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The objective of this campaign is to map the boundaries and inner regions of the magnetotail as a function of both upstream conditions in the solar wind and distance down the tail. In addition to its static configuration, the temporal response of the magnetotail to changes in the upstream solar wind will be explored. These considerations are scientifically interesting in their own right and are essential because all subsequent analyses of the detailed structure and dynamics within the tail need to be put within the framework of this global structure. Specific questions that will be addressed by this campaign include:
The simultaneous data to be provided by the IACG satellites--WIND, GEOTAIL, INTERBALL-TAIL, and INTERBALL-AURORAL--and associated satellite and ground observations present a unique opportunity to understand the global tail configuration and its response to changes in the upstream environment. This campaign will allow us to extend existing empirical magnetospheric models, based primarily on dayside and near tail observations, to the distant tail. These observations will be supplemented with numerical simulations to develop and test future theoretical tail models.
In order to characterize the upstream conditions quantitatively it is essential to have good observations of the bulk solar wind plasma parameters and IMF with a time resolution of ~1 minute. The instrument complements on either WIND or IMP-8, if appropriately positioned in the unperturbed solar wind, are ideal for obtaining these measurements.
The two following spacecraft configurations within the magnetotail are optimal for determining the motions and locations of the various boundaries. Simultaneous observations at two different distances down the center of the tail would be ideal for observing upward and downward motions of the neutral sheet. Conversely, observations near the flanks, at differing distances down the tail, would provide an ideal configuration for observing lateral motions of the tail boundary. In both cases, simultaneous observations of the plasma and magnetic field by INTERBALL-TAIL in the nearer tail and GEOTAIL in the deep tail would provide the required data sets for determining the tail regions being sampled. The INTERBALL-TAIL sub-satellite would provide additional information about the location and motion of the boundaries, as would remote sensing with energetic particle and radio observations. A temporal resolution of ~1 minute would be adequate for these measurements.
Simultaneous observations near Earth and at the polar cap would provide the required information on mapping the tail boundaries and configuration into the ionosphere. INTERBALL-AURORAL and POLAR, if it is launched in time, will provide auroral imaging for mapping the tail lobes. Ancillary information from these and other inner magnetospheric spacecraft, as well as from ground radars, will determine electric fields and convection patterns at the polar cap . An imaging time resolution as close as possible to the ~1 minute cadence of the in situ observations is desirable.
In support of the interpretation of the observations, global MHD simulations will be used to infer the magnetotail structure at high latitudes, and to simulate the time evolution of disturbances propagating through the tail. The combination of all of these coordinated resources will provide the opportunity to reach a quantitative understanding of solar wind effects on the global geomagnetic tail configuration.
2.2 Externally Driven Disturbances
The objectives of this research thrust are to examine experimentally and theoretically the macroscale and global dynamic response of the magnetospheric system to external short-term perturbations or to longer-term changes in boundary conditions. It is expected that the campaign will focus on the magnetospheric response to interplanetary shock waves, tangential discontinuities, and both short-term and extended changes in the pressure of the solar wind, elemental and charge state composition, heat flux, and orientation of the IMF.
An important feature of this campaign is to establish the modes, scale size, and propagation characteristics of the magnetospheric system response to each major class of perturbation in the solar wind. With the launch of GEOTAIL, these kinds of studies can be extended to the distant tail. With the confluence of the INTERBALL, WIND, and POLAR spacecraft, a multipoint, global view of responses can be obtained; and with the important inclusion of other spacecraft and ground-based observing arrays, a fresh and comprehensive picture of the system's response to each different class of stimulus should become clear.
We envision that rather complete measurements of plasmas and fields will be required throughout the system comprising the solar wind and magnetosphere. The initial general campaign period (late 1993 through mid-1994) is determined by the availability of GEOTAIL, INTERBALL, and WIND. Within that period it is assumed that suitable campaign intervals will be initially identified by distinctive dynamic changes occurring in the solar wind or the magnetosphere (solar wind shocks, major magnetospheric storms, etc.). Periods of interest are typically identified by the observation of a shock wave or southward turning of the IMF in the solar wind and/or the signature of a substantial substorm in magnetospheric and ground-based data records. Thus, there must be complete, relatively high-resolution measurements in the upstream region which reveal a suitable disturbance event. We require detailed knowledge of solar wind velocity, density, and temperature; composition knowledge is desirable although perhaps not essential. We also require IMF vector data. An important component of this campaign will be the capability to model the orientation and propagation of interplanetary shocks and other discontinuities. We must then model the propagation of these disturbances to the point where they strike the magnetospheric system. This will establish clearly when the disturbance initiates the magnetospheric system response.
We will also require auroral and near-Earth measurements for this campaign. This means that we will want to have images of the auroral zone and polar cap at high time resolution to determine the size and location of the auroral oval. We will want to know the state of disturbance of the auroras as a function of time. We will require good ground-based data including prompt geomagnetic records, auroral electrojet indices, and all-sky camera records from many stations. Global polar cap and auroral zone convection data from ground radar systems is also highly desirable. Data on plasmas, energetic particles, electric fields, and magnetic fields from low-altitude, polar orbiting spacecraft is of prime importance in this context.
