Investigating the Heating Mechanism of Hot Coronal Loops Using Spectral and Imaging Analysis of the Solar Moss Areas

Coronal heating mechanisms are generally classified as waves (AC: Alternating Current) and Direct Current (DC) models. Restricted evidences from both models have been observed over different structures of the solar atmosphere. However, a general model describing the global heating process in the whole solar atmosphere is missing. Active regions are the magnetic anchorages of the solar corona. It is believed that the Active Regions have important contribution in the heating of the corona. ARs are divided into two sub-regions: Warm coronal loops (~ 1 MK) and hot coronal loops (> 1.5 MK). There are strong observational evidence supporting the DC nanoflare model in the warm coronal loops. However, there is still no conclusive answer for the responsible heating mechanism of hot loops. There are some observational evidence indicating the existence of steady heating, some others indicating the impulsive heating, and very lately people have found MHD wave heating signatures in the hot loops. The intrinsic fuzzy nature of hot coronal emission lines made it impossible to isolate the hot loop structures and study their physical properties separately. Therefore, the only way to study the hot loops is to focus on their hot footpoints called “moss” areas, which was discovered in 1999 by Berger et al. using TRACE 171 Å images. The moss emission has a reticulated spongy structure. It is believed that when the hot coronal loops cover the cold plage regions, an inward radial thermal conduction gradient is formed which causes the plasma to heat up and radiate. This radiation is what we call ”moss”. The dark patches inside the bright moss areas are the cross sections of the cold spicular materials rising upward toward the corona.Using spectroscopic and imaging data of Interface Region Imaging Spectrograph (IRIS) and Solar Dynamic Observatory (SDO), dynamic properties of the moss areas over an AR is studied. Three boxes of moss regions are selected. The time variation of the intensity, Doppler shift, and line widths of C II 1335.7077 Å and Si IV 1402.770 Å emission lines are investigated. Time series of the intensities over the three selected moss regions are made from IRIS SJI 1400, and 2796, along with AIA/SDO 1700, 304, 1600, 171, 193, 211, 335, 94, and 131 channels. Using FFT technique we obtained oscillatory behavior over the all mentioned parameters. The results show oscillatory behaviors in the line width, Doppler shifts, and line intensities of C II and Si IV spectral lines with periods of 3.9 and 6.9 minutes over the moss areas. 3.9 min oscillations are observed over the AIA 211 passband, as well, which could be an indication of the presence of torsional Alfven waves coupled with kind mode. High frequency oscillations with 0.9 to 2 min periods are observed over the selected moss regions in AIA hot channels like 335, 131, and 94, as well as Si IV line. This could be an indication of occurring magnetic reconnections above the moss regions in the hot coronal lines, triggering the Alfven waves in this structure. Therefore, our results support the presence of MHD waves heating mechanisms in the studied moss structures.