R. M. Winglee

Critical Issues For Understanding Particle Acceleration in Impulsive Solar Flares

This paper, a review of the present status of existing models for particle acceleration during impulsive solar flares, was inspired by a week-long workshop held in the Fall of 1993 at NASA Goddard Space Flight Center. Recent observations from Yohkoh and the Compton Gamma Ray Observatory, and a reanalysis of older observations from the Solar Maximum Mission, have led to important new results concerning the location, timing, and efficiency of particle acceleration in flares. These are summarized in the first part of the review. Particle acceleration processes are then discussed, with particular emphasis on new developments in stochastic acceleration by magnetohydrodynamic waves and direct electric field acceleration by both sub- and super-Dreicer electric fields. Finally, issues that arise when these mechanisms are incorporated into the large-scale flare structure are considered. Stochastic and super-Dreicer acceleration may occur either in a single large coronal reconnection site or at multiple “fragmented” energy release sites. Sub-Dreicer acceleration requires a highly filamented coronal current pattern. A particular issue that needs to be confronted by all theories is the apparent need for large magnetic field strengths in the flare energy release region.

TIME-DEPENDENT ACCRETION BY MAGNETIC YOUNG STELLAR OBJECTS AS A LAUNCHING MECHANISM FOR STELLAR JETS

A time-dependent jet launching and collimating mechanism is presented. Initial results of numerical simulations of the interaction between an aligned dipole rotator and a conducting circumstellar accretion disk that is initially threaded by the dipole field show that differential rotation between the disk and the star leads to the rapid expansion of the magnetic loops that connect the star to the disk. The expansion of these magnetic loops above and below the disk produces a two-component outflow. A hot, well-collimated outflow is generated by the convergent flow of attached plasma toward the rotation axis, while a cool, slower outflow is produced on the disk side of the expanding loop. The expanding loop, which later forms a plasmoid, defines the boundary between the jetlike flow and the disk wind. Episodic magnetic reconnection above and below the disk releases the jet plasma from the system and allows the process to repeat, reinforcing the hot, well-collimated outflow.

Mini‐Magnetospheric Plasma Propulsion: Tapping the energy of the solar wind for spacecraft propulsion

Mini-Magnetospheric Plasma Propulsion is a potentially revolutionary plasma propulsion concept that could enable spacecraft to travel out of the solar system at unprecedented speeds of 50 to 80 km s -1 or could enable travel between the planets for low power requirements of ∼ 1 kW per 100 kg of payload and 0.5 kg fuel consumption per day for acceleration periods of several days to a few weeks. The high efficiency and specific impulse attained by the system are due to its utilization of ambient energy, in this case the energy of the solar wind, to provide the enhanced thrust. Coupling to the solar wind is produced through a large-scale magnetic bubble or mini-magnetosphere generated by the injection of plasma into the magnetic field supported by solenoid coils on the spacecraft. This inflation is driven by electromagnetic processes, so that the material and deployment problems associated with mechanical sails are eliminated.

Tearing instability, flux ropes, and the kinetic current sheet kink instability in the Earth's magnetotail: A three‐dimensional perspective from particle simulations

In this paper the tail current sheet is shown to be unstable to the kinetic current sheet kink instability (or simply the kinetic kink instability) in the crosstail plane (y–z plane) and under similar conditions that drive the tearing instability in the noon-midnight meridional plane (x–z plane). The tail current sheet is assumed to be a thin Harris current sheet (ρi/Lc ∼ 1) with equal ion and electron temperature. The kinetic kink instability develops due to the bending of the tail current sheet and the resulting pressure imbalance. The development of the kinetic kink instability including its growth rate and resultant distortion of the current sheet, is first examined using two-dimensional electromagnetic particle simulations. The coupling between the kinetic kink and tearing instabilities is then investigated via three-dimensional electromagnetic particle simulations. The results show that the tearing instability and the kinetic kink instability occur on the same timescale as what we expected from the two-dimensional simulations and that the projection of the field lines in the x–z plane reproduces a standard plasmoid shape. However, the three-dimensional plasmoid produced by the tearing instability is shown to consist of a series of flux ropes where the magnetic field lines are tightly wound as they cross the center of the current sheet. The entry and exit points of the field lines of the flux ropes are displaced in the dawn-dusk direction. Twist and displacement of the magnetic field lines arise from the magnetic field component By generated by a plasma current due to the differential motion between electrons and ions. This current and the associated flux ropes result from intrinsic particle effects. The kinetic kink instability bends the current sheet and the flux ropes along the y direction and generates large-scale cross-tail wavy structures. The wave fronts may eventually collide, causing a total distortion of the current sheet configuration and strong electron heating, while the tearing accounts for most of the ion heating.

Jets from Accreting Magnetic Young Stellar Objects. I. Comparison of Observations and High-Resolution Simulation Results

High-resolution numerical magnetohydrodynamic (MHD) simulations of a new model for the formation of jets from magnetic accreting young stellar objects (YSOs) are presented and compared with observations. The simulation results corroborate a previously laid out conceptual mechanism for forming jets, wherein the interaction of the stellar magnetosphere with a surrounding accretion disk leads to an outflow. The high resolution of the numerical simulation allows optical condensations, which form in the region close to the star to be seen. The optical condensations and the episodic behavior of the jet are effects that are inherent to the jet-launching mechanism itself. A disk wind arises as well. The simulated outflow is compared with observations, and it is shown that simulated images in the forbidden lines [S II] λλ6716+6731 have morphology consistent with recent observations of the jet source HH 30. Furthermore, velocity spectra of the simulated outflow in [S II] λλ6716+6731 and mass weighted by n clearly show a two-component outflow, in agreement with observed outflows from T Tauri stars such as DG Tauri. The mechanism produces a highly collimated fast jet and a slower disk wind. While the match between existing observations and the simulated system are not perfect (the time- and size scales of the jet differ from those in HH 30 by an order of magnitude), the morphology associated with both imagery and velocity spectra of the jet are matched well. A companion paper lays out the physics that control the timescale for knot production and defines the controlling parameters of the jet-launching mechanism in general.

