共查询到20条相似文献,搜索用时 437 毫秒
1.
S. T. Wu W. P. Guo M. D. Andrews G. E. Brueckner R. A. Howard M. J. Koomen C. M. Korendyke D. J. Michels J. D. Moses D. G. Socker K. P. Dere P. L. Lamy A. Llebaria M. V. Bout R. Schwenn G. M. Simnett D. K. Bedford C. J. Eyles 《Solar physics》1997,175(2):719-735
We present a qualitative and quantitative comparison of a single coronal mass ejection (CME) as observed by LASCO (July 28–29,
1996) with the results of a three-dimensional axisymmetric time-dependent magnetohydrodynamic model of a flux rope interacting
with a helmet streamer. The particular CME considered was selected based on the appearance of a distinct ‘tear-drop’ shape
visible in animations generated from both the data and the model.
The CME event begins with the brightening of a pre-existing coronal streamer which evolves into a ‘tear-drop’ shaped loop
followed by a Y-shaped structure. The brightening moves slowly outward with significant acceleration reaching velocities of
∼450 km s-1 at 30 R⊙.
The observed CME characteristics are compared with the model results. On the basis of this comparison, we suggested that the
observed features were caused by the evacuation of a flux rope in the closed field region of the helmet streamer (i.e., helmet
dome). The flux rope manifests itself as the cavity of the quasi-static helmet streamer and the whole system becomes unstable
when the flux rope reaches a threshold strength. The observed ‘tear-drop’ structure is due to the deformed flux rope. The
leading edge of the flux rope interacts with the helmet dome to form the typical loop-like CME. The trailing edge of this
flux rope interacts with the local bi-polar field to form the observed Y-shaped structure. The model results for the evolution
of the magnetic-field configurations, velocity, and polarization brightness are directly compared with observations.
Animations have been generated from both the actual data and the model to illustrate the good agreement between the observation
and the model. These animations can be found on the CD-ROM which accompanies this volume.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1004923016322 相似文献
2.
Daily images of the white light corona between 3 and 10 R ? have been recorded by a coronagraph aboard the OSO-7 unmanned satellite since October 3, 1971. Images for the years 1972 and 1973 have been examined for persistent coronal forms. For most of 1972 there passed over the Sun's east limb a regular alternation of northern and southern streamers separated frequently by equatorial fans. The alternation suggested the rotation of a stable four-sectored coronal structure produced by two northern streamers, 180° apart in longitude and a similar pair of southern streamers shifted 90° in longitude. Toward the end of 1972 this structure evolved into a two-sectored structure produced by a single northern streamer and a single southern streamer separated by 180° in longitude. This structure remained stable during most of 1973. Transition from a northern to southern streamer, converted to Earth passage dates, correlated with the passage of a -/+ sector boundary in the interplanetary magnetic field. Conversely, the transition from a southern to northern streamer was associated with a +/-boundary passage. These correlations support the recent observations of Hansen et al. (1973). 相似文献
3.
R. D. Jeffries 《Monthly notices of the Royal Astronomical Society》1998,295(4):825-833
The short-period, eclipsing, magnetically active binary system XY UMa has been observed over several orbital cycles at X-ray wavelengths. The X-ray light curves vary owing to obvious flares and other lower level activity on time-scales of days. These data caution against deducing coronal structure on the basis of a single orbit of X-ray data. In contrast to similar binary systems, XY UMa shows no significant X-ray eclipses. This is interpreted as evidence for either extended (> 1 R ⊙ ) coronae or a compact corona at high, uneclipsed latitudes on the primary star. The extended coronal scenario is favoured by some observational features of other systems such as extended radio coronae, long-duration X-ray flares and cool prominences, but unfavoured by others such as high coronal densities from EUV spectroscopy. A high-latitude compact corona might be associated with the high-latitude starspots seen in many active stars. Nearly simultaneous optical light curves suggest that some equatorial spots were present at the time of the X-ray observations, but fewer than in previous or subsequent years and that high-latitude spots must also be present. It is speculated that the lack of X-ray eclipses and dearth of equatorial, compact coronal regions are associated with a minimum in XY UMa's magnetic activity cycle. 相似文献
4.
