Establishing the mechanisms by which the solar wind enters Earth's magnetosphere is one of the biggest goals of magnetospheric physics, as it forms the basis of space weather phenomena such as magnetic storms and aurorae. It is generally believed that magnetic reconnection is the dominant process, especially during southward solar-wind magnetic field conditions when the solar-wind and geomagnetic fields are antiparallel at the low-latitude magnetopause. But the plasma content in the outer magnetosphere increases during northward solar-wind magnetic field conditions, contrary to expectation if reconnection is dominant. Here we show that during northward solar-wind magnetic field conditions-in the absence of active reconnection at low latitudes-there is a solar-wind transport mechanism associated with the nonlinear phase of the Kelvin-Helmholtz instability. This can supply plasma sources for various space weather phenomena.

[1] Strong interplanetary shock interactions with the Earth's magnetosphere have great impacts on energetic particle dynamics in the magnetosphere. An interplanetary shock on 7 November 2004 (with the maximum solar wind dynamic pressure of $70 nPa) was observed by the Cluster constellation to induce significant ULF waves in the plasmasphere boundary, and energetic electrons (up to 2 MeV) were almost simultaneously accelerated when the interplanetary shock impinged upon the magnetosphere. In this paper, the relationship between the energetic electron bursts and the large shock-induced ULF waves is studied. It is shown that the energetic electrons could be accelerated and decelerated by the observed ULF wave electric fields, and the distinct wave number of the poloidal and toroidal waves at different locations also indicates the different energy ranges of electrons resonating with these waves. For comparison, a rather weak interplanetary shock on 30 August 2001 (dynamic pressure $2.7 nPa) is also investigated. It is found that interplanetary shocks or solar wind pressure pulses with even small dynamic pressure change can have a nonnegligible role in the radiation belt dynamics.

For the first time, the Cluster spacecraft have collected 3‐D information on magnetic field structures at small to medium scales in the Earth's dayside magnetosphere. We focus here on the first application of the Curlometer (direct estimation of the electric current density from curl(B), using measured spatial gradients of the magnetic field) analysis technique. The applicability of this multipoint technique is tested, for selected events within the data set, in the context of various mission constraints (such as position, timing, and experimental accuracy). For the Curlometer, nonconstant spatial gradients over the spacecraft volume, time dependence, and measurement errors can degrade the quality of the estimate. The estimated divergence of the magnetic field can be used to monitor (indirectly) the effect of nonconstant gradients in the case of many magnetic field structures. For others, and at highly distorted spacecraft configurations, this test may not reflect the quality of the Curlometer well. The relative scales and relative geometry between the spacecraft array and the structures present, as well as measurement errors, all are critical to the quality of the calculation. We demonstrate that even when instrumental and other errors are known to contribute to the uncertainty in the estimate of the current, a number of current signatures within the magnetosphere can be plausibly determined in direction, if not absolute size. A number of examples show consistent currents at the magnetopause, both separate from, and nearby or in the cusp region. Field‐aligned currents near the polar cap boundary are also estimated reliably. We also demonstrate one example of an anomalous current arising from the effect of a highly distorted spacecraft configuration.

Individual multispacecraft case studies confirm that the underlying current sheets are tangential discontinuities, but most I-t•As have relatively small jumps in field magnitude from before to after and thus would fail traditional identification tests as definite tangential discontinuities. The combination of our results suggests that HFAs should occur at a rate of several per day, and thus they may play a significant role in the solar-terrestrial dynamics.

During Cassini's initial orbit, we observed a dynamic magnetosphere composed primarily of a complex mixture of water-derived atomic and molecular ions. We have identified four distinct regions characterized by differences in both bulk plasma properties and ion composition. Protons are the dominant species outside about 9 RS (where RS is the radial distance from the center of Saturn), whereas inside, the plasma consists primarily of a corotating comet-like mix of water-derived ions with approximately 3% N+. Over the A and B rings, we found an ionosphere in which O2+ and O+ are dominant, which suggests the possible existence of a layer of O2 gas similar to the atmospheres of Europa and Ganymede.

