SM53B-1277
Particle Acceleration to Ultrarelativistic Energies by Magnetic Reconnection in Electron- Positron Plasmas
Particle acceleration by magnetic reconnection in electron-positron (or pair) plasmas is of great interest not only because of its relevance to astrophysical objects such as pulsar winds and extragalactic jets, but also because in contrast with hydrogen plasmas, the generalized Ohm's law for pair plasmas contains no Hall currents. Hence, studies of particle acceleration in pair plasmas by collisionless magnetic reconnection enable one to focus on the essential role of particle pressure tensors and kinetic effects. We have carried out studies of particle acceleration by reconnection in pair plasmas by using 2D particle-in-cell simulations, and shown that fast reconnection is realized in non-relativistic [Phys. Rev. Lett., 95, 245001 (2005)] as well as relativistic regimes [Phys. Plasmas, 14, 056503 (2007)]. We have studied a long current sheet which breaks up into multiple magnetic islands and X-lines due to the tearing instability, and located sites where particles are accelerated. We have observed that ultrarelativistic particles (with Lorentz factors > 100), which produce non-thermal tails in distribution functions, are generated primarily by the reconnection electric field in the vicinity of an X-line. This acceleration mechanism appears to be independent of the presence of the guide field. We have shown that the parallel electric field is localized at the X-line when there is a guide field instead of localized at separatrices as shown in hydrogen plasmas, and this electric field accelerates particles to ultrarelativistic energies. However, the energy spectrum depends on the magnitude of the guide field and the background density, and we have observed a harder energy spectrum in the presence of a finite guide field than when the guide field is absent. Lessons learned from these studies for hydrogen plasmas in the corona and the magnetosphere will be discussed.
SM53B-1278
Impulsive Flare Energy Transport by Large-Scale Alfven Waves and the Electron Acceleration Problem
In this poster, we investigate an alternative scenario for solar flare energy transport. Usually, the energy stored in the solar corona and released during a flare is thought to be transported to the lower atmosphere by electron beams. However, based on microwave observations, magnetic fields of the order of a few 100 Gauss to a kilogauss are measured in the corona above the core of an active region. This implies an Alfvén speed on the order of 0.1c, meaning that Alfvén wave pulses become plausible agents for transporting the stored energy of the flare. We investigate this scenario, and the opportunities it presents for both heating the lower chromosphere and accelerating electrons to HXR-emitting energies.
SM53B-1279
Observation of Field-Aligned Particle Energization Using GOES-11
The evolution of high energy electron pitch angle distributions (PADs) can provide clues to help us distinguish the physical processes at work in the Earth's radiation belts. The shape of a PAD is affected by the geometric structure of the magnetic field environment and by the particle distributions. Changes in the form of magnetic field reconfiguration or the particle populations can result in specific changes in PAD. Using GOES-11 particle and magnetic field data, while the satellite was spinning in storage mode in geosynchronous orbit, we obtain pitch angle resolved electron measurements (>600 keV and >2MeV) and observe the evolution of the constructed PADs. In this study, we observe an unusual event occurring March 22 - 28, 2002, which suggests an acceleration mechanism which favors field-aligned electrons, resulting in clearly observed butterfly-type (minimum around 90 degrees) distributions at all local times over a 2 day period.
SM53B-1280
Are Electron Distributions Associated with Reconnection Electrostatically Unstable and do they lead to Electron Holes?
Model electron distributions arising in the late stages of 2-D PIC simulations1 of magnetic reconnection with a guide field are used to initialize 1-D and 2-D electrostatic Vlasov simulations, which are oriented to include the direction parallel to the (local) magnetic field. Transient electron holes are found to develop near the separatrix through a electron-ion (e.g., Buneman) instability. Restriction of the destabilizing current to a narrow sheet perpendicular to \mathbf{B} reduces the Buneman growth rate, possibly leading to more stable holes. Near the X-point, electron-electron two-stream instabilities can produce quasi-stable electron holes. These first-generation holes can modify the electron-ion interaction resulting in a second generation of holes, with a variety of subsequent hole-hole interactions that depend on the details of the initial distributions. When the holes are shallow and well-separated they can be analyzed using stationary solutions2 based on the condition eφ/Te \ll 1. *Research supported by DOE, NASA, and NSF. 1P.~L.~Pritchett, Phys.~Plasmas, 12, 062301 (2005). 2M.~V.~Goldman, D.~L.~Newman, and A.~Mangeney, Phys.~Rev.~Lett., in press, (2007).
