2,027 research outputs found
Active region evolution in the chromosphere and transtition region
Images in the C IV 1548 A and the Si II 1526 S lines taken with the ultraviolet spectrometer polarimeter (UVSP) instrument on board the Solar Maximum Mission (SMM) satellite were combined into movies showing the evolution of active regions and the neighboring supergranulation over several days. The data sets generally consist of 240 by 240 arc second rasters with 3 arc second pixels taken one per orbit (about every 90 minutes). The images are projected on a latitude/longitude grid to remove the forshortening as the region rotates across the solar disk and further processed to remove jitter and gain variations. Movies were made with and without differential rotation. Although there are occasional missing orbits, these series do not suffer from the long nighttime gaps that occur in observations taken at a single groundbased observatory and are excellent for studying changes on time scales of several hours. The longest sequence processed to date runs from 20 Oct. 1980 to 25 Oct. 1980. This was taken during an SMM flare buildup study on AR 2744. Several shorter sequences taken in 1980 and 1984 will also be shown. The results will be presented on a video disk which can be interactively controlled to view the movies
Convective Dynamos and the Minimum X-ray Flux in Main Sequence Stars
The objective of this paper is to investigate whether a convective dynamo can
account quantitatively for the observed lower limit of X-ray surface flux in
solar-type main sequence stars. Our approach is to use 3D numerical simulations
of a turbulent dynamo driven by convection to characterize the dynamic
behavior, magnetic field strengths, and filling factors in a non-rotating
stratified medium, and to predict these magnetic properties at the surface of
cool stars. We use simple applications of stellar structure theory for the
convective envelopes of main-sequence stars to scale our simulations to the
outer layers of stars in the F0--M0 spectral range, which allows us to estimate
the unsigned magnetic flux on the surface of non-rotating reference stars. With
these estimates we use the recent results of \citet{Pevtsov03} to predict the
level of X-ray emission from such a turbulent dynamo, and find that our results
compare well with observed lower limits of surface X-ray flux. If we scale our
predicted X-ray fluxes to \ion{Mg}{2} fluxes we also find good agreement with
the observed lower limit of chromospheric emission in K dwarfs. This suggests
that dynamo action from a convecting, non-rotating plasma is a viable
alternative to acoustic heating models as an explanation for the basal emission
level seen in chromospheric, transition region, and coronal diagnostics from
late-type stars.Comment: ApJ, accepted, 30 pages with 7 figure
A new Method to Constrain the Iron Abundance from Cooling Delays in Coronal Loops
Recent observations with TRACE reveal that the time delay between the
appearance of a cooling loop in different EUV temperature filters is
proportional to the loop length, dt_12 ~ L. We model this cooling delay in
terms of radiative loss and confirm this linear relationship theoretically. We
derive an expression that can be used to constrain the coronal iron enhancement
alpha_Fe=A_Fe^cor/A_Fe^Ph relative to the photospheric value as function of the
cooling delay dt_12, flux F_2, loop width w, and filling factor q_w < 1. With
this relation we find upper limits on the iron abundance enhancement of
alpha_Fe < 4.8+/-1.7 for 10 small-scale nanoflare loops, and alpha_Fe <
1.4+/-0.4 for 5 large-scale loops, in the temperature range of T~1.0-1.4 MK.
This result supports the previous finding that low-FIP elements, including Fe,
are enhanced in the corona. The same relation constitutes also a lower limit
for the filling factor, which is q_w > 0.2+/-0.1 and q_w > 0.8+/-0.2 for the
two groups of coronal loops.Comment: 2 Figure
Transverse oscillations of systems of coronal loops
We study the collective kinklike normal modes of a system of several
cylindrical loops using the T-matrix theory. Loops that have similar kink
frequencies oscillate collectively with a frequency which is slightly different
from that of the individual kink mode. On the other hand, if the kink frequency
of a loop is different from that of the others, it oscillates individually with
its own frequency. Since the individual kink frequency depends on the loop
density but not on its radius for typical 1 MK coronal loops, a coupling
between kink oscillations of neighboring loops take place when they have
similar densities. The relevance of these results in the interpretation of the
oscillations studied by \citet{schrijver2000} and \citet{verwichte2004}, in
which transverse collective loop oscillations seem to be detected, is
discussed. In the first case, two loops oscillating in antiphase are observed;
interpreting this motion as a collective kink mode suggests that their
densities are roughly equal. In the second case, there are almost three groups
of tubes that oscillate with similar periods and therefore their dynamics can
be collective, which again seems to indicate that the loops of each group share
a similar density. All the other loops seem to oscillate individually and their
densities can be different from the rest
The effect of magnetic activity saturation in chromospheric flux-flux relationships
We present a homogeneous study of chromospheric and coronal flux-flux
relationships using a sample of 298 late-type dwarf active stars with spectral
types F to M. The chromospheric lines were observed simultaneously in each star
to avoid spread due to long term variability. Unlike other works, we subtract
the basal chromospheric contribution in all the spectral lines studied. For the
first time, we quantify the departure of dMe stars from the general relations.
