共查询到20条相似文献，搜索用时 218 毫秒
1.
Wen-Rui Hu 《Astrophysics and Space Science》1983,92(2):373-394
The statistical correlation between an eruptive prominence and the coronal transient associated with this prominence implies that there should be a relationship between these two kinds of dynamical processes. This paper analyzes the dynamical effect of a plasma ‘piston’ in the corona, consisting of an eruptive prominence and/or a magnetic flux region (loop or arcade, or blob) in front of the prominence. Ahead of the piston, there is a compressed flow, which produces a shock front. This high-density region corresponds to the bright feature of the transient. Behind the piston, there is a rarefaction region, which corresponds to the dark feature of the transient. Therefore, both the bright and dark features of the transient may be explained at the same time by the dynamical process of the moving piston. Two local solutions, one perpendicular and one parallel to the direction of solar gravitational field, are discussed. The influence of gravity on the gas-dynamical process driven by the piston is discussed in terms of characteristic theory, and the flow field is given quantitatively. For a typical piston trajectory similar to the one for an eruptive prominence, the velocity of the shock front which locates ahead the transient front is nearly constant or slightly accelerated, and the width of the compressed flow region may be kept nearly constant or increased linearly, depending on the velocity distribution of the piston. Based on these results, the major features of the transient may be explained. Some of the fine structure of the transient is also shown, which may be compared in detail with observations. 相似文献
2.
Mukul R. Kundu 《Astrophysics and Space Science》1996,243(1):15-22
We present a summary of several studies of transient coronal phenomena based upon high spatial resolution radio imaging data along with Yohkoh SXT and HXT observations. In addition to normal flares the studies also involve such exotic events as active region transient brightenings (ARTB) and coronal jets and bright points. We provide evidence of nonthermal processes in flaring X-ray bright points from spatially resolved meter-wave data, existence and propagation of type II burst emitting electrons in coronal jets, radio signatures of ARTB's, and beaming of electrons producing microwave and hard X-rays. The implications of these observations are discussed. 相似文献
3.
This paper reports results of an analysis of Skylab observations of coronal bright points made in EUV spectral lines formed in the chromosphere, chromospheric-coronal transition region and corona. The most important result is that the observed bright points exhibited large variations in EUV emission over time scales as short as 5.5 min, the temporal resolution of the data. In most cases strong enhancements in the coronal line were accompanied by strong enhancements in the chromospheric and transition region lines. The intensity variations appear to take place within substructures of the bright points, which most likely consist of miniature loops evolving on time scales of a few minutes. Coronal cooling times derived from the data are consistent with an intermittent, impulsive coronal heating mechanism for bright points. 相似文献
4.
S. Lulić B. Vršnak T. Žic I. W. Kienreich N. Muhr M. Temmer A. M. Veronig 《Solar physics》2013,286(2):509-528
Magnetosonic wave formation driven by an expanding cylindrical piston is numerically simulated to obtain better physical insight into the initiation and evolution of large-scale coronal waves caused by coronal eruptions. Several very basic initial configurations are employed to analyze intrinsic characteristics of MHD wave formation that do not depend on specific properties of the environment. It turns out that these simple initial configurations result in piston/wave morphologies and kinematics that reproduce common characteristics of coronal waves. In the initial stage, the wave and the expanding source region cannot be clearly resolved; i.e. a certain time is needed before the wave detaches from the piston. Thereafter, it continues to travel as what is called a “simple wave.” During the acceleration stage of the source region inflation, the wave is driven by the piston expansion, so its amplitude and phase-speed increase, whereas the wavefront profile steepens. At a given point, a discontinuity forms in the wavefront profile; i.e. the leading edge of the wave becomes shocked. The time/distance required for the shock formation is shorter for a more impulsive source-region expansion. After the piston stops, the wave amplitude and phase speed start to decrease. During the expansion, most of the source region becomes strongly rarefied, which reproduces the coronal dimming left behind the eruption. However, the density increases at the source-region boundary, and stays enhanced even after the expansion stops, which might explain stationary brightenings that are sometimes observed at the edges of the erupted coronal structure. Also, in the rear of the wave a weak density depletion develops, trailing the wave, which is sometimes observed as weak transient coronal dimming. Finally, we find a well-defined relationship between the impulsiveness of the source-region expansion and the wave amplitude and phase speed. The results for the cylindrical piston are also compared with the outcome for a planar wave that is formed by a one-dimensional piston, to find out how different geometries affect the evolution of the wave. 相似文献
5.
