Some features of the streamer belt in the solar corona and at the Earth’s orbit

The rays of enhanced brightness making up the structure of the coronal-streamer belt can be traced to the lowest atmospheric layers in the Sun, with the angular size remaining nearly constant, d ≈ 2.5° ± 0.5°. This suggests that the physical mechanism generating the slow solar wind in the rays of the streamer belt differs from the mechanism giving rise to the fast solar wind from coronal holes. At distances of R < (4–5) R _⊙, the rays of the streamer belt are not radial in the plane of the sky and show deviations toward the corresponding pole. They then become essentially radial at R > (4–5) R _⊙. A transverse cross section of streamers in the corona and its continuation into the heliosphere—a plasma sheet—can be represented as two radially oriented, closely spaced rays (d ≈ 2.0°–2.5°) with enhanced density and an angular size of d. We also show that the ray structure of the streamer belt is involved in the development of coronal mass ejections (CMEs). The motion of a small-scale CME occurs within a magnetic flux tube (ray of enhanced brightness) and leads to an explosive increase in its angular size (rapid expansion of the tube). It seems likely that large-scale CMEs are the result of the simultaneous expansion of several magnetic tubes. We suggest that a small-scale CME corresponds to a “plasmoid” (clump of plasma of limited size with its own magnetic field) ejected into the base of a magnetic tube, which subsequently moves away from the Sun along the tube.     

太阳活动及其对地球环境的影响

太阳活动及其对地球环境影响的研究至今已发展成一门涉及太阳物理学、空间物理学和地球物理学的边缘学科,它研究三者的关系及相互作用的过程。本文将太阳活动分成缓变型和爆发型两类,分别介绍了它们的主要成员冕洞、总辐射、太阳黑子、太阳耀斑和日冕物质抛射的性质及特征;分别讨论了这两类太阳活动对地球环境的影响,还指出了太阳活动对固体地球的作用。     

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.     

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.     

Non-radial coronal streamers in the course of the solar cycle

Equatorward deviations of coronal streamers at solar minima and poleward deviations at solar maxima are interpreted as the effects of changes in the general topology of the global solar magnetic field. The streamer axis is located on the neutral surface of the radial magnetic field B _r = 0, and the neutral surfaces deviate toward the field null points. The magnetic configuration with a null point (line) located at the equator is typical for the solar minima, while the null points are located on the rotational axis of the Sun at the solar maxima.     

Two modes of the differential rotation of the solar corona

The differential rotation of the solar corona is studied using the brightness of the Fe XIV 530.3 nm green coronal line collected over 5.5 solar-activity cycles. The total observed velocity of the coronal rotation is analyzed as a superposition of two modes—fast and slow. A technique for separating two data series composing the initial data set and corresponding to the two differential-rotation modes of the solar corona is proposed. The first series is obtained by averaging the initial data set over six successive Carrington rotations; this series corresponds to long-lived, large-scale coronal regions. The second series is the difference between the initial data and the averaged series, and corresponds to relatively quickly varying coronal component. The coronal rotation derived from the first series coincides with the fast mode detected earlier using the initial data set; i.e., the synodic period of this mode is 27 days at the equator, then weakly increases with latitude, slightly exceeding 28 days at high latitudes. The second series describes a slow rotation displaying a synodic period of about 34 days. This coincides with the period of rotation of the high-latitude corona derived by M. Waldmeier for polar faculae. We expect that coronal objects corresponding to the fast mode are associated with magnetic fields on the scales typical for large activity complexes. The slow mode may be associated with weak fields on small scales.     

Origins and properties of the quasi-stationary slow solar wind

Comparisons of the brightness distributions of the white corona observed at distances of several solar radii with solar wind velocities derived from interplanetary-scintillation observations, as well as analyses of solar wind data obtained on spacecraft from December 1994 to June 1995, indicate that the fast solar wind can contain plasma with velocities V ≈ 300–450 km/s, approaching those typical for the slow solar wind that flows in the streamer belt and chains of streamers. At the same time, certain other parameters, first and foremost the plasma density N and ratio T/N ^0.5 (where T is the temperature), indicate that these two flows differ considerably. The slow solar wind flowing in the streamer belt and chains displays high densities N > 10 ± 2 cm^?3 and low T/N ^0.5 < 1.7 × 10⁴ K cm^3/2 at the Earth’s orbit. The number of slow solar-wind sources observed in chains can be comparable with the number observed in the belt. The fast solar wind flowing from coronal holes always displays low densities N≤ 8 cm^?3 and high T/N ^0.5 > 1.7 × 10⁴ K cm^3/2. These properties probably indicate different origins of the fast and slow solar winds.     