We show that dK and dKe stars also deviate from the general trend. Studying the
flux-colour diagrams we demonstrate that the stars deviating from the general
relations are those with saturated X-ray emission and that those stars also
present saturation in the H line. Using several age spectral
indicators, we show that they are younger stars than those following the
general relationships. The non-universality of flux-flux relationships found in
this work should be taken into account when converting between fluxes in
different chromospheric activity indicators.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Societ
Ca II H and K Chromospheric Emission Lines in Late K and M Dwarfs
We have measured the profiles of the Ca II H and K chromospheric emission
lines in 147 main sequence stars of spectral type M5-K7 (0.30-0.55 solar
masses) using multiple high resolution spectra obtained during six years with
the HIRES spectrometer on the Keck 1 telescope. Remarkably, the average FWHM,
equivalent widths, and line luminosities of Ca II H and K increase by a factor
of 3 with increasing stellar mass over this small range of stellar masses. We
fit the H and K lines with a double Gaussian model to represent both the
chromospheric emission and the non-LTE central absorption. Most of the sample
stars display a central absorption that is typically redshifted by ~0.1 km/s
relative to the emission, but the nature of this velocity gradient remains
unknown. The FWHM of the H and K lines increase with stellar luminosity,
reminiscent of the Wilson-Bappu effect in FGK-type stars. Both the equivalent
widths and FWHM exhibit modest temporal variability in individual stars. At a
given value of M_v, stars exhibit a spread in both the equivalent width and
FWHM of Ca II H and K, due both to a spread in fundamental stellar parameters
including rotation rate, age, and possibly metallicity, and to the spread in
stellar mass at a given M_v. The K line is consistently wider than the H line,
as expected, and its central absorption is more redshifted, indicating that the
H and K lines form at slightly different heights in the chromosphere where the
velocities are slightly different. The equivalent width of H-alpha correlates
with Ca II H and K only for stars having Ca II equivalent widths above ~2
angstroms, suggesting the existence of a magnetic threshold above which the
lower and upper chromospheres become thermally coupled.Comment: 40 pages including 12 figures and 17 pages of tables, accepted for
publication in PAS
Hydrostatic Modeling of the Integrated Soft X-Ray and EUV Emission in Solar Active Regions
Many studies of the solar corona have shown that the observed X-ray
luminosity is well correlated with the total unsigned magnetic flux.
In this paper we present results from the extensive numerical modeling of
active regions observed with the \textit{Solar and
Heliospheric Observatory} (\textit{SOHO}) Extreme Ultraviolet
Telescope (EIT), the \textit{Yohkoh} Soft X-ray Telescope (SXT), and the
\textit{SOHO} Michelson Doppler Imager (MDI). We use potential field
extrapolations to compute magnetic field lines and populate these field lines
with solutions to the hydrostatic loop equations assuming steady, uniform
heating. Our volumetric heating rates are of the form , where is the magnetic field strength
averaged along a field line and is the loop length. Comparisons between the
observed and simulated emission for 26 active regions suggest that coronal
heating models that scale as are in the closest
argreement the observed emission at high temperatures. The field-braiding
reconnection model of Parker, for example, is consistent with our results. We
find, however, that the integrated intensities alone are insufficent to
uniquely determine the parameterization of the volumetric heating rate.
Visualizations of the emission are also needed. We also find that there are
significant discrepancies between our simulation results and the lower
temperature emission observed in the EIT channels.Comment: Accepted for publication in ApJ; Replaced problem figur
Estimating the frequency of extremely energetic solar events, based on solar, stellar, lunar, and terrestrial records
The most powerful explosions on the Sun [...] drive the most severe
space-weather storms. Proxy records of flare energies based on SEPs in
principle may offer the longest time base to study infrequent large events. We
conclude that one suggested proxy, nitrate concentrations in polar ice cores,
does not map reliably to SEP events. Concentrations of select radionuclides
measured in natural archives may prove useful in extending the time interval of
direct observations up to ten millennia, but as their calibration to solar
flare fluences depends on multiple poorly known properties and processes, these
proxies cannot presently be used to help determine the flare energy frequency
distribution. Being thus limited to the use of direct flare observations, we
evaluate the probabilities of large-energy solar explosions by combining solar
flare observations with an ensemble of stellar flare observations. We conclude
that solar flare energies form a relatively smooth distribution from small
events to large flares, while flares on magnetically-active, young Sun-like
stars have energies and frequencies markedly in excess of strong solar flares,
even after an empirical scaling with the mean activity level of these stars. In
order to empirically quantify the frequency of uncommonly large solar flares
extensive surveys of stars of near-solar age need to be obtained, such as is
feasible with the Kepler satellite. Because the likelihood of flares larger
than approximately X30 remains empirically unconstrained, we present indirect
arguments, based on records of sunspots and on statistical arguments, that
solar flares in the past four centuries have likely not substantially exceeded
the level of the largest flares observed in the space era, and that there is at
most about a 10% chance of a flare larger than about X30 in the next 30 years.Comment: 14 pages, 3 figures (in press as of 2012/06/18); Journal of
Geophysical Research (Space Physics), 201
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