283 research outputs found
Macroscopic effects in attosecond pulse generation
We examine how the generation and propagation of high-order harmonics in a
partly ionized gas medium affect their strength and synchronization. The
temporal properties of the resulting attosecond pulses generated in long gas
targets can be significantly influenced by macroscopic effects, in particular
by the intensity in the medium and the degree of ionization. Under some
conditions, the use of gas targets longer than the absorption length can lead
to the generation of self-compressed attosecond pulses. We show this effect
experimentally, using long argon-filled gas cells as generating medium.Comment: 5 pages 4 figure
On the Lebesgue measure of Li-Yorke pairs for interval maps
We investigate the prevalence of Li-Yorke pairs for and
multimodal maps with non-flat critical points. We show that every
measurable scrambled set has zero Lebesgue measure and that all strongly
wandering sets have zero Lebesgue measure, as does the set of pairs of
asymptotic (but not asymptotically periodic) points.
If is topologically mixing and has no Cantor attractor, then typical
(w.r.t. two-dimensional Lebesgue measure) pairs are Li-Yorke; if additionally
admits an absolutely continuous invariant probability measure (acip), then
typical pairs have a dense orbit for . These results make use of
so-called nice neighborhoods of the critical set of general multimodal maps,
and hence uniformly expanding Markov induced maps, the existence of either is
proved in this paper as well.
For the setting where has a Cantor attractor, we present a trichotomy
explaining when the set of Li-Yorke pairs and distal pairs have positive
two-dimensional Lebesgue measure.Comment: 41 pages, 3 figure
Evidence of Impulsive Heating in Active Region Core Loops
Using a full spectral scan of an active region from the Extreme-Ultraviolet
Imaging Spectrometer (EIS) we have obtained Emission Measure EM
distributions in two different moss regions within the same active region. We
have compared these with theoretical transition region EMs derived for three
limiting cases, namely \textit{static equilibrium}, \textit{strong
condensation} and \textit{strong evaporation} from \cite{ebtel}. The EM
distributions in both the moss regions are strikingly similar and show a
monotonically increasing trend from . Using
photospheric abundances we obtain a consistent EM distribution for all ions.
Comparing the observed and theoretical EM distributions, we find that the
observed EM distribution is best explained by the \textit{strong condensation}
case (EM), suggesting that a downward enthalpy flux plays an important
and possibly dominant role in powering the transition region moss emission. The
downflows could be due to unresolved coronal plasma that is cooling and
draining after having been impulsively heated. This supports the idea that the
hot loops (with temperatures of 3{-}5 MK) seen in the core of active regions
are heated by nanoflares.Comment: 17 pages, 4 figures, Accepted for publication in The Astrophysical
Journa
Measurement and control of the frequency chirp rate of high-order harmonic pulses
The chirp rate of harmonics 13 to 23 in argon was measured by cross correlation with a 12 femtosecond probe pulse. The negative chirp was directly measured under low ionization conditions due to the atomic dipole phase. It was showed that the additional chirp on the pump pulse was transferred to the qth harmonic as q times the fundamental chirp. The attosecond pulse trains synthesized from these harmonics had a pulse spacing that increased linearly with time
Heating of the Solar Chromosphere and Corona by Alfven Wave Turbulence
A three-dimensional MHD model for the propagation and dissipation of Alfven
waves in a coronal loop is developed. The model includes the lower atmospheres
at the two ends of the loop. The waves originate on small spatial scales (less
than 100 km) inside the kilogauss flux elements in the photosphere. The model
describes the nonlinear interactions between Alfven waves using the reduced MHD
approximation. The increase of Alfven speed with height in the chromosphere and
transition region (TR) causes strong wave reflection, which leads to
counter-propagating waves and turbulence in the photospheric and chromospheric
parts of the flux tube. Part of the wave energy is transmitted through the TR
and produces turbulence in the corona. We find that the hot coronal loops
typically found in active regions can be explained in terms of Alfven wave
turbulence, provided the small-scale footpoint motions have velocities of 1-2
km/s and time scales of 60-200 s. The heating rate per unit volume in the
chromosphere is 2 to 3 orders of magnitude larger than that in the corona. We
construct a series of models with different values of the model parameters, and
find that the coronal heating rate increases with coronal field strength and
decreases with loop length. We conclude that coronal loops and the underlying
chromosphere may both be heated by Alfvenic turbulence.Comment: 28 pages (emulateapj style, 13 figures, 3 tables), ApJ, in pres
On Solving the Coronal Heating Problem
This article assesses the current state of understanding of coronal heating,
outlines the key elements of a comprehensive strategy for solving the problem,
and warns of obstacles that must be overcome along the way.Comment: Accepted by Solar Physics; Published by Solar Physic
Physics of Solar Prominences: II - Magnetic Structure and Dynamics
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations and
models, (2) the dynamics of prominence plasmas (formation and flows), (3)
Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and
large-scale patterns of the filament channels in which prominences are located.
Finally, several outstanding issues in prominence research are discussed, along
with observations and models required to resolve them.Comment: 75 pages, 31 pictures, review pape
The state of the Martian climate
60°N was +2.0°C, relative to the 1981–2010 average value (Fig. 5.1). This marks a new high for the record. The average annual surface air temperature (SAT) anomaly for 2016 for land stations north of starting in 1900, and is a significant increase over the previous highest value of +1.2°C, which was observed in 2007, 2011, and 2015. Average global annual temperatures also showed record values in 2015 and 2016. Currently, the Arctic is warming at more than twice the rate of lower latitudes
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