144 research outputs found
Modelling Fast-Alfv\'en Mode Conversion Using SPARC
We successfully utilise the SPARC code to model fast-Alfv\'en mode conversion
in the region via 3-D MHD numerical simulations of helioseismic
waves within constant inclined magnetic field configurations. This was achieved
only after empirically modifying the background density and gravitational
stratifications in the upper layers of our computational box, as opposed to
imposing a traditional Lorentz Force limiter, to ensure a manageable timestep.
We found that the latter approach inhibits the fast-Alfv\'en mode conversion
process by severely damping the magnetic flux above the surface.Comment: Proceedings of GONG 2012 / LWS/SDO-5 / SOHO 27 (Eclipse on the Coral
Sea: Cycle 24 Ascending) Conference, November 12 -16, 2012, Palm Cove,
Australi
Fast-to-Alfv\'en mode conversion mediated by Hall current. II Application to the solar atmosphere
Coupling between fast magneto-acoustic and Alfv\'en waves can be observe in
fully ionized plasmas mediated by stratification and 3D geometrical effects. In
Paper I, Cally & Khomenko (2015) have shown that in a weakly ionized plasma,
such as the solar photosphere and chromosphere, the Hall current introduces a
new coupling mechanism. The present study extends the results from Paper I to
the case of warm plasma. We report on numerical experiments where mode
transformation is studied using quasi-realistic stratification in thermodynamic
parameters resembling the solar atmosphere. This redresses the limitation of
the cold plasma approximation assumed in Paper I, in particular allowing the
complete process of coupling between fast and slow magneto-acoustic modes and
subsequent coupling of the fast mode to the Alfv\'en mode through the Hall
current. Our results confirm the efficacy of the mechanism proposed in Paper I
for the solar case. We observe that the efficiency of the transformation is a
sensitive function of the angle between the wave propagation direction and the
magnetic field, and of the wave frequency. The efficiency increases when the
field direction and the wave direction are aligned for increasing wave
frequencies. After scaling our results to typical solar values, the maximum
amplitude of the transformed Alfv\'en waves, for a frequency of 1 Hz,
corresponds to an energy flux (measured above the height of peak Hall coupling)
of , based on an amplitude of 500 at
, which is sufficient to play a major role in both quiet and active
region coronal heating
Directional Time-Distance Probing of Model Sunspot Atmospheres
A crucial feature not widely accounted for in local helioseismology is that
surface magnetic regions actually open a window from the interior into the
solar atmosphere, and that the seismic waves leak through this window, reflect
high in the atmosphere, and then re-enter the interior to rejoin the seismic
wave field normally confined there. In a series of recent numerical studies
using translation invariant atmospheres, we utilised a "directional
time-distance helioseismology" measurement scheme to study the implications of
the returning fast and Alfv\'en waves higher up in the solar atmosphere on the
seismology at the photosphere (Cally & Moradi 2013; Moradi & Cally 2014). In
this study, we extend our directional time-distance analysis to more realistic
sunspot-like atmospheres to better understand the direct effects of the
magnetic field on helioseismic travel-time measurements in sunspots. In line
with our previous findings, we uncover a distinct frequency-dependant
directional behaviour in the travel-time measurements, consistent with the
signatures of MHD mode conversion. We found this to be the case regardless of
the sunspot field strength or depth of its Wilson depression. We also isolated
and analysed the direct contribution from purely thermal perturbations to the
measured travel times, finding that waves propagating in the umbra are much
more sensitive to the underlying thermal effects of the sunspot.Comment: 9 pages, 8 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society Main Journa
Alfv\'en waves in simulations of solar photospheric vortices
Using advanced numerical magneto-hydrodynamic simulations of the magnetised
solar photosphere, including non-grey radiative transport and a non-ideal
equation of state, we analyse plasma motions in photospheric magnetic vortices.
We demonstrate that apparent vortex-like motions in photospheric magnetic field
concentrations do not exhibit "tornado"-like behaviour or a "bath-tub" effect.
