262 research outputs found
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
Exploring the Origin of Solar Energetic Electrons I: Constraining the Properties of the Acceleration Region Plasma Environment
Solar flare electron acceleration is an efficient process, but its properties
(mechanism, location) are not well constrained. Via hard X-ray (HXR) emission,
we routinely observe energetic electrons at the Sun, and sometimes we detect
energetic electrons in interplanetary space. We examine if the plasma
properties of an acceleration region (size, temperature, density) can be
constrained from in-situ observations, helping to locate the acceleration
region in the corona, and infer the relationship between electrons observed
in-situ and at the Sun. We model the transport of energetic electrons,
accounting for collisional and non-collisional effects, from the corona into
the heliosphere (to 1.0 AU). In the corona, electrons are transported through a
hot, over-dense region. We test if the properties of this region can be
extracted from electron spectra (fluence and peak flux) at different
heliospheric locations. We find that cold, dense coronal regions significantly
reduce the energy at which we see the peak flux and fluence for distributions
measured out to 1.0 AU, the degree of which correlates with the temperature and
density of plasma in the region. Where instrument energy resolution is
insufficient to differentiate the corresponding peak values, the spectral ratio
of [7-10) to [4-7) keV can be more readily identified and demonstrates the same
relationship. If flare electrons detected in-situ are produced in, and/or
transported through hot, over-dense regions close to HXR-emitting electrons,
then this plasma signature should be present in their lower-energy spectra
(1-20 keV), observable at varying heliospheric distances with missions such as
Solar Orbiter.Comment: 16 pages, 8 figure
On the variation of solar flare coronal x-ray source sizes with energy
Observations with {\em RHESSI} have enabled the detailed study of the
structure of dense hard X-ray coronal sources in solar flares. The variation of
source extent with electron energy has been discussed in the context of
streaming of non-thermal particles in a one-dimensional cold-target model, and
the results used to constrain both the physical extent of, and density within,
the electron acceleration region. Here we extend this investigation to a more
physically realistic model of electron transport that takes into account the
finite temperature of the ambient plasma, the initial pitch-angle distribution
of the accelerated electrons, and the effects of collisional pitch-angle
scattering. The finite temperature results in the thermal diffusion of
electrons, that leads to the observationally-inferred value of the acceleration
region volume being an overestimate of its true value. The different directions
of the electron trajectories, a consequence of both the non-zero injection
pitch-angle and scattering within the target, cause the projected propagation
distance parallel to the guiding magnetic field to be reduced, so that a
one-dimensional interpretation can overestimate the actual density by a factor
of up to . The implications of these results for the determination of
acceleration region properties (specific acceleration rate, filling factor,
etc.) are discussed.Comment: 45 pages, 9 figures, accepted for publication in Ap
Temperature sensitivity of the pyloric neuromuscular system and its modulation by dopamine
We report here the effects of temperature on the p1 neuromuscular system of the stomatogastric system of the lobster (Panulirus interruptus). Muscle force generation, in response to both the spontaneously rhythmic in vitro pyloric network neural activity and direct, controlled motor nerve stimulation, dramatically decreased as temperature increased, sufficiently that stomach movements would very unlikely be maintained at warm temperatures. However, animals fed in warm tanks showed statistically identical food digestion to those in cold tanks. Applying dopamine, a circulating hormone in crustacea, increased muscle force production at all temperatures and abolished neuromuscular system temperature dependence. Modulation may thus exist not only to increase the diversity of produced behaviors, but also to maintain individual behaviors when environmental conditions (such as temperature) vary
Spectral and Imaging Diagnostics of Spatially-Extended Turbulent Electron Acceleration and Transport in Solar Flares
Solar flares are efficient particle accelerators with a large fraction of
released magnetic energy (10-50%) converted into energetic particles such as
hard X-ray producing electrons. This energy transfer process is not well
constrained, with competing theories regarding the acceleration mechanism(s),
including MHD turbulence. We perform a detailed parameter study examining how
various properties of the acceleration region, including its spatial extent and
the spatial distribution of turbulence, affect the observed electron
properties, such as those routinely determined from X-ray imaging and
spectroscopy. Here, a time-independent Fokker-Planck equation is used to
describe the acceleration and transport of flare electrons through a coronal
plasma of finite temperature. Motivated by recent non-thermal line broadening
observations that suggested extended regions of turbulence in coronal loops, an
extended turbulent acceleration region is incorporated into the model. We
produce outputs for the density weighted electron flux, a quantity directly
related to observed X-rays, modelled in energy and space from the corona to
chromosphere. We find that by combining several spectral and imaging
diagnostics (such as spectral index differences or ratios, energy or
spatial-dependent flux ratios, and electron depths into the chromosphere) the
acceleration properties, including the timescale and velocity dependence, can
be constrained alongside the spatial properties. Our diagnostics provide a
foundation for constraining the properties of acceleration in an individual
flare from X-ray imaging spectroscopy alone, and can be applied to past,
current and future observations including those from RHESSI and Solar Orbiter.Comment: ApJ Accepte
Determination of the total accelerated electron rate and power using solar flare hard X-ray spectra
Solar flare hard X-ray spectroscopy serves as a key diagnostic of the
accelerated electron spectrum. However, the standard approach using the
collisional cold thick-target model poorly constrains the lower-energy part of
the accelerated electron spectrum, and hence the overall energetics of the
accelerated electrons are typically constrained only to within one or two
orders of magnitude. Here we develop and apply a physically self-consistent
warm-target approach which involves the use of both hard X-ray spectroscopy and
imaging data. The approach allows an accurate determination of the electron
distribution low-energy cutoff, and hence the electron acceleration rate and
the contribution of accelerated electrons to the total energy released, by
