17 research outputs found
Heliospheric Observations of Energetic Particles
Heliospheric observations of energetic particles have shown that, on long time averages, a consistent v^-5 power-law index arises even in the absence of transient events. This implies an ubiquitous acceleration process present in the solar wind that is required to generate these power-law tails and maintain them against adiabatic losses and coulomb-collisions which will cool and thermalize the plasma respectively. Though the details of this acceleration process are being debated within the community, most agree that the energy required for these tails comes from fluctuations in the magnetic field which are damped as the energy is transferred to particles. Given this source for the tail, is it then reasonable to assume that the turbulent LISM should give rise to such a power-law tail as well? IBEX observations clearly show a power-law tail of index approximately -5 in energetic neutral atoms. The simplest explanation for the origins of these ENAs are that they are energetic ions which have charge-exchanged with a neutral atom. However, this would imply that energetic ions possess a v^-5 power-law distribution at keV energies at the source of these ENAs. If the source is presumed to be the LISM, it provides additional options for explaining the, so called, IBEX ribbon. This presentation will discuss some of these options as well as potential mechanisms for the generation of a power-law spectrum in the LISM
Blazar Gamma-Rays, Shock Acceleration, and the Extragalactic Background Light
The observed spectra of blazars, their intrinsic emission, and the underlying
populations of radiating particles are intimately related. The use of these
sources as probes of the extragalactic infrared background, a prospect
propelled by recent advances in TeV-band telescopes, soon to be augmented by
observations by NASA's upcoming Gamma-Ray Large Area Space Telescope (GLAST),
has been a topic of great recent interest. Here, it is demonstrated that if
particles in blazar jets are accelerated at relativistic shocks, then gamma-ray
spectra with indices less than 1.5 can be produced. This, in turn, loosens the
upper limits on the near infrared extragalactic background radiation previously
proposed. We also show evidence hinting that TeV blazars with flatter spectra
have higher intrinsic TeV gamma-ray luminosities and we indicate that there may
be a correlation of flatness and luminosity with redshift.Comment: Version to appear in ApJ Letters, Vol. 667, 20 Sept. 200
Particle Acceleration at Relativistic Shocks in Extragalactic Systems
Diffusive shock acceleration (DSA) at relativistic shocks is expected to be
an important acceleration mechanism in a variety of astrophysical objects
including extragalactic jets in active galactic nuclei and gamma ray bursts.
These sources remain strong and interesting candidate sites for the generation
of ultra-high energy cosmic rays. In this paper, key predictions of DSA at
relativistic shocks that are salient to the issue of cosmic ray ion and
electron production are outlined. Results from a Monte Carlo simulation of such
diffusive acceleration in test-particle, relativistic, oblique, MHD shocks are
presented. Simulation output is described for both large angle and small angle
scattering scenarios, and a variety of shock obliquities including superluminal
regimes when the de Hoffman-Teller frame does not exist. The distribution
function power-law indices compare favorably with results from other
techniques. They are found to depend sensitively on the mean magnetic field
orientation in the shock, and the nature of MHD turbulence that propagates
along fields in shock environs. An interesting regime of flat spectrum
generation is addressed, providing evidence for its origin being due to shock
drift acceleration. The impact of these theoretical results on gamma-ray burst
and blazar science is outlined. Specifically, Fermi gamma-ray observations of
these cosmic sources are already providing significant constraints on important
environmental quantities for relativistic shocks, namely the frequency of
scattering and the level of field turbulence.Comment: 11 pages, 6 figures, to appear in Proc. of the 8th International
Astrophysics Conference "Shock Waves in Space and Astrophysical Environments"
(2010), eds. X. Ao, R. Burrows and G. P. Zank (AIP Conf. Proc., New York
SETH Technology Demonstration of Small Satellite Deep Space Optical Communications to aid Heliophysics Science and Space Weather Forecasting
Diversified, and high data rate communications are critical for the growing number of current and future small satellites providing the next generation of high-resolution science observations. Science Enabling Technologies for Heliophysics (SETH) is a small satellite mission concept1 that will utilize Fibertek’s low cost, Compact Laser Communication Terminal (CLCT) to demonstrate high rate optical communications from deep space. This cutting-edge technology will support the Helio Energetic Neutral Atom (HELENA) heliophysics instrument that demonstrates solar energetic neutral atom (ENA) and space weather observation capabilities, in alignment with NASA’s Moon to Mars exploration initiative. SETH will demonstrate data rates of at least 10 Mbps from 0.1 AU. The CLCT includes a telescope, Pointing, Acquisition and Tracking sensor, vibration isolation mounts, and a fine steering mirror, all fitting in a 2U commercially available stack. SETH will prove that deep space optical communications are now available also for small satellite missions. The mission utilizes public-private partnerships and multi-center NASA collaboration. Compatibility between ground and space segments is established by adopting the emerging Consultative Committee for Space Data Systems (CCSDS) High Photon Efficiency (HPE) standard
