Authors: Mingzhe Liu (SSL/UC Berkeley), John Bonnell (SSL/UC Berkeley), Marc Pulupa (SSL/UC Berkeley), Davin Larson (SSL/UC Berkeley), Stuart Bale (SSL/UC Berkeley), Jia Huang(SSL/UC Berkeley), Winry Ember (SSL/UC Berkeley), Ali Rahmati (SSL/UC Berkeley), Orlando Romeo (SSL/UC Berkeley), Phyllis Whittlesey (SSL/UC Berkeley), Roberto Livi(SSL/UC Berkeley), Samer Nussirat (SSL/UC Berkeley), FIELDS and SWEAP Teams
FIELDS/PSP makes use of five voltage probes V1–V5 combined to measure three components of the DC and fluctuating electric field. Four voltage probes V1–V4 can be combined to measure the difference between the antenna floating potential (𝛷A) and the spacecraft floating potential (𝛷SC). In order to minimize the current imbalances between opposite probes (the main source of error in the measurement) to produce the minimum error in the electric field measurement, a nominal bias current (Ib) is applied to bring their potential close to that of the nearby plasma. The optimum bias currents are determined by on-orbit bias sweeps during which we track the saturated (total) photoelectron current (Iph0) emitted from the electric field sensors (V1–V4) from E03 to E19. Preliminary results reveal that Iph0 may vary with both the solar cycle (2018-2024, solar minimum to maximum) and the daily timescales (possibly due to daily solar activity). Based on the Langmuir probe analysis of Iph, Ib and ambient thermal electron current Ie, we derive 𝛷A and therefore obtain 𝛷SC. Accurately derived 𝛷A help understand the biased shot noise, which is important to calibrate the quasi-thermal noise measurements. The calibrated 𝛷SC will later be used to calibrate SPAN-E/SWEAP/PSP and possibly electric field radial component (Ez), improve the high-frequency density measurements, and understand the dust impact dynamics. Recent analysis about Iph0, 𝛷A, and 𝛷SC will be presented.