33 research outputs found
Optical Spectral Observations of a Flickering White-Light Kernel in a C1 Solar Flare
We analyze optical spectra of a two-ribbon, long duration C1.1 flare that
occurred on 18 Aug 2011 within AR 11271 (SOL2011-08-18T15:15). The impulsive
phase of the flare was observed with a comprehensive set of space-borne and
ground-based instruments, which provide a range of unique diagnostics of the
lower flaring atmosphere. Here we report the detection of enhanced continuum
emission, observed in low-resolution spectra from 3600 \AA\ to 4550 \AA\
acquired with the Horizontal Spectrograph at the Dunn Solar Telescope. A small,
0''.5 ( cm) penumbral/umbral kernel brightens repeatedly in
the optical continuum and chromospheric emission lines, similar to the temporal
characteristics of the hard X-ray variation as detected by the Gamma-ray Burst
Monitor (GBM) on the Fermi spacecraft. Radiative-hydrodynamic flare models that
employ a nonthermal electron beam energy flux high enough to produce the
optical contrast in our flare spectra would predict a large Balmer jump in
emission, indicative of hydrogen recombination radiation from the upper flare
chromosphere. However, we find no evidence of such a Balmer jump in the
bluemost spectral region of the continuum excess. Just redward of the expected
Balmer jump, we find evidence of a "blue continuum bump" in the excess emission
which may be indicative of the merging of the higher order Balmer lines. The
large number of observational constraints provides a springboard for modeling
the blue/optical emission for this particular flare with radiative-hydrodynamic
codes, which are necessary to understand the opacity effects for the continuum
and emission line radiation at these wavelengths.Comment: 54 pages, 13 figures, accepted for publication in the Astrophysical
Journa
Science with Large Solar Telescopes: Overview of SpS 6
With several large aperture optical and IR telescopes just coming on-line, or scheduled for the near future, solar physics is on the verge of a quantum leap in observational capabilities. An efficient use of such facilities will require new and innovative approaches to both observatory operations and data handling.
This two-days long Special Session discussed the science expected with large solar telescopes, and started addressing the strategies necessary to optimize their scientific return. Cutting edge solar science as derived from state-of-the-art observations and numerical simulations and modeling was presented, and discussions were held on the role of large facilities in satisfying the demanding requirements of spatial and temporal resolution, stray-light correction, and spectro-polarimetric accuracy. Building on the experience of recently commissioned telescopes, critical issues for the development of future facilities were discussed. These included operational issues peculiar to large telecopes as well as strategies for their best use
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EVIDENCE FOR SHEET-LIKE ELEMENTARY STRUCTURES IN THE SUN'S ATMOSPHERE?
Narrow, thread-like structures in the Sun's chromosphere are currently understood to be plasma guided along narrow tubes of magnetic flux. We report on 1 s cadence imaging spectroscopic measurements of the Hα line with the IBIS Fabry-Perot instrument at the Dunn Solar Telescope, obtained +0.11 nm from line center. Rapid changes grossly exceeding the Alfven speed are commonly seen along the full extent of many chromospheric threads. We argue that only an optical superposition effect can reasonably explain the data, analogous to striations of curtains blowing in the wind. Other explanations appear to require significant contrivances to avoid contradicting various aspects of the data. We infer that the absorbing plasma exists in two-dimensional sheet-like structures within the three-dimensional magnetofluid, related perhaps to magnetic tangential discontinuities. This interpretation demands a re-evaluation of basic assumptions about low-β solar plasmas, as advocated by Parker, with broader implications in astrophysics and plasma physics. Diverse, high-cadence observations are needed to further define the relationship between magnetic field and thermal fine structure
Solar Chromospheric Temperature Diagnostics: a joint ALMA-H analysis
We present the first high-resolution, simultaneous observations of the solar
chromosphere in the optical and millimeter wavelength ranges, obtained with
