Large-scale dimmings produced by solar coronal mass ejections according to SOHO/EIT data in four EUV lines

SOHO/EIT data are used to analyze dimmings, or transient coronal holes (regions of reduced soft-X-ray and EUV emission), which are observed on the solar disk after halo-type coronal mass ejections (CMEs). Simultaneous observations in the 171 Å FeIX/X, 195 Å FeXII, and 284 Å FeIX coronal lines, which are sensitive to temperatures of T _e ≈1.2, 1.5, and 2.0 MK, respectively, are considered, together with the 304 Å HeII transition-region line (T _e ≈(0.02–0.08) MK). Difference images taken at intervals of six and twelve hours and compensated for solar rotation indicate that dimmings are normally strongly pronounced and have similar large-scale structures in the moderate-excitation-temperature 171 Å and 195 Å coronal lines, while the higher-temperature 284 Å line mainly display the deepest portions of the dimmings. In addition, clear dimmings with relatively small areas are visible in the 304 Å transition-region line during many CMEs, in particular, in regions adjacent to the source of the eruption. Moreover, dimmings in the transition region without coronal counterparts are observed during some events. These results suggest that the opening of magnetic-field lines and the resulting density reduction that occur during a CME can also involve cold plasma of the transition region. In addition, the effects of temperature variations cannot be ruled out for some dimming structures.     

Solar large-scale channeled dimmings produced by coronal mass ejections

A new type of dimmings, or transient coronal holes (i.e., regions of reduced soft-X-ray and EUV emission), is revealed in analyses of difference solar images obtained with the SOHO EIT ultraviolet telescope at 195 Å. Such features can be observed on the solar disk after halo-type coronal mass ejections (CMEs). If several active regions, filaments, and other structures are present on the disk during a major eruptive event, then strongly anisotropic, channel-shaped (“channeled”) dimmings coexist with relatively compact dimmings adjacent to the eruption center. The channeled dimmings are comparable to the compact dimmings in terms of their contrast; stretch along several narrow, extended features (channels); and can span nearly the entire visible disk. Coronal waves, which appear as fronts of enhanced brightness traveling ahead of the dimmings in some halo CME events, are also anisotropic. We argue that such transient phenomena are closely related to the strong disturbance and restructuring of large-scale magnetic fields involved in CMEs, and the channeled character of the dimmings reflects the complexity of the global solar magnetosphere, in particular, near the solar-activity maximum.     

Large-scale phenomena on the Sun associated with the eruption of filaments outside active regions: the event of September 12, 1999

The event of September 12, 1999 is used to analyze large-scale disturbances associated with coronal mass ejections during the eruption of filaments outside active regions. The analysis is based on Hα filtergrams, EUV and soft X-ray images, and coronograph data. The filament eruption occurred in relatively weak magnetic fields, but was accompanied by larger-scale phenomena than flare events. During several hours after the eruption, a large-scale arcade developed, whose bases formed diverging flare-like ribbons. The volume of the event was bounded by an “EIT wave”, which was quasi-stationary at the solar surface and expanded above the limb. The event did not have an impulsive component; therefore the “EIT wave” above the limb was a magnetic structure, identified as the front of a coronal mass ejection by virtue of its shape, structural features, and kinematics. Three types of dimmings were observed within the areal of the event, cause by (a) the evacuation of plasma, (b) heating of plasma with its subsequent evacuation, and (c) the absorption of radiation in a system of filaments activated by the eruption. The fact that a dimming appeared due to plasma heating was revealed by its presence in soft X-rays, whereas the four EIT channels did not demonstrate this. This brings into question the correctness of certain conclusions drawn earlier based purely on EIT data. A transformation of magnetic fields brought about by the eruption also occurred in a stationary coronal hole adjacent to the areal of the event. The expansion of the coronal mass ejection was self-similar and characterized by a rapidly decreasing acceleration, which is not taken into account in the widely used polynomial approximation.     

