共查询到20条相似文献，搜索用时 46 毫秒
1.
K. D. Leka Graham Barnes A. D. Crouch Thomas R. Metcalf G. Allen Gary Ju Jing Y. Liu 《Solar physics》2009,260(1):83-108
The objective testing of algorithms for performing ambiguity resolution in vector magnetic field data is continued, with an
examination of the effects of noise in the data. Through the use of analytic magnetic field models, two types of noise are
“added” prior to resolving: noise to simulate Poisson photon noise in the observed polarization spectra, and a spatial binning
to simulate the effects of unresolved structure. The results are compared through the use of quantitative metrics and performance
maps. We find that while no algorithm severely propagates the effects of Poisson noise beyond very local influences, some
algorithms are more robust against high photon-noise levels than others. In the case of limited spatial resolution, loss of
information regarding fine-scale structure can easily result in erroneous solutions. Our tests imply that photon noise and
limited spatial resolution can act so as to make assumptions used in some ambiguity resolution algorithms no longer consistent
with the observed magnetogram. We confirm a finding of the earlier comparison study that results can be very sensitive to
the details of the treatment of the observed boundary and the assumptions governing that treatment. We discuss the implications
of these findings, given the relative sensitivities of the algorithms to the two sources of noise tested here. We also touch
on further implications for interpreting observational vector magnetic field data for general solar physics research. 相似文献
2.
R. Presura V. V. Ivanov Y. Sentoku V. I. Sotnikov P. J. Laca N. Le Galloudec A. Kemp R. Mancini H. Ruhl A. L. Astanovitskiy T. E. Cowan T. Ditmire C. Chiu W. Horton P. Valanju S. Keely 《Astrophysics and Space Science》2005,298(1-2):299-303
An experimental simulation of planetary magnetospheres is being developed to investigate the formation of collisionless shocks
and their effects. Two experimental situations are considered. In both, the solar wind is simulated by laser ablation plasmas.
In one case, the “solar wind” flows across the magnetic field of a high-current discharge. In the other, a transverse magnetic
field is embedded in the plasma flow, which interacts with a conductive obstacle. The ablation plasma is created using the
“Tomcat” laser, currently emitting 5 J in a 6 ns pulse at 1 μm wavelength and irradiance above 1013 W/cm2. The “Zebra” z-pinch generator, with load current up to 1 MA and voltage up to 3.5 MV produces the magnetic fields. Hydrodynamic modeling
is used to estimate the plasma parameters achievable at the front of the plasma flow and to optimize the experiment design.
Particle-in-cell simulations reveal details of the interaction of the “solar wind” with an external magnetic field, including
flow collimation and heating effects at the stopping point. Hybrid simulations show the formation of a bow shock at the interaction
of a magnetized plasma flow with a conductor. The plasma density and the embedded field have characteristic spatial modulations
in the shock region, with abrupt jumps and fine structure on the skin depth scale. 相似文献
3.
“Elementary bursts” refer to fine time structures on scales of tens of milli-second to a few seconds in flare radiations.
In this paper, we investigate temporal and spatial properties of elementary bursts by exploiting high-cadence Hα (100 ms)
and hard X-ray (125 – 500 ms) observations of an impulsive flare on March 16, 2000. We find that the time scale of 2 – 3 s
is likely an upper limit of the elementary bursts in this event, at which hard X-ray emissions observed by different instruments
correlate, low energy (≤30 keV) hard X-rays and Hα flux correlate, and Hα emissions at conjugate flare kernels correlate.
From our methods, and also largely limited by instrument resolutions, there is a weak indication of existence of sub-second
structures. With the high-resolution Hα data, we also attempt to explore the spatial structure of “elementary bursts” by determining
the average spatial displacement of Hα peak emission between successive “elementary bursts” defined from hard X-ray light
curves. We find that, at the time scale of 3 s, the smallest spatial scale, as limited by the imaging resolution, is about
0.4″. We discuss these results with respect to mechanisms of fragmented magnetic energy release. 相似文献
4.
