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2. Scientific Objectives

When energetic ion mass spectrometers were first included on magnetospheric missions their primary scientific role was to aid in the identification of the origin, or origins, of the plasmas [e.g., Shelley et al., 1972]. We now know that the magnetosphere contains a mix of plasmas originating from both the solar wind and the ionosphere, varying with region, geomagnetic activity and solar activity [e.g. Young et al., 1982; Lennartsson and Shelley, 1986; Yau et al. 1985a, 1985b]. However, these results are based on large scale statistical data bases accumulated over periods of many years and show a great deal of variance. Little is known about large scale behavior on short time scales, even on the order of individual substorms.

The orbit of the POLAR spacecraft will carry it through some of the primary regions of the magnetosphere associated with plasma entry and transport. It will frequently cross the auroral zones and the cleft ion fountain regions which are known to be primary sources of ionospheric plasma for the magnetosphere (see Lockwood et al., 1985, Chappell et al., 1987, Yau et al., 1985b]. It will also cross the cusp/cleft region through which solar wind plasma from the magnetosheath gains access to the plasma mantle [Rosenbauer et al., 1975] and possibly to the plasma sheet through magnetotail reconnection. The POLAR spacecraft will spend the largest fraction of its time over the polar cap which is the transport region for ionospheric plasmas from the cleft ion fountain as well as that for the low energy component of the magnetosheath ions entering through the cleft and being convected onto magnetotail field lines. Thus the TIMAS measurements will not only provide frequent snapshots of key source regions for both ionospheric and solar wind plasmas but will provide a nearly continuous monitoring of a principal transport region. These measurements, taken together with similar measurements in the low altitude auroral region by the FAST satellite [Carlson, 1992], in the magnetotail by the Geotail spacecraft, and hopefully in the inner plasma sheet and ring current regions, will be critical to improving our knowledge of the entry and global transport of magnetospheric plasmas on substorm time scales.

As has been amply demonstrated, ion mass spectrometer measurements provide much more information than the simple quantification of ion composition, which is generally accepted as a key indicator of plasma origins. In order to investigate fundamental energization and transport processes it is necessary to transform flux measurements into phase space distributions. This requires a knowledge of the ion mass. Even when one species dominates the total plasma density, e.g. H+ in the magnetosheath, other species may contribute significantly to the flux at some energies. If only energy per charge is determined, the existence of a minor species could be interpreted erroneously as a high velocity or low velocity tail on the distribution. The potential error is most serious when mass density is a relevant parameter, such as in a test of the magnetopause as a rotational discontinuity [Hudson, 1970; Fuselier et al., 1993], since the conversion from flux to mass density goes as M^(3/2). Thus, by misinterpreting a mixture consisting of only 1% O+ and 99% H+ as entirely H<+ the error in mass density is more than 60%.

In the following paragraphs we outline several specific areas of scientific investigation to which the POLAR mission in general and the TIMAS measurements in particular will contribute significantly.

2.1 Plasma Entry

Perhaps the most important applications of mass resolved ion distribution function measurement are in the tests of fundamental physical processes that may depend on mass and/or charge. One clear example of this application is shown in Figure 2, [Fuselier et al. 1991], which gives the velocity distributions for H+, He+ and He2+ measured by the Hot Plasma Composition Experiment on the AMPTE/CCE spacecraft, both inside and outside the magnetopause. This particular set of data was selected because conditions were consistent with quasi-steady state reconnection at a rotational discontinuity. Sonnerup et al. [1990] make the general prediction that in the deHoffmann – Teller frame (i.e. the frame in which the electric field vanishes on both sides of the discontinuity) the bulk flow is field aligned and Alfvenic on both sides of the discontinuity. Specific predictions for the resulting velocity distributions were made by Cowley [1982] based on a phenomenological kinetic description of the reconnection process. His model included the mixture of transmitted and reflected hot, dense magnetosheath ions as well as cold, dense ionospheric components and energetic, low density ring current ions. Without going into detail, one can see that all of these components are individually identified and characterized in Figure 1. These measurements generally confirmed the predictions. No such confirmation could be provided, nor even could the existence of the minor species be inferred, for these conditions without mass per charge separation.

