51,444 research outputs found
Testing the Empirical Shock Arrival Model using Quadrature Observations
The empirical shock arrival (ESA) model was developed based on quadrature
data from Helios (in-situ) and P-78 (remote-sensing) to predict the Sun-Earth
travel time of coronal mass ejections (CMEs) [Gopalswamy et al. 2005a]. The ESA
model requires earthward CME speed as input, which is not directly measurable
from coronagraphs along the Sun-Earth line. The Solar Terrestrial Relations
Observatory (STEREO) and the Solar and Heliospheric Observatory (SOHO) were in
quadrature during 2010 - 2012, so the speeds of Earth-directed CMEs were
observed with minimal projection effects. We identified a set of 20 full halo
CMEs in the field of view of SOHO that were also observed in quadrature by
STEREO. We used the earthward speed from STEREO measurements as input to the
ESA model and compared the resulting travel times with the observed ones from
L1 monitors. We find that the model predicts the CME travel time within about
7.3 hours, which is similar to the predictions by the ENLIL model. We also find
that CME-CME and CME-coronal hole interaction can lead to large deviations from
model predictions.Comment: 17 pages, 4 figures, 3 table
A Hierarchical Relationship between the Fluence Spectra and CME Kinematics in Large Solar Energetic Particle Events: A Radio Perspective
We report on further evidence that solar energetic particles are organized by
the kinematic properties of coronal mass ejections (CMEs)[1]. In particular, we
focus on the starting frequency of type II bursts, which is related to the
distance from the Sun where the radio emission starts. We find that the three
groups of solar energetic particle (SEP) events known to have distinct values
of CME initial acceleration, also have distinct average starting frequencies of
the associated type II bursts. SEP events with ground level enhancement (GLE)
have the highest starting frequency (107 MHz), while those associated with
filament eruption (FE) in quiescent regions have the lowest starting frequency
(22 MHz); regular SEP events have intermediate starting frequency (81 MHz).
Taking the onset time of type II bursts as the time of shock formation, we
determine the shock formation heights measured from the Sun center. We find
that the shocks form on average closest to the Sun (1.51 Rs) in GLE events,
farthest from the Sun in FE SEP events (5.38 Rs), and at intermediate distances
in regular SEP events (1.72 Rs). Finally, we present the results of a case
study of a CME with high initial acceleration (~3 km s^-2) and a type II radio
burst with high starting frequency (~200 MHz) but associated with a minor SEP
event. We find that the relation between the fluence spectral index and CME
initial acceleration continues to hold even for this minor SEP event.Comment: 11 pages, 7 figures, 1 table, to appear in Journal of Physics:
Conference Series (JPCS), Proceedings of the 16th Annual International
Astrophysics Conference held in Santa Fe, NM, 201
The Peculiar Behavior of Halo Coronal Mass Ejections in Solar Cycle 24
We report on a remarkable finding that the halo coronal mass ejections (CMEs)
in cycle 24 are more abundant than in cycle 23, although the sunspot number in
cycle 24 has dropped by about 40%. We also find that the distribution of
halo-CME source locations is different in cycle 24: the longitude distribution
of halos is much flatter with the number of halos originating at central
meridian distance >/=60 degrees twice as large as that in cycle 23. On the
other hand, the average speed and the associated soft X-ray flare size are the
same in the two cycles, suggesting that the ambient medium into which the CMEs
are ejected is significantly different. We suggest that both the higher
abundance and larger central meridian longitudes of halo CMEs can be explained
as a consequence of the diminished total pressure in the heliosphere in cycle
24 (Gopalswamy et al. 2014). The reduced total pressure allows CMEs expand more
than usual making them appear as halos.Comment: 12 pages, 5 figures, accepted for publication in the Astrophysical
Journal Letters, April 7, 201
Magnetic ordering and structural phase transitions in strained ultrathin SrRuO/SrTiO superlattice
Ruthenium-based perovskite systems are attractive because their Structural,
electronic and magnetic properties can be systematically engineered.
SrRuO/SrTiO superlattice, with its period consisting of one unit cell
each, is very sensitive to strain change. Our first-principles simulations
reveal that in the high tensile strain region, it transits from a ferromagnetic
(FM) metal to an antiferromagnetic (AFM) insulator with clear tilted octahedra,
while in the low strain region, it is a ferromagnetic metal without octahedra
tilting. Detailed analyses of three spin-down Ru-t orbitals just below
the Fermi level reveal that the splitting of these orbitals underlies these
dramatic phase transitions, with the rotational force constant of RuO
octahedron high up to 16 meV/Deg, 4 times larger than that of TiO.
Differently from nearly all the previous studies, these transitions can be
probed optically through the diagonal and off-diagonal dielectric tensor
elements. For one percent change in strain, our experimental spin moment change
is -0.140.06 , quantitatively consistent with our theoretical value
of -0.1 .Comment: 3 figures, 1 supplementary material, accepted by Phys. Rev. Let
Ground Level Enhancement in the 2014 January 6 Solar Energetic Particle Event
We present a study of the 2014 January 6 solar energetic particle (SEP)
event, which produced a small ground level enhancement (GLE), making it the
second GLE of this unusual solar cycle 24. This event was primarily observed by
the South Pole neutron monitors (increase of ~2.5%) whereas a few other neutron
monitors recorded smaller increases. The associated coronal mass ejection (CME)
originated behind the western limb and had the speed of 1960 km/s. The height
of the CME at the start of the associated metric type II radio burst, which
indicates the formation of a strong shock, was measured to be 1.61 Rs using a
direct image from STEREO-A/EUVI. The CME height at the time of GLE particle
release (determined using the South Pole neutron monitor data) was directly
measured as 2.96 Rs, from the STEREO-A/COR1 white-light observations. These CME
heights are consistent with those obtained for the GLE71, the only other GLE of
the current cycle as well as cycle-23 GLEs derived using back-extrapolation.
GLE72 is of special interest because it is one of the only two GLEs of cycle
24, one of the two behind-the-limb GLEs and one of the two smallest GLEs of
cycles 23 and 24
Non-conditioned generation of Schroedinger cat states in a cavity
We investigate the dynamics of a two-level atom in a cavity filled with a
nonlinear medium. We show that the atom-field detuning and the
nonlinear parameter may be combined to yield a periodic dynamics
and allowing the generation of almost exact superpositions of coherent states
({\sl Schr\"odinger} cats). By analysing the atomic inversion and the field
purity, we verify that any initial atom-field state is recovered at each
revival time, and that a coherent field interacting with an excited atom
evolves to a superposition of coherent states at each collapse time. We show
that a mixed field state (statistical mixture of two coherent states) evolves
towards a pure field state ({\sl Schr\"odinger} cat) as well. We discuss the
validity of those results by using the field fidelity and the {\sl Wigner}
function.Comment: REVTeX4, 8 pages, 7 figures, link to an external animation fil
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