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1.
The SECCHI instruments aboard the recently launched STEREO spacecraft enable for the first time the continuous tracking of coronal mass ejections (CMEs) from the Sun to 1 AU. We analyze line-of-sight observations of the 24?–?25 January 2007 CMEs and fill the 20-hour gap in SECCHI coverage in 25 January by performing a numerical simulation using a three-dimensional magneto-hydrodynamic (MHD) code, the Space Weather Modeling Framework (SWMF). We show how the observations reflect the interaction of the two successive CMEs with each other and with the structured solar wind. We make a detailed comparison between the observations and synthetic images from our model, including time-elongation maps for several position angles. Having numerical simulations to disentangle observational from physical effects, we are able to study the three-dimensional nature of the ejections and their evolution in the inner heliosphere. This study reflects the start of a new era where, on one hand, models of CME propagation and interaction can be fully tested by using heliospheric observations and, on the other hand, observations can be better interpreted by using global numerical models. 相似文献
2.
C. O. Lee J. G. Luhmann D. Odstrcil P. J. MacNeice I. de Pater P. Riley C. N. Arge 《Solar physics》2009,254(1):155-183
We present results of solar-wind parameters generated by 3D MHD models. The ENLIL inner-heliosphere solar-wind model together
with the MAS or Wang – Sheeley – Arge (WSA) coronal models, describe the steady solar-wind stream structure and its origins
in the solar corona. The MAS/ENLIL and WSA/ENLIL models have been tuned to provide a simulation of plasma moments as well
as interplanetary magnetic-field magnitude and polarity in the absence of disturbances from coronal transients. To investigate
how well the models describe the ambient solar wind structure from the Sun out to 1 AU, the model results are compared to
solar-wind measurements from the ACE spacecraft. We find that there is an overall agreement between the observations and the
model results for the general large-scale solar-wind structures and trends, such as the timing of the high-density structures
and the low- and high-speed winds, as well as the magnetic sector structures. The time period of our study is the declining
phase of Solar Cycle 23 when the solar activity involves well-defined stream structure, which is ideal for testing a quasi-steady-state
solar-wind model. 相似文献
3.
We present some results from our submillimeter single-dish and aperture synthesis imaging surveys of protoplanetary disks
using the JCMT, CSO, and Submillimeter Array (SMA) on Mauna Kea, Hawaii. Employing a simple disk model, we simultaneously
fit the spectral energy distributions and spatially resolved submillimeter continuum emission from our SMA survey to constrain
disk structure properties, including surface density profiles and sizes. The typical disk structure we infer is consistent
with a fiducial accretion disk model with a viscosity parameter α≈0.01. Combined with a large, multiwavelength single-dish survey of similar disks, we show how these observations provide
evidence for significant grain growth and rapid evolution in the outer regions of disks, perhaps due to an internal photoevaporation
process. In addition, we discuss SMA observations of the disks in the Orion Trapezium (proplyds) in the context of disk evolution
in a more extreme environment. 相似文献
4.
José Antonio Belinchón 《Astrophysics and Space Science》2012,338(2):381-400
We study how the constants G and Λ may vary in different theoretical models (general relativity with a perfect fluid, scalar cosmological models (“quintessence”)
with and without interacting scalar and matter fields and a scalar-tensor model with a dynamical Λ) in order to explain some
observational results. We apply the program outlined in section II to study three different geometries which generalize the
FRW ones, which are Bianchi V, VII0 and IX, under the self-similarity hypothesis. We put special emphasis on calculating exact power-law solutions which allow
us to compare the different models. In all the studied cases we arrive at the conclusion that the solutions are isotropic
and noninflationary while the cosmological constant behaves as a positive decreasing time function (in agreement with the
current observations) and the gravitational constant behaves as a growing time function. 相似文献
5.
We present the observations of the Irr galaxy IC 10 at the 6-m SAO telescope with the panoramic Multi-Pupil Fiber Spectrograph
(MPFS). Based on the results of these observations and our long-slit spectroscopy performed previously, we have investigated
the ionized-gas emission spectrum in the region of intense star formation and refined the gas metallicity estimates. We show
that the “diagnostic diagrams” constructed from our observations agree best with the new improved ionization models by Martin-Manjon
et al. Using these models, we have determined the electron density and gas ionization parameter and ionizing-cluster characteristics,
the age and mass, from the spectra of the investigated HII regions. The cluster ages and masses are shown to be within the
ranges 2.5–5 Myr and (0.2–1) × 105
M
⊗, respectively. 相似文献
6.
