共查询到20条相似文献，搜索用时 31 毫秒
1.
The hydrodynamic properties of a steadily expanding corona are explored for situations in which departures from spherically symmetric outflow are large, in the sense that the geometrical cross section of a given flow tube increases outward from the Sun faster than r
2 in some regions. Assuming polytropic flow, it is shown that in certain cases the flow may contain more than one critical point. We derive the criterion for determining which of these critical points is actually crossed by the transonic solution which begins at the Sun and extends continuously outward. Next, we apply the theory to geometries which exhibit rapid spreading of the flow tubes in the inner corona, followed by more-or-less radial divergence at large distances. This is believed to be the type of geometry found in coronal hole regions. The results show that, if this initial divergence is sufficiently large, the outflow becomes supersonic at a critical point encountered low in the corona in the region of high divergence, and it remains supersonic at all greater heights in the corona. This feature strongly suggests that coronal hole regions differ from other open-field regions of the corona in that they are in a fast, low density expansion state over much of their extent. Such a dynamical configuration makes it possible to reconcile the low values of electron density observed in coronal holes with the large particle fluxes in the associated high speed streams seen in the solar wind.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
2.
By means of a photographic polarimeter, we attempted to measure both the amount and direction of linear polarization of all emission lines between 3400 and 9000 Å in the inner corona (1.034 r/r
0 1.085). Only the green and red coronal lines have been analyzed in detail. Neither of these lines shows polarization exceeding the probable error of 1.0% for 5303 and 1.8% for 6374. None of the other 17 coronal lines observed during the 7 March, 1970 solar eclipse show any obvious (>5%) polarization.Presently at the Sacramento Peak Observatory. 相似文献
3.
M. P. Nakada 《Solar physics》1977,51(2):327-343
Average solar wind properties at 1 AU either alone or together with the electron density distribution are used to obtain or review some results that relate coronal temperatures, temperature gradients, and compositions. Measured values of the temperature (T) and the temperature gradient parameter ( = -d ln T/d ln r) are used to find compositions that satisfy the equations used to obtain the results. The total energy equation may be satisfied if the thermal conductivity is reduced by considerable depletions of H+ in the corona. The electron energy equation only gives information on coronal compositions that are coupled with d/d ln r. The hydrostatic approximation (momentum equation) for the electron density distribution also appears to require considerable depletions of H+ in the corona. Results from the integrated momentum equation for the solar wind support the hydrostatic results; together, they give some information on the minimum value of in the outer corona. Some changes in assumptions or values of parameters that may modify these interpretations are discussed. 相似文献
4.
T. J. Bogdan 《Solar physics》1986,103(2):311-315
Previous efforts to construct solar coronal fields using surface magnetograph data have generally employed a least squares minimization technique in order to determine the spherical harmonic expansion coefficients of the magnetic scalar potential. Provided there is no source surface high up in the corona, we show that knowledge of the line-of-sight component of the surface magnetic field, B
i
= B
r
sin + B
cos , is sufficient to uniquely determine the potential coronal magnetic field by an explicit construction of the magnetic scalar potential for an arbitrary B
l
(, ).The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
5.
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
6.
Liouville's theorem for radiation, of which the generalized étendue is a consequence, implies 2 d2
d2
A = constant along the ray path, where is the refractive index and d2
and d2
A are the ranges, respectively, of solid angle and of area that define a ray (actually a bundle of rays). Implications of this concept on the propagation of radio waves from the actual to the apparent source in the solar corona (i.e., the scatter image of the true source) are discussed. The implications for sources of fundamental plasma radiation include: (1)The observed solid angle (defining the directivity) and apparent area A of the source are compatible with Liouville's theorem only if the apparent source (the scatter image of the true source) corresponds to the envelope of subsources with a small filling factor f. (2) The brightness temperature T
Bof the actual source is greater than that of the apparent source by f
-1. (3) For sources of fundamental plasma radiation the factor f is very small ( 10-2). (4) A long-standing discrepancy between the observed low value of T
B at meter/decameter wavelengths for the quiet Sun and the known coronal temperature may be resolved by noting that the implied coronal temperature is given by T
B
f and that the factor f must be significantly less than unity.A brief discussion is included of the relation between Liouville's theorem, the generalized étendue, Milne's laws, occupation numbers, extension in phase, and suppression of emission by a medium with refractive index unequal to unity. 相似文献
7.
Chen and Shawhan (1978) observed the brightness distribution of solar radio bursts at 26 MHz and fitted it to a double gaussian model. The small angle and large angle components were labelled core and halo. As an alternative to the scattering theory by coronal inhomogeneities, the authors introduced an interpretation of these components as the primary source and its reflected image on lower layers of the corona. We stress the difficulties with this interpretation because the corona is indeed very far from being spherically symmetric; the observed source structure may be due to a coronal scattering process involving both weak and strong inhomogeneities. But first of all we point out a relation between the halo dimensions and the local time which casts some doubt on the solar origin of the halo; we argue that it might result at least partially from instrumental or more probably from ionospheric effects.Laboratoire Associé du CNRS No. 264. 相似文献
8.
