1,387 research outputs found
Charge-exchange limits on low-energy α-particle fluxes in solar flares
This paper reports on a search for flare emission via charge-exchange radiation in the wings of the Lyα line of He II at 304 Å, as originally suggested for hydrogen by Orrall and Zirker. Via this mechanism a primary α particle that penetrates into the neutral chromosphere can pick up an atomic electron and emit in the He II bound-bound spectrum before it stops. The Extreme-ultraviolet Variability Experiment on board the Solar Dynamics Observatory gives us our first chance to search for this effect systematically. The Orrall-Zirker mechanism has great importance for flare physics because of the essential roles that particle acceleration plays; this mechanism is one of the few proposed that would allow remote sensing of primary accelerated particles below a few MeV nucleon<sup>–1</sup>. We study 10 events in total, including the γ-ray events SOL2010-06-12 (M2.0) and SOL2011-02-24 (M3.5) (the latter a limb flare), seven X-class flares, and one prominent M-class event that produced solar energetic particles. The absence of charge-exchange line wings may point to a need for more complete theoretical work. Some of the events do have broadband signatures, which could correspond to continua from other origins, but these do not have the spectral signatures expected from the Orrall-Zirker mechanism
Flows in the solar atmosphere due to the eruptions on the 15th July, 2002
<p>Which kind of flows are present during flares? Are they compatible with the present understanding of energy release and which model best describes the observations? We analyze successive flare events in order to answer these questions. The flares were observed in the magnetically complex NOAA active region (AR) 10030 on 15 July 2002. One of them is of GOES X-class. The description of these flares and how they relate to the break-out model is presented in Gary & Moore (2004). The Coronal Diagnostic Spectrometer on board SOHO observed this active region for around 14 h. The observed emission lines provided data from the transition region to the corona with a field of view covering more than half of the active region. In this paper we analyse the spatially resolved flows seen in the atmosphere from the preflare to the flare stages. We find evidence for evaporation occurring before the impulsive phase. During the main phase, the ongoing magnetic reconnection is demonstrated by upflows located at the edges of the flare loops (while downflows are found in the flare loops themselves). We also report the impact of a filament eruption on the atmosphere, with flows up to 300 km s<sup>-1</sup> observed at transition-region temperatures in regions well away from the location of the pre-eruptive filament. Our results are consistent with the predictions of the break out model before the impulsive phase of the flare; while, as the flare progresses, the directions of the flows are consistent with flare models invoking evaporation followed by cooling and downward plasma motions in the flare loops.</p>
Observation and Modeling of the Solar Transition Region: II. Solutions of the Quasi-Static Loop Model
In the present work we undertake a study of the quasi-static loop model and
the observational consequences of the various solutions found. We obtain the
most general solutions consistent with certain initial conditions. Great care
is exercised in choosing these conditions to be physically plausible (motivated
by observations). We show that the assumptions of previous quasi-static loop
models, such as the models of Rosner, Tucker and Vaiana (1978) and Veseckey,
Antiochos and Underwood (1979), are not necessarily valid for small loops at
transition region temperatures. We find three general classes of solutions for
the quasi-static loop model, which we denote, radiation dominated loops,
conduction dominated loops and classical loops. These solutions are then
compared with observations. Departures from the classical scaling law of RTV
are found for the solutions obtained. It is shown that loops of the type that
we model here can make a significant contribution to lower transition region
emission via thermal conduction from the upper transition region.Comment: 30 pages, 3 figures, Submitted to ApJ, Microsoft Word File 6.0/9
Extreme Ultra-Violet Spectroscopy of the Lower Solar Atmosphere During Solar Flares
The extreme ultraviolet portion of the solar spectrum contains a wealth of
diagnostic tools for probing the lower solar atmosphere in response to an
injection of energy, particularly during the impulsive phase of solar flares.
