Processing of ion VDFs from the SPAN-I measurement onboard Parker Solar Probe

The origin of the proton beam, a secondary proton population observed in the solar wind, remains unclear. Measurements made by the Solar Probe Cup (SPC) instrument onboard Parker Solar Probe (PSP), together with earlier observations from the Helios mission, suggest that the relative proton beam abundance increases from the Sun to Earth. In addition to the SPC, the PSP is equipped with the SPAN-I instrument which measures ion velocity distribution functions (VDFs) during the PSP’s perihelia that are not covered by the SPC instrument. However, the limited field of view of the SPAN-I instrument prevents direct observation of the full ion VDFs. We apply the Gyrotropic Slepian Reconstruction method (Das and Terres, 2025b) to recover the full ion VDFs and perform bi-Maxwellian fitting to derive the parameters of the proton core and beam populations. We observe that the drift velocity of the proton beam remains close to the local Alfvén speed, even at small heliocentric distances. This finding suggests that the proton beam formation may be related to the reconnection processes near the Sun. Thus, we focus on variations of the proton beam parameters across switchbacks. In addition, we investigate the radial evolution of the proton beam parameters using combined observations from PSP and Solar Orbiter.