Any scientific mission needs a firm, well-defined purpose. New technology and novel orbits add appeal, but scientific purpose comes first. Ideally that purpose should be defined not only in broad terms--understanding the causes of substorms, the origin of auroras, the topology of field lines, the flow of energy etc.--but a clear idea should also exist about how, and by what observations, this purpose would be accomplished.|
"Constellation" missions are a novel mode of observation, and therefore allow existing problems to be addressed in new ways--different ones at different times of the year, as orbits sample different parts of the magnetosphere. Such problems include the substorm (its start and its propagation), magnetic reconnection, global magnetic field structure and topology, convection patterns and the propagation of shocks and waves. The options of each mission--e.g. number, orbit, size and instrumentation of its satellites, duration, data handling and choices of spin and on-board propulsion--should be chosen to provide the best coverage of such scientific targets.
The "Profile" mission
One attractive magnetospheric constellation is the "Profile" mission, consisting of 12 small satellites in a near-equatorial orbit, with perigee around 7000 km (earth centered) and apogee at 20 or 25 RE (Figure 1, above) The satellites would spin around an axis perpendicular to the orbital plane, and each would carry a magnetometer and a high-rate "top hat" detector for ions and electrons of 0.1 to 30 keV. The detector would sample a narrow range of the rotation angle f but the full range 0 to p of the angle q to the spin axis. Using the spacecraft spin and comparing particle rates at diametrically opposite direction, one can then estimate the bulk velocity components in the spin plane. |
The satellites would be simple and small, with no on-board propulsion, and the mass of each, even with current technology, could be held down to about 17 kg. The mission, and the way it would use a "centrifugal slingshot" for dispensing the satellites from a "mother ship," are described elsewhere [Stern, 1998].
As a consequence of the "slingshot" release, the satellites would form two groups of 6 satellites each on two close orbits with slightly different orbital periods. Members of each group would pass perigee about 1 hour apart, but the faster (lower) group would periodically overtake the slower (higher) one--every 7 weeks for apogee at 20 RE (periods [46, 48] hours) and every 13 weeks for apogee at 25 RE (periods [66, 68] hours). Consequently the formations of the satellites change periodically, adding scientific versatility to the mission.
"Profile" would be a good choice for the initial constellation mission. The satellites are few enough to be manageable, and their basic formations allow some high-priority scientific goals to be addressed, as discussed further below. Its stable formations as well as the uniform spin of the satellites make analysis of the data relatively "clean" and straightforward, unlike the analysis of data from randomly-placed, randomly-spinning spacecraft.
The four basic formations
At some times the two groups would be spread out in different parts of their orbits: that is the "linear" configuration ("mode A"; the first in clockwise order from Earth in Fig. 2 above), useful for obtaining the radial dependence of fields and particles, and for studying the earthward propagation of disturbances. At other times the groups occupy different sides of their ellipses ("mode B"), providing some 2-dimensional coverage in their orbital plane.|
At still other times the formations overtake each other. If the orbital periods in each group are exactly the same--feasible with the slingshot release, and required if the groups are not to disperse--members of one group will pair up with all or part of the other, each pair slowly closing the gap between them down to a fraction of 1 RE, then pulling apart again ("mode C", third in Fig. 2). This not only allows an intercalibration of instruments, but also the study of 2-point correlation functions and of small-scale phenomena.
Finally, when the overtaking groups are near apogee, a dense "supercluster" is formed ("mode D", second in Fig. 2) in which 7-12 satellites are found within a radial spread delta-r of 2-3 RE . The original plans of "Profile" called for apogee at 20 RE, but after observations by GEOTAIL and ISEE suggested that substorms originated around 25 RE, an option of placing apogee at that distance was also studied, providing "supercluster" coverage for that interesting region.
The broad goal of "Profile" is to understand global plasma processes in the near-equatorial magnetosphere. The motion of the Earth around the Sun rotates the "Profile" orbit among magnetospheric regions in a yearly cycle, and the goals therefore change from season to season, depending on whether apogee is in the tail, on the flanks or on the day side.
Goals in the tail
"Profile" has gone beyond the definition of science goals, to spell out more than 20 "science tasks", selected "experiments" with specific targets. As far as we are aware, such explicit targeting has not been done before, except for single-purpose missions. Each task fits one of the 4 listed formations and is appropriate to the time of the year when the orbit covers a specific part of the magnetosphere.
The mission was also simulated on a computer, for different launch options, to make sure the formation required by each task actually occured frequently enough; details and results are given by Stern . A more detailed version of the list below was presented as poster SM12A-3 "Science Closure of the Profile Multiprobe Mission" at the 1997 Fall Meeting of the AGU (copies available from the author upon request). In each entry, a key phrase is set apart in bold type, to serve as a subhead.
Tasks in the Geotail
ConclusionThis study has examined the science planned for the " Profile" mission, going well beyond the usual description of scientific goals to explicitely outline various "experiments" by which such goals would be addressed. The list of those experiments should be compared to coverage statistics such as those of Table 1 of Stern , especially the last panel of that table, which suggest that the required formations do in fact occur frequently enough.
In the course of a year, as different regions of the magnetosphere rotate through the " Profile" orbit, different experiments become possible, relevant to tail convection, substorms, boundary layers, collision-free shocks and other key features. All these have been described in some detail. Not every mission allows such a specific listing of tasks, or simulations of their feasibility such as those conducted for " Profile". However, where such detailed planning is possible, it is highly recommended, since it helps focus the science and choose the proper options.
The nominal duration of " Profile" is two years, allowing two passes through every region. Yet such is the diversity of possible "experiments" and the wealth of information provided by them, that it may well be worth while to collect data for a longer time, which is why a 10-year lifetime was planned. The wide range of observations and experiments also suggests that this mission would probably do well as a shared facility, whose data are available throughout the scientific community. The principal investigators would be responsible for the operation of on-board instruments and for data retrieval, and would be supported accordingly, but the data analysis would be shared by the entire community.
APPENDIX: the 2001 AGU Fall Meeting
Session SM11-A: "Profile" discoveries in the geotail
Session SM12-A : Profile Results from the Magnetosphere's Flanks
Session SM21-A: Profile observations on the Dayside
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