6.
We present for the first time a three-dimensional reconstruction of the electron density in the corona at distances from 1.5R ⊙ to 4R ⊙ using COR1 STEREO observations. The reconstruction is performed using a regularized tomography inversion method for two biweekly periods corresponding to Carrington Rotations 2058 and 2066. Images from the two STEREO spacecraft are used to compare the reconstructed density structures with coronal features located by triangulation. We find that the location of a bright tip of a helmet streamer obtained from the tomographic reconstruction is in good agreement with the location obtained by triangulation. The reconstructed density structure of the equatorial streamer belt is largely consistent with the variation of the current sheet derived from a potential magnetic field extrapolation for most of the equatorial region and for an MHD model of the corona. A zero-value density region in the reconstruction is identified with a low-density region seen in an EUVI image below the reconstruction domain. 相似文献
7.
本文对盔状冕流底部磁通量喷发的大气响应进行数值模拟。数值结果表明:在喷发磁场的作用下,原冕流中的物质将受到压缩并向上运动,形成高密度亮环结构,同时在环的前方出现快磁声波,在环的下方出现低密度暗区。据此我们认为,磁通量喷发可能是触发环形日冕物质喷射的一种重要机制。 相似文献
8.
Maps of the brightness distribution of the ‘quiet Sun’ at 80 and 160 MHz reveal the presence of features both brighter and darker than average. The ‘dark’ regions are well correlated with dark regions on UV maps; we deduce that they result from ‘coronal holes’. The ‘bright’ regions are associated with quiescent filaments and not plages or bright regions on microwave or UV maps; we deduce that they result from ‘coronal helmets’.
When coronal holes appear near the centre of the disk we can estimate the density and kinetic temperature in the holes from the radio observations. For a hole observed on 1972 July 20–21, we find T ≈ 0.8 × 106 inside the hole and T ≈ 1.0 × 106 in average regions outside the hole. Inside the hole the density is estimated to be about one-quarter of that in Newkirk's model of the spherically symmetric corona.
Variations in brightness at a fixed height above the limb are generally well correlated with scans at a similar height made with a K-coronameter. Occasional differences may result from streamers protruding beyond the limb from the back of the Sun. These can be seen by the K-coronameter but, because of refraction of the radio rays, not by the radio-heliograph.
相似文献9.
This paper summarizes the results of a program of rocket observations of the solar corona with grazing incidence X-ray telescopes. A series of five flights of a Kanigen-surfaced telescope with a few arc seconds resolution, together with the first flight of a newer telescope have resulted in the identification of six classes of coronal structures observable in the X-ray photographs. These are: active regions, active region interconnections, large loop structures associated with unipolar magnetic regions, coronal holes, coronal bright points, and the structures surrounding filament cavities. Two solar flares have been observed. The methods involved in deriving coronal temperature and density information from X-ray photographs are described and the analysis of a bright active region (McMath plage 11035) observed at the west limb on November 24, 1970 is presented as an example of these techniques.This paper originated in an invited talk presented by one of us (G.V.) at the COSPAR Symposium on High Resolution Astronomical Observations from Space, Seattle, Washington, June 29, 1971. In addition, it includes material presented at the three NASA OSO workshops, as well as more recent work. 相似文献
10.
Coronal density, temperature, and heat-flux distributions for the equatorial and polar corona have been deduced from Saito’s model of averaged coronal white-light (WL) brightness and polarization observations. These distributions are compared with those determined from a kinetic collisionless/exospheric model of the solar corona. This comparison indicates similar distributions at large radial distances (>?7 R⊙) in the collisionless region. However, rather important differences are found close to the Sun in the acceleration region of the solar wind. The exospheric heat flux is directed away from the Sun, while that inferred from all WL coronal observations is in the opposite direction, i.e. conducting heat from the inner corona toward the chromosphere. This could indicate that the source of coronal heating extends up into the inner corona, where it maximizes at r>1.5 R⊙, well above the transition region. 相似文献
11.
