Plasma Beta Stratification in the Solar Atmosphere: A Possible Explanation for the Penumbra Formation

Abstract

Plasma beta is an important and fundamental physical quantity in order to understand plasma dynamics, particularly in the context of magnetically active stars, because it tells about the domination of magnetic versus thermodynamic processes on the plasma motion. We estimate the value ranges of plasma beta in different regions within the solar atmosphere and we describe a possible mechanism that helps forming a penumbra. For that we evaluate data from a 3D magnetohydrodynamic model of the solar corona above a magnetically active region. We compare our results with previously established data that is based on magnetic field extrapolations and that was matched for some observations. Our model data suggest that plasma beta in the photosphere should be considered to be larger than unity outside of sunspots. However, in the corona we also find that the beta value range reaches lower than previously thought, which coincides with a recent observation. We present an idea based on a gravity-driven process in a high-beta regime that might be responsible for the formation of the penumbra around sunspot umbra, where the vertical field strength reaches a given threshold. This process would also explain counter-Evershed flows. Regarding the thermal and magnetic pressure within the mixed-polarity solar atmosphere, including non-vertical magnetic field and quiet regions, plasma beta may reach unity at practically any height from the photosphere to the outer corona.

Transient Inverse-FIP Plasma Composition Evolution within a Solar Flare

“…The high shear in coalescing strands of the same magnetic polarity suggests that the strands with a non-zero component of anti-parallel magnetic field have the possibility for sub-chromospheric magnetic reconnection when brought together. In the highβ plasma regime of the photosphere/low chromosphere, upward-moving acoustic waves are generated which can mode convert to fast mode waves at ∼1 Mm above the photosphere in the chromosphere where plasma-β is equal to unity (Bourdin 2017). Over regions of high magnetic field concentration such as sunspots, the transition to a low-β plasma occurs at lower heights within the photosphere.…”

Magnetic Helicity Reversal in the Corona at Small Plasma Beta

“…To set the stage, we show in Figure 2 vertical profiles of H M xy , H M xy / B 2 xy , and the plasma beta, 2µ 0 P xy / B 2 xy (cf. Bourdin 2017). For comparison, we also plot the corresponding profiles for averages over the AR core and the complementary quiet-Sun (QS) area.…”

Estimating the Magnetic Structure of an Erupting CME Flux Rope From AR12158 Using Data-Driven Modeling

“…From a space weather forecasting perspective, a faster approach is to neglect plasma effects and use a non-linear force free field (NLFFF; Wiegelmann and Sakurai, 2012;James et al, 2018) approximation, i.e., it is assumed that electric currents and magnetic fields are parallel to each other and related by a scalar function that varies in space. The force-free assumption is generally justified in the low corona, in particular above active regions, where the plasma beta is low (e.g., Gary, 2001;Bourdin, 2017). The drawback in the NLFFF approach is however that it is static and does not describe the dynamics of the eruption.…”