39 research outputs found
Energy-Dependent Timing of Thermal Emission in Solar Flares
We report solar flare plasma to be multi-thermal in nature based on the
theoretical model and study of the energy-dependent timing of thermal emission
in ten M-class flares. We employ high-resolution X-ray spectra observed by the
Si detector of the "Solar X-ray Spectrometer" (SOXS). The SOXS onboard the
Indian GSAT-2 spacecraft was launched by the GSLV-D2 rocket on 8 May 2003.
Firstly we model the spectral evolution of the X-ray line and continuum
emission flux F(\epsilon) from the flare by integrating a series of isothermal
plasma flux. We find that multi-temperature integrated flux F(\epsilon) is a
power-law function of \epsilon with a spectral index (\gamma) \approx -4.65.
Next, based on spectral-temporal evolution of the flares we find that the
emission in the energy range E= 4 - 15 keV is dominated by temperatures of T=
12 - 50 MK, while the multi-thermal power-law DEM index (\gamma) varies in the
range of -4.4 and -5.7. The temporal evolution of the X-ray flux F(\epsilon,t)
assuming a multi-temperature plasma governed by thermal conduction cooling
reveals that the temperature-dependent cooling time varies between 296 and 4640
s and the electron density (n_e) varies in the range of n_e= (1.77-29.3)*10^10
cm-3. Employing temporal evolution technique in the current study as an
alternative method for separating thermal from non-thermal components in the
energy spectra, we measure the break-energy point ranging between 14 and
21\pm1.0 keV.Comment: Solar Physics, in pres
The Influence of Solar Flares on the Lower Solar Atmosphere: Evidence from the Na D Absorption Line Measured by GOLF/SOHO
Solar flares presumably have an impact on the deepest layers of the solar
atmosphere and yet the observational evidence for such an impact is scarce.
Using ten years of measurements of the Na D and Na D Fraunhofer
lines, measured by GOLF onboard SOHO, we show that this photospheric line is
indeed affected by flares. The effect of individual flares is hidden by solar
oscillations, but a statistical analysis based on conditional averaging reveals
a clear signature. Although GOLF can only probe one single wavelength at a
time, we show that both wings of the Na line can nevertheless be compared. The
varying line asymmetry can be interpreted as an upward plasma motion from the
lower solar atmosphere during the peak of the flare, followed by a downward
motion.Comment: 13 pages, 7 figure
Observations of quasi-periodic solar X-ray emission as a result of MHD oscillations in a system of multiple flare loops
We investigate the solar flare of 20 October 2002. The flare was accompanied
by quasi-periodic pulsations (QPP) of both thermal and nonthermal hard X-ray
emissions (HXR) observed by RHESSI in the 3-50 keV energy range. Analysis of
the HXR time profiles in different energy channels made with the Lomb
periodogram indicates two statistically significant time periods of about 16
and 36 seconds. The 36-second QPP were observed only in the nonthermal HXR
emission in the impulsive phase of the flare. The 16-second QPP were more
pronounced in the thermal HXR emission and were observed both in the impulsive
and in the decay phases of the flare. Imaging analysis of the flare region, the
determined time periods of the QPP and the estimated physical parameters of
magnetic loops in the flare region allow us to interpret the observations as
follows. 1) In the impulsive phase energy was released and electrons were
accelerated by successive acts with the average time period of about 36 seconds
in different parts of two spatially separated, but interacting loop systems of
the flare region. 2) The 36-second periodicity of energy release could be
caused by the action of fast MHD oscillations in the loops connecting these
flaring sites. 3) During the first explosive acts of energy release the MHD
oscillations (most probably the sausage mode) with time period of 16 seconds
were excited in one system of the flare loops. 4) These oscillations were
maintained by the subsequent explosive acts of energy release in the impulsive
phase and were completely damped in the decay phase of the flare.Comment: 14 pages, 4 figure
New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation
(abridged) The heating mechanism at high densities during M dwarf flares is
poorly understood. Spectra of M dwarf flares in the optical and
near-ultraviolet wavelength regimes have revealed three continuum components
during the impulsive phase: 1) an energetically dominant blackbody component
with a color temperature of T 10,000 K in the blue-optical, 2) a smaller
amount of Balmer continuum emission in the near-ultraviolet at lambda 3646
Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer
lines. These properties are not reproduced by models that employ a typical
"solar-type" flare heating level in nonthermal electrons, and therefore our
understanding of these spectra is limited to a phenomenological interpretation.
