118 research outputs found
Spectroscopic measurements of dynamic fibrils in the Ca {\small{II}} 8662 {\AA} line
We present high spatial resolution spectroscopic measurements of dynamic
fibrils (DFs) in the Ca {\small{II}} 8662 {\AA} line. These data show clear
Doppler shifts in the identified DFs, which demonstrates that at least a subset
of DFs are actual mass motions in the chromosphere. A statistical analysis of
26 DFs reveals a strong and statistically significant correlation between the
maximal velocity and the deceleration. The range of the velocities and the
decelerations are substantially lower, about a factor two, in our spectroscopic
observations compared to the earlier results based on proper motion in narrow
band images. There are fundamental differences in the different observational
methods; when DFs are observed spectroscopically the measured Doppler shifts
are a result of the atmospheric velocity, weighted with the response function
to velocity over an extended height. When the proper motion of DFs is observed
in narrow band images, the movement of the top of the DF is observed. This
point is sharply defined because of the high contrast between the DF and the
surroundings. The observational differences between the two methods are
examined by several numerical experiments using both numerical simulations and
a time series of narrow band H images. With basis in the simulations we
conclude that the lower maximal velocity is explained by the low formation
height of the Ca IR line. We conclude that the present observations support the
earlier result that DFs are driven by magneto-acoustic shocks exited by
convective flows and p-modes.Comment: 7 pages 5 figures, Submitted to Ap
What is the Nature of EUV Waves? First STEREO 3D Observations and Comparison with Theoretical Models
One of the major discoveries of the Extreme ultraviolet Imaging Telescope
(EIT) on SOHO were intensity enhancements propagating over a large fraction of
the solar surface. The physical origin(s) of the so-called `EIT' waves is still
strongly debated. They are considered to be either wave (primarily fast-mode
MHD waves) or non-wave (pseudo-wave) interpretations. The difficulty in
understanding the nature of EUV waves lies with the limitations of the EIT
observations which have been used almost exclusively for their study. Their
limitations are largely overcome by the SECCHI/EUVI observations on-board the
STEREO mission. The EUVI telescopes provide high cadence, simultaneous
multi-temperature coverage, and two well-separated viewpoints. We present here
the first detailed analysis of an EUV wave observed by the EUVI disk imagers on
December 07, 2007 when the STEREO spacecraft separation was .
Both a small flare and a CME were associated with the wave cadence, and single
temperature and viewpoint coverage. These limitations are largely overcome by
the SECCHI/EUVI observations on-board the STEREO mission. The EUVI telescopes
provide high cadence, simultaneous multi-temperature coverage, and two
well-separated viewpoints. Our findings give significant support for a
fast-mode interpretation of EUV waves and indicate that they are probably
triggered by the rapid expansion of the loops associated with the CME.Comment: Solar Physics, 2009, Special STEREO Issue, in pres
Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results
The chromosphere is a thin layer of the solar atmosphere that bridges the
relatively cool photosphere and the intensely heated transition region and
corona. Compressible and incompressible waves propagating through the
chromosphere can supply significant amounts of energy to the interface region
and corona. In recent years an abundance of high-resolution observations from
state-of-the-art facilities have provided new and exciting ways of
disentangling the characteristics of oscillatory phenomena propagating through
the dynamic chromosphere. Coupled with rapid advancements in
magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly
investigate the role waves play in supplying energy to sustain chromospheric
and coronal heating. Here, we review the recent progress made in
characterising, categorising and interpreting oscillations manifesting in the
solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review
Solar Spectroscopy and (Pseudo-)Diagnostics of the Solar Chromosphere
I first review trends in current solar spectrometry and then concentrate on
comparing various spectroscopic diagnostics of the solar chromosphere. Some are
actually not at all chromospheric but just photospheric or clapotispheric and
do not convey information on chromospheric heating, even though this is often
assumed. Balmer Halpha is the principal displayer of the closed-field
chromosphere, but it is unclear how chromospheric fibrils gain their large
Halpha opacity. The open-field chromosphere seems to harbor most if not all
coronal heating and solar wind driving, but is hardly seen in optical
diagnostics.Comment: To appear in "Recent Advances in Spectroscopy: Astrophysical,
Theoretical and Experimental Perspectives", eds. R.K. Chaudhuri, M.V.
Mekkaden, A.V. Raveendran and A. Satya Narayanan, Astrophysics and Space
Science Proceedings, Springer, Heidelberg, 2009. Revision: references
corrected, new references added, minor text correction
Physodes and the phenolic compounds of brown algae. Composition and significance of physodes in vivo
Origins of the Ambient Solar Wind: Implications for Space Weather
The Sun's outer atmosphere is heated to temperatures of millions of degrees,
and solar plasma flows out into interplanetary space at supersonic speeds. This
paper reviews our current understanding of these interrelated problems: coronal
heating and the acceleration of the ambient solar wind. We also discuss where
the community stands in its ability to forecast how variations in the solar
wind (i.e., fast and slow wind streams) impact the Earth. Although the last few
decades have seen significant progress in observations and modeling, we still
do not have a complete understanding of the relevant physical processes, nor do
we have a quantitatively precise census of which coronal structures contribute
to specific types of solar wind. Fast streams are known to be connected to the
central regions of large coronal holes. Slow streams, however, appear to come
from a wide range of sources, including streamers, pseudostreamers, coronal
loops, active regions, and coronal hole boundaries. Complicating our
understanding even more is the fact that processes such as turbulence,
stream-stream interactions, and Coulomb collisions can make it difficult to
unambiguously map a parcel measured at 1 AU back down to its coronal source. We
also review recent progress -- in theoretical modeling, observational data
analysis, and forecasting techniques that sit at the interface between data and
theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue
connected with a 2016 ISSI workshop on "The Scientific Foundations of Space
Weather." 44 pages, 9 figure
Limb Spicules from the Ground and from Space
We amassed statistics for quiet-sun chromosphere spicules at the limb using
ground-based observations from the Swedish 1-m Solar Telescope on La Palma and
simultaneously from NASA's Transition Region and Coronal Explorer (TRACE)
spacecraft. The observations were obtained in July 2006. With the 0.2 arcsecond
resolution obtained after maximizing the ground-based resolution with the
Multi-Object Multi-Frame Blind Deconvolution (MOMFBD) program, we obtained
specific statistics for sizes and motions of over two dozen individual
spicules, based on movies compiled at 50-second cadence for the series of five
wavelengths observed in a very narrow band at H-alpha, on-band and in the red
and blue wings at 0.035 nm and 0.070 nm (10 s at each wavelength) using the
SOUP filter, and had simultaneous observations in the 160 nm EUV continuum from
TRACE. The MOMFBD restoration also automatically aligned the images,
facilitating the making of Dopplergrams at each off-band pair. We studied 40
H-alpha spicules, and 14 EUV spicules that overlapped H-alpha spicules; we
found that their dynamical and morphological properties fit into the framework
of several previous studies. From a preliminary comparison with spicule
theories, our observations are consistent with a reconnection mechanism for
spicule generation, and with UV spicules being a sheath region surrounding the
H-alpha spicules
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Article discussing research on L- and M-shell x-ray production cross sections of Nd, Gd, Ho, Yb, Au, and Pb by 25-MeV carbon and 32-MeV oxygen ions
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