SM53B-1281
Jovian millisecond-bursts as markers of Alfvenic electron acceleration
Jupiter's radio emissions are dominated in intensity by decametric radio emissions due to the Io-Jupiter interaction. Previous analyses suggest that these emissions are cyclotron-maser emissions in the flux tubes connecting Io's wake to Jupiter, triggered by electrons accelerated from Io to Jupiter. We present simulations of a hot electron population under the assumption of acceleration by Alfvén waves in the Io flux tube. Outside of limited acceleration regions where parallel electric field associated with Alfvén waves are found, the electrons have an adiabatic motion along the magnetic field lines. Electron distribution functions are computed at various altitudes and times (relatively to the propagation of the Alfvén wave). Near Jupiter, a loss cone appears in the magnetically mirrored electron population; it amplifies extraordinary (X) mode radio waves. The X-mode growth rate (depending on time and altitude) is computed, and a theoretical dynamic spectrum of the resulting Jovian radio emissions is built. This spectrum is compared with those made from observations.
SM53B-1282
Position and velocity space diffusion of test particles in stochastic electromagnetic fields
The two-dimensional diffusive dynamics of test particles in a random electromagnetic field is studied. The synthetic electromagnetic fluctuations are generated through randomly placed magnetised "clouds" oscillating with a frequency ω. We investigate the mean square displacements of particles in both position and velocity spaces. As ω increases the particles undergo standard (Brownian-like) motion, anomalous diffusion and ballistic motion in position space. Although in general the diffusion properties in velocity space are not trivially related to those in position space, we find that energization is present only when particles display anomalous diffusion in position space. The anomalous character of the diffusion is only in the non-standard values of the scaling exponents while the process is Gaussian.
SM53B-1283
Particle acceleration by fluctuating electric fields at a magnetic field null point
Particle acceleration consequences from fluctuating electric fields superposed on an X-type magnetic field in collisionless solar plasma are studied. Such a system is chosen to mimic generic features of dynamic reconnection, or the reconnective dissipation of a linear disturbance. We explore numerically the consequences for charged particle distributions of fluctuating electric fields superposed on an X-type magnetic field. Particle distributions are obtained by numerically integrating individual charged particle orbits when a time varying electric field is superimposed on a static X-type neutral point. This configuration represents the effects of the passage of a generic MHD disturbance through such a system. Different frequencies of the electric field are used, representing different possible types of wave. The electric field reduces with increasing distance from the X-type neutral point as in linear dynamic magnetic reconnection.
SM53B-1284
Turbulent acceleration of superthermal electrons
The acceleration and heating of charged particles in the solar wind is one of the outstanding problems in contemporary solar and heliospheric physics. Electron distributions with various degrees of asymmetry associated with the energetic tail population are commonly detected in the solar wind near 1 AU. Traditional attempt to address the origin of superthermal electrons usually starts from the collisional dynamics near the Sun and their transport through interplanetary space. In the present paper, we look for collective mechanism(s) as an alternative explanation for the generation of superthermal population. By numerically solving one-dimensional electrostatic weak turbulence equations and by performing fully nonlinear particle-in-cell (PIC) simulations, this paper demonstrates that a wide variety of asymmetric energetic tail distribution may result. It is found that a wide variety of asymmetric tail formation becomes possible if one posits that the solar wind electrons are initially composed of thermal core plus field-aligned counter-streaming beams, instead of the customary thermal population plus a single beam. It is shown that the resulting nonlinear wave-wave and wave-particle interactions lead to asymmetric non-thermal tails.