Large-scale activity in major solar eruptive events of November 2004 according to SOHO data

Data obtained with the EIT UV telescope and LASCO coronagraph of the SOHO satellite are used to analyze large-scale solar disturbances associated with a series of major flares and coronal mass ejections that occurred in the late decline phase of cycle 23, on November 3–10, 2004, and gave rise to strong geomagnetic storms. Derotated fixed-base difference heliograms taken in the 195 Å coronal channel at 12-min intervals and in the various-temperature 171, 195, 284, and 304 Å channels at 6-h intervals indicate that these disturbances were global and homologous; i.e., they had similar characteristics and affected the same structures. Almost all of the nine events of this series included two recurrent systems of large-scale dimmings (regions of reduced intensity with lifetimes of 10–15 h): (a) transequatorial dimmings connecting a northern near-equatorial eruption center with a southern active region and (b) northern dimmings covering a large sector between two coronal holes. In this northern sector, coronal waves (brightenings propagated from the eruption center at speeds of several hundred km/s) were observed ahead of the expanding dimmings. The brightest, central part of the halo-type coronal mass ejection in each event corresponded to the northern dimming system. The properties of the dimmings and coronal waves and the relationship between them are discussed on the basis of the results obtained. We find that the eruption of large coronal mass ejections involves structures of the global solar magnetosphere with spatial scales far exceeding the sizes of active regions and normal activity complexes.     

Large-scale activity in solar eruptive events of October–November 2003 observed from SOHO/EIT data

Large-scale solar disturbances associated with powerful flares and coronal mass ejections (CMEs) during two passages of a grand system of three active regions in October–November 2003 are analyzed using data obtained with the SOHO/EIT EUV telescope. Dimmings (transient coronal holes) and, to a lesser extent, coronal waves (traveling emitting fronts) are studied using fixed-difference derotated images, in which a correction for the solar rotation is applied and a single heliogram preceding the event is subtracted from all subsequent heliograms. This method allows us to study difference heliograms in both the 195 Å line (with an interval of 12 min) and the various-temperature channels of 171, 195, 284, and 304 Å (with an interval of six hours). Our analysis shows, in particular, that the disturbances associated with CMEs demonstrated a global character and occupied almost the entire southern half of the disk in virtually all eruptive events during the two solar rotations. At the same time, the northern half of the disk, which had a large coronal hole, was only slightly disturbed. The dominant dimmings were observed on the disk as narrow, long features stretched mainly between three main, well-separated regions of the system and as long structures located along lines of solar latitude in the south polar sector. For repetitive events with intervals between them being not so long, the dominant dimmings demonstrated a clear homology in their forms and locations. During the very powerful event of October 28, one homologous global set of dimmings changed to another set. Many dimmings were observed to be identical or very similar in the three coronal channels and the transition-region line. It follows from the analysis that rapidly recovering global structures in the corona and transition region were involved in the eruption of running CMEs and the corresponding reconstruction of the large-scale magnetic fields.     

Motion of an eruptive prominence in the solar corona

A model for the nonradial motion of an eruptive prominence in the solar corona is proposed. Such motions, which can sometimes be inaccessible to observation, result in an apparent break in the causal link between eruptive prominences and coronal mass ejections. The global magnetic field of the Sun governs coronal plasma motions. The complex structure of this field can form prominence trajectories that differ considerably from a simple vertical rise (i.e., radial motion). A solar filament is modeled as a current-carrying ring or twisted toroidal magnetic rope in equilibrium with the coronal magnetic field. The global field is described using two spherical harmonics. A catastrophic violation of the filament equilibrium followed by its rapid acceleration—eruption—is possible in this nonlinear system. The numerical solution of the equations of motion corresponds well to the eruption pattern observed on December 14, 1997.     

