Feng Xueshang

GPU-accelerated computing of three-dimensional solar wind background

High-performance computational models are required to make the real-time or faster than real-time numerical prediction of adverse space weather events and their influence on the geospace environment. The main objective in this article is to explore the application of programmable graphic processing units (GPUs) to the numerical space weather modeling for the study of solar wind background that is a crucial part in the numerical space weather modeling. GPU programming is realized for our Solar-Interplanetary-CESE MHD model (SIP-CESE MHD model) by numerically studying the solar corona/interplanetary solar wind. The global solar wind structures are obtained by the established GPU model with the magnetic field synoptic data as input. Meanwhile, the time-dependent solar surface boundary conditions derived from the method of characteristics and the mass flux limit are incorporated to couple the observation and the three-dimensional (3D) MHD model. The simulated evolution of the global structures for two Carrington rotations 2058 and 2062 is compared with solar observations and solar wind measurements from spacecraft near the Earth. The MHD model is also validated by comparison with the standard potential field source surface (PFSS) model. Comparisons show that the MHD results are in good overall agreement with coronal and interplanetary structures, including the size and distribution of coronal holes, the position and shape of the streamer belts, and the transition of the solar wind speeds and magnetic field polarities.

Numerical Simulation of the 12 May 1997 CME Event

Our newly developed CESE MHD model is used to simulate sun-earth connection event with the well-studied 12 May 1997 CME event as an example. The main features and approximations of our numerical model are as follows: (1) The modified conservation element and solution element (CESE) numerical scheme in spherical geometry is implemented in our code. (2) The background solar wind is derived from a 3D time-dependent numerical MHD model by input measured photospheric magnetic fields. (3) Transient disturbances are derived from solar surface by introducing a mass flow of hot plasma. The numerical simulation has enabled us to predict the arrival of the interplanetary shock and provided us with a relatively satisfactory comparison with the WIND spacecraft observations.

A prediction method of geomagnetic disturbances based on IPS observations—dynamics—fuzzy mathematics

A prediction method for geomagnetic disturbances, based on the interplanetary scintillation (IPS) observations, dynamics of interplanetary disturbance propagation and fuzzy mathematics, is suggested. The membership functions are established by using observational data on the solar storm, interplanetary and geomagnetic disturbance data in 1966-1982. Prediction tests are performed on 37 geomagnetic disturbance events caused by the solar storm-associated interplanetary disturbances, which are identified by the IPS observations during the descending solar activity phase 1984-1985. The relative error of the magnetic disturbance onset time (DeltaT/T) is less than 10% for 50% of all the events and less than 20% for 70% of all the events. The relative error of the predicted magnetic disturbance magnitude (DeltaSigmaK(p)/SigmaK(p)) is less than 30% for 80% of all events and greater than 60% for only 15% of all the events. In addition, the prediction test of April-May 1998 event gives the relative errors DeltaTT/=7.4% and DeltaEK(p)/SigmaK(p)=15.3%. These results show that the method suggested in this paper has potential prospects for improving the accuracy of the geomagnetic disturbance prediction made from IPS observations.

New Evidence for Magnetic Reconnection in the Tail of Interplanetary Magnetic Cloud

We analyse the WIND data of an interplanetary magnetic cloud (MC) on 2 November 2001, and find new evidences for magnetic reconnection in the tail of this MC. In the MC tail, the largely dip and the large change of the orientation of the magnetic field occurred simultaneously, Δθ≈45°, and Δ changed from 90° to 320°. Correspondingly, the number density of ions increased, and the superthermal electrons were heated and accelerated, however its number density decreased. Meanwhile, inverse jets and Hall term were observed. The pitch-angle distributions of the electrons with lower energy and higher energy showed strong turbulence and bi-direction flow, respectively. The plasma wave activity enhanced near the electron plasma frequency, fpe and 2fpe. These important physical characteristics are new evidences for magnetic reconnection existing in interplanetary space.

Periodicities of solar activity and the surface temperature variation of the Earth and their correlations

Based on the well-calibrated systematiCmeasurements of sunspot numbers, the reconstructed data of the total solar irradiance (TSI), and the observed anomalies of the Earth’s averaged surface temperature (global, ocean, land), this paper investigates the periodicities of both solar activity and the Earth’s temperature variation as well as their correlations on the time scale of centuries using the wavelet and cross correlation analysis techniques. The main results are as follows. (1) Solar activities (including sunspot number and TSI) have four major periodic components higher than the 95% significance level of white noise during the period of interest, i.e. 11-year period, 50-year period, 100-year period, and 200-year period. The global temperature anomalies of the Earth have only one major periodic component of 64.3-year period, which is close to the 50-year cycle of solar activity. (2) Significant resonant periodicities between solar activity and the Earth’s temperature are focused on the 22- and 50-year period. (3) Correlations between solar activity and the surface temperature of the Earth on the long time scales are higher than those on the short time scales. As far as the sunspot number is concerned, its correlation coefficients to the Earth temperature are 0.31-0.35 on the yearly scale, 0.58-0.70 on the 11-year running mean scale, and 0.64-0.78 on the 22-year running mean scale. TSI has stronger correlations to the Earth temperature than sunspot number. (4) During the past 100 years, solar activities display a clear increasing tendency that corresponds to the global warming of the Earth (including land and ocean) very well. Particularly, the ocean temperature has a slightly higher correlation to solar activity than the land temperature. All these demonstrate that solar activity has a non-negligible forcing on the temperature change of the Earth on the time scale of centuries.

