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Energy, mass, and momentum entering the magnetosphere from the solar wind must cross the magnetopause. The magnetopause and associated boundary regions exhibit a complicated, multi-layered structure, upon which waves and transient events are often superimposed. The occurrence patterns of the various layers, waves, and transient events as functions of solar-wind conditions and downstream distance are poorly understood. Nor do we understand how these features connect and map to the ionosphere. The objective of this study is to use simultaneous observations of the solar wind, the near-Earth region, low-altitude phenomena, and the distant tail to address these issues.
There is a boundary layer just inside the magnetopause where magnetosheath-like plasma is found. At low and high latitudes the boundary layer is called the low-latitude boundary layer (LLBL) and high-latitude boundary layer (HLBL), or mantle, respectively. These boundary layers extend far into the magnetotail, where they represent potentially important sources of plasma and momentum. The overall configuration of these boundary layers in the tail, and the structure within them, are sensitive functions of conditions in the solar wind and particularly the interplanetary magnetic field (IMF).
Some previous research has suggested that on the dayside the LLBL tends to be thicker for northward IMF while the HLBL becomes thicker for southward IMF. However, there have been conflicting results on this point, both from theoretical and experimental work. Resolution of this controversy will help to resolve questions such as what fraction of the LLBL and HLBL are threaded by open field lines and what are the relative roles of reconnection and other processes such as diffusion and direct plasma penetration in the population of the magnetospheric boundary layers.
The dayside LLBL is observed to be highly structured both spatially and temporally. It is not known how far this structure extends into the magnetotail. Nor is it known how much of the structure arises from structure propagating with the solar wind versus that arising from the various coupling processes along the magnetopause.
It is important to measure how the thickness of the various boundary layers and their fine structure evolve with downstream distance and as functions of interplanetary conditions. These results will help to distinguish the relative roles of the low and high latitude boundary layers in supplying plasma to the plasma sheet. The fine structure of the boundary layers also evolves with downstream distance and with time following events. The transport properties within the boundary layers may depend on the development of large- and small-scale turbulence in these regions.
Strongly accelerated plasma flows, which may be interpreted as signatures of reconnection at the near-tail magnetopause, have been observed with International Sun-Earth Explorer (ISEE) in the presence of large magnetic-field shears along the flanks of the magnetopause. The evidence for reconnection is also strengthened by the sequential observation of oppositely-directed flows as the supposed reconnection site moves past the spacecraft. If reconnection does occur in this region, it may represent an important process for populating the LLBL and HLBL in the magnetotail with solar-wind plasma.
While it is known that the magnetosphere is magnetically open, with field lines that extend into the interplanetary medium, the characteristics of the regions of the tail magnetopause through which the open field lines pass are not known. Different merging models of the magnetosphere predict various configurations; for example, narrow "windows" of open field lines at different positions on the high-latitude tail magnetopause were suggested by David Stern in 1973. The open field regions of the magnetopause will, in any case, depend sensitively on the orientation of the IMF. Determination of the open-field regions will require the measurement of normal components of the magnetic field along the magnetopause at as many locations as possible and for as many different IMF orientations as possible, as well as determination of whether mantle plasma is present in the lobe adjacent to the high-latitude magnetopause.
Large-scale plasma flow vortices are a common feature of the near-Earth (<20 Re) plasma sheet. Although it has been suggested that these vortices arise from a Kelvin-Helmholtz instability at the magnetopause or from the earthward edge of the LLBL, this hypothesis has never been empirically demonstrated explicitly. Further, it is not known how these vortices evolve with increasing distance down the tail. A determination of the origin and evolution of these vortices is fundamental to our understanding of the solar wind's interaction with the magnetosphere.
Transient events, such as Kelvin-Helmholtz waves (at both the inner and outer edges of the LLBL), Flux Transfer Events (FTE's) and pressure-pulse- driven magnetopause motion, along with quasi-stationary reconnection, appear to facilitate the entry of mass, momentum, and energy from the solar wind into the magnetosphere. In future work, such events should be identified along with their dependence upon solar wind conditions, latitude, and downstream distance. Also important in assessing the entry of plasma and momentum from the solar wind into the magnetotail will be studies of the magnetosheath/magnetosphere interface at the flanks of the tail. Such studies will help to solve an important issue about the possibility of reconnection along the magnetopause flanks and its evolution with downstream distance.
Observations of sun-aligned arcs and cross-polar "theta" auroras in the polar caps by the Defense Meteorological Satellite Program (DMSP) and Dynamics Explorer (DE)-1 spacecraft led to the suggestion that the plasma sheet often develops a bifurcation that extends in north-south z directions to the HLBL. Subsequent measurements by ISEE lent some support to this prediction. Detailed study of this possibility is an important objective because it may represent the basic interaction mode for strongly northward IMF conditions.
