共查询到20条相似文献,搜索用时 31 毫秒
1.
The Murchison Widefield Array (MWA) is one of the new technology low frequency radio interferometers currently under construction at an extremely radio-quiet location in Western Australia. The MWA design brings to bear the recent availability of powerful high-speed computational and digital signal processing capabilities on the problem of low frequency high-fidelity imaging with a rapid cadence and high spectral resolution. Solar and heliosphere science are among the key science objectives of the MWA and have guided the array design from its very conception. We present here a brief overview of the design and capabilities of the MWA with emphasis on its suitability for solar physics and remote-sensing of the heliosphere. We discuss the solar imaging and interplanetary scintillation (IPS) science capabilities of the MWA and also describe a new software framework. This software, referred to as Haystack InterPlanetary Software System (HIPSS), aims to provide a common data repository, interface, and analysis tools for IPS data from all observatories across the world. 相似文献
2.
The direct propagation of acoustic waves, driven harmonically at the solar photosphere, into the three-dimensional solar atmosphere
is examined numerically in the framework of ideal magnetohydrodynamics. It is of particular interest to study the leakage
of 5-minute global solar acoustic oscillations into the upper, gravitationally stratified and magnetised atmosphere, where
the modelled solar atmosphere possesses realistic temperature and density stratification. This work aims to complement and
bring further into the 3D domain our previous efforts (by Erdélyi et al., 2007, Astron. Astrophys. 467, 1299) on the leakage of photospheric motions and running magnetic-field-aligned waves excited by these global oscillations.
The constructed model atmosphere, most suitable perhaps for quiet Sun regions, is a VAL IIIC derivative in which a uniform
magnetic field is embedded. The response of the atmosphere to a range of periodic velocity drivers is numerically investigated
in the hydrodynamic and magnetohydrodynamic approximations. Among others the following results are discussed in detail: i) High-frequency waves are shown to propagate from the lower atmosphere across the transition region, experiencing relatively
low reflection, and transmitting most of their energy into the corona; ii) the thin transition region becomes a wave guide for horizontally propagating surface waves for a wide range of driver periods,
and particularly at those periods that support chromospheric standing waves; iii) the magnetic field acts as a waveguide for both high- and low-frequency waves originating from the photosphere and propagating
through the transition region into the solar corona.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
3.
A set of smoothed temperature gradient profiles around overshooting layers at the solar convective zone bottom is considered. In classical local theories of convection the one point defined according to the Schwarzschild criterion is enough to describe a convective boundary. To get a sophisticated picture of the overshooting we use four points to compute the transition overshooting functions. Analyzing the transition gradient profiles we found that the overshooting convective flux may be either positive or negative. A negative overshooting flux appears in nonlocal convective theories and causes a steep temperature gradient profile. But we propose an evenly smoothed gradient which corresponds to a convective flux positive everywhere. To outline the effect of the temperature gradient on the solar oscillations the squared Brunt–Väisälä frequency N 2 is calculated. In local convective theories the N 2 profile shows the discontinuity of the first derivative at the convective boundary, while all smoothed profiles eliminate the break. 相似文献
4.
The Recovery of CME-Related Dimmings and the ICME’s Enduring Magnetic Connection to the Sun 总被引:1,自引:1,他引:0
G. D. R. Attrill L. van Driel-Gesztelyi P. Démoulin A. N. Zhukov K. Steed L. K. Harra C. H. Mandrini J. Linker 《Solar physics》2008,252(2):349-372
It is generally accepted that transient coronal holes (TCHs, dimmings) correspond to the magnetic footpoints of CMEs that
remain rooted in the Sun as the CME expands out into the interplanetary space. However, the observation that the average intensity
of the 12 May 1997 dimmings recover to their pre-eruption intensity in SOHO/EIT data within 48 hours, whilst suprathermal
unidirectional electron heat fluxes are observed at 1 AU in the related ICME more than 70 hours after the eruption, leads
us to question why and how the dimmings disappear whilst the magnetic connectivity is maintained. We also examine two other
CME-related dimming events: 13 May 2005 and 6 July 2006. We study the morphology of the dimmings and how they recover. We
find that, far from exhibiting a uniform intensity, dimmings observed in SOHO/EIT data have a deep central core and a more
shallow extended dimming area. The dimmings recover not only by shrinking of their outer boundaries but also by internal brightenings.
