385 research outputs found
Response of a marine-terminating Greenland outlet glacier to abrupt cooling 8200 and 9300 years ago
Long-term records of Greenland outlet-glacier change extending beyond the satellite era can inform future predictions of Greenland Ice Sheet behavior. Of particular relevance is elucidating the Greenland Ice Sheet's response to decadal- and centennial-scale climate change. Here, we reconstruct the early Holocene history of Jakobshavn IsbrĂŠ, Greenland's largest outlet glacier, using 10Be surface exposure ages and 14C-dated lake sediments. Our chronology of ice-margin change demonstrates that Jakobshavn IsbrĂŠ advanced to deposit moraines in response to abrupt cooling recorded in central Greenland ice cores ca. 8,200 and 9,300 years ago. While the rapid, dynamically aided retreat of many Greenland outlet glaciers in response to warming is well documented, these results indicate that marine-terminating outlet glaciers are also able to respond quickly to cooling. We suggest that short lag times of high ice flux margins enable a greater magnitude response of marine-terminating outlets to abrupt climate change compared to their land-terminating counterparts
Earliest Holocene south Greenland ice sheet retreat within its late Holocene extent
Early Holocene summer warmth drove dramatic Greenland ice sheet (GIS) retreat. Subsequent insolation-driven cooling caused GIS margin readvance to late Holocene maxima, from which ice margins are now retreating. We use 10Be surface exposure ages from four locations between 69.4°N and 61.2°N to date when in the early Holocene south to west GIS margins retreated to within these late Holocene maximum extents. We find that this occurred at 11.1â±â0.2 ka to 10.6â±â0.5 ka in south Greenland, significantly earlier than previous estimates, and 6.8â±â0.1 ka to 7.9â±â0.1 ka in southwest to west Greenland, consistent with existing 10Be ages. At least in south Greenland, these 10Be ages likely provide a minimum constraint for when on a multicentury timescale summer temperatures after the last deglaciation warmed above late Holocene temperatures in the early Holocene. Current south Greenland ice margin retreat suggests that south Greenland may have now warmed to or above earliest Holocene summer temperatures
Index
The interest in relativistic beam-plasma instabilities has been greatly rejuvenated over the past two decades by novel concepts in laboratory and space plasmas. Recent advances in this long-standing field are here reviewed from both theoretical and numerical points of view. The primary focus is on the two-dimensional spectrum of unstable electromagnetic waves growing within relativistic, unmagnetized, and uniform electron beam-plasma systems. Although the goal is to provide a unified picture of all instability classes at play, emphasis is put on the potentially dominant waves propagating obliquely to the beam direction, which have received little attention over the years. First, the basic derivation of the general dielectric function of a kinetic relativistic plasma is recalled. Next, an overview of two-dimensional unstable spectra associated with various beam-plasma distribution functions is given. Both cold-fluid and kinetic linear theory results are reported, the latter being based on waterbag and MaxwellâJĂŒttner model distributions. The main properties of the competing modes (developing parallel, transverse, and oblique to the beam) are given, and their respective region of dominance in the system parameter space is explained. Later sections address particle-in-cell numerical simulations and the nonlinear evolution of multidimensional beam-plasma systems. The elementary structures generated by the various instability classes are first discussed in the case of reduced-geometry systems. Validation of linear theory is then illustrated in detail for large-scale systems, as is the multistaged character of the nonlinear phase. Finally, a collection of closely related beam-plasma problems involving additional physical effects is presented, and worthwhile directions of future research are outlined.Original Publication: Antoine Bret, Laurent Gremillet and Mark Eric Dieckmann, Multidimensional electron beam-plasma instabilities in the relativistic regime, 2010, Physics of Plasmas, (17), 12, 120501-1-120501-36. http://dx.doi.org/10.1063/1.3514586 Copyright: American Institute of Physics http://www.aip.org/</p
On the nonthermal X-ray emission in blazar jets
We consider particle acceleration to high energy via diffusive shock
acceleration in a simple, self-consistent shock in jet model for blazars.
