Multicomponent solar wind and its evolution through the heliosphere

Advisor: Lubomir Prech and Zdenek Nemecek (DSPS FMF CUNI)

Funding: Fully funded (Supported by the grant of the advisors and by the DSPS).

Website: https://physics.mff.cuni.cz/kfpp/

Contact: zdenek.nemecek@mff.cuni.cz

The solar wind is fully ionized gaseous matter emanating from the solar corona that fills a whole interplanetary space. The speed of this flow does not change substantially during solar wind expansion and varies between 300 and 800 km/s whereas the ion number density decreases rapidly with the distance from the Sun being about 10 ions in cm-3 at the Earth orbit. Conductivity of the solar wind plasma is very high and thus the solar wind carries frozen-in solar magnetic field that, on the other hand, controls a motion of charged particles. This interaction results in two types of a solar wind – the fast wind that is typically observed in high solar latitudes where it generally propagates along the magnetic field lines, and the slow wind that dominates in low latitudes near the ecliptic plane. The mutual interaction of these streams can produce regions of highly compressed plasma.

The solar atmosphere is gravitationally stratified and thus the mass composition depends on the distance from the solar surface. The main constituent in all altitudes is ionized hydrogen (protons) and the relative abundance of heavier species decreases with the height and it is about 4 % at altitudes where the solar wind originates. It is generally believed that the variability of this abundance observed in the solar wind is thus connected with sources of a particular stream.

Early, Helios 1 and 2 solar wind observations between 0.3 and 1 AU [1] revealed that (1) alpha particles generally move faster than protons, however, alpha particles slower than protons can also be observed; (2) the differential speed of these two species is roughly proportional to that of protons and scales with the Alfvén speed; (3) the differential speed decreases with distance from the Sun; and (4) all these features are more distinct in fast solar wind streams. Later, these findings were confirmed by Ulysses observations up to 5 AU [2]. These features are consistent with the general view on the helium abundance controlled by the source region but Durovcova et al. [3] have shown that the abundance can change due to interaction of solar wind streams. Moreover, the solar wind contains also heavier species and their bulk speeds can differ from those of protons or alpha particles. Nevertheless, the experimental data on them were rather scarce up to now. To make things even more complicated, the velocity distribution of different ion species usually consists of two distinct populations – core and beam – that differ by their bulk velocities and, up to now, knowledge of their properties is limited.

Using remote sounding of the solar corona and in situ spacecraft measurements (Parker Solar Probe, Solar Orbiter, Wind, ACE), the applicants can concentrate on different aspects of evolution of ion species through the heliosphere. Their description will be based on two populations approach, and the subjects of studies will be their relative abundance, differential speeds and relations to the dominant proton population.

Literature:

[1] Marsch, E; Muhlhauser, K H; Rosenbauer, H; Schwenn, R; and Neubauer, F M, J. Geophys. Res., 87: (A1), 35-51, 1982.
[2] Neugebauer, M; Goldstein, B E; Smith, E J; and Feldman, W C, J. Geophys. Res., 101 (A8): 17047--17055, 1996.
[3] Durovcova, T; Nemecek, Z; Safrankova, J, Evolution of the α-proton differential motion across stream interaction regions, Astrophys. J., 873 (1): Art. No. 24, 2019.