We quantitatively demonstrate that the model developed by Fisk and Schwadron (Astrophys. J.
560, 425, 2001) of interchange reconnections between “open” magnetic field and small coronal loops is a strong candidate for the mechanism
facilitating the recovery of the dimmings. This process disperses the concentration of “open” magnetic field (forming the
dimming) out into the surrounding quiet Sun, thus recovering the intensity of the dimmings whilst still maintaining the magnetic
connectivity to the Sun.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
5.
Ehab E. El-Houssieny Robert J. Nemiroff Timothy E. Pickering 《Astrophysics and Space Science》2010,330(1):19-22
The outburst and subsequent brightness evolution of comet 17P/Holmes has been observed using the MMT Observatory’s All-Sky
Camera (Pickering 2006) on Mt. Hopkins near Tucson, Arizona, USA. The comet was picked up at the limiting visual magnitude of 5.5 on October 24.38
and tracked by the camera continuously until sunrise four hours later. During this time the comet brightened to visual magnitude
3.5. Comet Holmes was next observed just after sunset on October 25.23 at visual magnitude 2.5, where it remained approximately
constant over the next three days. The comet then began to dim slowly and was followed into the early months of 2008 with
periods of dense time coverage. 相似文献
6.
Recently observed oscillations in the solar atmosphere have been interpreted and modeled as magnetohydrodynamic wave modes.
This has allowed for the estimation of parameters that are otherwise hard to derive, such as the coronal magnetic-field strength.
This work crucially relies on the initial detection of the oscillations, which is commonly done manually. The volume of Solar Dynamics Observatory (SDO) data will make manual detection inefficient for detecting all of the oscillating regions. An algorithm is presented
that automates the detection of areas of the solar atmosphere that support spatially extended oscillations. The algorithm
identifies areas in the solar atmosphere whose oscillation content is described by a single, dominant oscillation within a
user-defined frequency range. The method is based on Bayesian spectral analysis of time series and image filtering. A Bayesian
approach sidesteps the need for an a-priori noise estimate to calculate rejection criteria for the observed signal, and it also provides estimates of oscillation frequency,
amplitude, and noise, and the error in all of these quantities, in a self-consistent way. The algorithm also introduces the
notion of quality measures to those regions for which a positive detection is claimed, allowing for simple post-detection discrimination by the user.
The algorithm is demonstrated on two Transition Region and Coronal Explorer (TRACE) datasets, and comments regarding its suitability for oscillation detection in SDO are made. 相似文献
7.
We analyze a special kind of temporal fine structure in microwave radio emission for the 25 August 1999 solar flare observed
by the PMO spectrometer over the range of 4.5 – 7.5 GHz. This flare displays continuum emission after a group of reverse-slope
type III bursts around 6 GHz. High-resolution dynamic spectra reveal three evolving emission lines (EELs) following the type
III group. They are characterized by isolated, narrow, and continuous emission strips, which display frequency fluctuations
with time. Their frequency-drift rates are between −2 and 3 GHz s−1. Distinct from the EELs at lower frequencies, three EELs have a very short duration of a few seconds. They show an average
bandwidth of Δf≈330 MHz and a relative bandwidth of Δf/f≈0.057. This is the first time that this kind of fine structure has been observed around 6 GHz. 相似文献
8.
Precursor techniques, in particular those using geomagnetic indices, often are used in the prediction of the maximum amplitude
for a sunspot cycle. Here, the year 2008 is taken as being the sunspot minimum year for cycle 24. Based on the average aa index value for the year of the sunspot minimum and the preceding four years, we estimate the expected annual maximum amplitude
for cycle 24 to be about 92.8±19.6 (1-sigma accuracy), indicating a somewhat weaker cycle 24 as compared to cycles 21 – 23.
Presuming a smoothed monthly mean sunspot number minimum in August 2008, a smoothed monthly mean sunspot number maximum is
expected about October 2012±4 months (1-sigma accuracy). 相似文献
9.
