Magnetic Field Dropouts and Associated Plasma Wave Emission near the Electron Plasma Frequency at Switchback Boundaries as Observed by the Parker Solar Probe

Abstract

The first solar encounters by the Parker Solar Probe revealed the magnetic field to be dominated by short field reversals in the radial direction, referred to as “switchbacks.” While radial velocity and proton temperature were shown to increase inside the switchbacks, ∣B∣ exhibits very brief dropouts only at the switchback boundaries. Brief intensifications in spectral density measurements near the electron plasma frequency, f pe, were also observed at these boundaries, indicating the presence of plasma waves triggered by current systems in the form of electron beams. We perform a correlative study using observations from the Parker FIELDS Radio Frequency Spectrometer and Fluxgate Magnetometer to compare occurrences of spectral density intensifications at the electron plasma frequency (f pe emissions) and ∣B∣ dropouts at switchback boundaries during Parker’s first and second solar encounters. We find that only a small fraction of minor ∣B∣ dropouts are associated with f pe emissions. This fraction increases with ∣B∣ dropout size until all dropouts are associated with f pe emissions. Brief spikes in the differential electron flux measured by the SWEAP Solar Probe Analyzer for Electron sensors also occur in conjunction with nearly all f pe emissions. This suggests that in the presence of strong ∣B∣ dropouts, electron currents that create the perturbation in ∣B∣ along the boundaries are also stimulating plasma waves such as Langmuir waves.