A chromospheric conundrum?

TL;DR

This study investigates chromospheric spectra to understand the apparent contradiction between observed bright network emissions and their inability to reach force balance, challenging existing models of chromospheric heating.

Contribution

It presents new observational evidence that bright chromospheric network patches expand little with wavelength, implying a potential conundrum in chromospheric force and energy balance.

Findings

Bright chromospheric network patches expand little with wavelength.

The observations suggest a mismatch between local heating and force balance.

The study raises questions about the mechanisms of chromospheric heating.

Abstract

We examine spectra of the Ca II H line, obtained under good seeing conditions with the VTT Echelle Spectrograph in June of 2007, and higher resolution data of the Ca II 8542 Angstrom line from Fabry-Perot instruments. The VTT targets were areas near disk center which included quiet Sun and some dispersed plage. The infrared data included quiet Sun and plage associated with small pores. Bright chromospheric network emission patches expand little with wavelength from line wing to line center, i.e. with increasing line opacity and height. We argue that this simple observation has implications for the force and energy balance of the chromosphere, since bright chromospheric network emission is traditionally associated with enhanced local mechanical heating which increases temperatures and pressures. Simple physical considerations then suggest that the network chromosphere may not be able to reach horizontal force balance with its surroundings, yet the network is a long-lived structure. We speculate on possible reasons for the observed behavior. By drawing attention to a potential conundrum, we hope to contribute to a better understanding of a long-standing unsolved problem: the heating of the chromospheric network.