Tomislav Jurkić Mariza Sarta Deković Dijana Dominis Prester Dubravka Kotnik-Karuza 《Astrophysics and Space Science》2011,335(1):113-120
We determined atmospheric parameters of the Galactic early B-supergiant HD 198478 (55 Cyg) from the UV silicon lines taken from the high-resolution 1150–1980 Å IUE spectra. TLUSTY numerical code was used to model the stellar atmosphere and to determine the temperature and surface gravity assuming a non-LTE plane parallel hydrostatic stellar atmosphere with microturbulence. The synthesized spectra were broadened by the IUE instrumental profile, rotational and macroturbulent velocity with ROTIN numerical code. The silicon 1264 Å, 1309 Å, 1312 Å, 1417 Å and 1294–1303 Å multiplet lines of different stages of ionization (Si II and Si III) and Balmer Hδ 4101 Å line were modeled, leading to the temperature, surface gravity, rotational and macroturbulent velocity values. Our results have shown that the line broadening cannot be explained by rotational velocity only, but additional macroturbulent velocity component should be taken into account. HD 198478 shows a significant degeneracy in velocity, which means that the individual contributions of the macroturbulence and rotation in the total velocity broadening cannot be distinguished. Adequate fit of TLUSTY models to the observed non-resonant silicon lines suggests that the non-LTE plane-parallel hydrostatic stellar model without wind contribution can be used to explain such lines. We have obtained similar results using the HST STIS spectra in the same procedure, showing that the IUE spectra, despite their lacking quality compared to the STIS spectra, are reliable enough in determination of the B supergiants’ photospheric parameters. 相似文献
10.
We demonstrate that a simple solar dynamo model, in the form of a Parker migratory dynamo with random fluctuations of the
dynamo governing parameters and algebraic saturation of dynamo action, can at least qualitatively reproduce all the basic
features of solar Grand Minima as they are known from direct and indirect data. In particular, the model successfully reproduces
such features as an abrupt transition into a Grand Minimum and the subsequent gradual recovery of solar activity, as well
as mixed-parity butterfly diagrams during the epoch of the Grand Minimum. The model predicts that the cycle survives in some
form during a Grand Minimum, as well as the relative stability of the cycle inside and outside of a Grand Minimum. The long-term
statistics of simulated Grand Minima appears compatible with the phenomenology of the Grand Minima inferred from the cosmogenic
isotope data. We demonstrate that such ability to reproduce the Grand Minima phenomenology is not a general feature of the
dynamo models but requires some specific assumption, such as random fluctuations in dynamo governing parameters. In general,
we conclude that a relatively simple and straightforward model is able to reproduce the Grand Minima phenomenology remarkably
well, in principle providing us with a possibility of studying the physical nature of Grand Minima. 相似文献
11.
A method for the full three-dimensional (3-D) reconstruction of the trajectories of coronal mass ejections (CMEs) using Solar TErrestrial RElations Observatory (STEREO) data is presented. Four CMEs that were simultaneously observed by the inner and outer coronagraphs (COR1 and 2) of the Ahead and Behind STEREO satellites were analysed. These observations were used to derive CME trajectories in 3-D out to ~?15?R ⊙. The reconstructions using COR1/2 data support a radial propagation model. Assuming pseudo-radial propagation at large distances from the Sun (15?–?240?R ⊙), the CME positions were extrapolated into the Heliospheric Imager (HI) field-of-view. We estimated the CME velocities in the different fields-of-view. It was found that CMEs slower than the solar wind were accelerated, while CMEs faster than the solar wind were decelerated, with both tending to the solar wind velocity. 相似文献
12.
The Heliospheric Imager (HI) instruments on the Solar TErrestrial RElations Observatory (STEREO) observe solar plasma as it streams out from the Sun and into the heliosphere. The telescopes point off-limb (from
about 4° to 90° elongation) and so the Sun is not in the field of view. Hence, the Sun cannot be used to confirm the instrument
pointing. Until now, the pointing of the instruments have been calculated using the nominal preflight instrument offsets from
the STEREO spacecraft together with the spacecraft attitude data. This paper develops a new method for deriving the instrument
pointing solutions, along with other optical parameters, by comparing the locations of stars identified in each HI image with
the known star positions predicted from a star catalogue. The pointing and optical parameters are varied in an autonomous
manner to minimise the discrepancy between the predicted and observed positions of the stars. This method is applied to all
HI observations from the beginning of the mission to the end of April 2008. For the vast majority of images a good attitude
solution has been obtained with a mean-squared deviation between the observed and predicted star positions of one image pixel
or less. Updated values have been obtained for the instrument offsets relative to the spacecraft, and for the optical parameters
of the HI cameras. With this method the HI images can be considered as “self-calibrating,” with the actual instrument offsets
calculated as a byproduct. The updated pointing results and their by-products have been implemented in SolarSoft. 相似文献
13.