C. E. DeForest J. T. Hoeksema J. B. Gurman B. J. Thompson S. P. Plunkett R. Howard R. C. Harrison D. M. Hasslerz 《Solar physics》1997,175(2):393-410
On 7 and 8 March 1996, the SOHO spacecraft and several other space- and ground-based observatories cooperated in the most
comprehensive observation to date of solar polar plumes. Based on simultaneous data from five instruments, we describe the
morphology of the plumes observed over the south pole of the Sun during the SOHO observing campaign. Individual plumes have
been characterized from the photosphere to approximately 15 R⊙ yielding a coherent portrait of the features for more quantitative
future studies. The observed plumes arise from small (∼ 2-5 arc sec diameter) quiescent, unipolar magnetic flux concentrations,
on chromospheric network cell boundaries. They are denser and cooler than the surrounding coronal hole through which they
extend, and are seen clearly in both Feix and Fexii emission lines, indicating an ionization temperature between 1.0–1.5 x
106 K. The plumes initially expand rapidly with altitude, to a diameter of 20–30 Mm about 30 Mm off the surface. Above 1.2 R⊙
plumes are observed in white light (as ‘coronal rays’) and extend to above 12 R⊙. They grow superradially throughout their
observed height, increasing their subtended solid angle (relative to disk center) by a factor of ∼10 between 1.05 R⊙ and 4–5
R⊙ and by a total factor of 20–40 between 1.05 R⊙ and 12 R⊙. On spatial scales larger than 10 arc sec, plume structure in
the lower corona (R < 1.3 R⊙) is observed to be steady-state for periods of at least 24 hours; however, on spatial scales
smaller than 10 arc sec, plume XUV intensities vary by 10–20% (after background subtraction) on a time scale of a few minutes.
(Dr. Hassler is now employed by Southwest Research Institute, Boulder, CO) 相似文献
5.
Type III solar radio bursts observed from 3.0 to 0.45 MHz with the ATS-II satellite over the period April–October 1967 have been analyzed to derive two alternative models of active region streamers in the outer solar corona. Assuming that the bursts correspond to radiation near the electron plasma frequency, pressure equilibrium arguments lead to streamer Model I in which the streamer electron temperature derived from collision damping time falls off much more rapidly than in the average corona and the electron density is as much as 25 times the average coronal density at heights of 10 to 50 solar radii (R
). In Model II the streamer electron temperature is assumed to equal the average coronal temperature, giving a density enhancement which decreases from a factor of 10 close to the Sun to less than a factor of two at large distances (> 1/4 AU). When the burst frequency drift is interpreted as resulting from the outward motion of a disturbance that stimulates the radio emission, Model I gives a constant velocity of about 0.35c for the exciting disturbance as it moves to large distances, while with Model II, there is a decrease in the velocity to less than 0.2c beyond 10 R
. 相似文献
6.
Švestka Zdeněk Fárník František Hick Paul Hudson Hugh S. Uchida Yutaka 《Solar physics》1997,176(2):355-371
We demonstrate several events where an eruptive flare close to the limb gave rise to a transient coronal streamer visible in X-rays in Yohkoh SXT images, and analyze one of these events, on 28–29 October 1992, in detail. A coronal helmet streamer began to appear 2 hours after the flare, high above rising post-flare loops; the streamer became progressively narrower, reaching its minimum width 7–12 hours after the flare, and widened again thereafter, until it eventually disappeared. Several other events behaved in a similar way. We suggest that the minimum width indicates the time when the streamer became fully developed. All the time the temperature in the helmet streamer structure was decreasing, which can explain the subsequent fictitious widening of the X-ray streamer. It is suggested that we may see here two systems of reconnection on widely different altitudes, one giving rise to the post-flare loops while the other creates (or re-forms) the coronal helmet streamer. A similar interpretation was suggested in 1990 by Kopp and Polettofor post-flare giant arches observed on board the SMM; indeed, there are some similarities between these post-flare helmet streamers and giant arches and, with the low spatial resolution of SMM instruments, it is possible that some helmet streamers could have been considered to be a kind of a giant arch. 相似文献
7.
