日冕瞬变的观测特征和理论解释

本文综述了日冕瞬变现象的主要观测特征及其理论模型.日冕瞬变是从太阳日冕中向行星际空间大量抛出物质的过程,每次事件可有5×1015克的物质在103秒的时间内以约500公里/秒的典型速度被驱动流到日球中.日冕瞬变与太阳耀斑和爆发日珥事件有密切的相关性.作为一种新的太阳活动现象,近年来对日冕瞬变提出了许多理论解释.一种数值模拟方法将瞬变看成是由于热力学量或磁力在日冕底部的脉冲增长所产生的结果.许多分析模型认为是由磁环内部的电磁力或外部的磁压力驱动所致,或者是环中磁浮力驱动的结果.考虑到瞬变与耀斑和爆发日珥的相关性,活塞驱动模型认为,瞬变是稠密等离子体喷射,像活塞驱动机制.观测和理论都有待于进一步的研究.      

中性电流片的形成和环形日冕瞬变

在双极背景场下,光球层反向磁通量的喷发将会在新老磁场之间形成中性电流片.本文从理想磁流体方程组出发,考虑磁场和日冕等离子体的相互作用,对上述电流片的形成过程进行了数值研究.结果表明,对亚音速喷发,将由里向外形成四个区域:(1)由喷发物质直接形成的低温,高密度日珥,位于最里层;(2)紧挨抛射日珥的低温稀疏区;(3)喷发物质和日冕物质向中性电流片集中形成的高温.高密度物质环;(４)在环的周围,由快磁声波形成的,密度略比日冕背景为高的前鞘区.上述结构与典型的环形日冕瞬变的观测特征相符.由此表明双极背景场下反向磁通量的喷发可能是触发这类瞬变的重要机制.      

Dynamical behaviour of the corona in association with radio emissions

From a combined analysis of radioheliographic observations and white light images of the Corona, it is shown that metric continua onsets or enhancements are associated with a great variety of dynamical changes of the Corona. It is found that if the most spectacular radio bursts (type IV) are associated with mass ejection transients, the more common noise storm onsets or enhancements are systematically associated with coronal changes which are not necessarly mass ejections.Simultaneous observations of Radio type III bursts and Hα mass ejecta are related. We are looking to the signature at low levels in the corona of the electron beam acceleration triggering type III bursts. The results deal with the relationship between the type III occurrence and optical features : the presence of velocities in Hα, the shape of Hα line which reveals trubulent motions and the probable existence of a shock wave.     

On upward motions of coronal hard X-ray sources in solar flares

Coronal hard X-ray (HXR) sources were discovered by the Yohkoh HXT telescope in about two dozen limb flares: Impulsive and gradual ones. On the basis of HXT data, we investigated the spatial evolution of coronal sources. Slow ascending motions of sources are seen in several flares. In five events, it was possible to estimate the velocity of the upward motion with values between 10 and 30 km/s. We present these observational results and conclude that coronal source motions should be studied statistically using the RHESSI high-resolution HXR imaging data. We discuss the possibility that coronal HXR emission is generated as bremsstrahlung of the fast electrons accelerated in collapsing magnetic traps due to joint action of the Fermi-type first-order mechanism and betatron acceleration.     

V.L.A. observations of flare build-up in coronal loops

Very Large Array (V.L.A.) measurements at 20 cm wavelength map emission from coronal loops with second-of-arc angular resolution at time intervals as short as 3.3 seconds. The total intensity of the 20 cm emission describes the evolution and structure of the hot plasma that is detected by satellite X-ray observations of coronal loops. The circular polarization of the 20 cm emission describes the evolution, strength and structure of the coronal magnetic field. Preburst heating and magnetic changes that precede burst emission on time scales of between 1 and 30 minutes are discussed. Simultaneous 20 cm and soft X-ray observations indicate an electron temperature $\text{T}{}_{\mathrm{e}}\textcolor{red}{}\text{2.5 × 10}{}^7\text{K}$ and electron density $\text{N}{}_{\mathrm{e}}\textcolor{red}{}\text{10}{}^{10}\text{cm}{}^{\mathrm{?3}}$ during preburst heating in a coronal loop that was also associated with twisting of the entire loop in space. We also discuss the successive triggering of bursts from adjacent coronal loops; highly polarized emission from the legs of loops with large intensity changes over a 32 MHz change in observing frequency; and apparent motions of hot plasma within coronal loops at velocities V > 2,000 kilometerspersecond.     

