23 research outputs found
Molecular absorption in transition region spectral lines
Aims: We present observations from the Interface Region Imaging Spectrograph
(IRIS) of absorption features from a multitude of cool atomic and molecular
lines within the profiles of Si IV transition region lines. Many of these
spectral lines have not previously been detected in solar spectra. Methods: We
examined spectra taken from deep exposures of plage on 12 October 2013. We
observed unique absorption spectra over a magnetic element which is bright in
transition region line emission and the ultraviolet continuum. We compared the
absorption spectra with emission spectra that is likely related to
fluorescence. Results: The absorption features require a population of sub-5000
K plasma to exist above the transition region. This peculiar stratification is
an extreme deviation from the canonical structure of the chromosphere-corona
boundary . The cool material is not associated with a filament or discernible
coronal rain. This suggests that molecules may form in the upper solar
atmosphere on small spatial scales and introduces a new complexity into our
understanding of solar thermal structure. It lends credence to previous
numerical studies that found evidence for elevated pockets of cool gas in the
chromosphere.Comment: accepted by A&A Letter
Thomson scattering above solar active regions and an ad-hoc polarization correction method for the emissive corona
Thomson scattered photospheric light is the dominant constituent of the lower
solar corona's spectral continuum viewed off-limb at optical wavelengths. Known
as the K-corona, it is also linearly polarized. We investigate the possibility
of using the a priori polarized characteristics of the K-corona, together with
polarized emission lines, to measure and correct instrument-induced polarized
crosstalk. First, we derive the Stokes parameters of Thomson scattering of
unpolarized light in an irreducible spherical tensor formalism. This allows
forward synthesis of the Thomson scattered signal for the more complex scenario
of symmetry-breaking features in the incident radiation field, which could
limit the accuracy of our proposed technique. For this, we make use of an
advanced 3D radiative magnetohydrodynamic coronal model. Together with
synthesized polarized signals in the Fe XIII 10746 Angstrom emission line, we
find that an ad hoc correction of telescope and instrument-induced polarization
crosstalk is possible under the assumption of a non-depolarizing optical
system.Comment: Accepted for publication in Ap
A Model-based Technique for Ad Hoc Correction of Instrumental Polarization in Solar Spectropolarimetry
We present a new approach for correcting instrumental polarization by
modeling the non-depolarizing effects of a complex series of optical elements
to determine physically realizable Mueller matrices. Provided that the Mueller
matrix of the optical system can be decomposed into a general elliptical
diattenuator and general elliptical retarder, it is possible to model the
cross-talk between both the polarized and unpolarized states of the Stokes
vector and then use the acquired science observations to determine the best-fit
free parameters. Here, we implement a minimization for solar
spectropolarimetric measurements containing photospheric spectral lines
sensitive to the Zeeman effect using physical constraints provided by polarized
line and continuum formation. This model-based approach is able to provide an
accurate correction even in the presence of large amounts of polarization
cross-talk and conserves the physically meaningful magnitude of the Stokes
vector, a significant improvement over previous ad hoc techniques.Comment: 16 pages, 4 figures, Accepted for publication in Ap
Evidence of Non-Thermal Particles in Coronal Loops Heated Impulsively by Nanoflares
The physical processes causing energy exchange between the Sun's hot corona
and its cool lower atmosphere remain poorly understood. The chromosphere and
transition region (TR) form an interface region between the surface and the
corona that is highly sensitive to the coronal heating mechanism. High
resolution observations with the Interface Region Imaging Spectrograph (IRIS)
reveal rapid variability (about 20 to 60 seconds) of intensity and velocity on
small spatial scales at the footpoints of hot dynamic coronal loops. The
observations are consistent with numerical simulations of heating by beams of
non-thermal electrons, which are generated in small impulsive heating events
called "coronal nanoflares". The accelerated electrons deposit a sizable
fraction of their energy in the chromosphere and TR. Our analysis provides
tight constraints on the properties of such electron beams and new diagnostics
for their presence in the nonflaring corona.Comment: Published in Science on October 17:
http://www.sciencemag.org/content/346/6207/1255724 . 26 pages, 10 figures.
Movies are available at: http://www.lmsal.com/~ptesta/iris_science_mov
Probing the Physics of the Solar Atmosphere with the Multi-slit Solar Explorer (MUSE). II. Flares and Eruptions
Current state-of-the-art spectrographs cannot resolve the fundamental spatial (subarcseconds) and temporal (less than a few tens of seconds) scales of the coronal dynamics of solar flares and eruptive phenomena. The highest-resolution coronal data to date are based on imaging, which is blind to many of the processes that drive coronal energetics and dynamics. As shown by the Interface Region Imaging Spectrograph for the low solar atmosphere, we need high-resolution spectroscopic measurements with simultaneous imaging to understand the dominant processes. In this paper: (1) we introduce the Multi-slit Solar Explorer (MUSE), a spaceborne observatory to fill this observational gap by providing high-cadence (<20 s), subarcsecond-resolution spectroscopic rasters over an active region size of the solar transition region and corona; (2) using advanced numerical models, we demonstrate the unique diagnostic capabilities of MUSE for exploring solar coronal dynamics and for constraining and discriminating models of solar flares and eruptions; (3) we discuss the key contributions MUSE would make in addressing the science objectives of the Next Generation Solar Physics Mission (NGSPM), and how MUSE, the high-throughput Extreme Ultraviolet Solar Telescope, and the Daniel K Inouye Solar Telescope (and other ground-based observatories) can operate as a distributed implementation of the NGSPM. This is a companion paper to De Pontieu et al., which focuses on investigating coronal heating with MUSE
An observational study of the formation and evolution of sunspots
Ph.D. University of Hawaii at Manoa 2011.Includes bibliographical references.This dissertation focuses on the problem of molecules and the horizontal balance of forces in sunspots. Sunspots are quasi-static features on the solar surface and can be considered to be in a state of equilibrium. The weaker gas pressure of the cool sunspot interior is horizontally supported against the higher pressure of the hotter quiet-Sun by a strong vertical magnetic field. However, some sunspots show a rapid increase in magnetic pressure relative to the temperature of the gas in the coolest regions of the sunspot, implying that an isothermal decrease in the gas pressure must have occurred. The current model of sunspots is unable to describe this deviation from the assumed equilibrium state of the magnetic field and thermal gas pressure observed in these sunspots. Another method of altering the pressure of the gas must be occurring. The formation of molecules in sunspots may be the key to solving this puzzle. The sunspot interior provides a cool environment where molecules can form in abundance. As atoms become bound into molecules the total particle number of the gas is decreased. A sufficiently large molecular fraction could significantly alter the properties of the sunspot plasma, and specifically provide a mechanism for concentrating the magnetic field by non-thermally lowering the gas pressure.
I have investigated the equilibrium condition of sunspots of different sizes and in a variety of evolutionary phases through a Milne-Eddington inversion of spectropolarimetric observations of the Zeeman-split Fe I lines at 6302 and 15650 A to obtain their thermal and magnetic topology. I carried out a calculation of the detailed radiative transfer and chemical equilibrium of model sunspot atmospheres to determine the molecular gas fraction. Several sunspots show unambiguous cases of isothermal magnetic field intensification, which can only be explained by the formation or destruction of a large molecular population. All sunspots with magnetic fields stronger than 2500 G and temperatures cooler than 5800 K consistently show a signature of magnetic field over-concentration, consistent with molecular hydrogen formation of a few percent of the total gas fraction. The formation of this large molecular population has widespread implications for sunspot physics