101 research outputs found
Recovering thermodynamics from spectral profiles observed by IRIS (II): improved calculation of the uncertainties based on Monte Carlo experiments
Observations by the Interface Region Imaging Spectrograph (IRIS) in the Mg II
h & k spectral lines have provided a new diagnostic window towards the
knowledge of the complex physical conditions in the solar chromosphere.
Theoretical efforts focused on understanding the behavior of these lines have
allowed us to obtain a better and more accurate vision of the chromosphere.
These efforts include forward modeling, numerical simulations, and inversions.
In this paper, we focus our attention on the uncertainties associated with the
thermodynamic model atmosphere obtained after the inversion of the Mg II h & k
lines. We have used ~ 50;000 synthetic representative profiles of the IRIS2
database to characterize the most important source of uncertainties in the
inversion process, viz.: the inherent noise of the observations, the random
initialization of process, and the selection criteria in a high-dimensional
space. We have applied a Monte Carlo approach to this problem. Thus, for a
given synthetic representative profile, we have created five randomized noise
realizations (representative of the most popular exposure times in the IRIS
observations), and inverted these profiles five times with different inversion
initializations. The resulting 25 inverted profiles, fits to noisy data, and
model atmospheres are then used to determine the uncertainty in the model
atmosphere, based on the standard deviation and empirical selection criteria
for the goodness of fit. With this approach, the new uncertainties of the
models available in the IRIS2 database are more reliable at the optical depths
where the Mg II h & k lines are sensitive to changes in the thermodynamics.Comment: 13 pages, 8 figures, and 1 tabl
On the Origin of a Sunquake during the 29 March 2014 X1 Flare
Helioseismic data from the HMI instrument have revealed a sunquake associated
with the X1 flare SOL2014-03-29T17:48 in active region NOAA 12017. We try to
discover if acoustic-like impulses or actions of the Lorentz force caused the
sunquake. We analyze spectro-polarimetric data obtained with the Facility
Infrared Spectrometer (FIRS) at the Dunn Solar Telescope (DST). Fortuitously
the FIRS slit crossed the flare kernel close to the acoustic source, during the
impulsive phase. The infrared FIRS data remain unsaturated throughout the
flare. Stokes profiles of lines of Si I 1082.7 nm and He I 1083.0 nm are
analyzed. At the flare footpoint, the Si I 1082.7 nm core intensity increases
by a factor of several, the IR continuum increases by 4 +/- 1%. Remarkably, the
Si I core resembles the classical Ca II K line's self-reversed profile. With
nLTE radiative models of H, C, Si and Fe these properties set the penetration
depth of flare heating to 100 +/- 100 km, i.e. photospheric layers. Estimates
of the non-magnetic energy flux are at least a factor of two less than the
sunquake energy flux. Milne-Eddington inversions of the Si I line show that the
local magnetic energy changes are also too small to drive the acoustic pulse.
Our work raises several questions: Have we "missed" the signature of downward
energy propagation? Is it intermittent in time and/or non-local? Does the 1-2 s
photospheric radiative damping time discount compressive modes?Comment: in pres
Recovering Thermodynamics from Spectral Profiles observed by IRIS: A Machine and Deep Learning Approach
Inversion codes allow reconstructing a model atmosphere from observations.
With the inclusion of optically thick lines that form in the solar
chromosphere, such modelling is computationally very expensive because a
non-LTE evaluation of the radiation field is required. In this study, we
combine the results provided by these traditional methods with machine and deep
learning techniques to obtain similar-quality results in an easy-touse, much
faster way. We have applied these new methods to Mg II h&k lines observed by
IRIS. As a result, we are able to reconstruct the thermodynamic state
(temperature, line-of-sight velocity, non-thermal velocities, electron density,
etc.) in the chromosphere and upper photosphere of an area equivalent to an
active region in a few CPU minutes, speeding up the process by a factor of
-. The open-source code accompanying this paper will allow the
community to use IRIS observations to open a new window to a host of solar
phenomena.Comment: 8 pages, 5 figure
Magnetic topology of a naked sunspot: Is it really naked?
The high spatial, temporal and spectral resolution achieved by Hinode
instruments give much better understanding of the behavior of some elusive
solar features, such as pores and naked sunspots. Their fast evolution and, in
some cases, their small sizes have made their study difficult. The moving
magnetic features, despite being more dynamic structures, have been studied
during the last 40 years. They have been always associated with sunspots,
especially with the penumbra. However, a recent observation of a naked sunspot
(one with no penumbra) has shown MMF activity. The authors of this reported
observation expressed their reservations about the explanation given to the
bipolar MMF activity as an extension of the penumbral filaments into the moat.
How can this type of MMFs exist when a penumbra does not? In this paper, we
study the full magnetic and (horizontal) velocity topology of the same naked
sunspot, showing how the existence of a magnetic field topology similar to that
observed in sunspots can explain these MMFs, even when the intensity map of the
naked sunspot does not show a penumbra.Comment: 6 pages, 2 figure
Chromospheric thermodynamic conditions from inversions of complex Mg II h & k profiles observed in flares
The flare activity of the Sun has been studied for decades, using both space- and ground-based telescopes. The former have mainly focused on the corona, while the latter have mostly been used to investigate the conditions in the chromosphere and photosphere. The Interface Region Imaging Spectrograph (IRIS) instrument has served as a gateway between these two cases, given its capability to observe quasi-simultaneously the corona, the transition region, and the chromosphere using different spectral lines in the near- and far-ultraviolet ranges. IRIS thus provides unique diagnostics to investigate the thermodynamics of flares in the solar atmosphere. In particular, the Mg II h&k and the Mg II UV triplet lines provide key information about the thermodynamics of low to upper chromosphere, while the C II 1334 & 1335 Å lines cover the upper-chromosphere and low transition region. The Mg II h&k and the Mg II UV triplet lines show a peculiar, pointy shape before and during the flare activity. The physical interpretation, i.e., the physical conditions in the chromosphere, that can explain these profiles has remained elusive. In this paper, we show the results of a non-LTE inversion of such peculiar profiles. To better constrain the atmospheric conditions, the Mg II h&k and the Mg II UV triplet lines are simultaneously inverted with the C II 1334 & 1335 Å lines. This combined inversion leads to more accurate derived thermodynamic parameters, especially the temperature and the turbulent motions (micro-turbulence velocity). We use an iterative process that looks for the best fit between the observed profile and a synthetic profile obtained by considering non-local thermodynamic equilibrium and partial frequency redistribution of the radiation due to scattered photons. This method is computationally rather expensive (≈6 CPU-hour/profile). Therefore, we use the k-means clustering technique to identify representative profiles and associated representative model atmospheres. By inverting the representative profiles with the most advanced inversion code (STiC), in addition to recover the main physical parameters, we are able to conclude that these unique, pointy profiles are associated with a large gradient in the line-of-sight velocity along the optical depth in the high chromosphere
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