The Influence of Coronal Background Structure with Nested and Closed Magnetic Fields on the Triggering of CME

The influence of coronal streamer background with nested and closed magnetic fields on the of the triggering of coronal mass ejections (CMEs) is investigated in the meridian plane. In the coronal streamers’ background magnetic structure there are three small-scale closed magnetic fields, of which the middle one has a direction opposite to that of the global dipolar field of coronal streamers. The trigger model of CMEs emerges from beneath this small-scale closed magnetic field and possesses a concentric circular structure with radius of a = 0.1R_s (R_s being the solar radius). The direction of the magnetic field in the front half of the CME trigger model is opposite to that of the small-scale closed field and is the same as that of the streamers’ global dipolar field. As revealed by numerical simulation, when the ratio of the plasma pressure at the center of the CME trigger model to the boundary pressure is m ≤ 2, then the emerging model can trigger CMEs. When m < 2, then it cannot. The error in this critical value of 2 is less than 1%.     

On the Tomographic Reconstruction of the 3D Electron Density for the Solar Corona from STEREO COR1 Data

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.     

A Statistical Study on the Morphology of Rays and Dynamics of Blobs in the Wake of Coronal Mass Ejections

In this paper, with a survey through the Large Angle and Spectrometric Coronagraph (LASCO) data from 1996 to 2009, we present 11 events with plasma blobs flowing outwards sequentially along a bright coronal ray in the wake of a coronal mass ejection. The ray is believed to be associated with the current-sheet structure that formed as a result of solar eruption, and the blobs are products of magnetic reconnection occurring along the current sheet. The ray morphology and blob dynamics are investigated statistically. It is found that the apparent angular widths of the rays at a fixed time vary in a range of 2.1?–?6.6^○ (2.0?–?4.4^○) with an average of 3.5^○ (2.9^○) at 3R _⊙ (4R _⊙), respectively, and the observed durations of the events vary from 12 h to a few days with an average of 27?h. It is also found, based on the analysis of blob motions, that 58% (26) of the blobs were accelerated, 20% (9) were decelerated, and 22% (10) moved with a nearly constant speed. Comparing the dynamics of our blobs and those that are observed above the tip of a helmet streamer, we find that the speeds and accelerations of the blobs in these two cases differ significantly. It is suggested that these differences of the blob dynamics stem from the associated magnetic reconnection involving different magnetic field configurations and triggering processes.     

DYNAMICAL EVOLUTION OF A CORONAL STREAMER – FLUX ROPE SYSTEM – II. A Self-Consistent Non-Planar Magnetohydrodynamic Simulation

The dynamical response of a helmet streamer to a flux-rope escape from the sub-photosphere is examined in a physically self-consistent manner within the approximation of axisymmetric three-dimensional magnetohydrodynamics (i.e., so-called 2 D). In contrast to the previous planar analyses of Paper I (Wu, Guo, and Wang, 1995), the present study shows, with the inclusion of out-of-plane components of magnetic and velocity fields, that the magnetic configuration represents a helical flux rope instead of a planar bubble as shown in Paper I. Because of this more physically realistic configuration, we are able to examine the dynamical evolution of the helical flux rope's interaction with the helmet streamer. This process leads to the formation of two parts of the solar mass ejection; (i) the expulsion of the helmet dome due to eruption of this flux rope, and (ii) the flux rope's eruption itself. When this two-part feature propagates out to the interplanetary space, it exhibits all the physical characteristics of observed interplanetary magnetic clouds. These numerical simulations also show that the dynamical behavior of the streamer – flux rope system has three distinct states: (i) quasi-equilibrium, (ii) non-equilibrium, and (iii) eruptive state depending on the energy level of the flux rope.     

Onset of hydrodynamic non-equilibrium in helmet streamers

The steady-state solar wind solution is examined for different geometries of the flow tube that mimics a helmet streamer. Onset of non-equilibrium is seen whenever the spatial variation of the flow geometry crosses critical values. It is suggested that the dynamical response of the flow to the onset of non-equilibrium can manifest as a coronal mass ejection.     

Observation of sectored structure in the outer solar corona: Correlation with interplanetary magnetic field

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).     

