88 research outputs found
EUV-TEC proxy to describe ionospheric variability using satellite-borne solar EUV measurements
An updated version of a proxy, termed EUV-TEC, describing the global total
primary photoionisation is calculated from satellite-borne EUV measurements
assuming a model atmosphere consisting of four major atmospheric
constituents. Regional number densities of the background atmosphere are
taken from the NRLMSISE-00 climatology. For calculation the Lambert-Beer law
is used to describe the decrease of the radiation along their way through the
atmosphere. The EUV-TEC proxy thus describes the ionospheric response to
solar EUV radiation and its variability. EUV-TEC is compared against the
global mean total electron content (TEC), a fundamental ionospheric parameter
created from vertical TEC maps derived from GPS data. Strong correlation
between these indices is found on different time scales. Results show that
the EUV-TEC proxy represents the ionsopheric variability better than the
conventional solar index F10.7 does, especially during high and moderate
solar activity
EUV-TEC - an index to describe ionospheric variability using satellite-borne solar EUV measurements: first results
Primary ionisation of major ionospheric constituents is calculated from satellite-borne solar EUV measurements. Number densities of the background atmosphere are taken from the NRLMSISE-00 climatology. From the calculated ionisation rates, an index termed EUV-TEC, which is based on the global total ionisation is calculated, and describes
the ionospheric response to solar EUV and its variability. The index is compared against global mean ionospheric total electron content (TEC) derived from GPS data. Results show that the EUV-TEC index provides a better overall representation of global TEC than conventional solar indices like F10.7 do. The EUV-TEC index may be used for scientific research, and to describe the ionospheric effects on radio communication and navigation systems
Downregulation of tumor necrosis factor (TNF) sensitivity via modulation of TNF binding capacity by protein kinase C activators.
Proxies to describe ionospheric variability and heating rates of the upper atmosphere: current progress
An updated version of the EUV-TEC proxy, describing the total primary ionisation of the upper atmosphere, is calculated from satellite-borne EUV measurements. Regional number densities of the background model atmosphere consisting of four major constituents are taken from the NRLMSISE-00 climatology. Furthermore, a first estimate of a global thermospheric heating rate is calculated from the absorbed energy.
For the calculations the Lambert-Beer law is used to describe the decrease of the radiation along their way through the atmosphere. The EUV-TEC proxy is compared against the global mean total electron content (TEC), obtained from vertical TEC maps derived from GPS data. Strong correlations between these indices are found on different time scales. Results show that the EUV-TEC proxy describes the ionospheric variability better than the conventional solar index F10.7, especially at short time scales of days to weeks
Solar activity and ionospheric response as seen from combined SolACES and SDO-EVE solar EUV spectra
Ionospheric response to solar EUV variability during late 2012 through mid 2013 is shown by the EUV-TEC proxy based on combined SolACES and SDO/EVE solar spectra. The results are compared with global TEC analyses. We found that EUV-TEC describes TEC variability better than the conventional F107 index, especially during periods of strong solar flare activity
Equilibrium abundances in hot DA white dwarfs as derived from self-consistent diffusion models I. Analysis of spectroscopic EUVE data
We present the first analysis of an EUV selected sample of hot DA white
dwarfs using a new type of atmospheric models. These models take into account
the interplay between gravitational settling and radiative acceleration to
predict the chemical stratification from an equilibrium between the two forces
while self-consistently solving for the atmospheric structure. In contrast to
atmospheric models with the assumption of chemical homogeneity, the number of
free parameters in the new models is reduced to the effective temperature and
surface gravity alone. The overall good reproduction of observed EUV spectra
reveals that these models are able to describe the physical conditions in hot
DA white dwarf atmospheres correctly. A comparison with previous analyses
highlights the improvements as well as the limits of our new models.Comment: 10 pages, 28 figures, accepted for publication by A&
Evolutionary and pulsational properties of white dwarf stars
Abridged. White dwarf stars are the final evolutionary stage of the vast
majority of stars, including our Sun. The study of white dwarfs has potential
applications to different fields of astrophysics. In particular, they can be
used as independent reliable cosmic clocks, and can also provide valuable
information about the fundamental parameters of a wide variety of stellar
populations, like our Galaxy and open and globular clusters. In addition, the
high densities and temperatures characterizing white dwarfs allow to use these
stars as cosmic laboratories for studying physical processes under extreme
conditions that cannot be achieved in terrestrial laboratories. They can be
used to constrain fundamental properties of elementary particles such as axions
and neutrinos, and to study problems related to the variation of fundamental
constants.
In this work, we review the essentials of the physics of white dwarf stars.
Special emphasis is placed on the physical processes that lead to the formation
of white dwarfs as well as on the different energy sources and processes
responsible for chemical abundance changes that occur along their evolution.
Moreover, in the course of their lives, white dwarfs cross different
pulsational instability strips. The existence of these instability strips
provides astronomers with an unique opportunity to peer into their internal
structure that would otherwise remain hidden from observers. We will show that
this allows to measure with unprecedented precision the stellar masses and to
infer their envelope thicknesses, to probe the core chemical stratification,
and to detect rotation rates and magnetic fields. Consequently, in this work,
we also review the pulsational properties of white dwarfs and the most recent
applications of white dwarf asteroseismology.Comment: 85 pages, 28 figures. To be published in The Astronomy and
Astrophysics Revie
Circumstellar features in hot DA white dwarfs
We present a phenomenological study of highly ionised, non-photospheric
absorption features in high spectral resolution vacuum ultraviolet spectra of
23 hot DA white dwarfs. Prior to this study, four of the survey objects (Feige
24, REJ 0457-281, G191-B2B and REJ 1614-085) were known to possess these
features. We find four new objects with multiple components in one or more of
the principal resonance lines: REJ 1738+665, Ton 021, REJ 0558-373 and WD
2218+706. A fifth object, REJ 2156-546 also shows some evidence of multiple
components, though further observations are required to confirm the detection.
We discuss possible origins for these features including ionisation of the
local interstellar environment, the presence of material inside the
gravitational well of the white dwarf, mass loss in a stellar wind, and the
existence of material in an ancient planetary nebula around the star. We
propose ionisation of the local interstellar medium as the origin of these
features in G191-B2B and REJ 1738+665, and demonstrate the need for higher
resolution spectroscopy of the sample, to detect multiple ISM velocity
components and to identify circumstellar features which may lie close to the
photospheric velocity.Comment: 20 figures, 21 page
Toward ensemble asteroseismology of ZZ Ceti stars with fully evolutionary models
ZZ Ceti stars form the most numerous group of degenerate variable stars. They
are otherwise normal DA (H-rich atmospheres) white dwarfs that exhibit
pulsations. Here, we present an asteroseismological analysis for 44 bright ZZ
Ceti stars based on a new set of fully evolutionary DA white dwarf models
characterized by detailed chemical profiles from the centre to the surface. One
of our targets is the archetypal ZZ Ceti star G117B15A, for which we obtain
an asteroseismological model with an effective temperature and a surface
gravity in excellent agreement with the spectroscopy. The asteroseismological
analysis of a set of 44 ZZ Ceti stars has the potential to characterize the
global properties of the class, in particular the thicknesses of the hydrogen
envelope and the stellar masses. Our results support the belief that white
dwarfs in the solar neighbourhood harbor a broad range of hydrogen-layer
thickness.Comment: 22 pages, 12 figures, 6 tables. Accepted for publication in MNRA
- …