Panagiota Petkaki

NONLINEAR DEPENDENCE OF ANOMALOUS ION-ACOUSTIC RESISTIVITY ON ELECTRON DRIFT VELOCITY

Collisionless magnetic reconnection requires the violation of ideal MHD by various kinetic-scale effects. Recent research has highlighted the potential importance of wave-particle interactions by showing that Vlasov simulations of unstable ion-acoustic waves predict an anomalous resistivity that can be significantly higher in the nonlinear regime than the quasi-linear estimate. Here, we investigate the dependence on the initial electron drift velocity of the current driven ion-acoustic instability and its resulting anomalous resistivity. We examine the properties of statistical ensembles of 10 Vlasov simulations with real mass ratios for a range of drift velocities and for electron to ion temperature ratios of 0.9, 1, and 2, relevant to both solar and magnetospheric physics. We show that the ion-acoustic anomalous resistivity depends nonlinearly on the electron drift velocity for the low temperature ratios examined, in contrast to the linear dependence predicted by theory and commonly assumed in models of magnetic reconnection. Specifically we find that (1) anomalous resistivity is a power-law function of the electron drift velocity (vde/θem), approximately with exponent β ~ 8–10, and (2) anomalous resistivity is a power-law function of the normalized drift velocity (vde − vcrit)/θem, approximately with exponent α ~ 2.5–6. An anomalous resistivity model consistent with our results could be important for simulations of magnetic reconnection in astrophysical plasmas.

Anomalous resistivity in non-Maxwellian plasmas

Vlasov simulations of the current-driven ion-acoustic instability produced in Maxwellian and non-Maxwellian (Lorentzian, kappa = 2) electron-ion plasma with number density 7 x 10(6) cm(-3), reduced mass ratio m(i)/m(e) = 25, and electron to ion temperature ratio T-e/T-i = 1 are presented and compared. A concise stability analysis of current-driven ion-acoustic waves in Maxwellian and non-Maxwellian plasmas modeled by generalized Lorentzian distribution function with index 2 less than or equal to kappa less than or equal to 7 and electron to ion temperature ratio 1 less than or equal to T-e/T-i less than or equal to 100 is also presented. The ion-acoustic instability is excited in low temperature ratio Lorentzian (kappa = 2) plasma for lower absolute electron drift velocity (up to half the critical electron drift velocity of a Maxwellian). The anomalous resistivity resulting from ion acoustic waves in a Lorentzian plasma is a strong function of the electron drift velocity and in the work presented here varies by a factor of similar to100 for a 1.5 increase in the electron drift velocity. Furthermore, ion-acoustic anomalous resistivity is excited for electron drift velocities that would be stable for Maxwellian plasmas. The magnitude of resistivity which can be generated by unstable ion-acoustic waves may be important for magnetic reconnection at the magnetopause.

Cluster observations of broadband electromagnetic waves in and around a reconnection region in the Earth's magnetotail current sheet

(1) We present an analysis of the electric and magnetic wave spectra on kinetic scales during several crossings of a reconnecting current sheet. The spectra were measured from 1 Hz or less up to 4096 Hz by the EFW, FGM and STAFF instruments onboard the Cluster spacecraft between 3 and 4 UT on 11 October 2001. During the event plasma flows of order of the local Alfven speed reversed from tailward to earthward, suggesting that a reconnection site moved over the spacecraft. We ordered the observed electric and magnetic field wave spectrum by the position within the current sheet using the magnitude of the magnetic field B. We found that the electric and magnetic wave power decreased considerably at all frequencies towards the center of the current sheet (B � 0 nT). The electric energy density decreases 5 orders of magnitude from the edge of the current sheet (B = 19 nT) to the center and the magnetic energy density peaks within the current sheet (B = 13 nT) and is decreased by 2.5 orders of magnitude at the center. Within the current sheet, the electric and magnetic wave spectra were dominantly broadband electromagnetic noise (i.e., power law spectra with exponents �� 1.4 and �� 2.4, respectively)throughoutthefrequencyrange � 0.1-1000Hz, spanning from MHD (i.e., ion cyclotron frequency � 0.2 Hz) to almost the electron plasma frequency (� 4000 Hz). We argue that the wave activity is likely to be whistler wave turbulence and discuss the implications of these results for reconnection from wave-particle interactions. Citation: Petkaki, P., M. P. Freeman, and A. P. Walsh (2006), Cluster observations of broadband electromagnetic waves in and around a reconnection region in the Earths magnetotail current sheet, Geophys. Res. Lett., 33, L16105, doi:10.1029/ 2006GL027066.

Topological Formation of Small Scales in Magnetohydrodynamics: A Fast Dissipation Mechanism

The dissipation of Alfven wave packets propagating in an inhomogeneous three-dimensional magnetic field is numerically studied in the Wentzel-Kramers-Brillouin (WKB) approximation. The dissipation rate is found to scale proportionally to the logarithm of viscosity and/or resistivity, i.e., much faster than the scaling found for two-dimensional configurations (phase mixing, resonance absorption). This phenomenon is related to the exponential separation of neighboring rays; estimations of the corresponding rate are consistent with the behavior of the Kolmogorov entropy in this kind of structure. The scaling law td ∝ ln S (td is the e-folding dissipation time and S is the Reynolds number) holds not only in an irregular magnetic field (which was studied by Similon & Sudan), but is also verified in quasi-uniform magnetic structures, which contain very small chaotic regions. This study can apply to the heating of low-collision plasmas, e.g., in astrophysical contexts such as the solar corona.

