129 research outputs found
Calculation of Spectral Darkening and Visibility Functions for Solar Oscillations
Calculations of spectral darkening and visibility functions for the
brightness oscillations of the Sun resulting from global solar oscillations are
presented. This has been done for a broad range of the visible and infrared
continuum spectrum. The procedure for the calculations of these functions
includes the numerical computation of depth-dependent derivatives of the
opacity caused by p modes in the photosphere. A radiative-transport code was
used for this purpose to get the disturbances of the opacities from temperature
and density fluctuations. The visibility and darkening functions are obtained
for adiabatic oscillations under the assumption that the temperature
disturbances are proportional to the undisturbed temperature of the
photosphere. The latter assumption is the only way to explore any opacity
effects since the eigenfunctions of p-mode oscillations have not been obtained
so far. This investigation reveals that opacity effects have to be taken into
account because they dominate the violet and infrared part of the spectrum.
Because of this dominance, the visibility functions are negative for those
parts of the spectrum. Furthermore, the darkening functions show a
wavelength-dependent change of sign for some wavelengths owing to these opacity
effects. However, the visibility and darkening functions under the assumptions
used contradict the observations of global p-mode oscillations, but it is
beyond doubt that the opacity effects influence the brightness fluctuations of
the Sun resulting from global oscillations
Far- and Extreme-UV Solar Spectral Irradiance and Radiance from Simplified Atmospheric Physical Models
The Ly<alpha> and Ly<beta> profiles in solar prominences and prominence fine structure
We present the first combined Ly and Ly profiles in solar
prominences obtained by the SOHO/SUMER instrument and discuss their important
spatial variability with respect to predictions from 1D and multithread models.Comment: Accepted in Solar Physics, 14 pages with 9 figures and 3 Table
Broadband Radio Spectral Observations of Solar Eclipse on 2008-08-01 and Implications on the Quiet Sun Atmospheric Model
Based on the joint-observations of the radio broadband spectral emissions of
solar eclipse on August 1, 2008 at Jiuquan (total eclipse) and Huairou (partial
eclipse) at the frequencies of 2.00 -- 5.60 GHz (Jiuquan), 2.60 -- 3.80 GHZ
(Chinese solar broadband radiospectrometer, SBRS/Huairou), and 5.20 -- 7.60 GHz
(SBRS/Huairou), the authors assemble a successive series of broadband spectrum
with a frequency of 2.60 -- 7.60 GHz to observe the solar eclipse
synchronously. This is the first attempt to analyze the solar eclipse radio
emission under the two telescopes located at different places with broadband
frequencies in the periods of total and partial eclipse. With these analyses,
the authors made a new semiempirical model of the coronal plasma density of the
quiet Sun and made a comparison with the classic models.Comment: 10 pages, 4 figures, published on Sci. China Ser. G, 2009, Vol.52,
page 1765-177
Signatures of transition region explosive events in hydrogen Ly-beta profiles
We search for signatures of transition region explosive events (EEs) in
hydrogen Ly-beta profiles. Two rasters made by the SUMER (Solar Ultraviolet
Measurements of Emitted Radiation) instrument on board SOHO in a quiet-Sun
region and an equatorial coronal hole are selected for our study. Transition
region explosive events are identified from profiles of C II 1037 Angstrom and
O VI 1032 Angstrom, respectively. We compare Ly-beta profiles during EEs with
those averaged in the entire quiet-Sun and coronal-hole regions. The
relationship between the peak emission of Ly-beta profiles and the wing
emission of C II and O VI during EEs is investigated. We find that the central
part of Ly-beta profiles becomes more reversed and the distance of the two
peaks becomes larger during EEs, both in the coronal hole and in the quiet Sun.
