双重电流片和日冕瞬变

当背景磁场在日冕中存在零磁场线时,反向新磁通量的喷发将会产生双重电流片,包括零场区附近的磁场受到挤压而形成的横向电流片和新喷发场、原背景场之间形成的拱形电流片、本文用一对线偶极子来模拟背景场,用一对线磁荷来模拟反向喷发场,讨论了上述双重电流片的形成和演变过程。在电流片形成过程中,物质将向电流片集中。拱形电流片物质主要来自过渡层和光球层,并通过辐射损失进一步冷却,形成低温日珥环;横向电流片的物质则全部来自日冕,从而形成高温日冕环。以上结果可用来解释1984年4月14日观测到的日冕瞬变。     

Model for flare loops,fast motions,and opening of magnetic field in the corona

A model is presented for the penetration into the corona of a new magnetic field of a developing bipolar region and for its interaction with an old large-scale coronal field. An important feature of the model is a reconnection of the old and new fields inside the current sheet arising along the zero line of the total magnetic field calculated in the potential approximation. The magnetic reconnection and accumulation of plasma inside the current sheet can explain the appearance of dense coronal loops and the energy source at their tops. The plasma together with the magnetic lines is flowed into the sheet from both its sides. This fact explains the appearance of coronal cavities above the loops. If the large-scale field gradually decreases with the height, the loop motion is slowed down. The account of the dipolar structure of the magnetic field at large heights explains the possibility of a rapid break of the new field through the corona and the appearance of transients and open field regions - the coronal holes. In this case a fast rising current sheet can be a source of accelerated particles and of type II radio burst, instead of the shock wave considered usually.     

Formation of neutral current sheet and loop coronal transients

An eruption of opposite magnetic flux into a bipolar background field is likely to lead to the formation of a natural current sheet between the new emerging field and the background. A numerical study is made on this process, based on the ideal MMD equations, taking into account the interaction between the magnetic field and the coronal plasma. The result shows that a subsonic eruption will give rise to a four region structure; 1) a cool and dense prominence made of the erupting material in the innermost region; 2) a cool and tenuous region further out; 3) a hot and dense loop formed by the concentration of both the erupting material and the coronal material in the neutral current sheet; and 4) a forerunner region outside the loop with density slightly above the background, due to fast magneto-acoustic waves. This structure agrees with the observed features of typical loop coronal transients. Therefore the eruption of opposite magnetic flux into a bipolar background is probably an important mechanism for triggering off such transients.     

Magnetohydrodynamic study of shock waves in the current sheet of a solar coronal magnetic loop

A Lagrangian Remap (LareXd) Code is employed to investigate the shock wave formation in the current sheet of a solar coronal magnetic loop and its effect on the magnetic reconnection. We constructed the slow shock structure in the presence of viscosity and heat conduction parallel and perpendicular to the magnetic field and pairs of slow shocks propagate away from the central current sheet, the so-called diffusion region. Significant jumps in plasma density, pressure, velocity and magnetic field occur across the main shock while the temperature appears in the foreshock. In the presence of dissipative effects, the distinct jumps disappear and the shock profiles show smooth transition between the downstream and the upstream regions while the plasma density and the pressure show very narrow and a sharp decrease with time. These results can be applied to the heating of the solar corona, the structure of the magnetic reconnection and the solar wind.     

Motions of Flare Ribbons and Loops in Various Magnetic Configurations

Kopp–Pneuman-type magnetic configurations, which include a vertical current sheet, with various background fields are investigated. Dissipation of the current sheet as a result of magnetic reconnection produces bright flare ribbons on the solar disk and a growing flare loop system in the corona. In principle, the growth of flare loop system is governed by a reconnection process only, and the behavior of flare ribbons is also controlled by the background field. The flare ribbons may appear either separate or attached to one another at the onset of the flare depending on the background field distribution on the boundary surface. We calculate the decrease in height that magnetic field lines undergo after they have reconnected to form closed loops. Following previous practice, we refer to this decrease as field line shrinkage. Unlike the motions of flare ribbons, the shrinkage of flare loops depends weakly on the background field. Individual loops always shrink fastest at the moment it is produced by reconnection and just starts to leave the current sheet. The earlier the loop forms, the more and faster it shrinks. The relevant observations are explained on the basis of our calculations, and the aspects of the explanation that need improvement are also discussed.     

