Whistler wave occurrence and the interaction with strahl electrons during the first encounter of Parker Solar Probe

Abstract

Aims. We studied the properties and occurrence of narrowband whistler waves and their interaction with strahl electrons observed between 0.17 and 0.26 au during the first encounter of Parker Solar Probe. Methods. We used Digital Fields Board band-pass filtered (BPF) data from FIELDS to detect the signatures of whistler waves. Additionally parameters derived from the particle distribution functions measured by the Solar Wind Electrons Alphas and Protons (SWEAP) instrument suite were used to investigate the plasma properties, and FIELDS suite measurements were used to investigate the electromagnetic (EM) fields properties corresponding to the observed whistler signatures. Results. We observe that the occurrence of whistler waves is low, nearly ~1.5% and less than 0.5% in the analyzed peak and average BPF data, respectively. Whistlers occur highly intermittently and 80% of the whistlers appear continuously for less than 3 s. The spacecraft frequencies of the analyzed waves are less than 0.2 electron cyclotron frequency (fce). The occurrence rate of whistler waves was found to be anticorrelated with the solar wind bulk velocity. The study of the duration of the whistler intervals revealed an anticorrelation between the duration and the solar wind velocity, as well as between the duration and the normalized amplitude of magnetic field variations. The pitch-angle widths (PAWs) of the field-aligned electron population referred to as the strahl are broader by at least 12 degrees during the presence of large amplitude narrowband whistler waves. This observation points toward an EM wave electron interaction, resulting in pitch-angle scattering. PAWs of strahl electrons corresponding to the short duration whistlers are higher compared to the long duration whistlers, indicating short duration whistlers scatter the strahl electrons better than the long duration ones. Parallel cuts through the strahl electron velocity distribution function (VDF) observed during the whistler intervals appear to depart from the Maxwellian shape typically found in the near-Sun strahl VDFs. The relative decrease in the parallel electron temperature and the increase in PAW for the electrons in the strahl energy range suggests that the interaction with whistler waves results in a transfer of electron momentum from the parallel to the perpendicular direction.

Whistler instability driven by the sunward electron deficit in the solar wind

“…This scenario corresponds to the results of a nonlinear evolution of the WHFI as shown with a particle-in-cell simulation by Kuzichev et al (2019). In this scenario, the WHFI itself is driven by relative drift between the core and the halo populations and generates quasi-parallel whistler waves propagating in the direction of the heat flux (Gary et al 1975(Gary et al , 1994Lacombe et al 2014;Kajdič et al 2016;Jagarlamudi et al 2021). These waves then deform the electron VDF through resonant damping forming the sunward deficit.…”

“…Enhanced fluctuations in this frequency band in the solar wind often correspond to whistler waves. The most common observed type of whistler waves are right-hand polarised, quasi-parallel whistler waves (Lacombe et al 2014;Jagarlamudi et al 2020Jagarlamudi et al , 2021. Zhang et al (1998); Stansby et al (2016) and Kretzschmar et al (2021) further find that the majority of the detected waves propagate in the anti-sunward direction.…”

Switchbacks: statistical properties and deviations from Alfvénicity

“…This suggests that some boundary regions may be unstable with respect to the Kelvin-Helmholtz instability (Kasper et al 2019;Mozer et al 2020). Finally, these boundaries show enhanced wave activity ranging from Magnetohydrodynamics (MHD) scales (as we are going to show) to the whistler frequency range (50-150 Hz) (Agapitov et al 2020;Jagarlamudi et al 2021) and beyond. There are still many open questions regarding the nature and origin of switchbacks, with a special interest in their role in the solar wind energy balance and plasma heating.…”

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

“…Numerous processes may alter the electron VDFs between the corona and interplanetary space, including Coulomb collisions (Scudder & Olbert 1979a, 1979bSalem et al 2003;Landi et al 2012;Horaites et al 2019;Berčič et al 2021) and a variety of plasma instabilities, such as electrostatic modes (Gary 1978;Roberg-Clark et al 2018;López et al 2020), quasi-parallel whistler modes (Gary et al 1975(Gary et al , 1994(Gary et al , 1999Saeed et al 2017;Shaaban et al 2018;López et al 2019), oblique whistler/magnetosonic instabilities (Horaites et al 2018;Vasko et al 2019;Verscharen et al 2019;Micera et al 2020), and the firehose instability (Shaaban et al 2018;Innocenti et al 2020). New observations from the Parker Solar Probe (PSP) (Fox et al 2016) have provided recent support for the importance of whistler waves, particularly oblique modes Jagarlamudi et al 2021;Halekas et al 2021). However, it appears highly likely that multiple mechanisms may play a role, possibly through a multi-step process (Micera et al 2020).…”