Space, Telecommunications, and Radioscience Laboratory, Stanford University, Stanford CA, 94305, phone (415) 723-3585, fax (415) 723-9251
Future high altitude spacecraft are expected to carry instruments for new types of photon or particle imaging and for radio sounding. Their missions will be to study the manner in which the components and regions of the Earth's inner magnetosphere interact as parts of the overall dynamic magnetospheric system. During such missions, a candidate region of observation would be the plasmasphere, a relatively high density (~100 el/cc) torus- like inner magnetospheric region whose configuration and dynamics are known to be highly sensitive to disturbance activity in the solar-terrestrial environment. The cutaway view of the magnetosphere sketched in Figure 1a shows the location of the plasmasphere, whose geomagnetic field-aligned outer boundary, the plasmapause, typically extends to several Earth radii at the equator. The factor of ~10 (or greater) difference in density between the plasmasphere and the tenuous plasma beyond is attributed to the comparatively stronger diffusive coupling of the plasmasphere to the dense ionosphere that underlies both regions.
What are the important science questions about the plasmasphere that future imaging and sounding missions will address? Many space scientists consider the plasmasphere to be a phenomenon of modest geophysical importance, one that has long been relatively well measured and understood. In this article I would like to challenge those views, arguing that the plasmasphere is geophysically important and that it offers many exciting and as yet largely unmet challenges to both the experimenter and the theorist. Consider, for example, that after thirty years of study we still do not know:
The features of the plasmasphere that theorists set out to explain nearly 30 years ago were: 1) an asymmetric shape, with a duskside "bulge", and 2) a substantial difference in total plasma density (by a factor of 10-100) between the regions interior to and exterior to the plasmapause. It had also been reported that as a result of some type of plasmasphere erosion process, the plasmasphere radius was typically smaller during periods of enhanced geomagnetic activity. These features were interpreted in fluid dynamical terms as the result of interplay between a regime of Earth-induced corotating plasma flow and a solar- wind-induced convection system. The latter involved antisunward flow along the flanks of the magnetosphere and over the polar cap regions and a generally sunward return flow in the interior regions (e.g. Nishida, 1966; Brice, 1967). Because of an increase in the relative strength of the solar-wind-induced return flow with increasing distance from the Earth, the combined flow would exhibit a separatrix between an inner region in which the lowest energy particles encircle the dipole and an outer regime in which the flow trajectories reach the outer limits of the magnetosphere. A combined flow pattern of this kind is illustrated in Figure 1b.
The inner region, correponding to the plasmasphere, was expected to be relatively dense, since its thermal (~1 eV) component would be in a state of quasi-equilibrium with the underlying ionosphere. In contrast, the thermal plasma of the outer region, the so-called plasmatrough, would be tenuous, because flow to the vicinity of the magnetopause would allow for plasma escape or expansion into high volume, low pressure, regions. There would be insufficient time for a state of quasi-equilibrium with the ionosphere to be reached (in terms of thermal plasma density). Thus the separatrix was interpreted as representing the plasmapause, or boundary between the two regimes.
Of particular interest in the early theory was the fact that since sunward convection and corotation opposed one another in the dusk sector (see Figure 1b), a stagnation point in the combined flow should occur there, and there should be a dawn-dusk asymmetry in the plasmasphere radius due to the outward extension of plasmasphere electric equipotentials or flow lines near dusk and their contraction near dawn. The reported duskside bulge in the plasmasphere thus appeared to be explained. Furthermore, if the intensity of the solar wind-induced flow were increased, portions of the outer plasmasphere would become entrained by that high latitude flow regime and would move along paths leading to the magnetopause. In an eventual steady state, the separatrix would appear closer to the Earth, thus accounting for the observed reductions in plasmasphere radius during magnetically disturbed periods.
In recognition of the basically unsteady nature of geomagnetic activity, modelers soon began to apply the then widely accepted theory to the simulation of time variations in plasmasphere configuration. This was usually accomplished by scaling the intensity of the high latitude flow regime according to some measure of substorm activity (or of the intensity of the solar wind dynamo) and also taking account of the integrated effects of refilling fluxes from the ionosphere (e.g. Chen and Wolf, 1974). In these terms the plasmapause was identified as the instantaneous boundary between flux tubes that had circulated within the magnetosphere long enough to be appreciably filled with plasma and flux tubes that had circulated sufficiently recently from the tail region such that they were as yet unfilled or only partially filled. Figure 2a shows examples of calculated plasmasphere configurations at 6 hour intervals during an initial period of steplike increases in disturbance (times t1 and t2) and subsequent quieting (t3 and t4). Among the effects illustrated are a taillike channel of dense plasma extending sunward in the afternoon sector and eastward rotation and inward spiraling of the taillike feature during quieting.
