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1.
A study of the background corona near solar minimum 总被引：2，自引：0，他引：2
The white light coronagraph data from Skylab is used to investigate the equatorial and polarK andF coronal components during the declining phase of the solar cycle near solar minimum. Measurements of coronal brightness and polarization brightness product between 2.5 and 5.5R
during the period of observation (May 1973 to February 1974) lead to the conclusions that: (1) the equatorial corona is dominated by either streamers or coronal holes seen in projections on the limb approximately 50% and 30% of the time, respectively; (2) despite the domination by streamers and holes, two periods of time were found which were free from the influences of streamers or holes (neither streamers nor holes were within 30° in longitude of the limb); (3) the derived equatorial background density model is less than 15% below the minimum equatorial models of Newkirk (1967) and Saito (1970); (4) a spherically symmetric density model for equatorial coronal holes yields densities one half those of the background density model; and (5) the inferred brightness of theF-corona is constant to within ±10% and ±5% for the equatorial and polar values, respectively, over the observation period. While theF-corona is symmetric at 2R
it begins to show increasing asymmetry beyond this radius such that at 5R
the equatorialF-coronal brightness is 25% greater than the polar brightness.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
2.
The radio observations of the coronal streamers obtained using Clark Lake radioheliograph at 73.8, 50.0, and 38.5 MHz during a period of minimum activity in September 1986 are presented. Streamers appear to correlate with two prominent disk sources whose intensities fluctuated randomly. The variations in half-power diameter of the radio Sun are found to correspond with the variations in the white-light extents of the coronal streamers. It appears that the shape of the radio Sun is not a function of the phase of the solar cycle; instead it depends on the relative positions of the streamers in the corona. The observed peak brightness temperatures,T
B
, of the streamers are found to be very low, being 6 × 104 K.We compute the brightness temperature distribution along the equator by tracing the rays in the coronal plasma. The rays are deflected away by the streamers before reaching the critical density level, whereas they penetrate deeper into the coronal hole for small angles between the line of sight and the streamer axis. As a consequence, it is found that the streamers and coronal holes appear in the calculated equatorial brightness distribution as irregular brightness depressions and enhancements, respectively. The fine structures are found to disappear when the scattering due to small-scale density inhomogeneities is included in the ray-tracing calculations. The required relative level of density fluctuations, 1 = N/N, is found to be greater than 12% to reduce the peak brightness temperature from 106 K to 6 × 104 K for all the three frequencies.On leave from Indian Institute of Astrophysics, Bangalore 560034, India. 相似文献
3.
The 1968 Abastumani Astrophysical Observatory Solar Eclipse Expedition obtained photographic records of the polarization and intensity of the solar corona on September 22 from a site of the South Urals (Western Siberia). A photometric study of the corona was carried out. Polarization has been computed both in a total corona and in some of its streamers. The coronal intensity I
k
and I
F
components are separated. Electron concentrations and temperatures are computed. 相似文献
4.
During a balloon flight in France on September 13, 1971, at altitude 32 000 m, the solar corona was cinematographed from 2 to 5R
during 5 hr, with an externally occulted coronagraph.Motions in coronal features, when they occur, exhibit deformations of structures with velocities not exceeding a few 10 km s–1; several streamers were often involved simultaneously; these variations are compatible with magnetic changes or sudden reorganizations of lines of forces.Intensity and polarization measurements give the electron density with height in the quiet corona above the equator. Electron density gradient for one of the streamers gives a temperature of 1.6 × 106 K and comparisons with the on-board Apollo 16 coronal observation of 31 July, 1971 are compatible with the extension of this temperature up to 25 R
bd.Three-dimensional structures and localizations of the streamers are deduced from combined photometry, polarimetry and ground-based K coronametry. Three of the four coronal streamers analysed have their axis bent with height towards the direction of the solar rotation, as if the upper corona has a rotation slightly faster than the chromosphere. 相似文献
5.
Large-scale coronal structures (helmet streamers) observed in the white-light corona during total solar eclipses and/or with
ground-based coronagraphs are mostly located only above quiescent types of prominences. These helmet streamers are maintained
due to the magnetic fields of the Sun. Time–latitudinal distribution of prominences during a solar cycle, however, shows both
the poleward and equatorward migrations, similar to the 530.3 nm emission corona (the green corona) intensities. Distribution
of observed coronal helmet streamers during total solar eclipses, enlarged with the helmet streamers as were obtained by the
ground-based coronagraph observations, are compared with the heliographic distribution of prominences and the green corona
intensities for the first time. It is shown that the distribution of above-mentioned helmet streamers, reflects – roughly
– the time–latitudinal distribution of prominences and emission corona branches, and migrates together with them over a solar
cycle. 相似文献
6.