SM53B-1285
Drift-Resonant Interaction of Magnetospheric Relativistic Electrons with Ultra-Low Frequency (ULF) Waves: Comparison between Observations and Simulations
By analyzing CRRES and GOES observations on Aug. 27 1991, Tan et al. [2004] reported evidence of magnetospheric relativistic electron acceleration by resonant interactions with PC5 ULF waves. The event showed strong ULF wave activities after a storm sudden commencement (SSC) and energetic electron fluxes were enhanced in 2 hours. The electron flux peak observed in energy channels (0.6 – 1.1 MeV) were modulated by local electric field observed by CRRES. In this study, we set up a drift-resonant interaction model between ULF wave and magnetospheric relativistic electrons to model the observed electron flux in the event. In this model, the poloidal mode wave is concentrated in the dayside and the toroidal mode wave is concentrated in two flanks. The toroidal mode waves in the dawn and dusk flanks are in anti-phase. We found that electron can be accelerated jointly by the poloidal wave in the dayside and toroidal wave in flanks. The dayside poloidal wave serves as the dominant source of electron acceleration. The simulated electron flux variations agree well with observations both in fine details and long period behavior. These agreements in electron behavior indicate that the ULF wave plays an important role in accelerating MeV relativistic electrons around the geosynchronous orbit. General applications of the model during SSC events are also discussed. Tan, L. C., S. F. Fung, and X. Shao, Observation of magnetospheric relativistic electrons accelerated by Pc-5 ULF waves, Geophys. Rev. Lett., L14802, doi: 10.1029/2004GL019459, 2004.
SM53B-1286
Ultra-relativistic Acceleration of Radiation Belt Electrons in Planetary Magnetospheres
The main mechanism for the formation of an electron radiation belt in a planetary magnetosphere is considered to be radial (cross-L) diffusion toward the planet from an external boundary. In this process the first and second adiabatic invariants are conserved and the third is violated. Typical electron energies in the Earth's outer radiation belt are E=400keV-10MeV ,while in Jupiter's inner magnetosphere electron energies are in the range E=1- 100MeV. Radial diffusion alone cannot explain the observed relativistic electron populations at Earth and Jupiter. For some time it has been thought that local acceleration due to electron cyclotron resonance with plasma waves(associated with violation of the first adiabatic invariant)could be important in energizing radiation belt electrons. We introduce a new particle acceleration mechanism called ultra-relativistic acceleration (URA). URA consists of electron energization due to a special form of nonlinear phase trapping by a coherent whistler-mode wave for electrons with an initial Lorentz factor exceeding a critical value; the critical value is the inverse of the wave frequency scaled by the cyclotron frequency at the magnetic equator of an assumed dipole field. Electrons that encounter multiple URA interactions together with relativistic turning acceleration(RTA) can undergo significant energization,e.g., for Earth(L=4) several-hundred keV electrons can be accelerated to a few MeV in about a second,while at Jupiter(L=8) several-hundred keV electrons can be accelerated to tens of MeV in seconds. We expect URA (and RTA) to be effective in the wider context of space and cosmic plasmas. Necessary conditions for significant energization by the URA and RTA mechanisms include a magnetic mirror geometry,a sufficient supply of seed electrons,and multiple whistler-mode wave packets of sufficient duration.
SM53B-1287
Simulation of auroral electron acceleration by inertial Alfven waves
A two-dimensional particle code, is used to model the acceleration of auroral electrons by inertial Alfven waves. The simulation domain is 20,000 km parallel to the magnetic field and some tens of kilometers across. Effects of a variable magnetic field and magnetic mirror force are included. The Alfven wave is launched by a moving bipolar potential wave-form with peak amplitudes of ±200V imposed at the top of the simulation domain. Electrons are accelerated to 3keV energy. Two types of acceleration processes are seen: one is impulsive acceleration from nonlinear shock structures that form and the other is from weaker parallel electric fields extending over several thousand kilometers. The shock structures are very small, and electrons accelerated by these structures can produce auroral forms with thicknesses considerably smaller than an electron inertial length. Electrons also tend to undergo multiple acceleration events. The code shows that electrons are more strongly accelerated upward. Half of these are destined to precipitate in the opposite hemisphere and half are destined to become trapped. This implies that much of the aurora seen in the northern hemisphere is due to electrons accelerated in the southern hemisphere. The simulations also suggest that auroral acceleration processes provide a likely source for radiation belt particles.
