52 research outputs found
Multimodal Differential Emission Measure in the Solar Corona
The Atmospheric Imaging Assembly (AIA) telescope on board the Solar Dynamics
Observatory (SDO) provides coronal EUV imaging over a broader temperature
sensitivity range than the previous generations of instruments (EUVI, EIT, and
TRACE). Differential emission measure tomography (DEMT) of the solar corona
based on AIA data is presented here for the first time. The main product of
DEMT is the three-dimensional (3D) distribution of the local differential
emission measure (LDEM). While in previous studies, based on EIT or EUVI data,
there were 3 available EUV bands, with a sensitivity range
MK, the present study is based on the 4 cooler AIA bands (aimed at studying the
quiet sun), sensitive to the range MK. The AIA filters allow
exploration of new parametric LDEM models. Since DEMT is better suited for
lower activity periods, we use data from Carrington Rotation 2099, when the Sun
was in its most quiescent state during the AIA mission. Also, we validate the
parametric LDEM inversion technique by applying it to standard bi-dimensional
(2D) differential emission measure (DEM) analysis on sets of simultaneous AIA
images, and comparing the results with DEM curves obtained using other methods.
Our study reveals a ubiquitous bimodal LDEM distribution in the quiet diffuse
corona, which is stronger for denser regions. We argue that the nanoflare
heating scenario is less likely to explain these results, and that alternative
mechanisms, such as wave dissipation appear better supported by our results.Comment: 52 pages, 18 figure
Time-Dependent Tomographic Reconstruction of the Solar Corona
Solar rotational tomography (SRT) applied to white-light coronal images
observed at multiple aspect angles has been the preferred approach for
determining the three-dimensional (3D) electron density structure of the solar
corona. However, it is seriously hampered by the restrictive assumption that
the corona is time-invariant which introduces significant errors in the
reconstruction. We first explore several methods to mitigate the temporal
variation of the corona by decoupling the "fast-varying" inner corona from the
"slow-moving" outer corona using multiple masking (either by juxtaposition or
recursive combination) and radial weighting. Weighting with a radial
exponential profile provides some improvement over a classical reconstruction
but only beyond 3 Rsun. We next consider a full time-dependent tomographic
reconstruction involving spatio-temporal regularization and further introduce a
co-rotating regularization aimed at preventing concentration of reconstructed
density in the plane of the sky. Crucial to testing our procedure and properly
tuning the regularization parameters is the introduction of a time-dependent
MHD model of the corona based on observed magnetograms to build a time-series
of synthetic images of the corona. Our procedure, which successfully reproduces
the time-varying model corona, is finally applied to a set of of 53 LASCO-C2 pB
images roughly evenly spaced in time from 15 to 29 March 2009. Our procedure
paves the way to a time-dependent tomographic reconstruction of the coronal
electron density to the whole set of LASCO-C2 images presently spanning 20
years.Comment: 24 pages, 18 figure
The WHI Corona from Differential Emission Measure Tomography
A three dimensional (3D) tomographic reconstruction of the local differential
emission measure (LDEM) of the global solar corona during the whole heliosphere
interval (WHI, Carrington rotation CR-2068) is presented, based on STEREO/EUVI
images. We determine the 3D distribution of the electron density, mean
temperature, and temperature spread, in the range of heliocentric heights 1.03
to 1.23 Rsun. The reconstruction is complemented with a potential field source
surface (PFSS) magnetic-field model. The streamer core, streamer legs, and
subpolar regions are analyzed and compared to a similar analysis previously
performed for CR-2077, very near the absolute minimum of the Solar Cycle 23. In
each region, the typical values of density and temperature are similar in both
periods. The WHI corona exhibits a streamer structure of relatively smaller
volume and latitudinal extension than during CR-2077, with a global
closed-to-open density contrast about 6% lower, and a somewhat more complex
morphology. The average basal electron density is found to be about 2.23 and
1.08 x 10^8 cm^-3, in the streamer core and subpolar regions, respectively. The
electron temperature is quite uniform over the analyzed height range, with
average values of about 1.13 and 0.93 MK, in the streamer core and subpolar
regions, respectively. Within the streamer closed region, both periods show
higher temperatures at mid-latitudes and lower temperatures near the equator.
Both periods show beta>1 in the streamer core and beta<1 in the surrounding
open regions, with CR-2077 exhibiting a stronger contrast. Hydrostatic fits to
the electron density are performed, and the scale height is compared to the
LDEM mean electron temperature. Within the streamer core, the results are
consistent with an isothermal hydrostatic plasma regime, with the temperatures
of ions and electrons differing by up to about 10% .. (continues)..Comment: 13 Figure
Millisecond Exoplanet Imaging, II: Regression Equations and Technical Discussion
The leading difficulty in achieving the contrast necessary to directly image
exoplanets and associated structures (eg. protoplanetary disks) at wavelengths
ranging from the visible to the infrared are quasi-static speckles, and they
are hard to distinguish from planets at the necessary level of precision. The
source of the quasi-static speckles is hardware aberrations that are not
compensated by the adaptive optics system. These aberrations are called
non-common path aberrations (NCPA). In 2013, Frazin showed how, in principle,
simultaneous millisecond (ms) telemetry from the wavefront sensor (WFS) and the
science camera behind a stellar coronagraph can be used as input into a
regression scheme that simultaneously and self-consistently estimates the NCPA
and the sought-after image of the planetary system (the exoplanet image). The
physical principle underlying the regression method is rather simple: the
wavefronts, which are measured by the WFS, modulate the speckles caused by the
NCPA and therefore can be used as probes of the optical system. The most
important departure from realism in the author's 2013 article was the
assumption that the WFS made error-free measurements. The simulations in Part I
provide results on the joint regression on the NCPA and the exoplanet image
from three different methods, called the ideal, the naive, and the
bias-corrected estimators. The ideal estimator is not physically realizable but
is a useful as a benchmark for simulation studies, but the other two are, at
least in principle. This article provides the regression equations for all
three of these estimators as well as a supporting technical discussion.
Briefly, the naive estimator simply uses the noisy WFS measurements without any
attempt to account for the errors, and the bias-corrected estimator uses
statistical knowledge of the wavefronts to treat errors in the WFS
measurements.Comment: 13 pages, 2 figures, submitted to JOSA
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