A higher-order multiscale analysis of the dissipation range of collisionless plasma turbulence is presented using in-situ high-frequency magnetic field measurements from the Cluster spacecraft in a stationary interval of fast ambient solar wind. The observations, spanning five decades in temporal scales, show a crossover from multifractal intermittent turbulence in the inertial range to non-Gaussian monoscaling in the dissipation range. This presents a strong observational constraint on theories of dissipation mechanisms in turbulent collisionless plasmas.PACS numbers: 94.05. Lk, 52.35.Ra, 96.60.Vg, 95.30.Qd The solar wind provides an ideal laboratory for the study of plasma turbulence [1]. In-situ spacecraft observations suggest well-developed turbulence at 1 AU with a magnetic Reynolds number ∼ O 10 5 [2,3]. These show an inertial range of Alfvénic turbulence on magnetohydrodynamic (MHD) scales which is an anisotropic and possibly compressible energy cascade [4,5,6] with intermittent magnetic field fluctuations described by statistical multifractals and a power spectral density (PSD) with a scaling exponent close to −5/3 [1]. An outstanding problem is how, in the absence of collisional viscosity in the solar wind, this inertial range of MHD turbulence terminates at smaller scales where one anticipates a cross-over to dissipative and/or dispersive processes via wave-particle resonances. Understanding the nature of the dissipation processes may also inform open questions such as how the solar wind and solar coronal plasmas are heated [7,8,9].It has long been known [10,11] that in collisionless plasmas there is a transition in the PSD at high wavenumber k from MHD to kinetic physics at approximately the ion gyroscale. High resolution in-situ magnetic field observations reveal that at these scales the turbulent solar wind shows a transition from a ∼ −5/3 power law in the inertial range to a steeper power-law at higher k with spectral exponents in the range (−4, −2) [12,13]. However, the relevant physical mechanism is much debated; having implications for phenomena as diverse as magnetic reconnection [14,15], neutron stars and accretion disks [16]. Theories which have been proposed range from nonlinear turbulent-like cascade processes [17,18,19] to weak turbulence theories with wave dispersion and resonant plasma interactions [20]. As well as studies of in-situ spacecraft measurements in the solar wind [21], foreshock [22] and magnetosheath [23,24] regions, these theories are explored using simulations ranging from Hall-MHD [25], electron-MHD [16,26], gyrokinetics [27], particle-in-cell simulations of whistler turbulence [28] and Vlasov-hybrid simulations [29].Both neutral fluid and MHD turbulence share a 'classic' statistical signature -namely an intermittent mul- Figure 1: PSD plots of the components of the magnetic field from both FGM (at frequencies lower than 1 Hz) and STAFF-SC (at frequencies above 1Hz) instruments. The PSD values for Bx and By have been shifted up for clarity. The 95% confidence intervals...

We have conducted a detailed analysis of a set of events termed short large-arnphtude magnetic structures (SLAMS) observed at an encounter of the quasi-parallel bow shock by the AMPTE UKS and IBM satellites. Both the satellite configuration and the solar wind conditions are favorable for the case study presented here. We have identified isolated SLAMS, surrounded by solar wind conditions, and embedded SLAMS, which lie within or form the boundary with regions of significant heating and deceleration. The duration, polarization, and other characteristics of SLAMS are all consistent with their growth directly out of the ULF wave field, including the common occurrence of an attached whistler as found in ULF shocklets. The plasma rest iYmne propagation speeds, where they can be determined, and two-spacecraft time delays for all cases show that the SLAMS attempt to propagate upstream against the oncoming flow, but are convected back downstream. The speeds and delays vary systematically with SLAMS amphtude in the way anticipated from nonlinear wave theory, as do their polarization features. Inter-SLAMS regions, and boundary regions with the solar wind, contain hot deflected ions of lesser density than within the SLAMS. The amplitude of the SLAMS requires an active growth mechanism. Following earlier inferences about the limited transverse extent of SLAMS, we highlight the importance of determining the thickness of the transition zone over which SLAMS grow and the bulk heating and deceleration is effected. From this case study it appears that, at least under some circumstances, the quasi-parallel shock cannot be regarded as an undulating, cychcally re-forming simply connected surface. Instead, the transition zone is better represented as a set of ULF waves, some of which grow to become SLAMS which gradually decelerate and merge to form the downstream state. conditions and that these instabilities cause the shock to cyclically re-form. pulsations was dictated primarily by the Larmor radii of the backstreaming particles found in abundance upstream of the bow shock but downstream of the foreshock boundary. This boundary is defined by the upstream field line which is tangent to the curved bow shock, or by the trajectory of field-aligned ion beams of a given energy originating at the point of tangency. The turbulence associated with the shock apparently saturates some distance, typically 10 R•, downstream of this ion foreshock boundary [Bonifazi et al., 1983], downstream of which nearly isotropic distributions of "diffuse" energetic ions are found. The curved nature of the bow shock led early workers to suggest that the turbulent appearance of the quasi-parallel shock was due to debris which originated under more quasi-perpendicular configura-4209

A new method is described to analyze the dimensional character of observed structures using multipoint magnetic field measurements of four or more spacecraft. The technique can provide three directions along which the magnetic field has the minimum, intermediate, and maximum derivatives if the magnetic gradient tensor G = ∇ at every moment has been estimated by multipoint measurements. It follows that the structure's dimensionality and the variation direction can be directly determined. Both Cluster observations and simulations have shown that it is feasible to obtain the invariant axis orientation for two‐dimensional structures such as flux tubes, and to find the normal directions for one‐dimensional structures such as discontinuities. One advantage of this method is that these directions can be determined instantaneously, point by point in the time series, and so can be tracked through each observed structure. The analysis tool provides us a new perspective of the observed structures in the space.