Sunward-propagating Whistler Waves Collocated with Localized Magnetic Field Holes in the Solar Wind: Parker Solar Probe Observations at 35.7 R_⊙ Radii

Abstract

Observations by the Parker Solar Probe mission of the solar wind at ~35.7 solar radii reveal the existence of whistler wave packets with frequencies below 0.1 (20-80 Hz in the spacecraft frame).These waves often coincide with local minima of the magnetic field magnitude or with sudden deflections of the magnetic field that are called switchbacks. Their sunward propagation leads to a significant Doppler frequency downshift from 200-300 Hz to 20-80 Hz (from 0.2 to 0.5 ). The polarization of these waves varies from quasi-parallel to significantly oblique with wave normal angles that are close to the resonance cone. Their peak amplitude can be as large as 2 to 4 nT. Such values represent approximately 10% of the background magnetic field, which is considerably more than what is observed at 1 a.u. Recent numerical studies show that such waves may potentially play a key role in breaking the heat flux and scattering the Strahl population of suprathermal electrons into a halo population.

Narrowband oblique whistler-mode waves: comparing properties observed by Parker Solar Probe at <0.3 AU and STEREO at 1 AU

“…We conclude, therefore, that the waves are most likely driven by the fan instability. This is consistent with the study of the electron heat flux and beta for all intervals inside 0.25 AU in the first five encounters by Halekas et al (2020b), and with the conclusion of Agapitov et al (2020) for a set of events in Encounter 1. We note, however, that there are a significant number of cases with large β e where the normalized heat flux is above the threshold (most are from Encounter 4).…”

“…The PSP whistlers occurred when beta was high, and the events for which Doppler shifts could be determined had frequencies of f / f ce of ∼0.2. Most cases, however, propagated within 20 degrees of the magnetic field, and none were propagating close to the resonance cone as was seen by Agapitov et al (2020) in a study of lower frequency whistlers in Encounter 1 and by Cattell et al (2020) at 1 AU. The magnetosonic-whistler mode is low beta, and the most unstable modes are at higher frequencies (f / f ce ∼ 0.5) than observed at PSP (Verscharen et al 2019).…”

“…7). The PSP data shown herein include both parallel and oblique waves, as is also described in Agapitov et al (2020). There is a tendency for more electrostatic whistlers (i.e., more oblique) to occur within regions of enhanced flow.…”

“…For the Encounter 1 events, for which the shifts could be determined, the shifts increased the average f / f ce to ∼0.2, which is comparable to the value seen at 1 AU (Cattell et al 2020). The lower frequency whistlers described by Agapitov et al (2020), utilizing the lower sample rate fields data set, had significantly larger relative Doppler shifts. The normalized frequency in the spacecraft frame, f / f ce , has a tendency to increase with distance from the Sun.…”

“…The occurrence frequency decreases with distance from the Sun, which is consistent with their absence in the STEREO waveform data at 1 AU. Lower frequency sunward propagating whistler-mode waves are also observed by PSP (Agapitov et al 2020), primarily in association with decreases in the magnetic field or the rapid change in magnetic field orientation called "switchbacks" or "jets" (Bale et al 2019;Kasper et al 2019).…”