Solar wind as a natural laboratory for study of turbulence

Advisor: Alexander Pitňa and Jana Šafránková (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: jana.safrankova@mff.cuni.cz

The solar wind is a weakly collisional plasma that is in a turbulent state and thus it represents a unique laboratory for study of turbulence in astrophysical plasmas. The results of solar wind studies can be scaled to diverse environments ranging from interstellar medium through plasmas in industrial applications up to controlled nuclear fusion in tokamaks. In-situ spacecraft observations have revealed that the solar wind as a magnetized plasma is characterized by a very broad electromagnetic power spectrum that includes time scales from several hours to about 0.01 s. Under the assumption of the Taylor hypothesis, it corresponds to a wide range of spatial scales. At time scales larger than the ion-gyroscale (i.e., in the inertial range), the power spectral density follows a power law decay −5/3 [1], at smaller time scales, where the ion physics becomes dominant, a second, steeper spectral range has been found [2].

In our group, investigations of solar wind turbulence from inertial to kinetic scales are facilitated by the high-time resolution data from the Bright Monitor of the Solar Wind (BMSW) instrument [3] that was developed at the faculty and that successfully operated in the solar wind from August 2011 to 2019. Using this and many spacecraft operating at various regions of the interplanetary space in past, at present and in a near future (Helios 1, Helios 2, Voyager, Wind, ACE, SOHO, Parker Solar Probe, Solar Orbiter, Bepi Colombo), the project addresses all scales accessible by the data series ranging from the solar cycle through the MHD scale up to ion dissipation range with motivation to clarify the understanding of the processes of dissipation (or dispersion) of turbulent energy that are still under debate. Clarification of these topics could solve the open problem of solar wind heating during its expansion through the heliosphere.

Turbulence in the solar wind is mainly Alfvenic and thus incompressible. In the inertial range, the compressive energy constitutes 10% or less of the total turbulent energy and consists of slow-wave-like fluctuations, with a negligible contribution from fast waves [4] but the compressive portion increases in the kinetic range due to the excitation of kinetic modes [5]. In detail, we will focus on an evolution of turbulence in the solar wind on different scales in both the low- and high latitudinal solar wind streams and at the various distances from the Sun. We will address also turbulent processes in other regions, as the Earth’s foreshock or magnetosheath where they play a significant role in transport of the solar wind mass and energy into the magnetosphere.

Literature:

[1] Matthaeus, W H; and Goldstein, M L, Measurement of the rugged invariants of magneto-hydrodynamic turbulence in the solar-wind, J. Geophys. Res., 87: 6011-6028, 1982.
[2] Leamon, R J; Smith, C W; Ness, N F; Matthaeus, W H, and Wong, H K, Observational constraints on the dynamics of the interplanetary magnetic field dissipation range, J. Geophys. Res., 103: 4775--4787, 1998.
[3] Safrankova, J; Nemecek, Z; Prech, L; Zastenker, G; Cermak, I; et al., Fast Solar Wind Monitor (BMSW): Description and first results, Space Sci. Rev., 175 (1-4): 165–182, 2013.
[4] Howes, G G; Bale, S D; Klein, K G; Chen, C H K; Salem, C S; and TenBarge, J M, The slow-mode nature of compressible wave power in solar wind turbulence, Astrophys. J. Lett., 753 (1): Art. No. L19, 2012.
[5] Safrankova, J; Nemecek, Z; Nemec, F; Verscharen, D; Chen, CHK; Durovcova, T; Riazantseva, MO, Scale-dependent polarization of solar wind velocity fluctuations at the inertial and kinetic scales, Astrophys. J., 870 (1): Art. No. 40, 2019.