The Quiet Sun Network at Subarcsecond Resolution: VAULT Observations and Radiative Transfer Modeling of Cool Loops

Abstract

One of the most enigmatic regions of the solar atmosphere is the transition region ( TR), corresponding to plasmas with temperatures intermediate of the cool, few thousand K, chromosphere and the hot, few million K, corona. The traditional view is that the TR emission originates from a thin thermal interface in hot coronal structures, connecting their chromosphere with their corona. This paradigm fails badly for cool plasmas (%T < 10 5 K ), since it predicts emission orders of magnitude less than what it is observed. It was therefore proposed that the ''missing'' TR emission could originate from tiny, isolated from the hot corona, cool loops at TR temperatures. A major problem in investigating this proposal is the very small sizes of the hypothesized cool loops. Here, we report the first spatially resolved observations of subarcsecond-scale looplike structures seen in the Ly line made by the Very High Angular Resolution Ultraviolet Telescope (VAULT). The subarcsecond (%0.3 00 ) resolution of VAULT allows us to directly view and resolve looplike structures in the quiet Sun network. We compare the observed intensities of these structures with simplified radiative transfer models of cool loops. The reasonable agreement between the models and the observations indicates that an explanation of the observed fine structure in terms of cool loops is plausible.

Solar low-lying cool loops and their contribution to the transition region EUV output

“…We find cool loops with lengths of 5-10 Mm, but with very low heights (in the range 10 −2 −10 −1 Mm). Loops this low in the solar atmosphere (effectively embedded in the chromosphere) cannot be visible in the Ly-α line; moreover, compared with the observations of Patsourakos et al (2007), our loops typically have lower pressures compared to their estimates.…”

“…Some recent observations with the VAULT instrument (Very High Angular Ultraviolet Telescope, Korendyke et al 2001) in the H i Ly-α line at very high spatial resolution showed looplike structures with estimated temperatures and densities (T = 10 4 −3 × 10 4 K, P = 0.1−0.3 dyne cm −2 ) that could be appropriate for the low-temperature end of cool loops (Patsourakos et al 2007;Vourlidas et al 2010). While this interpretation has been debated (Judge & Centeno 2008), those observations, together with the persistence of the problem of the excess emission in EUV lines below 10 5 K, call for additional investigations on the matter.…”

Hydrogen Lyman-α and Lyman-β spectral radiance profiles in the quiet Sun

“…It is still under debate if the TR emission originates from a thin thermal interface connecting the chromosphere and corona (Gabriel 1976;Dowdy et al 1986;Peter 2001), or from isolated unresolved tiny loops (Feldman 1983(Feldman , 1987Feldman & Laming 1994). Recently, Patsourakos et al (2007) reported the first spatially resolved observations of subarcsecond-scale (0.3 ) loop-like structures seen in the Ly-α line, as observed by the Very High Angular Resolution Ultraviolet Telescope (VAULT). As mentioned in that paper, flows can have distinct spectroscopic signatures, which can help to distinguish between the two possibilities of the TR emission origin.…”

Dynamic Lyαjets

“…The resulting signal is about 95% pure Lyα emission (Teriaca & Schühle, personal communication). More detail is given in Korendyke et al (2001) and Patsourakos et al (2007).…”

“…VAULT acquired Lyα images with much higher spatial and temporal resolution than the EUV imagery from the TRACE, SoHO, and STEREO satellites but only for a few minutes. The Lyα fine structure in these images is discussed by Patsourakos et al (2007) and Judge & Centeno (2008).…”