Valentine I. Makarov 《Solar physics》1994,150(1-2):359-374
Properties of even and odd 11-year solar cycles as part of the 22-year magnetic cycle have been studied on the basis of the
data on the zonal structure of the large-scale magnetic field, of polar faculae activity cycles, duration of 11-year cycles,
high-latitude prominence areas, inclinations of the coronal streamers, velocity of magnetic neutral line migration, and peculiarities
of the polar magnetic field reversal. It is shown that the properties of the odd cycle depend on those of the preceding even
cycle. The 22-year magnetic cycle, consisting of an even and odd cycle, is a unified dynamic process. The new data obtained
show that the poloidal magnetic fieldB(p) of ‘+’ and ‘−’ polarity for the new 22-year magnetic cycle is formed simultaneously, possibly in deep layers of the Sun
in the form of a certain magnetic configuration, containing alternating ‘+’ and ‘−’ polarities of the field. 相似文献
8.
J. David Bohlin 《Solar physics》1970,13(1):153-175
A unique combination of photographic and K-coronameter data were used to study the structure and evolution of two known coronal streamers. In addition, two other K-coronameter enhancements were studied as representing ideal second examples of the known streamers. As a general rule the observations indicate that these features were direct coronal manifestations of photospheric bipolar magnetic regions (BMR) and were of two basic types:active region, by which is meant a coronal streamer which develops radially over a low-latitude active region; andhelmet which denotes a streamer whose structure and development appear to be a consequence of a long-lived complex of activity, composed of both trailing magnetic fields and a parent center of disk activity.The similarity of growth rates during the first solar rotation of life led to derivation of a total streamer density of 4–5 × 108 cm–3 atr = 1.125R
. This density may represent a characteristic maximum density at the base of streamers. The intensity gradient of the inner (r1.5R
) corona was used to establish a qualitative evolutionary model of streamers which synthesizes the observations. Briefly, streamers initially develop over active regions; the streamer growth rate may be as rapid as the disk activity, or at worst lags flare activity by
solar rotation. The streamer can be the cause of interplanetary and geomagnetic effects at 1 AU within a solar rotation after birth. Thereafter the streamer follows an evolution dictated by the underlying solar magnetic fields. In any case the lowest level of the coronal enhancement has a lifetime not exceeding that of the solar disk activity. 相似文献
9.
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
10.
A phenomenological model of the interplay between the polar magnetic fields of the Sun and the solar sector structure is discussed. Current sheets separate regions of opposite polarity and mark the sector boundaries in the corona. The sheets are visible as helmet streamers. The solar sector boundary is tilted with respect to central meridian, and boundaries with opposite polarity change are oppositely tilted. The tilt of a given type of boundary [(+, ?) or (?, +)] changes systematically during the sunspot cycle as the polarity of the polar fields reverses. Similar reversals of the position of the streamers at the limbs takes place. If we consider (a) a sunspot cycle where the northern polar field is inward (?) during the early part of the cycle and (b) a (+, ?) sector boundary at central meridian then the model predicts the following pattern; a streamer at high northern latitudes should be observed over the west limb together with a corresponding southern streamer over the east limb. The current sheet runs now NW-SE. At sunspot maximum the boundary is more in the N-S direction; later when the polar fields have completed their reversal the boundary runs NE-SW and the northern streamer should be observed over the east limb and the southern streamer over the west limb. Observational evidence in support of the model is presented, especially the findings of Hansen, Sawyer and Hansen and Koomen and Howard that the K-corona is highly structured and related to the solar sector structure. 相似文献
11.
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
12.
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
13.