Relationship between the contrast of coronal holes and parameters of the solar wind streams

It is shown that the contrast of coronal holes (CH) determines the speed of the solar wind streams to the same extent as their area does. We analyzed more than 400 images obtained in the λ284 Å channel. The time interval under examination covers about 1500 days in the declining phase of cycle 23 (from 2002 to 2006). We considered all coronal holes recorded during that interval in the absence of coronal mass ejections (CME). Comparison was also made with some other parameters of the solar wind (e.g., density, temperature, and magnetic field). A fairly high correlation (0.70–0.89) was obtained with the velocity, especially during the periods of moderate activity, which makes this method useful for everyday forecast. The ratio of CH brightness to the mean brightness of the disk in the λ284 Å channel is about 25%.     

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.     

Additional plasma streams with steep fronts in structures of enhanced brightness in the coronal streamer belt

The presence of additional plasma streams with enhanced density in bright rays of the solar coronal-streamer belt is demonstrated. The streams have steep fronts, whose widths δ≈0.10R_⊙ (where R_⊙ is the solar radius) are comparable to the spatial resolution of the LASCO C3 instrument of the SOHO satellite. The additional streams are similar to streams of the slow, quasi-steady solar wind in the streamer belt in terms of their plasma density, directional velocity, and lifetime and are apparently one of the main sources of the slow solar wind.     

Relationship between the brightness in the coronal green line and magnetic fields on various scales

The relationship between the brightness in the FeXIV 530.3 nm coronal green line and magnetic fields on various scales in the corona is studied quantitatively. The cross-correlations of the corresponding synoptic maps for 1977–2001 have been calculated. Maps of the brightness of the coronal green line are constructed using daily monitoring data. Maps of the magnetic field are constructed separately for fields on large and small spatial scales, based on computations in a potential approximation using photospheric observations for distances of 1.1R _⊙ carried out at the Wilcox Solar Observatory. The correlations between the brightness in the coronal green line and the magnetic-field strengths on various scales as a function of latitude have a cyclic character. The correlation coefficients in the spot-formation zone are positive. Here, the green-line brightness corresponds mainly to the strength of small-scale fields, corresponding to the sizes of large active regions and activity complexes. The correlation coefficients are sign-variable above 40° latitude, and reach their greatest positive and negative values at the cyclemaximum and minimum. Larger-scale fields influence the green-line brightness at higher latitudes and near the phase of the cycle minimum. The results obtained can be used to investigate mechanisms for heating the corona. The relationship between the results obtained and the subsurface and deep solar dynamos are also discussed.     

The spectrum of solar cosmic rays: Data of observations and numerical simulation

Analysis of the relativistic proton spectra of solar flares occurring in the 23rd solar activity cycle derived from data of a worldwide neutron monitor network and numerical modeling both provide evidence for the acceleration of charged particles by an electric field that arises in coronal current sheets during reconnection. The method used to obtain the spectra is based on simulating the response of a neutron monitor to an anisotropic flux of relativistic solar protons with specified parameters and determining the characteristics of the primary relativistic solar protons by fitting model responses to the observations. Studies of the dynamics of the energy spectra distinguish two populations of relativistic protons in solar cosmic-ray events: the so-called fast component, which arrives at the flux front of the solar cosmic rays, followed by the delayed slow component. The fast component is characterized by strong anisotropy and an exponential energy spectrum, in agreement with the spectrum yielded by mathematical modeling of particle acceleration by an electric field directed along the X line of the magnetic field. The slow component, whose propagation is probably diffusive, has a power-law spectrum.     