A study of eruptive solar events with negative radio bursts

Solar events of June 15/16, 2000, June 1/2, 2002, February 6, 2002, and February 7, 2002, have been studied. These events probably belong to a poorly studied class of explosive eruptions. In such events disintegration of the magnetic structure of an eruptive filament and dispersing of its fragments as a cloud over a considerable part of the solar surface are possible. The analysis of SOHO/EIT extreme ultraviolet images obtained in the 195 Å and 304 Å channels has revealed the appearance of dimmings of various shapes and propagation of a coronal wave for June 1/2, 2002. In all the events the Nobeyama, Learmonth, and Ussuriysk observatories recorded negative radio bursts at several frequencies in the 1–10 GHz range. Most likely, these bursts were due to absorption of solar radio emission in clouds produced by fragments of filaments. Absorption of the solar background radiation can be observed as a depression of the emission in the 304 Å channel. A model has been developed, which permits one to estimate parameters of absorbing plasma such as temperature, optical thickness, area of the absorbing cloud, and its height above the chromosphere from the radio absorption observed at several frequencies. The obtained values of the temperature, 8000–9000 K, demonstrate that the absorber was the material of an erupted cool filament. The model estimate of the masses of the ejecta in the considered events were ～10¹⁵ g, which is comparable to masses of typical filaments and coronal mass ejections.     

Evidence for shock generation in the solar corona in the absence of coronal mass ejections

The solar event SOL2012–10–23T03:13, which was associated with a X1.8 flare without an accompanying coronal mass ejection (CME) and with a Type II radio burst, is analyzed. A method for constructing the spatial and temporal profiles of the difference brightness detected in the AIA/SDOUVand EUV channels is used together with the analysis of the Type II radio burst. The formation and propagation of a region of compression preceded by a collisional shock detected at distances R < 1.3R _⊙ from the center of the Sun is observed in this event (R _⊙ is the solar radius). Comparison with a similar event studied earlier, SOL2011–02–28T07:34 [1], suggests that the region of compression and shock could be due to a transient (impulsive) action exerted on the surrounding plasma by an eruptive, high-temperature magnetic rope. The initial instability and eruption of this rope could be initiated by emerging magnetic flux, and its heating from magnetic reconnection. The cessation of the eruption of the rope could result from its interaction with surrounding magnetic structures (coronal loops).     

Failed Eruptions of Solar Filaments

Solar filaments (prominences), which suddenly and swiftly ascend, i.e., become eruptive, sometimes decelerate and stop at comparatively low altitudes. Causes of failed eruptions generally remain uncertain. The present study analyzes two eruptive phenomena with very similar initial geometries and configurations of external magnetic fields; one of these eruptions evolves in a coronal mass ejection, but the other breaks off shortly after its start. The tension of curved magnetic field lines is the most probable force causing eruptions to stop. Significant external magnetic fields parallel to rope axes located in failed eruption regions can be a decisive factor. Such an effect has been revealed during laboratory experiments on plasma rope dynamics, which likely plays an important role in solar eruptive phenomena.     

Relationship between impulsive and post-eruptive processes in solar X-ray flares

Powerful solar flares contain one or more impulsive events, plasma ejection, and the subsequent development of gigant post-eruptive loops. In the middle of the 1980s, Jakimiec proposed an analysis of the flare loops based on log T-1/2log EM diagrams constructed from the observed soft X-rays (the so-called Jakimiec model). We have used this method to construct and analyze these diagrams not for various arbitrary events, but instead for similar flares within a single center of activity; in other words, for homological flares (two-ribbon flares observed in November 2000, powerful prolonged events observed in October–November 2003, etc.). This eliminated the effect of differences in the magnetic configurations, enabling us to find a new relationship: the slope (tan α) of the logT-1/2log EM line during the flare decay depends on the maximum temperature T _max at the source of the soft X-rays. The dependence of tan α on T _max gradually evolves from a series of short flares to a series of powerful, prolonged, nonstationary processes. Our results support the idea that the development of post-eruptive loops depends on the energy of the impulsive events for the phenomenon as a whole. Explosive evaporation simultaneously increases both the temperature and the density of the plasma at the loop top. The subsequent evolution of the post-eruptive formations depends on the difference in the initial conditions and on the degree of opening of the magnetic configuration. The importance of our analysis for the duration of flares and differences between dimmings is briefly discussed.     