An important baseline for these campaigns will be the geosynchronous orbit data sets. These can be used to monitor the passage of disturbance waves through the inner magnetosphere when shocks or sudden impulses occur. Also, such spacecraft can identify substorm growth and expansion phases. The injection phenomena of hot plasmas and energetic particles can help tell both when and where substorms initiate.
In the midtail region we will require complete 3-D measurements of plasmas and energetic particles. We will also require vector DC and AC electric and magnetic field measurements. Plasma wave and energetic particle anisotropy information can help remotely sense boundary motions near spacecraft in the lobes and plasma sheet. A great advantage in this campaign will be to have multiple spacecraft making measurements concurrently at several locations within the magnetotail.
The dramatically new and important feature required in this campaign is the measurement of complete particle and magnetic and electric field parameters in the distant magnetotail (~80-220 Re). This will allow us to detect the passage of wave disturbances down the tail, detect sheath motions over the spacecraft, sense changes in mantle thickness at various radial distances, remotely sense boundary motions, and determine magnetic connectivities over very large spatial scales (>100 Re). We will also be able to sense the possible growth in the magnetotail size during substorm growth phases and will be able to monitor plasmoids, flux ropes, current filaments, and other coherent plasma structures that may be produced by near-Earth substorm or geomagnetic storm processes. It will be very important, as well, to examine carefully the "restoration" of the distant magnetotail during the substorm recovery phase.
It is regarded as a matter of the highest priority for this IACG research campaign to have the POLAR spacecraft launched and in full operational configuration. Without the detailed POLAR imaging capability and its comprehensive plasma and field measurements, the full science potential to study global magnetospheric dynamics will be significantly diminished. In order to achieve optimal results, in phasing with GEOTAIL especially, we urge that POLAR be launched as early as possible in 1994, preferably by March.
In general, we envision that a prototypical arrangement for this campaign would have WIND or IMP-8 somewhere in the upstream solar wind to monitor the interplanetary conditions and sense the kind of disturbances/changes that will induce a global dynamical response in the magnetosphere. We would also want to have INTERBALL-AURORAL, AKEBONO, and/or FREJA in the polar regions to image the polar cap and monitor the low-altitude plasma population. We would further want to have good placement of ground-magnetometer chains, all-sky camera stations, and HF radar facilities. An operational set of geostationary orbit spacecraft at many different local times would also be highly desirable.
A key component of the configuration strategy is to have INTERBALL-TAIL in the mid-magnetotail during this campaign. We would optimally picture this being relatively near the midnight meridian, probably near apogee (~30 Re). However, the INTERBALL spacecraft could be at any local time in the tail--including near the flanks--and still return very useful information. Moreover, we would also note the exciting possibility that both IMP-8 and INTERBALL might be in the magnetotail at the same time. This would allow very interesting two-point comparisons of disturbance signatures at platforms simultaneously located at ~30 Re. If the spacecraft were separated by significant distances in Y and Z, we would get spatial/temporal information.
Finally, in this campaign we envision the GEOTAIL spacecraft to be in the relatively distant tail region, most probably near apogee. Notably, of course, apogee can range from ~80 Re for 1-month-class orbits to ~220 Re for 4-month-class orbits. Thus, a wide range of INTERBALL-GEOTAIL separation strategies are possible in this campaign. Moreover, if we imagine IMP-8 in the magnetotail along with INTERBALL and GEOTAIL, we have a very wide variety of 3-spacecraft location possibilities as disturbances propagate down the tail after their near-Earth initiation.
This global arrangement of observations of the solar wind, near Earth, and in the magnetotail must be tied together by the global modeling and theory tools presently under development. We envision that MHD, global-kinetic, and empirical models will all have a strong role to play in making sense of the global dynamical signatures.
Past studies of tail configuration and dynamics have concentrated on the characteristics of the tail during quiet and substorm intervals. In contrast, little attention has been paid to the activities in the tail region during geomagnetic storms. This is mostly due to the well-established fact that the major storm disturbances of the geomagnetic field on the ground are attributed to the formation of the storm time ring current within a geocentric distance of ~10Re.
It is well recognized that the polar cap enlarges and the auroral oval widens during the development of a storm, indicating that a large amount of magnetic energy and plasma are stored in the tail. The unprecedented fleet of spacecraft available is ideal to examine the tail configuration and dynamics during storm times to enhance our knowledge and understanding of the tail under the condition of extreme geomagnetic disturbance. The following four specific questions are designed to take advantage of the near- and far-tail satellite configurations offered by GEOTAIL, WIND, INTERBALL, and IMP-8, to address the general topic of the storm time configuration of the tail and its dynamical state, and determine the degree of control exercised on the tail by the solar wind.