A magnetic field model is constructed for the extremely slow rotator γEqu based on measurements of its magnetic field over
many years and using the “magnetic charge” method. An analysis of γEqu and of all the data accumulated up to the present on
the magnetic field parameters of chemically peculiar stars leads to some interesting conclusions, of which the main ones are:
the fact that the axis of rotation and the dipole axis are not parallel in γEqu and the other slowly rotating magnetic stars
which we have studied previously is one of the signs that the braking of CP stars does not involve the participation of the
magnetic field as they evolve “to the main sequence.” The axes of the magnetic field dipole in slow rotators are oriented
arbitrarily with respect to their axes of rotation. The substantial photometric activity of these CP stars also argues against
these axes being close. The well-known absence of sufficiently strong magnetic fields in the Ae/Be Herbig stars also presents
difficulties for the hypothesis of “magnetic braking” in the “pre-main sequence” stages of evolution. The inverse relation
between the average surface magnetic field Bs and the rotation period P is yet another fact in conflict with the idea that
the magnetic field is involved in the braking of CP stars. We believe that angular momentum loss involving the magnetic field
can hardly have taken place during evolution immediately prior “to the main sequence,” rather the slow rotation of CP stars
most likely originates from protostellar clouds with low angular momentum. Some of the slowly rotating stars have a central
dipole magnetic field configuration, while others have a displaced dipole configuration, where the displacement can be toward
the positive or the negative magnetic pole.
__________
Translated from Astrofizika, Vol. 49, No. 2, pp. 251–262 (May 2006). 相似文献
5.
H. P. Jones G. A. Chapman K. L. Harvey J. M. Pap D. G. Preminger M. J. Turmon S. R. Walton 《Solar physics》2008,248(2):323-337
Physical understanding of total and spectral solar irradiance variation depends upon establishing a connection between the
temporal variability of spatially resolved solar structures and spacecraft observations of irradiance. One difficulty in comparing
models derived from different data sets is that the many ways for identifying solar features such as faculae, sunspots, quiet
Sun, and various types of “network” are not necessarily consistent. To learn more about classification differences and how
they affect irradiance models, feature “masks” are compared as derived from five current methods: multidimensional histogram
analysis of NASA/National Solar Observatory/Kitt Peak spectromagnetograph data, statistical pattern recognition applied to
SOHO/Michelson Doppler Imager photograms and magnetograms, threshold masks allowing for influence of spatial surroundings
applied to NSO magnetograms, and “one-trigger” and “three-trigger” algorithms applied to California State University at Northridge
Cartesian Full Disk Telescope intensity observations. In general all of the methods point to the same areas of the Sun for
labeling sunspots and active-region faculae, and available time series of area measurements from the methods correlate well
with each other and with solar irradiance. However, some methods include larger label sets, and there are important differences
in detail, with measurements of sunspot area differing by as much as a factor of two. The methods differ substantially regarding
inclusion of fine spatial scale in the feature definitions. The implications of these differences for modeling solar irradiance
variation are discussed.
K.L. Harvey and S.R. Walton are deseased, to whom this paper is dedicated. 相似文献
6.
Yu. V. Glagolevskij 《Astrophysics》2010,53(4):536-543
Models of the magnetic field of the He-w star HD142301 are constructed. Observational data are well described by the model
of a dipole shifted by 0.6 stellar radii transverse to the axis. The phase dependence of the HeI λ4026? line, however, corresponds
better to a model assuming the presence of four monopoles (or two dipoles) shifted by 0.4 stellar radii from the center. The
distance between the monopoles in both models is comparable to the star’s radius, which indicates that the source of the magnetic
field is “long” dipoles, rather than “point” dipoles. 相似文献
7.
In a previous study (Cane and Richardson, J. Geophys. Res.
108(A4), SSH6-1, 2003), we investigated the occurrence of interplanetary coronal mass ejections in the near-Earth solar wind during 1996 – 2002,
corresponding to the increasing and maximum phases of solar cycle 23, and provided a “comprehensive” catalog of these events.
In this paper, we present a revised and updated catalog of the ≈300 near-Earth ICMEs in 1996 – 2009, encompassing the complete
cycle 23, and summarize their basic properties and geomagnetic effects. In particular, solar wind composition and charge state
observations are now considered when identifying the ICMEs. In general, these additional data confirm the earlier identifications
based predominantly on other solar wind plasma and magnetic field parameters. However, the boundaries of ICME-like plasma
based on charge state/composition data may deviate significantly from those based on conventional plasma/magnetic field parameters.
Furthermore, the much studied “magnetic clouds”, with flux-rope-like magnetic field configurations, may form just a substructure
of the total ICME interval. 相似文献
8.
Marc W. Pound Jave O. Kane Bruce A. Remington Dmitri D. Ryutov Akira Mizuta Hideaki Takabe 《Astrophysics and Space Science》2005,298(1-2):177-181
Over the past few years, our group has been developing hydrodynamic models to simulate formation of the Eagle Nebula pillars.
The true test of any model is, of course, how well it can reproduce the observations. Here, we discuss how we go about testing
our models against observations. We describe the process by which we “observe” the model data to create synthetic maps. We
show an example of this technique using one of our model runs and compare the resultant synthetic map to the real one. 相似文献
9.
