12 research outputs found
On Collisionless Electron-Ion Temperature Equilibration in the Fast Solar Wind
We explore a mechanism, entirely new to the fast solar wind, of electron
heating by lower hybrid waves to explain the shift to higher charge states
observed in various elements in the fast wind at 1 A.U. relative to the
original coronal hole plasma. This process is a variation on that previously
discussed for two temperature accretion flows by Begelman & Chiueh. Lower
hybrid waves are generated by gyrating minor ions (mainly alpha-particles) and
become significant once strong ion cyclotron heating sets in beyond 1.5 R_sun.
In this way the model avoids conflict with SUMER electron temperature
diagnostic measurements between 1 and 1.5 R_sun. The principal requirement for
such a process to work is the existence of density gradients in the fast solar
wind, with scale length of similar order to the proton inertial length. Similar
size structures have previously been inferred by other authors from radio
scintillation observations and considerations of ion cyclotron wave generation
by global resonant MHD waves.Comment: 32 pages including 11 figures, 4 tables, accepted by Ap
Accelerated Electrons in Cassiopeia A: An Explanation for the Hard X-ray Tail
We propose a model for the hard X-ray (> 10 keV) emission observed from the
supernova remnant Cas A. Lower hybrid waves are generated in strong (mG)
magnetic fields, generally believed to reside in this remnant, by shocks
reflected from density inhomogeneities. These then accelerate electrons to
energies of several tens of keV. Around 4% of the x-ray emitting plasma
electrons need to be in this accelerated distribution, which extends up to
electron velocities of order the electron Alfven speed, and is directled along
magnetic field lines. Bremsstrahlung from these electrons produces the observed
hard x-ray emission. Such waves and accelerated electrons have been observed in
situ at Comet Halley, and we discuss the viability of the extrapolation from
this case to the parameters relevant to Cas A.Comment: 20 pages, 3 figures, aasTeX502, accepted in Ap
Accelerated Electrons in Cassiopeia A: Thermal and Electromagnetic Effects
We consider in more detail a model previously proposed for the hard X-ray
(>10 keV) emission observed from the supernova remnant Cas A, whereby electrons
are accelerated by lower-hybrid waves and radiate bremsstrahlung. We consider
both cold and thermal plasma limits of the modified two-stream instability that
generates the lower-hybrid waves, and by studying time dependent ionization
balance for various components of the Cas A ejecta and shocked circumstellar
medium, find locations within the shell where one or other of the instabilities
may occur. Either instability can be effective, with the cold plasma
instability imposing fewer constraints on the shocked reflected ion population
responsible for exciting the waves. The instability must be located in the
ejecta shocked at the earliest times and therefore closest to the contact
discontinuity where magnetic fields are expected to be the strongest. The
energy deposited in this ejecta by collisions between accelerated and ambient
electrons is broadly consistent with that required to reheat this ejecta to
observed temperatures.Comment: 29 pages + 8 figures, accepted for publication by Ap
Wavelet bicoherence analysis of strong plasma turbulence at the Earth’s quasiparallel bow shock
International audienc
Late Cardiovascular Syphilis
A case of belatedly diagnosed late cardiovascular syphilis that had continued progressing despite two courses of ceftriaxone therapy is described. The patient had a combination of all possible complications of syphilitic mesaortitis including aortic regurgitation, coronary insufficiency, as well as fusiform aneurysm of the ascending aorta and the proximal aortic arch. Current issues of diagnosis and therapy of late cardiovascular syphilis are discussed