214 research outputs found
High energy particles accelerated during the large solar flare of 1990 May 24: X/γ-ray observations
The PHEBUS experiment aboard GRANAT observed γ-ray line emission and γ-ray continuum above 10 MeV from the 24 May, 1990 solar flare. Observations and interpretation of the high-energy continuum have been discussed previously. Here we re-examine these, combining the PHEBUS observations above 10 MeV with calculations of the pion decay continuum to quantitatively constrain the accelerated ion energy distribution at energies above 300 MeV. The uncertainty in the determination of the level of the primary electron bremsstrahlung as well as the lack of measurements on the γ-ray emission above 100 MeV combine to allow rather a wide range of energy distribution parameters (in terms of the number of protons above 30 MeV, the spectral index of the proton distribution and the high energy cut-off of the energetic protons). Nevertheless we are able to rule out some combinations of these parameters. Using the additional information provided by the γ-ray line observations we discuss whether it is possible to construct a consistent picture of the ions which are accelerated in a wide energy range during this flare. Our findings are discussed with respect to previous works on the spectrum of energetic protons in the 10 MeV to GeV energy range
Non-thermal recombination - a neglected source of flare hard X-rays and fast electron diagnostic
Context. Flare Hard X-Rays (HXRs) from non-thermal electrons are commonly
treated as solely bremsstrahlung (f-f), recombination (f-b) being neglected.
This assumption is shown to be substantially in error, especially in hot
sources, mainly due to recombination onto Fe ions.
Aims. We analyse the effects on HXR spectra and electron diagnostics by
including non-thermal recombination onto heavy elements in our model.
Methods. Using Kramers hydrogenic cross sections with effective Z, we
calculate f-f and f-b spectra for power-law electron spectra, in both thin and
thick target limits, and for Maxwellians, with summation over all important
ions.
Results. We find that non-thermal electron recombination, especially onto Fe,
must, in general, be included together with f-f, for reliable spectral
interpretation, when the HXR source is hot. f-b contribution is greatest when
the electron spectral index is large, and any low energy cut-off small. f-b
spectra recombination edges mean a cut-off in F(E) appears as a HXR feature at
Photon energy = Ec + Vz, offering an Ec diagnostic. Including f-b lowers,
greatly in some cases, the F(E) needed for prescribed HXR fluxes and, even when
small, seriously distorts F(E) as inferred by inversion or forward fitting
based on f-f alone.
Conclusions. f-b recombination from non-thermal electrons can be an important
contributor to HXR spectra and should be included in spectral analyses,
especially for hot sources. Accurate results will require use of better cross
sections than ours and consideration of source ionisation structure.Comment: 13 pages, 2 tables, 9 figures, Accepted for publication in A&
Problems and Progress in Flare Fast Particle Diagnostics
Recent progress in the diagnosis of flare fast particles is critically
discussed with the main emphasis on high resolution Hard X-Ray (HXR) data from
RHESSI and coordinated data from other instruments. Spectacular new photon data
findings are highlighted as are advances in theoretical aspects of their use as
fast particle diagnostics, and some important comparisons made with
interplanetary particle data. More specifically the following topics are
addressed
(a) RHESSI data on HXR (electron) versus gamma-ray line (ion) source
locations.
(b) RHESSI hard X-ray source spatial structure in relation to theoretical
models and loop density structure.
(c) Energy budget of flare electrons and the Neupert effect.
(d) Spectral deconvolution methods including blind target testing and results
for RHESSI HXR spectra, including the reality and implications of dips inferred
in electron spectra
(e) The relation between flare in-situ and interplanetary particle data.Comment: 15 pages, 13 figures, submitted to Advances in Space Researc
Inverse Compton X-rays from relativistic flare electrons and positrons
<p><b>Context:</b> In solar flares, inverse Compton scattering (ICS) of photospheric photons might give rise to detectable hard X-ray photon fluxes from the corona where ambient densities are too low for significant bremsstrahlung or recombination. γ-ray lines and continuum in some large flares imply the presence of the necessary ~100 MeV electrons and positrons, the latter as by-products of GeV energy ions. Recent observations of coronal hard X-ray sources in particular prompt us to reconsider here the possible contribution of ICS.</p>
<p><b>Aims:</b> We aim to evaluate the ICS X-ray fluxes to be expected from prescribed populations of relativistic electrons and positrons in the solar corona. The ultimate aim is to determine if ICS coronal X-ray sources might offer a new diagnostic window on relativistic electrons and ions in flares.</p>
<p><b>Methods:</b> We use the complete formalism of ICS to calculate X-ray fluxes from possible populations of flare primary electrons and secondary positrons, paying attention to the incident photon angular distribution near the solar surface and thus improving on the assumption of isotropy made in previous solar discussions.</p>
<p><b>Results:</b> Both primary electrons and secondary positrons produce very hard ICS X-ray spectra. The anisotropic primary radiation field results in pronounced centre-to-limb variation in predicted fluxes and spectra, with the most intense spectra, extending to the highest photon energies, expected from limb flares. Acceptable numbers of electrons or positrons could account for RHESSI coronal X/γ-ray sources.</p>
<p><b>Conclusions:</b> Some coronal X-ray sources at least might be interpreted in terms of ICS by relativistic electrons or positrons, particularly when sources appear at such low ambient densities that bremsstrahlung appears implausible.</p>
