What can observations tell us about coronal heating?

Abstract

The actual source of coronal heating is one of the longest standing unsolved mysteries in all of astrophysics, but it is only in recent years that observations have begun making significant contributions. Coronal loops, their structure and sub-structure, their temperature and density details, and their evolution with time, may hold the key to solving this mystery. Because spatial resolution of current observatories cannot resolve fundamental scale lengths, information about the heating of the corona must be inferred from indirect observations. Loops with unexpectedly high densities and multi-thermal cross-field temperatures were not consistent with results expected from steady uniform heating models. The hot ( T >5 MK) plasma component of loops may also be a key observation; a new sounding rocket instrument called the Marshall Grazing Incidence X-ray Spectrometer will specifically target this observable. Finally, a loop is likely to be a tangle of magnetic strands. The High Resolution Coronal Imager observed magnetic braids untwisting and reconnecting, dispersing enough energy to heat the surrounding plasma. The existence of multi-thermal, cooling loops and hot plasma provides observational constraints that all viable coronal heating models will need to explain.

Recent advances in coronal heating

“…Observations show that the properties of coronal loops differ substantially from expectations of simple scaling laws, with loops having unexpectedly constant temperature distributions and being over-dense [8]. The detection of very hot plasma (T > 5 MK) in active regions, as predicted from nanoflare heating models by Cargill [13], is very significant, but requires further study with new instruments.…”

“…Although there is still no consensus on which mechanism is responsible for heating the solar atmosphere, a few simple but relevant 'facts' are now supported by a strong body of observational evidence (see also the reviews by Klimchuk [7] and Schmelz & Winebarger [8]):…”

“…In fact, all heating mechanisms so far proposed will give impulsive heating to some extent [7], but one of the fundamental questions to be resolved is the 'degree of unsteadiness' [18]. Comparison of predictions from modelling with recent observations from missions such as SDO is beginning to shed light on this [8,18].…”

“…Present understanding of the constraints and input provided by the observations is summarized by Schmelz & Winebarger [8]. There have been substantial advances in recent years, especially owing to data with high spatial resolution at a range of wavelengths from the Solar Dynamic Observatory (SDO); also very high spatial resolution data from the HiC rocket flight, X-ray imaging from XRT/Hinode, spectroscopy from EIS/Hinode, and most recently, IRIS (Interface Region Imaging Spectrometer) [12] looking at lower layers of the atmosphere.…”

“…One approach to observational tests of models is to use scaling laws, investigating how heating rates vary with field strength and loop length [17]. Analysis of differential emission measure distributions is proving a powerful tool for potentially distinguishing between heating mechanisms [8,18]. The presence of non-thermal particles could provide an important signature of energy dissipation mechanisms, often being associated with magnetic reconnection, and recent results from IRIS [19] present exciting evidence, albeit indirect, of their creation in small-scale heating events.…”