SDO AIA and EVE observations and modelling of solar flare loops

We present imaging and spectroscopic observations of an isolated C1-class solar flare, obtained with the Atmospheric Imaging Assembly (AIA) and Extreme ultraviolet Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO). We obtain excellent agreement between the peak flare temperatures estimated using the EVE spectra with those obtained from GOES and, most importantly, from the ratio of the 94 A and 131 A AIA channels, which are found to be dominated by Fe xviii and Fe xxi .T hese results confirm that these two AIA bands can be reliably used to provide temperature diagnostics for the peak and gradual phases of solar flares. The flare kernels, probable sources of chromospheric evaporation, are seen as strong localised emission in the AIA bands at the footpoints of flare loops. The flare loops are close to isothermal during the gradual phase. We have run several hydrodynamic simulations (using the HYDRAD code) to study the cooling of the flare loops. We find good overall agreement between observed and predicted electron temperatures and densities when a gradual increase and decrease of the heating is assumed.

Particle acceleration by fluctuating electric fields at a magnetic field null point

Context. 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. Aims. We explore numerically the consequences for charged particle distributions of fluctuating electric fields superposed on an X-type magnetic field. Methods. 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. Results. The resulting particle distributions have properties that depend on the amplitude and frequency of the electric field. In many cases a bimodal form is found. Depending on the timescale for variation of the electric field, electrons and ions may be accelerated to different degrees and often have energy distributions of different forms. Protons are accelerated to γ-ray producing energies and electrons to and above hard X-ray producing energies in timescales of 1 s. The acceleration mechanism is possibly important for solar flares and solar noise storms but is also applicable to all collisionless plasmas.

Effect of binary collisions on electron acceleration in magnetic reconnection

Context. The presence of energetic X-ray sources in the solar corona indicates there are additional transport effects in the acceleration region. A prime method of investigation is to add collisions into models of particle behaviour at the reconnection region. Aims. We investigate electron test particle acceleration in a simple model of an X-type reconnection region. In particular, we explore the possibility that collisions will cause electrons to re-enter the acceleration more frequently, in turn causing particles to be accelerated to high energies. Methods. The deterministic (Lorentz) description of particle gyration and acceleration has been coupled to a model for the effects of collisions. The resulting equations are solved numerically using Honeycutt’s extension of the RK4 method to stochastic differential equations. This approach ensures a correct description of collisional energy loss and pitch-angle scattering combined with a sufficiently precise description of gyro-motion and acceleration. Results. Even with initially mono-energetic electrons, the competition between collisions and acceleration results in a distribution of electron energies. When realistic model parameters are used, electrons achieve X-ray energies. A possible model for coronal hard X-ray sources is indicated. Conclusions. Even in competition with energy losses, pitch-angle scattering results in a small proportion of electrons reaching higher energies than they would in a collisionless situation.

Waves close to the crossover frequency in the Jovian Middle Magnetosphere

The presence of heavy ions in a plasma intro- duces additional characteristic frequencies one of which is a crossover frequency between each adjacent pair of gyrofre- quencies of the ions present. The crossover frequency is controlled by the fractional ion charge densities. The phase speed of the L and R wave modes have the same phase ve- locity at this frequency. For oblique propagation waves be- come linearly polarized at the crossover frequency. They reverse their natural polarization from left to right (or vice versa) as the wave frequency transverses the crossover fre- quency. Here we present polarization analysis of a portion of the Ulysses magnetometer observations in the middle Jo- vian magnetosphere during the inbound pass and close to a plasma sheet approach. The analysis show transverse waves with frequency between the gyrofrequencies of SO + andS + . We observe polarization reversal at this frequency, which could be an indication of wave observations at the crossover frequency. On this assumption these observations can then be used to derive heavy ion composition of the resident plasma.

Particle acceleration and transport in reconnecting plasmas

We model the behaviour of particles in and around X-type magnetic configurations, a possible solar flare reconnection geometry. Particles are accelerated by a time-varying electric field close to the neutral point, and followed by integrating the equations of motion. When their motion becomes adiabatic a stochastic simulation is used to model their further transport in a collisional magnetised medium.

ULF Wave Observations in Jupiter's Magnetosphere

In previous work we have examined magnetometer data from the Ulysses Jupiter flyby, and in particular middle magnetosphere observations in the vicinity of the magnetodisk. Waves transverse to the background magnetic field were found in the heavy ion gyrofrequency regime, a signature of Ion Cyclotron (IC) waves. We examined the dayside and high latitude duskside high resolution 1 set data. Wave signatures were observed on several occasions both close to the magnetic equator and at some distance from it. Lower mass ion signatures were observed further away from the planet and the heaviest mass ion signatures appear closer to the 10 torus although still at some distance from it. IC waves and their propagation properties can be used as a diagnostic of the particle species in a multicomponent plasma. Here we present polarization analysis of a portion of the Ulysses magnetometer wave observations in the middle Jovian magnetosphere during the inbound pass and close to plasma sheet approaches. The analysis show transverse waves in the heavy ion gyrofrequency regime with left-hand polarization in some cases and right-hand polarisation in others. We discuss these results in terms of polarization reversal properties of ULF waves propagating in a multicomponent plasma. 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.