478 research outputs found
Bootstrapping the Coronal Magnetic Field with STEREO: I. Unipolar Potential Field Modeling
We investigate the recently quantified misalignment of between the 3-D geometry of stereoscopically triangulated
coronal loops observed with STEREO/EUVI (in four active regions) and
theoretical (potential or nonlinear force-free) magnetic field models
extrapolated from photospheric magnetograms. We develop an efficient method of
bootstrapping the coronal magnetic field by forward-fitting a parameterized
potential field model to the STEREO-observed loops. The potential field model
consists of a number of unipolar magnetic charges that are parameterized by
decomposing a photospheric magnetogram from MDI. The forward-fitting method
yields a best-fit magnetic field model with a reduced misalignment of
. We evaluate also stereoscopic
measurement errors and find a contribution of , which constrains the residual misalignment to
, which is likely
due to the nonpotentiality of the active regions. The residual misalignment
angle of the potential field due to nonpotentiality is found to
correlate with the soft X-ray flux of the active region, which implies a
relationship between electric currents and plasma heating.Comment: 12 figures, manuscript submitted to ApJ, 2010 Apr 2
Three-dimensional coronal slow modes: toward three-dimensional seismology
On 2008 January 10, the twin Solar Terrestrial Relations Observatory (STEREO) A and B spacecraft conducted a high time cadence study of the solar corona with the Extreme UltraViolet Imager (EUVI) instruments with the aim of investigating coronal dynamics. Observations of the three-dimensional propagation of waves within active region coronal loops and a measurement of the true coronal slow mode speed are obtained. Intensity oscillations with a period of approximately 12 minutes are observed to propagate outwards from the base of a loop system, consistent with the slow magnetoacoustic mode. A novel analysis technique is applied to measure the wave phase velocity in the observations of the A and B spacecraft. These stereoscopic observations are used to infer the three-dimensional velocity vector of the wave propagation, with an inclination of 37 +- 6 deg to the local normal and a magnitude of 132 +- 9 and 132 +- 11 km s-1, giving the first measurement of the true coronal longitudinal slow mode speed, and an inferred temperature of 0.84 +- 12 MK and 0.84 +- 15 MK
The Role of fast magnetosonic waves in the release and conversion via reconnection of energy stored by a current sheet
Using a simple two-dimensional, zero-beta model, we explore the manner by
which reconnection at a current sheet releases and dissipates free magnetic
energy. We find that only a small fraction (3%-11% depending on current sheet
size) of the energy is stored close enough to the current sheet to be
dissipated abruptly by the reconnection process. The remaining energy, stored
in the larger-scale field, is converted to kinetic energy in a fast
magnetosonic disturbance propagating away from the reconnection site, carrying
the initial current and generating reconnection-associated flows (inflow and
outflow). Some of this reflects from the lower boundary (the photosphere) and
refracts back to the X-point reconnection site. Most of this inward wave energy
is reflected back again, and continues to bounce between X-point and
photosphere until it is gradually dissipated, over many transits. This phase of
the energy dissipation process is thus global and lasts far longer than the
initial purely local phase. In the process a significant fraction of the energy
(25%-60%) remains as undissipated fast magnetosonic waves propagating away from
the reconnection site, primarily upward. This flare-generated wave is initiated
by unbalanced Lorentz forces in the reconnection-disrupted current sheet,
rather than by dissipation-generated pressure, as some previous models have
assumed. Depending on the orientation of the initial current sheet the wave
front is either a rarefaction, with backward directed flow, or a compression,
with forward directed flow
High energy cosmic-ray interactions with particles from the Sun
Cosmic-ray protons with energies above eV passing near the Sun may
interact with photons emitted by the Sun and be excited to a
resonance. When the decays, it produces pions which further decay to
muons and photons which may be detected with terrestrial detectors. A flux of
muons, photon pairs (from decay), or individual high-energy photons
coming from near the Sun would be a rather striking signature, and the flux of
these particles is a fairly direct measure of the flux of cosmic-ray nucleons,
independent of the cosmic-ray composition. In a solid angle within
around the Sun the flux of photon pairs is about \SI{1.3e-3}{}
particles/(kmyr), while the flux of muons is about \SI{0.33e-3}{}
particles/(kmyr). This is beyond the reach of current detectors like
the Telescope Array, Auger, KASCADE-Grande or IceCube. However, the muon flux
might be detectable by next-generation air shower arrays or neutrino detectors
such as ARIANNA or ARA. We discuss the experimental prospects in some detail.
Other cosmic-ray interactions occuring close to the Sun are also briefly
discussed.Comment: 8 pages, 11 figure
Time-Distance Seismology of the Solar Corona with CoMP
We employ a sequence of Doppler images obtained with the Coronal
Multi-channel Polarimeter (CoMP) instrument to perform time-distance seismology
of the solar corona. We construct the first k-omega diagrams of the region.
These allow us to separate outward and inward propagating waves and estimate
the spatial variation of the plane-of-sky projected phase speed, and the
relative amount of outward and inward directed wave power. The disparity
between outward and inward wave power and the slope of the observed power law
spectrum indicate that low-frequency Alfvenic motions suffer significant
attenuation as they propagate, consistent with isotropic MHD turbulence.Comment: In Press ApJ. 8 pages and 8 color figure
Observation of multiple sausage oscillations in cool postflare loop
Using simultaneous high spatial (1.3 arc sec) and temporal (5 and 10 s)
resolution H-alpha observations from the 15 cm Solar Tower Telescope at ARIES,
we study the oscillations in the relative intensity to explore the possibility
of sausage oscillations in the chromospheric cool postflare loop. We use
standard wavelet tool, and find the oscillation period of ~ 587 s near the loop
apex, and ~ 349 s near the footpoint. We suggest that the oscillations
represent the fundamental and the first harmonics of fast sausage waves in the
cool postflare loop. Based on the period ratio P1/P2 ~ 1.68, we estimate the
density scale height in the loop as ~ 17 Mm. This value is much higher than the
equilibrium scale height corresponding to H-alpha temperature, which probably
indicates that the cool postflare loop is not in hydrostatic equilibrium.