Thomas R. Metcalf Marc L. DeRosa Carolus J. Schrijver Graham Barnes Adriaan A. van Ballegooijen Thomas Wiegelmann Michael S. Wheatland Gherardo Valori James M. McTtiernan 《Solar physics》2008,247(2):269-299
We compare a variety of nonlinear force-free field (NLFFF) extrapolation algorithms, including optimization, magneto-frictional,
and Grad – Rubin-like codes, applied to a solar-like reference model. The model used to test the algorithms includes realistic
photospheric Lorentz forces and a complex field including a weakly twisted, right helical flux bundle. The codes were applied
to both forced “photospheric” and more force-free “chromospheric” vector magnetic field boundary data derived from the model.
When applied to the chromospheric boundary data, the codes are able to recover the presence of the flux bundle and the field’s
free energy, though some details of the field connectivity are lost. When the codes are applied to the forced photospheric
boundary data, the reference model field is not well recovered, indicating that the combination of Lorentz forces and small
spatial scale structure at the photosphere severely impact the extrapolation of the field. Preprocessing of the forced photospheric
boundary does improve the extrapolations considerably for the layers above the chromosphere, but the extrapolations are sensitive
to the details of the numerical codes and neither the field connectivity nor the free magnetic energy in the full volume are
well recovered. The magnetic virial theorem gives a rapid measure of the total magnetic energy without extrapolation though,
like the NLFFF codes, it is sensitive to the Lorentz forces in the coronal volume. Both the magnetic virial theorem and the
Wiegelmann extrapolation, when applied to the preprocessed photospheric boundary, give a magnetic energy which is nearly equivalent
to the value derived from the chromospheric boundary, but both underestimate the free energy above the photosphere by at least
a factor of two. We discuss the interpretation of the preprocessed field in this context. When applying the NLFFF codes to
solar data, the problems associated with Lorentz forces present in the low solar atmosphere must be recognized: the various
codes will not necessarily converge to the correct, or even the same, solution.
On 07/07/2007, the NLFFF team was saddened by the news that Tom Metcalf had died as the result of an accident. We remain grateful
for having had the opportunity to benefit from his unwavering dedication to the problems encountered in attempting to understand
the Sun’s magnetic field; Tom had completed this paper several months before his death, leading the team through the many
steps described above. 相似文献
10.
Recent studies of the solar-terrestrial environment for the past 500 years have necessitated the use of a variety of historical
databases: nitrates in ice cores, knowledge of large volcanic eruptions, sunspot numbers, mid-latitude aurora and geomagnetic
records. The nitrate data are being used to identify large solar proton fluence events. The volcanic record helps to provide
time markers for the ice core. The records of major geomagnetic storms and mid-latitude aurora have been used for additional
identification. We also know that the Earth’s magnetic field is evolving with a present rapid decrease in magnitude. In addition
the wandering magnetic pole must be considered in ascertaining what was “mid latitude” in historic times versus “mid latitude”
in 2000. We illustrate how these databases are being used in recent studies of historic solar proton events. 相似文献
11.
In this work a new information resource located at http://www.gao.spb.ru/database/esai and hereinafter referred to as ESAI (“Extended time series of Solar Activity Indices”) is presented. ESAI includes observational,
synthetic and simulated sets to study solar magnetic field variations and their influence on the Earth. ESAI extends the ordinary
lengths of some traditional indices, parameterizing time variations of physically different characteristics of solar activity.
In particular, long-term sets of the following indices are presented: sunspot areas, the Wolf numbers, polar faculae numbers,
sunspot mean latitudes and north-south asymmetry of hemispheres for different components of activity. Some methods for making
correct conclusions from incomplete data and some criteria to estimate the reliability of the obtained information are discussed. 相似文献
12.
An Evolving Synoptic Magnetic Flux map and Implications for the Distribution of Photospheric Magnetic Flux 总被引：1，自引：0，他引：1
We describe a procedure intended to produce accurate daily estimates of the magnetic flux distribution on the entire solar surface. Models of differential rotation, meridional flow, supergranulation, and the random emergence of background flux elements are used to regularly update unobserved or poorly observed portions of an initial traditional magnetic synoptic map that acts as a seed. Fresh observations replace model estimates when available. Application of these surface magnetic transport models gives us new insight into the distribution and evolution of magnetic flux on the Sun, especially at the poles where canopy effects, limited spatial resolution, and foreshortening result in poor measurements. We find that meridional circulation has a considerable effect on the distribution of polar magnetic fields. We present a modeled polar field distribution as well as time series of the difference between the northern and southern polar magnetic flux; this flux imbalance is related to the heliospheric current sheet tilt. We also estimate that the amount of new background magnetic flux needed to sustain the `quiet-Sun' magnetic field is about 1.1×1023 Mx d–1 (equivalent to several large active regions) at the spatial resolution and epoch of our maps. We comment on the diffusive properties of supergranules, ephemeral regions, and intranetwork flux. The maps are available on the NSO World Wide Web page. 相似文献
13.