Numerical Validation and Comparison of Three Solar Wind Heating Methods by the SIP-CESE MHD Model

We conduct simulations using the three-dimensional (3D) solar-interplanetary conservation element/solution element (SIP-CESE) magnetohydrodynamic (MHD) model and magnetogram data from a Carrington rotation (CR) 1897 to compare the three commonly used heating methods, i.e. the Wentzel-Kramers-Brillouin (WKB) Alfven wave heating method, the turbulence heating method and the volumetric heating method. Our results show that all three heating models can basically reproduce the bimodal structure of the solar wind observed near the solar minimum. The results also demonstrate that the major acceleration interval terminates about 4R(S) for the turbulence heating method and 10R(S) for both the WKB Alfven wave heating method and the volumetric heating method. The turbulence heating and the volumetric heating methods can capture the observed changing trends by the WIND satellite, while the WKB Alfven wave heating method does not.

A tentative study for the prediction of the CME related geomagnetic storm intensity and its transit time

Using 80 CME-ICME events during 1997.1–2002.9, based on the eruptive source locations of CMEs and solar magnetic field observation at the photosphere, a current sheet magnetic coordinate (CMC) system is established in order to study the propagation of CME and its geoeffectiveness. In context of this coordinate system, the effect of the eruptive source location and the form of heliospheric current sheet (HCS) at the eruptive time of CME on the geomagnetic storm intensity caused by CME and the CME’s transit time at the Earth is investigated in detail. Our preliminary conclusions are: 1) The geomagnetic disturbances caused by CMEs tend to have the so-called “same side-opposite side effect”, i.e. CMEs erupt from the same side of the HCS as the earth would be more likely to arrive at the earth and the geomagnetic disturbances associated with them tend to be of larger magnitude, while CMEs erupting from the opposite side would arrive at the earth with less probability and the corresponding geomagnetic disturbance magnitudes would be relatively weaker. 2) The angular separation between the earth and the HCS affect the corresponding disturbance intensity. That is, when our earth is located near the HCS, adverse space weather events occur most probably. 3) The erupting location of the CME and its nearby form of HCS will also affect its arrival time at the earth. According to these conclusions, in this context of CMC coordinate we arrive at new prediction method for estimating the geomagnetic storm intensity (Dstmin) caused by CMEs and their transit times. The application of the empirical model for 80 CME-ICME events shows that the relative error of Dst is within 30% for 59% events with Dstmin≤−50 nT, while the averaged absolute error of transit time is lower than 10 h for all events.

A New Hybrid Numerical Scheme for Two-Dimensional Ideal MHD Equations

We present a new hybrid numerical scheme for two-dimensional (2D) ideal magnetohydrodynamic (MHD) equations. A simple conservation element and solution element (CESE) method is used to calculate the flow variables, and the unknown first-order spatial derivatives involved in the CESE method are computed with a finite volume scheme that uses the solution of the derivative Riemann problem with limited reconstruction to evaluate the numerical flux at cell interface position. To show the validation and capacity of its application to 2D MHD problems, we study several benchmark problems. Numerical results verify that the hybrid scheme not only performs well, but also can retain the solution quality even if the Courant number ranges from close to 1 to less than 0.01.

Simulation of turbulent magnetic reconnection in the smallscale solar wind

Some observational examples for the possible occurrence of the turbulent magnetic reconnection in the solar wind are found by analysing Helios spacecrafts high resolution data. The phenomena of turbulent magnetic reconnections in small scale solar wind are simulated by introducing a third order accuracy upwind compact difference scheme to the compressible two-dimensional MHD flow. Numerical results verify that the turbulent magnetic reconnection process could occur in small scale interplanetary solar wind, which is a basic feature characterizing the magnetic reconnection in high-magnetic Reynolds number ( R M = 2 000-10 000) solar wind. The configurations of the magnetic reconnection could evolve from a single X-line to a multiple X-line reconnection, exhibiting a complex picture of the formation, merging and evolution of magnetic islands, and finally the magnetic reconnection would evolve into a low-energy state. Its life-span of evolution is about one hour order of magnitude. Various magnetic and flow signatures are recorded in the numerical test for different evolution stages and along different crossing paths, which could in principle explain and confirm the observational samples from the Helios spacecraft. These results are helpful for revealing the basic physical processes in the solar wind turbulence.

An improved CESE method and its application to steady-state coronal structure simulation

This paper presents an improved space-time conservation element and solution element (CESE) method by applying a non-staggered space-time mesh system and simply improving the calculation of flow variables and applies it to magnetohydrodynamics (MHD) equations. The improved CESE method can improve the solution quality even with a large disparity in the Courant number (CFL) when using a fixed global marching time. Moreover, for a small CFL (say < 0.1), the method can significantly reduce the numerical dissipation and retain the solution quality, which are verified by two benchmark problems. And meanwhile, comparison with the original CESE scheme shows better resolution of the improved scheme results. Finally, we demonstrate its validation through the application of this method in three-dimensional coronal dynamical structure with dipole magnetic fields and measured solar surface magnetic fields as the initial input.