UC Berkeley

Low-frequency $\nu < 10$ MHz astrophysical flux typically does not penetrate the Earth's ionosphere, so can only be precisely measured using space-based radio instruments. The restrictions on data volume and instrument size that this imposes have stimulated the development of specialized analysis techniques for radio measurements from spacecraft. One such technique is the decomposition of the spectra recorded by co-located electrically short antennas into the $l=0$ and $l=2$ spherical harmonic expansion coefficients of the measured brightness distribution.

I apply this decomposition to radio data from Parker Solar Probe (PSP) to analyze the diffuse galactic and extragalactic synchrotron background. The $\nu < 10$ MHz frequency regime notably features free-free absorption of this radiation by interstellar ionized hydrogen. Between $\nu = 10$ MHz and $\nu = 3$ MHz, the optical depth due to this plasma to a typical region in the galaxy increases from roughly zero to one. As a result, the brightness distribution on the sky at $\nu \lesssim 3$ MHz is determined as much by free-free absorption as by synchrotron emission. Measurements of the sky's $l=0$ and $l=2$ expansion coefficients from PSP provide new constraints on the agents of this radiation transfer, most significantly the free electrons in the galaxy.

The $l=2$ coefficients can also be used as a reference source for calibrating electrically short antennas. PSP data allowed this to be conducted in parallel with measurement of the coefficients. The calibration results will improve the accuracy of direction-finding analyses using PSP radio spectra. As typically implemented for spacecraft data, this technique can jointly determine the angular coordinates as well as the $I$ and $V$ Stokes parameters of point-source radiation, such as that from Jupiter or the Sun when these bodies are sufficiently distant.

Limited data telemetry also motivates close examination of the random statistical errors in spectral measurements and their correlations. Such an inquiry shows, for example, that these errors can sometimes impede direction-finding using two antennas, which in part prompted the calibration of additional antennas on PSP. Also, this calibration itself and the spherical harmonic coefficient results benefited from fitting procedures that compensated for the statistical errors.

Although basing radio instruments in space bypasses the ionosphere, ambient interplanetary plasma gives rise to novel sources of noise. Electrostatic plasma fluctuations produce so-called quasi-thermal noise, which presented the most severe impediment to the spherical harmonic analysis of PSP's measurements. Due to this noise's unique importance for space-based instruments, I start by approximating its spectral density in PSP's radio data.