Some general properties of helmeted coronal structures

A simplified analysis of helmeted coronal structures is carried out and some of the gross properties of such structures discussed. It is found that the magnetically closed region can have but a limited extension into the corona. For temperatures in excess of 1.5 × 10⁶ °K, the maximum height above the limb is about 1.6 R . The maximum possible extension of the helmet from the solar center is exactly one-half the distance to the critical point (where the flow velocity passes through the speed of sound). For this reason, a helmet streamer, at least out to a few solar radii, is essentially a magnetostatic structure - the flow adjacent to the helmet having little effect upon its properties. For given base dimensions, there is a maximum temperature for which a helmet streamer can exist - giving an indication of why such streamers do not appear over young active regions. If the temperature in the helmet and in the streaming region are approximately the same, the helmet height, helmet shape, external flow velocity, and rate of outward decline in the magnetic field are shown to be much more dependent upon the photospheric field distribution than upon the field strength. The density enhancement, however, is a strong function of the field strength. This enhancement is preserved out to the top of the helmet with both the density inside and outside decreasing approximately as predicted by hydrostatic equilibrium. The possible existence of both domed helmets and cusped helmets is demonstrated with the former existing at lower temperatures and the latter at higher temperatures. Cusped helmets occur, however, over a relatively narrow temperature range and are, hence, expected to be less common. The expansion velocity outside the helmet is higher than that predicted by radial flow but increases outward much more slowly. The magnetic field decreases outward proportionally to the square root of the density and inversely proportionally to the velocity - bearing, in general, no relation to a potential field since the rate of decline in field strength is determined by the temperature.On leave from AC Electronics Research Laboratories Santa Barvara, Calif., U.S.A.     

The Geometric Spreading of Coronal Plumes and Coronal Holes

The geometric spreading in plumes and in the interplume region in coronal holes is calculated, using analytic and numerical theoretical models, between 1.0 and 5.0 R. We apply a two-scale approximation that permits the rapid local spreading at the base of plumes (f_l) to be evaluated separately from the global spreading (f_g) imposed by coronal hole geometry. We show that f_l can be computed from a potential-field model and f_g can be computed from global magnetohydrodynamic simulations of coronal structure. The approximations are valid when the plasma beta is small with respect to unity and for a plume separation small with respect to a solar radius.     

Mhd Interpretation of LASCO Observations of a Coronal Mass Ejection as a Disconnected Magnetic Structure

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     

Observation of a possible neutral sheet in the corona

A linear coronal ray, centered on a large helmet, is prominent in white-light photographs taken at the 1922 eclipse. It extends from near the limb to 4 R , has minimum width 9 arc, and persisted for at least 35 min. Examination reveals that the ray marks the cleavage between the domes of a twin-arch streamer which in turn is associated with two large, distinct chromospheric active regions. The ray is interpreted as the edge-on aspect of a coronal neutral sheet which separates areas of presumed opposite magnetic polarity in the two surface regions.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

磁通量喷发的大气响应Ⅰ.日冕物质喷射机制

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。     

Coronal streamers

This work extends a previous analysis of helmet streamers into the somewhat higher range of coronal temperature where streamer geometries are shown to be open, in the sense that there is solar wind expansion everywhere. It is shown that, for a given photospheric field distribution, a certain minimum temperature is required for this type of streamer - this minimum temperature coinciding with the maximum temperature compatible with a helmet streamer. Near this minimum temperature, the streamer is very constricted and the critical point in the streamer core lies at the point of minimum cross-section. Hence the throat, under these conditions, becomes a true geometrical throat rather than the conventional gravitational throat. As the temperature is increased, the streamer shape becomes correspondingly more radial and the location of the throat becomes asymptotically more gravitationally determined. Residual manifestations of coronal streamers at large distances are investigated. It is found that lateral density variations at the earth's orbit tend to be small but velocity variations can become appreciable (100–200 km/sec) for streamers originating in regions where the photospheric magnetic field is strong. At large distances, either streamer or interstreamer regions can dominate, the former occurring at high temperature (2 × 10⁶K) and the latter being favored at lower temperature (1.5 × 10⁶K). In all cases the cross-section becomes essentially radial just above the point where it is a minimum. The marked sensitivity of these shapes to coronal temperature is pointed out - computations indicating that streamers can vary from helmet configurations to almost radial filaments for a very slight increase in temperature. This behavior suggests a strong solar cycle influence upon coronal form.     

First order latitude effects in the solar wind

The Weber-Davis model of the solar wind is generalized to include the effects of latitude. The principal assumptions of perfect electrical conductivity, rotational symmetry, a polytropic relation between pressure and density, and a flow aligned magnetic field in a system rotating with the Sun, are retained. A flow aligned magnetic field in the rotating system may be expressed in terms of the flow velocity and density. Rotational symmetry fixes the longitudinal flow velocity V_φ in terms of the flow in the r?θ plane. Thus, the original three dimensional magnetohydrodynamic flow problem is reduced to a two dimensional hydrodynamic flow problem in the r?θ plane.There are three critical surfaces associated with the equations which supply conditions to determine three of six required boundary conditions. The specified boundary conditions at the base of the corona are the temperature, density, and magnitude of the magnetic field. The equations are then expanded about the radial, nonrotating Parker solution and an analytic solution is obtained for the resulting first order equations. The results show that for constant coronal boundary conditions there is a latitudinal flow toward the solar poles, as a result of magnetic stresses, which persists out to large distances for the Sun. Associated with this flow is a latitudinal component of the magnetic field. The radial flow parameters are, to within small first order differences, in agreement with those of the Parker and the Weber-Davis models of the solar wind.The equations are further generalized to permit first order latitudinal variations in the specified coronal boundary conditions. Results at 1 a.u. are presented for 5 per cent latitudinal differences between the equatorial and polar values. These results show that the solution at 1 a.u. is most sensitive to a latitudinal dependence in the boundary temperature and least sensitive to a latitudinal dependence in the magnetic field magnitude.A solution is then obtained for an approximate dipolar variation in the coronal magnetic field magnitude. This solution predicts that the latitudinal flow is initially toward the Equator due to magnetic channeling; however, this effect is rapidly overcome and the latitudinal flow at 1 a.u. is toward the pole and not significantly different from the solution for constant boundary conditions.     

