arXiv subjects

M. Swisdak

Publications and source records attributed to M. Swisdak.

77 records · Page 5Linked to original sources

arXiv2005

Simulation Study of a Positive Ionospheric Storm Phase Observed at Millstone Hill

Simulation results from the NRL ionospheric model SAMI2 indicate that the changes in the F-region over Millstone Hill during the geomagnetic storm beginning on 3 April 2004 are primarily due to the influence of a long-lasting eastward electric field, as was previously suggested by {\it Huang et al.} [2005]. A simulation of the storm day agrees well with the observational data and shows that, compared with the ionosphere of the previous quiet day, the peak electron density in the F-region (NmF2) increased by a factor of $\approx 2$, the altitude of the peak density (hmF2) rose by $\approx 80$ km, and the F-region electron temperature decreased by $\approx 1000$ K. Further simulations in which either the neutral atmosphere and winds or the electric field were replaced by their quiet day counterparts clearly suggest that the electric field played the dominant, although not exclusive, role in producing these effects.

physics.space-ph↗

arXiv2005

The Transition from Anti-Parallel to Component Magnetic Reconnection

We study the transition between anti-parallel and component collisionless magnetic reconnection with 2D particle-in-cell simulations. The primary finding is that a guide field \approx 0.1 times as strong as the asymptotic reconnecting field -- roughly the field strength at which the electron Larmor radius is comparable to the width of the electron current layer -- is sufficient to magnetize the electrons in the vicinity of the x-line, thus causing significant changes to the structure of the electron dissipation region. This implies that great care should be exercised before concluding that magnetospheric reconnection is antiparallel. We also find that even for such weak guide fields strong inward-flowing electron beams form in the vicinity of the magnetic separatrices and Buneman-unstable distribution functions arise at the x-line itself. As in the calculations of {\it Hesse et al.} [2002] and {\it Yin and Winske} [2003], the non-gyrotropic elements of the electron pressure tensor play the dominant role in decoupling the electrons from the magnetic field at the x-line, regardless of the magnitude of the guide field and the associated strong variations in the pressure tensor's spatial structure. Despite these changes, and consistent with previous work, the reconnection rate does not vary appreciably with the strength of the guide field as it changes between 0 and a value equal to the asymptotic reversed field.

physics.space-ph↗

arXiv2004

Three species collisionless reconnection: Effect of O+ on magnetotail reconnection

The nature of collisionless reconnection in a three-species plasma composed of a heavy species, protons, and electrons is examined. Besides the usual two length scales present in two-species reconnection, there are two additional larger length scales in the system: one associated with a "heavy whistler" which produces a large scale quadrupolar out-of-plane magnetic field, and one associated with the "heavy Alfven" wave which can slow the outflow speed and thus the reconnection rate. The consequences for reconnection in the magnetotail with an O+ population present are discussed.

physics.plasm-ph↗

arXiv2003

Diamagnetic Suppression of Component Magnetic Reconnection at the Magnetopause

We present particle-in-cell simulations of collisionless magnetic reconnection in a system (like the magnetopause) with a large density asymmetry across the current layer. In the presence of an ambient component of the magnetic field perpendicular to the reconnection plane the gradient creates a diamagnetic drift that advects the X-line with the electron diamagnetic velocity. When the relative drift between the ions and electrons is of the order the Alfven speed the large scale outflows from the X-line necessary for fast reconnection cannot develop and the reconnection is suppressed. We discuss how these effects vary with both the plasma beta and the shear angle of the reconnecting field and discuss observational evidence for diamagnetic stabilization at the magnetopause.

physics.space-ph↗

arXiv1998

Effects of Large-Scale Convection on p-mode Frequencies

We describe an approach for finding the eigenfrequencies of solar acoustic modes (p modes) in a convective envelope in the WKB limit. This approximation restricts us to examining the effects of fluid motions which are large compared to the mode wavelength, but allows us to treat the three-dimensional mode as a localized ray. The method of adiabatic switching is then used to investigate the frequency shifts resulting from simple perturbations to a polytropic model of the convection zone as well as from two basic models of a convective cell. We find that although solely depth-dependent perturbations can give frequency shifts which are first order in the strength of the perturbation, models of convective cells generate downward frequency shifts which are second order in the perturbation strength. These results may have implications for resolving the differences between eigenfrequencies derived from solar models and those found from helioseismic observations.

astro-ph↗