These include temperature and density sensitive line ratios, Doppler shifted
emission lines and nonthermal broadening, abundance measurements, differential
emission measure profiles, and continuum temperatures and energetics, among
others. In this paper I shall review some of the advances made in recent years
using these techniques, focusing primarily on studies that have utilized data
from Hinode/EIS and SDO/EVE, while also providing some historical background
and a summary of future spectroscopic instrumentation.Comment: 34 pages, 8 figures. Submitted to Solar Physics as part of the
Topical Issue on Solar and Stellar Flare
Mitochondrial targeting adaptation of the hominoid-specific glutamate dehydrogenase driven by positive Darwinian selection
Many new gene copies emerged by gene duplication in hominoids, but little is known with respect to their functional evolution. Glutamate dehydrogenase (GLUD) is an enzyme central to the glutamate and energy metabolism of the cell. In addition to the single, GLUD-encoding gene present in all mammals (GLUD1), humans and apes acquired a second GLUD gene (GLUD2) through retroduplication of GLUD1, which codes for an enzyme with unique, potentially brain-adapted properties. Here we show that whereas the GLUD1 parental protein localizes to mitochondria and the cytoplasm, GLUD2 is specifically targeted to mitochondria. Using evolutionary analysis and resurrected ancestral protein variants, we demonstrate that the enhanced mitochondrial targeting specificity of GLUD2 is due to a single positively selected glutamic acid-to-lysine substitution, which was fixed in the N-terminal mitochondrial targeting sequence (MTS) of GLUD2 soon after the duplication event in the hominoid ancestor ~18–25 million years ago. This MTS substitution arose in parallel with two crucial adaptive amino acid changes in the enzyme and likely contributed to the functional adaptation of GLUD2 to the glutamate metabolism of the hominoid brain and other tissues. We suggest that rapid, selectively driven subcellular adaptation, as exemplified by GLUD2, represents a common route underlying the emergence of new gene functions
The dependence of the EIT wave velocity on the magnetic field strength
"EIT waves" are a wavelike phenomenon propagating in the corona, which were
initially observed in the extreme ultraviolet (EUV) wavelength by the EUV
Imaging Telescope (EIT). Their nature is still elusive, with the debate between
fast-mode wave model and non-wave model. In order to distinguish between these
models, we investigate the relation between the EIT wave velocity and the local
magnetic field in the corona. It is found that the two parameters show
significant negative correlation in most of the EIT wave fronts, {\it i.e.},
EIT wave propagates more slowly in the regions of stronger magnetic field. Such
a result poses a big challenge to the fast-mode wave model, which would predict
a strong positive correlation between the two parameters. However, it is
demonstrated that such a result can be explained by the fieldline stretching
model, \emph{i.e.,} that "EIT waves" are apparently-propagating brightenings,
which are generated by successive stretching of closed magnetic field lines
pushed by the erupting flux rope during coronal mass ejections (CMEs).Comment: 11 pages, 8 figures, accepted for publication in Solar Phy
The coronal Ne/O abundance of alpha Centauri
Recent improvements in the modeling of solar convection and line formation
led to downward revisions of the solar photospheric abundances of the lighter
elements, which in turn led to changes in the radiative opacity of the solar
interior and hence to conflicts with the solar convection zone depth as
inferred from helioseismic oscillation frequencies. An increase of the solar
Ne/O abundance to values as observed for nearby stars has been proposed as a
solution. Because of the absence of strong neon lines in the optical, neon
abundances are difficult to measure and the correct solar and stellar Ne/O
abundances are currently hotly debated. Based on X-ray spectra obtained with
XMM-Newton, we determine a reference value of Ne/O for the inactive, solar-like
star alpha Cen (primarily alpha Cen B, which is the dominant component in
X-rays), with three independent, line-based methods, using differential
emission measure reconstruction and an emission measure-independent method. Our
results indicate a value of approx. 0.28 for Ne/O in alpha Cen, approximately
twice the value measured for the Sun, but still below the average value
obtained for other stars. The low Ne/O abundance of the Sun is peculiar when
compared to alpha Cen and other stars; our results emphasize the necessity to
obtain more and accurate Ne/O abundance measurements of low activity stars.Comment: accepted by A&
Coronal properties of the EQ Peg binary system
The activity indicators of M dwarfs are distinctly different for early and
late types. The coronae of early M dwarfs display high X-ray luminosities and
temperatures, a pronounced inverse FIP effect, and frequent flaring to the
extent that no quiescent level can be defined in many cases. For late M dwarfs,
fewer but more violent flares have been observed, and the quiescent X-ray
luminosity is much lower. To probe the relationship between coronal properties
with spectral type of active M dwarfs, we analyze the M3.5 and M4.5 components
of the EQ Peg binary system in comparison with other active M dwarfs of
spectral types M0.5 to M5.5. We investigate the timing behavior of both
components of the EQ Peg system, reconstruct their differential emission
measure, and investigate the coronal abundance ratios based on emission-measure
independent line ratios from their Chandra HETGS spectra. Finally we test for
density variations in different states of activity. The X-ray luminosity of EQ
Peg A (M3.5) is by a factor of 6-10 brighter than that of EQ Peg B (M4.5). Like
most other active M dwarfs, the EQ Peg system shows an inverse FIP effect. The
abundances of both components are consistent within the errors; however, there
seems to be a tendency toward the inverse FIP effect being less pronounced in
the less active EQ Peg B when comparing the quiescent state of the two stars.
This trend is supported by our comparison with other M dwarfs. As the X-ray
luminosity decreases with later spectral type, so do coronal temperatures and
flare rate. The amplitude of the observed abundance anomalies, i.e. the inverse
FIP effect, declines; however, clear deviations from solar abundances remain.Comment: 14 pages, accepted by A&
An Intriguing Solar Microflare Observed with RHESSI, Hinode and TRACE
Investigate particle acceleration and heating in a solar microflare. In a
microflare with non-thermal emission to remarkably high energies ( keV),
we investigate the hard X-rays with RHESSI imaging and spectroscopy and the
resulting thermal emission seen in soft X-rays with Hinode/XRT and in EUV with
TRACE. The non-thermal footpoints observed with RHESSI spatially and temporally
match bright footpoint emission in soft X-rays and EUV. There is the
possibility that the non-thermal spectrum extends down to 4 keV. The hard X-ray
burst clearly does not follow the expected Neupert effect, with the time
integrated hard X-rays not matching the soft X-ray time profile. So although
this is a simple microflare with good X-ray observation coverage it does not
fit the standard flare model.Comment: 4 pages, 5 figures, accepted by A&
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