constraining the coronal plasma parameters. Using a solar flare observed in
X-rays by the {\em RHESSI} spacecraft, we demonstrate that using the standard
cold-target methodology, the low-energy cutoff (and hence the energy content in
electrons) is essentially undetermined. However, the warm-target methodology
can determine the low-energy electron cutoff with 7\% uncertainty at the
level and hence permits an accurate quantitative study of the
importance of accelerated electrons in solar flare energetics.Comment: Accepted for publication in the Astrophysical Journal, 18 pages, 5
figure
An Anisotropic Density Turbulence Model from the Sun to 1 au Derived from Radio Observations
© 2023. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/Solar radio bursts are strongly affected by radio-wave scattering on density inhomogeneities, changing their observed time characteristics, sizes, and positions. The same turbulence causes angular broadening and scintillation of galactic and extragalactic compact radio sources observed through the solar atmosphere. Using large-scale simulations of radio-wave transport, the characteristics of anisotropic density turbulence from 0.1 R ⊙ to 1 au are explored. For the first time, a profile of heliospheric density fluctuations is deduced that accounts for the properties of extrasolar radio sources, solar radio bursts, and in situ density fluctuation measurements in the solar wind at 1 au. The radial profile of the spectrum-weighted mean wavenumber of density fluctuations (a quantity proportional to the scattering rate of radio waves) is found to have a broad maximum at around (4–7) R ⊙, where the slow solar wind becomes supersonic. The level of density fluctuations at the inner scale (which is consistent with the proton resonance scale) decreases with heliocentric distance as 〈δni2〉(r)≃2×107r/R⊙−1−3.7 cm−6. Due to scattering, the apparent positions of solar burst sources observed at frequencies between 0.1 and 300 MHz are computed to be essentially cospatial and to have comparable sizes, for both fundamental and harmonic emission. Anisotropic scattering is found to account for the shortest solar radio burst decay times observed, and the required wavenumber anisotropy is q ∥/q ⊥ = 0.25–0.4, depending on whether fundamental or harmonic emission is involved. The deduced radio-wave scattering rate paves the way to quantify intrinsic solar radio burst characteristics.Peer reviewe
An Anisotropic Density Turbulence Model from the Sun to 1 au Derived From Radio Observations
Solar radio bursts are strongly affected by radio-wave scattering on density
inhomogeneities, changing their observed time characteristics, sizes, and
positions. The same turbulence causes angular broadening and scintillation of
galactic and extra-galactic compact radio sources observed through the solar
atmosphere. Using large-scale simulations of radio-wave transport, the
characteristics of anisotropic density turbulence from to
au are explored. For the first time, a profile of heliospheric density
fluctuations is deduced that accounts for the properties of extra-solar radio
sources, solar radio bursts, and in-situ density fluctuation measurements in
the solar wind at au. The radial profile of the spectrum-weighted mean
wavenumber of density fluctuations (a quantity proportional to the scattering
rate of radio-waves) is found to have a broad maximum at around , where the slow solar wind becomes supersonic. The level of density
fluctuations at the inner scale (which is consistent with the proton resonance
scale) decreases with heliocentric distance as cm. Due to scattering,
the apparent positions of solar burst sources observed at frequencies between
and MHz are computed to be essentially cospatial and to have
comparable sizes, for both fundamental and harmonic emission. Anisotropic
scattering is found to account for the shortest solar radio burst decay times
observed, and the required wavenumber anisotropy is , depending on whether fundamental or harmonic emission is involved.
The deduced radio-wave scattering rate paves the way to quantify intrinsic
solar radio burst characteristics.Comment: 27 pages, 12 figure
Retrospective Analysis of the 2014-2015 Ebola Epidemic in Liberia.
The 2014-2015 Ebola epidemic has been the most protracted and devastating in the history of the disease. To prevent future outbreaks on this scale, it is imperative to understand the reasons that led to eventual disease control. Here, we evaluated the shifts of Ebola dynamics at national and local scales during the epidemic in Liberia. We used a transmission model calibrated to epidemiological data between June 9 and December 31, 2014, to estimate the extent of community and hospital transmission. We found that despite varied local epidemic patterns, community transmission was reduced by 40-80% in all the counties analyzed. Our model suggests that the tapering of the epidemic was achieved through reductions in community transmission, rather than accumulation of immune individuals through asymptomatic infection and unreported cases. Although the times at which this transmission reduction occurred in the majority of the Liberian counties started before any large expansion in hospital capacity and the distribution of home protection kits, it remains difficult to associate the presence of interventions with reductions in Ebola incidence
Source positions of an interplanetary type III radio burst and anisotropic radio-wave scattering
Interplanetary solar radio type III bursts provide the means to remotely study and track energetic electrons propagating in the interplanetary medium. Due to the lack of direct radio source imaging, several methods have been developed to determine the source positions from space-based observations. Moreover, none of the methods consider the propagation effects of anisotropic radio-wave scattering, which would strongly distort the trajectory of radio waves, delay their arrival times, and affect their apparent characteristics. We investigate the source positions and directivity of an interplanetary type III burst simultaneously observed by Parker Solar Probe, Solar Orbiter, STEREO, and Wind and we compare the results of applying the intensity fit and timing methods with ray-tracing simulations of radio-wave propagation with anisotropic density fluctuations. The simulation calculates the trajectories of the rays, their time profiles at different viewing sites, and the apparent characteristics for various density fluctuation parameters. The results indicate that the observed source positions are displaced away from the locations where emission is produced, and their deduced radial distances are larger than expected from density models. This suggests that the apparent position is affected by anisotropic radio-wave scattering, which leads to an apparent position at a larger heliocentric distance from the Sun. The methods to determine the source positions may underestimate the apparent positions if they do not consider the path of radio-wave propagation and incomplete scattering at a viewing site close to the intrinsic source position
- …