The distribution function of electrons in the solar wind has an enhancement at energies above ~80 eV along the magnetic field away from the Sun. This enhancement carries the solar-wind heat flux. These highly beamed electrons, often called the "strahl," can enter the magnetosphere in the tail lobes and polar caps. The strahl evolves in the strong magnetic field at the polar cap into a broad pitch-angle distribution. The polarity of the IMF has been observed to correlate with the polar cap that is receiving rain, so that a study of this phenomenon in detail is important in understanding the entry of energy into the polar magnetosphere. Although the outline of this process is understood, more and better observations are required to quantify its details.
For the purposes of this study, accurate observations of the solar wind will be essential in order to estimate the location and shape of the magnetotail, as well as to determine the input IMF and occurrence of transient features. These observations will be supplied by the WIND spacecraft. Because transient features of the solar wind (T ~ 1-10 min) may have short spatial scale lengths, and because plasma parameters in the foreshock may differ greatly from those in the solar wind, it will be preferable to use WIND observations when that satellite is located directly upstream of Earth on the Earth-Sun line.
The IMP-8 spacecraft will supplement these measurements and provide additional information. In addition, IMP-8 provides field measurements when crossing the geotail at 30-40 Re downstream of Earth. INTERBALL provides field and particle information, including composition, in the LLBL, HLBL, lobes, and plasma sheet in the near-Earth region. With its subsatellite following INTERBALL-TAIL in a similar orbit, it plays an additional important role in studies of these two regions, since the satellite-subsatellite separation can vary between 1,000 and 10,000 km, and the pair of spacecraft can measure the different boundary thicknesses.
GEOTAIL will provide a complete set of high-time-resolution measurements of plasma, fields, and energetic particles. Its orbits near the ecliptic plane, combined with the Earth's motion and the wind-sock effect, allow it to traverse all the required regions of the magnetotail, reaching as far as 220 Re down the tail. Most of the objectives of this campaign call for the highest possible temporal resolution of the INTERBALL and GEOTAIL measurements. In general, plasma and particle instruments should remain in one data-taking operational mode rather than changing modes frequently. Furthermore, accurate empirical and/or magnetohydrodynamic (MHD) models of the geotail and magnetosheath will be essential in determining the position of the GEOTAIL spacecraft relative to the magnetopause and tail center. The models, in turn, will be evaluated and revised in response to GEOTAIL observations.
The first step in the study is to use the INTERBALL magnetotail and subsatellite pair to map occurrence patterns of the LLBL, mantle, depletion, halo, and energetic particle layers as functions of simultaneous solar wind conditions. It will be of great importance to examine the magnetopause crossings for evidence of magnetic merging (high-speed flows and open magnetic field lines void of energetic electrons). The spectral and composition (solar wind, magnetosphere, and ionosphere) transition from magnetosheath to magnetospheric plasmas must be examined in detail. Evidence must be sought for the plasma waves required to produce diffusion.
A similar procedure may be followed using the GEOTAIL observations. The resulting surveys will describe on an average (or statistical) basis how boundary layers vary with downstream distance. In particular, we wish to determine whether the LLBL expands to fill the plasma sheet and the mantle expands to fill the lobes. The LLBL and the mantle may be mapped to low altitudes and the ionosphere on the basis of similar plasma and convection characteristics. Optical images from low-altitude satellites will help identify correspondences between magnetospheric and ionospheric features. Such work will help improve existing models of magnetospheric magnetic fields. Using the mean boundary layer thickness and average parameters, one may estimate the quantity of mass, momentum, and energy crossing the magnetotail boundary as a function of downtail distance.
Of particular interest are those intervals when both INTERBALL and GEOTAIL are in the boundary layers. At these times, the statistical results may be compared with case studies. These studies will help determine how the properties of the magnetotail, the magnetopause, and the associated boundary layers vary with downstream distance. For example, one wishes to determine whether FTE's unravel with downstream distance, whether Kelvin-Helmholtz wave amplitudes grow with downstream distance, and whether LLBL flow velocities change with downstream distance.
The nature of the "interfaces" between the LLBL, the mantle, the magnetosheath, and the lobe has recently attracted great interest. INTERBALL's orbit will carry it into this region, where its observations will permit identification of each region. GEOTAIL observations will permit determination of the way in which this interface varies with downstream distance.
If we are to understand the global magnetospheric situation, supplementary data from geosynchronous and low-altitude spacecraft and from ground-based arrays are of major importance. It is necessary to determine how these phenomena in the magnetospheric boundary layers map to the ionosphere. Auroral images will be obtained from INTERBALL-AURORAL, AKEBONO, FREJA, and DMSP; and these spacecraft will also give information about, for instance, particle distributions, plasma boundaries, and field-aligned currents. Information about particle distributions from geostationary satellites will, in certain cases, consolidate the mapping.