Coronal general magnetic field evolution as a new parameter of the solar cycle

The magnetic field lines of the corona associated with the solar-cycle surface general magnetic field are calculated by a potential-field approximation to study the solar-cycle evolution of the geometry of the coronal field. The surface field evolution used here is the radial field evolution, predicted by a model of the solar cycle driven by the dynamo action of the global convection, and justified observationally using Mount Wilson magnetic synoptic chart data. The evolution of the calculated coronal general field is now good for comparison with observations and shows the following. (i) The field of the polar and high-altitude corona has dipolar structure in almost all phases of the solar cycle except in a short time interval around maximum phase despite the quadrupolar structure of the general magnetic field at the surface; quadrupolar field forms loop-like structure in the lower corona. The almost-dipolar structure of the polar and high-latitude corona and the loop formation of the equatorial lower corona explain the appearance of the undisturbed minimum corona observed at eclipses. (ii) The polar field lines are directed almost radially at the minimum phase, which should be responsible for polar plumes. The field lines slowly open up to participate in the loop-like structure of the equatorial lower corona, and rapidly change their structure and polarity at the maximum phase, to resume the almost radial configuration slowly, (iii) During the rapidly changing maximum phase, the field lines do not penetrate deep into the interplanetary space resulting in the absence of polar plumes and the appearance of the circular corona- the maximum corona. In this phase, the coronal field should not be approximated by a dipole field. The surface field evolution which can explain such behaviors of the corona is characteristic of the solar-cycle process dominated by the latitudinal gradient of the differential rotation. If the radial gradient dominated in the subsurface process, the coronal evolution would look quite different and would show latitudinal propagation of enhancement of activity. Although nonaxisymmetric features should be superposed on the axisymmetric general field to express the real corona, the general field can be a basic coronal field in studying long-term interaction between the convection zone and the interstellar space especially in studying the magnetic braking of the solar rotation.     

MHD Modeling of Coronal Large-Amplitude Waves Related to CME Lift-off

We have employed a two-dimensional magnetohydrodynamic simulation code to study mass motions and large-amplitude coronal waves related to the lift-off of a coronal mass ejection (CME). The eruption of the filament is achieved by an artificial force acting on the plasma inside the flux rope. By varying the magnitude of this force, the reaction of the ambient corona to CMEs with different acceleration profiles can be studied. Our model of the ambient corona is gravitationally stratified with a quadrupolar magnetic field, resulting in an ambient Alfvén speed that increases as a function of height, as typically deduced for the low corona. The results of the simulations show that the erupting flux rope is surrounded by a shock front, which is strongest near the leading edge of the erupting mass, but also shows compression near the solar surface. For rapidly accelerating filaments, the shock front forms already in the low corona. Although the speed of the driver is less than the Alfvén speed near the top of the atmosphere, the shock survives in this region as well, but as a freely propagating wave. The leading edge of the shock becomes strong early enough to drive a metric type II burst in the corona. The speed of the weaker part of the shock front near the surface is lower, corresponding to the magnetosonic speed there. We analyze the (line-of-sight) emission measure of the corona during the simulation and recognize a wave receding from the eruption site, which strongly resembles EIT waves in the low corona. Behind the EIT wave, we clearly recognize a coronal dimming, also observed during CME lift-off. We point out that the morphology of the hot downstream region of the shock would be that of a hot erupting loop, so care has to be taken not to misinterpret soft X-ray imaging observations in this respect. Finally, the geometry of the magnetic field around the erupting mass is analyzed in terms of precipitation of particles accelerated in the eruption complex. Field lines connected to the shock are further away from the photospheric neutral line below the filament than the field lines connected to the current sheet below the flux rope. Thus, if the DC fields in the current sheet accelerate predominantly electrons and the shock accelerates ions, the geometry is consistent with recent observations of gamma rays being emitted further out from the neutral line than hard X-rays.     