Electron Micro Bursts as a Mechanism of Electron Loss Via Wave-Particle Interactions
Electron microbursts are rapid fluctuations of electron fluxes occurring on time scales of milliseconds. They are thought be due to scattering into the loss cone by plasma waves of various types from chorus to the recently observed large amplitude whistlers. They may be a major process of loss of realtivistic electrons from the Earth's outer radiation belts. One of the key issues that new mission s such as RBSP will address is to understand the loss of relativistic electrons. The SAMPEX mission launched in 1992 and still collecting data has the HILT sensor onboard with the capability of measuring> 1 MeV electrons with a high time resolution of 20 milliseconds suited admirably for the study of microbursts. We will use the data collected by the HILT for over a decade to characterize the relationship between electron microbursts and macroscopic electron decay lifetimes. With the launch of RBSP it is expected that SAMPEX will continue to collect data and overlap with RBSP. The latter will provide valuable information regarding plasma waves which coupled with low altitude measurements of microbursts may help elucidate details of the physics of electron loss from the radiation belt
Particle Acceleration at Interplanetary Shocks
The acceleration of interstellar pick-up ions as well as solar wind species
has been observed at a multitude of interplanetary (IP) shocks by different
spacecraft. This paper expands upon previous work modeling the phase space
distributions of accelerated ions associated with the shock event encountered
on day 292 of 1991 by the Ulysses mission at 4.5 AU. A kinetic Monte Carlo
simulation is employed here to model the diffusive acceleration process. This
exposition presents recent developments pertaining to the incorporation into
the simulation of the diffusive characteristics incurred by field line
wandering (FLW), according to the work of Giacalone and Jokipii. For a pure
field-line wandering construct, it is determined that the upstream spatial ramp
scales are too short to accommodate the HI-SCALE flux increases for 200 keV
protons, and that the distribution function for H+ somewhat underpopulates the
combined SWICS/HI-SCALE spectra at the shock. This contrasts our earlier
theory/data comparison where it was demonstrated that diffusive transport in
highly turbulent fields according to kinetic theory can successfully account
for both the proton distributions and upstream ramp scales, using a single
turbulence parameter. The principal conclusion here is that, in a FLW scenario,
the transport of ions across the mean magnetic field is slightly less efficient
than is required to effectively trap energetic ions within a few Larmor radii
of the shock layer and thereby precipitate efficient acceleration. This
highlights the contrast between ion transport in highly turbulent shock
environs and remote, less-disturbed interplanetary regions.Comment: 6 pages, 1 embedded figure, to appear in Proc. of the 7th IGPP
International Astrophysics Conference "Particle Acceleration and Transport in
the Heliosphere and Beyond" (2008), eds. G. Li, et al. (AIP Conf. Proc., New
York
Electrostatic Potentials in Supernova Remnant Shocks
Recent advances in the understanding of the properties of supernova remnant
shocks have been precipitated by the Chandra and XMM X-ray Observatories, and
the HESS Atmospheric Cerenkov Telescope in the TeV band. A critical problem for
this field is the understanding of the relative degree of dissipative
heating/energization of electrons and ions in the shock layer. This impacts the
interpretation of X-ray observations, and moreover influences the efficiency of
injection into the acceleration process, which in turn feeds back into the
thermal shock layer energetics and dynamics. This paper outlines the first
stages of our exploration of the role of charge separation potentials in
non-relativistic electron-ion shocks where the inertial gyro-scales are widely
disparate, using results from a Monte Carlo simulation. Charge density spatial
profiles were obtained in the linear regime, sampling the inertial scales for
both ions and electrons, for different magnetic field obliquities. These were
readily integrated to acquire electric field profiles in the absence of
self-consistent, spatial readjustments between the electrons and the ions. It
was found that while diffusion plays little role in modulating the linear field
structure in highly oblique and perpendicular shocks, in quasi-parallel shocks,
where charge separations induced by gyrations are small, and shock-layer
electric fields are predominantly generated on diffusive scales.Comment: 7 pages, 2 embedded figures, Accepted for publication in Astrophysics
and Space Science, as part of the HEDLA 2006 conference proceeding
Diffusive Acceleration of Particles at Oblique, Relativistic, Magnetohydrodynamic Shocks
Diffusive shock acceleration (DSA) at relativistic shocks is expected to be
an important acceleration mechanism in a variety of astrophysical objects
including extragalactic jets in active galactic nuclei and gamma ray bursts.