A. Papaioannou O. Malandraki A. Belov R. Skoug H. Mavromichalaki E. Eroshenko A. Abunin S. Lepri 《Solar physics》2010,266(1):181-193
In this work an analysis of a series of complex cosmic ray events that occurred between 17 January 2005 and 23 January 2005
using solar, interplanetary and ground based cosmic ray data is being performed. The investigated period was characterized
both by significant galactic cosmic ray (GCR) and solar cosmic ray (SCR) variations with highlighted cases such as the noticeable
series of Forbush effects (FEs) from 17 January 2005 to 20 January 2005, the Forbush decrease (FD) on 21 January 2005 and
the ground level enhancement (GLE) of the cosmic ray counter measurements on 20 January 2005. The analysis is focusing on
the aforementioned FE cases, with special attention drawn on the 21 January 2005, FD event, which demonstrated several exceptional
features testifying its uniqueness. Data from the ACE spacecraft, together with GOES X-ray recordings and LASCO CME coronagraph
images were used in conjunction to the ground based recordings of the Worldwide Neutron Monitor Network, the interplanetary
data of OMNI database and the geomagnetic activity manifestations denoted by K
p and D
st indices. More than that, cosmic ray characteristics as density, anisotropy and density gradients were also calculated. The
results illustrate the state of the interplanetary space that cosmic rays crossed and their corresponding modulation with
respect to the multiple extreme solar events of this period. In addition, the western location of the 21 January 2005 solar
source indicates a new cosmic ray feature, which connects the position of the solar source to the cosmic ray anisotropy variations.
In the future, this feature could serve as an indicator of the solar source and can prove to be a valuable asset, especially
when satellite data are unavailable. 相似文献
14.
Hasmukh K. Tank 《Astrophysics and Space Science》2010,330(2):203-205
Keeping apart the problem, whether Modified Newtonian Dynamics [MOND] can replace ‘dark matter’, this letter considers seven different theoretical recurrences of ‘critical acceleration’ of MOND noticed by Sivaram in various physical situations; adds five more observational recurrences to the list; and arrives at a set of laws which seem to be followed by all the systems bound by different fundamental forces; suggesting a clue to unification of fundamental forces. This attempt proposes a general explanation for ‘flattening of galaxies’ rotation-curves’ as well as the ‘expansion of the universe’. 相似文献
15.
M. K. Ammar 《Astrophysics and Space Science》2007,312(1-2):17-22
In a previous paper (Ammar in Proc. Math. Phys. 77:99, 2002) the statement of the problem was formulated and the basic equations of motion were formed in terms of variables suitable
for the applications in the problem of asteroid motion close to 2:1 commensurability. The short period terms has been eliminated
up to the first order in masses O(μ), using a perturbation approach based on the Lie series, the problem is reduced to that of secular resonance one. In the
present work the extended Delaunay method has been applied to develop the Hamiltonian and the generator as a power series
in
rather than the power of ε, where ε is a small parameter of order of the relative mass of the perturber. Hamilton–Jacobi method were used as a method of integration
of the equations of the dynamical system in order to build a formal solution for the resonant problem of the type 2:1 with
one degree of freedom. 相似文献
16.
Jeffrey L. Linsky 《Astrophysics and Space Science》2009,320(1-3):3-9
In this summary of the conference Space Astronomy: the UV Window to the Universe, held in El?Escorial, Spain, May 28 to June 1, 2007, I identify the important scientific questions posed by the speakers and the corresponding discoveries that future ultraviolet space instruments should enable. The science objectives described by the various speakers naturally fall into groups according to the needed instrumental requirements: wavelength coverage, spectral resolution, sensitivity, rapid access to targets, monitoring, and signal/noise. Although most of the science objectives presented during the conference require UV spectra in the 1,170–3,200 Å range, there are important science objectives that require spectra in the 912–1,170 Å range and at shorter wavelengths. I identify the limitations of present instruments for meeting these requirements. To avoid the upcoming UV dark age, important work must be done to properly build the World Space Observatory (WSO) and to plan future space missions. 相似文献
17.
