248 research outputs found
Seismic Emissions from a Highly Impulsive M6.7 Solar Flare
On 10 March 2001 the active region NOAA 9368 produced an unusually impulsive
solar flare in close proximity to the solar limb. This flare has previously
been studied in great detail, with observations classifying it as a type 1
white-light flare with a very hard spectrum in hard X-rays. The flare was also
associated with a type II radio burst and coronal mass ejection. The flare
emission characteristics appeared to closely correspond with previous instances
of seismic emission from acoustically active flares. Using standard local
helioseismic methods, we identified the seismic signatures produced by the
flare that, to date, is the least energetic (in soft X-rays) of the flares
known to have generated a detectable acoustic transient. Holographic analysis
of the flare shows a compact acoustic source strongly correlated with the
impulsive hard X-ray, visible continuum, and radio emission. Time-distance
diagrams of the seismic waves emanating from the flare region also show faint
signatures, mainly in the eastern sector of the active region. The strong
spatial coincidence between the seismic source and the impulsive visible
continuum emission reinforces the theory that a substantial component of the
seismic emission seen is a result of sudden heating of the low photosphere
associated with the observed visible continuum emission. Furthermore, the
low-altitude magnetic loop structure inferred from potential--field
extrapolations in the flaring region suggests that there is a significant
inverse correlation between the seismicity of a flare and the height of the
magnetic loops that conduct the particle beams from the corona.Comment: 16 pages, 7 figures, Solar Physics Topical Issue: SOHO 19/GONG 2007
"Seismology of Magnetic Activity", Accepte
Properties of Flares-Generated Seismic Waves on the Sun
The solar seismic waves excited by solar flares (``sunquakes'') are observed
as circular expanding waves on the Sun's surface. The first sunquake was
observed for a flare of July 9, 1996, from the Solar and Heliospheric
Observatory (SOHO) space mission. However, when the new solar cycle started in
1997, the observations of solar flares from SOHO did not show the seismic
waves, similar to the 1996 event, even for large X-class flares during the
solar maximum in 2000-2002. The first evidence of the seismic flare signal in
this solar cycle was obtained for the 2003 ``Halloween'' events, through
acoustic ``egression power'' by Donea and Lindsey. After these several other
strong sunquakes have been observed. Here, I present a detailed analysis of the
basic properties of the helioseismic waves generated by three solar flares in
2003-2005. For two of these flares, X17 flare of October 28, 2003, and X1.2
flare of January 15, 2005, the helioseismology observations are compared with
simultaneous observations of flare X-ray fluxes measured from the RHESSI
satellite. These observations show a close association between the flare
seismic waves and the hard X-ray source, indicating that high-energy electrons
accelerated during the flare impulsive phase produced strong compression waves
in the photosphere, causing the sunquake. The results also reveal new physical
properties such as strong anisotropy of the seismic waves, the amplitude of
which varies significantly with the direction of propagation. The waves travel
through surrounding sunspot regions to large distances, up to 120 Mm, without
significant decay. These observations open new perspectives for helioseismic
diagnostics of flaring active regions on the Sun and for understanding the
mechanisms of the energy release and transport in solar flares.Comment: 12 pages, 4 figures, submitted to Ap
Generalized pseudo-Newtonian potential for studying accretion disk dynamics in off-equatorial planes around rotating black holes: Description of a vector potential
We prescribe a pseudo-Newtonian vector potential for studying accretion disks
around Kerr black holes. The potential is useful to study the inner properties
of disk not confined to the equatorial plane where general relativistic effect
is indispensable. Therefore, we incorporate the essential properties of the
metric at the inner radii through the pseudo-Newtonian potential derived from
the general Kerr spacetime. The potential, reproducing most of the salient
features of the general-relativity, is valid for entire regime of Kerr
parameter. It reproduces the last stable circular orbit exactly as that in the
Kerr geometry. It also reproduces last bound orbit and energy at last stable
circular orbit with a maximum error ~7% and ~15% respectively upto an orbital
inclination 30 degree.Comment: 22 AASTeX pages including 5 postscript figures; Accepted for
publication in The Astrophysical Journa
Incorporating chemical signalling factors into cell-based models of growing epithelial tissues
In this paper we present a comprehensive computational framework within which the effects of chemical signalling factors on growing epithelial tissues can be studied. The method incorporates a vertex-based cell model, in conjunction with a solver for the governing chemical equations. The vertex model provides a natural mesh for the finite element method (FEM), with node movements determined by force laws. The arbitrary Lagrangian–Eulerian formulation is adopted to account for domain movement between iterations. The effects of cell proliferation and junctional rearrangements on the mesh are also examined. By implementing refinements of the mesh we show that the finite element (FE) approximation converges towards an accurate numerical solution. The potential utility of the system is demonstrated in the context of Decapentaplegic (Dpp), a morphogen which plays a crucial role in development of the Drosophila imaginal wing disc. Despite the presence of a Dpp gradient, growth is uniform across the wing disc. We make the growth rate of cells dependent on Dpp concentration and show that the number of proliferation events increases in regions of high concentration. This allows hypotheses regarding mechanisms of growth control to be rigorously tested. The method we describe may be adapted to a range of potential application areas, and to other cell-based models with designated node movements, to accurately probe the role of morphogens in epithelial tissues
