Proposal

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    Intercomparison of Global Models and Measurements of the Martian Plasma Environment


    D. Brain, M. Holmstrom, A. Boesswetter, S. Bougher, S. Brecht, E. Dubinin, A. Fedorov, M. Fraenz, E. Kallio, A. Nagy, M. Liemohn, Y. Ma, R. Modolo, H. Nilsson

    Abstract

    At least eight different modeling groups are presently active in simulating the global Martian plasma environment. The models are used for a variety of purposes, including estimate of the present and past rates of escape of the Martian atmosphere. However, they are seldom compared to observations in the same manner, or to each other. We propose to rectify this situation by undertaking a model challenge activity. The goals of the activity are to:
    Advance our understanding of the basic processes governing the interaction of solar wind plasma with the Martian atmosphere through a combination of modeling and comparison to observations.
    Intercompare the many different models of the global plasma interaction at Mars using (for the first time) identical input conditions. This will help place published data comparisons and science conclusions drawn from the different model results on a common footing.
    Six simulation groups with models ranging from MHD to hybrid have committed to participate in this activity at ISSI. Several additional experts will assist with determining the input conditions for the models, and with comparing the models to spacecraft observations. We will meet at ISSI two times, allowing us to refine the activity and run additional input conditions based on the results of the first iteration. Results of this activity will be presented at meetings such as EGU, EPSC, AGU, and DPS, and are expected to stimulate publication of several manuscripts by the entire group and individual members. Finally, our group will establish a list of desired model runs and data comparisons for future activities in the field.

    Scientific Rationale, Goals, and Timeliness

    The first simulation of the global Martian solar wind interaction was published by Spreiter et al. in 1970. Since this initial gasdynamic model, the last 35 years (and especially the last decade) have seen continued advances in the number, variety, and sophistication of global models of the Martian plasma environment. Simulations today can be classified according to the physical assumptions they employ: MHD, Hall MHD, multi-fluid, hybrid, and test particle. Within these classifications models can be distinguished by whether they are 2D or 3D, the number of species they follow, their implementation, and the assumptions employed at the model boundaries. We count at least eight different modeling groups that have been actively simulating the Martian interaction in the past several years, with one or two additional groups coming on line now. These are: MHD (Ma/Nagy, Terada); Hall MHD (Ma/Nagy); multi-fluid (Harnett/Winglee); hybrid (Brecht/Ledvina, Modolo/Chanteur, Boesswetter/Motschmann, Kallio/Liu, Terada/Shinagawa); test particle (Liemohn/Fang).

    These models have been used for a variety of purposes. They have been used to study atmospheric escape rates [e.g. Ma et al., 2002; 2004; Modolo et al., 2005; Boesswetter et al., 2006; Brecht and Ledvina, 2007; Kaneda et al., 2007; Harnett and Winglee, 2006] the structure and topology of magnetic fields in the Martian system [e.g. Brecht,, 1997; Harnett and Winglee,2005, 2007; Kallio et al., 2008], the influence of the solar wind on the ionosphere [e.g. Ma et al., 2002, 2004] the global shapes of plasma boundaries [e.g. Liu et al., 1999, 2001; Brecht et al., 1991, 1993; Boesswetter et al., 2004], and particle transport near Mars [e.g. Liemohn et al., 2006]. They can be used to simulate the interaction considering conditions that may have existed in the past to trace the evolution of the system over history [e.g. Barabash et al., 2007]. In short, models are powerful tools that can be used to study the effects of different drivers on the Martian system (i.e. variability), reveal the physical processes important in the interaction, and place existing observations in context.

    A number of challenges face simulationists as recent spacecraft results are analyzed and digested. Foremost among these is to identify the source of discrepancies between similar models. For example, several groups run 3D multi-species simulations. Yet the predicted atmospheric escape fluxes differ by at least an order of magnitude for similar input conditions. The reasons for this difference lie in the different computational schemes and model resolution, and differences in the methods for addressing the ionospheric lower boundary. Secondly, the models must be compared more rigorously to spacecraft observations in order to determine where the model assumptions are valid. This includes comparisons to individual orbits of spacecraft data for a variety of different conditions and geometries, and comparison to statistical results derived from many orbits of observation. These issues must be resolved before the simulations can reliably and believably be applied to the problems mentioned above,.

    An effective way to address the issues above is for the community to undertake a model challenge activity where different groups run their models for identical input conditions and compare the results. "Model challenge" activities are common in other disciplines [e.g Birn et al.,, 2001], and provide valuable opportunity to both find errors in the models and probe the physics responsible for differences between the models. We propose to undertake such an activity at ISSI, with involvement from the modeling community (including representatives at ISSI from six different groups), the data community (including representatives from ion, electron, and magnetic field instruments on Mars Express and Mars Global Surveyor), and experts in the Martian upper atmosphere and plasma environment.