We present a qualitative and quantitative comparison of a single coronal mass ejection (CME) as observed by LASCO on 5 October
1996 with the results of a two-dimensional magnetohydrodynamic (MHD) model. This event was selected as a clear example of
a CME that is not caused by the disruption of a helmet streamer. This CME occurs against the background of multiple bright
streamers on the west limb. The CME is first seen as a brightening of the entire west limb. The CME has a bright, sharp front
that moves outward with no significant change in shape. The CME moves outward with roughly constant velocity that is approximately
twice as fast at high latitude as near the streamer. The measured CME mass is 1.2×1016 g. There are two parts to the MHD model. The pre-event corona was calculated using a 2-dimensional bi-modal model. The CME
is simulated using a time dependent perturbation at the base of the corona. The model successfully reproduces the observed
morphology, velocity profiles, and change in coronal mass. The observed velocity asymmetry is a natural consequence of the
structure of the pre-event corona. Animations have been generated from both the data and model to illustrate the good agreement
between the observations and simulation. 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:1005178630316 相似文献
12.
W. J. Wagner R. M. E. Illing C. B. Sawyer L. L. House N. R. Sheeley Jr. R. A. Howard M. J. Koomen D. J. Michels R. N. Smartt M. Dryer 《Solar physics》1983,83(1):153-166
Multi-telescope observations of the coronal transient of 15–16 April, 1980 provide simultaneous data from the Solar Maximum Mission Coronagraph/Polarimeter, the Solwind Coronagraph, and the new Emission Line Coronagraph of the Sacramento Peak Observatory. An eruptive prominence-associated white light transient is for the first time seen as an unusual wave or brightening in Fe x
gl6374 (but not in Fe xiv
gl5303). Several interpretations of this fleeting enhancement are offered.The prominence shows a slowly increasing acceleration which peaks at the time of the Fe event. The white light loop transient surrounding the prominence expands at a well-documented constant speed to 10R
, with an extrapolated start time at zero height coincident with the surface activity.This loop transient exemplifies those seen above 1.7R
in that leading the disturbance is a bright (N
e-enhanced) loop rather than dark. This is consistent with a report of the behavior of another eruptive event observed by Fisher and Poland (1981) which began as a density depletion in the lower corona, with a bright loop forming at greater altitudes. The top of the bright loop ultimately fades in the outer corona while slow radial growth continues in the legs.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
13.
The very steep decrease in density with heliocentric distance makes imaging of coronal density structures out to a few solar
radii challenging. The radial gradient in brightness can be reduced using numerous image processing techniques, thus quantitative
data are manipulated to provide qualitative images. We introduce a new normalizing-radial-graded filter (NRGF): a simple filter
for removing the radial gradient to reveal coronal structure. Applied to polarized brightness observations of the corona,
the NRGF produces images which are striking in their detail. Total-brightness, white-light images include contributions from
the F corona, stray light, and other instrumental contributions which need to be removed as effectively as possible to properly
reveal the electron corona structure. A new procedure for subtracting this background from LASCO C2 white-light, total-brightness
images is introduced. The background is created from the unpolarized component of total-brightness images and is found to
be remarkably time-invariant, remaining virtually unchanged over the solar cycle. By direct comparison with polarized-brightness
data, we show that the new background-subtracting procedure is superior in depicting coronal structure accurately, particularly
when used in conjunction with the NRGF. The effectiveness of the procedures is demonstrated on a series of LASCO C2 observations
of a coronal mass ejection (CME). 相似文献
14.
