1,073 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
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Virtual communities and professional learning across a distributed remote membership
Headteachers, or Principals, of schools work in isolation from each other yet share common practice and domain of leadership and management. They exhibit the characteristics of a community of practice yet are remote from other members of their community. Similar communities of practice can be identified for other types of school leaders, subject co-ordinators for example, and for professionals in other disciplines – consultant registrars in health, optometrists working in dispensing opticians, museum curators, and so on.
This paper explores ways of using virtual communities to develop professional learning in these communities of practice. We discuss our work in the context of education and formal and informal learning communities of school leaders and explore how the lessons learnt have general application. We present a model for professional learning through online collaboration and communication, and look, in particular, at the concept of time and its effects in the virtual community
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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