Solar empirical models based on regression of two variability indices for radiation from the photosphere and chromosphere
fit total solar irradiance (TSI) observations with accuracy comparable to the precision reported for the observations themselves.
However, the physical meaning of the fitting coefficients and their stability during different phases of the solar cycle has
not been examined in detail. We test the stability of the coefficients in regression models of the VIRGO TSI observations
over the nine years from the minimum of Cycle 23 in 1996 through the maximum to 2005. We also show how the coefficients converge
to the ‘`best fit’' using a search in the coefficient space. Analysis of TSI variability in different phases of this cycle
shows little change in regression models as long as the time periods used in the regression are long enough to show the slow
solar cycle variation in TSI. We extend our analysis to TSI observations from ERB, ACRIM2, ACRIM3, DIARAD, and TIM. The regression
models from these time series show large systematic differences in fitting coefficients for the plage and sunspot indices
that we used. These differences are significantly larger than the estimated uncertainties in the coefficients and point to
the difficulty of combining observations from different instruments to create an accurate composite TSI record over several
solar cycles. Our results clearly demonstrate the improvement in precision of TSI measurements from the Nimbus 7 ERB in Cycle
22 to the latest SORCE TIM data in Cycle 23. 相似文献
7.
Anirudh Pradhan Rekha Jaiswal Kanti Jotania Rajeev Kumar Khare 《Astrophysics and Space Science》2012,337(1):401-413
We present two dark energy (DE) models with an anisotropic fluid in Bianchi type-VI
0 space-time by considering time dependent deceleration parameter (DP). The equation of state (EoS) for dark energy ω is found to be time dependent and its existing range for derived models is in good agreement with the recent observations.
Under the suitable condition, the anisotropic models approach to isotropic scenario. We also find that during the evolution
of the universe, the EoS parameter for DE changes from ω>−1 to ω=−1 in first model whereas from ω>−1 to ω<−1 in second model which is consistent with recent observations. The cosmological constant Λ is found to be a positive decreasing
function of time and it approaches a small positive value at late time (i.e. the present epoch) which is corroborated by results
from recent type Ia supernovae observations. The cosmic jerk parameter in our derived models is also found to be in good agreement
with the recent data of astrophysical observations. The physical and geometric aspects of both the models are also discussed
in detail. 相似文献
8.
José Antonio Belinchón 《Astrophysics and Space Science》2009,323(2):185-195
We study a perfect fluid Bianchi II models with time varying constants under the self-similarity approach. In the first of
the studied model, we consider that only vary G and Λ. The obtained solution is more general that the obtained one for the classical solution since it is valid for an equation
of state ω∈(−1,∞) while in the classical solution ω∈(−1/3,1). Taking into account the current observations, we conclude that G must be a growing time function while Λ is a positive decreasing function. In the second of the studied models we consider
a variable speed of light (VSL). We obtain a similar solution as in the first model arriving to the conclusions that c must be a growing time function if Λ is a positive decreasing function. 相似文献
9.
Spectral continua observed during solar flares may contain information about both thermal and non-thermal heating mechanisms.
Using two semi-empirical flare models F2 and FLA, we synthesize the thermal continua from optical to mm–radio domains and
compare their intensities with quiet-Sun values computed from a recent model C7. In this way, the far-infrared and sub-mm/mm
continua are studied for the first time, and we present our results as a benchmark for further modeling and for planning new
observations, especially with the ALMA instrument. Finally, we demonstrate how these continua are formed and show a close
correspondence between their brightness temperature and the kinetic-temperature structure of the flaring atmosphere. 相似文献
10.