S. F. Smerd G. A. Dulk R. M. MacQueen J. T. Gosling A. Magun R. T. Stewart K. V. Sheridan R. D. Robinson S. Jacques 《Solar physics》1976,49(2):369-394
Observations of a coronal transient event were obtained in white light by the Skylab coronagraph and at metric wavelengths by the radioheliograph and spectrograph at Culgoora and the spectrograph-interferometer at Boulder. The continuum radio burst was found to originate above the outward-moving white light loop - a region of compressed material headed by a bow wave. The computed density in the region of radio emission, based on either gyro-synchrotron or harmonic plasma radiation mechanisms, was approximately 10 times the ambient coronal density; this is compatible with the density deduced from the white light observations. The magnetic energy density derived from the radio observations was greater than 10 times the thermal energy density, marginally larger than the kinetic energy density in the fastest moving portion of the transient, and considerably larger in most other regions. The ambient medium, the white light front, the compression region, the loop, and the slower, massive flow of material behind are each examined. It is found that the plasma was magnetically controlled throughout, and that magnetic forces provided the principal mechanism for acceleration of the transient material from the Sun.Also, High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado.Now at Los Alamos Scientific Laboratory, Los Alamos, New Mexico.The National Center for Atmospheric Research is sponsored by the National Science Foundation.On leave from Institute of Applied Physics, University of Berne, Switzerland.Also, Division of Radiophysics, CSIRO, Sydney, Australia. 相似文献
15.
In coronal holes the electron (proton) density is low, and heating of the proton gas produces a rapidly increasing proton temperature in the inner corona. In models with a reasonable electron density in the upper transition region the proton gas becomes collisionless some 0.2 to 0.3 solar radii into the corona. In the collisionless region the proton heat flux is outwards, along the temperature gradient. The thermal coupling to electrons is weak in coronal holes, so the heat flux into the transition region is too small to supply the energy needed to heat the solar wind plasma to coronal temperatures. Our model studies indicate that in models with proton heating the inward heat conduction may be so inefficient that some of the energy flux must be deposited in the transition region to produce the proton fluxes that are observed in the solar wind. If we allow for coronal electron heating, the energy that is needed in the transition region to heat the solar wind to coronal temperatures, may be supplied by heat conduction from the corona. 相似文献
16.
S. Krucker M. Battaglia P. J. Cargill L. Fletcher H. S. Hudson A. L. MacKinnon S. Masuda L. Sui M. Tomczak A. L. Veronig L. Vlahos S. M. White 《Astronomy and Astrophysics Review》2008,16(3-4):155-208
This review surveys hard X-ray emissions of non-thermal electrons in the solar corona. These electrons originate in flares and flare-related processes. Hard X-ray emission is the most direct diagnostic of electron presence in the corona, and such observations provide quantitative determinations of the total energy in the non-thermal electrons. The most intense flare emissions are generally observed from the chromosphere at footpoints of magnetic loops. Over the years, however, many observations of hard X-ray and even γ-ray emission directly from the corona have also been reported. These coronal sources are of particular interest as they occur closest to where the electron acceleration is thought to occur. Prior to the actual direct imaging observations, disk occultation was usually required to study coronal sources, resulting in limited physical information. Now RHESSI has given us a systematic view of coronal sources that combines high spatial and spectral resolution with broad energy coverage and high sensitivity. Despite the low density and hence low bremsstrahlung efficiency of the corona, we now detect coronal hard X-ray emissions from sources in all phases of solar flares. Because the physical conditions in such sources may differ substantially from those of the usual “footpoint” emission regions, we take the opportunity to revisit the physics of hard X-radiation and relevant theories of particle acceleration. 相似文献
17.
We present observations of the formation process of a small-scale filament on the quiet Sun during 5?–?6 February 2016 and investigate its formation cause. Initially, a small dipole emerged, and its associated arch filament system was found to reconnect with overlying coronal fields accompanied by numerous extreme ultraviolet bright points. When the bright points faded, many elongated dark threads formed and bridged the positive magnetic element of the dipole and the external negative network fields. Interestingly, an anticlockwise photospheric rotational motion (PRM) set in within the positive endpoint region of the newborn dark threads following the flux emergence and lasted for more than 10 hours. Under the drive of the PRM, these dispersive dark threads gradually aligned along the north-south direction and finally coalesced into an inverse S-shaped filament. Consistent with the dextral chirality of the filament, magnetic helicity calculations show that an amount of negative helicity was persistently injected from the rotational positive magnetic element and accumulated during the formation of the filament. These observations suggest that twisted emerging fields may lead to the formation of the filament via reconnection with pre-existing fields and release of its inner magnetic twist. The persistent PRM might trace a covert twist relaxation from below the photosphere to the low corona. 相似文献
18.