The foregoing line of interpretive work has provided and today still provides the basis for discussions of the plasmapause phenomenon in introductory texts on magnetospheric physics. Yet our understanding of that phenomenon is surprisingly incomplete; for example, there are no reported observations or model calculations that describe the establishment of a new plasmapause boundary within an existing plasmasphere. Experimenters have found new boundary formation to be observationally elusive, while modelers have been inhibited by a need to depend upon poorly known initial conditions on plasmasphere morphology and in general have not had access to plasma flow models that are sufficiently realistic at the subauroral latitudes of interest. Thus there are serious restrictions on our ability to understand the important phenomenon of plasmasphere erosion and the complex thermal plasma distributions to which erosion evidently leads.
Experimenters have long been challenged by the great size and complexity of the plasmasphere. In recent years, thanks to the rich ISEE-1, CRRES, DE-1, and synchronous satellite records of plasmapause crossings, as well as earlier satelllite records and whistler data from midlatitude ground stations, they have learned much about the various states of the plasma density profile. However, they have not had tools capable of: 1) observing the density profile as it rapidly evolves in time, or 2) mapping global changes with time in the thermal plasma distribution. In spite of such limitations, enough has been learned about the apparent consequences of plasmapause formation and plasmasphere erosion to allow speculation about the circumstances involved therein. For example, the evidence suggests that the formation mechanism is operative on the nightside of the Earth, probably in the premidnight sector and during periods of substorm activity. The mechanism should be capable of producing both "smooth" plasmapause density dropoffs (on radial distance scales of ~150 km or more) as well as profiles with embedded irregularities in the form of secondary peaks or plateaus a few hundred km in width.
The importance of hot plasma dynamics in plasmapause formation is suggested by past reports of a spatial relation between the plasmapause and the inner edge of the plasma sheet (Horwitz et al., 1984). Furthermore, new plasmapause boundaries are often found in the 57-62 degree invariant latitude range, the same range in which latitudinally narrow (~1 degree) westward subauroral ion drifts (SAIDs) are regularly observed at ionospheric heights in the premidnight sector some tens of minutes after the onset of substorms (e.g. Anderson et al., 1993). Differential inward penetration of the nightside magnetosphere by plasma sheet ions and electrons is believed to be important for the development of SAIDs, and whatever their role in plasmapause formation may be, the narrow ~2-4 km/s SAID flows would appear to play an important role in the plasmasphere erosion process that must accompany that formation.
A major challenge to theoretical modelers of plasmasphere dynamics is the development of a realistic description of the subauroral electric field (and associated low energy plasma flow pattern) that results when the solar-wind induced electric field penetrates to subauroral latitudes during disturbed periods. The outer plasmasphere is generally located at subauroral latitudes; hence it is there that processes involving new boundary formation and plasma erosion must be specified. Modelers must take account of narrow flow channels such as those involved in the fast ion drift phenomenon (SAID) noted above and of the coupled magnetosphere-ionosphere interactions that appear to be involved in their development (e.g. Anderson et al. 1993). And they should not neglect instabilities as mechanisms for shedding plasma from the outer plasmasphere. Satellite observations of irregularities within and just beyond the plasmapause region suggest that such mechanisms are operative, as Lemaire (1975) argued many years ago.
Other challenges to modelers arise with regard to plasmasphere erosion and the resultant plasma flow into magnetospheric boundary layers. Observations suggest that some cold plasma of the plasmasphere is indeed entrained and carried to the magnetopause during periods of enhanced disturbance activity, as the MHD models discussed above predict. However, such models fail to predict the manner in which the thermal plasma distribution in the afternoon-dusk sector evolves during such periods. There is a need, for example, to account for a kind of "pileup" of cool, dense plasma in the afternoon-dusk magnetosphere near and beyond synchronous orbit, such that only when very deep quieting occurs does the region become devoid of "outliers" (Carpenter et al., 1993). This pileup suggests that the convective transport of eroded plasma to the magnetopause is far less efficient than has been supposed. Figure 2b shows an ISEE profile in which a variety of outlying dense plasma features were detected. Are these often substantial accumulations of cold plasma inside the magnetopause the result of decoupling of the low and high altitude convection systems, as has been suggested (Carpenter et al., 1993)? A limited study of the morphology of dense outliers has led to the inference that some are connected to the main plasmasphere, like the tail-like extensions in MHD simulations (see Figure 2a), while others are "detached," as suggested in the work on OGO 5 data by Chappell et al. (1971).