SM53B-1288
Self-Consistent Hybrid Technique for Modeling Particle Acceleration in Space Plasma Outflow
Plasma outflows exist in various regions in space. For instance, the solar wind is an outflow of plasma along the open magnetic field lines emanating from the sun. In the polar cap region of the Earth, there is the polar wind, which features particle outflow along the open geomagnetic field lines. In the auroral zone, conic velocity distributions of ions are frequently observed as those particles travel upward after being heated in the directions transverse to the magnetic field. The primary physical mechanisms responsible for these phenomena of plasma outflow may be all different. We discuss a self-consistent hybrid modeling technique that is capable of incorporating various kinetic effects in space plasma outflows. The modeling technique is hybrid in that it combines fluid and kinetic calculations. By applying the technique to the polar wind to model the effects of photoelectrons, and to the solar wind to study the effects of ion cyclotron waves, we have demonstrated in both applications that the kinetic effects considered may provide an explanation for the observed qualitative features in the outflow. For the auroral zone, we have used a preliminary model to consider ion energization due to intermittent electric field fluctuations. Our studies have shown that such fluctuations can effectively energize the ions and lead to the formation of ion conics. The degree of ion energization would depend on the intermittency of the electric field. Recent analyses of the auroral zone electric field have shown that such fluctuations are indeed intermittent in nature, suggesting the relevance of our studies for the region. Here, we describe the salient features and utilities of the self-consistent hybrid technique, and discuss how the technique can be applied to take into account the effects of intermittent fluctuations on plasma outflows.
SM53B-1289
Effect of temperature anisotropy on quick magnetic reconnection triggering
Magnetic reconnection is an important process to convert electromagnetic energy into plasma kinetic/thermal energy in the plasma universe. However, trigger mechanisms of the reconnection have not been fully understood well yet. The tearing mode is thought to be a direct cause of triggering the magnetic reconnection. Previous two- dimensional (2-D) full-particle simulations of the tearing mode have revealed that global reconnection picture is attained at the thickness far thinner than the ion-inertial scale. In contrast, in-situ magnetotail observations have shown the existence of the ion-scale current sheet. Recently, two key processes attracted attentions in the sense that they might play important role in explosive triggering of magnetic reconnection. The first process is the effect of the lower-hybrid drift instability (LHDI) at the edges of the current sheet. In the three-dimensional (3-D) situation, this fastest growing mode quickly leads to the spontaneous quick magnetic reconnection triggering due to the quick current sheet modification. Indeed, recent 3-D full-particle simulations have shown that even in an ion-scale current sheet, the magnetic reconnection can be triggered more quickly than 2-D situations. Can be this process operative in much thicker current sheet? The second process is the effect of the electron temperature anisotropy on the tearing mode. It has been shown that when the electron temperature perpendicular to the magnetic field is higher than that of parallel, this anisotropy boosts up the growth rate at high wave number modes of the tearing mode. In a thick current sheet favored for the electron temperature anisotropy, coalescence of small magnetic islands may lead to quick triggering of global reconnection. However, recent 2-D full-particle simulations have shown that there is a critical current sheet thickness, above which no significant saturation level was attained in the existence of the electron anisotropy. It is natural to be asked if the "ion" anisotropy might play an important role in quick magnetic reconnection triggering. In this talk, it will be reported that there is a critical thickness above which the spontaneous quick magnetic reconnection triggering ceases to exist in 3-D situation. Then, it will be reported that introduction of both the ion and electron temperature anisotropy to the 3-D system is required to revive the quick magnetic reconnection process.
