[P-EM16] Heliosphere and Interplanetary Space

[PEM16-P08] Linear analysis of the parametric decay instability in the expanding solar corona considering temperature anisotropy

*Hayato Saguchi¹, Yuto Katoh¹, Yohei Kawazura¹ (1.Department of Geophysics, Graduate School of Science, Tohoku University)

Keywords:Sun, Corona, Parametric decay instability, Linear analysis

Alfvén waves play an important role in coronal heating and the acceleration of the solar wind. Despite the recent progress of theoretical and observational studies on the Alfvén waves, their dissipation processes have not been fully understood. Parametric Decay Instability (PDI) is a nonlinear interaction between waves in which a large-amplitude, forward-propagating Alfvén wave resonates and decays into a backward-propagating Alfvén wave and a forward-propagating slow magnetosonic wave. The slow magnetosonic waves generated by PDI contribute to the efficient heating of the solar wind ions through the formation of shock waves and to the turbulent heating due to Alfvénic turbulence enhanced by Alfvén waves reflected by the density fluctuations[e.g., Shoda et al., 2018]. Therefore, PDI is an important physical process in coronal heating and solar wind acceleration. Recent studies have advanced our understanding of PDI in the solar atmosphere. Theoretical and observational studies showed that PDI operates in the solar wind [e.g., Tenerani and Velli, 2013; Bowen et al., 2018], and in the near-transition layer of the lower solar atmosphere [e.g., Hahn et al., 2022]. Previous studies have shown that the growth rate of PDI increases with higher temperature perpendicular to the magnetic field than parallel, lower plasma beta parallel to the magnetic field, and larger amplitude of the parent wave [e.g., Tenerani et al., 2017]. In this study, using the dispersion relation derived from the linearized CGL equations, we evaluated the linear growth rate of PDI in the solar corona. We investigated the effect of the expansion of plasma volume to PDI quantitatively. In the computation of the linear growth rate, we used the parameter representing the ratio of the temperature perpendicular to the magnetic field to the temperature parallel (\xi), the parallel beta (\beta_para), and the normalized amplitude of the parent wave (B_perp^2). In a case that (\xi, \beta_para, B_perp^2) = (10, 0.01, 1), we obtain the maximum growth rate \omega_i = 0.59 \omega_0, where \omega_0 is the frequency of the parent waves, suggesting that PDI occurs significantly in the corona. Additionally, we consider the expansion of the plasma volume by assuming B_perp^2 is constant and \xi and \beta_para change with radius as r^-2 and r^2, respectively. We obtained 0.54 \omega_0 for the case of \xi=5 and \beta_para = 0.02 and 0.45 \omega_0 for the case of \xi=1 and \beta_para = 0.1, showing that the maximum growth rate decreased with expansion. We discuss properties of PDI based on the computed linear growth rate, by referring to the radial dependence of the three parameters \xi, \beta_para, and B_perp^2 due to expansion, and the values of these parameters in the corona.