Hugh R. A. Jones Yakiv Pavlenko Serena Viti R. J. Barber Larisa A. Yakovina David Pinfield Jonathan Tennyson 《Monthly notices of the Royal Astronomical Society》2005,358(1):105-112
We compare high-resolution infrared observations of the CO 2–0 bands in the 2.297–2.310 μm region of M dwarfs and one L dwarf with theoretical expectations. We find a good match between the observational and synthetic spectra throughout the 2000–3500 K temperature regime investigated. None the less, for the 2500–3500 K temperature range, the temperatures that we derive from synthetic spectral fits are higher than expected from more empirical methods by several hundred kelvin. In order to reconcile our findings with the empirical temperature scale, it is necessary to invoke warming of the model atmosphere used to construct the synthetic spectra. We consider that the most likely reason for the back-warming is missing high-temperature opacity due to water vapour. We compare the water vapour opacity of the Partridge–Schwenke line list used for the model atmosphere with the output from a preliminary calculation by Barber & Tennyson. While the Partridge–Schwenke line list is a reasonable spectroscopic match for the new line list at 2000 K, by 4000 K it is missing around 25 per cent of the water vapour opacity. We thus consider that the offset between empirical and synthetic temperature scales is explained by the lack of hot water vapour used for computation of the synthetic spectra. For our coolest objects with temperatures below 2500 K, we find best fits when using synthetic spectra which include dust emission. Our spectra also allow us to constrain the rotational velocities of our sources, and these velocities are consistent with the broad trend of rotational velocities increasing from M to L. 相似文献
11.
Richard A. Battye Jochen Weller 《Monthly notices of the Royal Astronomical Society》2005,362(1):171-183
We consider the optimum depth of a cluster survey selected using the Sunyaev–Zel'dovich effect. By using simple models for the evolution of the cluster mass function and detailed modelling for a variety of observational techniques, we show that the optimum survey yield is achieved when the average size of the clusters selected is close to the size of the telescope beam. For a total power measurement, we compute the optimum noise threshold per beam as a function of the beam size and then discuss how our results can be used in more general situations. As a by-product we gain some insight into what is the most advantageous instrumental set-up. In the case of beam switching observations one is not severely limited if one manages to set the noise threshold close to the point which corresponds to the optimum yield. Considering a variety of alternative scenarios, we discuss how robust our conclusions are to modifications in the cluster model and cosmological parameters. The precise optimum is particularly sensitive to the amplitude of fluctuations and the profile of the gas in the cluster. 相似文献
12.
Over the past thirty years, two bodies of literature have developed in parallel presenting mutually exclusive views of the
Sun’s upper transition region. One model holds that the Sun’s upper-transition-region plasmas are confined primarily in hydrostatic
funnels with a substantial backheating component. The other model holds that discrete structures, which are effectively isolated
from the corona, predominate in the Sun’s upper transition region. Purveyors of the latter position have recently begun to
present near-resolved observations of discrete structures. The funnel scenario, in contrast, has only been addressed by modeling
unresolved upper transition region emission. To address this paradox we have constructed hydrostatic funnel models and tested
them against a wider set of solar observations than previously performed. We reproduce the results of the previous analyses,
yet find that the hydrostatic funnels are unable to self-consistently match the wider set of observations against which we
test the models. We show that it is not possible for a class of funnels having peak temperatures in the transition region
or in the corona to match the observations. We conclude that it is implausible that a class of hydrostatic funnels constitutes
the dominant emitting component of the Sun’s upper-transition-region plasmas as has been suggested. 相似文献
13.
N. Capitaine P. M. Mathews V. Dehant P. T. Wallace S. B. Lambert 《Celestial Mechanics and Dynamical Astronomy》2009,103(2):179-190
In this paper, we discuss the fundamental aspects of the semi-analytical precession–nutation models that were adopted by IAU
Resolutions in 2000 and 2006. We show that no significant discrepancies appear between those models (Mathews et al., J Geophys
Res 107:B4, ETG 3-1–3-26, 2002, Capitaine et al., Astron Astrophys 412:567– 586, 2003) and other semi-analytical solutions
or the INPOP06 numerical integration (Fienga et al., Astron Astrophys 477:315–327, 2008), especially for the quadratic terms.
We also report on the most recent comparisons of the models with VLBI observations. We have employed different empirical models
to fit the residuals, in attempting to characterize the nature of the observed curvature. The efficiencies of those empirical
models are compared and their interpretations in terms of physical mechanisms are discussed. We show that a combination of
linear and 18.6-year corrections is the most credible model for explaining the currently observed residuals, but that a longer
span of observations is required before the true character of the effect can be determined. We note that the predictions from
the ERA-2005 theory (Krasinsky, Celest Mech Dyn Astron 96:169–217, 2006) have diverged from recent VLBI results and suggest
that the empirical nature of the ERA model is responsible. 相似文献
14.
