740 research outputs found
On the Decades-Long Stability of the Interstellar Wind through the Solar System
We have revisited the series of observations recently used to infer a
temporal variation of the interstellar helium flow over the last forty years.
Concerning the recent IBEX-Lo direct detection of Helium neutrals, there are
two types of precise and unambiguous measurements which do not rely on the
exact response of the instrument: the count rate maxima as a function of the
spin angle, which determines the ecliptic latitude of the flow, and the count
rate maxima as a function of IBEX longitude, which determines a tight
relationship between the ecliptic longitude of the flow and its velocity far
from the Sun. These measurements provide parameters (and couples of parameters
in the second case) remarkably similar to the canonical, old values. In
contrast, the preferential choice of a lower velocity and higher longitude
reported before from IBEX data is based only on the count rate variation (at
each spin phase maximum) as a function of the satellite longitude, when
drifting across the region of high fluxes. We have examined the consequences of
dead time counting effects, and conclude that their inclusion at a realistic
level is sufficient to reconcile the data with the old parameters, calling for
further investigations. We discuss the analyses of the STEREO pickup ion (PUI)
data and argue that the statistical method that has been preferred to infer the
neutral flow longitude (instead of the more direct method based on the PUI
maximum flux directions), is not appropriate. Moreover, transport effects may
have been significant at the very weak solar activity level of 2007-2009, in
which case the longitudes of the PUI maxima are only upper limits on the flow
longitude. Finally, we found that the use of some flow longitude determinations
based on UV glow data are not adequate. At variance with recent conclusions we
find no evidence for a temporal variability of the interstellar helium flow.Comment: 8 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
The solar wind as seen by SOHO/SWAN since 1996: comparison with SOHO/LASCO C2 coronal densities
We update the SOHO/SWAN H Lyman-alpha brightness analysis to cover the
1996-2008 time interval. A forward model applied to the intensity maps provides
the latitude and time dependence of the interstellar Hydrogen ionisation rate
over more than a full solar cycle. The hydrogen ionisation, being almost
entirely due to charge-exchange with solar wind ions, reflects closely the
solar wind flux. Our results show that the solar wind latitudinal structure
during the present solar minimum is strikingly different from the previous
minimum, with a much wider slow solar wind equatorial belt which persists until
at least the end of 2008. We compute absolute values of the in-ecliptic H
ionisation rates using OMNI solar wind data and use them to calibrate our
ionisation rates at all heliographic latitudes. We then compare the resulting
fluxes with the synoptic LASCO/C2 electron densities at 6 solar radii. The two
time-latitude patterns are strikingly similar over all the cycle. This
comparison shows that densities at 6 solar radii can be used to infer the solar
wind type close to its source, with high (resp. low) densities tracing the slow
(resp. fast) solar wind, simply because the density reflects at which altitude
occurs the acceleration. The comparison between the two minima suggests that
the fast polar wind acceleration occurs at larger distance during the current
minimum compared to the previous one. This difference, potentially linked to
the magnetic field decrease or(and) the coronal temperature decrease should be
reproduced by solar wind expansion models.Comment: Proceedings of the SOHO-23 conference, Sept 21-25, 200
Interplanetary Lyman line profiles: variations with solar activity cycle
Interplanetary Lyman alpha line profiles are derived from the SWAN H cell
data measurements. The measurements cover a 6-year period from solar minimum
(1996) to after the solar maximum of 2001. This allows us to study the
variations of the line profiles with solar activity. These line profiles were
used to derive line shifts and line widths in the interplanetary medium for
various angles of the LOS with the interstellar flow direction. The SWAN data
results were then compared to an interplanetary background upwind spectrum
obtained by STIS/HST in March 2001. We find that the LOS upwind velocity
associated with the mean line shift of the IP \lya line varies from 25.7 km/s
to 21.4 km/s from solar minimum to solar maximum. Most of this change is linked
with variations in the radiation pressure. LOS kinetic temperatures derived
from IP line widths do not vary monotonically with the upwind angle of the LOS.
