27 research outputs found
The Flare-energy Distributions Generated by Kink-unstable Ensembles of Zero-net-current Coronal Loops
It has been proposed that the million degree temperature of the corona is due
to the combined effect of barely-detectable energy releases, so called
nanoflares, that occur throughout the solar atmosphere. Alas, the nanoflare
density and brightness implied by this hypothesis means that conclusive
verification is beyond present observational abilities. Nevertheless, we
investigate the plausibility of the nanoflare hypothesis by constructing a
magnetohydrodynamic (MHD) model that can derive the energy of a nanoflare from
the nature of an ideal kink instability. The set of energy-releasing
instabilities is captured by an instability threshold for linear kink modes.
Each point on the threshold is associated with a unique energy release and so
we can predict a distribution of nanoflare energies. When the linear
instability threshold is crossed, the instability enters a nonlinear phase as
it is driven by current sheet reconnection. As the ensuing flare erupts and
declines, the field transitions to a lower energy state, which is modelled by
relaxation theory, i.e., helicity is conserved and the ratio of current to
field becomes invariant within the loop. We apply the model so that all the
loops within an ensemble achieve instability followed by energy-releasing
relaxation. The result is a nanoflare energy distribution. Furthermore, we
produce different distributions by varying the loop aspect ratio, the nature of
the path to instability taken by each loop and also the level of radial
expansion that may accompany loop relaxation. The heating rate obtained is just
sufficient for coronal heating. In addition, we also show that kink instability
cannot be associated with a critical magnetic twist value for every point along
the instability threshold
Review article: MHD wave propagation near coronal null points of magnetic fields
We present a comprehensive review of MHD wave behaviour in the neighbourhood
of coronal null points: locations where the magnetic field, and hence the local
Alfven speed, is zero. The behaviour of all three MHD wave modes, i.e. the
Alfven wave and the fast and slow magnetoacoustic waves, has been investigated
in the neighbourhood of 2D, 2.5D and (to a certain extent) 3D magnetic null
points, for a variety of assumptions, configurations and geometries. In
general, it is found that the fast magnetoacoustic wave behaviour is dictated
by the Alfven-speed profile. In a plasma, the fast wave is focused
towards the null point by a refraction effect and all the wave energy, and thus
current density, accumulates close to the null point. Thus, null points will be
locations for preferential heating by fast waves. Independently, the Alfven
wave is found to propagate along magnetic fieldlines and is confined to the
fieldlines it is generated on. As the wave approaches the null point, it
spreads out due to the diverging fieldlines. Eventually, the Alfven wave
accumulates along the separatrices (in 2D) or along the spine or fan-plane (in
3D). Hence, Alfven wave energy will be preferentially dissipated at these
locations. It is clear that the magnetic field plays a fundamental role in the
propagation and properties of MHD waves in the neighbourhood of coronal null
points. This topic is a fundamental plasma process and results so far have also
lead to critical insights into reconnection, mode-coupling, quasi-periodic
pulsations and phase-mixing.Comment: 34 pages, 5 figures, invited review in Space Science Reviews => Note
this is a 2011 paper, not a 2010 pape
Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results
The chromosphere is a thin layer of the solar atmosphere that bridges the
relatively cool photosphere and the intensely heated transition region and
corona. Compressible and incompressible waves propagating through the
chromosphere can supply significant amounts of energy to the interface region
and corona. In recent years an abundance of high-resolution observations from
state-of-the-art facilities have provided new and exciting ways of
disentangling the characteristics of oscillatory phenomena propagating through
the dynamic chromosphere. Coupled with rapid advancements in
magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly
investigate the role waves play in supplying energy to sustain chromospheric
and coronal heating. Here, we review the recent progress made in
characterising, categorising and interpreting oscillations manifesting in the
solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review
Time signatures of impulsively generated coronal fast wave trains
Impulsively generated short-period fast magneto-acoustic wave trains, guided by solar and stellar coronal loops, are numerically modelled. In the developed stage of the evolution, the wave trains have a characteristic quasi-periodic signature. The quasi-periodicity results from the geometrical dispersion of the guided fast modes, determined by the transverse profile of the loop. A typical feature of the signature is a tadpole wavelet Spectrum: a narrow-spectrum tail precedes a broad-band head. The instantaneous period of the oscillations in the wave train decreases gradually with time. The period and the spectral amplitude evolution are shown to be determined by the steepness of the transverse density profile and the density contrast ratio in the loop. The propagating wave trains recently discovered with the Solar Eclipse Coronal Imaging System (SECIS) instrument are noted to have similar wavelet spectral features, which strengthens the interpretation of SECIS results as guided fast wave trains
Neuroticism, education and self-assessed health in the general population of the United States: Can smoking behaviour explain the associations?
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