哈雷彗星在日彗距较大时出现长而直的主彗尾(尾流),这是很有趣的。尾流一般是指等离子体尾流;但是,当地球接近彗星轨道面时,尘埃尾流可能叠加到主彗尾上。在一般感光波段宽的彗星底片上很难区分这两种尾流。本文选取哈雷彗星在不同日彗距的5条主尾流,作了光度测量和比较分析。得出沿各尾轴及其垂直方向几个截面的亮度分布、亮度半极大全宽、尾轴的视风差角和真风差角及彗尾长度。在所分析的蓝敏底片上,过近日点前的2个尾流肯定是等离子体尾流,而5个尾流的相似性以及其他证据说明它们主要都是等离子体尾流,尘埃彗尾的污染是次要的。 相似文献
14.
Recent observations of the solar corona with the LASCO coronagraph on board of the SOHO spacecraft have revealed the occurrence of triple helmet streamers even during solar minimum, which occasionally go unstable and give rise to large coronal mass ejections. There are also indications that the slow solar wind is either a combination of a quasi-stationary flow and a highly fluctuating component or may even be caused completely by many small eruptions or instabilities. As a first step we recently presented an analytical method to calculate simple two-dimensional stationary models of triple helmet streamer configurations. In the present contribution we use the equations of time-dependent resistive magnetohydrodynamics to investigate the stability and the dynamical behaviour of these configurations. We particularly focus on the possible differences between the dynamics of single isolated streamers and triple streamers and on the way in which magnetic reconnection initiates both small scale and large scale dynamical behaviour of the streamers. Our results indicate that small eruptions at the helmet streamer cusp may incessantly accelerate small amounts of plasma without significant changes of the equilibrium configuration and might thus contribute to the non-stationary slow solar wind. On larger time and length scales, large coronal eruptions can occur as a consequence of large scale magnetic reconnection events inside the streamer configuration. Our results also show that triple streamers are usually more stable than a single streamer. 相似文献
15.
Sabine Mengel 《Astrophysics and Space Science》2002,281(1-2):105-108
We obtained various sets of near infrared observations of the prototypical merger, NGC 4038/4039 (‘the Antennae’). Integral
field spectroscopy and broad- and narrow band imaging aimed at obtaining age and extinction estimates of the young star clusters
seen in large numbers distributed throughout the disks of the interacting galaxies. High resolution spectroscopy led to estimates
of the dynamical masses of the clusters. The clusters have ages ranging from 3.7 to ≈ 20 Myrs. Those in the ‘overlap region’
are very young (below 8 Myrs), while in the nothwestern loop ages are above that limit, and the nuclear starbursts are much
older (∼ 100 Myrs). Some photometric cluster masses lie above 106 M⊙. The stellar velocity dispersions determined from the medium- to high resolution spectra yielded virial cluster masses again
up to a few 106 M⊙. Large differences in the estimated photometric and virial masses suggest a variation of the IMF between the clusters. At
least some of the clusters have masses, concentrations and IMFs that could allow them to evolve into globular clusters.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
16.
A comparison is made of polarization brightness distributions of the white-light corona based on the data from Mark III (MLSO) with calculated magnetic configurations in the corona (a potential-field approximation) between adjacent coronal holes (or associated open magnetic tubes) with magnetic fields of single polarity. It is shown that in these coronal regions, which were referred to as chains of streamers in earlier papers of these authors, magnetic structures in meridional and azimuthal cross-sections typically have the aspect of helmet streamers. The calculated magnetic field configurations under the helmets of chains of streamers have an even number of systems of arched features closely adjacent to each other (unlike the structure of the streamer belt with a neutral line where their number is odd). The height of the helmet top of chains of streamers remains smaller than that of the source surface but changes along the length of the chain and reaches maximum values at the ends of the chain. 相似文献
17.