Seismologically estimated Alfv\'en speed outside the loop is ~ 300-330 km/s.
The observation of multiple oscillations may play a crucial role in
understanding the dynamics of lower solar atmosphere, complementing such
oscillations already reported in the upper solar atmosphere (e.g., hot flaring
loops).Comment: 13 pages, 4 figures, accepted in MNRA
The State of Self-Organized Criticality of the Sun During the Last 3 Solar Cycles. I. Observations
We analyze the occurrence frequency distributions of peak fluxes , total
fluxes , and durations of solar flares over the last three solar cycles
(during 1980--2010) from hard X-ray data of HXRBS/SMM, BATSE/CGRO, and RHESSI.
From the synthesized data we find powerlaw slopes with mean values of
for the peak flux, for the total
flux, and for flare durations. We find a systematic
anti-correlation of the powerlaw slope of peak fluxes as a function of the
solar cycle, varying with an approximate sinusoidal variation
, with a
mean of , a variation of , a solar cycle
period yrs, and a cycle minimum time . The
powerlaw slope is flattest during the maximum of a solar cycle, which indicates
a higher magnetic complexity of the solar corona that leads to an
overproportional rate of powerful flares.Comment: subm. to Solar Physic
Stereoscopic Polar Plume Reconstructions from Stereo/Secchi Images
We present stereoscopic reconstructions of the location and inclination of
polar plumes of two data sets based on the two simultaneously recorded images
taken by the EUVI telescopes in the SECCHI instrument package onboard the
\emph{STEREO (Solar TErrestrial RElations Observatory)} spacecraft. The ten
plumes investigated show a superradial expansion in the coronal hole in 3D
which is consistent with the 2D results. Their deviations from the local
meridian planes are rather small with an average of . By
comparing the reconstructed plumes with a dipole field with its axis along the
solar rotation axis, it is found that plumes are inclined more horizontally
than the dipole field. The lower the latitude is, the larger is the deviation
from the dipole field. The relationship between plumes and bright points has
been investigated and they are not always associated. For the first data set,
based on the 3D height of plumes and the electron density derived from
SUMER/\emph{SOHO} Si {\sc viii} line pair, we found that electron densities
along the plumes decrease with height above the solar surface. The temperature
obtained from the density scale height is 1.6 to 1.8 times larger than the
temperature obtained from Mg {\sc ix} line ratios. We attribute this
discrepancy to a deviation of the electron and the ion temperatures. Finally,
we have found that the outflow speeds studied in the O {\sc vi} line in the
plumes corrected by the angle between the line of sight and the plume
orientation are quite small with a maximum of 10 . It is
unlikely that plumes are a dominant contributor to the fast solar wind.Comment: 25 pages, 13 figure
Reconstructing the Local Twist of Coronal Magnetic Fields and the Three-Dimensional Shape of the Field Lines from Coronal Loops in EUV and X-Ray Images
Non-linear force-free fields are the most general case of force-free fields,
but the hardest to model as well. There are numerous methods of computing such
fields by extrapolating vector magnetograms from the photosphere, but very few
attempts have so far made quantitative use of coronal morphology. We present a
method to make such quantitative use of X-Ray and EUV images of coronal loops.
Each individual loop is fit to a field line of a linear force-free field,
allowing the estimation of the field line's twist, three-dimensional geometry
and the field strength along it.
We assess the validity of such a reconstruction since the actual corona is
probably not a linear force-free field and that the superposition of linear
force-free fields is generally not itself a force-free field. To do so, we
perform a series of tests on non-linear force-free fields, described in Low &
Lou (1990). For model loops we project field lines onto the photosphere. We
compare several results of the method with the original field, in particular
the three-dimensional loop shapes, local twist (coronal alpha), distribution of
twist in the model photosphere and strength of the magnetic field. We find
that, (i) for these trial fields, the method reconstructs twist with mean
absolute deviation of at most 15% of the range of photospheric twist, (ii) that
heights of the loops are reconstructed with mean absolute deviation of at most
5% of the range of trial heights and (iii) that the magnitude of non-potential
contribution to photospheric field is reconstructed with mean absolute
deviation of at most 10% of the maximal value.Comment: submitted to Ap
Continuous upflows and sporadic downflows observed in active regions
We present a study of the temporal evolution of coronal loops in active
regions and its implications for the dynamics in coronal loops. We analyzed
images of the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics
Observatory (SDO) at multiple temperatures to detect apparent motions in the
coronal loops. Quasi-periodic brightness fluctuations propagate upwards from
the loop footpoint in hot emission at 1MK, while sporadic downflows are seen in
cool emission below 1MK. The upward motion in hot emission increases just after
the cool downflows. The apparent propagating pattern suggests a hot upflow from
the loop footpoints, and is considered to supply hot plasma into the coronal
loop, but a wavelike phenomenon cannot be ruled out. Coronal condensation
occasionally happens in the coronal loop, and the cool material flows down to
the footpoint. Emission from cool plasma could have a significant contribution
to hot AIA channels in the event of coronal condensation.Comment: 5 pages, 6 figures, A&A in pres
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