The structure of solar filaments. Prominences in the corona free from external magnetic field

The new approach to the modeling of quiescent solar prominences is proposed. We solve the inverse magnetohydrostatic problem, when the pressure, density and temperature of plasma in the filament are calculated from the equilibrium equations using the given magnetic structure (magnetic flux function is proposed to be known). The new exact nonlinear solutions for dense (n ≈ (2?3) × 10¹¹ cm^?3) and cold (T ≈ (5?10) × 10³ K) filaments, embedded in the plan, vertically stratified atmosphere (hot solar corona) free of magnetic field, are derived. The filaments are stretched along the horizontal axisy(the translational symmetry is assumed: ?/?y = 0) and located parallel to and above a photospheric, magnetic polarity reversal line. The magnetic field lines have a structure of magnetic flux rope with helical field lines in three-dimensional space; the strength of magnetic field falls rapidly with distance from a rope axis. No external longitudinal magnetic field is needed to equilibrate the prominence. The net electric current along the filament is equal to zero. The model of magnetic arcade with the deflection (sag) on the top, proposed by Pikelner (1971) as a basic form of normal prominence, is calculated also using the method proposed. It is shown that such magnetic arcade, having the magnetic field strength of few gauss only, can effectively maintain the equilibrium of cool dense filament at the heights about 50–60 Mm.     

Height of a solar filament before eruption

The relationship between the height of a solar filament observed above the photosphere before the eruption on October 21, 2010, and the critical height of a stable equilibrium of magnetic flux ropes in the coronal magnetic field is analyzed. Data from the SDO, SOHO, and STEREO space observatories observing at different viewing angles makes it possible to deduce these parameters with high accuracy. It is shown that the filament height slowly increased over several days, with the eruption occuring when the height reached the critical value of 80 Mm.     

Three types of flows in the structure of the solar wind

An experimental study of the source and formation of large-scale streams in the solar wind is presented. Radio-astronomical data from 1998 are compared with optical SOHO observations and solar coronal magnetic fields calculated from Zeeman data obtained at the Wilcox Observatory. A correlation between the geometry of the solar-wind transition region and the strength of coronal magnetic fields is revealed. For the moderate heliolatitudes studied, this correlation divides into three branches corresponding to three types of coronal magnetic-field structures: open structures with field lines escaping into interplanetary space, closed structures with loop-like field lines, and intermediate structures including both open and closed configurations. High-speed streams of solar wind originate in regions with open magnetic structures. These structures are connected with the lateral lobes of streamers at moderate heliolatitudes. Low-speed flows originate above closed magnetic structures, typical of the main bodies of streamers. The lowest-speed solar-wind flows are not associated with coronal streamer structures, and originate in coronal regions with intermediate magnetic configurations simultaneously containing open and closed field lines. In these regions, the white-light corona becomes an extended and amorphous area with high luminosity, which stratifies into a radial structure with narrow stripes at higher resolution.     

A model of a solar flare: Comparisons with observations of high-energy processes

New data for the energy and location of the hard-emission centers of a solar flare agree with an electrodynamic model of a solar flare based on the idea of the accumulation of free magnetic energy in the field of a current sheet. Three-dimensional MHD simulations are used to show that the energy stored in the preflare magnetic field of the current sheet is sufficient for the development of a flare and a coronal mass ejection. The flare and coronal mass ejection result from the explosive decay of the current sheet. The position of the brightness-temperature maximum of the radio emission during the flare coincides with the maximum of the current in the current sheet. The exponential spectrum of relativistic protons generated during the flare is consistent with acceleration by the electric field during the current-sheet decay.     

The structure of the coronal-streamer belt

We analyze calibrated white-light coronal images from the LASCO-C2/SOHO experiment (processing level L1), focusing on quasistationary events without coronal mass ejections or their manifestations in the solar wind. The previous result that the streamer belt forms a set of rays of increased brightness is confirmed. The cross section of the streamer belt is frequently observed as two closely spaced rays differing in brightness. It is difficult to explain this in terms of ordinary bending of the belt. We suggest that the belt is normally a set of pairs of rays with enhanced brightness (or two close rows of rays). The distance between the rays in each pair is comparable to the ray size. The ray brightnesses in any pair can, in general, be different. The magnetic field has opposite directions in the rays forming a pair, so that the neutral line of the radial component of the solar magnetic field probably runs along the strip between the pairs of rays.     

