860 research outputs found
The magnetic connectivity of coronal shocks from behind-the-limb flares to the visible solar surface during -ray events
Context. The observation of >100 MeV {\gamma}-rays in the minutes to hours
following solar flares suggests that high-energy particles interacting in the
solar atmosphere can be stored and/or accelerated for long time periods. The
occasions when {\gamma}-rays are detected even when the solar eruptions
occurred beyond the solar limb as viewed from Earth provide favorable viewing
conditions for studying the role of coronal shocks driven by coronal mass
ejections (CMEs) in the acceleration of these particles.
Aims: In this paper, we investigate the spatial and temporal evolution of the
coronal shocks inferred from stereoscopic observations of behind-the-limb
flares to determine if they could be the source of the particles producing the
{\gamma}-rays.
Methods: We analyzed the CMEs and early formation of coronal shocks
associated with {\gamma}-ray events measured by the Fermi-Large Area Telescope
(LAT) from three eruptions behind the solar limb as viewed from Earth on 2013
Oct. 11, 2014 Jan. 06 and Sep. 01. We used a 3D triangulation technique, based
on remote-sensing observations to model the expansion of the CME shocks from
above the solar surface to the upper corona. Coupling the expansion model to
various models of the coronal magnetic field allowed us to derive the
time-dependent distribution of shock Mach numbers and the magnetic connection
of particles produced by the shock to the solar surface visible from Earth.
Results: The reconstructed shock fronts for the three events became
magnetically connected to the visible solar surface after the start of the
flare and just before the onset of the >100 MeV {\gamma}-ray emission. The
shock surface at these connections also exhibited supercritical Mach numbers
required for significant particle energization.
[...] (Abridged)Comment: 20 pages, 15 figures, version published in A&
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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 Ă
, 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>â1</sup>. We study 10 events in total, including the Îł-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
Global Energetics of Thirty-Eight Large Solar Eruptive Events
We have evaluated the energetics of 38 solar eruptive events observed by a
variety of spacecraft instruments between February 2002 and December 2006, as
accurately as the observations allow. The measured energetic components
include: (1) the radiated energy in the GOES 1 - 8 A band; (2) the total energy
radiated from the soft X-ray (SXR) emitting plasma; (3) the peak energy in the
SXR-emitting plasma; (4) the bolometric radiated energy over the full duration
of the event; (5) the energy in flare-accelerated electrons above 20 keV and in
flare-accelerated ions above 1 MeV; (6) the kinetic and potential energies of
the coronal mass ejection (CME); (7) the energy in solar energetic particles
(SEPs) observed in interplanetary space; and (8) the amount of free
(nonpotential) magnetic energy estimated to be available in the pertinent
active region. Major conclusions include: (1) the energy radiated by the
SXR-emitting plasma exceeds, by about half an order of magnitude, the peak
energy content of the thermal plasma that produces this radiation; (2) the
energy content in flare-accelerated electrons and ions is sufficient to supply
the bolometric energy radiated across all wavelengths throughout the event; (3)
the energy contents of flare-accelerated electrons and ions are comparable; (4)
the energy in SEPs is typically a few percent of the CME kinetic energy
(measured in the rest frame of the solar wind); and (5) the available magnetic
energy is sufficient to power the CME, the flare-accelerated particles, and the
hot thermal plasma
Distribution of Positron Annihilation Radiation
The SPI instrument on-board the ESA/INTEGRAL satellite is engaged in a
mission-long study of positron annihilation radiation from the Galaxy. Early
results suggest that the disk component is only weakly detected at 511 keV by
SPI. We review CGRO/OSSE, TGRS and SMM studies of 511 keV line and positronium
continuum emission from the Galaxy in light of the early INTEGRAL/SPI findings.
We find that when similar spatial distributions are compared, combined fits to
the OSSE/SMM/TGRS data-sets produce bulge and disk fluxes similar in total flux
and in B/D ratio to the fits reported for SPI observations. We further find
that the 511 keV line width reported by SPI is similar to the values reported
by TGRS, particularly when spectral fits include both narrow-line and
broad-line components. Collectively, the consistency between these four
instruments suggests that all may be providing an accurate view of positron
annihilation in the Galaxy.Comment: Accepted for publication in conference proceedings of "Astronomy with
Radioactivities 5", 8 page
Operation and performance of the OSSE instrument
The Oriented Scintillation Spectrometer Experiment (OSSE) on the Arthur Holly Compton Gamma Ray Observatory is described. An overview of the operation and control of the instrument is given, together with a discussion of typical observing strategies used with OSSE and basic data types produced by the instrument. Some performance measures for the instrument are presented that were obtained from pre-launch and in-flight data. These include observing statistics, continuum and line sensitivity, and detector effective area and gain stability
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Orthographic facilitation in oral vocabulary acquisition
An experiment investigated whether exposure to orthography facilitates oral vocabulary learning. A total of 58 typically developing children aged 8-9 years were taught 12 nonwords. Children were trained to associate novel phonological forms with pictures of novel objects. Pictures were used as referents to represent novel word meanings. For half of the nonwords children were additionally exposed to orthography, although they were not alerted to its presence, nor were they instructed to use it. After this training phase a nonword-picture matching posttest was used to assess learning of nonword meaning, and a spelling posttest was used to assess learning of nonword orthography. Children showed robust learning for novel spelling patterns after incidental exposure to orthography. Further, we observed stronger learning for nonword-referent pairings trained with orthography. The degree of orthographic facilitation observed in posttests was related to children's reading levels, with more advanced readers showing more benefit from the presence of orthography
INTEGRAL/IBIS search for e-e+ annihilation radiation from the Galactic Center Region
Electron-positron annihilation radiation from the Galactic Center region has
been detected since the seventies, but its astrophysical origin is still a
topic of a scientific debate. We have analyzed data of the gamma-ray imager
IBIS/ISGRI onboard of ESA's INTEGRAL platform in the ee line.
During the first year of the missions Galactic Center Deep Exposure no evidence
for point sources at 511 keV has been found in the ISGRI data; the
upper limit for resolved single point sources is estimated to be .Comment: 6 pages, 3 figures; Cospar 2004. To be published in: Advances in
Space Researc
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