Project: Plasma kinetics of the expanding solar wind, dp189. Data Intensive Cambridge, CSD3
PI: Lorenzo Matteini, Imperial
Recent near-Sun observations by the NASA Parker Solar Probe have demonstrated the diffuse presence of interplanetary magnetic field reversals, called “switchbacks”, whose origin is under debate. It’s believed that these structures can play an important role in solar wind acceleration and also bring signatures of processes responsible for Coronal heating (Raouafi, Matteini et al. SSR 2024).
The solar wind is permeated by Alfvén waves launched by the Sun. Spacecraft observations at various distances from the Sun show that such large-amplitude plasma waves continuously modify the local orientation of the interplanetary magnetic field B, but with the surprising property that |B| remains constant. Geometrically, measurements of the magnetic field vector B sit on a spherical surface; this is referred as a “spherical polarisation” state for the fluctuations.
To capture the emergence of spherical polarisation in the solar wind and investigate its radial evolution and possible connection to switchbacks, we performed simulations of Alfvén waves in the solar wind using a Hybrid-PIC code with an Expanding Box model. The model describes the evolution of the plasma during its spherical expansion away from the Sun starting at a distance R0 and captures the self-consistent evolution of fields and particles as a function of the radial distance R.
In the simulations, magnetic fluctuations – initially only in the transverse component BT inR0 –gradually become arc-shaped (left panel), then introducing large changes also in the radial component BR. As the B strength decreases with R, the polarisation becomes spherical (middle), like in-situ observations (right). When the polarisation sphere is more extended (red), some BR sign reversals appear, corresponding to the magnetic “switchbacks” observed by PSP (right panel).
This study supports the idea that switchbacks can be generated/amplified by expansion and suggests that they will be less frequent in regions even closer to the Sun, at the beginning of the expansion. This prediction seems to be confirmed by recent PSP observations in the sub-Alfvénic solar wind.
Fig.1: Evolution of spherical polarisation in an expanding hybrid simulation (Matteini et al, 2024). Left panel: Projection of the magnetic fluctuations in the (BT , BR) plane where T is one of the
transverse directions, for different radial distances encoded by different colors. Middle panel: projection in the plane (B⊥,BR), where fluctuations are normalized to the local average magnitude ⟨B⟩ for each distance R; a subset of distances from the left panel is shown, with the same color code. Right panel: magnetic field data from PSP first perihelion at 35 solar radii from the Sun.
These results have been published in the special topic: “Plasma physics of the Sun in honor of Eugene Parker”, in Physics of Plasma: Matteini et al. 2024 (doi: 10.1063/5.0177754). The article was chosen as the cover of the PoP March 2024 Issue and indicated as a 2024 highlight by the Editor-in-Chief.
The article was also picked for an Editor’s “News & Views” highlight by Nature Astronomy:
Gaire, B. Maintaining spherical polarization in solar wind plasma. Nat Astron 8, 410 (2024). https://doi.org/10.1038/s41550-024-02265-0