Evidence for a solar coronal thick-target hard X-ray source observed by RHESSI

We study a solar flare hard X-ray (HXR) source observed by the Reuven Ramaty high energy solar spectroscopic imager (RHESSI) in which the HXR emission is almost entirely in a coronal loop so dense as to be collisionally thick at electron energies up to ∼45−60 keV. This contrasts with most events previously reported in which the HXR emission is primarily from the loop footpoints in the collisionally dense chromosphere. In particular, we show that the high loop column densities inferred from the GOES and RHESSI soft X-ray emission measure and the volume of the flare loop are consistent with the coronal thick-target interpretation of the HXR images and spectra. The high column densities observed already at the very beginning of the impulsive phase are explained by chromospheric evaporation during a preflare which, as Nobeyama 17 GHz radio images reveal, took place in the same set of nested loops as the main flare.     

Synoptic observations of coronal transients and their interplanetary consequences

A small coronagraph has been placed in orbit to monitor the sun's outer corona from 2.5 to 10.0 solar radii, and five years of nearly continuous synoptic observations have now been completed. Rapid and sensitive image processing techniques have been developed to screen the data for transient phenomena, particularly coronal mass ejections (CMEs). About 50,000 coronal images have been examined, out of a five-year total of 68,000, and a standardized listing of more than 1,200 coronal transients for the period 1979–1982 has been prepared. These data have been analysed in the light of other available information, particularly on conditions in the interplanetary plasma. The dynamical characteristics of the active corona, as they are beginning to emerge from the data, are presented. We find that coronal mass ejections exercise significant influence on the interplanetary solar wind. They are the source of disturbances that are frequent and energetic, that tend to be somewhat focussed, that often reach shock intensity, and that propagate to large heliocentric distances, sometimes causing major geomagnetic storms.     

Evidence for a shock wave in visible light and radio observations of the 1980 June 29 event

Shock waves, as evidenced by type II radio bursts, often accompany flares and coronal mass ejection transients. At present, the density enhancements observed by coronagraphs are believed by some to be ejected matter from the low corona, and by others to be the compressed material behind a shock front. If the former is correct, one would expect in some cases to see a density enhancement, associated with the compression region of the shock, some distance ahead of the transient ejecta. Such a density enhancement has not been previously reported.The coronal transient of 1980 June 29 (0233 UT) was observed with the High Altitude Observatory's Coronagraph/Polarimeter aboard SMM. This flare-associated coronal transient event was well observed with the Culgoora Radioheliograph, including a well-developed type II burst. Visible on the coronagraph images is a faint circular arc moving out well ahead of the transient loops. This arc is moving at more than 900 km s^?1 while the transient itself is moving at a speed of about 600 km s^?1. Both the arc and transient appear to have originated either prior to the X-ray flare or at some height above the flare at the time of the flare. The type II burst observed at Culgoora is associated with the transient loops, and no type II emission is identified with the faint arc.Due to its great speed, we interpret the faint arc as a manifestation of a shock wave, but also envision a separate shock wave associated with the transient loops as evidenced by the type II emission. Preliminary density measurements are consistent with this interpretation, and show the outer shock wave associated with the faint arc to have a Mach number M_A ≤ 1.7. At present we have no convincing explanation for the lack of a type II burst in association with the arc.This work was supported in part by NASA through grants NSG-7287 and NAGW-91 to the University of Colorado, Boulder, and S-55989 to the High Altitude Observatory, National Center for Atmospheric Research. The National Center for Atmospheric Research, NCAR, is sponsored by the National Science Foundation.     

Dynamical models of coronal transients and interplanetary disturbances

We review the status of the best “off-the-shelf” tool available for the study of dynamical behavior of coronal transients and traveling interplanetary disturbances. This tool involves numerical solution of the initial-boundary value problem of multi-dimensional time-dependent magnetohydrodynamics. While this tool cannot address questions of turbulence and kinetic behavior, we suggest that deeper understanding of large scale phenomena can be obtained by direct comparison of the MHD models with multi-disciplinary synoptic observations of specific events on the sun, and in the corona and interplanetary space. Conclusions reached after a recent critique (based on a limited set of observational and numerical data) of the MHD paradigm's application to coronal transients are examined and found to have limited validity. Substantial observational progress was achieved during SMY through ground- and space-based observations of solar and interplanetary events. Many of these observations can confidently be associated with one another for specific events. These associations can be combined into a reasonable scenario of geometrical extent and mass, energy and momentum transfer in the framework of the solar-terrestrial chain of cause and effect. Several of these events during STIP Interval VII in August 1979 are used to provide test cases for an MHD simulation that is described with some details. The bringing-together of diverse observations is necessary in order to outline a program for the testing of dynamical models and their more physically-restricted approximations.     