The initial trajectories of eruptive solar prominences

Trajectories of eruptive prominences are compared with the shapes of coronal neutral surfaces calculated in a potential approximation using photospheric measurements. Space-based Solar Dymamics Observatory and STEREO observations carried out at different viewing angles enable a precise determination of a prominence’s position at successive times during its eruption. In the initial segments of their trajectories, eruptive prominences move along neutral surfaces (B_r = 0) of the potential coronal magnetic field. This can be used to predict the directions of subsequent coronal mass ejections and to estimate their geoefficiency.     

Coronal mass ejections and eruptive prominences: Mutual spatial arrangement

The mutual spatial arrangement of coronal mass ejections and eruptive prominences on the Sun is considered. These phenomena occur on different scales and are observed at different heights above the solar surface. In spite of the presumed causal connection between them, they are often widely separated in position angle at epochs of solar minimum. This means that the motion of a prominence in the corona is not strictly radial and has an appreciable component along the surface. This behavior can be explained in a model of a filament as a magnetic flux rope in equilibrium in the coronal magnetic field. The initial trajectory of the filament is determined by the structure of the global field.     

Effects of coronal mass ejections on distant coronal streamers

The effects of a large coronal mass ejection (CME) on a solar coronal streamer located roughly 90° from the main direction of the CME propagation observed on January 2, 2012 by the SOHO/LASCO coronograph are analyzed. Radial coronal streamers undergo some bending when CMEs pass through the corona, even at large angular distances from the streamers. The phenomenon resembles a bending wave traveling along the streamer. Some researchers interpret these phenomena as the effects of traveling shocks generated by rapid CMEs, while others suggest they are waves excited inside the streamers by external impacts. The analysis presented here did not find convincing arguments in favor of either of these interpretations. It is concluded that the streamer behavior results from the effect of the magnetic field of a moving magnetic flux rope associated with the coronal ejection. The motion of the large-scale magnetic flux rope away from the Sun changes the surrounding magnetic field lines in the corona, and these changes resemble the half-period of a wave running along the streamer.     

Studying coronal holes through observations of an HeI infrared line and the Hα line

New results from electrophotometric scanning of the solar disk in the HeI λ 10830 Å and H_α lines are presented. The intensity at the center of the HeI λ 10830.30 Å line is 1–3% higher in the regions of coronal holes than in quiescent regions; this is accompanied by a decrease in the size and contrast of the chromospheric network compared to the network in quiescent regions. Our observations in the HeI line revealed chains of “dark points” surrounding coronal holes. The H_α±0.5 observations show increased velocities of ascent near the dark points compared to the velocities inside coronal holes and in quiescent regions. It is proposed that the intensification and acceleration of the flows of solar plasma from the dark points are due to reconnection of the magnetic fields of the bipolar chromospheric network and the predominantly unipolar magnetic field inside the coronal holes. Our observations suggest that the same reconnection process takes place near the temperature minimum, in the presence of certain conditions at the boundary between coronal holes and bipolar active regions. The reconnection process produces plasma flows from the chromosphere to the corona, which are sufficient to form prominences.     

MHD simulations of current-sheet formation over a bipolar active region

We present the results of numerical simulations of the development of a current sheet in the solar corona over a bipolar region during the emergence of two new sunspots arranged collinearly with older spots. Two fronts of increased plasma density form at the boundary of the rising new magnetic flux. One of these is due to the generation of a current sheet, whose magnetic field accumulates energy for a flare. The other front is a branch of the density perturbation, and separates the old and new magnetic fluxes in a region where the magnetic field lines have the same direction on both sides of the boundary. The development of this perturbation is not associated with the energy accumulation in the corona, and hinders observation of the preflare state and complicates analysis of the results. This second front can be interpreted as the eruption of a filament before the onset of the flare. A scheme conservative with respect to magnetic flux was introduced in the Peresvet code that solves the MHD equations, in order to suppress numerical instabilities in regions of large magnetic-field gradients.