6,452 research outputs found
Focused acceleration of cosmic-ray particles in non-uniform magnetic fields
The Fokker–Planck equation for cosmic-ray particles in a spatially varying guide magnetic field in a turbulent plasma is analyzed. An expression is derived for the mean rate of change of particle momentum, caused by the effect of adiabatic focusing in a non-uniform guide field. Results of an earlier diffusion-limit analysis are confirmed, and the physical picture is clarified by working directly with the Fokker–Planck equation. A distributed first-order Fermi acceleration mechanism is identified, which can be termed focused acceleration. If the forward and backward-propagating waves have equal polarizations, focused acceleration operates when the net cross helicity of an Alfvenic slab turbulence is either negative in a diverging guide field or positive in a converging guide field. It is suggested that focused acceleration can contribute to the formation of the anomalous cosmic-ray spectrum at the heliospheric termination shock
A Ulysses Detection of Secondary Helium Neutrals
The Interstellar Boundary EXplorer (IBEX) mission has recently studied the
flow of interstellar neutral He atoms through the solar system, and discovered
the existence of a secondary He flow likely originating in the outer
heliosheath. We find evidence for this secondary component in Ulysses data. By
coadding hundreds of Ulysses He beam maps together to maximize signal-to-noise,
we identify a weak signal that is credibly associated with the secondary
component. Assuming a laminar flow from infinity, we infer the following He
flow parameters: V=12.8+/-1.9 km/s, lambda=74.4+/-1.8 deg, beta=-10.5+/-4.1
deg, and T=3000+/-1100 K; where lambda and beta are the ecliptic longitude and
latitude direction in J2000 coordinates. The secondary component has a density
that is 4.9+/-0.9% that of the primary component. These measurements are
reasonably consistent with measurements from IBEX, with the exception of
temperature, where our temperature is much lower than IBEX's T=9500 K. Even the
higher IBEX temperature is suspiciously low compared to expectactions for the
outer heliosheath source region. The implausibly low temperatures are due to
the incorrect assumption of a laminar flow instead of a diverging one, given
that the flow in the outer heliosheath source region will be deflecting around
the heliopause. As for why the IBEX and Ulysses T values are different,
difficulties with background subtraction in the Ulysses data are a potential
source of concern, but the discrepancy may also be another effect of the
improper laminar flow assumption, which could affect the IBEX and Ulysses
analyses differently.Comment: 9 pages, 4 figures, to appear in The Astrophysical Journa
Revisiting Ulysses Observations of Interstellar Helium
We report the results of a comprehensive reanalysis of Ulysses observations
of interstellar He atoms flowing through the solar system, the goal being to
reassess the interstellar He flow vector and to search for evidence of
variability in this vector. We find no evidence that the He beam seen by
Ulysses changes at all from 1994-2007. The direction of flow changes by no more
than ~0.3 deg and the speed by no more than ~0.3 km/s. A global fit to all
acceptable He beam maps from 1994-2007 yields the following He flow parameters:
V_ISM=26.08+/-0.21 km/s, lambda=75.54+/-0.19 deg, beta=-5.44+/-0.24 deg, and
T=7260+/-270 K; where lambda and beta are the ecliptic longitude and latitude
direction in J2000 coordinates. The flow vector is consistent with the original
analysis of the Ulysses team, but our temperature is significantly higher. The
higher temperature somewhat mitigates a discrepancy that exists in the He flow
parameters measured by Ulysses and the Interstellar Boundary Explorer, but does
not resolve it entirely. Using a novel technique to infer photoionization loss
rates directly from Ulysses data, we estimate a density of n_He=0.0196+/-0.0033
cm^-3 in the interstellar medium.Comment: to appear in The Astrophysical Journa
Hubble Space Telescope Constraints on the Winds and Astrospheres of Red Giant Stars
We report on an ultraviolet spectroscopic survey of red giants observed by
the Hubble Space Telescope, focusing on spectra of the Mg II h & k lines near
2800 A in order to study stellar chromospheric emission, winds, and
astrospheric absorption. We focus on spectral types between K2 III and M5 III,
a spectral type range with stars that are noncoronal, but possessing strong,
chromospheric winds. We find a very tight relation between Mg II surface flux
and photospheric temperature, supporting the notion that all K2-M5 III stars
are emitting at a basal flux level. Wind velocities (V_w) are generally found
to decrease with spectral type, with V_w decreasing from ~40 km/s at K2 III to
~20 km/s at M5 III. We find two new detections of astrospheric absorption, for
Sigma Pup (K5 III) and Gamma Eri (M1 III). This absorption signature had
previously only been detected for Alpha Tau (K5 III). For the three
astrospheric detections the temperature of the wind after the termination shock
correlates with V_w, but is lower than predicted by the Rankine-Hugoniot shock
jump conditions, consistent with the idea that red giant termination shocks are
radiative shocks rather than simple hydrodynamic shocks. A full hydrodynamic
simulation of the Gamma Eri astrosphere is provided to explore this further.Comment: 16 pages, 8 figures, to appear in The Astrophysical Journa
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