Magnetic reconnection in the temperature minimum region and prominence formation

Magnetic reconnection at the photospheric boundary is an essential part of some theories for prominence formation. We consider a simple model for reconnection in this region. Parameters of the reconnecting current sheet are expressed in terms of the concentration and temperature of the outside dense and cold plasma, magnetic field intensity, and velocity of convective flows at the photosphere. The reconnection process is shown to be most efficient in a layer several hundred kilometers thick coinciding with the temperature minimum region of the solar atmosphere. The calculated upward flux of matter through the current sheet ( 10¹¹–10¹² g s^–1) is amply sufficient for prominence formation in the upper chromosphere or lower corona.     

Thermal instability of coronal neutral sheets and the formation of quiescent prominences

It is argued that the quiscent prominences are a natural consequence of the formation and thermal instability of current sheets in the corona. Thus observation and theory of prominences can give vital information on the presence of currents and the topology of magnetic fields in the corona. Conversely by developing the theory of the structure and evolution of current sheets under coronal conditions we can attempt to gain a comprehensive understanding of the structure, evolution, and mass and energy balance of quiescent prominences. A stability analysis for coronal material permeated by a vertical magnetic field rooted in the photosphere, indicates that a condensation will take the form of a thin vertical wedge of cool matter. The development of a finite condensation is followed and it is shown that photospheric line tying is only important in the initial stages. A perturbation analysis of vertical motions at the neutral sheet shows that thermal instability can lead to overstable oscillations. Cooling of coronal material can lead to both upward and downward mass motions, and gravitational energy release is important to the thermal balance of prominences. Relevant optical and radio observations are discussed. Synoptic observations of the development of active regions and magnetic fields are needed to test the basic hypothesis of the formation of prominences from neutral sheets.     

Magnetic field and current sheets in the corona above active regions

A new method for the calculation of coronal magnetic field is proposed and it is shown to reproduce the EUV features in the corona as observed by Skylab experiments satisfactorily well. One of the remarkable points is that it reproduces the loopy threads in the active region corona and also the large scale field lines connecting active regions. The existence of coronal current is expected wherever the present coronal-current-free model fails to represent the feature. A method of calculating the coronal sheet-current is also developed with the purpose of knowing the shape of the current sheet and the amount of magnetic stress energy stored due the the presence of it by comparing the calculated field configuration with the observed local distortion of the EUV threads. This may be used in pinning down the possible site of the flare and in discussing the flare occurrence in terms of the energy stored there.During the preparation of this work, Poletto et al. (1975) calculated the magnetic field shape in Schmidt's method to compare with the soft X-ray feature obtained by Skylab.     

A model of hot loops associated with solar flares

A dynamical model is proposed for the formation of soft X-ray emitting hot loops in solar flares. It is examined by numerical simulations how a solar model atmosphere in a magnetic loop changes its state and forms a hot loop when the flare energy is released in the form of heat liberation either at the top part or around the transition region in the loop.When the heat liberation takes place at the top part of the loop which arches in the corona, the plasma temperature around the loop apex rises rapidly and, as the result, the downward thermal conductive flux is increased along the magnetic tube of force. Soon after the thermal conduction front rushes into the upper chromosphere, a local peak of pressure is produced near the conduction front and the chromospheric material begins to expand into the corona to form a high-temperature (10⁷ K-3 × 10⁷ K at the loop apex) and high-density (10¹⁰ cm^–3-10¹¹ cm^–3 at the loop apex) loop. The velocity of the expanding material can reach a few hundred kilometres per second in the coronal part. The thermal conduction front also plays a role of piston pushing the chromospheric material downward and gives birth to a shock wave which propagates through the minimum temperature region into the photosphere. If, on the other hand, the heat source is placed around the transition region in the loop, the expansion of the material into the corona occurs from the beginning of the flare and the formation process of the hot loop differs somewhat from the case with the heat source at the top part of the loop.Thermal components of radiations emitted from flare regions, ranging from soft X-rays to radio wavelengths, are interpreted in a unified way by using physical quantities obtained as functions of time and position in our flare loop model as will be discussed in detail in a following paper.     