    The two main goals of this activity, stated in the abstract, are to advance our understanding of the physical processes governing different regions of the Martian plasma interaction and to intercompare the different models for identical input conditions. These goals will be accomplished by having several different Mars global plasma models run identical test cases, comparing the results to each other and to spacecraft data. Table 1 identifies candidate test cases to run, and a minimum set of comparisons to be made. Note that each comparison addresses specific science questions pertaining to the goals of our activity. With these comparisons in hand, we will identify common features of the different simulation results, and key differences. We will use this information to contrast the influence of model assumptions on structure and plasma flows in the interaction region in order to identify the physical processes governing different parts of the interaction. We will further compare the results from the different sets of input conditions to identify their influence (e.g. of solar activity) on the plasma environment, comparing also to observations.

    Table 1: Possible test cases and comparisons for the model challenge
    Sample Test Cases Sample Comparisons / Science Questions Addressed
    • Solar maximum (equinox)
    • Solar moderate (equinox)
    • Solar minimum (equinox)
    • Solar minimum (aphelion)
    • Solar maximum (perihelion)
    • 1D cuts of pressure (dynamic, thermal, magnetic)
      How and where is pressure converted in the interaction region?
    • 2D cuts of solar wind and planetary particle density
      What are the pathways for particle escape / penetration?
    • Particle fluxes along a spacecraft orbit trajectory
      Can spatial and temporal effects be separated in observations?
    • Total atmospheric escape flux
      What are the error bars on model predictions of escape?

    A model challenge activity is (in our opinion) long overdue for the Mars plasma community. With the recent spacecraft datasets from Mars Express, it has become increasingly possible to apply a combination of observations and models toward a broad understanding of the physics of solar wind interactions with unmagnetized planets. And with the recent proliferation of published model results for Mars it has become increasingly important that standard methods of comparison be established for the models. A precursor model challenge activity was undertaken for the recent AGU Chapman Conference of the Solar Wind Interaction with Mars (SWIM), involving four of the same simulation groups participating in this proposal. This activity was very well received at the meeting, with the understanding that it was just the start of this important effort that will now be pursued in a more concentrated and organized manner. Initiation of an ISSI activity now, while there is still momentum and enthusiasm for this effort in the community, is in our opinion very important.


    2D cuts in the noon-midnight plane of escaping oxygen fluxes from Mars, simulated for similar input conditions by an MHD model (left - Y. Ma, 2008) and hybrid model (right - E. Kallio and K. Liu, 2008). The color table ranges from .003-30 cm-3. What are the reasons (physics and/or implementation) for the striking differences?
    Finally, we anticipate that this activity will have benefits beyond the stated goals. First, it will provide seed ideas for future comparisons and future problems for the individual modelers to tackle. Second, it will prompt scientists affiliated with spacecraft instruments to think about the different data products (in some cases requiring combinations of instrument data sets) that would be most useful for global modelers. Third, this activity will establish a set of baseline input conditions and data products for comparison with any new model of Mars. Finally, our exercise will benefit communities simulating plasma interactions at other unmagnetized bodies such as Venus, Titan, or comets. This includes both adoption of standard comparison data products and investigation of the physics governing the Martian interaction as applied to other bodies.

    Expected Output

    The main output of the proposed activity will be a series of scientific manuscripts submitted for publication to reputable journals in planetary science or space physics (candidate journals include Planetary and Space Science, Icarus, JGR, Annales Geophysicae, and Space Science Reviews) and presentations at scientific meetings such as AGU, EGU, DPS and EPSC. We anticipate two publications authored by our entire group outlining the model challenge and the main results. Additionally, it is expected that the modeling and data analysis will lead to separate publications and presentations from individual team members or modeling groups.

    A final outcome of meeting two will be an itemized list of major results of our study, and a prioritized list of data comparisons and model runs should the activity be continued in the future. This information will be maintained on the Team website at ISSI.

    Value Provided by ISSI

    A precursor model challenge activity was initiated at the recent Chapman Conference on the Solar Wind Interaction with Mars (SWIM), and included participation from six modeling groups. Remote coordination of the activity was challenging and inefficient, requiring many email exchanges in order to establish input conditions and perform the model comparisons. In the end, identical input conditions were not run for every model. In addition, there was no time for discussion of the activity among all the participants, so that little insight was gained about the underlying physics responsible for model differences. It became very clear at the SWIM meeting that the goals of this activity are best accomplished when all participants are in the same location at the same time for a period of at least several days, and have no outside interruptions. An ISSI Team meeting provides these conditions. The two meetings will allow the group to have detailed discussion of the model assumptions and implementation, discuss the similarities and differences in the data comparisons as a group, and probe the results and perform new comparisons in "real time". These benefits are simply not possible at larger meetings.