The Asymmetrical Eruption of a Quiescent Filament and Associated Halo CME

We will present detailed observations of the asymmetrical eruption of a large quiescent filament on 24 November 2002, which was followed by a two-ribbon flare, three coronal dimmings, endpoint brightenings, and a very fast halo-type coronal mass ejection (CME). Before the eruption, the filament lay along the main neutral line (MNL) underneath a single-arcade helmet streamer with a simple bipolar configuration. However, photospheric magnetic fields on both sides of the filament showed an asymmetrical distribution, and the filament and MNL were not located just at the center of the streamer base but were closer to the eastern leg of the streamer arcade. Therefore, instead of erupting along the streamer’s symmetrical axis, the filament showed a nonradial and asymmetrical eruption. It lifted from the eastern flank of the streamer arcade to impact the western leg directly, leading to an asymmetrical CME that expanded westward; eventually the streamer was disrupted significantly. Accordingly, the opposite-polarity coronal dimmings at both sides of the filament forming in the eruption also showed an asymmetrical area distribution. We thus assume that the streamer arcade could guide the filament at the early eruption phase but failed to restrain it later. Consistent with previous results, these observations suggest that the global background magnetic field can impose additional action on the initial eruption of the filament and CME, as well as the dimming configuration.     

Large-Scale Active Coronal Phenomena in YOHKOH SXT Images. III. Enhanced Post-Flare Streamer

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.     

Solar coronal streamers

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.     

Evolution of coronal helmets during the ascending phase of solar cycle 20

The principal polar-crown coronal helmet structures were selected from nearly three years (May, 1965–January, 1968) of K-coronameter observations made at Haleakala and Mauna Loa, Hawaii. Six isolated and long-lived helmet systems were found at latitudes of 45° and above. Their developments are compared with underlying chromospheric and photospheric activity and a simple phenomenological model is presented showing that a coronal system is formed over an active region. Thereafter the center of gravity of the system gradually drifts poleward with the trailing unipolar magnetic region (UMR), and it becomes a high latitude coronal helmet, arched over a polar crown filament.By comparison of these coronal helmets with observations of the outer corona (to circa 4 R ) made at solar eclipse, lunar sunset, and with balloon and rocket-borne externally occulted corona-graphs, it appears that ground-based K-coronameter measurements to a distance of 1.5–2.0 R are sufficient to detect the coronal streamers.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The Solar Origin of the 6 January 1997 Coronal Mass Ejection

By using observations from the satellites of the International Solar Terrestrial Physics (ISTP) Observatories, the relationships among the coronal mass ejection (CME), the helmet streamer and the disappearing filament (DSF) have been studied. Our main conclusions are as follows: (1) The DSF disrupted the streamer, thus resulting in the restructuring of coronal field and causing the mass in the helmet streamer to form the CME. (2) The DSF under a helmet streamer and the sigmoid soft X-ray loop are possibly the precursors of the 6 January 1997 CME. (3) The energy stored in the filament circuit and the energy of the CME (include kinetic, potential and magnetic energies) are estimated and it is found that there was enough energy stored in the filament to provide the CME of 6 January 1997. (4) The CME's speed in response to the DSF is calculated. It is showed that the DSF can drive the CME to the observed speed.     

Magnetic configurations of streamer structures in the solar atmosphere

We consider two types of streamer structures observed in the solar atmosphere. Structures of the first type are medium-scale configurations with scale lengths comparable to the scale height in the corona, kT/mg = 100 thousand km, which appear as characteristic plasma structures in the shape of a dome surrounding the active region with thin streamers emanating from its top. In configurations of this type, gravity plays no decisive role in the mass distribution. The plasma density is constant on magnetic surfaces. Accordingly, the structure of the configurations is defined by the condition ψ = const, where ψ is the flux function of the magnetic field. Structures of the second type are large-scale configurations (coronal helmets, loops, and streamers), which differ from the above structures in that their scale lengths exceed the scale height in the corona. For them, gravity plays a decisive role; as a result, instead of the magnetic surfaces, the determining surface is BgradΦ = 0. We constructed three-dimensional images of these structures. Some of the spatial curves called “visible contours” of the B_r = 0 surface are shown to be brightest in the corona. We assume that the helmet boundaries and polar plumes are such curves.     

The evolution of a coronal streamer and the photospheric magnetic field

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.