Radio Propagation Diagnostics of the Inner Heliosphere in the Era of the Parker Solar Probe

The solar wind offers an extraordinary laboratory for studying turbulence, turbulent dissipation, and heating. The Parker Solar Probe (PSP) was launched in early August to study these and other important processes in the inner heliosphere. Extensive and complementary observations from the ground or from other space based platforms are in the planning stages or are underway. One type of observation that will complement those of PSP are radio propagation measurements of solar wind turbulence in the outer corona and the inner heliosphere. One such measurement is the angular broadening of distant spatially coherent background sources that transilluminate the foreground solar wind plasma. This well-known technique can be used to measure the spatial spectrum of electron density inhomogeneities in the solar wind on scales of 100s of meters to 10s of kilometers inside of 10-15 solar radii over a wide range of position angles.

This paper reports the results of a pilot study of background sources using the Jansky Very Large Array (JVLA) in summer 2015. Unlike previous studies of this kind, the JVLA's much greater sensitivity allows fainter and more numerous sources to be used as probes of the foreground medium. We observed 12 background sources in 24 sessions at apparent radial distances of 2-7 solar radii. We confirm previous findings: that the spectrum is flatter than Kolmogorov and that is highly anisotropic. Unlike previous studies we find breaks in the spectrum for certain sources to steeper spectra on short spatial scales, suggestive of a transition to dissipation.

Looking forward, we describe observations planned in August 2019 in support of the third PSP perihelion passage (35.7 solar radii). The VLA will be used to observe the corona and inner heliosphere along 70 pierce points <10 solar radii. These observations will not only provide global context about the state of the inner heliosphere at time of perihelion passage, they will also baseline key solar wind parameters that can be compared directly with PSP measurements. These include turbulence level, spectral index, degree of anisotropy, and the orientation of the magnetic field. The PSP measurements will, in turn, provide measurements that will validate key assumptions made in interpreting the radio data.