J. David Bohlin 《Solar physics》1971,18(3):450-457
Two-dimensional isophotes of the extreme solar corona (r
max 45 R
) have been derived from integrated vidicon pictures taken from the Moon's surface by the unmanned probes Surveyors 6 and 7. These data were calibrated through use of previously published values for the coronal brightness gradient along the ecliptic. The resulting structure of the outer corona is compared to ground-based observations of the innermost corona 1.125 r/R
2.0 made by the High Altitude Observatory K-coronameter. The possible existence of a streamer seen by Surveyor 7 is analyzed over the region 15 r 22.5 R
. 相似文献
9.
We present observations of the corona at 169 MHz with the Nançay Radioheliograph during the summer of 1984. We compare synoptic maps of the metric radio emission on the solar disk with synoptic charts of the K-corona as well as of the green and the red lines. Local sources of radio emission are not located near regions of enhanced green or red line emission which, in turn, are in general above chromospheric faculae. Thus the radio emissions located in the surroundings of faculae are apparently related to different loop systems, with lower density. The comparison of the radio data with the K-corona showed one radio source associated with enhanced emission both at 1.3 and at 1.7 R
, apparently a streamer. Other radio sources did not show any clear associations, but were nevertheless located within the coronal plasma sheet, delineated by the large-scale K-corona emission. Moreover the large-scale structure of the corona at 169 MHz was quite similar to the coronal plasma sheet observed at 1.3 R
above the limb. The extent of the radio emission in latitude is very similar to that of the K-corona, while the coronal line emission is more concentrated near the solar equator. 相似文献
10.
In this study we analyse the positions of major flares from 1978 and 1979, with respect to the magnetic structure of the solar corona, as described by a potential field model. We find that major flares exhibit no strong association with the neutral line at the chromospheric level. However, when we calculate the neutral line's position at higher and higher altitudes in the corona, we find that major flares show an increasing tendency to be found close to these high-altitude coronal neutral lines. The correlation between flares and higher-altitude coronal neutral lines reaches a maximum at an altitude of 0.35R
, and thereafter decreases as the neutral line is moved out to the source surface at an altitude of 1.50R
. This indicates that major flares are strongly associated with coronal structure at the 0.35R
level ( 250 000 km) - an altitude surprisingly high in the corona. This reinforces the idea that flares are associated with large-scale coronal magnetic fields and also indicates that the region of coronal magnetic topology important to solar flare processes may be larger than previously thought. 相似文献
11.
The brightness distribution of the Sun near the north pole at = 3 and 21 cm is obtained using the results of the partial eclipse of February 25, 1971. The solution obtained by expanding B
(r) in orthogonal polynomials does not show any brightening at either frequency, but a sensible darkening. If this result is explained by the presence of spicules, the coronal emission must be negligible with respect to the emission coming from the transition region. This is possible, at = 21 cm, only if we assume the presence of a coronal hole near the pole. 相似文献
12.
V. A. Kovalenko 《Solar physics》1981,73(2):383-403
The energy balance of open-field regions of the corona and solar wind and the influence of the flow geometry in the corona upon the density and temperature, are analyzed. It is found that the energy flux arriving at the corona is constant for the corona's open regions with different flow geometries. For the waves heating the corona and solar wind, the dependence of the absorption coefficient on the corona's plasma density is found to be within the range of distances r=1.05–1.5R
. It is shown that the wave absorption is more dependent on electron density than the coronal emission. It is this difference that causes lower-density coronal holes to be colder than quiet regions. It is found that the additional energy flux necessary for providing energy balance of the corona and for producing solar wind is a flux of Alfvén waves, which can provide the energy needed for producing quasi-stationary high-speed solar wind streams. Theoretical models of coronal holes and the question of why the high-speed solar wind streams are precisely flowing out of coronal holes, are discussed. 相似文献
13.
G. M. Webb 《Astrophysics and Space Science》1981,80(2):323-335
Two spherically symmetric time-dependent Green's functions of the equation of transport for cosmic rays in the interplanetary region are derived by transform techniques. The solar wind velocity is assumed radial and of constant speedV. In the first model the radial diffusion coefficient =0
r (0 constant), and in the second solution =0= constant. The solutions are for monoenergetic, impulsive release of particles from a fixed heliocentric radius. Integration of the solutions over timet, fromt=0 tot=, gives the steady-state Green's functions obtained previously. 相似文献
14.
A mechanism of excitation of radial oscillations of a magnetic tube is proposed for the interpretation of a periodic modulation of type IV radio burst intensity in the meter and decimeter range. After the flare a configuration with denser plasma extended along the magnetic field can be formed in the corona. Eigenoscillations of such a system are damped by MHD-wave emission into the external coronal plasma. However, if high energy protons with 0.2 are trapped by this configuration, the damping of oscillations can be made up for by an amplification due to bounce-resonant plasma instability. The regularity of the pulse period is explained by presence of a maximum in the wave growth rate dependence on the frequency. 相似文献
15.
