The Sun is an extremely complex radio source. While there is no shortage of signal - in fact, it must often be attenuated by large factors - its radio emission has structure on angular scales (shortward of a few cm) to more than 0.5 deg. Furthermore, the intrinsic brightness distribution varies on timescales ranging from less than one second, to minutes, hours, days, and years. Additional problems are introduced by the Sun's (differential) rotation and apparent motion on the sky.
Aside from being a challenging imaging problem for modern Fourier synthesis telescopes, the Sun presents us with different requirements on the observables. In general, observers of continuum radio emission from cosmic sources are interested in mapping the four Stokes polarization parameters (I, Q, U, and V) with a high degrees of sensitivity, angular resolution, and accuracy at a number of discrete frequencies. Spectral line observers are generally interested in mapping the strength and profile of one or more spectral lines, and the continuum baseline, again with high degrees of sensitivity, angular resolution, and accuracy. In addition, they generally wish to do so with sufficient velocity resolution to resolve the relevant kinematics.
While some solar radio sources are expected to be intrinsically linearly polarized, the linearly polarized component is subsequently washed out by strong differential Faraday rotation as the radiation propagates through the relatively dense, magnetized corona. The observed radiation is unpolarized or circularly polarized, and any information concerning the magnetic field is embodied in the Stokes I and V parameters. Furthermore, spectral lines are absent at wavelengths longer than a few m - strong collisional and (where relevant) Zeeman broadening make the line profile so broad and shallow that it becomes indistinguishable from the continuum. Hence, high resolution spectral line observations are of limited interest except at meter and decimeter wavelengths (about which, see Radio Bursts in 2.1.1). Solar observers gain little by employing state-of-the-art, high sensitivity receivers. The system temperature is overwhelmingly dominated by the source contribution. For example, at cm, the antenna temperature is K while the receiver temperature is K. Finally, solar observations are unlikely to benefit from increasing the angular resolution of the VLA. At wavelengths longward of a few cm, observations are limited in angular resolution by angular broadening on turbulent inhomogeneities in the solar corona to an extent that the Sun is over-resolved by the A and B configurations!
Given the lack of a need for sensitivity, for support of full Stokes polarimetry, for a high resolution spectral line system (at cm wavelengths, at least), or for increased angular resolution, how may the solar physics community benefit from the VLA Development Plan ?