P. Song

A numerical study of solar wind—magnetosphere interaction for northward interplanetary magnetic field

The solar wind-magnetosphere interaction for northward interplanetary magnetic field (IMF) is studied using a newly developed three-dimensional adaptive mesh refinement (AMR) global MHD simulation model. The simulations show that for north-ward IMF the magnetosphere is essentially closed. Reconnection between the IMF and magnetospheric field is limited to finite regions near the cusps. When the reconnection process forms newly closed magnetic field lines on the day side, the solar wind plasma trapped on these reconnected magnetic field lines becomes part of the low-latitude boundary layer (LLBL) plasma and it convects to the nightside along the magnetopause. The last closed magnetic field line marks the topological boundary of the magnetospheric domain. When the last closed magnetic field line disconnects at the cusps and reconnects to the IMF, its plasma content becomes part of the solar wind. Plasma convection in the outer magnetosphere does not directly contribute to the reconnection process. On the dayside the topological boundary between the solar wind and the magnetosphere is located at the inner edge of the magnetopause current layer. At the same time, multiple current layers are observed in the high-altitude cusp region. Our convergence study and diagnostic analysis indicate that the details of the diffusion and the viscous interaction do not play a significant role in controlling the large-scale configuration of the simulated magnetosphere. It is sufficient that these dissipation mechanisms exist in the simulations. In our series of simulations the length of the magnetotail is primarily determined by the balance between the boundary layer driving forces and the drag forces. With a parametric study, we find that the tail length is proportional to the magnetosheath plasma beta near the magnetopause at local noon. A higher solar wind density, weaker IMF, and larger solar wind Mach number results in a longer tail. On the nightside downstream of the last closed magnetic field line the plasma characteristics are similar to that in the magnetotail, posing an observational challenge for identification of the topological status of the corresponding field lines.

A model of solar wind–magnetosphere–ionosphere coupling for due northward IMF

Abstract A solar wind–magnetosphere–ionosphere coupling model for due northward IMF is proposed. The magnetosphere couples with the solar wind through reconnection nightside of the cusps. Other than the two small regions where reconnection takes place, the magnetosphere is closed. There are three plasma regions in the magnetosphere. The inner core is dominated by corotation. The outer magnetosphere contains two convection cells, and maps to the ionospheric viscous cells and Region I currents. The boundary layer and magnetotail region consists of a pair of flow channels, and maps to the ionospheric reverse cells. The three regions are separated by separatrix surfaces. Energy coupling across the surfaces can be facilitated by non-ideal-MHD processes such as ionospheric coupling, viscous and diffusive interactions, and waves and instabilities, although only the ionospheric coupling is essential to the model. This model is consistent with most established characteristics from observations and MHD computer simulations in both the ionosphere and magnetosphere. There are two specific features that need to be further confirmed from observations. The model expects that the ionospheric NBZ currents and reverse cells are maximized in the region sunward of the pole near the dayside cusps, and that in the tail there is a region which separates the earthward and tailward flows although the field and plasma characteristics are magnetospheric on both sides.

On the processes in the terrestrial magnetosheath 1. Scheme development

We propose a new method to study the structure of the magnetosheath and thereby determine the underlying processes that create this structure. This method provides a systematic means of separating perturbations due to the solar wind variations from those generated within the magnetosheath. As a result, we are able to study the magnetosheath processes as well as the dynamic solar wind-magnetopause interaction. We use the solar wind measurements from an upstream monitor as the input to the gasdynamic convected field model and then compare the model output with the in situ magnetosheath observations. We introduce three parameters to scale the model prediction to match the timings of the magnetopause crossing, bow shock crossing, and upstream variations. With this procedure the relationship between the upstream measurements and the magnetosheath observations and the location of the magnetosheath satellite relative to the magnetopause and bow shock boundaries are highly constrained. We then introduce a series of normalization procedures that provide the means to remove the effects of the solar wind variations. The systematic differences between the model prediction and observation indicate physical processes that are not included in the gasdynamic model. An application of this approach is presented in a companion paper.

The extreme compression of the magnetosphere on May 4, 1998, as observed by the POLAR spacecraft

Abstract On May 4, 1998 the velocity and density of the solar wind were high and the interplanetary magnetic field was strong and southward. The POLAR spacecraft crossed the dayside magnetopause well inside geosynchronous orbit, at 5.3 R E and a solar zenith angle of 19°. After this crossing, POLAR spent most of the rest of its outbound orbit in the magnetosheath and for brief periods crossed into the solar wind at distances from 7.3 R E and a solar zenith angle of 32° to a distance of only 8.5 R E and a solar zenith angle of 45°. This corresponds to subsolar distances of only 6.8 to 7.5 R E for the shock. During this very disturbed period of time, predictions of the locations of the magnetopause by both Shue and co-workers and by Petrinec and co-workers indicate extreme distortions of the magnetopause location. Because of the importance of such events to the understanding of space weather, we recommend that this event be pursued as a special IACG 2 campaign.

On the processes in the terrestrial magnetosheath: 2. Case study

We test a new scheme to study the magnetosheath. The scheme uses the solar wind measurements as the input into the gasdynamic convected field model, and the model output is compared with magnetosheath observations. In our four test cases there is a significant overall success in the model prediction. This scheme works better than other methods in magnetosheath studies and is potentially useful for space weather forecasts and nowcasts. The direction of the magnetic field is modeled most accurately. The prediction of the size of the magnetosphere is accurate within a few percent. The predicted thickness of the magnetosheath is accurate up to 90%. With a double-normalization procedure developed in this study, we are able to separate the processes intrinsic in the magnetosheath from those due to large-scale upstream temporal variations. The test cases confirm the existence of a compressional front one third of the distance from the magnetopause to the bow shock near the stagnation streamline. The magnetosheath density profile near the stagnation streamline is consistent with the models that add a compressional front between the two depletion processes described by the plasma depletion model. A major unexpected feature is that the magnetosheath flow pattern is very different from that described by the model and maybe by most other models, including MHD models. The magnetosheath flow near the stagnation streamline does not slow down gradually toward the stagnation point. It moves rapidly until reaching a very small region near the magnetopause.

