Xuepu Zhao

Prediction of the interplanetary magnetic field strength

A new model of the coronal and interplanetary magnetic field can predict both the interplanetary magnetic field strength and its polarity from measurements of the photospheric magnetic field. The model includes the effects of the large-scale horizontal electric currents flowing in the inner corona, of the warped heliospheric current sheet in the upper corona, and of volume currents flowing in the region where the solar wind plasma totally controls the magnetic field. The model matches the MHD solution for a simple dipole test case better than earlier source surface and current sheet models. The strength and polarity of the radial interplanetary magnetic field component predicted for quiet time samples in each year from 1977 to 1986 agree with observations made near the Earths orbit better than the hybrid MHD-source surface model (Wang and Sheeley, 1988). The results raise the question of whether coronal holes are the only solar source of the interplanetary magnetic field in the solar wind. If some interplanetary flux originates outside coronal holes, the model can match the observed field using the accepted 1.8 saturation correction factor for λ5250 A magnetograph observations. Requiring open flux to come exclusively from coronal holes requires an additional factor of two.

A coronal magnetic field model with horizontal volume and sheet currents

When globally mapping the observed photospheric magnetic field into the corona, the interaction of the solar wind and magnetic field has been treated either by imposing source surface boundary conditions that tacitly require volume currents outside the source surface (Schatten, Wilcox, and Ness, 1969) or by limiting the interaction to thin current sheets between oppositely directed field regions (Wolfson, 1985). Yet observations and numerical MHD calculations suggest the presence of non-force-free volume currents throughout the corona as well as thin current sheets in the neighborhoods of the interfaces between closed and open field lines or between oppositely directed open field lines surrounding coronal helmet-streamer structures. This work presents a model including both horizontal volume currents and streamer sheet currents. The present model builds on the magnetostatic equilibria developed by Bogdan and Low (1986) and the current-sheet modeling technique developed by Schatten (1971). The calculation uses synoptic charts of the line-of-sight component of the photospheric magnetic field measured at the Wilcox Solar Observatory. Comparison of an MHD model with the calculated model results for the case of a dipole field and comparison of eclipse observations with calculations for CR 1647 (near solar minimum) show that this horizontal current-current-sheet model reproduces polar plumes and axes of corona streamers better than the source-surface model and reproduces coronal helmet structures better than the current-sheet model.

A DATA-DRIVEN MODEL FOR THE GLOBAL CORONAL EVOLUTION

This work is devoted to the construction of a data-driven model for the study of the dynamic evolution of the global corona that can respond continuously to the changing of the photospheric magnetic field. The data-driven model consists of a surface flux transport (SFT) model and a global three-dimensional (3D) magnetohydrodynamic (MHD) coronal model. The SFT model is employed to produce the global time-varying and self-consistent synchronic snapshots of the photospheric magnetic field as the input to drive our 3D numerical global coronal AMR-CESE-MHD model on an overset grid of Yin-Yang overlapping structure. The SFT model and the 3D global coronal model are coupled through the boundary condition of the projected characteristic method. Numerical results of the coronal evolution from 1996 September 4 to October 29 provide a good comparison with multiply observed coronal images.

Changes of the boot‐shaped coronal hole boundary during Whole Sun Month near sunspot minimum

The August 27, 1996, boot-shaped coronal hole is shown to rotate nearly rigidly at a rate of 13.25°/day, greater than the equatorial rotation rate of bipolar magnetic regions such as active regions and plages. The day-to-day variation of the coronal hole border is determined by comparing the rigid rotation projection of the disk-center hole boundary to coronal hole boundaries observed in successive daily coronal images. To determine the influence of the changing photospheric field on the location of the coronal hole boundary, a better approximation of the instantaneous global magnetic field distribution is developed and used as input to a potential-field source-surface model to compute the foot-point areas of open field lines. Day-to-day variations of the coronal hole boundary may be caused by changes of the magnetic field and plasma properties in the corona, as well as by the changing photospheric field.

Effect of coronal mass ejections on the structure of the heliospheric current sheet

The existence of a relatively stable large-scale heliospheric current sheet (HCS) structure near sunspot maximum has recently been questioned [Hundhausen, 1992]. We consider this question here by determining the effect of coronal mass ejections (CMEs) on the spiral characteristics of the interplanetary magnetic field (IMF) and on the HCS. In general, CMEs do not have long-term effects on the location of the HCS. The evidence shows that (1) the coronal streamer belt locally disrupted or blown out by CMEs reforms in a time interval shorter than the lifetime of the HCS structure ; (2) the internal structure of IMF sector boundaries is temporarily changed during the passage of the interplanetary counterpart of CMEs ; (3) even in the Carrington rotation just 1 month after the sunspot maximum of solar cycle 21 the IMF spiral characteristics are maintained, and the calculated sector pattern agrees very well with that observed at 1 AU ; and (4) the fact that the calculated closed field regions correspond to the helmet streamers observed in the February 16, 1980, solar eclipse confirms the validity of the three-dimensional model even at high activity, giving additional confidence in the predicted HCS location. The rapid reformation of disrupted helmet structures may explain the existence of a structured HCS during intervals when CMEs occur frequently and several coronal helmet streamers along the base of the HCS are disrupted or blown out. Ulysses observations at the next sunspot maximum may finally answer the question.

