117 research outputs found
A new population of terrestrial gamma-ray flashes in the RHESSI data
Terrestrial gamma-ray flashes (TGFs) are the most energetic photon phenomenon
occurring naturally on Earth. An outstanding question is as follows: Are these
flashes just a rare exotic phenomenon or are they an intrinsic part of
lightning discharges and therefore occurring more frequently than previously
thought? All measurements of TGFs so far have been limited by the dynamic range
and sensitivity of spaceborne instruments. In this paper we show that there is
a new population of weak TGFs that has not been identified by search
algorithms. We use the World Wide Lightning Location Network (WWLLN) to
identify lightning that occurred in 2006 and 2012 within the 800 km field of
view of Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). By
superposing 740,210 100 ms RHESSI data intervals, centered at the time of the
WWLLN detected lightning, we identify at least 141 and probably as many as 191
weak TGFs that were not part of the second RHESSI data catalogue. This supports
the suggestion that the global TGF production rate is larger than previously
reported
Meter-scale spark X-ray spectrumstatistics
X-ray emission by sparks implies bremsstrahlung from a population of
energetic electrons, but the details of this process remain a mystery. We
present detailed statistical analysis of X-ray spectra detected by multiple
detectors during sparks produced by 1 MV negative high-voltage pulses with 1
s risetime. With over 900 shots, we statistically analyze the signals,
assuming that the distribution of spark X-ray fluence behaves as a power law
and that the energy spectrum of X-rays detectable after traversing 2 m of
air and a thin aluminum shield is exponential. We then determine the parameters
of those distributions by fitting cumulative distribution functions to the
observations. The fit results match the observations very well if the mean of
the exponential X-ray energy distribution is 86 7 keV and the spark X-ray
fluence power law distribution has index -1.29 0.04 and spans at least 3
orders of magnitude in fluence
The Hall current system revealed as a statistical significant pattern during fast flows
We have examined the dawn-dusk component of the magnetic field, <I>B<sub>Y</sub></I>, in the night side current sheet during fast flows in the neutral sheet. 237 h of Cluster data from the plasma sheet between 2 August 2002 and 2 October 2002 have been analysed. The spatial pattern of <I>B<sub>Y</sub></I> as a function of the distance from the centre of the current sheet has been estimated by using a Harris current sheet model. We have used the average slopes of these patterns to estimate earthward and tailward currents. For earthward fast flows there is a tailward current in the inner central plasma sheet and an earthward current in the outer central plasma sheet on average. For tailward fast flows the currents are oppositely directed. These observations are interpreted as signatures of Hall currents in the reconnection region or as field aligned currents which are connected with these currents. Although fast flows often are associated with a dawn-dusk current wedge, we believe that we have managed to filter out such currents from our statistical patterns
Dynamic effects of restoring footpoint symmetry on closed magnetic field lines
Here we present an event where simultaneous global imaging of the aurora from both hemispheres reveals a large longitudinal shift of the nightside aurora of about 3 h, being the largest relative shift reported on from conjugate auroral imaging. This is interpreted as evidence of closed field lines having very asymmetric footpoints associated with the persistent positive y component of the interplanetary magnetic field before and during the event. At the same time, the Super Dual Auroral Radar Network observes the ionospheric nightside convection throat region in both hemispheres. The radar data indicate faster convection toward the dayside in the dusk cell in the Southern Hemisphere compared to its conjugate region. We interpret this as a signature of a process acting to restore symmetry of the displaced closed magnetic field lines resulting in flux tubes moving faster along the banana cell than the conjugate orange cell. The event is analyzed with emphasis on Birkeland currents (BC) associated with this restoring process, as recently described by Tenfjord et al. (2015). Using data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) during the same conditions as the presented event, the large-scale BC pattern associated with the event is presented. It shows the expected influence of the process of restoring symmetry on BCs. We therefore suggest that these observations should be recognized as being a result of the dynamic effects of restoring footpoint symmetry on closed field lines in the nightside
How the IMF Induces a Local Component During Northward IMF and Characteristic Timescales
We use the Lyon-Fedder-Mobarry global magnetohydrodynamics model to study the
effects of the interplanetary magnetic field (IMF) component
on the coupling between the solar wind and magnetosphere-ionosphere system when
IMF 0. We describe the evolution of how a magnetospheric
component is induced on closed field lines during these
conditions. Starting from dayside lobe reconnection, the magnetic tension on
newly reconnected field lines redistribute the open flux asymmetrically between
the two hemispheres. This results in asymmetric magnetic energy density in the
lobes. Shear flows are induced to restore equilibrium, and these flows are what
effectively induces a local component. We show the radial
dependence of the induced and compare the results to the
induced during southward IMF conditions. We also show the
response and reconfiguration time of the inner magnetosphere to IMF
reversals during northward IMF . A superposed
epoch analysis of magnetic field measurements from seven Geostationary
Operational Environmental Satellite spacecraft at different local times both
for negative-to-positive and positive-to-negative IMF
reversals is presented. We find that the induced responds
within 16 min of the arrival of IMF at the bow shock, and it
completely reconfigures within 47 min
Competition of ferromagnetic and antiferromagnetic spin ordering in nuclear matter
In the framework of a Fermi liquid theory it is considered the possibility of
ferromagnetic and antiferromagnetic phase transitions in symmetric nuclear
matter with Skyrme effective interaction. The zero temperature dependence of
ferromagnetic and antiferromagnetic spin polarization parameters as functions
of density is found for SkM, SGII effective forces. It is shown that in the
density domain, where both type of solutions of self--consistent equations
exist, ferromagnetic spin state is more preferable than antiferromagnetic one.Comment: 9p., 3 figure
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