Characteristics of the exterior cusp for steady southward interplanetary magnetic field: Interball observations

High-latitude observations of plasma flow across the magnetopause are made by the Interball-Tail satellite near the polar cusp for steady southward and dawnward interplanetary magnetic field. Three-dimensional proton distribution functions measured in this region contain the features predicted by the existing reconnection models. D-shaped distributions in magnetospheric parts of the field lines with cutoff near zero velocity parallel to the magnetic field and tablet-shaped distributions tailward from the cusp are observed and can be interpreted in terms of a gradually diminishing ion injection. Besides these predicted signatures, a new type of proton distribution with large pitch angles is also observed. We present observations of suprathermal ions and electrons with peaks at ∼ 90° pitch angles on the high-latitude magnetospheric field lines adjacent to the magnetopause. These particles show a sharp change in fluxes on the magnetospheric side, indicating that they are probably quasi-trapped in the cusp field. We also present the results of a simulation that uses the Toffoletto-Hill model of the open magnetosphere to trace the particle transport from the magnetosheath to magnetospheric locations of Interball. The simulation successfully reproduces the main predicted features of distribution functions as well as distributions peaked at large pitch angles. The results show that for steady southward interplanetary magnetic field with a large dawnward component, the entry of magnetosheath plasma into the magnetosphere can be described in terms of open topology of field lines near the high-altitude cusp.

Polar-Interball coordinated observations of plasma and magnetic field characteristics in the regions of the northern and southern distant cusps

[1]xa0The structure of the high-latitude magnetosphere near the polar cusps is studied by using coordinated observations from the Polar and Interball satellites. While Polar surveyed the northern polar cusp region, Interball sampled the high-altitude region in the vicinity of the southern cusp in similar local times. A “magnetic turbulent region” that is distinct from the magnetosphere and magnetosheath and consists of recurrent small-scale events is observed in both hemispheres. The small-scale events often contain a steep ramp followed by a wave train. The events are characterized by a steep increase in the ion energy followed by dispersed velocity filter features. Kinetic and fluid characteristics of ions and a test of the Walen relation show that the events are associated with reconnection processes. S-shaped polarization of the magnetic field variations is a characteristic feature in many cases indicating the existence of intermediate shocks. The change in the low-energy cutoff of injected ions indicates that the reconnection site is about a few Earth radii from Polar. Reflected ions on the reconnected magnetospheric field lines, which are not predicted by simple reconnection scenarios, are also observed. Coexistence of transmitted and reflected ions gives rise to a “stagnant” plasma on the magnetospheric field lines. Characteristics of ions and electrons strongly support the suggestion that the field lines threading the plasma in these regions, which extend from the magnetosheath to the magnetosphere, are connected, at least at one end, with the Earth. In other words, the boundary layer is located, at least partly, on reconnected field lines. In contrast, a turbulent region in the indentation of the cusps has been thought to be in the magnetosheath and not to be connected with the Earth. Although the events observed by both satellites have a certain similarity, the probability of conjugated events being recorded by both satellites is small. It is believed that the visible similarity between the observations from the two hemispheres is due to structures that are characteristic features of a single transient reconnection event.

Structure of the flank magnetopause for horizontal IMF: INTERBALL 1 observations

A case study is presented of a flank duskside magnetopause crossing by the INTERBALL 1 spacecraft during an interval of duskward horizontal interplanetary magnetic field. There is strong evidence that reconnection takes place in the flank magnetopause, where the draped sheath field has a large shear relative to the magnetospheric field. In our case the satellite was on the duskside above the equator. The assumed reconnection site is on the duskside of the northern flank slightly sunward at higher latitudes with respect to the satellite. The observations show energetic particles streaming along the field in the magnetosheath from the reconnection site, as expected. The deHoffman-Teller frame analysis shows that propagation of the magnetopause perturbations originates from the derived reconnection site. Observations are, in general, consistent with reconnection models that are based on the one-dimensional assumption. However, there is some significant deviation from the model prediction. We attribute the deviation to the presence of sheared plasma flow. The magnetopause in this case could not be described as a simple one-dimensional rotational discontinuity.

Shape of the low-latitude magnetopause: Comparison of models

Abstract The location and shape of the magnetopause are among the important parameters in space physics because they specify the size of the magnetosphere and respond to the physical processes occurring therein. In the past few years, several empirical models of the magnetopause shape have been developed using large in situ data sets of magnetopause crossings. These models were derived from best-fits to observed magnetopause locations; however, the data sets, the functional forms of the magnetopause, and the specific dependence of the shape on the upstream solar wind conditions used by these models are different, so are their ranges of validity. In this paper, we provide a comprehensive review of these models, compare the differences among them, and discuss their limitations. In addition, we also show the results of validation of these models for space weather forecasts using the January 1997 magnetic cloud event.

Plasma characteristics of high-altitude cusp for steady southward-dawnward IMF

Abstract High-altitude observations of plasma flow across the magnetopause are made by the Interball-Tail satellite near the polar cusp for steady Bz