CDS/SoHO multi-line observation of a solar active region: Detection of a hot stable loop and of a cool dynamic loop

Abstract

Abstract. We analyze a space-, time-and spectral-resolved SoHO/CDS observation of the evolution of an active region over a time lapse of approximately three hours in various spectral lines emitted in the interval of temperature 1.3 × 10 4 < T < 2.5 × 10 6 K. We identify and characterize two structures of interest: a longer coronal loop (≈5.5 × 10 9 cm), relatively steady and well visible in lines forming at coronal temperatures (e.g. Fe XIV 334.17 Å, Fe XVI 360.76 Å) and a smaller one (≈1.8 × 10 9 cm), transient and visible only in cooler lines (O IV 554.51 Å, O V 629.73 Å). In the hot lines, the longer loop has a bright apex and an emission distribution of constant shape, but of moderately variable absolute intensity; the region around the loop apex shows a distinct brightening practically in all lines. In the hot lines, the brightening appears as a minor perturbation over a steadily high emission level. In the same region the emission measure vs temperature of the hottest lines indicates a temperature of ∼2 MK, lower than the temperature obtained from Yohkoh data taken just before the CDS observation. Comparison with steady-state loop scaling laws and with plasma time scales, and connection to cooling or heating episodes are discussed. As for the cool loop, its whole evolution, from ignition to disappearance, is directly observed, confirming the highly transient nature of such structures. The O V line is blue-shifted at one footpoint, indicating an upflow associated with the loop ignition.

All Coronal Loops Are the Same: Evidence to the Contrary

“…The loop is also visible in slightly cooler lines (Fe xii, Fe xiii, Fe xiv), but only the footpoints are visible in even cooler lines (Mg viii and Mg ix line). This characteristic makes this loop similar to the loop observed by Di Giorgio et al (2003), but different from ours. Landi & Landini (2004) used EM loci plots to show that the plasma is isothermal along most of the lines of sight investigated, but not at the loop footpoints.…”

“…Background-subtracted CDS loop results Di Giorgio et al 2003;Landi & Landini 2004) are sometimes (but not always) consistent with isothermal plasma along the line of sight, along the length of the loop, or both. The results described in Paper I implied multithermal plasma in both directions but did not include background subtraction.…”

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

“…This class of loops was first studied by Foukal (1976) and observed by Brekke et al (1997). From observations, they are generally detected in the UV lines and appear to be steady for long times even if they are more variable and dynamic than hot coronal loops, probably because of substantial flows (Brekke et al 1997;Di Giorgio et al 2003;Reale 2010). Their estimated densities are in the range 10 9 −10 10 cm −3 (e.g., Brown 1996).…”

Models for Solar Magnetic Loops. III. Dynamic Models and Coronal Diagnostic Spectrometer Observations

“…The only attempts have been made by Dr. J. T. Schmelz and coworkers (Schmelz et al 2001;Schmelz 2002;Martens et al 2002), Brković et al (2002, hereafter Paper I), and Di Giorgio et al (2003) using CDS observations of active region loops at the limb or the disk. Schmelz et al (2001) find contradictory results from an active region at the limb: while line intensity ratios pointed toward a constant temperature along the loop, a differential emission measure (DEM) analysis showed a multithermal behavior across the loop section.…”

“…Different results have been found in Paper I, where an emission measure (EM) analysis was used to show that the loop plasma is isothermal both along and across the loop shape, thus putting into question the presence of multithermal subresolution filaments. Di Giorgio et al (2003) analyzed time-resolved CDS and Yohkoh observations of an active region loop, detecting moderate activity; they propose several scenarios that might account for it but conclude that higher spatial and temporal resolution is needed. Paper I showed that uniform heating was inadequate in a static loop scenario and proposed footpoint-concentrated heating as a possible solution.…”