These sources remain good candidate sites for the generation of ultra-high
energy cosmic rays. In this paper, key predictions of DSA at relativistic
shocks that are germane to production of relativistic electrons and ions are
outlined. The technique employed to identify these characteristics is a Monte
Carlo simulation of such diffusive acceleration in test-particle, relativistic,
oblique, magnetohydrodynamic (MHD) shocks. Using a compact prescription for
diffusion of charges in MHD turbulence, this approach generates particle
angular and momentum distributions at any position upstream or downstream of
the shock. Simulation output is presented for both small angle and large angle
scattering scenarios, and a variety of shock obliquities including superluminal
regimes when the de Hoffmann-Teller frame does not exist. The distribution
function power-law indices compare favorably with results from other
techniques. They are found to depend sensitively on the mean magnetic field
orientation in the shock, and the nature of MHD turbulence that propagates
along fields in shock environs. An interesting regime of flat spectrum
generation is addressed; we provide evidence for it being due to shock drift
acceleration, a phenomenon well-known in heliospheric shock studies. The impact
of these theoretical results on blazar science is outlined. Specifically,
Fermi-LAT gamma-ray observations of these relativistic jet sources are
providing significant constraints on important environmental quantities for
relativistic shocks, namely the field obliquity, the frequency of scattering
and the level of field turbulence.Comment: 25 pages, 12 figures, accepted for publication in The Astrophysical
Journa
Modeling Accelerated Pick-up Ion Distributions at an Interplanetary Shock
The acceleration of interstellar pick-up ions as well as solar wind species
has been observed at a multitude of interplanetary (IP) shocks by different
spacecraft. The efficiency of injection of the pick-up ion component differs
from that of the solar wind, and is expected to be strongly enhanced at highly
oblique and quasi-perpendicular shock events, in accord with inferences from
{\it in situ} observations. This paper explores theoretical modeling of the
phase space distributions of accelerated ions obtained by the Ulysses mission
for the Day 292, 1991 shock associated with a corotating interaction region,
encountered before Ulysses' fly-by of Jupiter. A Monte Carlo simulation is used
to model the acceleration process, adapting a technique that has been
successfully tested on earlier IP shocks possessing minimal pick-up ion
presence. Phase space distributions from the simulation technique for various
low mass ions are compared with SWICS and HI-SCALE data to deduce values of a
``turbulence parameter'' that controls the efficiency of injection, and the
degree of cross-field diffusion. Acceptable fits are obtained for the and
populations using standard prescriptions for the pick-up ion
distribution; spectral data was only fit well for scenarios very
close to the Bohm diffusion limit. It is also found that the simulation
successfully accounts for the observation of energetic protons farther upstream
of the forward shock than lower energy pick-up protons, using the same
turbulence parameter that is required to achieve reasonable spectral fits.Comment: 15 pages, 2 embedded figures, Advances Space Research, in pres