Conditions for Photospherically Driven Alfvenic Oscillations to Heat the Solar Chromosphere by Pedersen Current Dissipation

TL;DR

This study models how photospheric Alfvenic oscillations can heat the solar chromosphere through Pedersen current dissipation, showing conditions where this process matches observed chromospheric radiative losses.

Contribution

It provides a detailed MHD model including a complete conductivity tensor to estimate chromospheric heating from Alfvenic oscillations driven by photospheric flux tubes.

Findings

Chromospheric heating flux $F_{Ch} extasciitilde 10^7 - 10^8$ ergs-cm$^{-2}$-sec$^{-1}$ at 100-1000 mHz frequencies.

Heating is most significant in the photosphere and regulated by electron current dissipation.

Oscillations are likely normal modes of small flux tubes excited by magnetic reconnection.

Abstract

A magnetohydrodynamic model that includes a complete electrical conductivity tensor is used to estimate conditions for photospherically driven, linear, non-plane Alfvenic oscillations extending from the photosphere to the lower corona to drive a chromospheric heating rate due to Pedersen current dissipation that is comparable to the net chromospheric net radiative loss of $\sim 10^7$ ergs-cm$^{-2}$-sec$^{-1}$. The heating rates due to electron current dissipation in the photosphere and corona are also computed. The wave amplitudes are computed self-consistently as functions of an inhomogeneous background (BG) atmosphere. The effects of the conductivity tensor are resolved numerically using a resolution of 3.33 m. The oscillations drive a chromospheric heating flux $F_{Ch} \sim 10^7 - 10^8$ ergs-cm$^{-2}$-sec$^{-1}$ at frequencies $\nu \sim 10^2 - 10^3$ mHz for BG magnetic field strengths $B \gtrsim 700$ G and magnetic field perturbation amplitudes $\sim 0.01 - 0.1$ $B$. The total resistive heating flux increases with $\nu$. Most heating occurs in the photosphere. Thermalization of Poynting flux in the photosphere due to electron current dissipation regulates the Poynting flux into the chromosphere, limiting $F_{Ch}$. $F_{Ch}$ initially increases with $\nu$, reaches a maximum, and then decreases with increasing $\nu$ due to increasing electron current dissipation in the photosphere. The resolution needed to resolve the oscillations increases from $\sim 10$ m in the photosphere to $\sim 10$ km in the upper chromosphere, and is proportional to $\nu^{-1/2}$. Estimates suggest that these oscillations are normal modes of photospheric flux tubes with diameters $\sim 10-20$ km, excited by magnetic reconnection in current sheets with thicknesses $\sim 0.1$ km.