While at each time instance the velocity field lines in the upper layers of the
solar photosphere show swirls, the test particles moving with the
time-dependent velocity field do not demonstrate such structures. Instead, they
move in a wave-like fashion with rapidly changing and oscillating velocity
field, determined mainly by magnetic tension in the magnetised intergranular
downflows. Using time-distance diagrams, we identify horizontal motions in the
magnetic flux tubes as torsional Alfv\'en perturbations propagating along the
nearly vertical magnetic field lines with local Alfv\'en speed.Comment: 5 pages, 4 figures, accepted to ApJ
Three Dimensional MHD Wave Propagation and Conversion to Alfven Waves near the Solar Surface. I. Direct Numerical Solution
The efficacy of fast/slow MHD mode conversion in the surface layers of
sunspots has been demonstrated over recent years using a number of modelling
techniques, including ray theory, perturbation theory, differential eigensystem
analysis, and direct numerical simulation. These show that significant energy
may be transferred between the fast and slow modes in the neighbourhood of the
equipartition layer where the Alfven and sound speeds coincide. However, most
of the models so far have been two dimensional. In three dimensions the Alfven
wave may couple to the magneto-acoustic waves with important implications for
energy loss from helioseismic modes and for oscillations in the atmosphere
above the spot. In this paper, we carry out a numerical ``scattering
experiment'', placing an acoustic driver 4 Mm below the solar surface and
monitoring the acoustic and Alfvenic wave energy flux high in an isothermal
atmosphere placed above it. These calculations indeed show that energy
conversion to upward travelling Alfven waves can be substantial, in many cases
exceeding loss to slow (acoustic) waves. Typically, at penumbral magnetic field
strengths, the strongest Alfven fluxes are produced when the field is inclined
30-40 degrees from the vertical, with the vertical plane of wave propagation
offset from the vertical plane containing field lines by some 60-80 degrees.Comment: Accepted for the HELAS II/ SOHO 19/ GONG 2007 Topical Issue of Solar
Physic
Resonant Absorption as Mode Conversion?
Resonant absorption and mode conversion are both extensively studied
mechanisms for wave "absorption" in solar magnetohydrodynamics (MHD). But are
they really distinct? We re-examine a well-known simple resonant absorption
model in a cold MHD plasma that places the resonance inside an evanescent
region. The normal mode solutions display the standard singular resonant
features. However, these same normal modes may be used to construct a ray
bundle which very clearly undergoes mode conversion to an Alfv\'en wave with no
singularities. We therefore conclude that resonant absorption and mode
conversion are in fact the same thing, at least for this model problem. The
prime distinguishing characteristic that determines which of the two
descriptions is most natural in a given circumstance is whether the converted
wave can provide a net escape of energy from the conversion/absorption region
of physical space. If it cannot, it is forced to run away in wavenumber space
instead, thereby generating the arbitrarily small scales in situ that we
recognize as fundamental to resonant absorption and phase mixing. On the other
hand, if the converted wave takes net energy way, singularities do not develop,
though phase mixing may still develop with distance as the wave recedes.Comment: 23 pages, 8 figures, 2 tables; accepted by Solar Phys (July 9 2010
Probing sunspots with two-skip time-distance helioseismology
Previous helioseismology of sunspots has been sensitive to both the
structural and magnetic aspects of sunspot structure. We aim to develop a
technique that is insensitive to the magnetic component so the two aspects can
be more readily separated. We study waves reflected almost vertically from the
underside of a sunspot. Time-distance helioseismology was used to measure
travel times for the waves. Ray theory and a detailed sunspot model were used
to calculate travel times for comparison. It is shown that these large distance
waves are insensitive to the magnetic field in the sunspot. The largest travel
time differences for any solar phenomena are observed. With sufficient modeling
effort, these should lead to better understanding of sunspot structure
Helioseismic analysis of the solar flare-induced sunquake of 2005 January 15
We report the discovery of one of the most powerful sunquakes detected to
date, produced by an X1.2-class solar flare in active region 10720 on 2005
January 15. We used helioseismic holography to image the source of seismic
waves emitted into the solar interior from the site of the flare. Acoustic
egression power maps at 3 and 6 mHz with a 2 mHz bandpass reveal a compact
acoustic source strongly correlated with impulsive hard X-ray and
visible-continuum emission along the penumbral neutral line separating the two
major opposing umbrae in the -configuration sunspot that predominates
AR10720. The acoustic emission signatures were directly aligned with both hard
X-ray and visible continuum emission that emanated during the flare. The
visible continuum emission is estimated at J,
approximately 500 times the seismic emission of J. The
flare of 2005 January 15 exhibits the same close spatial alignment between the
sources of the seismic emission and impulsive visible continuum emission as
previous flares, reinforcing the hypothesis that the acoustic emission may be
driven by heating of the low photosphere. However, it is a major exception in
that there was no signature to indicate the inclusion of protons in the
particle beams thought to supply the energy radiated by the flare. The
continued strong coincidence between the sources of seismic emission and
impulsive visible continuum emission in the case of a proton-deficient
white-light flare lends substantial support to the ``back -- warming''
hypothesis, that the low photosphere is significantly heated by intense Balmer
and Paschen continuum-edge radiation from the overlying chromosphere in
white-light flares.Comment: 12 pages, 7 figures, published in MNRA
- …