Solar and late-type dwarfs

The Einstein Observatory and the IUE satellite have provided the observational basis for a major restructuring in theories of coronal formation for late-type stars. For the first time, coronal and transition region emission from a large sample of low mass (1 M_o) dwarf stars has been directly observed, with the unexpected result that essentially all such stars are x-ray emitters. The Sun, which was previously assumed to be typical, is now known to be at the low end of the x-ray luminosity function for solar-type stars. K- and M-dwarfs are observed to have nearly the same luminosity distributions as G-dwarfs and all of these spectral types have a large spread in x-ray luminosity.Observationally, there is a strong correlation between the strength of coronal emission in stars with outer convective zones and the rotation rates of these stars. At the present time we have only the beginnings of a satisfactory theoretical explanation for this correlation; although we are beginning to understand the connection between coronal emission strength and the magnetic field, we do not yet understand the stellar dynamo which generates the magnetic field. Studies of the coronal emission of stars may lead to a better understanding of stellar dynamos.     

A POSSIBLE MECHANISM FOR CORONAL MASS EJECTIONS

This paper examines a possible mechanism for the initiation of coronal mass ejections by means of the eruption of magnetic flux beneath a closed magnetic structure analogous to a coronal helmet structure. Anumerical computation shows that such an eruption deforms the helmet structure into an expanding bright "loop", followed by a dark cavity due to the expansion of the erupted magnetic field. The morphology and the internal structure of the loop are mainly determined by the background magnetic field whereas its ejective speed is largely controlled by the rate of flux ejection. The present model identifies the loop-like mass ejections with the above plasma structures moving with the frozen-in magnetic field and it may well explain some general properties of such ejections.      

The relationship between coronal mass ejections and solar flares

X-ray observations show that at a time consistent with a coronal mass ejection onset there is a small, soft X-ray burst (precursor). Generally this is followed some 20–30m later by a more significant flare. At the onset time there is frequently simultaneous activity from widely separated points on the Sun (>10⁵km). We present a model which accounts for the relationship between the coronal mass ejection and the precursor using 10²–10³ keV protons as the energy transfer agent. The protons (1) heat the high coronal loop. Inferred from the simultaneous activity, destabilizing the pressure balance to produce the ejection and (2) are guided by the magnetic field to below the transition region where they heat the chromospheric plasma to produce the precursor X-rays. High correlation between these events and a subsequent flare suggests that there may be a feedback mechanism operating from the coronal mass ejection.     

Origin and location of chromospheric evaporation in flares

Observation of two flares obtained with the Solar Maximum Mission spectrometers indicate that at flare onset the emission in soft (3.5 – 8 keV) and hard (16 – 30 keV) X-rays is predominant at the footpoints of the flaring loops. Since, at the same time, blue-shifts are observed in the soft X-ray spectra from the plasma at temperature of 10⁷ K, we infer that material is injected at high velocity into the coronal loops from the footpoints. These areas are also the sites of energy deposition, since their emission in hard X-rays is due to non-thermal electrons penetrating in the denser atmosphere. Hence, chromospheric evaporation occurs where energy is deposited. During the impulsive phase, the configuration of the flare region changes indicating that the flaring loop is progressively filled by hot plasma.     

日冕扰动传播的数值模拟(Ⅲ)太阳风的影响

本文数值研究了二维磁流体动力学平衡基态下开场区日面冷物质径向喷射所引起的日冕动力学响应。结果表明:(1)在高密度环前方有一弱扰动区近似以Alfvén速度向外传播;(2)高密度环前缘移动速度随着径向距离而增加, 其增加值近似为局地太阳风速度;(3)高密度环中等离子体的最大径向速度约在4个太阳半径处趋于局地逃逸速度;(4)对于强开放场, 环形结构在θ方向上没有明显的扩张。这些结果可以更好地解释伴随有日珥的日冕物质抛射事件。      

Asymmetric precipitation in a coronal loop as explanation of a singular observed spectrum

Almost 10 years of solar submillimeter observations have shown new aspects of solar activity, such as the presence of rapid solar spikes associated with the launch of coronal mass ejections and an increasing submillimeter spectral component in flares. We analyse the singular microwave–submillimeter spectrum of an M class solar flare on 20 December, 2002. Flux density observations measured by Sun patrol telescopes and the Solar Submillimeter Telescope are used to build the radio spectrum, which is fitted using Ramaty’s code. At submillimeter frequencies the spectrum shows a component different from the microwave classical burst. The fitting is achieved proposing two homogeneous sources of emission. This theoretical fitting is in agreement with differential precipitation through a magnetically asymmetric loop or set of loops. From a coronal magnetic field model we infer an asymmetric magnetic structure at the flare location. The model proposed to quantify the differential precipitation rates due to the asymmetry results in a total precipitation ratio Q₂/Q₁≈10⁴–10⁵, where Q₁(Q₂) represents the total precipitation in the loop foot with the high (low) magnetic field intensity. This ratio agrees with the electron total number ratio of the two sources proposed to fit the radio spectrum.     

