Signatures of Steady Heating in Time Lag Analysis of Coronal Emission

TL;DR

This paper uses time lag analysis of coronal emission to distinguish between steady and impulsive heating, demonstrating that zero time lag signals are indicative of evolving plasma rather than steady heating.

Contribution

It shows that zero time lag in coronal emission is caused by plasma evolution, not steady heating, refining the interpretation of time lag analysis in solar physics.

Findings

Zero time lag indicates plasma evolution, not steady heating.

Most time lags are consistent with impulsive cooling of heated plasma.

Highly correlated zero lag light curves are incompatible with equilibrium solutions.

Abstract

Among the many ways of investigating coronal heating, the time lag method of Viall & Klimchuk (2012) is becoming increasingly prevalent as an analysis technique complementary to those traditionally used. The time lag method cross correlates light curves at a given spatial location obtained in spectral bands that sample different temperature plasmas. It has been used most extensively with data from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory. We have previously applied the time lag method to entire active regions and surrounding quiet Sun and create maps of the results (Viall & Klimchuk 2012; Viall & Klimchuk 2015). We find that the majority of time lags are consistent with the cooling of coronal plasma that has been impulsively heated. Additionally, a significant fraction of the map area has a time lag of zero. This does not indicate a lack of variability. Rather, strong variability must be present, and it must occur in phase in the different channels. We have shown previously that these zero time lags are consistent with the transition region response to coronal nanoflares (Viall & Klimchuk 2015; Bradshaw & Viall 2016), but other explanations are possible. A common misconception is that the zero time lag indicates steady emission resulting from steady heating. Using simulated and observed light curves, we demonstrate here that highly correlated light curves at zero time lag are not compatible with equilibrium solutions. Such light curves can only be created by evolution.