Recently, a tachyonic field was presented as a dark energy model to represent the present acceleration of the Universe. In this paper, we consider a mixture of tachyonic fluid with a perfect fluid. For this purpose we consider barotropic fluid and Generalized Chaplygin gas (GCG). We present a particular form of the scale factor. We solve the equations of motion to get exact solutions of the density, tachyonic potential and the tachyonic field. We introduce a coupling term to show that the interaction decays with time. We also show that the nature of the potentials vary, so the interaction term reduces the potential in both cases. 相似文献
18.
Lorenzo Iorio 《Astrophysics and Space Science》2008,315(1-4):335-340
In this paper we explore the consequences of the recent determination of the mass m=(8.7±0.8)M ⊙ of Cygnus X-1, obtained from the Quasi-Periodic Oscillation (QPO)-photon index correlation scaling, on the orbital and physical properties of the binary system HDE 226868/Cygnus X-1. By using such a result and the latest spectroscopic optical data of the HDE 226868 supergiant star we get M=(24±5)M ⊙ for its mass. It turns out that deviations from the third Kepler law significant at more than 1-sigma level would occur if the inclination i of the system’s orbital plane to the plane of the sky falls outside the range ≈41–56 deg: such deviations cannot be due to the first post-Newtonian (1PN) correction to the orbital period because of its smallness; interpreted in the framework of the Newtonian theory of gravitation as due to the stellar quadrupole mass moment Q, they are unphysical because Q would take unreasonably large values. By conservatively assuming that the third Kepler law is an adequate model for the orbital period we obtain i=(48±7) deg which yields for the relative semimajor axis a=(42±9)R ⊙ (≈0.2 AU). 相似文献
19.
The electron distribution functions from the solar corona to the solar wind are determined in this paper by considering the
effects of the external forces, of Coulomb collisions and of the wave – particle resonant interactions in the plasma wave
turbulence. The electrons are assumed to be interacting with right-handed polarized waves in the whistler regime. The acceleration
of electrons in the solar wind seems to be mainly due to the electrostatic potential. Wave turbulence determines the electron
pitch-angle diffusion and some characteristics of the velocity distribution function (VDF) such as suprathermal tails. The
role of parallel whistlers can also be extended to small altitudes in the solar wind (the acceleration region of the outer
corona), where they may explain the energization and the presence of suprathermal electrons. 相似文献
20.
P. S. Negi 《Astrophysics and Space Science》2011,332(1):145-153
We construct for the first time, the sequences of stable neutron star (NS) models capable of explaining simultaneously, the
glitch healing parameters, Q, of both the pulsars, the Crab (Q≥0.7) and the Vela (Q≤0.2), on the basis of starquake mechanism of glitch generation, whereas the conventional NS models cannot give such consistent
explanation. Furthermore, our models also yield an upper bound on NS masses similar to those obtained in the literature for
a variety of modern equations of state (EOSs) compatible with causality and dynamical stability. If the lower limit of the
observational constraint of (i) Q≥0.7 for the Crab pulsar and (ii) the recent value of the moment of inertia for the Crab pulsar (evaluated on the basis of
time-dependent acceleration model of the Crab Nebula), I
Crab,45≥1.93 (where I
45=I/1045 g cm2), both are imposed together on our models, the models yield the value of matching density, E
b
=9.584×1014 g cm−3 at the core-envelope boundary. This value of matching density yields a model-independent upper bound on neutron star masses,
M
max≤2.22M
⊙, and the strong lower bounds on surface redshift z
R
≃0.6232 and mass M≃2.11M
⊙ for the Crab (Q≃0.7) and the strong upper bound on surface redshift z
R
≃0.2016, mass M≃0.982M
⊙ and the moment of inertia I
Vela,45≃0.587 for the Vela (Q≃0.2) pulsar. However, for the observational constraint of the ‘central’ weighted mean value Q≈0.72, and I
Crab,45>1.93, for the Crab pulsar, the minimum surface redshift and mass of the Crab pulsar are slightly increased to the values
z
R
≃0.655 and M≃2.149M
⊙ respectively, whereas corresponding to the ‘central’ weighted mean value Q≈0.12 for the Vela pulsar, the maximum surface redshift, mass and the moment of inertia for the Vela pulsar are slightly decreased
to the values z
R
≃0.1645, M≃0.828M
⊙ and I
Vela,45≃0.459 respectively. These results set an upper and lower bound on the energy of a gravitationally redshifted electron-positron
annihilation line in the range of about 0.309–0.315 MeV from the Crab and in the range of about 0.425–0.439 MeV from the Vela
pulsar. 相似文献