Thermalisation of self-interacting solar flare fast electrons
Most theoretical descriptions of the production of solar flare bremsstrahlung
radiation assume the collision of dilute accelerated particles with a cold,
dense target plasma, neglecting interactions of the fast particles with each
other. This is inadequate for situations where collisions with this background
plasma are not completely dominant, as may be the case in, for example,
low-density coronal sources. We aim to formulate a model of a self-interacting,
entirely fast electron population in the absence of a dense background plasma,
to investigate its implications for observed bremsstrahlung spectra and the
flare energy budget. We derive approximate expressions for the time-dependent
distribution function of the fast electrons using a Fokker-Planck approach. We
use these expressions to generate synthetic bremsstrahlung X-ray spectra as
would be seen from a corresponding coronal source. We find that our model
qualitatively reproduces the observed behaviour of some flares. As the flare
progresses, the model's initial power-law spectrum is joined by a lower energy,
thermal component. The power-law component diminishes, and the growing thermal
component proceeds to dominate the total emission over timescales consistent
with flare observations. The power-law exhibits progressive spectral hardening,
as is seen in some flare coronal sources. We also find that our model requires
a factor of 7 - 10 fewer accelerated electrons than the cold, thick target
model to generate an equivalent hard X-ray flux. This model forms the basis of
a treatment of self-interactions among flare fast electrons, a process which
affords a more efficient means to produce bremsstrahlung photons and so may
reduce the efficiency requirements placed on the particle acceleration
mechanism. It also provides a useful description of the thermalisation of fast
electrons in coronal sources.Comment: 9 pages, 7 figures, accepted for Astronomy & Astrophysics; this
version clarifies arguments around Eqs. (11) and (20
Divergence between the high rate of p53 mutations in skin carcinomas and the low prevalence of anti-p53 antibodies
Circulating anti-p53 antibodies have been described and used as tumoural markers in patients with various cancers and strongly correlate with the p53 mutated status of the tumours. No study has yet looked at the prevalence of such antibodies in skin carcinoma patients although these tumours have been shown to be frequently p53 mutated. Most skin carcinoma can be diagnosed by examination or biopsy, but aggressive, recurrent and/or non-surgical cases' follow up would be helped by a biological marker of residual disease. We performed a prospective study looking at the prevalence of anti-p53 antibodies using an ELISA technique in a series of 105 skin carcinoma patients in comparison with a sex- and age-matched control skin carcinoma-free group (n = 130). Additionally, p53 accumulation was studied by immunohistochemistry to confirm p53 protein altered expression in a sample of tumours. Anti-p53 antibodies were detected in 2.9% of the cases, with a higher prevalence in patients suffering from the more aggressive squamous cell type (SCC) of skin carcinoma (8%) than for the more common and slowly growing basal cell carcinoma type or BCC (1.5%). p53 protein stabilization could be confirmed in 80% of tumours studied by IHC. This low level of anti-p53 antibody detection contrasts with the high rate of p53 mutations reported in these tumours. This observation shows that the anti-p53 humoral response is a complex and tissue-specific mechanism. © 2001 Cancer Research Campaign http://www.bjcancer.co
Impact of the 26-30 May 2003 solar events on the earth ionosphere and thermosphere.
During the last week of May 2003, the solar active region AR 10365 produced a large number of flares, several of which were accompanied by Coronal Mass Ejections (CME). Specifically on 27 and 28 May three halo CMEs were observed which had a significant impact on geospace. On 29 May, upon their arrival at the L1 point, in front of the Earth's magnetosphere, two interplanetary shocks and two additional solar wind pressure pulses were recorded by the ACE spacecraft. The interplanetary magnetic field data showed the clear signature of a magnetic cloud passing ACE. In the wake of the successive increases in solar wind pressure, the magnetosphere became strongly compressed and the sub-solar magnetopause moved inside five Earth radii. At low altitudes the increased energy input to the magnetosphere was responsible for a substantial enhancement of Region-1 field-aligned currents. The ionospheric Hall currents also intensified and the entire high-latitude current system moved equatorward by about 10°. Several substorms occurred during this period, some of them - but not all - apparently triggered by the solar wind pressure pulses. The storm's most notable consequences on geospace, including space weather effects, were (1) the expansion of the auroral oval, and aurorae seen at mid latitudes, (2) the significant modification of the total electron content in the sunlight high-latitude ionosphere, (3) the perturbation of radio-wave propagation manifested by HF blackouts and increased GPS signal scintillation, and (4) the heating of the thermosphere, causing increased satellite drag. We discuss the reasons why the May 2003 storm is less intense than the October-November 2003 storms, although several indicators reach similar intensities
Submillimeter and X-ray observations of an X Class flare
The GOES X1.5 class flare that occurred on August 30,2002 at 1327:30 UT is
one of the few events detected so far at submillimeter wavelengths. We present
a detailed analysis of this flare combining radio observations from 1.5 to 212
GHz (an upper limit of the flux is also provided at 405 GHz) and X-ray.
