Xueshang Feng

A NOVEL NUMERICAL IMPLEMENTATION FOR SOLAR WIND MODELING BY THE MODIFIED CONSERVATION ELEMENT/SOLUTION ELEMENT METHOD

In this paper, the spacetime conservation element and solution element (CESE) method is applied to threedimensional magnetohydrodynamics (MHD) equations in Cartesian coordinates for solar wind plasma, with the purpose of modeling the steady state solar atmospheric study. To illustrate this newly developed scheme we have studied two examples: (1) two-dimensional coronal dynamical structure with multipole magneticfields and (2) threedimensional coronal dynamical structure, using measuredsolar surface magnetic fields and the empirical values of the plasma properties on the solar surface as the initial conditions for the set of MHD equations and then the relaxationmethod toachieve aq uasiYsteadystate.From these examples we have shown that the newlydeveloped modified spacetime CESE scheme possesses the ability to model the Sun-Earth environment and other astrophysical flows. Subject headingg methods: numerical — MHD — solar wind

Effects of the interaction and evolution of interplanetary shocks on “background” solar wind speeds

[1]xa0Solar wind speed plays an important role in the study of space weather prediction. Some workers have used it for measuring the arrival time of solar disturbances at 1 AU. The purpose of this work is to extend our previous study (Wu et al., 2005c) of some Halloween 2003 events by presenting additional physical effects of multiple shock interactions on the solar wind profile during a complex compound event. In order to achieve this goal, we track a group of specific solar events plasma and magnetic field output as they propagate into interplanetary space. A one-dimensional, time-dependent adaptive grid MHD code is used to study the evolution and interaction of shocks from Sun through the heliosphere. The MHD simulation results demonstrate that the solar wind speed might increase about ≈25% after two shocks collide with each other. This kind of interaction can affect the accuracy of the identification of the solar source that causes the interplanetary event (e.g., magnetic cloud, coronal mass ejection, interplanetary shock, or some other interplanetary discontinuity.) In this study we further simulate part of the famous Halloween 2003 events that contain at least four major solar events (flares) during 28 October to 1 November 2003. These major events, simulated by pressure pulses, generated shocks that matched well with ACE (Advanced Composition Explorer) observations as reported by Wu et al. (2005c) in our previous study. The present work presents new details concerning the interplay (such as sunward and antisunward traveling compression and rarefaction waves) between fast forward and fast reverse interplanetary shocks.

A new non‐pressure‐balanced structure in interplanetary space: Boundary layers of magnetic clouds

Here is analyzed the observational data of 70 magnetic cloud boundary layers (BLs) from the Three-Dimensional (3-D) Plasma and Energetic Particle (3DP) and 50 BLs from the Solar Wind Experiment (SWE) instruments on Wind spacecraft from February 1995 to June 2003. From this analysis, we discover that the boundary layer of a magnetic cloud is a new non-pressure-balanced structure different from the jump layer (i.e., shocked front) of an interplanetary shock wave. The main results are that ( 1) the BL is often a non-pressure-balanced structure with the magnetic pressure decrease associated with the abrupt variation of field direction angle (theta, phi) for about 90% and more than 85% of the BLs investigated from 3DP and SWE data, respectively; ( 2) the events of heated and accelerated plasma in the BLs are about 90%, 85% and 85%, 82% of the BLs investigated, respectively, from 3DP and SWE data; ( 3) the reversal flows are observed and their occurrence ratio is as high as 80% and 90% of the BLs investigated from 3DP and SWE data, respectively; and ( 4) the plasma and field characteristics for the BLs are also obviously different from those in the jump layers (JLs) of shock waves. These results show that there exist important dynamic interactions inside the BLs. As a preliminary interpretation, this could be associated with the magnetic reconnection process possibly occurring inside the BLs. Thus the study of the BLs, as a new non-pressure-balanced structure in interplanetary space, could open a new window for revealing some important physical processes in interplanetary space.