Interplanetary Signatures of Unipolar Streamers and the Origin of the Slow Solar Wind

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.     

Coronal Magnetic Field Context of Simple CMEs Inferred from Global Potential Field Models

Most work on coronal mass ejection (CME) interpretation focuses on the involved active region rather than on the large-scale coronal context. In this paper a global potential-field source-surface model of the coronal magnetic field is used to evaluate the sensitivity of the coronal field configuration to the location, orientation, and strength of a bipolar active region relative to a background polar field distribution. The results suggest that the introduction of antiparallel components between the field of the active region and the background field can cause significant topological changes in the large-scale coronal magnetic field resembling observations during some simple CMEs. Antiparallel components can be introduced in the real corona by the diffusion and convection of photospheric fields, flux emergence, or erupted or shear-induced twist of active-region fields. Global MHD models with time-dependent boundary conditions could easily test the stability of such configurations and the nature of any related transients.     

Similarities and Differences between Coronal Holes and the Quiet Sun: Are Loop Statistics the Key?

Coronal holes (CH) emit significantly less at coronal temperatures than quiet-Sun regions (QS), but can hardly be distinguished in most chromospheric and lower transition region lines. A key quantity for the understanding of this phenomenon is the magnetic field. We use data from SOHO/MDI to reconstruct the magnetic field in coronal holes and the quiet Sun with the help of a potential magnetic model. Starting from a regular grid on the solar surface we then trace field lines, which provide the overall geometry of the 3D magnetic field structure. We distinguish between open and closed field lines, with the closed field lines being assumed to represent magnetic loops. We then try to compute some properties of coronal loops. The loops in the coronal holes (CH) are found to be on average flatter than in the QS. High and long closed loops are extremely rare, whereas short and low-lying loops are almost as abundant in coronal holes as in the quiet Sun. When interpreted in the light of loop scaling laws this result suggests an explanation for the relatively strong chromospheric and transition region emission (many low-lying, short loops), but the weak coronal emission (few high and long loops) in coronal holes. In spite of this contrast our calculations also suggest that a significant fraction of the cool emission in CHs comes from the open flux regions. Despite these insights provided by the magnetic field line statistics further work is needed to obtain a definite answer to the question if loop statistics explain the differences between coronal holes and the quiet Sun.     

Magnetic field, helicity and the 2000 July 14 flare in solar active region 9077

In this paper we analyse the non-potential magnetic field and the relationship with current (helicity) in the active region NOAA 9077 in 2000 July, using photospheric vector magnetograms obtained at different solar observatories and also coronal extreme-ultraviolet 171-Å images from the TRACE satellite.
We note that the shear and squeeze of magnetic field are two important indices for some flare-producing regions and can be confirmed by a sequence of photospheric vector magnetograms and EUV 171-Å features in the solar active region NOAA 9077. Evidence on the release of magnetic field near the photospheric magnetic neutral line is provided by the change of magnetic shear, electric current and current helicity in the lower solar atmosphere. It is found that the 'Bastille Day' 3B/5.7X flare on 2000 July 14 was triggered by the interaction of the different magnetic loop systems, which is relevant to the ejection of helical magnetic field from the lower solar atmosphere. The eruption of the large-scale coronal magnetic field occurs later than the decay of the highly sheared photospheric magnetic field and also current in the active region.     

