Ya-Hui Yang

Models for the size and shape of the earth's magnetopause and bow shock

New models for the size and shape of the Earths magnetopause and bow shock are derived, based on a criterion for selecting the crossing events and their corresponding up-stream solar wind parameters. In this study, we emphasize the importance of accurate interplanetary parameters for, predicting the size and shape of the magnetopause and bow shock. The time lag of the solar wind between the solar wind monitor and the location ofcrossings is carefully considered, ensuring more reliable up-stream solar wind parameters. With this database new functional forms for the magnetopause and bow shock surfaces are deduced. In this paper, we briefly present the preliminary results. For a given up-stream solar wind dynamic pressure Dp, an IMF norths-south compoent Bz, a solar wind β and a magnetosconic mach number Mms, the parameters that describe the magnetopause and bow shock surfaces ro and α can be expressed in terms of a set of coefficients determined with a multi-parameter fitting. Applications of these models to extreme solar wind conditions are demonstrated. For convenience, we have assumed that ro, Bz and Dp retain their units, except in equations where they are normalized by 1 RE(Earth radius), 1 nT and 1 nPa, respectively.

Toward predicting the position of the magnetopause within geosynchronous orbit

Although the average magnetopause is ∼10 R E from the Earth, the magnetopause moves inside the geosynchronous orbit during extreme solar wind conditions. Under these circumstances some geosynchronous satellites suddenly enter the magnetosheath and are exposed to the plasma and fields of the magnetosheath. In this study we evaluate the predictive capabilities of magnetopause location models in forecasting geosynchronous magnetopause crossings. We predict periods during which geosynchronous satellites enter the magnetosheath using the Petrinec and Russell [1996] and Shue et al. [1998] magnetopause location models driven by data from Interplanetary Monitor Platform (IMP) 8. These predictions are then verified with in situ observations from Geosynchronous Operational Environment Satellite (GOES) 5, 6, and 7. We estimate the false alarm rate, probability of detection, and probability of false prediction for the two models. The estimation shows that false alarm rate for a forecast with a 20-min separation cadence is ∼62% (80%) for the Shue et al. [1998] model (the Petrinec and Russell [1996] model). The probability of detection is very high for both prediction models. These results suggest that both models work well in predicting magnetosheath periods for geosynchronous satellites. Predictions from the models provide a prerequisite condition for geosynchronous magnetopause crossings. Further examination of unsuccessful events indicates that preconditioning by the interplanetary magnetic field B z needs to be included in the forecasting procedure for a better forecast. This finding provides us with a guide to improving future magnetopause location models.

CHARACTERISTICS OF THE PHOTOSPHERIC MAGNETIC FIELD ASSOCIATED WITH SOLAR FLARE INITIATION

The physical environment governing the solar flare initiation is not fully understood, although there are significant efforts to address the relationship between magnetic non-potential parameters and early flare signatures. In this study, we attempt to characterize the flare initiation based on the processed Helioseismic and Magnetic Imager vector magnetograms, Atmospheric Imaging Assembly 1600 A, and RHESSI hard X-ray observations. Three flare events, the M6.6 flare on 2011 February 13, the X2.2 flare on 2011 February 15, and the X2.1 flare on 2011 September 6, in two active regions AR 11158 and AR 11283 are investigated. We analyze the source field strength in the photosphere, which is defined as the magnitude of the observed magnetic field deviation from the potential field. It is found that one of the strong source field regions above the magnetic polarity inversion line well connects the initial bright kernels of two conjugate ribbons. The results imply that the distribution of the photospheric source field strength can be used to locate the initiation site of flaring loops regardless of the configuration of pre-flare magnetic fields or the evolution of active regions. Moreover, the field configuration in the strong source field regions tends to become more inclined after flares, which is consistent with the coronal implosion scenario. We also employ a fast method to derive the total current density from the photospheric vector magnetogram in the framework of force-free field. This method can provide fast estimation of photospheric current density within a reasonable accuracy without appealing for the more accurate calculation from a model extrapolation.

