[Loops] New version of EBTEL code

[Peter Cargill]

Dear Loops friends

We have recently updated the EBTEL loop model to include area 
variations. This can be found at:

http://arxiv.org/abs/2111.09339

and at:

https://academic.oup.com/mnras/article-abstract/509/3/4420/6414539

with abstract:

The Enthalpy Based Thermal Evolution of Loops (EBTEL) approximate model 
for static and dynamic coronal loops is developed to include the effect 
of a loop cross-sectional area which increases from the base of the 
transition region (TR) to the corona. The TR is defined as the part of a 
loop between the top of the chromosphere and the location where thermal 
conduction changes from an energy loss to an energy gain. There are 
significant differences from constant area loops due to the manner in 
which the reduced volume of the TR responds to conductive and enthalpy 
fluxes from the corona. For static loops with modest area variation the 
standard picture of loop energy balance is retained, with the corona and 
TR being primarily a balance between heating and conductive losses in 
the corona, and downward conduction and radiation to space in the TR. As 
the area at the loop apex increases, the TR becomes thicker and the 
density in TR and corona larger. For large apex areas, the coronal 
energy balance changes to one primarily between heating and radiation, 
with conduction playing an increasingly unimportant role, and the TR 
thickness becoming a significant fraction of the loop length. 
Approximate scaling laws are derived that give agreement with full 
numerical solutions for the density, but not the temperature. For 
non-uniform areas, dynamic loops have a higher peak temperature and are 
denser in the radiative cooling phase by of order 50% than the constant 
area case for the examples considered. They also show a final rapid 
cooling and draining once the temperature approaches 1 MK. Although the 
magnitude of the emission measure will be enhanced in the radiative 
phase, there is little change in the important observational diagnostic 
of its temperature dependence.

The EBTEL code is also updated and can be found at:

https://github.com/rice-solar-physics/EBTEL

Before using the new version of EBTEL, please read both the paper and 
the user notes for limitations of the validity of the model. These are 
NEW limitations introduced by the non-uniform area.

Peter