The link between gas dynamical models and observations is crucial. The general progress in numerical simulations must be accompanied
by predictions for observable quantities, which not only allow to test the models or parts of them against observations but
which also improve the understanding of observational data. In this paper we focus on predictions for observations, for three
examples of 3Dhydrodynamical simulations of binary star systems, and the techniques required for their derivation. The examples
include synthetic, optically thin Doppler broadened line profiles for colliding wind symbiotic binaries, the UV light curve
of an accretion model for the symbiotic binary RW Hya, and the X-ray light curve of the WR+O binaryγ Velorum. The main purpose
is to emphasize the importance of such studies and to illustrate the capabilities of the emploiedtools. The tools are all
contained in the A-MAZE code package we have developed and are publicly available.
Seminar for Applied Mathematics
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
J. A. Pons J. F. Pérez-Azorín J. A. Miralles G. Miniutti 《Astrophysics and Space Science》2007,308(1-4):247-257
We propose a self–consistent model to explain all observational properties reported so far on the isolated neutron star (INS)
RX J0720-3125 with the aim of giving a step forward towards our understanding of INSs. For a given magnetic field structure,
which is mostly confined to the crust and outer layers, we obtain theoretical models and spectra which account for the broadband
spectral energy distribution (including the apparent optical excess), the X-ray pulsations, and for the spectral feature seen
in the soft X-ray spectrum of RX J0720-3125 around 0.3 keV. By fitting our models to existing archival X-ray data from 6 different
XMM–Newton observations and available optical data, we show that the observed properties are fully consistent with a normal neutron star, with a proper radius of about 12 km, a temperature at the magnetic pole of about 100 eV, and a magnetic field
strength of 2–3×1013 G. Moreover, we are able to reproduce the observed long–term spectral evolution in terms of free precession which induces
changes in the orientation angles of about 40 degrees with a periodicity of 7 years. In addition to the evidence of internal
toroidal components, we also find strong evidence of non–dipolar magnetic fields, since all spectral properties are better
reproduced with models with strong quadrupolar components.
相似文献
16.
C. O. Lee J. G. Luhmann J. T. Hoeksema X. Sun C. N. Arge I. de Pater 《Solar physics》2011,269(2):367-388
The solar cycle 23 minimum period has been characterized by a weaker solar and interplanetary magnetic field. This provides
an ideal time to study how the strength of the photospheric field affects the interplanetary magnetic flux and, in particular,
how much the observed interplanetary fields of different cycle minima can be understood simply from differences in the areas
of the coronal holes, as opposed to differences in the surface fields within them. In this study, we invoke smaller source
surface radii in the potential-field source-surface (PFSS) model to construct a consistent picture of the observed coronal
holes and the near-Earth interplanetary field strength as well as polarity measurements for the cycles 23 and 22 minimum periods.
Although the source surface value of 2.5 R
⊙ is typically used in PFSS applications, earlier studies have shown that using smaller source surface heights generates results
that better match observations during low solar activity periods. We use photospheric field synoptic maps from Mount Wilson
Observatory (MWO) and find that the values of ≈ 1.9 R
⊙ and ≈ 1.8 R
⊙ for the cycles 22 and 23 minimum periods, respectively, produce the best results. The larger coronal holes obtained for the
smaller source surface radius of cycle 23 somewhat offsets the interplanetary consequences of the lower magnetic field at
their photospheric footpoints. For comparison, we also use observations from the Michelson Doppler Imager (MDI) and find that
the source surface radius of ≈ 1.5 R
⊙ produces better results for cycle 23, rather than ≈ 1.8 R
⊙ as suggested from MWO observations. Despite this difference, our results obtained from MWO and MDI observations show a qualitative
consistency regarding the origins of the interplanetary field and suggest that users of PFSS models may want to consider using
these smaller values for their source surface heights as long as the solar activity is low. 相似文献
17.