Multi‐fluid simulations of the magnetosphere: The identification of the geopause and its variation with IMF

A new multi-fluid global treatment is presented that separately tracks the solar wind and ionospheric plasmas in the magnetosphere. The model is used to identify the density and pressure geopauses, i.e. the boundaries where the contributions to the magnetospheric density or pressure from the ionosphere equals that from the solar wind. The density geopause is primarily restricted to the inner magnetosphere and central lobes for northward interplanetary magnetic field (IMF) but during southward IMF it can extend into middle magnetosphere due to enhanced ionospheric outflow and convection. This variation is consistent with recent statistical studies that show that the plasma sheet density is correlated with the solar wind density, particularly during northward IMF.

Jets from Accreting Magnetic Young Stellar Objects. II. Mechanism Physics

This paper addresses the physical principles underpinning a new jet-launching mechanism described in a companion paper. In this new jet formation model, magnetic loops that connect the star to the disk become twisted and expand via helicity injection. This expansion drives an outflow, with the axial symmetry of the disk leading to a concentration of outflowing plasma along the rotation axis, forming the jet. In the companion paper, it is found that the radial location of the inner edge of the disk undergoes oscillations. In this paper, the physical causes of the disk oscillations are investigated. This investigation leads to the conclusion that there are three classes of flows that can arise, depending on the role of diffusive instabilities. The most diffusive flows allow the stellar magnetic field to slip through the accretion disk and yield steady accretion flows. Such configurations are unlikely to produce outflows. The flows with intermediate diffusivity have been described by Lovelace, Romanova, & Bisnovatyi-Kogan and represent conditions in which the field is effectively frozen into the accretion disk azimuthally but slips radially. In the absence of magnetic reconnection, such configurations are predicted to produce steady flows with logarithmically collimated disk winds. The least diffusive flows, in which the bulk radial disk velocities are greater at times than the speeds with which magnetic field lines can diffuse into the disks, lead to the formation of the collimated unsteady jets described in the companion paper and are the primary interest of this paper. The jet velocity is also addressed.

Simulation and laboratory validation of magnetic nozzle effects for the high power helicon thruster

The efficiency of a plasma thruster can be improved if the plasma stream can be highly focused, so that there is maximum conversion of thermal energy to the directed energy. Such focusing can be potentially achieved through the use of magnetic nozzles, but this introduces the potential problem of detachment of plasma from the magnetic field lines tied to the nozzles. Simulations and laboratory testing are used to investigate these processes for the high power helicon (HPH) thruster, which has the capacity of producing a dense (1018−1020m−3) energetic (tens of eV) plasma stream which can be both supersonic and super-Alfvenic within a few antenna wavelengths. In its standard configuration, the plasma plume generated by this device has a large opening angle, due to relatively high thermal velocity and rapid divergence of the magnetic field. With the addition of a magnetic nozzle system, the plasma can be directed/collimated close to the pole of the nozzle system causing an increase in the axial velocity of t...

Three‐dimensional multifluid simulations of ionospheric loss at Mars from nominal solar wind conditions to magnetic cloud events

conditions. Ionospheric losses on the order of 10 25 O2 ions per second are found for quiet solar wind conditions. This is of the same order as that estimated from Phobos 2 measurements. Varying the orientation of Mars’ magnetic anomalies relative to the incident solar wind direction leads to only minor variation in the ionospheric loss rates of O2 for each set of solar wind conditions studied. Solar wind parameters were varied from nominal solar wind conditions to conditions with high-speed flows, high densities, and large IMF magnitudes. Outflow rates on the order of 10 26 O2 ions per second were seen for storm-like conditions. The simulations indicate that ionospheric outflow rates increase by a larger percentage for high solar wind number density when compared to high solar wind speed or strong IMF conditions alone. This is due to the higher solar wind density and temperature of the precipitating ions. The results also indicate a significant influence of pickup on ionospheric loss.

Disk Formation by Asymptotic Giant Branch Winds in Dipole Magnetic Fields

We present a simple, robust mechanism by which an isolated star can produce an equatorial disk. The mechanism requires that the star have a simple dipole magnetic field on the surface and an isotropic wind acceleration mechanism. The wind couples to the field, stretching it until the field lines become mostly radial and oppositely directed above and below the magnetic equator, as occurs in the solar wind. The interaction between the wind plasma and magnetic field near the star produces a steady outflow in which magnetic forces direct plasma toward the equator, constructing a disk. In the context of a slow (10 km s-1) outflow (10-5 M☉ yr-1) from an asymptotic giant branch star, MHD simulations demonstrate that a dense equatorial disk will be produced for dipole field strengths of only a few Gauss on the surface of the star. A disk formed by this model can be dynamically important for the shaping of planetary nebulae.