component merging model, though antiparallel merging cannot be excluded because the distance from the X-line is not known.

The two inbound traversals of the Saturn's magnetosheath by Voyagers 1 and 2 have been studied using plasma and magnetic field data. In a great portion of the subsolar magnetosheath, large‐amplitude compressional waves are observed at low frequency (∼0.1 fp) in a high‐β plasma regime. The fluctuations of the magnetic field magnitude and ion density are anticorrelated, as are those of the magnetic and thermal pressures. The normals to the structures are almost orthogonal to the background field, and the Doppler ratio is on the average small. Even though the data do not allow the determination of the ion thermal anisotropy, the observations are consistent with values of T⊥/T∥ > 1, producing the onset of the mirror instability. All the above features indicate that the waves should be most probably identified with mirror modes. One of the two magnetopause crossings is of the high‐shear type and the above described waves are seen until the magnetopause. The other crossing is of the low‐shear type and, similarly to what has been observed at Earth, a plasma depletion occurs close to the magnetopause. In this layer, waves with smaller amplitude, presumably of the mirror mode, are present together with higher‐frequency waves showing a transverse component.

Abstract. Mirror modes have been systematically observed by Voyagers 1 and 2 in wide portions of Jupiter's and Saturn's magnetosheaths. In particular, in one crossing of Saturn•s subsolar magnetosheath, mirror waves are present almost continuously from the bow shock to the magnetopause. Therefore in this crossing, taking advantage also of relatively steady interplanetary conditions, we can track the evolution of mirror structures from a quasi-perpendicular bow shock to a low-shear magnetopause. We find that these structures evolve from quasi-sinusoidal waves to nonperiodic structures, consisting of both magnetic field enhancements and wells, and, finally, to dips in the plasma depletion layer (PDL) close to the magnetopause. Both the arnplitude and wavelength of the fluctuations tend to increase with increasing distance from the bow shock, except in the PDL, where they decrease toward the magnetopause. The waves are always compressional, and the direction of maximum variance forms an angle of •30 ø with B in the outer magnetosheath and a smaller angle in the inner magnetosheath. A comparison with the predictions of a nonlinear theory of the mirror instability shows some discrepancies, indicating that further theoretical studies are necessary.