The Solar “Exosphere” and Electric Field

[Solar]

Can stars have their very own version of a Global Electric Circuit? I've wondered this question for quite some time. There is something about these next series of papers that seem to fit the concept although none of them make such an assertion. Nevertheless, the papers are interesting in their own right. They posit an "Exosphere" with electric field for the Sun in order to accelerate charged particles ie the solar wind. What is an ...

Exosphere: "the outermost region of a planet's atmosphere." - where the atmosphere thins, gas-like Brownian motion and particle collisions decrease and become weakly collisional and/or collisionless. This is the upper region of a stratified atmospheric envelope where particles are often characterized as "leaking" from a celestial body. Neutrals might do this, but charged particles can be easily accelerated from the surface out into the surrounding space. Earth has one, even airless bodies such as the Moon, Comets, Mars, and asteroids have some sort of dynamic atmospheric envelope.

This first paper was referenced earlier in another thread by Paladin (here). The paper is available in full here:

Electrons in the Young Solar Wind: First Results from the Parker Solar Probe
J. S. Halekas, P. Whittlesey, D. E. Larson, et al

Draw attention to paragraph two of the “Introduction” (pg. 3) where the authors note that, based on Exospheric Models, in order to maintain quasi-neutrality relative to gravitational forces that “… a significant sunward electric field must exist in the solar wind”.

Notice also that, in that one paragraph, the paper cites five different references to Exopheric models that posit the existence of such an electric field. In other words even though these Exopheric models put forth an electric field in the the solar version of an exosphere and heliosphere they may not have correctly described how said electric field works. However regardless of that, the authors are still under impression that this feature “must exist”. I took the liberty of following up on some of the references in that paragraph. Solar Exospheric (Kinetic) models are still being worked on today and the basic principle is included in NASA's Community coordinated Modeling Center

Exospheric Model (note the electric field as accelerating force)

These are interesting models because Here follows examples of work being done in this area:

Abstract
The solar coronal exosphere (fixed at a level where the mean free path is comparable with the scale height) is shown to act like the sheath in a laboratory discharge that builds up an excess positive charge and electric field. Equality of flux for the charged particles (protons and electrons) at the exosphere is assumed. The exospheric electric potential (for a coronal temperature of 106 °K) is found to be about 325 V, which makes the protons virtually weightless. The exospheric sheath potential accelerates the thermal protons to a velocity of 258 km/sec with which they coast to the earth. The proton density in transit varies approximately as the inverse square law, and is 3 cm-3 near the earth. Higher velocities and densities are obtained for higher coronal temperatures. An estimate of the exospheric transition region is obtained by applying the hydrodynamic theory of ambipolar diffusion in a gravitational field to the “observed” coronal density distribution. It is found that the ratio of the electric to the gravitational force on the proton builds up from its hydrostatic value of 12 at 2·4R⊚ to 1 at 8R⊚ from the solar center. - The electric field in the solar coronal exosphere and the solar wind - Hari K.Sen 1969 (Paywalled)

Very interesting. I’ve gathered a few papers on this topic and am still trying to study them. Consider this next one: What would happen if small electrostatic potential drops across Coherent Electrostatic Waveforms (CEW's) detected by the WIND spacecraft at Lagrange L1 were interpreted as the signature of “weak double layers” between Earth and Sun as opposed to being "waves"? Here open access to:

Abstract. In the solar wind at 1 AU, coherent electrostatic
waveforms in the ion acoustic frequency range ('1 kHz) have been observed by the Time Domain Sampler (TDS) instrument on the Wind spacecraft. Small drops of electrostatic potential (18 10−3 V) have been found across some of these waveforms, which can thus be considered as weak double layers (Mangeney et al., 1999). The rate of occurrence of these potential drops, at 1AU, is estimated by a comparison of the TDS data with simultaneous data of another Wind instrument, the Thermal Noise Receiver (TNR), which measures continuously the thermal and non-thermal electric spectra above 4 kHz. We assume that the potential drops have a constant amplitude and a constant rate of occurrence between the Sun and the Earth. The total potential drop between the Sun and the Earth, which results from a succession of small potential drops during the Sun-Earth travel time, is then found to be about 300V to 1000V, of the same order of magnitude as the interplanetary potential implied by a two-fluid or an exospheric model of the solar wind: the interplanetary potential may manifest itself as a succession of weak double layers. We also find that the hourly average of the energy of the non-thermal ion acoustic waves, observed on TNR between 4 and 6 kHz, is correlated to the interplanetary electrostatic field, parallel to the spiral magnetic field, calculated with a two-fluid model: this is another evidence of a relation between the interplanetary electrostatic field and the electrostatic fluctuations in the ion acoustic range. We have yet to discuss the role of the Doppler effect, which is strong for ion acoustic waves in the solar wind, and which can bias the measure of the ion acoustic wave energy in the narrow band 4–6 kHz. - Evidence for the interplanetary electric potential? WIND observations of electrostatic fluctuations - C. Lacombe, C. Salem, A. Mangeney, D. Hubert et al, 2002

Lastly another open access paper does a very good job of mentioning other models and their problems relative to Exospheric models:

Abstract. A kinetic model of the solar wind based on Kappa velocity distribution functions for the electrons and protons escaping out of the corona is presented. The high velocity particles forming the tail of these distribution functions have an enhanced phase space density compared to a Maxwellian. The existence of such velocity distribution functions have been introduced in the pioneering work of Scudder (1992a,b) to explain the high temperature of the coronal plasma. The first results obtained with this new kinetic model of the solar wind are very encouraging, indeed they fit better many major features observed in the solar wind than earlier models: e.g. the large bulk velocities observed in high speed streams emitted out of coronal regions where the plasma temperature is smaller, and the low speed solar wind originating in the hotter equatorial regions of the solar corona. This new kinetic model is also able to predict the high speed solar wind streams without unreasonably large coronal temperatures and without additional heating of the outer region of the corona, as it is needed in hydrodynamic models to achieve the same solar wind speed. -A kinetic model of the solar wind with Kappa distribution functions in the corona M. Maksimovic, V. Pierrard, and J.F. Lemaire

Hmmm...

[dren]

Thanks Solar, especially that last article. It does do a great job explaining the idea behind the hydrodynamic and exospheric models and the issues both have describing the solar wind measured at the Earth.