Calculations of periodicity from H<i>α</i> profiles of Proxima Centauri
We investigate retrieval of the stellar rotation signal for Proxima Centauri. We make use of high-resolution spectra taken with UVES and HARPS of Proxima Centauri over a 13-yr period as well as photometric observations of Proxima Centauri from ASAS and HST. We measure the Hα equivalent width and Hα index, skewness and kurtosis and introduce a method that investigates the symmetry of the line, the peak ratio, which appears to return better results than the other measurements. Our investigations return a most significant period of 82.6 ± 0.1 days, confirming earlier photometric results and ruling out a more recent result of 116.6 days which we conclude to be an alias induced by the specific HARPS observation times. We conclude that whilst spectroscopic Hα measurements can be used for period recovery, in the case of Proxima Centauri the available photometric measurements are more reliable. We make 2D models of Proxima Centauri to generate simulated Hα, finding that reasonable distributions of plage and chromospheric features are able to reproduce the equivalent width variations in observed data and recover the rotation period, including after the addition of simulated noise and flares. However the 2D models used fail to generate the observed variety of line shapes measured by the peak ratio. We conclude that only 3D models which incorporate vertical motions in the chromosphere can achieve this
No Evidence Supporting Flare Driven High-Frequency Global Oscillations
The underlying physics that generates the excitations in the global
low-frequency, < 5.3 mHz, solar acoustic power spectrum is a well known process
that is attributed to solar convection; However, a definitive explanation as to
what causes excitations in the high-frequency regime, > 5.3 mHz, has yet to be
found. Karoff and Kjeldsen (Astrophys. J. 678, 73-76, 2008) concluded that
there is a correlation between solar flares and the global high-frequency solar
acoustic waves. We have used the Global Oscillations Network Group (GONG)
helioseismic data in an attempt to verify Karoff and Kjeldsen (2008) results as
well as compare the post-flare acoustic power spectrum to the pre-flare
acoustic power spectrum for 31 solar flares. Among the 31 flares analyzed, we
observe that a decrease in acoustic power after the solar flare is just as
likely as an increase. Furthermore, while we do observe variations in acoustic
power that are most likely associated with the usual p-modes associated with
solar convection, these variations do not show any significant temporal
association with flares. We find no evidence that consistently supports flare
driven high-frequency waves.Comment: 20 pages, 9 figures, Accepted for publication in Solar Physic
FIC/FEM formulation with matrix stabilizing terms for incompressible flows at low and high Reynolds numbers
The final publication is available at Springer via http://dx.doi.org/10.1007/s00466-006-0060-yWe present a general formulation for incompressible fluid flow analysis using the finite element method. The necessary stabilization for dealing with convective effects and the incompressibility condition are introduced via the Finite Calculus method using a matrix form of the stabilization parameters. This allows to model a wide range of fluid flow problems for low and high Reynolds numbers flows without introducing a turbulence model. Examples of application to the analysis of incompressible flows with moderate and large Reynolds numbers are presented.Peer ReviewedPostprint (author's final draft
Newly identified properties of surface acoustic power
The cause of enhanced acoustic power surrounding active regions, the acoustic
halo, is not as yet understood. We explore the properties of the enhanced
acoustic power observed near disk center from 21 to 27 January 2002, including
AR 9787. We find that (i) there exists a strong correlation of the enhanced
high frequency power with magnetic-field inclination, with greater power in
more horizontal fields, (ii) the frequency of the maximum enhancement increases
along with magnetic field strength, and (iii) the oscillations contributing to
the halos show modal ridges which are shifted to higher wavenumber at constant
frequency in comparison to the ridges of modes in the quiet-Sun.Comment: 16 pages, 10 figures, submitted to solar physic
Implications of Cosmological Gamma-Ray Absorption II. Modification of gamma-ray spectra
Bearing on the model for the time-dependent metagalactic radiation field
developed in the first paper of this series, we compute the gamma-ray
attenuation due to pair production in photon-photon scattering. Emphasis is on
the effects of varying the star formation rate and the fraction of UV radiation
assumed to escape from the star forming regions, the latter being important
mainly for high-redshift sources. Conversely, we investigate how the
metagalactic radiation field can be measured from the gamma-ray pair creation
cutoff as a function of redshift, the Fazio-Stecker relation. For three
observed TeV-blazars (Mkn501, Mkn421, H1426+428) we study the effects of
gamma-ray attenuation on their spectra in detail.Comment: 10 pages, 6 figures, accepted by A&
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