Cedar-GEM Magnetosphere-Ionosphere Coupling Campaign

Introduction

At the GEM 2000 Workshop, the Magnetosphere-Ionosphere Coupling Working Group began the formal process of transitioning to a GEM Campaign. To accomplish this, two MI Coupling Working Groups were formed. W. K. Peterson and T. E. Moore accepted responsibility for Working Group 1, directed toward "Ionospheric Plasma in the Magnetosphere" and B. Anderson and W. Lotko accepted responsibility for Working Group 2, directed toward the Electrodynamics of M-I Coupling. Each working group had two splinter sessions at which critical issues in the identified theme areas were vigorously discussed and a short list of "Challenges" was identified. In addition, the MI Coupling Working Group shared splinter sessions with the Inner Magnetosphere/Storm Campaign and the General Geospace Circulation Model (GGCM) Campaign. These sessions were particularly well attended and served to substantiate the Theme Areas identified by the MI Coupling Working Groups. R. Strangeway presented an MI Coupling Tutorial in one of the GEM plenary sessions. His title: "Outstanding Issues in Magnetosphere-Ionosphere Coupling: The Three Dimensional Ionosphere" further touched upon many of the issues underlying the MI-Coupling Challenges.

Three challenges were identified by each of the working groups. These challenges were identified under the assertion of modelers that global MHD models will advance over the coming yield to include realistic Region 2 currents systems derived from more complete ring current physics. In the following we present a brief description of each of these challenges.

The most readily attained task within this challenge is the development of empirical maps of ionospheric mass in the magnetosphere as functions of solar and solar wind parameters. The raw material for this characterization already exists in the literature and the main task consists of modularizing these results into a form suitable for use in computer simulations. Such a module will deliver ionospheric plasma parameters such as density, velocity, temp, by species, and net mass flux, at any LT and latitude, based upon requests specifying solar and solar wind conditions. The module will resolve only coarse regions, e.g. sunlit auroral zone, dark auroral zone, polar wind (above 60 deg. ILAT) and the plasmasphere will be assumed to be filled.

In a more developed form, the mass characterization could be based upon empirical local relationships given as functions of MHD parameters at the ionospheric boundary, rather than in terms of solar wind conditions. Again, the module could deliver density, velocity, temperature by species, based upon requests specifying the local conditions required by the module. These will likely include such items as solar zenith angle, electrodynamic energy flux, plasma density, precipitation heat flux (electrons and ions), field-aligned current, and fluctuation spectrum.

This challenge has empirical and theoretical forks. To motivate the theoretical efforts and provide a basis for testing its results, statistical databases will be developed to document the impacts of ionospheric plasmas on the magnetosphere. These will consist primarily of maps of global spatial and temporal variability of plasma composition both by density and energy density (pressure). The plasma sheet versions of these maps will be developed in coordination with the Magnetotail Substorm Campaign. The ring current and plasmasphere version of these maps would be developed in coordination with the Inner Magnetosphere/Storm Campaign.

The theory fork of this challenge is the principal short-term focus of this working group. Here the effort is to develop a GGCM enhancement that includes extracted ionospheric plasma as a dynamic mechanical element as well as electrodynamic element. Coordination with the Magnetotail/Substorm and Inner Magnetosphere/Storm Campaigns would be important for the GGCM effort.

A long-term challenge is to model the ionospheric outflows self-consistently within the context of the GGCM. Ultimately, this may require a "global" 3D ionosphere-thermosphere model that would evolve in response to momentum and energy inputs from its upper boundary, which will be provided by the GGCM. It seems clear from the spatial and temporal scales over which the topside ionosphere evolves as each flux tube circulates within the high latitude regions that there will be substantial hysteresis effects (flux tube history and preconditioning). Thus, the ultimate goal must be cooperative code runs in which the ITM codes are run within a context provided by the GGCM. The combined code may be envisioned as a Heliosphere-Geosphere interaction model.

In the near term, there is a need for theory-based local responses to MHD drivers, in a form similar to that targeted in Challenge 2 for empirical local condition responses, i.e., as functions of (P//, J//, T, dP, dV, ...). Among the processes that such a code must take account of are:

1. Frictional (F-region) ion heating (can do now, apart from neutral wind affects).
2. Joule electron heating (FAC heating of electrons).
3. Polar rain effects (adjustments to the ambipolar electric field).
4. Precipitation heating of ions and electrons.
5. Cyclotron, lower hybrid, and other wave heating effects (generating local wave environment from MHD specified conditions will be most challenging aspect here).
6. Auroral acceleration region physics (The production of E_// and associated effects by J_//).

1. Ionospheric Conductance

Ionospheric conductivities, Hall and Pedersen, are critical because they regulate the energy transport to the ionosphere from magnetospheric dynamos. Specifically: the production and depletion of conductivities need to be better characterized; the advection of regions of enhanced ionization must be properly accounted for, e.g. in a manner analogous to the coupled global MHD-TICGM; the structure of conductivity enhancements due to auroral acceleration processes needs to be understood both on the large scale, particularly with respect to gradients, and on the small scale, in association with arcs and surges; the global distribution of conductivities is central to global energy transport but we have only rudimentary knowledge of the distributions based largely on statistical averages which are not adequate for assessing global energy dissipation.

Auroral plasma energization is a major unresolved issue because it acts to modify the conductivities and hence facilitate electrodyamical feedback from the ionosphere, in response to magnetospheric dynamo processes, especially field-aligned currents. To improve the global MHD treatment of these processes, we need to understand the relationship between precipitating electron flux and field aligned currents, their distributions, and other relevant parameters such as the high altitude (above the accelerator) electron temperature. Moreover, to be applicable to global MHD, the precipitating flux needs to be expressed in MHD variables. Describing the electron temperature in terms of single fluid MHD is expected to be particularly challenging. In addition, electron precipitation is not the sole sink for electromagnetic energy into the acceleration region, ion acceleration/heating and upward electron beams account for an additional component of the net dissipation occurring in this region and these processes should be considered as well.

3. Multi-scale Processes

The presence and development of multiple spatial scales is a basic problem as well. Discrete auroral arcs develop filamentary structures and are associated with layered acceleration processes. How these structures arise is not understood. The structuring and dynamics necessarily lead to structured ionization, so treatment of the ionospheric conductivities problem is necessarily related to the multi-scale character of the interaction. The presence of multiple spatial scales raises a fundamental issue of assessing energy dissipation because the dissipation estimated from average distributions of electric field and conductivity is not the same as the average dissipation, i.e. <S ><E>² vs <S E²>. We don't know how much the non-linear nature of dissipation skews our estimates of dissipation or what effect localized regions of intense dissipation affect the dynamics of the system in ways that could not be predicted from the average.

One specific problem was also identified, the model/data discrepancies in cross polar cap potential and its apparent saturation at high solar-wind electric fields. This is related to a basic problem in present global MHD models - the absence of a properly defined region 2 current system. Because these currents are underestimated, there may be overestimation of the cross polar cap potential (with reasonable region 1 currents). The provisional conclusion reached in splinter-group discussions is that the region 2 currents will play a central role in addressing the saturation process as well as reconciling the MHD models. Incorporation of ring-current drift physics and the concomitant region 2 currents in global MHD is underway. However, an outstanding issue that is not being addressed is the characterization of the ionospheric conductivity in the region 2 current region and its dependence on ring current properties.