26 research outputs found
A statistical correlation of sunquakes based on their seismic and white-light emission
Several mechanisms have been proposed to explain the transient seismic emission, i.e. “sunquakes,” from some solar flares. Some theories associate high-energy electrons and/or white-light emission with sunquakes. High-energy charged particles and their subsequent heating of the photosphere and/or chromosphere could induce acoustic waves in the solar interior. We carried out a correlative study of solar flares with emission in hard X-rays, enhanced continuum emission at 6173 Å, and transient seismic emission. We selected those flares observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) with a considerable flux above 50 keV between 1 January 2010 and 26 June 2014. We then used data from the Helioseismic and Magnetic Imager onboard the Solar Dynamic Observatory to search for excess visible-continuum emission and new sunquakes not previously reported. We found a total of 18 sunquakes out of 75 flares investigated. All of the sunquakes were associated with an enhancement of the visible continuum during the flare. Finally, we calculated a coefficient of correlation for a set of dichotomic variables related to these observations. We found a strong correlation between two of the standard helioseismic detection techniques, and between sunquakes and visible-continuum enhancements. We discuss the phenomenological connectivity between these physical quantities and the observational difficulties of detecting seismic signals and excess continuum radiation
Interpreting Helioseismic Structure Inversion Results of Solar Active Regions
Helioseismic techniques such as ring-diagram analysis have often been used to
determine the subsurface structural differences between solar active and quiet
regions. Results obtained by inverting the frequency differences between the
regions are usually interpreted as the sound-speed differences between them.
These in turn are used as a measure of temperature and magnetic-field strength
differences between the two regions. In this paper we first show that the
"sound-speed" difference obtained from inversions is actually a combination of
sound-speed difference and a magnetic component. Hence, the inversion result is
not directly related to the thermal structure. Next, using solar models that
include magnetic fields, we develop a formulation to use the inversion results
to infer the differences in the magnetic and thermal structures between active
and quiet regions. We then apply our technique to existing structure inversion
results for different pairs of active and quiet regions. We find that the
effect of magnetic fields is strongest in a shallow region above 0.985R_sun and
that the strengths of magnetic-field effects at the surface and in the deeper
(r < 0.98R_sun) layers are inversely related, i.e., the stronger the surface
magnetic field the smaller the magnetic effects in the deeper layers, and vice
versa. We also find that the magnetic effects in the deeper layers are the
strongest in the quiet regions, consistent with the fact that these are
basically regions with weakest magnetic fields at the surface. Because the
quiet regions were selected to precede or follow their companion active
regions, the results could have implications about the evolution of magnetic
fields under active regions.Comment: Accepted for publication in Solar Physic
Magneto--Acoustic Energetics Study of the Seismically Active Flare of 15 February 2011
Multi--wavelength studies of energetic solar flares with seismic emissions
have revealed interesting common features between them. We studied the first
GOES X--class flare of the 24th solar cycle, as detected by the Solar Dynamics
Observatory (SDO). For context, seismic activity from this flare
(SOL2011-02-15T01:55-X2.2, in NOAA AR 11158) has been reported in the
literature (Kosovichev, 2011; Zharkov et al., 2011). Based on Dopplergram data
from the Helioseismic and Magnetic Imager (HMI), we applied standard methods of
local helioseismology in order to identify the seismic sources in this event.
RHESSI hard X-ray data are used to check the correlation between the location
of the seismic sources and the particle precipitation sites in during the
flare. Using HMI magnetogram data, the temporal profile of fluctuations in the
photospheric line-of-sight magnetic field is used to estimate the magnetic
field change in the region where the seismic signal was observed. This leads to
an estimate of the work done by the Lorentz-force transient on the photosphere
of the source region. In this instance this is found to be a significant
fraction of the acoustic energy in the attendant seismic emission, suggesting
that Lorentz forces can contribute significantly to the generation of
sunquakes. However, there are regions in which the signature of the
Lorentz-force is much stronger, but from which no significant acoustic emission
emanates.Comment: Submitted to Solar Physic
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
Transient Magnetic and Doppler Features Related to the White-light Flares in NOAA 10486
Rapidly moving transient features have been detected in magnetic and Doppler
images of super-active region NOAA 10486 during the X17/4B flare of 28 October
2003 and the X10/2B flare of 29 October 2003. Both these flares were extremely
energetic white-light events. The transient features appeared during impulsive
phases of the flares and moved with speeds ranging from 30 to 50 km s.
These features were located near the previously reported compact acoustic
\cite{Donea05} and seismic sources \cite{Zharkova07}. We examine the origin of
these features and their relationship with various aspects of the flares, {\it
viz.}, hard X-ray emission sources and flare kernels observed at different
layers - (i) photosphere (white-light continuum), (ii) chromosphere (H
6563\AA), (iii) temperature minimum region (UV 1600\AA), and (iv) transition
region (UV 284\AA).Comment: 26 pages, 13 figures, 2 tables, accepted for publication in Solar
Physic
Active region formation through the negative effective magnetic pressure instability
The negative effective magnetic pressure instability operates on scales
encompassing many turbulent eddies and is here discussed in connection with the
formation of active regions near the surface layers of the Sun. This
instability is related to the negative contribution of turbulence to the mean
magnetic pressure that causes the formation of large-scale magnetic structures.
For an isothermal layer, direct numerical simulations and mean-field
simulations of this phenomenon are shown to agree in many details in that their
onset occurs at the same depth. This depth increases with increasing field
strength, such that the maximum growth rate of this instability is independent
of the field strength, provided the magnetic structures are fully contained
within the domain. A linear stability analysis is shown to support this
finding. The instability also leads to a redistribution of turbulent intensity
and gas pressure that could provide direct observational signatures.Comment: 19 pages, 10 figures, submitted to Solar Physic
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
We review basic theoretical concepts in particle acceleration, with
particular emphasis on processes likely to occur in regions of magnetic
reconnection. Several new developments are discussed, including detailed
studies of reconnection in three-dimensional magnetic field configurations
(e.g., current sheets, collapsing traps, separatrix regions) and stochastic
acceleration in a turbulent environment. Fluid, test-particle, and
particle-in-cell approaches are used and results compared. While these studies
show considerable promise in accounting for the various observational
manifestations of solar flares, they are limited by a number of factors, mostly
relating to available computational power. Not the least of these issues is the
need to explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A brief
prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
An Observational Overview of Solar Flares
We present an overview of solar flares and associated phenomena, drawing upon
a wide range of observational data primarily from the RHESSI era. Following an
introductory discussion and overview of the status of observational
capabilities, the article is split into topical sections which deal with
different areas of flare phenomena (footpoints and ribbons, coronal sources,
relationship to coronal mass ejections) and their interconnections. We also
discuss flare soft X-ray spectroscopy and the energetics of the process. The
emphasis is to describe the observations from multiple points of view, while
bearing in mind the models that link them to each other and to theory. The
present theoretical and observational understanding of solar flares is far from
complete, so we conclude with a brief discussion of models, and a list of
missing but important observations.Comment: This is an article for a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011