Leonard Strachan 《Journal of Astrophysics and Astronomy》2008,29(1-2):167-170
Measurements made with the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory can be used to determine physical parameters in the solar corona such as hydrogen and ion kinetic temperatures, electron densities, and absolute elemental abundances. Hydrogen and ion outflow velocities can be determined by combining the UV spectroscopic measurements with white light polarized brightness measurements. These combined measurements can be used to reveal physical characteristics of coronal streamers. To date we have studied plasma properties, such as the variation of plasma outflows in quiescent streamers, primarily in classic helmet streamers at solar minimum. Out-flows have not been observed in the centers of coronal streamers suggesting that these are closed magnetic field regions. We propose to study all of the coronal streamers in the UVCS synoptic dataset in order to investigate different types of streamers and their long-term evolution. 相似文献
7.
Unipolar streamers (also known as pseudo-streamers) are coronal structures that, at least in coronagraph images, and when
viewed at the correct orientation, are often indistinguishable from dipolar (or “standard”) streamers. When interpreted with
the aid of a coronal magnetic field model, however, they are shown to consist of a pair of loop arcades. Whereas dipolar streamers
separate coronal holes of the opposite polarity and whose cusp is the origin of the heliospheric current sheet, unipolar streamers
separate coronal holes of the same polarity and are therefore not associated with a current sheet. In this study, we investigate
the interplanetary signatures of unipolar streamers. Using a global MHD model of the solar corona driven by the observed photospheric
magnetic field for Carrington rotation 2060, we map the ACE trajectory back to the Sun. The results suggest that ACE fortuitously
traversed through a large and well-defined unipolar streamer. We also compare heliospheric model results at 1 AU with ACE
in-situ measurements for Carrington rotation 2060. The results strongly suggest that the solar wind associated with unipolar streamers
is slow. We also compare predictions using the original Wang–Sheeley (WS) empirically determined inverse relationship between
solar wind speed and expansion factor. Because of the very low expansion factors associated with unipolar streamers, the WS
model predicts high speeds, in disagreement with the observations. We discuss the implications of these results in terms of
theories for the origin of the slow solar wind. Specifically, premises relying on the expansion factor of coronal flux tubes
to modulate the properties of the plasma (and speed, in particular) must address the issue that while the coronal expansion
factors are significantly different at dipolar and unipolar streamers, the properties of the measured solar wind are, at least
qualitatively, very similar. 相似文献
8.
Gary D. Parker 《Solar physics》2009,257(1):155-167
During the descent of Ulysses following the 2001 solar north pole passage, the SOHO LASCO C2 telescope recorded a particularly strong sequence of recurrent
polarization brightness (pB) features at latitudes of around 55°. As Ulysses passed overhead, solar rotation swept the interplanetary extensions of these persistent coronal structures over the spacecraft.
Comparison of solar remote sensing and Ulysses
in situ observations through 2002 reveals the solar wind effects of very bright and recurrent K-coronal structures at high solar
latitudes and of a steeply inclined heliospheric neutral sheet (HNS). Despite the high level of solar activity, the HNS at
high latitude still organizes solar wind stream structure much as it did near the previous solar minimum. The recurrent coronal
streamers originate slow solar wind and mark the northern extremity of a very tilted HNS whose passage at Ulysses is accompanied by slow, dense solar wind, enhanced temperature, depressed α abundance, enhanced magnetic fields, and magnetic field directional changes that evolve with spacecraft latitude. 相似文献
9.
The results of the white-light polarization measurements performed during three solar eclipses (1973, 1980, 1991) are presented. The eclipse images were processed and analysed by the same technique and method and, consequently, the distributions of the polarization and coronal intensity around the Sun were obtained in unified form for all three solar eclipses. The mutual comparisons of our results, and their comparison with the distributions found by other authors, allowed the real accuracy of the current measurements of the white-light corona polarization, which is not worse than ±5%, to be estimated. We have investigated the behaviour of the polarization in dependence on heliocentric distance in helmet streamers and coronal holes. Simultaneous interpretation of the data on polarization and intensity in white-light helmet streamers is only possible if a considerable concentration of coronal matter (plasma) towards the plane of the sky is assumed. The values obtained for the coronal hole regions can be understood within the framework of a spherically symmetrical model of the low density solar atmosphere. A tendency towards increasing polarization in coronal holes, connected with the decrease of the hole's size and with the transition from the minimum to the maximum of the solar cycle, was noticed. The problem of how the peculiarities of the large-scale coronal structures are related to the orientation of the global (dipole) solar magnetic field and to the degree of the goffer character of the coronal and interplanetary current sheet is discussed briefly. 相似文献
10.