The average Ly-beta profile of the EEs detected by C II has an obvious stronger
blue peak. During EEs, there is a clear correlation between the increased peak
emission of Ly-beta profiles and the enhanced wing emission of the C II and O
VI lines. The correlation is more pronounced for the Ly-beta peaks and C II
wings, and less significant for the Ly-beta blue peak and O VI blue wing. We
also find that the Ly-beta profiles are more reversed in the coronal hole than
in the quiet Sun. We suggest that the jets produced by EEs emit Doppler-shifted
Ly-beta photons, causing enhanced emission at positions of the peaks of Ly-beta
profiles. The more-reversed Ly-beta profiles confirm the presence of a larger
opacity in the coronal hole than in the quiet Sun. The finding that EEs modify
the Ly-beta line profile in QS and CHs implies that one should be careful in
the modelling and interpretation of relevant observational data.Comment: accepted for publication in Astronomy and Astrophysics; 8 pages, 2
tables, 5 figure
The Structure and Dynamics of the Upper Chromosphere and Lower Transition Region as Revealed by the Subarcsecond VAULT Observations
The Very high Angular resolution ULtraviolet Telescope (VAULT) is a sounding
rocket payload built to study the crucial interface between the solar
chromosphere and the corona by observing the strongest line in the solar
spectrum, the Ly-a line at 1216 {\AA}. In two flights, VAULT succeeded in
obtaining the first ever sub-arcsecond (0.5") images of this region with high
sensitivity and cadence. Detailed analyses of those observations have
contributed significantly to new ideas about the nature of the transition
region. Here, we present a broad overview of the Ly-a atmosphere as revealed by
the VAULT observations, and bring together past results and new analyses from
the second VAULT flight to create a synthesis of our current knowledge of the
high-resolution Ly-a Sun. We hope that this work will serve as a good reference
for the design of upcoming Ly-a telescopes and observing plans.Comment: 28 pages, 11 figure
Solar irradiance variability: a six-year comparison between SORCE observations and the SATIRE model
Aims: We investigate how well modeled solar irradiances agree with
measurements from the SORCE satellite, both for total solar irradiance and
broken down into spectral regions on timescales of several years. Methods: We
use the SATIRE model and compare modeled total solar irradiance (TSI) with TSI
measurements between 2003 and 2009. Spectral solar irradiance over 200-1630nm
is compared with the SIM instrument on SORCE between 2004 and 2009 during a
period of decline from moderate activity to the recent solar minimum in 10 nm
bands and for three spectral regions of significant interest: the UV integrated
over 200-300nm, the visible over 400-691nm and the IR between 972-1630 nm.
Results: The model captures 97% of observed TSI variation. In the spectral
comparison, rotational variability is well reproduced, especially between 400
and 1200 nm. The magnitude of change in the long-term trends is many times
larger in SIM at almost all wavelengths while trends in SIM oppose SATIRE in
the visible between 500 and 700nm and between 1000 and 1200nm. We discuss the
remaining issues with both SIM data and the identified limits of the model,
particularly with the way facular contributions are dealt with, the limit of
flux identification in MDI magnetograms during solar minimum and the model
atmospheres in the IR employed by SATIRE. It is unlikely that improvements in
these areas will significantly enhance the agreement in the long-term trends.
This disagreement implies that some mechanism other than surface magnetism is
causing SSI variations, in particular between 2004 and 2006, if the SIM data
are correct. Since SATIRE was able to reproduce UV irradiance between 1991 and
2002 from UARS, either the solar mechanism for SSI variation fundamentally
changed around the peak of cycle 23, or there is an inconsistency between UARS
and SORCE UV measurements. We favour the second explanation.Comment: 14 pages, 13 figure
Magnetosphere-Ionosphere Coupling Through E-region Turbulence 1: Energy Budget
During periods of intense geomagnetic activity, strong electric fields and
currents penetrate from the magnetosphere into high-latitude ionosphere where
they dissipate energy, form electrojets, and excite plasma instabilities in the
E-region ionosphere. These instabilities give rise to plasma turbulence which
induces non-linear currents and strong anomalous electron heating (AEH) as
observed by radars. These two effects can increase the global ionospheric
conductances. This paper analyzes the energy budget in the electrojet, while
the companion paper applies this analysis to develop a model of anomalous
conductivity and frictional heating useful in large-scale simulations and
models of the geospace environment. Employing first principles, this paper
proves for the general case an earlier conjecture that the source of energy for
plasma turbulence and anomalous heating equals the work by external field on
the non-linear current. Using a two-fluid model of an arbitrarily magnetized
plasma and the quasilinear approximation, this paper describes the energy
conversion process, calculates the partial sources of anomalous heating, and
reconciles the apparent contradiction between the inherently 2-D non-linear
current and the 3-D nature of AEH.Comment: 13 pages, 1 figure; 1st of two companion paper
Observed Effect of Magnetic Fields on the Propagation of Magnetoacoustic Waves in the Lower Solar Atmosphere
We study Hinode/SOT-FG observations of intensity fluctuations in Ca II H-line
and G-band image sequences and their relation to simultaneous and co-spatial
magnetic field measurements. We explore the G-band and H-line intensity
oscillation spectra both separately and comparatively via their relative phase
differences, time delays and cross-coherences. In the non-magnetic situations,
both sets of fluctuations show strong oscillatory power in the 3 - 7 mHz band
centered at 4.5 mHz, but this is suppressed as magnetic field increases. A
relative phase analysis gives a time delay of H-line after G-band of 20\pm1 s
in non-magnetic situations implying a mean effective height difference of 140
km. The maximum coherence is at 4 - 7 mHz. Under strong magnetic influence the
measured delay time shrinks to 11 s with the peak coherence near 4 mHz. A
second coherence maximum appears between 7.5 - 10 mHz. Investigation of the
locations of this doubled-frequency coherence locates it in diffuse rings
outside photospheric magnetic structures. Some possible interpretations of
these results are offered.Comment: 19 pages, 6 figure
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