This is not compatible with calculations of IP line profiles based on hot model
distributions of interplanetary hydrogen. We also find that the line profiles
get narrower during solar maximum. The results obtained on the line widths (LOS
temperature) show that the IP line is composed of two components scattered by
two hydrogen populations with different bulk velocities and temperature. This
is a clear signature of the heliospheric interface on the line profiles seen at
1 AU from the sun.Comment: 9 pages, 9 figure
The Nature and Frequency of the Gas Outbursts in Comet 67P/Churyumov-Gerasimenko observed by the Alice Far-ultraviolet Spectrograph on Rosetta
Alice is a far-ultraviolet imaging spectrograph onboard Rosetta that, amongst
multiple objectives, is designed to observe emissions from various atomic and
molecular species from within the coma of comet 67P/Churyumov-Gerasimenko. The
initial observations, made following orbit insertion in August 2014, showed
emissions of atomic hydrogen and oxygen spatially localized close to the
nucleus and attributed to photoelectron impact dissociation of H2O vapor.
Weaker emissions from atomic carbon were subsequently detected and also
attributed to electron impact dissociation, of CO2, the relative H I and C I
line intensities reflecting the variation of CO2 to H2O column abundance along
the line-of-sight through the coma. Beginning in mid-April 2015, Alice
sporadically observed a number of outbursts above the sunward limb
characterized by sudden increases in the atomic emissions, particularly the
semi-forbidden O I 1356 multiplet, over a period of 10-30 minutes, without a
corresponding enhancement in long wavelength solar reflected light
characteristic of dust production. A large increase in the brightness ratio O I
1356/O I 1304 suggests O2 as the principal source of the additional gas. These
outbursts do not correlate with any of the visible images of outbursts taken
with either OSIRIS or the navigation camera. Beginning in June 2015 the nature
of the Alice spectrum changed considerably with CO Fourth Positive band
emission observed continuously, varying with pointing but otherwise fairly
constant in time. However, CO does not appear to be a major driver of any of
the observed outbursts.Comment: 6 pages, 4 figures, accepted for publication in the Astrophysical
Journal Letter
Rosetta-Alice Observations of Exospheric Hydrogen and Oxygen on Mars
The European Space Agency's Rosetta spacecraft, en route to a 2014 encounter
with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on
25 February 2007, closest approach being at 01:54UT. The Alice instrument on
board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for
in situ cometary spectroscopy in the 750-2000 A spectral band, was used to
study the daytime Mars upper atmosphere including emissions from exospheric
hydrogen and oxygen. Offset pointing, obtained five hours before closest
approach, enabled us to detect and map the HI Lyman-alpha and Lyman-beta
emissions from exospheric hydrogen out beyond 30,000 km from the planet's
center. These data are fit with a Chamberlain exospheric model from which we
derive the hydrogen density at the 200 km exobase and the H escape flux. The
results are comparable to those found from the the Ultraviolet Spectrometer
experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen
emission at 1304 A is detected at altitudes of 400 to 1000 km above the limb
during limb scans shortly after closest approach. However, the derived oxygen
scale height is not consistent with recent models of oxygen escape based on the
production of suprathermal oxygen atoms by the dissociative recombination of
O2+.Comment: 17 pages, 8 figures, accepted for publication in Icaru
Ultraviolet Spectroscopy of Comet 9P/Tempel 1 with Alice/Rosetta during the Deep Impact Encounter
We report on spectroscopic observations of periodic comet 9P/Tempel 1 by the
Alice ultraviolet spectrograph on the Rosetta spacecraft in conjunction with
NASA's Deep Impact mission. Our objectives were to measure an increase in
atomic and molecular emissions produced by the excavation of volatile
sub-surface material. We unambiguously detected atomic oxygen emission from the
quiescent coma but no enhancement at the 10% (1-sigma) level following the
impact. We derive a quiescent water production rate of 9 x 10^27 molecules per