Emission gradient curves for extreme ultraviolet (EUV) resonance lines of O vi and Mg x have been constructed from spectroheliograms of quiet limb regions observed with the Harvard experiment on Skylab. An analysis of these data suggests that the coronal temperature rises throughout the height range 1.03R
r1.3R
. This result implies that in quiet regions there is significant coronal heating beyond r = 1.3R
.Now at E.O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, D.C., U.S.A. 相似文献
16.
Slit spectrograms of a quiescent prominence and the inner corona (h2.5 arc min) in the range 3400–7000 Å (dispersion 6–10 Å/mm) were obtained. From an analysis of the Stark effect on the Balmer lines (up to number 36) the electron density in the prominence n
e = (7 ± 3) × 1010 cm–3 was deduced. The kinetic temperature T
k and the non-thermal velocities
t, found from a simultaneous consideration of the Balmer and metal lines, are T
k 10 000 K and v
t6 km/s. Also the emission measure of the prominence along the line-of-sight was found: ME = 1031 cm–5.In the coronal spectrum 24 coronal lines were found. Thirteen of these lines were identified and measured photometrically to get their absolute intensities, profiles and halfwidths. For nine lines the intensities as a function of the height were studied and on this basis the coronal lines were divided into a few groups. The line-of-sight and non-thermal velocities are
r 10 km/s and
t 25 km/s. The coronal lines originate in at least three types of regions with different temperatures. The emission measure as a function of the ionization temperature was determined. The abundances of four elements of the iron group (V, Cr, Mn, Co) were estimated. The abundances of the other elements of the same group (A, Ca, Fe, Ni), found from EUV-data, are in a good agreement with our observations. The degree of inhomogeneity in the corona was estimated:
. 相似文献
17.
Very faint H emission in the solar corona registered on over-exposed photographs made by a coronagraph and an H filter is studied. The over-exposed filtergrams have been processed by a Joyce Loebl automated microdensitometer and the two-dimensional scans have been analysed by the residual image method. A classification of the faint H emission objects, revealed on the isodensity maps, is proposed and the latitudinal distribution, the morphology, and the location with respect to the other active phenomena are analysed. Taking into account some possible plasma effects that could be caused by coronal magnetic field changes, a hydromagnetic interpretation of the faint H emission is proposed. 相似文献
18.
Imad A. Ahmad 《Solar physics》1977,53(2):409-415
The intensity of the He+ 304 coronal line relative to the H0 1216 line, including the dominant contribution due to resonance scattering, is presented. All physical processes important in the corona are included. It is found that He+ 304 is a major contributor to the XUV corona, and that the sensitivity of the He+ 304/H0 1216 intensity ratio to coronal temperature is very weak, supporting the belief that this ratio is a good indicator of the coronal helium abundance. 相似文献
19.
The photometry of coronal colour negatives is carried out. The films were obtained at the March 7, 1970 and July 10, 1972 eclipses. A distribution of the coronal brightness in the red (635 nm), green (545 nm), and blue (455 nm) wavelength intervals up to distances of (6–7)R
is deduced (Figure 1). Colour indexes of the corona (the emission ratio red/blue - C
rb and green/blue - C
gb) have been obtained. We assume C
rb = C
gb = 1 in the inner corona (2R
). The maximum of colour indexes of the 1970 corona are at the distances of about 4R
(C
rb 1.9 and C
gb 1.7). A slight reddening within the limits of the errors was found in the 1972 corona.There is a correlation between colour indexes and diffuse external reinforcements (RED) brightness. The analysis of the results leads to the conclusion that RED consists of dust grains with radii 1 m. RED brightness is evaluated to be 4 × 10-10
B
. There is 1 grain of dust in the elementary volume with cross section of 1 cm2 along the line of sight. The intensity of dust emission in wavelength interval 10 m deduced by the authors is approximately 1 W cm-2 s m–1. That is in agreement with Mankin et al. (1974) and Léna et al. (1974) observations. The whole dust mass of RED is 1% of the coronal gas mass contained within RED region. The dust grain number density is about 10-11 cm-3.Determinations of the colour of the solar corona have been made by a number of scientists (Tikhov, 1940, 1957; Allen, 1946; Blackwell, 1952; Michard, 1956; Sharonov, 1958; Nay et al., 1961). The corona colour was found to be somewhat redder than the Sun's. However this question is not finally settled to date. 相似文献
20.
The electron density distribution of the inner solar corona (r 2 R
) as a function of latitude, longitude, and radial distance is determined from K-coronameter polarization-brightness (pB) data. A Legendre polynomial is assumed for the electron density distribution, and the coefficients of the polynomial are determined by a least-mean-square regression analysis of several days of pB-data. The calculated electron density distribution is then mapped as a function of latitude and longitude. The method is particularly useful in determining the longitudinal extent of coronal streamers and enhancements and in resolving coronal features whose projections on the plane of the sky overlap.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献