Since mass resolving instruments on past missions were basically two dimensional and had to sequentially scan in both mass and energy per charge, such measurements could be attempted only under special circumstances. Assumptions had to be made about the third dimension in velocity space and the data were subject to time aliasing because of the sequential measurement approach. By contrast, the TIMAS will measure simultaneously the three dimensional velocity distributions for all species; thus most constraints on the selection of conditions other than those necessary for the occurrence of the phenomena of interest will be removed. While the orbit of the POLAR spacecraft will not frequently carry it to the magnetopause, it will regularly pass through the cusp/cleft region where many of the signatures of reconnection as well as the signatures of processes occurring at the bowshock and within the magnetosheath remain [e.g. Shelley et al., 1976]. The nearly continuous monitoring of the near earth upstream solar wind and IMF conditions by the WIND spacecraft will aid significantly in future studies of these processes.

One of the specific studies we will undertake, in conjunction with the other plasma and field measurements, is the investigation of the signatures of flux transfer events (FTE) at high latitude. A previous investigation of FTEs near the subsolar region [Klumpar et al., 1990] revealed ion distributions, ion composition and flow characteristics unique to the FTE, i.e. distinct from the adjacent magnetospheric and nearby boundary layer and magnetosheath plasmas. Ions of both solar wind and ionospheric origin within the FTE exhibited reversed temperature anisotropies relative to the adjacent regions. It will be very useful if these unique features can be identified in candidate FTEs at high latitude and mid-to-low altitude where they should be encountered by the POLAR spacecraft. The fast, three dimensional character of the TIMAS data, together with other advanced measurements should provide definitive answers to the fate of FTEsin the ionospheric region.

2.2 Auroral Processes

On the basis of our current knowledge, both experimental and theoretical, the acceleration of electrons and ions in the auroral region appears to involve multiple fundamental processes, occurring independently, in conjunction with one another, and sequentially. An excellent collection of reviews on this subject are found in Auroral Physics, edited by Meng et al., [1991]. In particular, the contributions that ion composition has and can make to unraveling these processes, is reviewed by Shelley and Collin [1991]. We know that even when both O+ and H+ ions are observed flowing along the field lines with beam-like distributions, consistent with parallel electrostatic acceleration, the energies of the two species vary systematically as shown in Figure 3 taken from Collin et al., [1987]. Relatively detailed distributions obtained by the Energetic Ion Composition Spectrometer on DE-1 (see Figure 4) appear to be consistent with the two stream instability that is set up by the acceleration of H+ and O+ to the same energy through a parallel potential drop [e.g. Reiff et al., 1988].

Much of the energetic ion outflow from the auroral regions appears to be dominated by transverse acceleration and heating that is probably not driven by a two stream instability set up by parallel acceleration [e.g. Kintner and Gorney, 1984]. Moreover it has also been suggested [Bryant, 1992] that parallel electrostatic acceleration is not responsible for the electron and ion beams associated with inverted-V structures. The distinction among the various processes requires detailed measurements of the particle distributions as well as the accompanying wave and field environment. This has been attempted on previous missions but has been hampered by a combination of instrumental limitations. Specifically these have included two-dimensional plasma measurements with limited temporal resolution as well as incomplete instrument complements. The instrumental limitations will be essentially eliminated on the POLAR mission. The primary limitation will be the infrequent encounters with the auroral acceleration region, due to the high apogee, and the lack of a planned two point measurement along the same field line. The former problem will be significantly mitigated by the quality of the measurements that will be obtained. Thus a much larger fraction of events encountered will be amenable to interpretation. Further, since the FAST mission is scheduled to closely parallel the POLAR mission, every effort should be made to maximize coordinated measurements. This should include the selection of initial orbit conditions if this can be accomplished without jeopardizing the prime objectives of the FAST mission. Where coordinated measurements are obtained we plan to amplify on investigations of the type previously carried out with DE-1 and DE-2 [Reiff et al., 1988]. In particular, the availability of simultaneous , three-dimensional distributions of electrons and multiple ion species will provide significantly tighter constraints on the proposed models.

2.3 Ionospheric Ion Outflow and Transport

The DE-1 mission provided excellent coverage of the polar cap regions through which the ions from the cleft ion fountain must be transported, and indeed established the existence of this potentially important plasma source. However, those measurements were severely compromised, not only by the lack of three-dimensional plasma measurements that are particularly critical in a regime where convection velocities can be comparable to streaming and thermal velocities, but by the lack of spacecraft potential control. Both of these problems are alleviated on the POLAR mission. The combination of the TIDE and TIMAS instruments, including significant overlap in energy coverage, will guarantee the complete measurement of ion distributions without degradation due to significant spacecraft potentials relative to the plasma. These complete distributions, which will be obtained over much of the polar cap, will be used as a test of various transport models [e.g. Cladis, 1986].

Last modified February 1996 by Bill Peterson

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