Specific Scientific Questions
Because we cannot yet control or predict when a magnetic storm will occur, this campaign must be done in a retrospective mode. To monitor conditions in the solar wind, one must confine attention to storm times during which WIND and/or IMP-8 are in the solar wind. In that situation, one can estimate the travel time of solar wind features and determine their arrival at the frontside magnetosphere. Measurements of solar wind number density, velocity, temperature, and magnetic field at a resolution of ~1 min. would be adequate. In addition, information on the ion composition and charge state, and the intensity of the strahl component are useful because they can be used as tracers in the magnetosphere. To monitor the response of the near and far tail, data from at least two spacecraft, viz. GEOTAIL and INTERBALL-TAIL, are required. To determine the transport parameters (including current density) of the plasma in the tail, the velocity distribution functions of plasma ions and electrons are required. Ion composition and charge state measurements can be used as tracers to infer the origin of the plasma. Electric and magnetic field measurements are essential for estimating flow and storage of electromagnetic energy in the tail. Velocity dispersion effects of high-energy particles can be used to deduce source locations. Similarly, high-frequency waves can assist in characterizing the plasma regions.
Studies of the orientation, configuration, and response of the magnetotail to IMF/solar wind changes are best done when INTERBALL and GEOTAIL are located close to the boundary layers of the tail, either inside or in the solar wind close to the tail, in which case the tail can sweep over the satellites. Alternatively, when the spacecraft are located near the central tail axis, we can determine the relationship of storm time features to such substorm phenomena as current disruption, plasmoid and/or flux rope formation and propagation, neutral line retreat, and plasma sheet refilling. The study of the coupling between the ionosphere and the storm-time tail necessitates global and detailed measurements of inner magnetosphere conditions, ionospheric parameters, and the potential and size of the polar cap. Consequently, data from geostationary satellites would be useful to determine the state of the inner magnetosphere and to time processes affecting the plasma sheet inner edge to provide comparisons with dynamical features seen in the far tail. Auroral imagers such as INTERBALL-AURORAL, POLAR, AKEBONO, DMSP, and FREJA are very important, as are radars such as SuperDARN, incoherent scatter radars, and the entire CANOPUS network. The campaign will also require complete catalogues of the geomagnetic storm index (Dst) and the auroral electrojet index (AE). Identification of the regions in which the satellites are located can be facilitated if high-resolution and accurate global magnetosphere models are available. Further enhancements of data interpretation can be realized with the use of hybrid (fluid electron, kinetic ion), particle and Vlasov equation model simulations of local phenomena. Various research groups in Japan, Europe, and the US have developed such capabilities.
Because this is a retrospective analysis study, the key parameters disseminated routinely by the CDHF will be sufficient for the initial survey of the data sets. Requests by individual scientists for higher time- resolution data in conducting specific studies are anticipated and can often be accommodated.
2.3 Internal Magnetospheric Instabilities
The stretched magnetic field in the tail of the magnetosphere, associated with the cross-tail current, represents a large reservoir of magnetic energy. Magnetospheric substorms get their energy from this reservoir through disruption of the tail-current and the explosive development of magnetic reconfiguration. This process is associated with enhancements of particle energies and strong magnetic and electric wave fields.
Many processes have been associated with the substorm onset in the magnetotail, including the tearing mode instabilities, Kelvin-Helmholtz instability, cross-field current instabilities, the fire-hose instability, and the ballooning mode. Ion orbits in the near-Earth tail could be essentially chaotic due to nonlinear effects. Transient events when electrons are chaotized could also occur in the near-Earth tail and could have an effect on triggering of instabilities. Chaotization of particle trajectories could have clear manifestations in particle distribution functions, which could be observed in situ by IACG spacecraft. Several of these processes may operate concurrently. The present incomplete picture of substorm development places the triggering region--the tail current interruption--at a tailward radial distance of 8-15 Re. The effects of the substorm are manifested in the auroral zones of the ionosphere as intensifications of auroras caused by energetic particle precipitation, surges of field-aligned and ionospheric currents, and changes in the electric field configuration. Another distinct manifestation of substorm activity is the generation of energetic particle bursts and transient beams. IACG spacecraft are well equipped for measurement of these bursts including the determination of the time of burst onset by measuring the dispersion of particle velocities.
Such measurements provide important information about the location and mechanisms of substorm initiation. In the distant magnetotail, plasmoids may have been identified moving tailward following substorm onsets. An important goal of the IACG tail campaign is to identify more clearly the physics of the current disruption process and the consequent effects in the ionosphere as well as the propagation of substorm-triggered disturbances--such as plasmoids--down the distant tail.
This research can be addressed with the combination of the INTERBALL and GEOTAIL spacecraft in the magnetotail, respectively at distances of 8-15 Re and the deep tail, augmented by solar wind observations by WIND. The INTERBALL-AURORAL and POLAR spacecraft will provide inner magnetosphere observations as well as images of substorm development in the auroral oval. Observations by geosychronous spacecraft are very important. IMP-8 data could be complementary, either in the solar wind or in the magnetotail. Finally, ground-based observations of currents and electric fields will define the ionospheric response.