Three‐dimensional Stereoscopic Analysis of Solar Active Region Loops. II. SOHO /EIT Observations at Temperatures of 1.5–2.5 MK

Abstract

In this paper we study the three-dimensional structure of hot (Te ≈ 1.5-2.5 MK) loops in solar active region NOAA 7986, observed on 1996 August 30 with the Extreme-ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO). This complements a first study (Paper I) on cooler (Te ≈ 1.0-1.5 MK) loops of the same active region, using the same method of Dynamic Stereoscopy to reconstruct the three-dimensional geometry. We reconstruct the three-dimensional coordinates x(s), y(s), z(s), the density ne(s), and temperature profile Te(s) of 35 individual loop segments (as a function of the loop coordinate s) using EIT 195 and 284 Å images. The major findings are as follows. (1) All loops are found to be in hydrostatic equilibrium, in the entire temperature regime of Te = 1.0-2.5 MK. (2) The analyzed loops have a height of 2-3 scale heights, and thus only segments extending over about one vertical scale height have sufficient emission measure contrast for detection. (3) The temperature gradient over the lowest scale height is of order dT/ds ≈ 1-10 K km-1. (4) The radiative loss rate is found to exceed the conductive loss rate by about two orders or magnitude in the coronal loop segments, implying that the loops cannot be in quasi-static equilibrium, since standard steady-state loop models show that radiative and conductive losses are comparable. (5) A steady state could only be maintained if the heating rate EH matches exactly the radiative loss rate in hydrostatic equilibrium, requiring a heat deposition length λH of the half density scale height λ. (6) We find a correlation of p L-1 between loop base pressure and loop length, which is not consistent with the scaling law predicted from steady-state models of large-scale loops. All observational findings indicate consistently that the energy balance of the observed EUV loops cannot be described by steady-state models.