A piston-driven shock in the solar corona     

Alan Maxwell  Murray Dryer  Patrick McIntosh 《Solar physics》1985,97(2):401-413

A solar flare that occurred on the west limb at 1981, March 25, 2038 UT generated a massive, rapidly-expanding optical coronal transient, which moved outward with an approximately constant velocity of 800 km s^–1. An associated magnetohydrodynamic shock travelled out ahead of the transient with a velocity estimated to be approximately 1000 km s^–1. The optical and radio data on the transient and shock fit well with general theories concerning piston-driven shocks and with current MHD models for propagation of such shocks through the solar corona.  相似文献   

Two-dimensional,time-dependent MHD description of interplanetary disturbances: Simulation of high speed solar wind interactions     

S.T. Wu  S.M. Han  M. Dryer 《Planetary and Space Science》1979,27(3):255-264

A two-dimensional, time-dependent, magnetohydrodynamic, numerical model is used to investigate multiple, transient solar wind flows which start close to the Sun and then extend into interplanetary space. The initial conditions are assumed to be appropriate for steady, homogeneous solar wind conditions with an average, spiral magnetic field configuration. Because both radial and azimuthal dimensions are included, it is possible to place two or more temporally-developing streams side-by-side at the same time. Thus, the evolution of the ensuing stream interaction is simulated by this numerical code. Advantages of the present method are as follows: (1) the development and decay of asymmetric MHD shocks and their interactions are clearly indicated; and (2) the model allows flexibility in the specification of evolutionary initial conditions in the azimuthal direction, thereby making it possible to gain insight concerning the interplanetary consequences of real physical situations more accurately than by use of the one-dimensional approach. Examples of such situations are the occurrence of near-simultaneous solar flares in adjacent active regions and the sudden appearance or enlargement of coronal holes as a result of a transient re-arrangement from a closed to an open magnetic field topology.  相似文献   

Evolution of fast and slow shock interactions in the inner heliosphere     

C. -C. Wu  S. T. Wu  M. Dryer 《Solar physics》2004,223(1):259-282

We use a one-dimensional, time-dependent adaptive grid MHD code to study the interaction between fast and slow shocks in the solar wind. Our results show that: (1) a forward slow shock (FSS) can be destroyed by a forward fast shock (FFS) that overtakes it from behind; (2) two propagating FSSs can merge into a stronger FSS; (3) a strong FSS can survive by following a strong forward fast shock; and (4) the strength of a FSS is decreased by following an FFS. These simulation results reproduce an important feature of the Helios observations (Richter, 1987) where transient fast shocks were more frequently followed within a few hours by slow shock ype discontinuities rather than by fast reverse shocks.  相似文献   

THREE-DIMENSIONAL MHD SIMULATION OF INTERPLANETARY MAGNETIC FIELD CHANGES AT 1 AU CAUSED BY A SIMULATED SOLAR DISTURBANCE AND A TILTED HELIOSPHERIC CURRENT/PLASMA SHEET     

Wu  Chin-Chun  Dryer  Murray 《Solar physics》1997,173(2):391-408

A fully three-dimensional (3D), time-dependent, MHD interplanetary model has been used, for the first time, to study the relationship between one form of solar activity and transient variations of the north–south component, Bz, of the interplanetary magnetic field (IMF) at 1 AU during the active period of a representative solar cycle. Four cases of initial steady-state solar wind conditions, with different tilt angles of the heliospheric current sheet/plasma sheet (HCS/HPS) which is known to be inclined at solar maximum, are used to study the relationship between the location of solar activity and transient variations of the north–south IMF Bz component at 1 AU. We simulated the initialization of the disturbance as a density pulse at different locations near the solar surface for each case of initial steady-state condition and observed the simulated IMF evolution of B (= –Bz) at 1 AU. The results show that, for a given density pulse, the orientation of the corresponding transient variation of Bz has a strong relationship to the location of the density pulse and the initial conditions of the IMF. A recipe for prediction of the initial Bz turning direction is also presented in this study.In previous studies that used this recipe with only a flat HCS/HPS that was coincident with the solar equatorial plane, we found a prediction accuracy of 83% from a data set of 73 events during solar maximum. The present study that incorporates more realistic HCS/HPS tilt angles confirms the earlier work.Our study leads us to suggest that significant Bz values, associated with substantial post-shock temporal periods of hours at 1 AU, could be achieved if large energies (say, 10 32–10 33 erg) were released at the Sun in a flare or helmet de-stabilization process.  相似文献   