How To Use Magnetic Field Information For Coronal Loop Identification

The structure of the solar corona is dominated by the magnetic field because the magnetic pressure is about four orders of magnitude higher than the plasma pressure. Due to the high conductivity the emitting coronal plasma (visible, e.g., in SOHO/EIT) outlines the magnetic field lines. The gradient of the emitting plasma structures is significantly lower parallel to the magnetic field lines than in the perpendicular direction. Consequently information regarding the coronal magnetic field can be used for the interpretation of coronal plasma structures. We extrapolate the coronal magnetic field from photospheric magnetic field measurements into the corona. The extrapolation method depends on assumptions regarding coronal currents, e.g., potential fields (current-free) or force-free fields (current parallel to magnetic field). As a next step we project the reconstructed 3D magnetic field lines on an EIT-image and compare with the emitting plasma structures. Coronal loops are identified as closed magnetic field lines with a high emissivity in EIT and a small gradient of the emissivity along the magnetic field.     

Magnetospheric substorms and solar flares

Assuming that basic plasma processes associated with magnetospheric substorms and solar flares are similar and thus assuming also that a flare ribbon is produced by the impact of field-aligned current-carrying electrons on the chromosphere, a chain of processes leading to solar flares is considered, including the dynamo process in the photospheric level in the vicinity of bipolar sunspots, the formation of a sheet current in the lower coronal level, the interruption of the sheet current, the subsequent diversion of it to the chromosphere, the development of a potential drop along magnetic field lines, the acceleration of current-carrying electrons and their impact on the chromosphere, producing a pair of flare ribbons.     

Connections Between the White-Light Eclipse Corona and Magnetic Fields over the Solar Cycle

Observations of ten solar eclipses (1973–1999) enabled us to reveal and describe mutual relations between the white-light corona structures (e.g., global coronal forms and most conspicuous coronal features, such as helmet streamers and coronal holes) and the coronal magnetic field strength and topology. The magnetic field strength and topology were extrapolated from the photospheric data under the current-free assumption. In spite of this simplification the found correspondence between the white-light corona structure and magnetic field organization strongly suggests a governing role of the field in the appearance and evolution of local and global structures. Our analysis shows that the study of white-light corona structures over a long period of time can provide valuable information on the magnetic field cyclic variations. This is particularly important for the epoch when the corresponding measurements of the photospheric magnetic field are absent.     

The formation of solar prominences by thermal instability in a current sheet

The energy balance equation for the upper chromosphere or lower corona contains a radiative loss term which is destabilizing, because a slight decrease in temperature from the equilibrium value causes more radiation and hence a cooling of the plasma; also a slight increase in temperature has the effect of heating the plasma. In spite of this tendency towards thermal instability, most of the solar atmosphere is remarkably stable, since thermal conduction is very efficient at equalizing any temperature irregularity which may arise. However, the effectiveness of thermal conduction in transporting heat is decreased considerably in a current sheet or a magnetic flux tube, since heat can be conducted quickly only along the magnetic field lines. This paper presents a simple model for the thermal equilibrium and stability of a current sheet. It is found that, when its length exceeds a certain maximum value, no equilibrium is possible and the plasma in the sheet cools. The results may be relevant for the formation of a quiescent prominence.     

Coronal holes as indicators of large-scale magnetic fields in the corona

It is shown that coronal holes may be used as indicators to trace the location of the neutral line on the source surface in the corona. At the same time, coronal holes are shown to concentrate in regions of enhanced magnetic field at the source surface. This provides us with a simple method for predicting the interplanetary current sheet and sector structure which, in turn, determine the location of the proton complexes and the outflow regions of high-velocity streams. Rotation of coronal holes has been studied. Rather than being rigid, it displays the same reduced differentiallity as the rest of the corona. However, there are particular periods 2 or 3 years before the cycle minimum when the solid-body type of rotation is settled for both the coronal holes and the corona as a whole.