Ground-based arrays have the unparalleled advantage of extended temporal and spatial coverage. It is expected that optical imaging systems like CANOPUS and ALIS, ground-based magnetometer arrays like CANOPUS, the Alaska chain, the Greenland chain, the Scandinavian magnetometer array, the Kara chain, and the SuperDARN HF radar system will be of particular importance. Since the ionospheric footprint of the low- and high-latitude boundary layers should be located at high geomagnetic latitudes, extensive ground-based data from high-latitude stations like those on Svalbard and Sondre Stromfjord are also likely to be of interest. For the ground-based optical measurements, it would be advantageous for the campaign period to be in the winter season of the northern hemisphere.
Space research has clearly demonstrated different modes of interaction between the solar wind and the magnetosphere. When the IMF is southward, the coupling is strong, geomagnetic activity is high, and energy transfer to the magnetosphere is maximized. Under certain types of weak or northward IMF and/or low velocities of the solar wind, coupling is weak, geomagnetic activity is low, and energy transfer is diminished. Much less is known about the quiet magnetosphere, partly because the observable effects are less dramatic and have not been frequently studied, and partly because the necessary intervals of northward field required to produce this state are rather infrequent. It is, however, important to understand the quiet magnetosphere because under ideal quiescent interplanetary conditions it may be a ground state, and because alternate modes of energy transfer may occur at these times.
The structure of the magnetotail is determined largely by the amount and location of magnetic flux crossing the magnetotail current sheet. When more field lines cross the equatorial plane in the inner magnetotail, more plasma is apt to be confined in a thicker plasma sheet closer to the Earth; fewer field lines remain in the tail lobes to form the distant tail. Evidence suggests that, on average, northward Bz in the plasma sheet is larger during quiet times and hence it might be expected that the radius of the distant tail might be reduced at these times. GEOTAIL, INTERBALL, and geosynchronous spacecraft can measure both equatorial Bz and the degree of plasma confinement and determine their variations in response to the IMF as measured by WIND.
The source of magnetotail plasma is another important problem that can be attacked by INTERBALL and GEOTAIL. Studying the composition of the plasma is an effective way of distinguishing between solar wind and ionospheric sources. In addition, the INTERBALL-AURORAL spacecraft should be able to measure upflowing plasma from the latter source to determine its relative contribution to the magnetotail under quiet and disturbed conditions.
The high-latitude ionosphere exhibits distinctive behavior under quiet conditions which mirrors the activity in and configuration of the magnetotail. High-latitude convection patterns change dramatically, and high-latitude auroral arcs appear in response to processes occurring on magnetotail field lines. One of the goals of a quiet-time campaign is to determine the magnetotail sources of these high-latitude arcs as well as the sources and sinks of the associated field-aligned currents and electric fields. Another goal is to determine the large-scale ionospheric convection patterns during quiet times and relate them to circulation in the magnetotail. Studies of the latter type will identify the types of solar wind-magnetosphere coupling that drive quiet-time convection and improve our understanding of magnetospheric topology.
Another question concerns the origin and the role of MHD waves in transferring energy through the magnetosphere under quiet conditions. Long-period waves are frequently observed in the quiet-time, high-latitude ionosphere with radars, magnetometers, and other instrumentation. Simultaneous observations by IACG satellites will allow the structure of these waves in the outer magnetosphere to be determined and will aid in the identification of the sources of excitation.
The preferred configuration of the IACG spacecraft is to have both GEOTAIL and the INTERBALL-TAIL spacecraft near the center of the magnetotail with global images being provided by the INTERBALL-AURORAL spacecraft. The WIND spacecraft should be in the upstream solar wind where it can provide data on both the solar wind and the IMF. Geosynchronous spacecraft will provide important information concerning the near-Earth magnetotail. Other low-altitude spacecraft capable of providing data on convection and precipitation boundaries should be used where possible. Data on ionospheric convection and current patterns will be provided by a network of radars and magnetometers, the most extensive of these being in North America, Northern Scandinavia, and Russia.
The quiet tail campaign is a campaign that will use data from the quietest geomagnetic intervals during the coordinated IACG study. These periods will be selected according to a combination of the following criteria: the absolute value of the auroral index (|AL|) should be less than 50 nT; there are prolonged periods of northward IMF; and other interplanetary and geomagnetic conditions should obtain that are yet to be determined.
1.3 Origin of Plasma in the Plasma Sheet
Plasma of the solar wind could be transported into the plasma sheet either via the LLBL or via the plasma mantle. Although entry through the LLBL does not seem to have found observational support so far, this possibility needs to be examined thoroughly with the IACG data sets obtained by a focused campaign, since the plasma supply process is fundamentally important in tail dynamics. If the LLBL plasma does not flow into the plasma sheet, it is intriguing to know how the closed field lines of the LLBL dispose of the anti-Sunward flowing plasma which they carry.
