1,659 research outputs found
The mean ionic charges of N, Ne, MG, SI and S in solar energetic particle events
The mean ionic charges of nitrogen, neon, magnesium, silicon, and sulfur in solar flare particle events were determined for 12 flares during the time interval from September 1978 to September 1979. The observations were carried out with the MPI/UoMd ULEZEQ Sensor on the ISEE-3 satellite comparing the results with mean charge states established in a hot coronal plasma under equilibrium conditions, different temperatures for different elements are discussed. These range from approx. 2 million K to 7 million K in a single flare. From flare to flare the variation in temperature for each element is less than the variation between different ion species
MeV magnetosheath ions energized at the bow shock
A causal relationship between midlatitude magnetosheath energetic ions and bow shock magnetic geometry was previously established for ion energy up to 200 keV e−1 for the May 4, 1998, storm event. This study demonstrates that magnetosheath ions with energies above 200 keV up to 1 MeV simply extend the ion spectrum to form a power law tail. Results of cross-correlation analysis suggest that these ions also come directly from the quasi-parallel bow shock, not the magnetosphere. This is confirmed by a comparison of energetic ion fluxes simultaneously measured in the magnetosheath and at the quasi-parallel bow shock when both regions are likely connected by the magnetic field lines. We suggest that ions are accelerated at the quasi-parallel bow shock to energies as high as 1 MeV and subsequently transported into the magnetosheath during this event
Analytic solution for nonlinear shock acceleration in the Bohm limit
The selfconsistent steady state solution for a strong shock, significantly
modified by accelerated particles is obtained on the level of a kinetic
description, assuming Bohm-type diffusion. The original problem that is
commonly formulated in terms of the diffusion-convection equation for the
distribution function of energetic particles, coupled with the thermal plasma
through the momentum flux continuity equation, is reduced to a nonlinear
integral equation in one variable. Its solution provides selfconsistently both
the particle spectrum and the structure of the hydrodynamic flow. A critical
system parameter governing the acceleration process is found to be , where , with a suitably
normalized injection rate , the Mach number M >> 1, and the cut-off
momentum . We particularly focus on an efficient solution, in which
almost all the energy of the flow is converted into a few energetic particles.
It was found that (i) for this efficient solution (or, equivalently, for
multiple solutions) to exist, the parameter
must exceed a critical value ( is the injection
momentum), (ii) the total shock compression ratio r increases with M and
saturates at a level that scales as $ r \propto \Lambda_1 (iii) the downstream
power-law spectrum has the universal index q=3.5 over a broad momentum range.
(iv) completely smooth shock transitions do not appear in the steady state
kinetic description.Comment: 39 pages, 3 PostScript figures, uses aasms4.sty, to appear in Aug.
20, 1997 issue ApJ, vol. 48
Acceleration of Solar Wind Ions by Nearby Interplanetary Shocks: Comparison of Monte Carlo Simulations with Ulysses Observations
The most stringent test of theoretical models of the first-order Fermi
mechanism at collisionless astrophysical shocks is a comparison of the
theoretical predictions with observational data on particle populations. Such
comparisons have yielded good agreement between observations at the
quasi-parallel portion of the Earth's bow shock and three theoretical
approaches, including Monte Carlo kinetic simulations. This paper extends such
model testing to the realm of oblique interplanetary shocks: here observations
of proton and alpha particle distributions made by the SWICS ion mass
spectrometer on Ulysses at nearby interplanetary shocks are compared with test
particle Monte Carlo simulation predictions of accelerated populations. The
plasma parameters used in the simulation are obtained from measurements of
solar wind particles and the magnetic field upstream of individual shocks. Good
agreement between downstream spectral measurements and the simulation
predictions are obtained for two shocks by allowing the the ratio of the
mean-free scattering length to the ionic gyroradius, to vary in an optimization
of the fit to the data. Generally small values of this ratio are obtained,
corresponding to the case of strong scattering. The acceleration process
appears to be roughly independent of the mass or charge of the species.Comment: 26 pages, 6 figures, AASTeX format, to appear in the Astrophysical
Journal, February 20, 199
Electron Injection at High Mach Number Quasi-Perpendicular Shocks : Surfing and Drift Acceleration
Electron injection process at high Mach number collisionless
quasi-perpendicular shock waves is investigated by means of one-dimensional
electromagnetic particle-in-cell simulations. We find that energetic electrons
are generated through the following two steps: (1) electrons are accelerated
nearly perpendicular to the local magnetic field by shock surfing acceleration
at the leading edge of the shock transition region. (2) the preaccelerated
electrons are further accelerated by shock drift acceleration. As a result,
energetic electrons are preferentially reflected back to the upstream. Shock
surfing acceleration provides sufficient energy required for the reflection.
Therefore, it is important not only for the energization process by itself, but
also for triggering the secondary acceleration process. We also present a
theoretical model of the two-step acceleration mechanism based on the
simulation results, which can predict the injection efficiency for subsequent
diffusive shock acceleration process. We show that the injection efficiency
obtained by the present model agrees well with the value obtained by Chandra
X-ray observations of SN 1006. At typical supernova remnant shocks, energetic
electrons injected by the present mechanism can self-generate upstream Alfven
waves, which scatter the energetic electrons themselves.Comment: 35 pages, 9 figures, accepted by Ap
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