Electron Bernstein waves and narrowband plasma waves near the electron cyclotron frequency in the near-Sun solar wind

Abstract

Context. Recent studies of the solar wind sunward of 0.25 AU reveal that it contains quiescent regions, with low-amplitude plasma and magnetic field fluctuations, and a magnetic field direction similar to the Parker spiral. The quiescent regions are thought to have a more direct magnetic connection to the solar corona than other types of solar wind, suggesting that waves or instabilities in the quiescent regions are indicative of the early evolution of the solar wind as it escapes the corona. The quiescent solar wind regions are highly unstable to the formation of plasma waves near the electron cyclotron frequency (fce). Aims. We examine high time resolution observations of these waves in an effort to understand their impact on electron distribution functions of the quiescent near-Sun solar wind. Methods. High time resolution waveform captures of near-fce waves were examined to determine variations of their amplitude and frequency in time as well as their polarization properties. Results. We demonstrate that the near-fce wave intervals contain several distinct wave types, including electron Bernstein waves and extremely narrowband waves that are highly sensitive to the ambient magnetic field orientation. Using the properties of these waves, we suggest possible plasma wave mode classifications and possible instabilities that generate these waves. The results of this analysis indicate that these waves may modify the cold core of the electron distribution functions in the quiescent near-Sun solar wind.

Parker Solar Probe observations of near-$f_{\rm ce}$ harmonics emissions in the near-Sun solar wind and their dependence on the magnetic field direction

“…Regarding the generation mechanism, the observations presented in this work are inconclusive and do not favor any of the emission mechanisms proposed in Malaspina et al (2021). The present report on dependence on the magnetic field direction for the emission of the near-f ce harmonics is, to our knowledge, an unprecedented observation in the free solar wind plasma, which suggests that these waves may be connected to a distinctive, underlying property of the near-Sun environment.…”

“…In this study, similar to earlier studies, we could not identify the generation mechanism of the nearf ce harmonics waves. Different mechanisms have been suggested: electron cyclotron drift instability, loss-cone instability, electromagnetic pump wave, and wave-wave interaction (Malaspina et al 2021). The observations reported in this study, that the waves favor a particular range of magnetic field directions, do not support any of the previously proposed wave generation mechanisms.…”

“…As a result, Malaspina et al (2020) concludes that a near-radial ambient magnetic field, though necessary, is not a sufficient condition for the growth of near-f ce waves. Earlier studies have suggested that the near-f ce harmonics structures are composed of electron Bernstein waves, and at least two other wave modes that have not been completely identified so far (Malaspina et al 2020(Malaspina et al , 2021Ma et al 2021;Shi et al 2022). The generation mechanism of these waves is still an open question.…”

The Sunward Electron Deficit: A Telltale Sign of the Sun’s Electric Potential

“…Alternatively, an instability related to the sunward deficit itself could operate, as suggested by Berčič et al (2021b). Malaspina et al (2021) also discussed a loss cone instability driven by sunward electrons as a candidate to generate narrowband plasma waves observed by PSP near the Sun.…”

Parker Solar Probe Observations of Near-f Ce Harmonic Emissions in the Near-Sun Solar Wind and Their Dependence on the Magnetic Field Direction

“…As a result, Malaspina et al (2020) conclude that a near-radial ambient magnetic field, though necessary, is not a sufficient condition for the growth of near-f ce waves. Earlier studies have suggested that near-f ce harmonic structures are composed of electron Bernstein waves, and at least two other wave modes that have not been completely identified so far (Malaspina et al 2020;Ma et al 2021;Malaspina et al 2021;Shi et al 2022). The generation mechanism of these waves is still an open question.…”