Dynamical magnetic reconnection in Parker''s coronal heating model

The excitation (flares, ejections, heating, …) of the corona can be understood in terms of the dynamics of the confectively driven magnetized plasma. In particular, anomalous ohmic heating may be a consequence of the formation and rapid dissipation of small-scale magnetic fields in the corona. We have performed numerical simulations of the loop heating model proposed by Parker (1972, 1994), and have studied its dynamics and global power balance in order to assess its viability as a coronal heating candidate, with promising results. Our results suggest the following view of the small-scale dynamics of coronal loops. First of all, photospheric granular motions quasi-statically twist the magnetic field of the corona in a random-walk fashion. In topologically closed structures, the perpendicular magnetic energy increases, causing magnetic shear to build up at the quasi-separatrices of the resulting close-packed magnetic flux tubes. At some point, the boundary driving causes this stressed configuration to cross the threshold of an ideal time-scale MHD instability (possibly magnetic coalescence or resistive tearing) or a point of nonequilibrium and the field lines pinch toward a small-scale sheared configuration. It then becomes energetically favorable for dynamic reconnection to occur, producing narrow current sheets and an Ohmic heating rate sufficient to balance the input Poynting flux.     

两冕流间物质抛射事件的数值模拟

二维磁流体力学方程数值敢两冕流间的日冕物质抛射事件。为了能重现两冕流间ＣＭＥ的基本特征,本文在加入一定强度的扰动热压的同时,叠加了一个由圆线电流产生的闭合磁场。模拟结果表明,此类扰动能形成明显的高密度等离子体环形结构,且外环后随一暗脸。它们在径向和横向 都存在不同程度的膨胀。     

Signatures of magnetic reconnection in solar radio observations?

Magnetic reconnection occurs during eruptive processes (flares, CMEs) in the solar corona. This leads to a change of magnetic connectivity. Nonthermal electrons propagate along the coronal magnetic field thereby exciting dm- and m-wave radio burst emission after acceleration during reconnection or other energy release processes in heights of some Mm to ⩾700 Mm. We summarize the results of some case studies which can be interpreted as radio evidence of magnetic reconnection: under certain conditions, simple spectral structures (pulsation pulses, reverse drift bursts) are formed by simultaneously acting but widely spaced radio sources. Narrowband spikes are emitted as a side-effect during large-scale coronal loop collisions. In dynamic radio spectra, the lower fast mode shock formed in the reconnection outflow appears as type II burst-like but nondrifting emission lane. It has been several times observed at the harmonic mode of the local plasma frequency between 250 and 500 MHz and at heights of ≈200 Mm.     

Double plasma resonance and fine spectral structure of solar radio bursts

The process of generation of upper-hybrid waves by electron beam with loss-cone distribution is considered. The necessary conditions of the double plasma resonance effect, which is considered to be one of the most probable formation mechanisms of the zebra patterns in the spectra of solar radio emission, are investigated. It is shown that this effect considerably affects excitation of waves by electrons with power-law energetic spectrum. Interpretation of observations and diagnostics of plasma for the April 21, 2002 event are performed. It is found that the zebra stripes consist of separate short pulses; there is a good correlation between the separate stripes (with a certain temporal shift). The conclusion about the impulsive mode of injection of energetic particles into the coronal loop is made.     

The 1980 April 12 flare and transient: Report on progress in interpretation

A multidisciplinary study of this solar-interplanetary event is summarized by two main points: this flare was an incident in a process that began days before the flare, and continued after the flare; and the chain of events can be interpreted most simply in terms of energy input over scales of time and space that are large compared to the flare seen in the light of Hα. In support of these points, 5 aspects of the flare are described here: (1) hours before the flare, slow changes in coronal structure were associated with radio continuum emission, suggesting large-scale magnetic-field changes and the presence of energetic electrons; (2) long-lived X-ray loops require sustained energy input for at least an hour after the flare start; (3) interplanetary disturbance near earth is probably related to this limb flare, although the (expected) absence of a shock makes identification uncertain; (4) the coronal mass ejection overlay decaying magnetic field; (5) speed derived from frequency drift of the type II radio burst in the low corona, and from the travel time of the disturbance to 1 a.u., are about twice as great as the observed speed of the coronal mass ejection and of the disturbed solar-wind speed.