Electrons are assumed to be accelerated at a shock front in relativistic jets
and radiate synchrotron emission in a post-shock region. The full time, space
and momentum dependence of the electron distribution function is used for a
calculation of the nonthermal synchrotron spectra. We discuss the evolution of
the spectral index by varying the rate at which particles enter the
acceleration process. The results indicate that the synchrotron spectral index
displays a characteristic looplike behaviour with intensity (as has been
observed in several blazars), where the orientation of the loop depends on
whether the acceleration time scale is comparable to the synchrotron cooling
time scale or not. We show that our model provides a good fit to the observed
evolution of the spectral index of Mkn 421 during a flare in 1994.Comment: 6 pages, 3 figures, conference proceedin
Plasma Wakefield Acceleration for Ultrahigh Energy Cosmic Rays
A cosmic acceleration mechanism is introduced which is based on the
wakefields excited by the Alfven shocks in a relativistically flowing plasma,
where the energy gain per distance of a test particle is Lorentz invariant. We
show that there exists a threshold condition for transparency below which the
accelerating particle is collision-free and suffers little energy loss in the
plasma medium. The stochastic encounters of the random
accelerating-decelerating phases results in a power-law energy spectrum: f(e)
1/e^2. The environment suitable for such plasma wakefield acceleration can be
cosmically abundant. As an example, we discuss the possible production of
super-GZK ultra high energy cosmic rays (UHECR) through this mechanism in the
atmosphere of gamma ray bursts. We show that the acceleration gradient can be
as high as G ~ 10^16 eV/cm. The estimated event rate in our model agrees with
that from UHECR observations.Comment: 11 pages, 1 figure, submitted to Phys. Rev. Let
High Energy Cosmic Rays From Supernovae
Cosmic rays are charged relativistic particles that reach the Earth with
extremely high energies, providing striking evidence of the existence of
effective accelerators in the Universe. Below an energy around
eV cosmic rays are believed to be produced in the Milky Way while above that
energy their origin is probably extragalactic. In the early '30s supernovae
were already identified as possible sources for the Galactic component of
cosmic rays. After the '70s this idea has gained more and more credibility
thanks to the the development of the diffusive shock acceleration theory, which
provides a robust theoretical framework for particle energization in
astrophysical environments. Afterwards, mostly in recent years, much
observational evidence has been gathered in support of this framework,
converting a speculative idea in a real paradigm. In this Chapter the basic
pillars of this paradigm will be illustrated. This includes the acceleration
mechanism, the non linear effects produced by accelerated particles onto the
shock dynamics needed to reach the highest energies, the escape process from
the sources and the transportation of cosmic rays through the Galaxy. The
theoretical picture will be corroborated by discussing several observations
which support the idea that supernova remnants are effective cosmic ray
factories.Comment: Final draft of a chapter in "Handbook of Supernovae" edited by Athem
W. Alsabti and Paul Murdi
A priori estimates for 3D incompressible current-vortex sheets
We consider the free boundary problem for current-vortex sheets in ideal
incompressible magneto-hydrodynamics. It is known that current-vortex sheets
may be at most weakly (neutrally) stable due to the existence of surface waves
solutions to the linearized equations. The existence of such waves may yield a
loss of derivatives in the energy estimate of the solution with respect to the
source terms. However, under a suitable stability condition satisfied at each
point of the initial discontinuity and a flatness condition on the initial
front, we prove an a priori estimate in Sobolev spaces for smooth solutions
with no loss of derivatives. The result of this paper gives some hope for
proving the local existence of smooth current-vortex sheets without resorting
to a Nash-Moser iteration. Such result would be a rigorous confirmation of the
stabilizing effect of the magnetic field on Kelvin-Helmholtz instabilities,
which is well known in astrophysics
Primary proton spectrum between 200 TeV and 1000 TeV observed with the Tibet burst detector and air shower array
Since 1996, a hybrid experiment consisting of the emulsion chamber and burst
detector array and the Tibet-II air-shower array has been operated at
Yangbajing (4300 m above sea level, 606 g/cm^2) in Tibet. This experiment can
detect air-shower cores, called as burst events, accompanied by air showers in
excess of about 100 TeV. We observed about 4300 burst events accompanied by air
showers during 690 days of operation and selected 820 proton-induced events
with its primary energy above 200 TeV using a neural network method. Using this
data set, we obtained the energy spectrum of primary protons in the energy
range from 200 to 1000 TeV. The differential energy spectrum obtained in this
energy region can be fitted by a power law with the index of -2.97 0.06,
which is steeper than that obtained by direct measurements at lower energies.
We also obtained the energy spectrum of helium nuclei at particle energies
around 1000 TeV.Comment: 25 pages, 22 figures, Accepted for publication in Phys. Rev.
Understanding hadronic gamma-ray emission from supernova remnants
We aim to test the plausibility of a theoretical framework in which the
gamma-ray emission detected from supernova remnants may be of hadronic origin,
i.e., due to the decay of neutral pions produced in nuclear collisions
involving relativistic nuclei. In particular, we investigate the effects
induced by magnetic field amplification on the expected particle spectra,
outlining a phenomenological scenario consistent with both the underlying
Physics and the larger and larger amount of observational data provided by the
present generation of gamma experiments, which seem to indicate rather steep
spectra for the accelerated particles. In addition, in order to study to study
how pre-supernova winds might affect the expected emission in this class of
sources, the time-dependent gamma-ray luminosity of a remnant with a massive
progenitor is worked out. Solid points and limitations of the proposed scenario
are finally discussed in a critical way.Comment: 30 pages, 5 figures; Several comments, references and a figure added.
Some typos correcte
Challenges and research priorities to understand interactions between climate, ice sheets and global mean sea level during past interglacials
Quaternary interglacials provide key observations of the Earth system's responses to orbital and greenhouse gas forcing. They also inform on the capabilities of Earth system models, used for projecting the polar ice-sheet and sea-level responses to a regional warmth comparable to that expected by 2100 C.E. However, a number of uncertainties remain regarding the processes and feedbacks linking climate, ice-sheet and sea-level changes during past warm intervals. Here, we delineate the major research questions that need to be resolved and future research directions that should be taken by the paleoclimate, sea-level and ice-sheet research communities in order to increase confidence in the use of past interglacial climate, ice-sheet and sea-level reconstructions to constrain future predictions. These questions were formulated during a joint workshop held by the PAGES-INQUA PALSEA (PALeo constraints on SEA level rise) and the PAGES-PMIP QUIGS (QUaternary InterGlacialS) Working Groups in September 2018.PAGE
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