Modelers are also challenged to consider and account for a virtually unknown effect in which dense plasma just inside a newly created plasmasphere boundary disappears, apparently by dumping into the underlying ionosphere (Park, 1973). The loss tends to occur in a belt with a sharply defined inner limit, and may involve a reduction in outer plasmasphere density by a factor of up to 3. The amount of plasma thus lost from the plasmasphere is comparable to, if not greater than, the amounts carried outward by bulk flow normal to the geomagnetic field! Is the inward extent of this region of loss related to the depth of penetration of the outer plasmasphere by the hot plasma of the ring current?
Thus far I have stressed total electron density and have mentioned only a few of the challenges presented to the space physics community by the dynamic plasmasphere. There are many other challenges, in areas such as: 1) plasmapause formation in regions exterior to a still well defined plasmapause, 2) energy storage and flow within the plasmasphere, and 3) relations between the plasmapause in electron density and transitions in the low energy ion pitch angle characteristics.
It seems clear that the dynamic plasmasphere remains a theater of discovery as well as an arena for problem solving on many levels. The region is geophysically important; the evolution of the main plasmasphere body appears to involve the physics of hot plasma penetration of the middle magnetosphere during substorms, while the complex outlying features of the bulge region appear to reflect as yet unknown properties of the high latitude plasma flow regime. It is appropriate to focus attention upon the still poorly explored plasmasphere at a time when new tools for remote sensing of the EarthÕs thermal plasma such as EUV imaging and high altitude radio sounding are being considered.
Anderson, P. C., W. B. Hanson, R. A. Heelis, J. D. Craven, D. N. Baker, and L. A. Frank, A proposed production model of rapid subauroral ion drifts and their relationship to substorm evolution, J. Geophys. Res., 98, 6069, 1993.
Brice, N. M., Bulk motion of the magnetosphere, J. Geophys. Res., 72, 5193, 1967.
Carpenter, D. L., B. L. Giles, C. R. Chappell, P. M. E. Dˇcrˇau, R. R. Anderson, A. M. Persoon, A. J. Smith, Y. Corcuff, and P. Canu, Plasmasphere dynamics in the duskside bulge region: a new look at an old topic, J. Geophys. Res., 98, 19243, 1993.
Chappell, C. R., K. K. Harris, and G. W. Sharp, The dayside of the plasmasphere, J. Geophys. Res., 76, 7632, 1971.
Chen, A. J., and R. A. Wolf, Effects on the plasmasphere of a time-varying convection electric field, Planet. Space Sci., 20, 483, 1972.
Horwitz, J. L., S. Menteer, J. Turnley, J. L. Burch, J. D. Winningham, C. R. Chappell, J. D. Craven, L. A. Frank, and D. W. Slater, Plasma boundaries in the inner magnetosphere, J. Geophys. Res., 91, 8861, 1986.
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Nishida, A., Formation of plasmapause, or magnetospheric plasma knee, by the combined action of magnetospheric convection and plasma escape from the tail, J. Geophys. Res., 71, 5669, 1966.
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1. (a) Cutaway sketch (not to scale) of the EarthÕs duskside magnetosphere, showing the plasmasphere. (b) Electric equipotentials and associated streamlines of zero energy plasma flow calculated by combining the potentials associated with a solar wind-induced dawn- dusk electric field and those associated with the EarthÕs corotation electric field.
2. (a) MHD calculation of the equatorial cross section of the plasmasphere at four successive intervals spaced by 6 hours, the top two involving step-like increases in activity levels and the bottom two a period of quieting. Adapted from Kurita and Hayakawa (1985). (b) Equatorial density profile in the afternoon sector obtained by ISEE-1, showing an irregular plasmapause near L=3.5 and a variety of outlying high density structures. The dashed lines are a reference profile based on empirical modeling. From Carpenter et al. (1993).