The structure and the shape of the solar corona during the total solar eclipse on March 29, 2006 were studied. The corona was of the intermediate pre-minimal type, it had northern and southern polar ray systems above the polar coronal holes and six streamers of different brightness in the middle and low heliographic latitudes. All the coronal structural features were found to have counterparts at the photosphere-chromosphere level on the limb and near it. The corona’s photometric flattening index was equal to 0.17. 相似文献
11.
G. D. Parker 《Solar physics》1987,108(1):77-87
Long-lived brightness structures in the solar electron corona persist over many solar rotation periods and permit an observational determination of coronal magnetic tracer rotation as a function of latitude and height in the solar atmosphere. For observations over 1964–1976 spanning solar cycle 20, we compare the latitude dependence of rotation at two heights in the corona. Comparison of rotation rates from East and West limbs and from independent computational procedures is used to estimate uncertainty. Time-averaged rotation rates based on three methods of analysis demonstrate that, on average, coronal differential rotation decreases with height from 1.125 to 1.5 R
S. The observed radial variation of differential rotation implies a scale height of approximately 0.7 R
S for coronal differential rotation.Model calculations for a simple MHD loop show that magnetic connections between high and low latitudes may produce the observed radial variations of magnetic tracer rotation. If the observed tracer rotation represents the rotation of open magnetic field lines as well as that of closed loops, the small scale height for differential rotation suggests that the rotation of solar magnetic fields at the base of the solar wind may be only weakly latitude dependent. If, instead, closed loops account completely for the radial gradients of rotation, outward extrapolation of electron coronal rotation may not describe magnetic field rotation at the solar wind source. Inward extrapolations of observed rotation rates suggest that magnetic field and plasma are coupled a few hundredths of a solar radius beneath the photosphere. 相似文献
12.
A rocket-borne coronagraph utilizing external occulting disks was used to photograph the solar corona from 3 to 9 R
s at 1931 UT on 7 March, 1970. Comparison of the rocket and ground-based observations shows a one-to-one correspondence between major streamers from the inner to the outer corona. In particular streamers over the poles are clearly visible against the background corona from 3 to 8 R
s. These rocket data had a scattered light level of 1.2 × 10–10
B
s. The derived quiet equatorial and polar K + F corona was within 10% of the absolute brightness of standard coronal models and displayed identical radial gradients to those models. The photometric profiles of the NE limb streamer were analyzed assuming a model in which the core density follows a Gaussian distribution in directions perpendicular to the radius vector. This streamer was assumed to be rooted on the visible disk at 55 to 60° from the plane-of-the-sky as based on K-coronameter and XUV data. An uncertainty of as much as a factor of three still remains for the value of the axis density owing to uncertainty in the line-of-sight dimension of the streamer. 相似文献
13.
Shape and structure of the solar corona during the August 1, 2008, total solar eclipse is reported. The August 1, 2008, corona
is classified as of near-minimum type with well developed northern and southern polar ray systems over polar coronal holes
and several streamers of different brightness at the middle and low heliographic latitude. The flattening index was found
to be 0.21. 相似文献
14.
Photographs of the corona in the continuum spectrum (580–700 nm) have been obtained with a doublet camera (F=4m) through rotating sector polarizers with the vibrational directions oriented radially and tangentially to the solar limb. Isophotes of the total emission and its polarized component, as well as diagrams giving the degree of polarization for the (K+F)-corona and the electron corona proper (P
K), have been plotted up to the distance of 1R
from the limb. In some inner regions the polarizationP
K exceeds the highest value possible for Thomson scattering under the given conditions. The reality of this anomally is dubious; systematic photometrical errors may account for it. Three-dimensional forms of 15 different coronal rays have been ascertained by comparing the course of polarization along the rays to the family of Baumach curves. The rays are found to deflect substantially from the radial directions out of the plane of the sky. Mean values of the coronal brightness and polarization versus the distance from the Sun have been determined. Contrary to well-known models (Van de Hulst) the mean polarization of the electron corona (P
K) decreases with distance after reaching the maximum (50%) at (1.4–1.6)R
from the Sun's centre. This decrease can be explained by deflection of the streamers from radial directions. 相似文献
15.