Nelson L. Reginald Joseph M. Davila O. C. St. Cyr Douglas M. Rabin Madhulika Guhathakurta Donald M. Hassler Hadi Gashut 《Solar physics》2011,270(1):235-251
An experiment was conducted in conjunction with the total solar eclipse on 29 March 2006 in Libya to measure both the electron
temperature and its flow speed simultaneously at multiple locations in the low solar corona by measuring the visible K-coronal
spectrum. Coronal model spectra incorporating the effects of electron temperature and its flow speed were matched with the
measured K-coronal spectra to interpret the observations. Results show electron temperatures of (1.10±0.05) MK, (0.70±0.08) MK,
and (0.98±0.12) MK, at 1.1 R
⊙ from Sun center in the solar north, east and west, respectively, and (0.93±0.12) MK, at 1.2 R
⊙ from Sun center in the solar west. The corresponding outflow speeds obtained from the spectral fit are (103±92) km s−1, (0+10) km s−1, (0+10) km s−1, and (0+10) km s−1. Since the observations were taken only at 1.1 R
⊙ and 1.2 R
⊙ from Sun center, these speeds, consistent with zero outflow, are in agreement with expectations and provide additional confirmation
that the spectral fitting method is working. The electron temperature at 1.1 R
⊙ from Sun center is larger at the north (polar region) than the east and west (equatorial region). 相似文献
18.
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
19.
S. Koutchmy 《Solar physics》1972,24(2):373-384
A Hydrodynamical Study of the Large NE Coronal Streamer Observed during the 7th March Eclipse.
The photometry of the large coronal streamer located on the N-E limb of the Sun observed on the 7th March 1970 in Mexico has been performed using compensated plates in unpolarized light as well as in tangentially and radially polarized light. The measured polarizations permit us to locate the streamer in space using a generally used method. The density distribution in the core of the streamer and for r > 1.5 R
is given as well as the half-widths of the density distribution perpendicular to the axis of the streamer. Densities n(r) in the 1(R
) < r < 1.5(R
) region are obtained assuming hydrostatic equilibrium (4) for 1.5 < r < 2.The axial values of n(r) are given which agree with those of others models. The values of parameters for the model given are tabulated.The variation of the expansion velocity has been derived from the hydrodynamical continuity equation. This velocity becomes supersonic [v > 150(km s–1)] above the height of minimum cross section of the streamer, r = 3.5 (R
). This result is verified by a simple calculation based on the observed curvature of the streamer's stalk. The velocity corresponding to the garden hose effect considered has a lower value equal to 60(km s–1). The radial distribution of the expansion velocity is shown. Our model allows us to extrapolate in order to obtain the value of v(r) at the Sun's surface. This value, v 0.07 (km s–1) can be compared with the value of the differential rotation velocity of the foot of the streamer. The values of the corpuscular flux of the streamer are calculated [F=2.4 × 1034( s-1] and agree with the value of the flux corresponding to the density enhancements of the solar wind, measured in situ. This shows the importance of large streamers for the coronal expansion processus. 相似文献
20.
C. V. S. R. Sarma M. B. K. Sarma N. B. Sanwal 《Journal of Astrophysics and Astronomy》1989,10(3):307-346
The extensiveUBV observations of SV Camelopardalis by Patkos (1982) have been analysed to derive the orbital elements of the system. The data
were corrected for the effect of third body (Sarma, Sarma & Abhyankar 1985) and for the ‘RS CVn’ distortion wave (Sarma, Vivekanandarao
& Sarma 1988). The cleaned data were used to obtain a preliminary solution by a modified version of Wellmann method (Sarma
& Abhyankar 1979) from which we concluded that the primary eclipse is a transit. The final orbital elements of SV Cam were
obtained by the modified version (Sarma 1988; Sarmaet al. 1987) of WINK program by Wood (1972). The colour and median brightness variation are discussed. From the spectroscopic mass
functionf(m) = 0.118 M⊙ (Hiltner 1953), the absolute dimensions of the components are found to be 0.826 Mbd & 0.592 M⊙ and 1.236 R⊙ & 0.778 R⊙ for the primary and secondary components, respectively. The age of the binary system is estimated to be 6.0 ± 1.0 × 108 years 相似文献