The propagation characteristics of MHD fast-mode disturbances, which can emanate from flare regions, are computed for realistic conditions of the solar corona at the times of particular flares. The path of a fast-mode disturbance is determined by the large-scale (global) coronal distributions of magnetic field and density, and can be computed by a general raytracing procedure (eikonal equation) adapted to MHD. We use the coronal (electron) density distribution calculated from daily K-coronameter data, and the coronal magnetic field calculated under the current-free approximation from magnetograph measurements of the photospheric magnetic field. We compare the path and time-development of an MHD fast-mode wavefront emitted from the flare region (as calculated from a realistic model corona for the day of the observed Moreton wave event) with actual observations of the Moreton wave event, and find that the Moreton wave can be identified with the rapidly moving intersection of the coronal fast-mode wavefront and the chromosphere (as hypothesized in our previous paper); the directivity (anisotropic propagation), as well as other characteristics of the propagation of the Moreton wave can be successfully explained.sponsored by the National Science Foundation. 相似文献
19.
Very Large Array (VLA) observations at 91-cm wavelength are combined with data from the SOHO EIT, MDI and LASCO and used to study the evolving coronal magnetic environment in which Type I noise storms and large-scale coronal loops occur. On one day, we have shown the early evolution of a coronal mass ejection (CME) in projection in the disk by tracing its decimetric continuum emission. The passage of the CME and an associated EUV ejection event coincided with an increase in the 91-cm brightness temperature of an extended coronal loop located a significant distance away and with the displacement of the 91-cm source during the early stage of the CME. We suggest that the energy deposited into the corona by the CME may have caused a local increase in the thermal or nonthermal electron density or in the electron temperature in the middle corona resulting in a transient increase in the brightness of the 91-cm loop. On a second observing day, we have consolidated the known association between magnetic changes in the photosphere and low corona with noise storm enhancements in an overlying radio source well in advance of a flare event in the same region. We find anti-correlated changes in the brightness of a bipolar 91-cm Type I noise storm that appear to be associated with the cancellation and emergence of magnetic flux in the underlying photosphere. In this case, the evolving fields may have led to magnetic instabilities and reconnection in the corona and the acceleration of nonthermal particles that initiated and sustained the Type I noise storm. 相似文献
20.
Neupert W.M. Newmark J. Delaboudinière J.-P. Thompson B.J. Catura R.C. Moses J.D. Gurman J.B. Portier-Fozzani F. Maucherat A.J. Defise J.M. Jamar C. Rochus P. Dere K.P. Howard R.A. Michels D.J. Freeland S. Lemen J.R. Stern R.A. 《Solar physics》1998,183(2):305-321
Solar EUV images recorded by the EUV Imaging Telescope (EIT) on SOHO have been used to evaluate temperature and density as a function of position in two largescale features in the corona observed in the temperature range of 1.0–2.0 MK. Such observations permit estimates of longitudinal temperature gradients (if present) in the corona and, consequently, estimates of thermal conduction and radiative losses as a function of position in the features. We examine two relatively cool features as recorded in EIT's Feix/x (171 Å) and Fexii (195 Å) bands in a decaying active region. The first is a long-lived loop-like feature with one leg, ending in the active region, much more prominent than one or more distant footpoints assumed to be rooted in regions of weakly enhanced field. The other is a near-radial feature, observed at the West limb, which may be either the base of a very high loop or the base of a helmet streamer. We evaluate energy requirements to support a steady-state energy balance in these features and find in both instances that downward thermal conductive losses (at heights above the transition region) are inadequate to support local radiative losses, which are the predominant loss mechanism. The requirement that a coronal energy deposition rate proportional to the square of the ambient electron density (or pressure) is present in these cool coronal features provides an additional constraint on coronal heating mechanisms. 相似文献