G. Djurašević O. Demircan S. Özdemir M. Tanriver Z. Müyesseroglu H. Ak B. Albayrak 《Astrophysics and Space Science》1997,253(2):265-274
The UBV observations of the massive binary BF Aur were made at the Ankara University Observatory during 1988, 1989 and 1996.
Asymmetry of the light curves, arising from unequal height of successive maxima, indicates that the system is active. By analysing
these observations in the framework of the Roche model (including the presence of bright regions on the components) one obtains
a semidetached configuration of the system, with the cooler secondary component filling its Roche lobe. The analysis of the
light curves yields consistent solutions for mass ratio q = m2/m1 somewhat less than one. The influence of the mass transfer on the change of the system-orbital-period is relatively small.
The upward parabolic character of the O–C diagram (Zhang et al., 1993) indicates a mass transfer from the less massive secondary
to the more massive primary. This inturn requires the less massive secondary to fill its Roche lobe. This is consistent with
our solution. Based on these facts we introduced the following working hypothesis. At the place where the gas stream from
the secondary falls on the primary, relatively small in size but a high temperature contrast active hot-spot (hs) region is
formed. As a result of the heating effect caused by the irradiation of the hot-spot region, on the secondary's side facing
the hot spot a bright-spot (bs) region is formed. The bright-spot region is larger in size but with significantly lower temperature
than the hot spot. This region can be treated as a ‘reflection cap’. By analysing the light curves in the framework of this
working hypothesis the basic parameters of the system and the active regions are estimated. The problem is solved in two stages:
by obtaining a synthetic light curve in the case when the parameters of the corresponding Close Binary (CB) Roche model (Djurašević,
1992a) are given a priori (the direct problem) and by determining the parameters of the given model for which the best fit
between the synthetic light curve and the observations is achieved (the inverse problem) (Djurašević, 1992b).
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
In this paper we discuss the two mechanisms by which solar prominences on the limb can manifest themselves when observed in
coronal UV – EUV lines and in the soft X-ray continuum. These mechanisms are the absorption in the resonance continua of hydrogen
and helium on one hand and the reduction of the emissivity in a part of the coronal volume occupied by a prominence on the
other one. We briefly describe earlier observations made with SOHO/SUMER, EIT and Yohkoh/SXT. We then discuss how the instruments on the new Japanese satellite Hinode can be used for more detailed studies of prominences. We also propose some combined observations between the Hinode satellite and the SOHO/SUMER instrument. 相似文献
19.
Rachel S. Somerville † Joel R. Primack 《Monthly notices of the Royal Astronomical Society》1999,310(4):1087-1110
Using semi-analytic models of galaxy formation, we investigate galaxy properties such as the Tully–Fisher relation, the B - and K -band LFs, cold gas contents, sizes, metallicities and colours, and compare our results with observations of local galaxies. We investigate several different recipes for star formation and supernova feedback, including choices that are similar to the treatment by Kauffmann, White & Guiderdoni and Cole et al., as well as some new recipes. We obtain good agreement with all of the key local observations mentioned above. In particular, in our best models, we simultaneously produce good agreement with both the observed B - and K -band LFs and the I -band Tully–Fisher relation. Improved cooling and supernova feedback modelling, inclusion of dust extinction and an improved Press–Schechter model all contribute to this success. We present results for several variants of the CDM family of cosmologies, and find that models with values of Ω0 ≃0.3–0.5 give the best agreement with observations. 相似文献
20.
We calculate the polarization of the radiation from an optically thick accretion disk with a vertical averaged magnetic field.
The polarization arises from the scattering of light by free electrons in a magnetized disk plasma. The Faraday rotation of
the polarization plane during the propagation of a photon in a medium with a magnetic field is considered as the main effect.
We discuss various models of optically thick accretion disks with a vertical averaged magnetic field. Our main goal is to
derive simple asymptotic formulas for the polarization of radiation in the case where the Faraday rotation angle Ψ ≫ 1 at
the Thomson optical depth τ = 1. The results of our calculations allow the magnetic field strength in the region of the marginally
stable orbit near a black hole to be estimated from polarimetric observations, including X-ray observations expected in the
future. Since the polarization spectrum of the radiation strongly depends on the accretion disk model, a realistic physical
model of the accretion disk can be determined from data on the polarization of its radiation. 相似文献