Unique determination of model coronal magnetic fields using photospheric observations

We show that the non-radial field-boundary condition (or the line-of-sight boundary condition) for the Laplacian-like equation developed by Bogdan and Low (1986) is sufficient to uniquely determine the model coronal magnetic field provided the electric currents are horizontal (or zero, the current-free case) at the solar surface as well as in the solar atmosphere between the photosphere and the source surface. The derived recursion formulae for the spherical harmonic coefficients can be used to determine the spherical harmonic coefficients in the solutions of the horizontal current models very efficiently.

The Three-dimensional Coronal Magnetic Field during Whole Sun Month

Combining models and observations, we study the three-dimensional coronal magnetic field during a period of extensive coordinated solar observations and analysis known as the Whole Sun Month (WSM) campaign (1996 August 10-September 8). The two main goals of the WSM campaign are addressed in this paper, namely, (1) to use the field configuration to link coronal features observed by coronagraphs and imaging telescopes to solar wind speed variations observed in situ and (2) to study the role of the three-dimensional coronal magnetic field in coronal force balance. Specifically, we consider how the magnetic field connects the two fastest wind streams to the two regions that have been the main foci of the WSM analysis: the equatorial extension of the north coronal hole (known as the Elephants Trunk) and the axisymmetric streamer belt region on the opposite side of the Sun. We then quantitatively compare the different model predictions of coronal plasma and solar wind properties with observations and consider the implications for coronal force balance and solar wind acceleration.

Time‐dependent MHD modeling of the global solar corona for year 2007: Driven by daily‐updated magnetic field synoptic data

In this paper, we develop a time-dependent MHD model driven by the daily-updated synoptic magnetograms (MHD-DUSM) to study the dynamic evolution of the global corona with the help of the 3D Solar-Interplanetary (SIP) adaptive mesh refinement (AMR) space-time conservation element and solution element (CESE) MHD model (SIP-AMR-CESE MHD Model). To accommodate the observations, the tangential component of the electric field at the lower boundary is specified to allow the flux evolution to match the observed changes of magnetic field. 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 3D MHD model. The simulated evolution of the global coronal structure during 2007 is compared with solar observations and solar wind measurements from both Ulysses and spacecrafts near the Earth. The MHD-DUSM model is also validated by comparisons with the standard potential field source surface (PFSS) model, the newly improved Wang-Sheeley-Arge (WSA) empirical formula, and the MHD simulation with a monthly synoptic magnetogram (MHD-MSM). Comparisons show that the MHD-DUSM results have good overall agreement with coronal and interplanetary structures, including the sizes and distributions of coronal holes, the positions and shapes of the streamer belts, and the transitions of the solar wind speeds and magnetic field polarities. The MHD-DUSM results also display many features different from those of the PFSS, the WSA, and the MHD-MSM models.

Interaction of fast steady flow with slow transient flow: A new cause of shock pair and interplanetary Bz event

The occurrence of the nonspiral magnetic field, high helium density, “cold magnetic enhancement” and counterstreaming suprathermal electron flux in the slow flow around a forward shock indicates that the slow flow is the interplanetary counterpart of the coronal mass ejection (ICME). The characteristics of the field and plasma in the fast flow around the reverse shock are typical for a fast steady flow. Thus the shock pair here appears to be caused by interaction of a fast steady flow with a slow ICME. The fact that the slow ICME possesses a planar magnetic structure and a large −Bz component suggests that the slow ICME may be disconnected from the Sun. It is shown that compression alone appears to be adequate to explain the large southward interplanetary magnetic field component within the shocked slow plasma because of the large southward field component present in the ICME ahead of the forward shock. In addition, a new method to infer the shock angle and Mach number from the observed upstream plasma β and the jump ratios of proton density and total magnetic flux density across a shock is suggested.

Is the geoeffectiveness of the 6 January 1997 CME predictable from solar observations

We present a prediction scheme for specifying the duration and maximum strength of the southward IMF within a magnetic cloud from observations of the disappearing filament associated with the coronal mass ejection and the photospheric magnetic field made near the filament disappearing. Using this scheme we were able to predict that the Earth directed 6 January 1997 coronal mass ejection would be geoeffective. We expected that the southward IMF interval would have a maximum strength of −13±5 nT and a duration of 14±5 hours. This compares favorably with the WIND observations of −15 nT and 13 hours.