MHD modeling of coronal loops: the transition region throat

Abstract

Context. The expansion of coronal loops in the transition region may considerably influence the diagnostics of the plasma emission measure. The cross-sectional area of the loops is expected to depend on the temperature and pressure, and might be sensitive to the heating rate. Aims. The approach here is to study the area response to slow changes in the coronal heating rate, and check the current interpretation in terms of steady heating models. Methods. We study the area response with a time-dependent 2D magnetohydrodynamic (MHD) loop model, including the description of the expanding magnetic field, coronal heating and losses by thermal conduction, and radiation from optically thin plasma. We run a simulation for a loop 50 Mm long and quasi-statically heated to about 4 MK. Results. We find that the area can change substantially with the quasi-steady heating rate, e.g., by ∼40% at 0.5 MK as the loop temperature varies between 1 MK and 4 MK, and, therefore, affects the interpretation of the differential emission measure vs. temperature (DEM(T )) curves.

Magnetic Reconnection Turbulence in Strong Guide Fields: Basic Properties and Application to Coronal Heating

“…It may be translated into a volumetric heating rate via division by the coronal loop length L ref , yielding 10 −3 erg cm −3 s −1 , in good agreement with Equation (18). Ofman et al (1998), for T e0 = 10 6 K, reports some 10 −4 erg cm −3 s −1 , also rather compatible with the present results, while Guarrasi et al (2014) uses a maximum heating rate of 2 × 10 −3 erg cm −3 s −1 , again in good agreement with the findings here.…”

3d MHD Modeling of Twisted Coronal Loops

“…−5 erg cm −3 s −1 is a volumetric heating rate sufficient to sustain a static corona with an apex temperature of about 8×10 5 K, namely a background atmosphere adopted as initial conditions, according to the hydrostatic loop model by Serio et al (1981; see also Guarrasi et al 2014). An estimate of this heating rate can be derived from loop scaling laws Reale 2014) that can be rearranged into~- 2 , where T 6 and L 9 are the temperature and the loop half-length in units of 10 6 K and 10 9 cm, respectively.…”

Modelling nanoflares in active regions and implications for coronal heating mechanisms

“…The inability to accurately describe the coronal field leads to uncertainties in the loop length, which has important implications for many loop properties, such as the predicted EUV intensity and total loop cooling time. However, preliminary modelling efforts that incorporate magnetic field variation as a function of height in a controlled manner within the three-dimensional MHD equations are starting to quantify these issues [23]. (5) The internal structure of coronal loops presents another complication.…”