ALMA and the IBIS instrument at the Dunn Solar Telescope. In this paper we
concentrate on the comparison between the brightness temperature observed in
ALMA Band 3 (3 mm; 100 GHz) and the core width of the H 656.3 nm line,
previously identified as a possible diagnostic of the chromospheric
temperature. We find that in the area of plage, network and fibrils covered by
our FOV the two diagnostics are well correlated, with similar spatial
structures observed in both. The strength of the correlation is remarkable,
given that the source function of the mm-radiation obeys local thermodynamic
equilibrium, while the H line has a source function that deviates
significantly from the local Planck function. The observed range of ALMA
brightness temperatures is sensibly smaller than the temperature range that was
previously invoked to explain the observed width variations in H. We
employ analysis from forward modeling with the RH code to argue that the strong
correlation between H width and ALMA brightness temperature is caused
by their shared dependence on the population number of the first excited
level of hydrogen. This population number drives millimeter opacity through
hydrogen ionization via the Balmer continuum, and H width through a
curve-of-growth-like opacity effect. Ultimately, the population is
regulated by the enhancement or lack of downward Ly flux, which
coherently shifts the formation height of both diagnostics to regions with
different temperature, respectively.Comment: Accepted for publication in Ap
The energy flux of internal gravity waves in the lower solar atmosphere
Stably stratified fluids, such as stellar and planetary atmospheres, can support and propagate gravity waves. On Earth these waves, which can transport energy and momentum over large distances and can trigger convection, contribute to the formation of our weather and global climate. Gravity waves also play a pivotal role in planetary sciences and modern stellar physics. They have also been proposed as an agent for the heating of stellar atmospheres and coronae, the exact mechanism behind which is one of the outstanding puzzles in solar and stellar physics. Using a combination of high-quality observations and 3D numerical simulations we have the first unambiguous detection of propagating gravity waves in the Sun's (and hence a stellar) atmosphere. Moreover, we are able to determine the height dependence of their energy flux and find that at the base of the Sun's chromosphere it is around 5 kW m−2. This amount of energy is comparable to the radiative losses of the entire chromosphere and points to internal gravity waves as a key mediator of energy into the solar atmosphere
LEMUR: Large European Module for solar Ultraviolet Research. European contribution to JAXA's Solar-C mission
Understanding the solar outer atmosphere requires concerted, simultaneous
solar observations from the visible to the vacuum ultraviolet (VUV) and soft
X-rays, at high spatial resolution (between 0.1" and 0.3"), at high temporal
resolution (on the order of 10 s, i.e., the time scale of chromospheric
dynamics), with a wide temperature coverage (0.01 MK to 20 MK, from the
chromosphere to the flaring corona), and the capability of measuring magnetic
fields through spectropolarimetry at visible and near-infrared wavelengths.
Simultaneous spectroscopic measurements sampling the entire temperature range
are particularly important.
These requirements are fulfilled by the Japanese Solar-C mission (Plan B),
composed of a spacecraft in a geosynchronous orbit with a payload providing a
significant improvement of imaging and spectropolarimetric capabilities in the
UV, visible, and near-infrared with respect to what is available today and
foreseen in the near future.
The Large European Module for solar Ultraviolet Research (LEMUR), described
in this paper, is a large VUV telescope feeding a scientific payload of
high-resolution imaging spectrographs and cameras. LEMUR consists of two major
components: a VUV solar telescope with a 30 cm diameter mirror and a focal
length of 3.6 m, and a focal-plane package composed of VUV spectrometers
covering six carefully chosen wavelength ranges between 17 and 127 nm. The
LEMUR slit covers 280" on the Sun with 0.14" per pixel sampling. In addition,
LEMUR is capable of measuring mass flows velocities (line shifts) down to 2
km/s or better.
LEMUR has been proposed to ESA as the European contribution to the Solar C
mission.Comment: 35 pages, 14 figures. To appear on Experimental Astronom