    Team Membership

    Table 2: Mars Plasma Environment Model Challenge Team
    Team Member Institution Nationality Role
    Dave Brain UC Berkeley SSL USA Co-leader
    Mats Holmstrom IRF Kiruna Sweden Co-leader
    Andrew Nagy U. Michigan USA Senior advisor
    Yingjuan Ma * UCLA USA MHD modeling
    Alex Boesswetter * ITP Braunschweig Germany Hybrid modeling
    Steve Brecht Bay Area Research USA Hybrid modeling
    Esa Kallio Finnish Meteorological Inst. Finland Hybrid modeling
    Ronan Modolo* IRF Uppsala Sweden Hybrid modeling
    Mike Liemohn U. Michigan USA MHD + test particles
    Steve Bougher U. Michigan USA Atmospheric input
    Eduard Dubinin Max Planck Germany Data comparison
    Andrei Fedorov CESR France Data comparison
    Markus Fraenz Max Planck Germany Data comparison
    Hans Nilsson IRF Kiruna Sweden Data comparison

    The team has three basic sub-groups. A leadership team consisting of Dave Brain, Mats Holmstrom, and Andy Nagy will coordinate the model inputs and comparisons, data comparisons, and group discussion while at ISSI. A modeling team consists of representatives from six different modeling groups (Boesswetter, Brecht, Kallio, Liemohn, Ma, and Modolo) who will be responsible for running the simulations and extracting relevant information from the model results. A support team consisting of Bougher, Dubinin, Fedorov, Fraenz, and Nilsson (with assistance from Brain and Holmstrom) will provide atmospheric and ionospheric input conditions for the activity, extract spacecraft observations for comparison to model results, and compare the different model results using common display methods.

    In addition to these team members, two other simulation groups have expressed interest in being involved in this activity without attending the meetings at ISSI. These are Naoki Terada (MHD model) and Erika Harnett (multi-fluid model). There may be other simulation groups that opt to participate in this manner as well. We do not rely on their participation to make this activity successful, but will welcome additional input from their models if they are able to supply it by the time of each meeting, with the philosophy that the more active models that can be intercompared using identical input conditions the better it is for the community as a whole.

    Project Schedule

    Our project has five phases centered around two meetings at ISSI. Prior to Meeting 1 the entire team will participate in choosing appropriate input conditions for the models to run (2-3 cases). The modeling team will run the simulations and extract information for comparisons, while the support team will choose relevant spacecraft results for comparison. The leadership team will gather the results and perform the comparisons.

    Meeting 1 will last four days at ISSI, preferably in Spring 2009. During this meeting the support team will first present the chosen model inputs and relevant spacecraft observations. The modeling team will present basic information about each model (assumptions, limitations, and implementation choices) and results of the model runs. The leadership team will present the results of the intercomparison between models and spacecraft data. The entire team will then discuss the results and underlying physics revealed for different regions, perform new data comparisons in response, and choose 2-3 new input conditions to be run for the next meeting.

    Between meetings the support team will choose relevant spacecraft observations for the new input conditions, and the modeling team will run the new simulations and extract the results for comparison. The leadership team will gather the results and perform comparisons.

    During Meeting 2 (Fall 2009 or Spring 2010 preferred) the leadership team will review the results and outcome of Meeting 1 and the different teams will present the outcomes of the tasks undertaken between meetings (support team: input conditions and spacecraft observations; modeling team: model results; leadership team: model comparisons). Then the entire group will discuss the results and the underlying physics revealed by the comparisons, perform new comparisons as necessary, and create a list of accomplishments and priorities for the future.

    After Meeting 2 the entire team will refine the comparisons if necessary, submit a summary group manuscript for publication, and interested parties will submit supporting publications.

    We have included an expert in modeling of the plasma interaction at Mars as part of our team (Andrew Nagy). However, we welcome addition of other experts at ISSI's discretion.

    Required Facilities

    Our group requires a room that can seat all participants, a computer, projector, and screen so that the results can be presented and discussed, and internet access (preferably wireless) for all participants. Other facilities that are not strictly required (but are preferred) include: a white board, easel with paper, or second computer projector and screen; and a telephone with conference capability (videoconferencing options may also be desirable).

    Financial Support

    We request financial support to cover per diem and accommodation for 14 participants to attend two week-long meetings at ISSI (28 man-weeks). Additionally, we request travel costs to and from the meetings for the team co-leaders - Mats Holmstrom and Dave Brain.

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