Philip L. Pritchett

does ion tearing exist

In a quasi-neutral sheet the tearing mode is strongly stabilized by electron compressibility. The authors demonstrate that previous attempts to remove the compressibility by wave turbulent pitch-angle scattering or by non-adiabatic stochastic first invariant diffusion are incorrect since these processes do not change the pertubed number of particles on a flux tube. Only spatial diffusion of electrons across magnetic flux surfaces can remove the stabilizing effect of compressibility; in this case, the electron, but not the ion, tearing mode may exist.

Electron-Cyclotron Maser Driven by Charged-Particle Acceleration from Magnetic Field-aligned Electric Fields

We present a detailed description of the auroral kilometric radiation (AKR) source region based on observations from the Fast Auroral SnapshoT (FAST) satellite and discuss how these new results may pertain to solar and stellar radio sources. FAST satellite observations are directly within the AKR source region and have unprecedented spatial and temporal resolution. They confirm many of the fundamental elements of the electron-cyclotron maser mechanism but with substantial modification. The most important modification is that the emissions do not draw their energy from a loss-cone instability; rather, the radiation results from an unstable horseshoe or shell distribution. The most far-reaching implication is that the electron-cyclotron maser is directly associated with a particular type of charged particle acceleration, a magnetic field-aligned (parallel) electric field in a dipole magnetic field. These findings change several of the characteristics of the electron-cyclotron maser mechanism and may necessitate reanalysis of some astrophysical radio sources. Under the shell instability, radio emissions with brightness temperatures ~1014 K, the steady state limit of the loss-cone instability, may be continuous. Through observations, we demonstrate that source brightness may be as high as 1020 K in steady state. A moderately or strongly relativistic beam may result in broadband emissions. A loss cone is not required, so the radiation source may be high above the stellar or planetary surface. Although the generation is in the X mode with k|| = 0, we suggest that the radiation, guided by a density cavity that is created by the parallel electric field, efficiently converts to the R mode, which experiences substantially lower absorption at higher harmonics. These findings also suggest that parallel electric fields may be a fundamental particle acceleration mechanism in astrophysical plasmas.

Collisionless magnetic reconnection in a three-dimensional open system

Two-dimensional (2-D) simulations of collisionless magnetic reconnection have demonstrated the importance of the Hall term for the structure of the diffusion region and the existence of a very thin (∼ c/ωpe) electron current layer and sharp density/pressure gradients on c/ωpi scales. The present work explores the effects of the third dimension (along the direction of the equilibrium current) and of an open geometry along the magnetic field direction on these 2-D structures within a fully kinetic treatment based on particle-in-cell simulations. The thin electron current layer is found to remain a 2-D structure. The electric field components Ez and E‖ in the diffusion region each possess a complex structure on the c/ωpi and c/ωpe spatial scales. The Ez fields are needed to maintain pressure balance on the ion and electron species in the current sheet, while E‖ results in nearly Alfvenic (for the ions) and supra-Alfvenic (for the electrons) flows out of the diffusion region. The Hall-generated quadrupolar By field extends to large distances away from the neutral line and is a characteristic signature of collisionless reconnection. No evidence is found for the development of small-scale turbulent modes; the electron and ion flows are highly ballistic, and the currents and density remain sharply defined. The outflow region, however, can become unstable to an ideal pressure gradient instability of relatively long wavelength (kyw ∼ 0.25, where w is the half thickness of the current sheet) with the character of a kink or interchange mode. Attempts to induce localized reconnection by imposing a localized (in y) convection electric field at the asymptotic (lobe) boundaries do not alter appreciably the basic 2-D configuration.

Energetic electron acceleration during multi-island coalescence

The mechanisms for the production of relativistic electrons associated with the coalescence/reconnection of multiple magnetic islands are investigated using two-dimensional particle-in-cell simulations for the case where the initial island half width L is comparable to the ion inertia length. Configurations without and with a uniform magnetic guide field are considered. Significant energization occurs only when the number of islands is reduced to 2 or 3 with wavelength satisfying kxL≲0.2. The energization proceeds in two distinct stages. In the first stage, a small number of electrons are accelerated to relativistic energies at the X-line by the inductive electric field, corresponding to perpendicular acceleration in the absence of the guide field and parallel/anti-parallel acceleration with a guide field. The second stage is associated with the final coalescence into one large island and produces a considerably larger number of relativistic electrons. With a guide field, this stage is dominated by the fo...

Vlasov simulations of electron holes driven by particle distributions from PIC reconnection simulations with a guide field

[1] Parallel velocity particle distributions taken from 2-D PIC simulations of magnetic reconnection with a guide field, Bg are used to initialize 1-D and 2-D electrostatic Vlasov simulations which include the direction parallel to the (local) magnetic field, B. Electron holes develop near the separatrix from an electron-ion (e.g., Buneman) instability. Restriction of the destabilizing current to a narrow sheet perpendicular to B reduces the Buneman growth rate but leads to more stable holes. Near the x-point, B is almost parallel to B g . Here, electron-electron kinetic two-stream instabilities lead to holes moving parallel to B, which can modify the electron-ion interaction. This results in a second generation of (slower) electron phase space perturbations which can reduce the current.

