847 research outputs found
The Cooling of Coronal Plasmas. iv: Catastrophic Cooling of Loops
We examine the radiative cooling of coronal loops and demonstrate that the
recently identified catastrophic cooling (Reale and Landi, 2012) is due to the
inability of a loop to sustain radiative / enthalpy cooling below a critical
temperature, which can be > 1 MK in flares, 0.5 - 1 MK in active regions and
0.1 MK in long tenuous loops. Catastrophic cooling is characterised by a rapid
fall in coronal temperature while the coronal density changes by a small
amount. Analytic expressions for the critical temperature are derived and show
good agreement with numerical results. This effect limits very considerably the
lifetime of coronal plasmas below the critical temperature
Enthalpy-based Thermal Evolution of Loops: II. Improvements to the Model
This paper develops the zero-dimensional (0D) hydrodynamic coronal loop model
"Enthalpy-based Thermal Evolution of Loops" (EBTEL) proposed by Klimchuk et al
(2008), which studies the plasma response to evolving coronal heating,
especially impulsive heating events. The basis of EBTEL is the modelling of
mass exchange between the corona and transition region and chromosphere in
response to heating variations, with the key parameter being the ratio of
transition region to coronal radiation. We develop new models for this
parameter that now include gravitational stratification and a physically
motivated approach to radiative cooling. A number of examples are presented,
including nanoflares in short and long loops, and a small flare. The new
features in EBTEL are important for accurate tracking of, in particular, the
density. The 0D results are compared to a 1D hydro code (Hydrad) with generally
good agreement. EBTEL is suitable for general use as a tool for (a) quick-look
results of loop evolution in response to a given heating function, (b)
extensive parameter surveys and (c) situations where the modelling of hundreds
or thousands of elemental loops is needed. A single run takes a few seconds on
a contemporary laptop
Diagnosing the time-dependence of active region core heating from the emission measure: II. Nanoflare trains
The time-dependence of heating in solar active regions can be studied by
analyzing the slope of the emission measure distribution cool-ward of the peak.
In a previous study we showed that low-frequency heating can account for 0% to
77% of active region core emission measures. We now turn our attention to
heating by a finite succession of impulsive events for which the timescale
between events on a single magnetic strand is shorter than the cooling
timescale. We refer to this scenario as a "nanoflare train" and explore a
parameter space of heating and coronal loop properties with a hydrodynamic
model. Our conclusions are: (1) nanoflare trains are consistent with 86% to
100% of observed active region cores when uncertainties in the atomic data are
properly accounted for; (2) steeper slopes are found for larger values of the
ratio of the train duration to the post-train cooling and draining
timescale , where depends on the number of heating events,
the event duration and the time interval between successive events ();
(3) may be diagnosed from the width of the hot component of the
emission measure provided that the temperature bins are much smaller than 0.1
dex; (4) the slope of the emission measure alone is not sufficient to provide
information about any timescale associated with heating - the length and
density of the heated structure must be measured for to be uniquely
extracted from the ratio
X-ray Source Heights in a Solar Flare: Thick-target versus Thermal Conduction Front Heating
Observations of solar flares with RHESSI have shown X-ray sources traveling
along flaring loops, from the corona down to the chromosphere and back up. The
28 November 2002 C1.1 flare, first observed with RHESSI by Sui et al. 2006 and
quantitatively analyzed by O'Flannagain et al. 2013, very clearly shows this
behavior. By employing numerical experiments, we use these observations of
X-ray source height motions as a constraint to distinguish between heating due
to a non-thermal electron beam and in situ energy deposition in the corona. We
find that both heating scenarios can reproduce the observed light curves, but
our results favor non-thermal heating. In situ heating is inconsistent with the
observed X-ray source morphology and always gives a height dispersion with
photon energy opposite to what is observed.Comment: Accepted to Ap
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