Plasma entry through the high-latitude mantle is expected to depend on the polarity of the IMF, and it is important to see whether or not the supply through this channel is maintained when the IMF is northward. Measurements of tenuous plasma and its convection in the tail lobe will provide a vital clue.
Heated plasma from the ionosphere has also been recognized as a second possible source of the plasma sheet population. The relative importance of this source in the distant tail and its variability with magnetospheric activity will be studied by the campaign. Charge state and ionic mass composition will be vital clues to the distribution between internal and external plasma sources.
Because the nature of the plasma sheet source most likely varies with radial distance as well as with quiet/stable versus substorm conditions for this dynamic system, extended measurements will be required to cover the range of radial positions and magnetospheric activity levels. Variations in large- scale convection and the configuration of the plasma sheet must also be studied for their effects on the relative strengths of sources and sinks. In addition to sources, sinks such as precipitation and flow down the tail must be investigated: in particular their relative strengths must be established. Specific ionospheric sources such as the cusp ion fountain, the polar wind, and the auroral oval need to be fully explored and the detailed plasma physics underlying them understood. Detailed studies of underlying mechanisms must be conducted to understand fully the dynamics of the plasma sheet. In this regard, observations of refilling of the post-substorm plasma sheet as the neutral line retreats are needed.
Fundamental questions that remain to be resolved are:
An unprecedentedly close working relationship between observation, analysis, and theory is required to resolve these issues.
Although initial progress can be made in elucidating the origin of plasma in the plasma sheet prior to the launch of POLAR, the spacecraft will play a critical role in resolving a number of issues by means of its advanced in situ and remote-sensing capabilities, and hence the launch of POLAR is vital to the ultimate success of this campaign.
Observations needed will require several spacecraft and ground-based observatories. These include GEOTAIL, WIND, POLAR, IMP-8, INTERBALL (AURORAL and TAIL), geosynchronous satellites, AKEBONO, FREJA, DMSP, SuperDARN, SONDRESTROM, EISCAT, CANOPUS, and SESAME. The campaign would be enhanced if it were to extend beyond the launch of POLAR and when INTERBALL is in the magnetotail.
The WIND measurements will provide a complete description of the solar wind scalar and vector parameters to high precision. IMP-8 will supplement the WIND measurements and partially perform the role of WIND prior to WIND launch. When it is inside the magnetosphere, IMP-8 will provide a needed increase in spatial coverage of magnetospheric observations. The INTERBALL-TAIL spacecraft will be important in measuring field topology, plasma distribution and composition in the lobe, in the plasma mantle, and in the mid-magnetotail plasma sheet. INTERBALL-AURORAL can provide images of the auroral zone to provide a remote signature of plasma sheet particle precipitation as well as measuring local dynamics in the polar region. GEOTAIL will provide a complete set of field, plasma, and energetic particle measurements. POLAR will evaluate in a similar detailed manner the entry of plasma into the magnetotail. Geosynchronous satellites will measure magnetic field topology and electron flux as tracers of activity near the inner edge of the plasma sheet. Finally, POLAR, AKEBONO, DMSP, FREJA, and INTERBALL-AURORAL images of precipitation due to plasma sheet losses will yield an indication of plasma sheet dynamics in both the southern and northern auroral regions. A nearly continuous description of the ground-based signature of relevant large-scale convection processes may be obtained by the ground-based magnetometer, camera, and radar arrays, including those of SuperDARN, Sonderstorm, EISCAT, CANOPUS, and in the southern auroral zone, by the SESAME radar.
A key unifier of these measurements will be mission-oriented theory. Contributions will include magnetospheric 3-D MHD simulations, which will extend to 300 Re and hence cover the entire range of observational interest. These simulations, although they lack the detailed physics to understand the mechanisms underlying origins of the plasma sheet, will provide a magnetospheric road map with which to connect the various spacecraft measurements. Empirical models will also be central to the analysis by providing a phenomenological framework for understanding the mechanisms. Modeling of large-scale kinetic processes in populating the plasma sheet from sources in both the solar wind and the ionosphere including detailed heating and acceleration processes is needed. In addition, microscale kinetic simulations will provide insight into the overall role of smaller scale processes such as plasma-wave turbulence and the resulting pitch-angle scattering and hence precipitation and loss from the plasma sheet. Insights into transport across boundary layers will also be obtained. For example, the LLBL is usually a transition layer consisting of an admixture of plasmas from the magnetosheets and the plasma sheet. It is expected that more complete theoretical scenarios will be constructed for wave-particle interactions, which may help to shape our view of the nature of the plasma sheet sinks.