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P. Rodriguez, N. E. Kassim, K. W. Weiler (NRL), M. L. Kaiser, M. J. Reiner, R. A. MacDowall (GSFC), D. L. Jones, S. C. Unwin, T. B. Kuiper (JPL), N. Gopalswamy (CUA)
Since the 1960s, ground-based radio astronomers have had the capability to image solar radio emissions using aperture synthesis techniques. These images show the two-dimensional time history of CME-driven shock fronts, for example. However, ground-based observations are limited by the terrestrial ionosphere to frequencies above about 15 MHz, which corresponds to a maximum solar altitude of about one solar radius. To probe the altitude range from one solar radius to one AU requires space-based radio telescopes. Many space-based low-frequency radio telescopes have flown over the past three decades, but they all suffered from lack of angular resolution because they consisted of single spacecraft carrying simple dipole antennas. Even with this limitation, much progress has been made by utilizing spacecraft spin to deduce source location and size, thereby permitting tracking of solar radio sources between the sun and Earth. However, no structural detail is available from this technique, only source centroids and approximate angular size. To provide a greater level of structural detail, we need to take the next logical step in low-frequency solar radio astronomy: aperture synthesis from space. We believe this can be done with currently-available hardware as a medium-class Explorer (MIDEX) mission. This mission would consist of approximately 16 identical and quite simple micro-spacecraft in a spherical array approximately 100 km in diameter. The array should be situated relatively far from Earth (to lessen terrestrial interference), such as at the distant retrograde class of orbits at about one million kilometers from Earth. This array will be capable of imaging not only solar transient events to unprecedented altitudes, but also the quiet sun, the entire terrestrial magnetosphere via scattering in the magnetosheath, and the low-frequency cosmic background, the latter being a totally unexplored radio window. This work is supported in part by ONR and NASA.
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