Multiple Type II Solar Radio Bursts     

A. Shanmugaraju  Y.-J. Moon  K.-S. Cho  Y.-H. Kim  M. Dryer  S. Umapathy 《Solar physics》2005,232(1-2):87-103

We report on the detailed analysis of a set of 38 multiple type II radio bursts observed by Culgoora radio spectrograph from January 1997 to July 2003. These events were selected on the basis of the following criteria: (i) more than one type II were reported within 30 min interval, (ii) both fundamental and harmonic were identified for each of them. The X-ray flares and CMEs associated with these events are identified using GOES, Yohkoh SXT, SOHO/EIT, and SOHO/LASCO data. From the analysis of these events, the following physical characteristics are obtained: (i) In many cases, two type IIs with fundamental and harmonic were reported, and the time interval between the two type IIs is within 15 min; (ii) The mean values of starting frequency, drift rate, and shock speed of the first type II are significantly higher than those of the second type II; (iii) More than 90% of the events are associated with both X-ray flares and CMEs; (iv) Nearly 75% of the flares are stronger than M1 X-ray class and 50% of CMEs have their widths larger than 200^∘ or they are halo CMEs; (v) While most of the first type IIs started within the flare impulsive phase, 22 out of 38 second type IIs started after the flare impulsive phase. Weak correlations are found between the starting and ending frequencies of these type II events. On the other hand, there was no correlation between two shock speeds between the first and the second type II. Since most of the events are associated with both the flares and CMEs, and there are no events which are only associated with multiple impulsive flares or multiple mass ejections, we suggest that the flares and CMEs (front or flank) both be sources of multiple type IIs. Other possibilities on the origin of multiple type IIs are also discussed.  相似文献   

Numerical simulation of slow shocks in the solar wind     

Chin-Chun Wu  J. K. Chao  S. T. Wu  Murray Dryer 《Solar physics》1996,165(2):377-393

The evolutionary state of slow forward shock waves is examined with the use of two MHD numerical codes. Our study is intended to be exploratory rather than a detailed parametric one. The first code is one-dimensional (with three components of velocity and magnetic field) which is used to follow a slow shock that propagates into a positive gradient of density versus distance. It is found that the slow shock evolves into an extraneous (intermediate) shock wave. The second code has a spherical, one-dimensional, planar geometry (with two velocity and magnetic field components) which is used to follow a spiral interplanetary magnetic field. It is found that a slow shock type perturbation can generate a forward slow shock; a fast forward shock is generated in the front of the slow shock; a contact discontinuity is formed behind the slow shock, and a compound nonlinear MHD wave is formed behind the contact discontinuity with a fast reverse shock formed further behind. Thus, we demonstrate that the evolution of a slow shock into (solely) a fast shock, as suggested by Whang (1987), is much more complicated.  相似文献   

MHD study of temporal and spatial evolution of simulated interplanetary shocks in the ecliptic plane within 1 AU   总被引：5，自引：0，他引：5  