A practical database method for predicting arrivals of ''average'' interplanetary shocks at Earth

[1]xa0A practical database method for predicting the interplanetary shock arrival time at L1 point is presented here. First, a shock transit time database (hereinafter called Database-I) based on HAFv.1 (version 1 of the Hakamada-Akasofu-Fry model) is preliminarily established with hypothetical solar events. Then, on the basis of the prediction test results of 130 observed solar events during the period from February 1997 to August 2002, Database-I is modified to create a practical database method, named Database-II, organized on a multidimensional grid of source location, initial coronal shock speed, and the year of occurrence of the hypothetical solar event. The arrival time at L1 for any given solar event occurring in the 23rd solar cycle can be predicted by looking up in the grid of Database-II according to source location, the initial coronal shock speed, and the year of occurrence in cycle 23. Within the hit window of ±12 h, the success rate of the Database-II method for 130 solar events is 44%. This could be practically equivalent to the shock time of arrival (STOA) model, the interplanetary shock propagation model (ISPM), and the HAFv.2 model. To explore the capability of this method, it is tested on new data sets. These tests give reasonable results. In particular, this methods performance for a set of events in other cycles is as good as that of the STOA and ISPM models. This gives us confidence in its application to other cycles. From the viewpoint of long-term periodicity for solar activity, it is expected that the Database-II method can be applicable to the next solar cycle 24.

Observations of an interplanetary slow shock associated with magnetic cloud boundary layer

The observations of the slow shocks associated with the interplanetary coronal mass ejections near 1 AU have seldom been reported in the past several decades. In this paper we report the identification of an interplanetary slow shock observed by Wind on September 18, 1997. This slow shock is found to be just the front boundary of a magnetic cloud boundary layer. A self-consistent method based on the entire R-H relations is introduced to determine the shock normal. It is found that the observations of the jump conditions across the shock are in good agreement with the R-H solutions. The intermediate Mach number M-I = U-n/(V(A)cos theta(Bn)) is less than 1 on both sides of the shock. In the upstream region, the slow Mach number M-s1 = U-n1/V-s1 is 1.44 ( above unity), and in the downstream region, the slow Mach number M-s2 = U-n2/ V-s2 is 0.8 ( below unity). Here V-s and V-A represent the slow magnetoacoustic speed and Alfven speed respectively. In addition, the typical interior magnetic structure inside the shock layer is also analyzed using the 3s time resolution magnetic field data since the time for the spacecraft traversing the shock layer is much longer ( about 17s). As a potential explanation to the formation of this kind of slow shock associated with magnetic clouds, this slow shock could be a signature of reconnection that probably occurs inside the magnetic cloud boundary layer.

Characteristics of solar flares associated with interplanetary shock or nonshock events at Earth

Solar flares and metric type II radio bursts are one kind of preliminary manifestations of solar disturbances and they are fundamental for predicting the arrival of associated interplanetary (IP) shocks at Earth. We statistically studied 347 solar flare type II radio burst events during 1997.2 - 2002.8 and found ( 1) only 37.5% of them were followed by the IP shocks at L1 ( in other words, at Earth), the others without such IP shocks account for 62.5%; ( 2) the IP shocks associated with intense flares have large probability to arrive at Earth; ( 3) the IP shocks associated with central flares are more likely to arrive at Earth than those associated with the limb flares, and the most probable location for flares associated with IP shocks at Earth is W20 degrees; and ( 4) there exists a east-west asymmetry in the distribution of geoeffectiveness of flare-associated IP shocks along the flare longitude. Most severe geomagnetic storms (Dst(min) <= - 100 nT) are usually caused by flare-associated shocks originating from western hemisphere or middle regions near central meridian, and the most probable location for strong flares associated with more intense geomagnetic storms is W20 degrees as well. These results could provide some criteria to estimate whether the associated shock would arrive at Earth and corresponding geomagnetic storm intensity.

Prediction method for October 2003 solar storm

Aiming at two intense shock events on October 28 and 29, 2003, this paper presents a two-step method, which combines synoptic analysis of space weather — “observing” and quantitative prediction — “palpating”, and then uses it to test predictions. In the first step of “observing”, on the basis of observations of the solar source surface magnetic field, interplanetary scintillation (IPS) and ACE spacecraft, we find that the propagation of the shocks is asymmetric relative to the normal direction of their solar sources, and the Earth is located near the direction of the fastest speed and the greatest energy of the shocks. As the two fast ejection shock events, the fast explosion of coronal mass of the extremely high temperature, the strong magnetic field, and the high speed background solar wind are also helpful to their rapid propagation. In the second step of “palpating”, we adopt a new membership function of the fast shock events for the ISF method. The predicted results show that for the onset time of the geomagnetic disturbance, the relative errors between the observational and the predicted results are 1.8% and 6.7%; and for the magnetic disturbance magnitude, the relative errors are 4.1% and 3.1%, respectively. Furthermore, the comparison among the predicted results of our two-step method with those of five other prevailing methods shows that the two-step method is advantageous. The results tell us that understanding the physical features of shock propagation thoroughly is of great importance in improving the prediction precision.