Evolution of the magnetic field structure outside the magnetopause under radial IMF conditions

We use the Time History of Events and Macroscale Interactions during Substorms data to investigate the magnetic field structure just outside the magnetopause and its time evolution for radial interplanetary magnetic field (IMF) events. When the magnetic field drapes around the magnetopause in the magnetosheath region, an asymmetric magnetic field orientation in different hemispheres is expected. Our two-case study reveals some conflicts with the predicted draped field configuration in the Southern Hemisphere. The magnetosheath Bz component had a different sign depending on the upstream IMF Bx components polarity at the beginning of the radial IMF intervals. With time, the observed Bz became northward in both cases with increasing positive values through the events. The increasing value of the Bz component may be explained by two possible mechanisms: by a change of the upstream IMF and by a reconnection between magnetosheath and magnetospheric field lines. Our study shows that both mechanisms contributed to the observed changes. Thus, there was a correlation between the change of the upstream IMF conditions and an increase in the magnetosheath northward magnetic field component. The observed formation of the boundary layer near the magnetopause proves that the reconnection process was ongoing at least for a part of the time. We suggest two possible reconnection scenarios: one near subsolar point and another tailward of the one cusp due to lobe reconnection. The asymmetry of reconnection locations causes rearrangement of the magnetic field structure near the magnetopause and turns the observed magnetosheath Bz component even further into positive values.

Low‐frequency electromagnetic cyclotron waves in and around magnetic clouds: STEREO observations during 2007–2013

Wave activities in the solar wind are an important topic and magnetic clouds (MCs) are a common phenomenon in interplanetary space, though waves activities associated with MCs have not been well documented. Based on a survey of 120 MCs observed by STEREO spacecraft during the years 2007–2013, this work studies electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency in and around MCs. For total 7807 ECW events, 24% of them occurred in the regions within MCs while 76% occurred in the regions around MCs. Statistics indicate that ECWs around MCs have higher frequencies, wider bandwidths, and stronger powers relative to the waves in MCs. More ECWs, on the other hand, tend to be related to a plasma with higher temperature, lower density, and larger velocity. In particular, it is found that there exist positive power law correlations between plasma betas and the wave frequencies, bandwidths, and powers. The results imply that the plasma beta should play an important role in determining the properties of ECWs, which is consistent with previous theory studies and the recent simulation results.

Asymmetry of Hard X-Ray Emissions at Conjugate Footpoints in Solar Flares

The chromospheric double hard X-ray (HXR) sources generally appear at the conjugate footpoints of flaring loops with asymmetric flux distributions. The behavior of such HXR footpoint asymmetry should be affected by several effects simultaneously and cannot be attributed to a single effect easily. In this study, we attempt to address the properties of photospheric magnetic fields in the areas coinciding with asymmetric HXR footpoints based on RHESSI observations during 2002-2009. A total of 172 time intervals in 22 flares closed to the solar disk center with available pre-flare MDI magnetograms are investigated. The strong HXR footpoint is found to preferentially (75%) locate at the region with weak magnetic field strength, which is qualitatively consistent with the asymmetric magnetic mirror scenario. The HXR footpoint fluxes become more asymmetric when the footpoints move to the areas with more asymmetric field strength. A feature of asymmetry reversal between different energy ranges is observed in some flares, although no significant energy dependence of footpoint asymmetry is found in our statistical results. We also investigated the possible causes of time-dependent HXR footpoint asymmetry by examining the 2004 November 4 M5.4 flare and the 2004 November 6 M3.6 flare. By comparing the estimated asymmetry quantities with the HXR light curves, the asymmetry reversal in the late period of the M5.4 flare is mainly attributed to the difference of coronal energy release or acceleration processes in different periods, while it is associated with the location changes of HXR footpoints moving to different magnetic field regions in the M3.6 flare.

