Ambipolar Electric Field and Potential in the Solar Wind Estimated from Electron Velocity Distribution Functions

Berčič, Laura; Maksimović, M.; Halekas, J. S.; Landi, Simone; Owen, C. J.; Verscharen, Daniel; Larson, D. E.; Whittlesey, P. L.; Badman, S. T.; Bale, S. D.; Case, A. W.; Goetz, K.; Kasper, J. C.; Korreck, K. E.; Livi, R.; MacDowall, R. J.; Malaspina, D.; Pulupa, M.; Stevens, Michael L.

doi:10.3847/1538-4357/ac1f1c

Cited by 19 publications

(20 citation statements)

References 75 publications

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“…If collisions and other scattering mechanisms are neglected, a sharp cut-off is thus expected on the sunward side of the electron distribution, marking the separation between the reflected and the (missing) electrons that have escaped the potential. This cut-off has been observed in the form of a sunward deficit in the electron distribution function in data from Parker Solar Probe (Berčič et al, 2021a;Halekas et al, 2021a).…”

Section: Electron-deficit Whistler Instabilitymentioning

confidence: 79%

Electron-Driven Instabilities in the Solar Wind

Verscharen

Chandran²,

Boella³

et al. 2022

Front. Astron. Space Sci.

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The electrons are an essential particle species in the solar wind. They often exhibit non-equilibrium features in their velocity distribution function. These include temperature anisotropies, tails (kurtosis), and reflectional asymmetries (skewness), which contribute a significant heat flux to the solar wind. If these non-equilibrium features are sufficiently strong, they drive kinetic micro-instabilities. We develop a semi-graphical framework based on the equations of quasi-linear theory to describe electron-driven instabilities in the solar wind. We apply our framework to resonant instabilities driven by temperature anisotropies. These include the electron whistler anisotropy instability and the propagating electron firehose instability. We then describe resonant instabilities driven by reflectional asymmetries in the electron distribution function. These include the electron/ion-acoustic, kinetic Alfvén heat-flux, Langmuir, electron-beam, electron/ion-cyclotron, electron/electron-acoustic, whistler heat-flux, oblique fast-magnetosonic/whistler, lower-hybrid fan, and electron-deficit whistler instability. We briefly comment on non-resonant instabilities driven by electron temperature anisotropies such as the mirror-mode and the non-propagating firehose instability. We conclude our review with a list of open research topics in the field of electron-driven instabilities in the solar wind.

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Section: Electron-deficit Whistler Instabilitymentioning

confidence: 79%

Electron-Driven Instabilities in the Solar Wind

Verscharen

Chandran²,

Boella³

et al. 2022

Front. Astron. Space Sci.

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“…They guarantee quasi-neutrality and provide the solar wind with a significant heat flux through the nonthermal properties of the electron velocity distribution functions (VDFs; Marsch 2006). In addition, the electrons generate a global ambipolar electric field through their thermal pressure gradient (Jockers 1970;Lemaire & Scherer 1970, 1971Pierrard et al 1999;Scudder 2019;Berčič et al 2021).…”

Section: Introductionmentioning

confidence: 99%

The Stability of the Electron Strahl against the Oblique Fast-magnetosonic/Whistler Instability in the Inner Heliosphere

Jeong,

Abraham,

Verscharen

et al. 2022

Preprint

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We analyze the micro-kinetic stability of the electron strahl in the solar wind depending on heliocentric distance. The oblique fast-magnetosonic/whistler (FM/W) instability has emerged in the literature as a key candidate mechanism for the effective scattering of the electron strahl into the electron halo population. Using data from Parker Solar Probe (PSP) and Helios, we compare the measured strahl properties with the analytical thresholds for the oblique FM/W instability in the low-and high-β c regimes, where β c is the ratio of the core parallel thermal pressure to the magnetic pressure. Our PSP and Helios data show that the electron strahl is on average stable against the oblique FM/W instability in the inner heliosphere. Our analysis suggests that the instability, if at all, can only be excited sporadically and on short timescales. We discuss the caveats of our analysis and potential alternative explanations for the observed scattering of the electron strahl in the solar wind. Furthermore, we recommend the numerical evaluation of the stability of individual distributions in the future to account for any uncertainties in the validity of the analytical expressions for the instability thresholds.

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“…However, recent progress has been made by considering the impact of the electric potential on the eVDF. In Berčič et al (2021b), abbreviated here as B21, the ambipolar potential was indirectly inferred from eVDFs measured by the Parker Solar Probe (PSP) SPAN-E electron instrument (Whittlesey et al 2020). These measurements are based on two signals associated with the large-scale potential: 1) the "deficit" of sunward-moving electrons in the Maxwellian core (Halekas et al 2020(Halekas et al , 2021, and 2) the "breakpoint energy" (Scudder & Olbert 1979;Bakrania et al 2020) that delineates the core from the suprathermal electrons.…”

Section: Introductionmentioning

confidence: 99%

The Heliospheric Ambipolar Potential Inferred from Sunward-Propagating Halo Electrons

Horaites,

Boldyrev

2022

Preprint

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We provide evidence that the sunward-propagating half of the solar wind electron halo distribution evolves without scattering in the inner heliosphere. We assume the particles conserve their total energy and magnetic moment, and perform a "Liouville mapping" on electron pitch angle distributions measured by the Parker Solar Probe SPAN-E instrument. Namely, we show that the distributions are consistent with Liouville's theorem if an appropriate interplanetary potential is chosen. This potential, an outcome of our fitting method, is compared against the radial profiles of proton bulk flow energy. We find that the inferred potential is responsible for nearly 100% of the proton acceleration in the solar wind at heliocentric distances 0.18-0.79 AU. These observations combine to form a coherent physical picture: the same interplanetary potential accounts for the acceleration of the solar wind protons as well as the evolution of the electron halo. In this picture the halo is formed from a sunward-propagating population that originates somewhere in the outer heliosphere by a yet-unknown mechanism.

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Ambipolar Electric Field and Potential in the Solar Wind Estimated from Electron Velocity Distribution Functions

Cited by 19 publications

References 75 publications

Electron-Driven Instabilities in the Solar Wind

Electron-Driven Instabilities in the Solar Wind

The Stability of the Electron Strahl against the Oblique Fast-magnetosonic/Whistler Instability in the Inner Heliosphere

The Heliospheric Ambipolar Potential Inferred from Sunward-Propagating Halo Electrons

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