Although the observations of radio emission up to 212 GHz indicates that
relativistic electrons with energies of a few MeV were accelerated, no
significant hard X-ray emission was detected by RHESSI above ~ 250 keV. Images
at 12--20 and 50--100 keV reveal a very compact, but resolved, source of about
~ 10" x 10". EUV TRACE images show a multi-kernel structure suggesting a
complex (multipolar) magnetic topology. During the peak time the radio spectrum
shows an extended flatness from ~ 7 to 35 GHz. Modeling the optically thin part
of the radio spectrum as gyrosynchrotron emission we obtained the electron
spectrum (spectral index delta, instantaneous number of emitting electrons). It
is shown that in order to keep the expected X-ray emission from the same
emitting electrons below the RHESSI background at 250 keV, a magnetic field
above 500 G is necessary. On the other hand, the electron spectrum deduced from
radio observations >= 50 GHz is harder than that deduced from ~ 70 - 250 keV
X-ray data, meaning that there must exist a breaking energy around a few
hundred keV. During the decay of the impulsive phase, a hardening of the X-ray
spectrum is observed which is interpreted as a hardening of the electron
distribution spectrum produced by the diffusion due to Coulomb collisions of
the trapped electrons in a medium with an electron density of n_e ~ 3E10 - 5E10
cm-3.Comment: Accpeted in Astronomy & Astrophysics. 9 Pages, 6 Figures ADDED
REFERENCE
Observations of Shock Propagation through Turbulent Plasma in the Solar Corona
Eruptive activity in the solar corona can often lead to the propagation of shock waves. In the radio domain the primary signature of such shocks are type II radio bursts, observed in dynamic spectra as bands of emission slowly drifting toward lower frequencies over time. These radio bursts can sometimes have an inhomogeneous and fragmented fine structure, but the cause of this fine structure is currently unclear. Here we observe a type II radio burst on 2019 March 20th using the New Extension in Nançay Upgrading LOFAR, a radio interferometer observing between 10–85 MHz. We show that the distribution of size scales of density perturbations associated with the type II fine structure follows a power law with a spectral index in the range of α = −1.7 to −2.0, which closely matches the value of −5/3 expected of fully developed turbulence. We determine this turbulence to be upstream of the shock, in background coronal plasma at a heliocentric distance of ∼2 R⊙. The observed inertial size scales of the turbulent density inhomogeneities range from ∼62 Mm to ∼209 km. This shows that type II fine structure and fragmentation can be due to shock propagation through an inhomogeneous and turbulent coronal plasma, and we discuss the implications of this on electron acceleration in the coronal shock
Spatially resolved observations of a split-band coronal type-II radio burst
Context. The origin of coronal type-II radio bursts and of their
band-splitting are still not fully understood. Aims. To make progress in
solving this problem on the basis of one extremely well observed solar eruptive
event. Methods. The relative dynamics of multi-thermal eruptive plasmas,
observed in detail by the SDO/AIA and of the harmonic type-II burst sources,
observed by the NRH at ten frequencies from 445 to 151 MHz, is studied for the
partially behind the limb event on 3 November 2010. Special attention is given
to the band-splitting of the burst. Analysis is supplemented by investigation
of coronal hard X-ray (HXR) sources observed by the RHESSI. Results. It is
found that the flare impulsive phase was accompanied by the formation of a
double coronal HXR source, whose upper part coincided with the hot (T~10 MK)
eruptive plasma blob. The leading edge (LE) of the eruptive plasmas (T~1-2 MK)
moved upward from the flare region with the speed of v=900-1400 km/s. The type
II burst source initially appeared just above the LE apex and moved with the
same speed and in the same direction. After about 20 s it started to move about
twice faster, but still in the same direction. At any given moment the low
frequency component (LFC) source of the splitted type-II burst was situated
above the high frequency component (HFC) source, which in turn was situated
above the LE. It is also found that at a given frequency the HFC source was
located slightly closer to the photosphere than the LFC source. Conclusions.
The shock wave, which could be responsible for the observed type-II radio
burst, was initially driven by the multi-temperature eruptive plasmas, but
later transformed to a freely propagating blast shock wave. The most preferable
interpretation of the type-II burst splitting is that its LFC was emitted from
the upstream region of the shock, whereas the HFC - from the downstream region.Comment: 14 pages, 10 figure
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