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Steinolfson, R. S.

Publications and source records attributed to Steinolfson, R. S..

72 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Magnetohydrodynamic shock propagation in the vicinity of a magnetic neutral sheet

This paper reports a numerical investigation of the propagation of magnetohydrodynamic (MHD) shocks in the vicinity of magnetic neutral sheets. The attenuation of a shock after passing through a neutral sheet has been evaluated (assuming infinite electrical conductivity). In a parameter study, values of shock speed, polytropic index, plasma beta, and neutral-sheet thickness which are representative of solar coronal conditions have been examined. If solar cosmic rays are accelerated in association with a flare-induced shock (as seems most likely), then our results suggest that the spatial structure of solar particle sources will be influenced by helmet streamers. Such streamers are most readily detectable by H alpha filaments in the underlying chromosphere.

Steinolfson, R. S.↗
NASA NTRS1980

Dynamic simulation of coronal mass ejections

A model is developed for the formation and propagation through the lower corona of the loop-like coronal transients in which mass is ejected from near the solar surface to the outer corona. It is assumed that the initial state for the transient is a coronal streamer. The initial state for the streamer is a polytropic, hydrodynamic solution to the steady-state radial equation of motion coupled with a force-free dipole magnetic field. The numerical solution of the complete time-dependent equations then gradually approaches a stationary coronal streamer configuration. The streamer configuration becomes the initial state for the coronal transient. The streamer and transient simulations are performed completely independent of each other. The transient is created by a sudden increase in the pressure at the base of the closed-field region in the streamer configuration. Both coronal streamers and coronal transients are calculated for values of the plasma beta (the ratio of thermal to magnetic pressure) varying from 0.1 to 100.

Steinolfson, R. S.↗
NASA NTRS1980

The dependence of coronal hole size on large scale magnetic field strength

The importance of mathematical models of the coronal structure for studies of coronal energetics, to simulate global flows of the solar wind, and to obtain reliable solar terrestrial predictions is discussed. Previous coronal models, including an example of a coronal MHD flow model, are reviewed. The development of a coronal model which is a logical extension of earlier models and which allows a closer relationship to the photospheric magnetic field as it is observed daily is described. The calculations are outlined. The assumptions of the model are: axisymmetric flow with no rotation, resulting in two dimensional flow in a meridional plane; zero viscosity and infinite electrical conductivity; polytropic, single fluid flow; and no momentum addition.

Suess, S. T.↗
NASA NTRS1980

Mass ejections

Observations and model simulations of solar mass ejection phenomena are examined in an investigation of flare processes. Consideration is given to Skylab and other observations of flare-associated sprays, eruptive prominences, surges and coronal transients, and to MHD, gas dynamic and magnetic loop models developed to account for them. Magnetic forces are found to confine spray material, which originates in preexisting active-region filaments, within steadily expanding loops, while surges follow unmoving, preexisting magnetic field lines. Simulations of effects of a sudden pressure pulse at the bottom of the corona are found to exhibit many characteristics of coronal transients associated with flares, and impulsive heating low in the chromosphere is found to be able to account for surges. The importance of the magnetic field as the ultimate source of energy which drives eruptive phenomena as well as flares is pointed out.

Rust, D. M.↗
NASA NTRS1979

Hydrodynamic simulations of flare/surge events

A one-dimensional, hydrodynamic, time-dependent model which simulates the major observed dynamics of flare associated surges was developed. The thermodynamics, the surge mass, the time scales, the physical dimensions, and the velocities of typical surge events are reproduced. The surge is created by a sudden pressure increase at the top of the chromosphere, and this pressure pulse produces a disturbance which is followed, with a time-dependent numerical solution, as it propagates upward through the transition region and into the corona. The leading edge of the disturbance is a weak shock which has only a slight effect on the original transition region and coronal thermodynamics. The major effect occurs behind the shock where the temperature is decreased and the density is increased, and this cool, dense region moves upward then falls downward. After the material begins returning to the chromosphere, a second shock is formed which propagates upward, brings the infalling material to rest, and returns the atmosphere to hydrostatic equilibrium.

Steinolfson, R. S.↗
NASA NTRS1979

Magnetohydrodynamic models of coronal transients in the meridional plane. II - Simulation of the coronal transient of 1973 August 21

A two-dimensional planar MHD model of solar atmospheric transient events is used to simulate the flare-associated events of August 21, 1973. This event, observed in H-alpha, He II 304 A, soft X-ray, and coronal white light, provided sufficient information (especially in the latter diagnostic) for an assessment to be made of the model's ability to simulate major features of an actual solar event. It was found that a thermodynamic input pulse based on data provided by the NASA Marshall Space Flight Center-Aerospace Corporation X-ray telescope (S-056) on Skylab was sufficient to produce the global geometry, shock and contact-surface velocities, excess mass contours, and energy budget which were, for the most part, observed by the High Altitude Observatory white-light coronagraph (S-052) on Skylab in the form of a forerunner and coronal transient.