Zdenka Smith  Murray Dryer 《Solar physics》1990,129(2):387-405

We utilize a 21/2-D MHD time-dependent model to perform a parametric study of interplanetary shock propagation to 1 AU. The input conditions are represented by the following variables:(1) initial shock velocity, (2) duration of the driving pulse, and (3) width of the pulse at the near-Sun position (18 solar radii). The total net energy added to the solar wind was calculated for each pulse. The forward shock's travel time to, and the peak dynamic pressure at, 1 AU as a function of location along the shock front have been studied over a range of total input pulse energies from 10²⁹ to 10³² ergs. For input pulses with modest angular width and temporal duration, we find that the propagation of the resulting interplanetary fast forward shock waves depends primarily upon the net input energy. The dependence of the transit time upon energy is a power law with a -1/3 index which corresponds to the classical, piston driven case. Reverse shocks are also formed behind all but the lowest energy shocks. Their properties, although also a function of input energy, depend upon the specific values of the input pulse shock velocity, width and duration. We also briefly discuss the propagation of the shocks out to 1 AU, and the conditions for which the interplanetary shocks depart from being symmetric about the input pulse central meridian due to magnetic and dynamic effects.  相似文献   

Interplanetary disturbances produced by a simulated solar flare and equatorially-fluctuating heliospheric current sheet     

G. Gislason  M. Dryer  Z. K. Smith  S. T. Wu  S. M. Han 《Astrophysics and Space Science》1984,98(1):149-161

A recently developed nonplanar, time-dependent magnetohydrodynamic (MHD) model (Wuet al., 1983) was used to study the interplanetary disturbances produced by a compound event in the heliosphere. That is, a steady-state interplanetary medium is first disturbed by a simulated equatorially-fluctuating current sheet. After a few days (100 hr), the disturbed interplanetary medium is again perturbed by a solar-flare-generated shock wave. Attention is directed toward the differences that are caused by the presence of the equatorially-fluctuating (warped) current sheet.  相似文献   

Interplanetary disturbances in the solar wind produced by density,temperature, or velocity pulses at 0.08 AU     

S. T. Wu  Murray Dryer  S. M. Han 《Solar physics》1976,49(1):187-204

Time-dependent solutions of a one-fluid model of the interplanetary medium are investigated. This set of unsteady hydrodynamic equations has been written in conservation form in order to apply the Lax-Wendroff method for the solution of this problem. The initial disturbance is specified by a pulse at 0.08 AU (astronomical units). Physically, this pulse can be interpreted as having been caused by a solar flare, surge, or any other solar disturbance. The equilibrium condition is determined to be the steady solution of the governing equations and represents the quiet solar wind. The results are presented in terms of density, temperature, and velocity profiles of the interplanetary gas flow at heliocentric distances up to 6 AU at several times. Also, the trajectories of disturbances for various initial pulses are shown. Finally, we have used some June 1972 interplanetary observational data to compare with these theoretical calculations. On the basis of these results, the effects of solar disturbances on the interplanetary environment (such as the generation of large non-linear wave trains in the shocks' wakes) can be inferred.  相似文献   

On the kinematic evolution of flare-associated cmes     

A. Shanmugaraju  Y.-J. Moon  M. Dryer  S. Umapathy 《Solar physics》2003,215(1):185-201

We report a common tendency of the kinematic evolution of three flare-associated coronal mass ejections (CMEs). Their kinematic evolutions are examined using well-observed data (eruptive filaments, X-ray structures, and prominences) very close to the solar surface as well as SOHO/LASCO C2-C3 data. Their height–time data are fitted using three analytical models (exponential, power-law, and linear) to examine their kinematic behaviors. The speed and acceleration of the CMEs are then obtained from the analytical expressions of height–time data. From this analysis, it is found that the kinematic patterns of these three CMEs have a typical tendency; that is, the speed of the CMEs very close to the surface (lower corona) is approximately exponential in form, but it is nearly constant in the upper corona. The peak of the acceleration is found to occur within 2–3 solar radii and during the eruptive phase of the associated flare. It is also noted that the observed kinematic patterns are quite similar to those predicted by two flux rope emergence models: (i) a theoretical, electrodynamic model (Chen, 1996); and (ii) a numerical simulation, self-consistent, 2 ;D MHD model (Wu, Guo, and Dryer, 1997).  相似文献