Decay of activity complexes and the formation of coronal holes

Analysis of long-term measurements of solar magnetic fields and the flux of UV radiation from the Sun indicates a cause-effect relationship between activity complexs, their residual magnetic fields, and coronal holes. A comparison of the background magnetic fields of the Sun and the evolution of former activity complexes reveals unipolar magnetic regions that form after the decay of these complexes. The latitude and time evolution of unipolar magnetic regions in solar cycles 21–24 is studied. A North-South asymmetry in solar activity is manifest in the distribution of unipolar regions migrating toward higher latitudes. It is shown that, when residual magnetic fields of the opposite polarity reach the polar regions, this leads to a sign change of the polar magnetic field and a decrease in the area of polar coronal holes, or even their complete disappearance. These interactions can explain the triple sign change of the polar magnetic field of the Sun in cycle 21 and the short-term polarity reversals observed in 2010 and 2011.     

The rotation of the Sun as a star from the green-line emission of the entire corona

A new representation for the database created by J. Sykora on the 5303 Å Fe XIV line emission observed from 1939 to 2001 is proposed. Observations of the corona at an altitude of 60″ above the limb reduced to a uniform photometric scale provide estimates of the emission of the entire visible solar surface. It is proposed to use the resulting series of daily measurements as a new index of the solar activity, GLSun (The Green-Line Sun). This index is purely observational and is free of the model-dependent limitations imposed on other indices of coronal activity. GLSun describeswell both the cyclic activity and the rotational modulation of the brightness of the corona of the Sun as a star. The GLSun series was subject to a wavelet analysis similar to that applied to long-term variability in the chromospheric emission of late-type active stars. We obtain that the brightness inhomogeneities in the solar corona rotate more slowly during epochs of high activity than their average rotational rate over the entire time observations. The time interval of slower rotation of the inhomogeneities is close to the epoch when the Sun’s field represents a horizontal magnetic dipole in each activity cycle, but is somewhat longer than the duration of the polarity reversal in both hemispheres. The difference between the periods for the slower and mean rotation exceeds three days, as is typical for some stars with higher but less regular activity than solar one. The importance of these findings for dynamo theory for the origin and evolution of the magnetic fields of the Sun and other late-type stars is briefly discussed.     

Solar modulation of galactic cosmic rays in the three-dimensional heliosphere according to meteorite data

Cosmogenic radionuclides with distinctive half-lives from chondritic falls were used as natural detectors of galactic cosmic rays (GCR). A unique series of uniform data was obtained for variations in the integral gradients of GCR with a rigidity of R > 0.5 GV in 1955–2000 on heliocentric distances of 1.5–3.3 AU and heliographic latitudes between 23° S and 16° N. Correlation analysis was performed for the variations in GCR gradients and variations in solar activity (number of sunspots, SS, and intensity of the green coronal line, GCL), the intensity of the interplanetary magnetic field (IMF), and the inclination of the heliospheric current sheet (HCS). Distribution and variations of GCR were analyzed in 11-year solar cycles and during a change in 22-year magnetic cycles. The detected dependencies of GCR gradients on the intensity of IMF and HCS inclination provided insight into the differences in the processes of structural transformation of IMF during changes between various phases of solar and magnetic cycles. The investigated relationships lead to the conclusion that a change of secular solar cycles occurred during solar cycle 20; moreover, there is probably still an increase in the 600-year solar cycle, which can be among the major reasons for the observed global warming.     

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.     

Cyclic variation in the spatial distribution of the coronal green line brightness

The spatial and temporal brightness distributions of the Fe XIV 530.3 nm coronal green line (CGL) and cyclic variations of these distributions are analyzed for a long time interval covering more than five 11-year cycles (1943–2001). The database of line brightnesses is visually represented in the form of a movie. Substantial restructuring of the spatial distribution of the CGL brightness occur over fairly short time intervals near the so-called reference points of the solar cycle; such points can be identified based on various sets of solar-activity indices. Active longitudes are observed in the CGL brightness over 1.5–3 yr. Antipodal and “alternating” active longitudes are also detected. The movie can be used to compare the CGL brightness data with other indicators of solar activity, such as magnetic fields. The movie is available at http://helios.izmiran.rssi.ru/hellab/Badalyan/green/.     

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 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.     

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.