Implicit predictor-corrector central finite difference scheme for the equations of magnetohydrodynamic simulations

Abstract Nowadays most of supercomputers are based on the frame of PC cluster; therefore, the efficiency of parallel computing is of importance especially with the increasing computing scale. This paper proposes a high-order implicit predictor–corrector central finite difference (iPCCFD) scheme and demonstrates its high efficiency in parallel computing. Of special interests are the large scale numerical studies such as the magnetohydrodynamic (MHD) simulations in the planetary magnetosphere. An iPCCFD scheme is developed based on fifth-order central finite difference method and fourth-order implicit predictor–corrector method in combination with elimination-of-the-round-off-errors (ERE) technique. We examine several numerical studies such as one-dimensional Brio–Wu shock tube problem, two-dimensional Orszag–Tang vortex system, vortex type K–H instability, kink type K–H instability, field loop advection, and blast wave. All the simulation results are consistent with many literatures. iPCCFD can minimize the numerical instabilities and noises along with the additional diffusion terms. All of our studies present relatively small numerical errors without employing any divergence-free reconstruction. In particular, we obtain fairly stable results in the two-dimensional Brio–Wu shock tube problem which well conserves ∇ ⋅ B = 0 throughout the simulation. The ERE technique removes the accumulation of roundoff errors in the uniform or non-disturbed system. We have also shown that iPCCFD is characterized by the high order of accuracy and the low numerical dissipation in the circularly polarized Alfven wave tests. The proposed iPCCFD scheme is a parallel-efficient and high precision numerical scheme for solving the MHD equations in hyperbolic conservation systems.

Understanding Magnetic Cloud Structure From Shock/Discontinuity Analysis

We reexamine the magnetic cloud (MC) event during the period of 21-23 May 2007. In this event, the axis of the MC has a high inclination to the ecliptic plane and the heliospheric current sheet (HCS) happens to be on the ecliptic plane. Therefore, we can use the feature of zero north-south component of interplanetary magnetic field to identify the MC boundaries. Inside the MC, there is an enhanced pressure/density region enclosed by two discontinuities. We verified these discontinuities through multiple spacecraft in-situ observations. The front one is a forward fast shock, which is a quasi-perpendicular shock at STEREO B but a quasi-parallel shock at Wind location. The discontinuity at the rear part of the enhanced pressure region resembles a reverse slow shock. However, we verify it is a tangential discontinuity (TD) using multi-spacecraft observations. Furthermore, we analyze the successive TDs inside the MC based on the TD signature of no normal magnetic field component to estimate the magnetic field morphology along the spacecraft trajectories. A novel method to evaluate the uncertainties of those TDs in this study has been given. It is found that the errors of the TD normal are much smaller than that calculated by conventional methods.

A STATISTICAL STUDY OF FLARE PRODUCTIVITY ASSOCIATED WITH SUNSPOT PROPERTIES IN DIFFERENT MAGNETIC TYPES OF ACTIVE REGIONS

It is often believed that intense flares preferentially originate from the large-size active regions (ARs) with strong magnetic fields and complex magnetic configurations. This work investigates the dependence of flare activity on the AR properties and clarifies the influence of AR magnetic parameters on the flare productivity, based on two data sets of daily sunspot and flare information as well as the GOES soft X-ray measurements and HMI vector magnetograms. By considering the evolution of magnetic complexity, we find that flare behaviors are quite different in the short- and long-lived complex ARs and the ARs with more complex magnetic configurations are likely to host more impulsive and intense flares. Furthermore, we investigate several magnetic quantities and perform the two-sample Kolmogorov–Smirnov test to examine the similarity/difference between two populations in different types of ARs. Our results demonstrate that the total source field strength on the photosphere has a good correlation with the flare activity in complex ARs. It is noted that intense flares tend to occur at the regions of strong source field in combination with an intermediate field-weighted shear angle. This result implies that the magnetic free energy provided by a complex AR could be high enough to trigger a flare eruption even with a moderate magnetic shear on the photosphere. We thus suggest that the magnetic free energy represented by the source field rather than the photospheric magnetic complexity is a better quantity to characterize the flare productivity of an AR, especially for the occurrence of intense flares.