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Among the many ways of investigating coronal heating, the time lag method ofViall & Klimchuk (2012) is becoming increasingly prevalent as an analysistechnique complementary to those traditionally used. The time lag method crosscorrelates light curves at a given spatial location obtained in spectral bandsthat sample different temperature plasmas. It has been used most extensivelywith data from the Atmospheric Imaging Assembly on the Solar DynamicsObservatory. We have previously applied the time lag method to entire activeregions and surrounding quiet Sun and create maps of the results (Viall &Klimchuk 2012; Viall & Klimchuk 2015). We find that the majority of time lagsare consistent with the cooling of coronal plasma that has been impulsivelyheated. Additionally, a significant fraction of the map area has a time lag ofzero. This does not indicate a lack of variability. Rather, strong variabilitymust be present, and it must occur in phase in the different channels. We haveshown previously that these zero time lags are consistent with the transitionregion response to coronal nanoflares (Viall & Klimchuk 2015; Bradshaw & Viall2016), but other explanations are possible. A common misconception is that thezero time lag indicates steady emission resulting from steady heating. Usingsimulated and observed light curves, we demonstrate here that highly correlatedlight curves at zero time lag are not compatible with equilibrium solutions.Such light curves can only be created by evolution.

Signatures of Steady Heating in Time Lag Analysis of Coronal Emission