[1] We present a comparison between a simple but general model of solar windmagnetosphere-ionosphere coupling (the Hill model) and the output of a global magnetospheric MHD code, the Integrated Space Weather Prediction Model (ISM). The Hill model predicts transpolar potential and region 1 currents from environmental conditions specified at both boundaries of the magnetosphere: at the solar wind boundary, electric field strength, ram pressure, and interplanetary magnetic field direction; at the ionospheric boundary, conductance and dipole strength. As its defining feature, the Hill model predicts saturation of the transpolar potential for high electric field intensities in the solar wind, which accords with observations. The model predicts how saturation depends on boundary conditions. We compare the output from ISM runs against these predictions. The agreement is quite good for non-storm conditions (differences less than 10%) and still good for storm conditions (differences up to 20%). The comparison demonstrates that global MHD codes (like ISM) can also exhibit saturation of transpolar potential for high electric field intensities in the solar wind. We use both models to explore how the strength of solar wind-magnetosphere-ionosphere coupling depends on the strength of Earth's magnetic dipole, which varies on short geological timescales. As measured by power into the ionosphere, these models suggest that magnetic storms might be considerably more active for high dipole strengths. [2] Total region 1 current, I 1 , and transpolar potential, È pc , epitomize solar wind-magnetosphere-ionosphere (SW-M-I) coupling. Progress in understanding this subject can almost be measured by how well the field predicts these quantities. (Region 2 currents, which this paper does not treat, are also an important aspect of the story. In section 7 we discuss how they might affect results presented here.) First models of SW-M-I coupling, reviewed by Reiff and Luhmann [1986], assumed one-way coupling from the solar wind to the ionosphere in which magnetic reconnection at the magnetopause taps a fraction of the solar wind potential across the magnetosphere, È sw , to yield an available magnetospheric convection potential È m . È m is then impressed via equipotential magnetic field lines onto the ionosphere, where it becomes the È pc that generates region 1 currents. The envisioned process was therefore linear. Empirical formulas based on this linear assumption work fairly well, except they tend to overpredict È pc for big values of È sw . This tendency has been called saturation of the transpolar potential at high values [Reiff and Luhmann, 1986;Russell et al., 2000].[3] Hill et al. [1976] presented a model of SW-M-I coupling that manifests saturation intrinsically and at about the observed value. (Hill [1984] developed the implications of the model further. We therefore refer to it as the Hill model.) Saturation is a nonlinear process that, in the Hill model, results from a feedback in which the magnetic field generated by region 1 cu...
Dayside and nightside merging rates usually differ; first one dominates and then the other to maintain long‐term flux balance. The polar cap, defined as the area in the ionosphere penetrated by open field lines, expands when closed field lines open during dayside merging and contracts when open field lines close during tail merging. The patterns of convection, electric field and current for polar cap inflation and deflation differ from those of steady state convection. This report models the flow and electrical parameters of polar cap inflation and deflation. We treat the idealized case of uniform conductivity, a planar ionosphere, and “pure Bz” merging (i.e., no By effect, although we note how the approach used here is readily modified to account for the By effect). For this idealized case, the potential associated with polar cap inflation and deflation is seen to be described by the same equations that govern the two‐dimensional hydrodynamic flow into and out of an expanding and contracting cylinder with a gap in its side. The gap corresponds to the breach in the polar cap boundary through which magnetic flux passes as a result of merging. The calculated potential pattern displays the usual two cell convection configuration, but the foci of the two cells are the edges of the flux gaps. Thus the cells are displaced sunward during dayside merging and tailward during tail merging.
This chapter provides an overview of current efforts in the theory and modeling of CMEs.Five key areas are discussed: (1) CME initiation; (2) CME evolution and propagation; (3) the structure of interplanetary CMEs derived from flux rope modeling; (4) CME shock formation in the inner corona; and (5) particle acceleration and transport at CME driven shocks. In the section on CME initiation three contemporary models are highlighted. Two of these focus on how energy stored in the coronal magnetic field can be released violently to drive CMEs. The third model assumes that CMEs can be directly driven by currents from below the photosphere. CMEs evolve considerably as they expand from the magnetically dominated lower corona into the advectively dominated solar wind. The section on evolution and propagation presents two approaches to the problem. One is primarily analytical and focuses on the key physical processes involved. The other is primarily numerical and illustrates the complexity of possible interactions between the CME and the ambient medium. The section on flux rope fitting reviews the accuracy and reliability of various methods. The section on shock formation considers the effect of the rapid decrease in the magnetic field and plasma density with height. Finally, in the section on particle acceleration and transport, some recent developments in the theory of diffusive particle acceleration at CME shocks are discussed. These include efforts to combine self-consistently the process of particle acceleration in the vicinity of the shock with the subsequent escape and transport of particles to distant regions.
The flux density of ions created by ionization of interstellar neutral particles in the solar system and picked up by the solar wind is calculated as a function of the neutral particles. A very broad maximum occurs at an angle of 0 and a distance that depends on the density and speed of the neutral particles and on the ionization time but is typically in the general region of 10 AU. For atomic hydrogen the flux density is estimated to exceed 104 cm−2 s−1 over the distance range from a few to nearly 100 AU. The velocity space distribution of the interstellar ions is calculated under the assumption of no significant energy diffusion but with inclusion of adiabatic effects as well as a possible strong pitch angle diffusion. The energy spectrum is highly nonthermal and much broader than that of the solar wind ions; under the assumed conditions, interstellar protons are easily distinguishable from solar wind protons by their location in velocity space. If charge exchange is an important contributor to the ionization of hydrogen, the observed local intensity of interstellar protons should exhibit time variations correlated with the density changes of the solar wind stream structure.
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