We present a new 1D radiative-hydrodynamic model of an M dwarf flare from
precipitating nonthermal electrons with a large energy flux of erg
cm s. The simulation produces bright continuum emission from a
dense, hot chromospheric condensation. For the first time, the observed color
temperature and Balmer jump ratio are produced self-consistently in a
radiative-hydrodynamic flare model. We find that a T 10,000 K
blackbody-like continuum component and a small Balmer jump ratio result from
optically thick Balmer and Paschen recombination radiation, and thus the
properties of the flux spectrum are caused by blue light escaping over a larger
physical depth range compared to red and near-ultraviolet light. To model the
near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer
lines, we include the extra Balmer continuum opacity from Landau-Zener
transitions that result from merged, high order energy levels of hydrogen in a
dense, partially ionized atmosphere. This reveals a new diagnostic of ambient
charge density in the densest regions of the atmosphere that are heated during
dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar
Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015):
updated to include comments by Guest Editor. The final publication is
available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-
Muscle fatigue resistance during stimulated contractions is reduced in young male smokers.
Aim: To determine whether muscle function is compromised in healthy smokers in comparison with activity-matched non-smokers. Methods: Nine male smokers (aged 22.2 ± 2.5 years: mean ± SD) with a smoking history of 2.5 ± 3.1 pack years, and ten male control participants (25.4 ± 2.9 years) matched for physical activity level participated in this study. Knee extensor strength was measured using isometric maximal voluntary contractions. Voluntary activation of the quadriceps and co-activation of the biceps femoris were determined using interpolated twitches and surface electromyography respectively. The frequency-torque relationship and fatigue resistance were assessed with electrically evoked contractions. A fatigue index was determined as the ratio of final torque to initial torque during a series of isometric contractions (2 min; 30 Hz; 1 s contraction/1 s rest). Quadriceps anatomical cross sectional area was measured with MRI at 50% of femur length. Results: Maximal voluntary contraction torque, quadriceps anatomical cross sectional area, knee extensor torque/quadriceps cross sectional area, activation, co-activation and force-frequency relationship were similar, whereas the fatigue index was 17% lower in smokers than non-smokers. Conclusion: In young men smoking does not significantly affect quadriceps muscle mass and contractile properties, but does reduce fatigue resistance of the quadriceps muscle, which was not attributable to differences in physical activity. © 2007 The Authors
Imaging Observations of Quasi-Periodic Pulsatory Non-Thermal Emission in Ribbon Solar Flares
Using RHESSI and some auxiliary observations we examine possible connections
between spatial and temporal morphology of the sources of non-thermal hard
X-ray (HXR) emission which revealed minute quasi-periodic pulsations (QPPs)
during the two-ribbon flares on 2003 May 29 and 2005 January 19. Microwave
emission also reveals the same quasi-periodicity. The sources of non-thermal
HXR emission are situated mainly inside the footpoints of the flare arcade
loops observed by the TRACE and SOHO instruments. At least one of the sources
moves systematically both during the QPP-phase and after it in each flare that
allows to examine the sources velocities and the energy release rate via the
process of magnetic reconnection. The sources move predominantly parallel to
the magnetic inversion line or the appropriate flare ribbon during the
QPP-phase whereas the movement slightly changes to more perpendicular regime
after the QPPs. Each QPP is emitted from its own position. It is also seen that
the velocity and the energy release rate don't correlate well with the flux of
the HXR emission calculated from the sources. The sources of microwaves and
thermal HXRs are situated near the apex of the loop arcade and are not
stationary either. Almost all QPPs and some spikes of HXR emission during the
post-QPP-phase reveal the soft-hard-soft spectral behavior indicating separate
acts of electrons acceleration and injection, rather than modulation of
emission flux by some kinds of magnetohydrodynamic (MHD) oscillations of
coronal loops. In all likelihood, the flare scenarios based on the successively
firing arcade loops are more preferable to interpret the observations, although
we can not conclude exactly what mechanism forces these loops to flare up.Comment: 22 pages, 10 figure
Fine structure in the α decay of 223U
Fine structure in the α decay of 223U was observed in the fusion-evaporation reaction 187Re(40Ar, p3n) by using fast digital pulse processing technique. Two α-decay branches of 223U feeding the ground state and 244 keV excited state of 219Th were identified by establishing the decay chain 223U →α1 219Th →α2 215Ra →α3 211Rn. The α-particle energy for the ground-state to ground-state transition of 223U was determined to be 8993(17) keV, 213 keV higher than the previous value, the half-life was updated to be 62−10+14 μs. Evolution of nuclear structure for N = 131 even-Z isotones from Po to U was discussed in the frameworks of nuclear mass and reduced α-decay width, a weakening octupole deformation in the ground state of 223U relative to its lighter isotones 219Ra and 221Th was suggested
On the Frequency Distributions of Solar Soft X-ray Flares
By giving three different definitions of peak flux(original peak flux, corrected peak flux, and scaled peak flux), we analyze the frequency distributions of 14057 flares recorded by GOES from 2000 to 2005. We find a significant result that the frequency distribution of the scaled peak flux for all the sample flares can be approximated by a perfect power-law function with an index of 2.18, without any usually deviating from the power-law at the lower fluxes