Evidence of Solar Coronal Heating by Nanoflares Based on Time-Lag Measurements in EUV Light Curves from EIS

Abstract:

The nanoflare model is a major contender to explain solar coronal heating. The model is based on the idea that ubiquitous tiny, independent heating events occur on individual sub-resolution strands within coronal loops. Each heating event raises its strand plasma to temperatures (6 - 10 MK) that are greater than the average active region temperature (~2 MK). Compelling evidence for this mechanism is pervasive faint emission at flare-like temperatures, such as that detected in an active region by the EUNIS sounding rocket. After the impulsive energy release, the loop strand cools by conduction and radiation, during which it spends more time at higher density and at colder temperatures than it does at hotter temperatures. Thus, even when observed on spatial scales larger than the unresolvable individual strands, the solar atmosphere is expected to exhibit an overall cooling trend. Evidence for this cooling trend has been sought and found based on correlations among light curves from AIA’s six EUV channels. While this provides further support for the nanoflare model of coronal heating, AIA’s lack of temperature fidelity means that precise cooling information for small locations or single events are less than conclusive. Here we report preliminary results from an investigation of time-lag diagnostics based on EUV light curves from Hinode/EIS spectra. We present results for non-flaring active regions and quiet-sun areas derived from stare spectra obtained with several different EIS studies that observe unblended emission lines formed at temperatures that range from 0.14 to 14 MK. We performed time-lag diagnostics on light curves of Fe XXIII, Fe XVII, Fe XVI, Fe XIV, S X, Si VII, Mg VI, O IV, and other lines. For example, for a 2014 March 11 observing run on AR 12002, EIS observed fan loops that cooled slowly between 1.4 and 0.6 MK on timescales of ~3000s; microflares that cooled from 14 to 2 MK on timescales of ~1000 s; and core loops that cooled from about 4 to 0.1 MK on timescales ~1500 s. Properties such as peak temperature, the timescales of the cooling, and a determination of whether the cooling is full or partial all provide valuable constraints on nanoflares as a source of coronal heating.