Magnetic Helicity Reversal in the Corona at Small Plasma Beta

Abstract

Solar and stellar dynamos shed small-scale and large-scale magnetic helicity of opposite signs. However, solar wind observations and simulations have shown that some distance above the dynamo both the smallscale and large-scale magnetic helicities have reversed signs. With realistic simulations of the solar corona above an active region now being available, we have access to the magnetic field and current density along coronal loops. We show that a sign reversal in the horizontal averages of the magnetic helicity occurs when the local maximum of the plasma beta drops below unity and the field becomes nearly fully force free. Hence, this reversal is expected to occur well within the solar corona and would not directly be accessible to in-situ measurements with the Parker Solar Probe or SolarOrbiter. We also show that the reversal is associated with subtle changes in the relative dominance of structures with positive and negative magnetic helicity.

On the influence of magnetic helicity on X-rays emission of solar and stellar coronae

“…In particular, as we are mostly interested in the scaling of magnetic helicity to X-ray emission, the gauge will not affect our results. Furthermore, the work of Bourdin et al (2018) have shown that calculating magnetic helicity density using the resistive gauge, the magnetic helicity spectrum and the relative helicity by Berger & Field (1984) give consistent results for a simulation of the solar corona, very similar to the ones used in our work. We are therefore convinced that calculation of magnetic helicity in our gauge is meaningful and we can use the observed values of magnetic helicity (Zhang et al 2014(Zhang et al , 2016 as an motivation to our photospheric helicity injection.…”

“…Nindos et al 2003;Pariat et al 2017). Furthermore, there are indication that magnetic helicity also plays an important role the heating in coronal loops (Warnecke et al 2017;Bourdin et al 2018). However, how the coronal heating scales with magnetic helicity have not been studied so far.…”

Advances in mean-field dynamo theory and applications to astrophysical turbulence

“…If, however, the sign reversal occurs near the point where the plasma is unity, as now predicted by Bourdin et al. (2018), we would need to measure even closer to the surface. This requires remote sensing via polarimetry.…”

“…NASA's Parker Solar Probe mission will have a magnetometer on board as well and will be able to approach the Sun to within 0.04 AU = 6000 Mm. If, however, the sign reversal occurs near the point where the plasma beta is unity, as now predicted by Bourdin et al (2018), we would need to measure even closer to the surface. This requires remote sensing via polarimetry.…”

“…Thus, for the Sun, we expect a similar sign change to occur somewhere above the surface, and perhaps already within the corona. Realistic corona simulations by Bourdin, Bingert & Peter (2013) have now shown that this magnetic helicity reversal occurs when the magnetic plasma drops below unity (Bourdin, Singh & Brandenburg 2018), i.e. when the plasma becomes dominated by magnetic pressure compared with the gas pressure.…”