second with an estimated uncertainty of 30%. Our upper limits to the volatiles
produced by the impact are consistent with other estimates.Comment: 11 pages, 4 postscript figures. Accepted for publication in Icarus
special issue on Deep Impac
An extrasolar planetary system with three Neptune-mass planets
Over the past two years, the search for low-mass extrasolar planets has led
to the detection of seven so-called 'hot Neptunes' or 'super-Earths' around
Sun-like stars. These planets have masses 5-20 times larger than the Earth and
are mainly found on close-in orbits with periods of 2-15 days. Here we report a
system of three Neptune-mass planets with periods of 8.67, 31.6 and 197 days,
orbiting the nearby star HD 69830. This star was already known to show an
infrared excess possibly caused by an asteroid belt within 1 AU (the Sun-Earth
distance). Simulations show that the system is in a dynamically stable
configuration. Theoretical calculations favour a mainly rocky composition for
both inner planets, while the outer planet probably has a significant gaseous
envelope surrounding its rocky/icy core; the outer planet orbits within the
habitable zone of this star.Comment: 17 pages, 3 figures, preprint of the paper published in Nature on May
18, 200
Photolysis of sulphuric acid as the source of sulphur oxides in the mesosphere of Venus
The sulphur cycle plays fundamental roles in the chemistry and climate of Venus. Thermodynamic equilibrium chemistry at the surface of Venus favours the production of carbonyl sulphide and to a lesser extent sulphur dioxide. These gases are transported to the middle atmosphere by the Hadley circulation cell. Above the cloud top, a sulphur oxidation cycle involves conversion of carbonyl sulphide into sulphur dioxide, which is then transported further upwards. A significant fraction of this sulphur dioxide is subsequently oxidized to sulphur trioxide and eventually reacts with water to form sulphuric acid. Because the vapour pressure of sulphuric acid is low, it readily condenses and forms an upper cloud layer at altitudes of 60–70 km, and an upper haze layer above 70 km (ref. 9), which effectively sequesters sulphur oxides from photochemical reactions. Here we present simulations of the fate of sulphuric acid in the Venusian mesosphere based on the Caltech/JPL kinetics model, but including the photolysis of sulphuric acid. Our model suggests that the mixing ratios of sulphur oxides are at least five times higher above 90 km when the photolysis of sulphuric acid is included. Our results are inconsistent with the previous model results but in agreement with the recent observations using ground-based microwave spectroscopy and by Venus Express
A multi-site campaign to measure solar-like oscillations in Procyon. II. Mode frequencies
We have analyzed data from a multi-site campaign to observe oscillations in
the F5 star Procyon. The data consist of high-precision velocities that we
obtained over more than three weeks with eleven telescopes. A new method for
adjusting the data weights allows us to suppress the sidelobes in the power
spectrum. Stacking the power spectrum in a so-called echelle diagram reveals
two clear ridges that we identify with even and odd values of the angular
degree (l=0 and 2, and l=1 and 3, respectively). We interpret a strong, narrow
peak at 446 muHz that lies close to the l=1 ridge as a mode with mixed
character. We show that the frequencies of the ridge centroids and their
separations are useful diagnostics for asteroseismology. In particular,
variations in the large separation appear to indicate a glitch in the
sound-speed profile at an acoustic depth of about 1000 s. We list frequencies
for 55 modes extracted from the data spanning 20 radial orders, a range
comparable to the best solar data, which will provide valuable constraints for
theoretical models. A preliminary comparison with published models shows that
the offset between observed and calculated frequencies for the radial modes is
very different for Procyon than for the Sun and other cool stars. We find the
mean lifetime of the modes in Procyon to be 1.29 +0.55/-0.49 days, which is
significantly shorter than the 2-4 days seen in the Sun.Comment: accepted for publication in Ap
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