Particle-in-cell simulations of magnetosphere electrodynamics

This paper reviews the basic principles and techniques involved in formulating particle-in-cell (PIC) simulation models which can be used to address medium- and large-scale problems in magnetosphere electrodynamics. The limitations imposed by the underlying kinetic physics of a plasma are emphasized, and representative algorithms are described for full particle and hybrid (particle ions, fluid electrons) models. Issues related to the choice of initial and boundary conditions and the implementation of PIC models on massively parallel computers are discussed. Explicit examples involving the diffusion region in collisionless reconnection, plasma sheet convection, and large scale structure in magnetic reconnection are presented to illustrate the current capabilities of PIC models.

Accelerated electrons as the source of Auroral Kilometric Radiation

Abstract Data from the Fast Auroral Snapshot Explorer (FAST) have confirmed that Auroral Kilometric Radiation (AKR) is generated by the accelerated electrons in the auroral density cavity. FAST observations of the electron distribution within the AKR source region show a characteristic horse-shoe distribution expected from the parallel electric field acceleration and magnetic mirroring of the electrons, the precipitating portion of which constitute the auroral electrons. Within the density cavity it is likely that the hot electrons carry most, if not all, of the plasma density. The AKR wave characteristics within the source region show that the lower frequency cut-off is below the cold electron gyro-frequency, and near the cut-off, the wave electric and magnetic field polarization is consistent with perpendicularly propagating X-mode waves. Taken together these observations are consistent with the cyclotron maser instability, but with the inclusion of relativistic effects in the wave dispersion, as well as the resonance condition. This allows the accelerated electrons to generate AKR, and further the replenishment of the distribution by the parallel electric field ensures that AKR can be generated over a large altitude range on auroral zone field lines

Kinetic properties of magnetic merging in the coalescence process

The magnetic merging process associated with pairwise magnetic island coalescence is investigated using two-dimensional particle-in-cell simulations for the case where the initial island separation ζ is in the range of 3–12c∕ωpi, where c∕ωpi is the ion inertia length. In this regime the coalescence process is driven by the electrons, the electron and ion bulk flows decouple on the global island scale (the electron flows are much larger than those for the ions), there is no magnetic flux pileup near the merging line, and the X-O line separation drops smoothly to zero on a time scale of the order of twice the linear e-folding time for the coalescence instability. For fixed island aspect ratio, the scaling of the merging electric field Ey as a function of ζ is rather weak; i.e., ∼ζ−0.5. The magnitude of Ey, however, is strongly dependent on the magnitude of the current concentration at the initial O lines, suggesting that driven merging does not exhibit a universal rate. These kinetic results support the exi...

Short‐burst auroral radiations in Alfvénic acceleration regions: FAST observations

(1) We examined particle and field data during the first 3 years (1997-1999) of the FAST mission searching for short-burst auroral kilometric radio (AKR) emissions in Alfvenic auroral acceleration regions. Eight such events were found at altitudes between 2500 and 3600 km in the midnight local time sector during winter months when the AE index was observed to be greater than 150. The emission source regions are estimated to be � 300-900 km below the satellite. The frequency of observed short-bursts is in the range between 400 and 600 kHz. The average bandwidth of the burst, Df/f, is approximately 2 � 10 � 2 . The reoccurrence frequency of discrete short bursts is estimated to be between 7 and 18 Hz, which is similar to that of Jovian S-bursts. The negative drift of each discrete burst may be associated with upward moving electrons. As in the case for ordinary AKR, short-bursts can be explained by the electron cyclotron maser instability due to the positive gradient in the phase space of an unstable electron distribution. Shell, conic, and ring distributions, and a combination of these three, were observed in each of the eight events. Snapshots of observed multiple shells and rings are suggested to be the results of the temporal effect, where low energy electrons are accelerated by the Alfvenic perturbation at an earlier time, while high energy electrons are newly accelerated by later Alfvenic pulses. Results presented within this paper suggest multiple Alfvenic disturbances in the magnetotail as the source of the multiple shell (and ring) distributions. Multiple Alfvenic disturbances may also explain the observed reoccurrence pattern of short-bursts.

Reconnection flow jets in 3D as a source of structured dipolarization fronts

Three-dimensional electromagnetic particle-in-cell simulations are used to investigate the propagation and breakup of a reconnection flow jet of initial cross-tail extent 24 di (∼1.5RE; di is the ion inertial length). Such a front is found to separate into two segments, with the dawnward portion propagating ahead of the duskward one. Both segments expand duskward, reaching separate lengths of 18–25 di, and both segments develop internal structures on east–west scales of 1–2 di. The currents responsible for the ramp up of Bz at the fronts are confined to narrow (≲di) ribbons whose localization is primarily associated with the electron Uey flow. The incoming ion flow is slowed down and deflected duskward at the front, and ambient ions are reflected back from the moving front. These processes create regions of enhanced Tixx both downstream and upstream of the front, while there is a local minimum at the front itself. These results help to explain the prevalence of ∼1RE flow jets in the plasma sheet.