SM51D-01 INVITED
Particle Acceleration in relativistic plasmas
We review our recent results on particle acceleration in shocks [1], in reconnection [2] and via wave-particle interactions in microinstabilites [3]. After reviewing the fundamental issues in particle acceleration in the three classes of problems mentioned above, we focus specifically on the interactions of waves and particles during the evolution of plasma microinstabilites. We consider a new mechanism responsible for exceedingly strong acceleration events in relativistic plasmas. We conduct simulations of streaming plasmas (generated for example in astrophysical processes such as jets or in shocks) and consider the evolution of instabilities, comparing the classical and the relativistic evolution. The cause of the new processes is discussed in ration with the properties of the Minkowski space-time itself [3]. [1] G. Lapenta, J. King, JGR, to appear. [2] W. Wan, G. Lapenta, GRL, submitted [3] G. Lapenta et al, ApJ, to appear.
SM51D-02 INVITED
Production of energetic electrons and ions during magnetic reconnection
Simulations and observations suggest that the formation of magnetic islands is generic to reconnection with a guide field and therefore that particle acceleration in a multi-island environment is of greatest interest. Electrons are accelerated by parallel electric fields localized near the magnetic x-line and through Fermi acceleration in the contracting islands. A contracting island model of electron acceleration has been developed to explain the powerlaw spectra of energetic electrons seen in the Earth's magnetotail and inferred from the solar flare observations. We find that ion acceleration occurs primarily as ions cross from upstream into the Alfvenic outflow jet. Depending on the strength of the guide field, protons and higher mass particles behave like pickup particles in that they abruptly cross a narrow boundary layer and find themselves in a region of Alfvenic outflow. Their motion then mimics that of a classic pickup ion, gaining an Alfvenic ExB flow in the jet and a thermal speed close to the Alfven speed. The resulting ion energy is therefore proportional to mass as in many of the solar energetic particle observations. Preliminary analysis of Wind in-situ observations of solar wind reconnection exhausts supports this picture. A model of solar energetic ion production based on multiple encounters with reconnection exhausts is being pursued.
SM51D-03
Particle Acceleration Mediated by Magnetic Islands: Observations and Simulations
Magnetic reconnection is the underlying process that impulsively release an enormous amount of magnetic energy in solar and stellar flares, and substorms in the Earth's magnetosphere. Studies of energy release during solar flares, in particular, indicate that up to 50 percent of the released energy is carried by accelerated 20-100 keV suprathermal electrons. How so many electrons can gain so much energy during reconnection has been a long-standing question. In this study (to appear in Nature Physics 2007), we use multi-spacecraft measurements of energetic electrons from the Earth's magnetotail to demonstrate that the presence of energetic electrons is linked to the dynamics of magnetic islands. Specifically, we show that the peaks of energetic electron fluxes occur at sites of compressed density within islands. The strong density sub-structures within islands suggest that the islands may have undergone coalescence, which presents a potential new mechanism for electron acceleration during reconnection. We will compare these observations with Hall MHD and PIC simulations to assess the particle acceleration efficiency of this mechanism.
SM51D-04
Ion Acceleration in Vlasov Simulations of Double Layers, Electron Holes, and Associated Waves in Earth's Auroral Ionosphere
The localized unipolar electric field of strong double layers (DLs), such as those observed1 by FAST in the downward current region of the auroral ionosphere, can produce significant acceleration of anti-earthward electrons and earthward ions. However, DLs can also contribute to the energization of ion perpendicular to the geomagnetic field through several different processes: If the DL itself develops structure perpendicular to \mathbf{B}0, strong local ion heating can result. On the high-potential (high-altitude) side of the DL, electron holes (nonlinear structures with bipolar electric fields resulting from the saturation of a DL-driven electron two-stream instability) can also contribute to the acceleration of ions \perp to \mathbf{B}0. Finally, oblique wave modes with E\perp\gg E\parallel (e.g., lower-hybrid and ion-Bernstein waves) can contribute to perpendicular ion heating both above and below the DL. Two-dimensional Vlasov simulations are employed to study the interactions of DLs, holes, and oblique waves, together with their contributions to perpendicular ion heating rates. Fully kinetic algorithms for unmagnetized ions as well as reduced2 algorithms for magnetized ions, are used in this study. *Research supported by NASA, NSF, and DOE 1 R.~E.~Ergun, et al., Phys.~Rev.~Lett., 87, 045003 (2001). 2 D.~L.~Newman, et al., Phys.~Plasmas, 14, 055907 (2007).
SM51D-05
What Supports Parallel Electric Fields in Birkeland Current Regions?
Quasi-steady electric fields parallel to the background magnetic field exist in both upward and downward Birkeland current regions above the aurora. These electric fields, together with the turbulence found in the auroral region, energize ionospheric plasma as it flows upward into the magnetosphere. These parallel electric fields are split among one or more double layers, a transition region above the double layer, and a long-range potential region. Recently, we have developed a new kinetic and multimoment fluid theory for the Birkeland current system which includes the effect of plasma turbulence [1,2]. Using that theory, we derive a generalized Ohm's law for the Birkeland currents and find that these parallel electric fields are supported by some combination of (1) anomalous resistivity, (2) pressure gradients, and (3) the mirror force. Most theoretical efforts have focused on anomalous resistivity as the dominant supporting factor in parallel electric fields. Applying this theory to observations in the long range potential region of the downward Birkeland current, however, we show that the anomalous resistivity accounts for only a small portion of the parallel electric fields (<10%) and that the contributions of the other two terms are much more important. This result has important implications for other regions of space, such as reconnection sites and near solar flares, where parallel electric fields are likely to exist in inhomogeneous plasmas. [1] J. R. Jasperse et al. (2006), Phys. Plasmas 13, 072903 [2] J. R. Jasperse et al. (2006), Phys. Plasmas 13, 112902
SM51D-06
Cluster observations of electron acceleration in near-Earth reconnection events
Cluster observations of magnetic reconnection events in the near-Earth magnetotail are studied in detail, in an attempt to answer open questions regarding the acceleration of electrons in the reconnection region, such as the exact location of acceleration. We identify the Cluster positions relative to the X-line throughout the events, by investigation of plasma bulk flow and magnetic field. The events show a wide range of energetic electron flux intensities, both within individual events and between different events. Some of this variation might be understood in terms of spacecraft location relative to the X-line (Inflow region, Outflow region, earthward and tailward of X-line and separatrices). We will also investigate other possible factors such as local time (or YGSM) of observations, concurrent wave activity and overall geomagnetic activity. Both plasma sheet- and lobe-field line reconnection are studied, where differences can be expected. The intensities of energetic electrons (>40 keV) are put into context of those in the plasma sheet, before and after the events.
SM51D-07
The Microphysics of Magnetic Reconnection Observed by the Cluster Spacecraft
Magnetic reconnection leads to energy conversion in large volumes in space but is initiated in small regions. We report on Cluster observations of reconnection separatrices and diffusion regions. We concentrate on fundamentals of the microphysics on spatial scales at and below the ion scale. We compare detailed observations of reconnection obtained in large-scale regions such as the magnetopause and the magnetotail with recent observations in the turbulent magnetosheath. All the observations are compared to theoretical predictions. The microphysics of reconnections seems independent of the large-scale structure of the surrounding plasma.