Flux transfer events: Scale size and interior structure

We report the first direct investigation of the spatial properties of flux transfer events (FTEs) at the Earth's dayside magnetopause. Simultaneous magnetometer and plasma data from the ISEE 1 and 2 satellites are combined to show that magnetosheath FTEs can have a scale size of order an Earth radius in the magnetopause normal direction. We confirm that the magnetic field within the events appears to be twisted, this twisting corresponding to a core field‐aligned current of magnitude a few × 105 A. We also show evidence for plasma vorticity in FTEs. The transverse flow and field perturbations accompanying the three events studied obey approximately the Walén relation for a propagating Alfvén wave

Non-Maxwellian Proton Velocity Distributions in Nonradiative Shocks

The Balmer line profiles of nonradiative supernova remnant shocks provide the means to measure the post-shock proton velocity distribution. While most analyses assume a Maxwellian velocity distribution, this is unlikely to be correct. In particular, neutral atoms that pass through the shock and become ionized downstream form a nonthermal distribution similar to that of pickup ions in the solar wind. We predict the H alpha line profiles from the combination of pickup protons and the ordinary shocked protons, and we consider the extent to which this distribution could affect the shock parameters derived from H alpha profiles. The Maxwellian assumption could lead to an underestimate of shock speed by up to about 15%. The isotropization of the pickup ion population generates wave energy, and we find that for the most favorable parameters this energy could significantly heat the thermal particles. Sufficiently accurate profiles could constrain the strength and direction of the magnetic field in the shocked plasma, and we discuss the distortions from a Gaussian profile to be expected in Tycho's supernova remnant.Comment: 13 pages, 6 figure

Structure of the low latitude boundary layer

Observations at high temporal resolution of the frontside magnetopause and plasma boundary layer, made with the LASL/MPE fast plasma analyzer onboard the ISEE 1 and 2 spacecraft, revealed a complex quasiperiodic structure of some of the observed boundary layers. A cool tailward streaming boundary layer plasma was seen intermittently, with intervening periods of hot tenuous plasma which has properties similar to the magnetospheric population. While individual encounters with the boundary layer plasma last only a few minutes, the total observation time may extend over one hour or more

Fluid and particle signatures of dayside reconnection

International audienceUsing measurements of the AMPTE/IRM spacecraft, we study reconnection signatures at the dayside magnetopause. If the magnetopause is open, it should have the properties of a rotational discontinuity. Applying the fluid concept of a rotational discontinuity, we check for the existence of a de Hoffmann-Teller frame and the tangential stress balance (Walén relation). For 13 out of 40 magnetopause crossings in a statistical survey we find a reasonable agreement between observed plasma flows and those predicted by the Walén relation. In addition, we check if the measured distribution functions show single particle signatures which are expected on open field lines. We find the following types of signatures: field-aligned streaming of ring current particles, "D-shaped" distributions of solar wind particles, counterstreaming of solar wind and cold ionospheric ions, two-beam distributions of solar wind ions, and distributions of solar wind particles associated with field-aligned heat flux. While a particular type of particle signature is observed only for the minority of magnetopause crossings, 24 of the 40 crossings show at least one type of signature. Both the particle signatures and the fit to the Walén relation can be used to infer the sign of the normal magnetic field, Bn. We find that the two ways of inferring the sign of Bn lead primarily to the same result. Thus, both the particle signatures and a reasonable agreement with the Walén relation can, in a statistical sense, be considered as a useful indicator of open field lines. On the other hand, many crossings do not show any reconnection signatures. We discuss the possible reasons for their absence

Dual spacecraft determinations of magnetopause motion

We examine the motion of Earth's magnetopause for 16 dawnside traversals of this boundary by the sister spacecraft AMPTE/UKS and IRM in December, 1984, when their separation was 400-900 km. We compare magnetopause normal vectors, n, and speeds of motion, u(n), obtained separately from each spacecraft by use of three different methods, and also compare those u(n) to corresponding speeds, u(n)*, obtained from observed time lags between the two spacecraft. Agreement between u, values and n vectors determined from the three methods ranges from poor to excellent. Comparing u(n)* and u(n) values, we find a clear tendency for \ u(n)*\ to be larger than \ u(n)\: While slightly less than half of the results show reasonable agreement (0.5 < u(n)/u(n)* < 2), there are about as many results in the range 0 < u(n)/u(n)* < 0.5, and a few cases give the wrong sign of u(n)

Nonthermal Electrons at High Mach Number Shocks: Electron Shock Surfing Acceleration

We study the suprathermal electron acceleration mechanism in a perpendicular magnetosonic shock wave in a high Mach number regime by using a particle-in-cell simulation. We find that shock surfing/surftron acceleration producing the suprathermal electrons occurs in the shock transition region where a series of large amplitude electrostatic solitary waves (ESWs) are excited by Buneman instability under the interaction between the reflected ions and the incoming electrons. It is shown that the electrons are likely to be trapped by ESWs, and during the trapping phase they can be effectively accelerated by the shock motional/convection electric field. We discuss that suprathermal electrons can be accelerated up to $m_i c^2 (v_0/c)$, where $m_i c^2$ is the ion rest mass energy and $v_0$ is the shock upstream flow velocity. Furthermore, some of these suprathermal electrons may be effectively trapped for infinitely long time when Alfv\'en Mach number $M_A$ exceeds several 10, and they are accelerated up to the shock potential energy determined by the global shock size.Comment: 21 pages, 6 figure