S.T. Wu

Joule heating and winds due to geomagnetic disturbances

Abstract A self-consistent theoretical model of the dynamical processes occurring in the Earths upper atmosphere has been developed based on magnetohydrodynamic concepts. The May 1967 geomagnetic storm data recorded at College, Alaska, have been used in the resulting set of non-linear, partial differential magnetohydrodynamic equations, to calculate variations due to the storm in the amount of Joule heating and the winds at an altitude of approximately 140 km as a function of time. It is assumed that initially (1) the vertical wind velocity is zero everywhere, and (2) the horizontal pressure gradient is balanced by the Coriolis force where the background vertical pressure gradient is computed using the 1964 Jacchia model atmosphere. A finite difference numerical integration scheme is used to obtain a numerical solution of the set of governing equations. Presented here are the vertical and horizontal wind speeds, and amount of Joule heating as a function of time during the period of the geomagnetic storm.

A multiple type-II burst associated with a coronal transient and its MHD simulation

A large coronal transient took place on May 8, 1981. The transient was related to an M7.7/2B flare and was associated with at least two coronal type-II bursts. The velocities of the type-II bursts were in the range 1100-1800 km/s, in excess of the transient velocity of 1000 km/s. Two dimensional positions of the type-II radio sources are available from both the Clark Lake and the Culgoora Radio Observatories. Two dimensional MHD simulations of the event are carried out, taking into account the observed velocity, position, and size of the type-II bursts. The multiple shocks observed during the event and their interaction are simulated, and results of the simulation are discussed. 13 references.

A transient MHD model applicable for the source of solar cosmic ray acceleration

Abstract A two-dimensional, time-dependent magnetohydrodynamic (MHD) model is used to describe the possible mechanisms for the source of solar cosmic ray acceleration following a solar flare. The hypothesis is based on the propagation of fast mode MHD shocks following a sudden release of energy. This model has already been used with some success for simulation of some major features of type II shocks and white light coronal transients. In this presentation, we have studied the effects of initial magnetic topology and strength on the formation of MHD shocks. We consider the plasma beta (thermal pressure/magnetic pressure) as a measure of the initial , relative strength of the field. During dynamic mass motion, the Alfven Mach number is the more appropriate measure of the magnetic fields ability to control the outward motion. We suggest that this model (computed self-consistently) provides the shock wave and the disturbed mass motion behind it as likely sources for solar cosmic ray acceleration.

Numerical modeling of the energy storage and release in solar flares

Abstract This paper reports on investigation of the photospheric magnetic fieldline footpoint motion (usually referred to as shear motion) and magnetic flux emerging from below the surface in relation to energy storage in a solar flare. These causality relationships are demonstrated by using numerical magnetohydrodynamic simulations. From these results, one may conclude that the energy stored in solar flares is in the form of currents. The dynamic process through which these currents reach a critical value is discussed as well as how these currents lead to energy release, such as the explosive events of solar flares.

MHD simulation of mass injection: A mechanism for the formation of active region loops

Abstract We have used a 2-D nonlinear MHD numerical code to simulate the formation and dynamic evolution of active regions loops subjected to mass injections at the footpoints. We also calculated the UV and X-ray signatures of the plasmas. We find that it is possible to form loops in a low beta plasma that occur in the solar active regions.

Numerical simulation of flare energy build-up and release via joule dissipation

Abstract A new numerical magnetohydrodynamic (MHD) model is developed to study the evolution of an active region due to photospheric converging motion, which leads to magnetic energy buildup in the form of electric current. Because this new MHD model has incorporated finite conductivity, the energy conversion occurs from magnetic mode to thermal mode through joule dissipation. In order to test the causality relationship between the occurence of flare and photospheric motion, a multiple pole configuration with neutral point is used for this numerical simulation study. Using these numerically simulated results we find that in addition to the converging motion, the initial magnetic field configuration and the redistribution of the magnetic flux at photospheric level will enhance the possibility for the development of a flare.

On the heating mechanism of magnetic flux loops in the solar atmosphere

Abstract Hα filtergrams and magnetograms indicate that bright features (such as plages and granulation boundaries) correspond to areas of strong vertical magnetic fields and dark features (such as fibrils and filaments) are associated with strong horizontal magnetic field. It was suggested by /1/ that there is an excess dissipation of waves, available for heating, in regions of vertical magnetic fields. With this suggestion in mind, we have investigated the physical heating mechanism due to ponderomotive forces exerted by turbulent waves along curved magnetic flux loops. Results show that the temperature difference (ΔT) between the inside and outside of the flux loop can be classified into three parts; ΔT = ΔT 1 + ΔT 2 + ΔT 3 ; in which ΔT 1 and ΔT 3 represent the heating or cooling effect from the ponderomotive force, and ΔT 2 is the heating effect due to conversion of turbulent energy from the localized plasma. The specific physical mechanism (i.e., the ponderomotive forces exerted by turbulent waves), is used to illustrate solar atmospheric heating via an example leading to the formulation of plages.