A. N. Afanasiev 《Solar physics》2006,238(1):87-104
Using numerical simulation, we investigate the possibility that echo components of type IIId solar decameter bursts can be
produced through refraction of radio emission in a coronal plasma with large-scale regular electron density inhomogeneities.
It is shown that the observed time profiles of radio burst intensity with three or four maxima can be related to the presence
in the middle corona of streamers and a localized regular nonuniformity whose electron density exceeds background electron
density by a factor of more than 3 – 4. The nonuniformity scale in this case is comparable with the optical diameter of the
solar disk. The observed radio burst echo components with delays longer than 3 s are explained by the production of additional
radio emission propagation modes within a “transverse" refraction waveguide arising between the localized electron density
nonuniformity and deeper layers in the corona. As these additional modes are reflected from the streamers, they can reach
the Earth. Calculations of the time profile of radio burst intensity take into consideration the influence of scattering by
turbulent coronal inhomogeneities and of collisional absorption. Comparison of the modeling results with the observational
data shows that the calculated values of some profile parameters differ from the observed values. One of the possible reasons
is that the method used does not take into account the diffraction leakage of radio emission through the large-scale nonuniformity. 相似文献
16.
In order to study the solar corona during eclipses, a new telescope was constructed. Three coronal images were obtained simultaneously
through a single objective of the telescope as the coronal radiation passed through three polarizers (whose transmission directions
were turned 0°, 60°, and 120° in the chosen direction); one image was obtained without a polarizer. The telescope was used
to observe the solar corona during the eclipse of 1 August 2008. We obtained the distributions of polarization brightness,
K-corona brightness, the degree of K-corona polarization and the total polarization degree; the polarization direction, depending
on the latitude and radius in the plane of the sky, was also obtained. We calculated the radial distributions of electron
density depending on the latitude. The properties of all these distributions were compared for different coronal structures.
We determined the temperature of the coronal plasma in different coronal structures assuming hydrostatic equilibrium. 相似文献
17.
During the eclipse of 12 November 1966, the solar corona was photographed at an effective wavelength of 6500 Å with an f/16, 11.1 cm aperture camera. Reduction of the observations yields coronal radiances and polarizations from 1.4 to 3.5 solar radii. Standard techniques are used for the separation of F and K-coronas, determination of coronal electron densities and temperatures, and estimation of the orientation of the major streamers in space.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
18.
A large equatorial coronal streamer observed in the outer corona (3R
) grew in brightness and size during successive limb passages between October 6, 1973 and January 10, 1974 (solar rotations 1606–1611). Unlike previous studies of streamers and their photospheric associations, no definite surface feature could be identified in the present case. This suggests that the streamer is associated with the large scale photospheric magnetic field. Comparison of the streamer growth with observed underlying photospheric magnetic flux changes indicated that as the streamer increased in brightness, areal extent, and density, the photospheric magnetic flux decreased. Three possible explanations for the streamer's growth are presented; the conceptually simplest being that the decrease in photospheric field results in an opening of the flux tubes under the streamer which permits an increased mass flux through the streamer.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
19.
K. R. Sivaraman M. Jayachandran K. K. Scaria G. S. D. Babu S. P. Bagare A. P. Jayarajan 《Journal of Astrophysics and Astronomy》1984,5(2):149-158
During the eclipse of 1980 February 16 we photographed the solar corona at an effective wavelength of 6300 å. Using a quadruple camera we also obtained the coronal pictures in polarized light for four Polaroid orientations. We have used these observations to derive the coronal brightness and polarization and from these the electron densities in the corona out to a distance of about 2.5 R⊙ from the centre of the disc. The coronal brightness matches well with that of the corona of 1958 October 12. 相似文献
20.
We observed Faraday rotation of linearly polarized radio waves from the Crab Nebula (Tau A) at 4170 MHz during solar coronal occultations in June 1971–75. Mean amplitudes of the variations of position angle are larger in an active phase of the solar cycle than in a quiet phase. In occultations in 1971 and 1973, the position angle of the polarization varied oscillatory by 20–50 degrees due to local magnetic structures in the corona with a typical scale-length of about 0.5 R
. In 1974, we observed a typical variation of position angle of polarization which is expected from a Y-shaped field configuration in coronal streamers.The Faraday rotation is enhanced when the line of sight to Tau A passes through strong coronal magnetic fields computed from magnetograph observations, while the rotation is suppressed when the line of sight passes through large coronal holes observed in X-rays. Short-time oscillation of the rotation angle observed in 1971 and 1973 suggests that neutral sheets in coronal streamers oscillate at a period of 3 hours with an amplitude of 1 R
at a distance of 10 R
from the Sun. 相似文献