8,714 research outputs found
Solar Flare Measurements with STIX and MiSolFA
Solar flares are the most powerful events in the solar system and the
brightest sources of X-rays, often associated with emission of particles
reaching the Earth and causing geomagnetic storms, giving problems to
communication, airplanes and even black-outs. X-rays emitted by accelerated
electrons are the most direct probe of solar flare phenomena. The Micro
Solar-Flare Apparatus (MiSolFA) is a proposed compact X-ray detector which will
address the two biggest issues in solar flare modeling. Dynamic range
limitations prevent simultaneous spectroscopy with a single instrument of all
X-ray emitting regions of a flare. In addition, most X-ray observations so far
are inconsistent with the high anisotropy predicted by the models usually
adopted for solar flares. Operated at the same time as the STIX instrument of
the ESA Solar Orbiter mission, at the next solar maximum (2020), they will have
the unique opportunity to look at the same flare from two different directions:
Solar Orbiter gets very close to the Sun with significant orbital inclination;
MiSolFA is in a near-Earth orbit. To solve the cross-calibration problems
affecting all previous attempts to combine data from different satellites,
MiSolFA will adopt the same photon detectors as STIX, precisely quantifying the
anisotropy of the X-ray emission for the first time. By selecting flares whose
footpoints (the brightest X-ray sources, at the chromosphere) are occulted by
the solar limb for one of the two detectors, the other will be able to study
the much fainter coronal emission, obtaining for the first time simultaneous
observations of all interesting regions. MiSolFA shall operate on board of a
very small satellite, with several launch opportunities, and will rely on
moir\'e imaging techniques.Comment: Invited talk, N30-8, Astrophysics and Space Instrumentation session,
2014 Nuclear Science Symposium and Medical Imaging Conference, 11 Nov 201
Measuring X-ray anisotropy in solar flares. Prospective stereoscopic capabilities of STIX and MiSolFA
During the next solar maximum, two upcoming space-borne X-ray missions, STIX
on board Solar Orbiter and MiSolFA, will perform stereoscopic X-ray
observations of solar flares at two different locations: STIX at 0.28 AU (at
perihelion) and up to inclinations of , and MiSolFA in a
low-Earth orbit. The combined observations from these cross-calibrated
detectors will allow us to infer the electron anisotropy of individual flares
confidently for the first time. We simulated both instrumental and physical
effects for STIX and MiSolFA including thermal shielding, background and X-ray
Compton backscattering (albedo effect) in the solar photosphere. We predict the
expected number of observable flares available for stereoscopic measurements
during the next solar maximum. We also discuss the range of useful spacecraft
observation angles for the challenging case of close-to-isotropic flare
anisotropy. The simulated results show that STIX and MiSolFA will be capable of
detecting low levels of flare anisotropy, for M1-class or stronger flares, even
with a relatively small spacecraft angular separation of 20-30{\deg}. Both
instruments will directly measure the flare X-ray anisotropy of about 40 M- and
X-class solar flares during the next solar maximum. Near-future stereoscopic
observations with Solar Orbiter/STIX and MiSolFA will help distinguishing
between competing flare-acceleration mechanisms, and provide essential
constraints regarding collisional and non-collisional transport processes
occurring in the flaring atmosphere for individual solar flares
Density-functional Theory for f electron Systems: the {\alpha}-{\gamma} Phase Transition in Cerium
The isostructural {\alpha}-{\gamma} phase transition in cerium is analyzed
using density-functional theory with different exchange-correlation
functionals, in particular the PBE0 hybrid functional and the exact- exchange
plus correlation in the random-phase approximation [(EX+cRPA)@PBE0] approach.
We show that the Hartree-Fock exchange part of the hybrid functional actuates
two distinct solutions at zero temperature that can be associated with the
{\alpha} and {\gamma} phases of cerium. However, despite the relatively good
structural and magnetic properties, PBE0 predicts the {\gamma} phase to be the
stable phase at ambient pressure and zero temperature, in contradiction with
low temperature experiments. EX+cRPA reverses the energetic ordering, which
emphasizes the importance of correlation for rare- earth systems
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