We compare magnetic field measurements taken by the FIELDS instrument on board Parker Solar Probe (PSP) during its first solar encounter to predictions obtained by potential field source surface (PFSS) modeling. Ballistic propagation is used to connect the spacecraft to the source surface. Despite the simplicity of the model, our results show striking agreement with PSP’s first observations of the heliospheric magnetic field from ∼0.5 au (107.5 R _⊙) down to 0.16 au (35.7 R _⊙). Further, we show the robustness of the agreement is improved both by allowing the photospheric input to the model to vary in time, and by advecting the field from PSP down to the PFSS model domain using in situ PSP/Solar Wind Electrons Alphas and Protons measurements of the solar wind speed instead of assuming it to be constant with longitude and latitude. We also explore the source surface height parameter (R _SS) to the PFSS model, finding that an extraordinarily low source surface height (1.3–1.5 R _⊙) predicts observed small-scale polarity inversions, which are otherwise washed out with regular modeling parameters. Finally, we extract field line traces from these models. By overlaying these on extreme ultraviolet images we observe magnetic connectivity to various equatorial and mid-latitude coronal holes, indicating plausible magnetic footpoints and offering context for future discussions of sources of the solar wind measured by PSP.

The Radio Frequency Spectrometer (RFS) is a two‐channel digital receiver and spectrometer, which will make remote sensing observations of radio waves and in situ measurements of electrostatic and electromagnetic fluctuations in the solar wind. A part of the FIELDS suite for Solar Probe Plus (SPP), the RFS is optimized for measurements in the inner heliosphere, where solar radio bursts are more intense and the plasma frequency is higher compared to previous measurements at distances of 1 AU or greater. The inputs to the RFS receiver are the four electric antennas mounted near the front of the SPP spacecraft and a single axis of the SPP search coil magnetometer (SCM). Each RFS channel selects a monopole or dipole antenna input, or the SCM input, via multiplexers. The primary data products from the RFS are autospectra and cross spectra from the selected inputs. The spectra are calculated using a polyphase filter bank, which enables the measurement of low amplitude signals of interest in the presence of high‐amplitude narrowband noise generated by spacecraft systems. We discuss the science signals of interest driving the RFS measurement objectives, describe the RFS analog design and digital signal processing, and show examples of current performance.

We present initial results from the Radio Frequency Spectrometer, the high-frequency component of the FIELDS experiment on the Parker Solar Probe (PSP). During the first PSP solar encounter (2018 November), only a few small radio bursts were observed. During the second encounter (2019 April), copious type III radio bursts occurred, including intervals of radio storms where bursts occurred continuously. In this paper, we present initial observations of the characteristics of type III radio bursts in the inner heliosphere, calculating occurrence rates, amplitude distributions, and spectral properties of the observed bursts. We also report observations of several bursts during the second encounter that display circular polarization in the right-hand-polarized sense, with a degree of polarization of 0.15–0.38 in the range from 8 to 12 MHz. The degree of polarization can be explained either by depolarization of initially 100% polarized o-mode emission or by direct generation of emission in the o-mode and x-mode simultaneously. Direct in situ observations in future PSP encounters could provide data that can distinguish these mechanisms.