A. Runov

Dynamics of thin current sheets associated with magnetotail reconnection

[1] We have examined the detailed structure of thin current sheets and their evolution during a substorm interval on 24 August 2003, when Cluster experienced several rapid current sheet crossings within a couple of ion gyrotimes. These crossings took place during an interval of high-speed ion flows with BZ reversals and signatures of accelerated electrons, suggesting crossing of the reconnection region. On the basis of four-point observations with a tetrahedron scale of � 200 km, we could quantify for the first time the thickness of the current sheet, which was comparable to or less than one ion inertia length, and resolve some internal structures such as multiple peaks within these thin current sheets. Different patterns in jX and in electron anisotropy were identified during the current sheet crossings: two crossings during tailward flow interval exhibited a quadrupole-type Hall current in the ion diffusion region without a guide field, while one crossing during earthward flow showed a current system as predicted in the ion diffusion region under the presence of a guide field. Multiple flux rope type signatures or transient skewed structures are observed in the thin current sheets, particularly in regions where signatures of electron acceleration are observed. These observations suggest that threedimensional localized/transient structures could play an essential role in the dynamics of the thin current sheets, while a gross X-line picture can be established only in an average sense.

Electron flat-top distributions around the magnetic reconnection region

[1] Cluster multisatellite observations provide snapshots of electron distributions around the magnetic neutral line. An isotropic flat-top-type electron distribution in phase space is frequently observed around the X line, together with large ion velocities and a Hall quadrupole-like magnetic field inside the hot and tenuous plasma sheet in the magnetotail. The flat-top distributions are also associated with a finite magnetic field in the direction normal to the neutral sheet, and the cross-tail current density is sometimes very small. These results indicate that the flat-top-type distribution is mainly located near the outer boundary of the ion diffusion region in the plasma sheet outflow region, before reaching the pileup region with large normal component of the magnetic field. Simultaneously with the flat-top distributions, strong field-aligned electron beams mainly toward the X line are occasionally observed. This type of beam is mainly observed in the off-equatorial plasma sheet and also appears well inside the plasma sheet. Typical energies of the beam are 4–10 keV, which is comparable to the upper edge of flat-top energy. These highly accelerated electron distributions have a steep decrease in phase space density at the high-energy end, and it is found that they are not correlated with the increase of the higher-energy electrons related to suprathermal acceleration (>30 keV). This result indicates that the electron acceleration processes for the flat-top-type distributions are different from the suprathermal components, both of which are beyond the conventional MHD outflow acceleration and considered to be associated with some kinetic processes.

Study of reconnection-associated multiscale fluctuations with cluster and double star

The objective of the paper is to asses the specific spectral scaling properties of magnetic reconnection associated fluctuations/turbulence at the Earthward and tailward outflow regions observed simultaneously by the Cluster and Double Star (TC-2) spacecraft on September 26, 2005. Systematic comparisons of spectral characteristics, including variance anisotropy and scale-dependent spectral anisotropy features in wave vector space were possible due to the well-documented reconnection events, occurring between the positions of Cluster (X = -14--16

SCALE-DEPENDENT ANISOTROPY OF MAGNETIC FLUCTUATIONS IN THE EARTH'S PLASMA SHEET

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Multipoint observations of plasma distributions around an X line

) and TC-2 (X = -6.6

Scales in a thinning plasma sheet

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Local structure of the magnetotail current sheet: 2001 Cluster observations

). Another factor of key importance is that the magnetometers on the spacecraft are similar. The comparisons provide further evidence for asymmetry of physical processes in Earthward/tailward reconnection outflow regions. Variance anisotropy and spectral anisotropy angles estimated from the multi-scale magnetic fluctuations in the tailward outflow region show features which are characteristic for magnetohydrodynamic cascading turbulence in the presence of a local mean magnetic field. The multi-scale magnetic fluctuations in the Earthward outflow region are exhibiting more power, lack of variance and scale dependent anisotropies, but also having larger anisotropy angles. In this region the magnetic field is more dipolar, the main processes driving turbulence are flow breaking/mixing, perhaps combined with turbulence ageing and non-cascade related multi-scale energy sources.

Electric current and magnetic field geometry in flapping magnetotail current sheets

Abstract We use high resolution magnetic field data from the Cluster spacecraft to analyse the occurrence of specific anisotropy characteristics of magnetic fluctuations at two adjacent scales near the dissipation scale, during non-flow and bursty bulk flow (BBF) associated periods. The obtained anisotropy patterns show signatures of scale dependent anisotropy and can be explained by different physical processes. During non-flow periods the main source of the magnetic anisotropy are anisotropic ion populations within the plasma sheet boundary layer (PSBL). BBF-associated periods are characterized mainly by strong interaction of the plasma flow with the magnetic field. In both cases the local mean background magnetic field has a decisive role in the development of anisotropy in magnetic fluctuations.

Survey of large-amplitude flapping motions in the midtail current sheet

Using electron and magnetic field data obtained from the Cluster satellites, we identify the spatial distribution of highly accelerated electron distributions up to 10 keV. They are generally isotropic and form flat‐top distributions in the phase space. These distributions are observed in the vicinity of the X line associated with the quadrupole‐like magnetic field and energetic ions, throughout the plasma sheet. In some cases, these distributions are quasi‐stable, continuously observed for a few minutes with a stable Bz polarity and low current density in the center of the plasma sheet.

Multi-spacecraft observation of plasma dipolarization/injection in the inner magnetosphere

With Cluster observations in the magnetotail, we study the dynamics of plasma sheet thinning and stretching in a typical growth phase event of September 12, 2001. The thinning and stretching proceed in parallel, with transient variations. The pre‐onset value is Bz∼1.5 nT, J∼8 nA/m2. The current density increase is not accompanied with a corresponding number density increase. A large (>5 nT) guide field along the cross‐tail current direction was registered. An embedded current sheet structure was detected and, therefore, caution is required if making thickness estimations.