“EIT waves” are a wavelike phenomenon propagating in the corona, which was initially observed in the extreme ultraviolet (EUV)
wavelength by the EUV Imaging Telescope (EIT). Their nature is still elusive, with the debate on-going between fast-mode wave
model and non-wave model. In order to distinguish between these models, we investigate the relation between the EIT wave velocity
and the local magnetic field in the corona. It is found that the two parameters show significant negative correlation in most
of the EIT wave fronts, i.e., the EIT wave propagates more slowly in the regions of stronger magnetic field. Such a result poses a big challenge to the
fast-mode wave model, which would predict a strong positive correlation between the two parameters. However, it is demonstrated
that such a result can be explained by the fieldline stretching model, i.e., that “EIT waves” are the propagation of apparent brightenings, which are generated by successive stretching of closed magnetic
field lines pushed by the erupting flux rope during coronal mass ejections (CMEs). 相似文献
14.
Martín López-Corredoira 《Journal of Astrophysics and Astronomy》2007,28(2-3):101-116
The Galactic foreground contamination in CMBR anisotropies, especially from the dust component, is not easily separable from
the cosmological or extragalactic component. In this paper, some doubts will be raised concerning the validity of the methods
used until now to remove Galactic dust emission and will show that none of them achieves its goal.
First, I review the recent bibliography on the topic and discuss critically the methods of foreground subtraction: the cross-correlation
with templates, analysis assuming the spectral shape of the Galactic components, the “maximum entropy method”, “internal linear
combination”, and “wavelet-based high resolution fitting of internal templates”. Second, I analyse the Galactic latitude dependence
from WMAP data. The frequency dependence is discussed with data in the available literature. The result is that all methods
of subtracting the Galactic contamination are inaccurate. The Galactic latitude dependence analysis or the frequency dependence
of the anisotropies in the range 50–250 GHz put a constraint on the maximum Galactic contribution in the power spectrum to
be less than ∼ 10% (68% C. L.) for an ∼ 1 degree scale, and possibly higher for larger scales.
The origin of most of the signals in the CMBR anisotropies is not Galactic. In any case, the subtraction of the galaxy is
not accurate enough to allow a “precision Cosmology”; other sources of contamination (extragalactic, solar system) are also
present. 相似文献
15.
Christopher M. Wright 《Astrophysics and Space Science》2007,311(1-3):47-55
The position angle of mid-infrared polarisation can be directly related to the magnetic field direction projected onto the
plane of the sky. Such observations, from both ground and space-based platforms, have been used to investigate the relation
between the magnetic field and other “symmetry axes” associated with embedded young stars, such as the interstellar magnetic
field, bipolar outflow and disk/toroid axes. Interpretation of the results in terms of hydromagnetic driving mechanisms of
bipolar outflow is discussed. 相似文献
16.
The vortex structure of the “npe” phase of neutron stars with a 3P2 superfluid neutron condensate of Cooper pairs is discussed. It is shown that, as the star rotates, superfluid neutron vortex
filaments described by a unitary ordering parameter develop in the “npe” phase. The entrainment of superconducting protons
by the rotating superfluid neutrons is examined. The entrainment effect leads to the appearance of clusters of proton vortices
around each neutron vortex and generates a magnetic field on the order of 1012 G. 3P2 neutron vortex filaments combine with quark semi-superfluid vortex filaments at the boundary of the “npe” and “CFL” phases.
At the boundary of the “Aen” and “npe” phases, they combine with 1S0 neutron vortex filaments. In this way, a unified vortex structure is formed. The existence of this structure and its collective
elastic oscillations explain the observed oscillations in the angular rotation velocity of pulsars. 相似文献
17.
