No abstract

Kinetic simulations of magnetic reconnection typically employ periodic boundary conditions that limit the duration in which the results are physically meaningful. To address this issue, a new model is proposed that is open with respect to particles, magnetic flux and electromagnetic radiation. The model is used to examine undriven reconnection in a neutral sheet initialized with a single x-point. While at early times the results are in excellent agreement with previous periodic studies, the evolution over longer intervals is entirely different. In particular, the length of the electron diffusion region is observed to increase with time resulting in the formation of an extended electron current sheet. As a consequence, the electron diffusion region forms a bottleneck and the reconnection rate is substantially reduced. Periodically, the electron layer becomes unstable and produces a secondary island, breaking the diffusion region into two shorter segments. After growing for some period, the island is ejected and the diffusion region again expands until a new island is formed. Fast reconnection may still be possible provided that the generation of secondary islands remains sufficiently robust. These results indicate that reconnection in a neutral sheet may be inherently unsteady and raise serious questions regarding the standard model of Hall mediated reconnection.

[1] Kinetic simulations of magnetic reconnection indicate that the electron diffusion region (EDR) can elongate into a highly stretched current layer with a width on the electron scale and a length that exceeds tens of ion inertial lengths. The resulting structure has no fluid analogue and consists of two regions in the exhaust direction. The inner region is characterized by the locale where electrons reach a peak outflow speed near the electron Alfvén velocity. Ions also approach $80% of their peak velocity in this inner region but remain sub-Alfvénic. There exists a large electrostatic potential that can temporarily trap electrons within this inner region. The electron frozen-in condition is violated over a wider outer region characterized by highly collimated electron jets that are gradually decelerated and thermalized. Reconnection proceeds continuously but the rate is modulated in time as the EDR elongates into an extended layer. The elongation of the EDR is controlled by the competition between the outward convection of magnetic flux and the non-ideal term involving the divergence of the electron pressure tensor. The occasional balance between these two terms leads to periods of quasi-steady reconnection. However, over longer time scales, a natural feature of the reconnection process appears to be frequent formation of plasmoids due to the instability of the elongated EDR which leads to larger variations in the reconnection rate. These new findings provide testable predictions and indicate the need to reconsider the diagnostics for identification of the diffusion region and interpretation of observational data.

A complete set of ISEE plasma wave, plasma, and field data are used to identify the plasma instability responsible for the generation of extremely low frequency (ELF) electromagnetic lion roars. Lion roars detected close to the magnetopause are generated by the cyclotron instability of anisotropic (Tz-/Tll-= 1.2) thermal electrons When the local plasma critical energy, EM = B2/8zrN, falls to values (EM --10-30 eV) close to or below the electron thermal energy, 25 eV, as a result of decreases in B. A companion theoretical paper, Thorne and Tsurutani (1981), demonstrates that the convective growth rates of lion roars under these conditions is greater than 100 dB RE -•, The lion roars are terminated by increases in the ambient magnetic field magnitude and consequential increases in E• to values greater than 100 eV. Because there are few resonant particles at these high energies, the growth rate decreases by 3 orders of magnitude and measurable growth ceases. The value of the absolute upper limit of the frequency of unstable waves predicted by theory, 60ma x --A-f•-/(A-+ 1), is compared with observations. The predictions and observations are found to be in general, but not exact, agreement. Several possible explanations are explored. The quasi-periodic, --•20-s magnetic and plasma oscillations which cause the variations in E• and hence alternately drive the cyclotron waves unstable and then stable are also investigated. The plasma and field pressures are shown to be out of phase, while the total pressure (electron + ion + field) remains relatively constant. Most of the pressure is associated with the particle thermal motion. The large 2:1 variations in field strength cause large oscillations in/3 (8•rP/B2), from/3 = 1-2 at field maximum to/3 = 10-25 at field minimum. Analysis of the high-resolution magnetic fields at the two closely separated spacecraft, ISEE 1 and 2, rule out the possibility that these field and plasma oscillations could be due to magnetopause motion. Crosscorrelation analyses of the magnetic fields at the two spacecraft and the time delays for maximum correlation are shown to be consistent with the magnetic structures being quasi-static in nature. The temporal variations of the plasma and fields are due to spatial structures convecting past the spacecraft at the magnetosheath flow speed. The quasi-periodic structures are -20 proton gyroradii in scale in the plasma rest frame. Magnetic structures with similar scale lengths are also shown to exist in the magnetosheaths of Jupiter and Saturn (Pioneer 11 data). The results are consistent with the interpretation that these magnetohydrodynamic structures are nonoscillatory 'waves' generated by the drift mirror instability. The condition for instability,/3_d/311 > 1 + (1//3•), is met for the cases studied in this paper. The electron and ion instabilities are intimately coupled. The generation of high/3 (> 10), low critical energy (E• = 10-30 eV) regions by the drift mirror instability leads to the electrons becoming cyclotron unstable. The consequential w...

[1] We present evidence based on measurements from the Polar spacecraft for the existence of small-scale, large-amplitude kinetic Alfvén waves/spikes at the plasma sheet boundary layer (PSBL) at altitudes of 4-6 R E . These structures coincide with larger-scale Alfvénic waves that carry a large net Poynting flux along magnetic field lines toward the Earth. Both structures are typically observed in the PSBL but have also been observed deeper in the plasma sheet. The small-scale spikes have electric field amplitudes up to 300 mV m À1 and associated magnetic field variations between 0.5 and 5 nT. Previous analysis has shown that the larger-scale Alfvén waves have periods of $20-60 s and carry enough Poynting flux to explain the generation of the most intense auroral structures observed in the Polar Ultraviolet Imager data set. In this paper it is shown that the smaller-scale waves have durations in the spacecraft frame of 250 ms to 1 s (but may have shorter time durations since the Nyquist frequency of the magnetic field experiment is $4 Hz.). The characteristic ratio of the amplitudes of the electric to magnetic field fluctuations is strong evidence that the waves are kinetic Alfvén waves with scale sizes perpendicular to the magnetic field on the order of 20-120 km (with an electron inertial length c/w pe $10 km and an ion gyroradius $20 km). Theoretical analysis of the observed spikes suggests that these waves should be very efficient at accelerating electrons parallel to the magnetic field. Simultaneously measured electron velocity space distribution functions from the Polar Hydra instrument include parallel electron heating features and earthward electron beams, indicating strong parallel energization. The characteristic parallel energy is on the order of $1 keV, consistent with estimates of the parallel R Edl associated with small-scale kinetic Alfvén wave structures. The energy flux in the electron ''beams'' is $0.7 ergs cm À2 s À1 . These observations suggest that the small-scale kinetic Alfvén waves are generated from the larger-scale Alfvén waves through one or more of a variety of mechanisms that have been proposed to result in the filamentation of large-amplitude Alfvén waves. The observations presented herein provide strong evidence that in addition to the auroral particle energization processes known to occur at altitudes between 0.5 and 2 R E , there are important heating and acceleration mechanisms operating at these higher altitudes in the plasma sheet.