Robert M. Winglee

The magnetopause current sheet is known to have a thickness comparable to an ion gyroradius/skin depth where the magnetic and electric field can differ markedly from that assumed in MHD treatments. Multifluid/multiscale simulations are used to provide the first investigation of these processes in a global simulation that includes high‐resolution (200 km) gridding around the magnetopause. It is shown that the model is able to capture the quadrupole core magnetic field and the fast (tens of ion cyclotron periods) reconnection seen in idealized studies reconnection for a Harris current sheet. Within a global magnetosphere, multiple X‐line reconnection occurs for southward IMF due to localized pinching of the magnetopause current sheet via the convection of the magnetosheath plasma against a three‐dimensional magnetopause. Localized flux ropes with a thickness of a few hundred to a few thousand kilometers develop and can expand laterally due to current sheet acceleration of ions that have a gyroradius comparable to the current sheet thickness. These flux ropes are shown to have essentially the same properties as flux transfer events (FTEs), including being quasi‐periodic with a curvature greater on the magnetosheath side than on the magnetospheric side, a strong core magnetic field, and a mixture of magnetospheric and magnetosheath plasma. The speeds of the plasma flows associated with flux ropes are also similar to those observed with FTEs. The presence of multiple X‐line reconnection is shown to produce the rippling of the magnetopause and gives a nature explanation to the multiple magnetopause encounters that typically occur for slow moving spacecraft. These small‐scale processes are shown to have global effects with a reduction of the cross‐polar cap by as much as 20% seen between simulations with and without high resolution about the magnetopause.

We present the results of time-dependent, numerical magnetohydrodynamic simulations of a realistic young stellar object outflow model with the addition of a disk-associated magnetic field. The outflow produced by the magnetic star-disk interaction consists of an episodic jet plus a wide-angle wind with an outflow speed comparable to that of the jet (100--200 km/s). An initially vertical field of << 0.1 Gauss, embedded in the disk, has little effect on the wind launching mechanism, but we show that it collimates the entire flow (jet + wide wind) at large (several AU) distances. The collimation does not depend on the polarity of the vertical field. We also discuss the possible origin of the disk-associated field.

We examine, parametrically, the interaction between the magnetosphere of a rotating, young stellar object (YSO) and a circumstellar accretion disk using 2.5-D (cylindrically symmetric) numerical magnetoydrodynamic simulations. The interaction drives a collimated outflow, and we find that the jet formation mechanism is robust. For variations in initial disk density of a factor of 16, variations of stellar dipole strength of a factor of 4, and for various initial conditions with respect to the disk truncation radius and the existence of a disk field, outflows with similar morphologies were consistently produced. Secondly, the system is self-regulating, where the outflow properties depend relatively weakly on the parameters above. The large scale magnetic field structure rapidly evolves to a configuration that removes angular momentum from the disk at a rate that depends most strongly on the field and weakly on the rotation rate of the foot-points of the field in the disk and the mass outflow rate. Third, the simulated jets are episodic, with the timescale of jet outbursts identical to the timescale of magnetically induced oscillations of the inner edge of the disk. To better understand the physics controlling these disk oscillations, we present a semi-analytical model and confirm that the oscillation period is set by the spin down rate of the disk inner edge. Finally, our simulations offer strong evidence that it is indeed the interaction of the stellar magnetosphere with the disk, rather than some primordial field in the disk itself, that is responsible for the formation of jets from these systems.

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.

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) outflow (10^{-5} M_sun/yr) from an AGB 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.

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 outÑow. 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 e †ects that are inherent to the jet-launching mechanism itself. A disk wind arises as well. The simulated outÑow is compared with observations, and it is shown that simulated images in the forbidden lines [S II] jj6716]6731 have morphology consistent with recent observations of the jet source HH 30. Furthermore, velocity spectra of the simulated outÑow in [S II] jj6716]6731 and mass weighted by n clearly show a two-component outÑow, in agreement with observed outÑows 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 di †er 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 deÐnes the controlling parameters of the jet-launching mechanism in general.

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.