A number of independent arguments indicate that the toroidal flux system responsible for the sunspot cycle is stored at the
base of the convection zone in the form of flux tubes with field strength close to 105 G. Although the evidence for such strong fields is quite compelling, how such field strength can be reached is still a topic
of debate. Flux expulsion by convection should lead to about the equipartition field strength, but the magnetic energy density
of a 105-G field is two orders of magnitude larger than the mean kinetic energy density of convective motions. Line stretching by
differential rotation (i.e., the “Ω effect” in the classical mean-field dynamo approach) probably plays an important role, but arguments based on energy considerations show that it does not seem feasible
that a 105-G field can be produced in this way. An alternative scenario for the intensification of the toroidal flux system in the overshoot
layer is related to the explosion of rising, buoyantly unstable magnetic flux tubes, which opens a complementary mechanism for magnetic-field intensification.
A parallelism is pointed out with the mechanism of “convective collapse” for the intensification of photospheric magnetic
flux tubes up to field strengths well above equipartition; both mechanisms, which are fundamentally thermal processes, are
reviewed. 相似文献
18.
The Transition Region and Coronal Explorer (TRACE) instrument includes a “white light” imaging capability with novel characteristics.
Many flares with such white-light emission have been detected, and this paper provides an introductory overview of these data.
These observations have 0.5″ pixel size and use the full broad-band response of the CCD sensor; the images are not compromised
by ground-based seeing and have excellent pointing stability as well as high time resolution. The spectral response of the
TRACE white-light passband extends into the UV, so these data capture, for the first time in images, the main radiative energy
of a flare. This initial survey is based on a sample of flares observed at high time resolution for which the Reuven Ramaty
High-Energy Solar Spectroscopic Imager (RHESSI) had complete data coverage, a total of 11 events up to the end of 2004. We
characterize these events in terms of source morphology and contrast against the photosphere. We confirm the strong association
of the TRACE white-light emissions - which include UV as well as visual wavelengths – with hard X-ray sources observed by
RHESSI. The images show fine structure at the TRACE resolution limit, and often show this fine structure to be extended over
large areas rather than just in simple footpoint sources. The white-light emission shows strong intermittency both in space
and in time and commonly contains features unresolved at the TRACE resolution. We detect white-light continuum emission in
flares as weak as GOES C1.6. limited by photon statistics and background solar fluctuations, and support the conclusion of
Neidig (1989) that white-light continuum occurs in essentially all flares. 相似文献
19.
Location and parameters of a microwave millisecond spike event 总被引：1，自引：0，他引：1
A typical microwave millisecond spike event on November 2, 1997 was observed by the radio spectrograph of National Astronomical
Observatories (NAOs) at 2.6–3.8 GHz with high time and frequency resolution. This event was also recorded by Nobeyama Radio
Polarimeters (NoRP) at 1–35 GHz and Radio Heliograph (NoRH) at 17 GHz. The source at 17 GHz is located in one foot-point of
a small bright coronal loop of YOHKOH SXT and SOHO EIT images with strong photospheric magnetic field in SOHO MDI magnetograph.
It is assumed that the electron cyclotron maser instability and gyro-resonance absorption dominate, respectively, the rising
and decay phase of the spike event. For different harmonic number of gyro-frequency or magnetic field strength, a fitting
program with free plasma parameters is used to minimize the difference between the observational and theoretical values of
the exponential growth and decay rates for a given spike. The plasma parameters at third harmonic number are more comparable
to their typical values in solar corona. Hence, it is able to provide a diagnosis for the source parameters (magnetic field,
density, and temperature), the properties of radiations (wave vector and propagation angle), and the properties of non-thermal
electrons (density, pitch angle, and energy). The results are also comparable with the diagnosis of the gyro-synchrotron radiation
model, the frequency drift rates and a dipole magnetic field model, as well as the YOHKOH SXT and SOHO MDI data.
This study is supported by the NFSC project nos. 10333030 and 10273025, and “973” program with no. G2000078403. 相似文献
20.
Pranab Ghosh 《Journal of Astrophysics and Astronomy》1995,16(2):289-305
I review our understanding of the evolution of the spin periods of neutron stars in binary stellar systems, from their birth
as fast, spin-powered pulsars, through their middle life as accretion-powered pulsars, upto their recycling or “rebirth” as
spin-powered pulsars with relatively low magnetic fields and fast rotation. I discuss how the new-born neutron star is spun
down by electromagnetic and “propeller” torques, until accretion of matter from the companion star begins, and the neutron
star becomes an accretion-powered X-ray pulsar. Detailed observations of massive radio pulsar binaries like PSR 1259-63 will
yield valuable information about this phase of initial spindown. I indicate how the spin of the neutron star then evolves
under accretion torques during the subsequent phase as an accretion-powered pulsar. Finally, I describe how the neutron star
is spun up to short periods again during the subsequent phase of recycling, with the accompanying reduction in the stellar
magnetic field, the origins of which are still not completely understood. 相似文献