M. B. Bavassano Cattaneo

Evolution of mirror structures in the magnetosheath of Saturn from the bow shock to the magnetopause

Mirror modes have been systematically observed by Voyagers 1 and 2 in wide portions of Jupiters and Saturns magnetosheaths. In particular, in one crossing of Saturns 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 amplitude 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° 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.

Structure of the separatrix region close to a magnetic reconnection X-line: Cluster observations

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.

Occurrence of reconnection jets at the dayside magnetopause: Double Star observations

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 Walen 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 Alfven Mach number, the plasma beta, 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.

Observations of mirror waves and plasma depletion layer upstream of Saturn's magnetopause

The two inbound traversals of the Saturns 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.

Energetic magnetospheric oxygen in the magnetosheath and its response to IMF orientation: Cluster observations

[1] We present Cluster observations made during an outbound orbit on 10 December 2000. After exiting the magnetosphere at midlatitude, Cluster spent a long time skimming the magnetopause moving to lower latitude along an orbit approximately in the ZY GSM plane on the dusk flank of the magnetopause. During this time, magnetospheric oxygen with energy >10 keV was observed continuously both in the magnetosphere and in the magnetosheath by the Cluster Ion Spectrometry (CIS) plasma experiment. While the oxygen density is roughly constant in the magnetosheath throughout the event, its velocity shows a strong dependence on the magnetosheath magnetic field orientation: low speeds, corresponding to almost isotropic distribution functions, occur for northward magnetic field, and high speeds, corresponding to beam-like distribution function occur for southward magnetic field. Mainly, two different processes have been discussed to explain the energetic particles escaping from the magnetosphere: flow along reconnected magnetospheric and magnetosheath field lines or crossing of the magnetopause when the particle gyroradii are comparable with the magnetopause thickness. The presence of the oxygen population cannot be readily explained in the framework of the reconnection theory. Instead, the observations are successfully reproduced by a model based on magnetopause crossing by finite gyroradius, provided the magnetosheath convection is taken into account together with the magnetosheath magnetic field orientation. Moreover, the presence of quasi-periodic motion of the magnetopause surface with period of approximately 5 min are evidenced by the analysis.

Evidence for interplanetary magnetic field By controlled large‐scale reconnection at the dayside magnetopause

We report evidence of a long-lasting reconnection event during which the accelerated plasma flow direction changes in response to an interplanetary magnetic field (IMF) By reversal, indicating a change in the reconnection site location. The observations were made by Equator-S on the dawn flank of the magnetopause and consist of a large number of plasma jets detected mostly within magnetospheric flux transfer events. The plasma jets were found in quantitative agreement with the theoretical predictions for reconnection. The reversal of the plasma flow direction in the jets following the reversal of the By component not only confirms that the dayside reconnection configuration is controlled by the IMF, as opposed to local control, but also stresses the importance of the IMF dawn-dusk component, in addition to the north–south component, in determining the global configuration of the reconnection.

Extended SuperDARN and IMAGE observations for northward IMF: Evidence for dual lobe reconnection

[1]xa0We present observations of ionospheric convection in the Northern Hemisphere made by the SuperDARN radar network during a 3 h period on 3 December 2001. The interplanetary magnetic field (IMF) during the time of observations is predominately northward with the By component changing from positive to slightly negative. During this period Cluster is skimming the southern high latitude dusk magnetopause and reveals that reconnection is going on quasi-continuously with the reconnection site being most of the time tailward of the southern cusp and always near the satellite location (Retino, et al., 2005). Detailed analysis of the three dimensional distribution function indicates that Cluster samples magnetosheath lines connected with geomagnetic field lines tailward of the cusps in both hemispheres (Bavassano Cattaneo et al., 2006). The evolution of the ionospheric convection measured by SuperDARN, together with IMAGE FUV observations of aurorae and DMSP particle precipitation data, confirms Cluster observations and shows that simultaneous reconnection poleward of both the northern and southern cusps occurs at a variable rate on the dusk part of the magnetosphere when the IMF clock angle is small.

Upstream waves in Saturn's foreshock

Waves in the foreshock of Saturn have been studied using data from the plasma and magnetic field experiments on board Voyager 1. The appearance of the waves, in the plasma as well as in the magnetic field data, is related to magnetic connection of the spacecraft to the bow shock. Their relative amplitude {Delta}B/B lies between 0.3 and 0.8 and their period in the spacecraft frame is {approximately}500 s. The waves are elliptically polarized and propagate at {approximately}30{degree} to the average magnetic field direction.

MHD turbulence in Saturn's magnetosheath downstream of a quasi‐parallel bow shock

The origin of the MHD turbulence downstream of a quasi-parallel shock is investigated using plasma and magnetic field data taken by Voyagers 1 and 2 during their outbound traversals of the Saturns magnetosheath. The characteristics of the waves observed in that region are compared with those of the waves present (1) downstream of a quasi-perpendicular shock and (2) in the foreshock. The last comparison supplies direct evidence that the upstream waves, filling the foreshock, are the main source of the magnetosheath turbulence, immediately downstream of a quasi-parallel shock.