Dryer, M.↗
NASA NTRS1978

Magnetohydrodynamic models of coronal transients in the meridional plane. I - The effect of the magnetic field

The propagation of coronal transients through the lower corona in the meridional plane is studied on the basis of numerical solutions of the time-dependent MHD equations of motion. The importance of the topology of the initial coronal magnetic field is demonstrated by considering two configurations: one which is essentially radial (open), and another which is essentially parallel to the solar surface (closed). The effect of coronal field magnitude is investigated by using values for the beta parameter of 1 and 0.1 at the coronal base. The consequences of including radiative losses and of varying the polytropic index are also considered. The solar event is simulated by a step-function increase in the pressure at the base of an initially hydrostatic atmosphere by a factor of 5, lasting for a period of 5 min.

Steinolfson, R. S.↗
NASA NTRS1978

Magnetohydrodynamic simulation of coronal mass ejections into the solar wind

A state of the art fluid continuum technique to describe the MHD transient respose of the corona below 10 solar radii during two well observed events (flares on 21 August 1973 and 5 September 1973) is reviewed. It is concluded that the computer simulation should be subjected to in situ verification of as many of its initial assumptions as possible. Its ability to provide a rational basis for physical understanding of mass ejections suggests its use as one of the tools used in the planning and analysis of such encounter missions.

Dryer, M.↗
NASA NTRS1978

Dynamic MHD modeling of solar wind corotating stream interaction regions observed by Pioneer 10 and 11

The use of the Pioneer 10 and 11 projects to test an MHD one-dimensional time-dependent model of corotating solar wind streams during the period from Sept. 30 to Nov. 25, 1973 is described. During this period, five or six corotating interaction regions streamed past the two spacecraft, and, as a result of multiple-spacecraft radial alignment and temporally varying conditions at the solar wind source, the pattern predicted by the Steinolfson et al. (1975) model could be compared with observations. The results, in general, support the validity of the model, although the neglect of thermal energy exchange leads to incorrect values for the proton temperature. A detailed analysis of a stream is discussed.

Dryer, M.↗
NASA NTRS1978

Numerical simulation of MHD shock waves in the solar wind

The effects of the interplanetary magnetic field on the propagation speed of shock waves through an ambient solar wind are examined by numerical solutions of the time-dependent nonlinear equations of motion. The magnetic field always increases the velocity of strong shocks. Although the field may temporarily slow down weak shocks inside 1 AU, it eventually also causes weak shocks to travel faster than they would without the magnetic field at larger distances. Consistent with the increase in the shock velocity, the gas pressure ratio across a shock is reduced considerably in the presence of the magnetic field. The numerical method is used to simulate (starting at 0.3 AU) the large deceleration of a shock observed in the lower corona by ground-based radio instrumentation and the more gradual deceleration of the shock in the solar wind observed by the Pioneer 9 and Pioneer 10 spacecraft.

Steinolfson, R. S.↗
NASA NTRS1978

Dynamic MHD modeling of the solar wind disturbances during the August 1972 events

A time-dependent one-dimensional MHD theoretical model is tested by using plasma and magnetic field observations of Pioneer 9 and Pioneer 10 during the August 1972 events on the sun and in the interplanetary medium. These spacecraft were nearly aligned along a common heliocentric radius during these events, considered now to be the most spectacular and best-documented events during solar cycle 20. The observations of Pioneer 9 at 0.78 AU were used as input for the theoretical model. The plasma and magnetic field forcing functions were superimposed upon a preexisting ambient solar wind at this inner boundary, and the response was simulated as far as 8 AU. The simulated output at 2.2 AU is compared directly with the Pioneer 10 observations at 2.2 AU. Qualitative comparison is good, although several limitations of the one-dimensional theory are noted.

Dryer, M.↗
NASA NTRS1977

Thermally conductive flows in coronal holes

A treatment of polytropic solar wind flows in non-radial expansion regions, developed by Kopp and Holzer (1976), is extended to include the effect of thermal conduction. Thermal conductive and polytropic flows in the lower corona under specified high-speed stream conditions at 1 AU are compared; the thermally conductive flows more closely model the observed phenomena, though predicted electron density is still too low and the predicted temperature too high. It is suggested that another mechanism (such as wave pressure), in conjunction with thermal conduction, may provide an accurate explanation for solar wind flows originating in coronal holes.

