Solar wind particles organized by the flow speed

Space Physics

Two new solar missions, Parker Solar Probe (NASA) launched in 2018 and Solar Orbiter (ESA) launched in February 2020, are taking the closest ever images of the Sun, observing the particles of the solar wind and the Sun's polar regions like never before. Combined with previous observations, they allow us to determine the presence of very energetic (suprathermal) particles close to the Sun at the origin of the acceleration of the solar wind and possibly the heating of the corona.

Measured in-situ, the velocity distributions of different solar wind species are used to determine their macroscopic properties (the moments), such as number densities n, bulk velocities u, temperatures T …. Solar wind core populations are still controlled by binary collisions, and hence they are following a Maxwellian-like distribution. Suprathermal halo populations, on the other hand, are practically collisionless (they are more dilute and much hotter) and hence they rather follow a so-called Kappa power law distribution.

To ensure the convergence of the moments, fitting an empirical Kappa distribution to the observations would require a sufficiently large kappa parameter. However, using a new regularized Kappa distribution with a Maxwellian-like cut-off of the power tails instead, ensures the convergence of the moments of any order and restrains the velocities to values below the speed of light in vacuum.

We determined links, direct- or anti-correlations between the solar wind bulk speed and physical properties of solar wind particles at 0,4 and 1 Astronomy Unit (AU, i.e. the distance between the Sun and the Earth) and found that:

The anti-correlation observed at 0,4 AU between the bulk velocity and the average number density appeared to remain valid also at 1 AU for both the core and halo electrons.
On the contrary, only the core electrons manifest a clear anti-correlation of temperature with the bulk velocity, while the halo temperature does not vary much.
Ions, protons and helium particles have a more reduced mobility and their properties exhibit different variations with the solar wind speed.

These results are used to shed more light on the mechanisms leading to a differential acceleration of these species and the origin of slow and fast wind modulation. Such results are also important to determine the evolution of the distribution functions of the particles with the radial distance and to better understand the physical mechanisms implicated in their variations by including them in the models.

Further reading

Pierrard, V., Lazar, M., and Štverák, S. (2020). Solar wind plasma particles organized by the flow speed. Solar Physics, 295, A151. https://doi.org/10.1007/s11207-020-01730-z
Lazar, M., Pierrard, V., Poedts, S., and Fichtner, H. (2020). Characteristics of solar wind suprathermal halo electrons. Astronomy & Astrophysics, 642, A130. https://doi.org/10.1051/0004-6361/202038830
Lazar, M., Scherer, K., Fichtner, H., and Pierrard, V. (2020). Towards a realistic macroscopic parametrization of space plasmas with regularized Kappa-distribution. Astronomy & Astrophysics, 634, A20. https://doi.org/10.1051/0004-6361/201936861

Number density in m-3 of solar wind core electrons as a function of the bulk velocity at 0,35-0,45 AU. Diamonds corresponds to averaged values and vertical lines to standard error bars (Figure from Pierrard et al., 2020).

Number density in m-3 of halo electrons as a function of the bulk velocity at 0,35-0,45 AU. Diamonds corresponds to averaged values and vertical lines to standard error bars (Figure from Pierrard et al., 2020).

2021