SM53B-1290
Laboratory and Field Experiments on Expulsion of Selected Ions along Divergent Polar Geomagnetic Fields
Laboratory and Field Experiments on Expulsion of Selected Ions along Divergent Polar Geomagnetic Fields. Laboratory experiments have shown significant gyro-resonance acceleration of minority ion species in a magnetized plasma. Field aligned elctron drifts can provide free energy needed to make this process efficient. The linear magnetized device has a uniform magnetic field linked to two adjustable mirrors at the ends. Outdoor experiments at HIPAS Facility Ak(1) ( 84 MW ERP ) are used to test this process in the earth's "chimneys" at the two poles. The divergent polar geomagnetic field converts the perpendicular ion velocity into an upward motion. Satellites and ground-based ELF receivers,supplemented by UHF radars, LIDARs and infrared diagnostics , will monitor low-frequency EM waves and upflows of ions. The upward transport of ions in the lower atmosphere by field-induced diffusion and convection and the coupling to the free energy in the auroral region will be discussed. Computer modeling and theoeries complement our experiments. 1. Wong, A.Y. et al. AIP CIP 96-27719, Chap 3, pp 41-75, 1997 http://www.nidnano.com
SM53B-1291
Electron acceleration during magnetic islands coalescence in magnetic reconnection
Production of energetic electrons is one of the most interesting topics in space plasma physics. Magnetic reconnection has a significant role to produce energetic electrons. In the Earth's magnetosphere, magnetotail reconnection has been thought to be a generation mechanism of the observed energetic electrons. The electron acceleration mechanism during magnetic reconnection has been surveyed by theoretical and observational studies up to today. However, the detailed production mechanism of energetic electron still remains an open question. In this study, we focused magnetic islands coalescence process and carried out two dimensional electromagnetic full particle simulations. In our simulation, multi magnetic islands are given initially in magnetic field. In the result, we found production of energetic electrons corresponded coalescence phase from time evolution of electron energy spectra. These accelerated electrons spatially formed multi ring distribution. Earlier studies had indicated accelerated electrons formed single ring distribution, multi ring distribution was first to be found in our simulations. We also find out magnetic islands coalescence region was electron accelerate region as well as near X-line and pileup region. These results indicate big effect of coalescence on electron acceleration mechanism. In this presentation, we discuss where, when, and how energetic electrons are produced during magnetic islands coalescence.
SM53B-1292
Diffusive Compression Acceleration of Energetic Particles in the Magnetosphere
Diffusive compress acceleration of energetic charged particle is similar to diffusive shock acceleration. The particle acceleration occurs through either drift acceleration or Fermi process. Based on the adiabatic theory of particle motion and the frozen-in-law of space plasma, we can write the rate of particle energy increase and particle pitch-angle change in terms of the divergence of plasma flow. Particles are accelerated whenever the plasma with its embedded magnetic fields is compressed. This is a first-order particle acceleration mechanism. Shock is just an extreme profile of compression, but particle acceleration does not have to have a shock present. In this acceleration theory, diffusion is only needed to keep the particles near the acceleration site long enough to get the particles to much higher energies than their initial energies. Inside the magnetosphere, a shock is unlikely, but plasma compression still occurs. Using a model map of magnetospheric convection pattern, we found that the compression of magnetospheric plasma can play important roles in particle acceleration and trapping. It is a large-scale phenomenon. The strongest compression acceleration occurs ~6-12 Re on the night-side and there particles can double their energies in matter of a few minutes. The acceleration has a correlation with the speed of external solar wind that drives the magnetospheric convection and the location of plasmapause that indicates the level of magnetospheric convection. Energetic electrons accelerated at near- Earth tail will drift to the dawn side and emit whistler waves or other electron-related waves, and energetic ions will drift to dusk side and emit ion cyclotron waves and form partial ring current there. When magnetospheric convection is enhanced, particularly during a fast sunward flow in the tail, the acceleration becomes stronger and particle acceleration site gets closer to the Earth. These could change the radiation belt content and disrupt the magnetospheric current system.