Whistler waves generated inside magnetic dips in the young solar wind: Observations of the search-coil magnetometer on board Parker Solar Probe

Abstract

Context. Whistler waves are electromagnetic waves produced by electron-driven instabilities, which in turn can reshape the electron distributions via wave–particle interactions. In the solar wind they are one of the main candidates for explaining the scattering of the strahl electron population into the halo at increasing radial distances from the Sun and for subsequently regulating the solar wind heat flux. However, it is unclear what type of instability dominates to drive whistler waves in the solar wind. Aims. Our goal is to study whistler wave parameters in the young solar wind sampled by Parker Solar Probe (PSP). The wave normal angle (WNA) in particular is a key parameter to discriminate between the generation mechanisms of these waves. Methods. We analyzed the cross-spectral matrices of magnetic field fluctuations measured by the search-coil magnetometer (SCM) and processed by the Digital Fields Board (DFB) from the FIELDS suite during PSP’s first perihelion. Results. Among the 2701 wave packets detected in the cross-spectra, namely individual bins in time and frequency, most were quasi-parallel to the background magnetic field; however, a significant part (3%) of the observed waves had oblique (> 45°) WNA. The validation analysis conducted with the time series waveforms reveal that this percentage is a lower limit. Moreover, we find that about 64% of the whistler waves detected in the spectra are associated with at least one magnetic dip. Conclusions. We conclude that magnetic dips provide favorable conditions for the generation of whistler waves. We hypothesize that the whistlers detected in magnetic dips are locally generated by the thermal anisotropy as quasi-parallel and can gain obliqueness during their propagation. We finally discuss the implications of our results for the scattering of the strahl in the solar wind.

Whistler Wave Observations by Parker Solar Probe During Encounter 1: Counter-propagating Whistlers Collocated with Magnetic Field Inhomogeneities and their Application to Electric Field Measurement Calibration

“…Wave amplitudes peaked at 2 nT representing up to 0.05 of the background magnetic field magnitude. The polarization of these waves was found to be predominantly quasi-parallel to the background magnetic field confirming the statistics reported by Cattell et al (2020) and Froment et al (2022). The quasi-parallel propagation appears to be the predominant mode of the observed whistler waves in the solar wind in all the range of heliocentric distances covered by observations by PSP and Solar Orbiter: the probability to observe waves with wave normal angle above 50°is less than 0.01 (Tong et al 2019;Kretzschmar et al 2021;Cattell et al 2022).…”

“…There was strong whistler wave activity throughout much of PSP's Encounter 1 and several authors have reported on these observations (Agapitov et al 2020;Cattell et al 2021aCattell et al , 2021bCattell et al , 2022Dudok de Wit et al 2022;Froment et al 2022). Recently, whistler waves observed during Encounter 1 have been linked to the scattering of strahl at distances less than 50R e (less than 0.25 au) from the Sun (Cattell et al 2021a(Cattell et al , 2021bJagarlamudi et al 2021) including some of the cases reported in this article.…”

Particle-in-Cell Simulations of Sunward and Anti-sunward Whistler Waves in the Solar Wind

“…These indications consist in statistically significant observations of the halo population with perpendicular temperature anisotropy (Pierrard et al 2016;Wilson et al 2019;Jagarlamudi et al 2020;Wilson et al 2020;Salem et al 2023) and preferential occurrence of whistler waves in association with isotropic or perpendicular anisotropic halo (Tong et al 2019a(Tong et al , 2019bJagarlamudi et al 2020). The recent reports of sunward and anti-sunward-propagating whistler waves in the near-Sun solar wind are also of relevance (Mozer et al 2020;Froment et al 2023).…”

“…Since whistler waves in the solar wind have typical amplitudes below a few percent of the background magnetic field, we expect the typical efficiency of local electron heat flux suppression to be within about 10%. The recent reports of sunward whistler waves with amplitudes of 0.1B 0 (Mozer et al 2020;Froment et al 2023) indicate, however, that the local electron heat flux suppression may occasionally exceed 60%.…”

Reconstruction of Polarization Properties of Whistler Waves From Two Magnetic and Two Electric Field Components: Application to Parker Solar Probe Measurements

“…The statistical study of whistler properties by Froment et al. (2023) revealed that most of the whistler wave packets recorded during Encounter 1 were quasi‐parallel to the background magnetic field: 97% had WNA between 0 and 25°. Whistler waves were observed in the frequency range from the local lower hybrid frequency f lh up to 0.2 f ce .…”