Steinolfson, R. S.↗
NASA NTRS1977

Dynamical response of the solar corona. III - Numerical simulation of the 1973 June 10 coronal transient

A spherically symmetric adiabatic single-fluid model is outlined for simulating the nonlinear time-dependent response of the corona to solar events that are simulated by perturbations in the appropriate physical variables from their steady-state values at the coronal base. Several observed features of the coronal transient that occurred on June 10, 1973, are simulated by using a particular steady-state solar wind, a specific combination of density and temperature perturbations, and a particular time dependence of the perturbations. A different steady-state solar wind, a perturbation of shorter duration, and other perturbation combinations are also employed to simulate the same transient so that the effect of each quantity can be determined. It is found that the model cannot adequately simulate all the observational results for the investigated transient, that the steady-state solar wind is relatively unimportant in such numerical simulations, and that studies which attempt only to reproduce observed shock trajectories may lead to erroneous conclusions regarding the physics of the solar event that produced the transient.

Steinolfson, R. S.↗
NASA NTRS1976

MHD solution of interplanetary disturbances generated by simulated velocity perturbations

An MHD time-dependent numerical simulation, restricted to the solar equatorial plane, is used to demonstrate the interplanetary disturbances caused by several simplified coronal holes. Each 'hole' is assumed to have a configuration such that the higher solar wind velocity produced within their longitudinal extent is Gaussian over a 7-day period at the inner boundary (0.3 AU) of the numerical simulation. A second, twin coronal hole is assumed to rotate on the solar disk behind its predecessor. It is shown that the first coronal hole-produced interplanetary shock ensemble is overtaken by the second ensemble because of the higher velocity, lower density environment into which the latter propagates. A number of features predicted by MHD similarity theory are confirmed by the numerical simulation. These features include (1) strong azimuthal magnetic and plasma density compression, accompanied with average temperature depression, at the contact surface between forward and reverse shock ensembles, and (2) increasing spatial separation distance between forward and reverse shocks.

Dryer, M.↗
NASA NTRS1976

Interplanetary disturbances caused by the August 1972 solar flares as observed by Pioneer 9

Pioneer 9 plasma and field observations at 0.78 AU were used as the basis of the analysis of the dynamic behavior of the interplanetary medium during early August, 1972. The following investigations were carried out: (1) energy and mass estimates for the solar flares of Aug. 2, 4, and 7; (2) shock wave characteristics; and (3) a numerical simulation of the first two flare-generated disturbances on Aug. 2, 4, and 7.

Dryer, M.↗
NASA NTRS1976

On build-up of magnetic energy in the solar atmosphere

The dynamic response of the solar atmosphere is examined with the use of self-consistent numerical solutions to the complete set of nonlinear two-dimensional hydromagnetic equations. Of particular interest are the magnetic-energy buildup and the velocity field established by emerging flux at the base of an existing magnetic loop structure in a stationary atmosphere. For a plasma with a relatively low beta (0.03), the magnetic-energy buildup is approximately twice that of the kinetic energy, while the buildup in magnetic energy first exceeds but is eventually overtaken by the kinetic energy for a plasma with an intermediate beta (3). The increased magnetic flux causes the plasma to flow upward near the loop center and downward near the loop edges for the low-beta plasma. The plasma eventually flows downward throughout the lower portion of the loop carrying the magnetic field with it for the intermediate beta plasma. It is hypothesized that this latter case, and possibly the other case as well, may provide a reasonable simulation of the disappearance of prominences by flowing down into the chromosphere (a form of disparition brusque).

Nakagawa, Y.↗
NASA NTRS1976

Simulated traveling interplanetary disturbances initiated by various solar phenomena

Earlier studies have shown that interplanetary disturbances observed by spacecraft at large heliocentric distances from the sun can be directly associated with various solar activities such as surges, sprays, eruptive prominences, and flares. A digital computer code to simulate these phenomena is developed. This numerical code solves a set of conservation equations for an adiabatic time-dependent spherically-symmetric fluid flow. The solar events are simulated by perturbations in the dependent variables at the lower boundary of the quiet sun corona, and the time-dependent numerical solution simulates the coronal response. Three physical cases are simulated by using the proposed numerical code: (1) part of a surge, spray and prominence; (2) a single flare; and (3) two flares whose interplanetary consequences interact in space. Results for the first two cases are compared with both ground-based and space probe observations. Fairly good agreement with observations is found. A comparison of the third case with observations is presently under way.

Dryer, M.↗