The interplanetary medium is composed of a tenuous, magnetized plasma which originates with the supersonic solar wind. The IPM is of interest for many reasons. The supersonic outflow interacts with planetary magnetospheres and drives a number of magnetospheric phenomena, including geomagnetic storms on Earth. The IPM modulates the flux of cosmic rays in the inner heliosphere and the details of cosmic ray diffusion and drift through the IPM must be understood in detail in order to infer the spectrum and composition of the cosmic rays incident on the heliosphere. The IPM is itself modulated by solar activity, leading to fast and slow solar wind streams, corotating shocks, and in situ particle acceleration. Finally, the details of the structure of the IPM - the rôle of turbulence, waves, and the magnetic field, and their bearing on the spatial spectrum of the IPM - are of great interest for their own sake.
The VLA has played an important rôle in ground-based observations of the solar wind using indirect means. Briefly, interferometers such as those at the VLA are ideally suited for measuring the angular broadening and intensity scintillations of background sources viewed through the solar wind, from which properties of the solar wind turbulence may be deduced: the spectral index, the turbulence level, its degree of anisotropy, and the dissipation scale. In principal, the solar wind velocity can be measured by observing the source diffraction pattern sweeping over the array.
The enhanced VLA will improve studies of this kind in two key
respects. Studies of turbulence in the solar wind exploit observations
of background cosmic sources which, in the absence of the IPM, would
be unresolved by the array. Angular broadening and scintillation are
then due solely to the intervening turbulent medium. Such studies
therefore rely upon the presence of suitable background sources. The
increased continuum sensitivity will enable many more background
sources to be exploited as probes of the IPM. Source counts at 5 GHz
suggest that there are roughly a factor 15 more sources at 100 mJy
than at 1 Jy. To date, the properties of the solar wind turbulence
have only been determined for a handful of sources at various
elongations and position angles due to the paucity of suitable
sources. Access to large numbers of background sources will enable
observers to determine the properties of the solar wind turbulence
along many lines of sight over the course of a single observing run,
thus characterizing the state of turbulence in the outer corona in a
more global fashion. Second, since the scattering measure (the
line-of-sight integral over the turbulence level) falls of with radius
roughly as 
, the A+ configuration will allow the effects of
scattering to be observed on small angular scales. The effects of
scattering will therefore be observable over a much larger range of
solar elongation, effectively bridging the gap between the VLA A
configuration and the VLBA.