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Laura Bercic (MSSL): Using in situ measured electron VDF to estimate solar coronal temperature and ambipolar electric field in the solar wind

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Abstract:

The solar coronal plasma which escapes the Sun’s gravity and expands through our solar system is called the solar wind. It consists mainly of electrons and protons, carries the Sun’s magnetic field and, at most heliocentric distances, remains weakly-collisional. Due to their small mass, the solar wind electrons have much higher thermal velocity than their positively charged counterpart, and play an important role in the solar wind energetics by carrying the heat flux away from the Sun. Their velocity distribution functions (VDFs) are complex, usually modeled by three components. While the majority of electrons belong to the low-energetic thermal Maxwellian core population, some reach higher velocities, forming either the magnetic field aligned strahl population, or an isotropic high-energy halo population. This shape of the electron VDF is a product of the interplay between Coulomb collisions, adiabatic expansion, global and local electro-magnetic fields and turbulence.

 

During the talk I will focus on the effects of two of these: global ambipolar electric field and Coulomb collisions. We compare the results of BiCoP kinetic solar wind model with the in situ solar wind measurements provided by the Parker Solar Probe.

 

Following the kinetic model results we can use features of in situ VDF to estimate plasma conditions in the solar corona and some global properties of the solar wind. We demonstrate that the fast streaming electrons forming the strahl population do not strongly interact with the surrounding plasma and partially preserve the shape of the VDF they had in the solar corona. This allows us to estimate coronal electron temperature at their origin. The interplay between ambipolar electron field and Coulomb collisions defines the location of the transitions between separate electron populations in the velocity space. Recognizing the strahl break-point and the electron cutoff velocity in the in situ VDFs, thus gives estimations about the strength of the ambipolar electric field and potential in the solar wind.

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