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.     

Solar magnetic fields and the intensity of the green coronal line

Synoptic maps of the intensity of the λ530.5 nm FeXIV green coronal line and maps of computed coronal magnetic fields for the period 1977–2001 are compared. For quantitative comparisons, the correlation coefficients r for the correlation between these two parameters at corresponding points of the synoptic maps are calculated. This coefficient exhibits cyclic variations in the spot-formation zone, ±30° and the zone above 30° and is in antiphase in these two zones. In the low-latitude zone, the correlation coefficient is always positive, reaches its maximum at activity minimum, and strongly decreases by activity maximum. Above 30°, r reaches maximum positive values at activity maximum and then gradually decreases, passing through zero near the beginning of the phase of activity minimum and becoming negative during this phase. A Fourier analysis of r as a function of time reveals a wavelike variation with a period close to 1.3 yr (known also from helioseismological data for the tachoclinic region of magnetic-field generation), as well as a pronounced wave with a period of about 5 yr. The latitude dependence of r seems to be related to variations in the contributions from local, large-scale, and global fields. Our analysis suggests an approach to studying the complex problem of mechanisms for coronal heating.     

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/.     

Polarization of the solar corona in the green line: Contradiction between theory and observations

The main results of polarization observations in the 530.3-nm line and their role in studying the physical conditions, structure, and magnetic field in the solar corona are discussed. A serious discrepancy between the observations and widely-accepted theoretical concepts was revealed: the theory predicts that the orientation of the polarization electric vector should be nearly radial, in contradiction with the observational results. In particular, the polarization vectors for both the green line and white-light corona in high-latitude streamers were tangential during the eclipse of July 11, 1991. The dependence of the degree of polarization on the angle between the radial direction and the magnetic-field vector was calculated without any a priori assumptions about the configuration of coronal fields. This theoretical analysis of the polarization-vector orientation for magnetic-dipole emission in the green line are in agreement with results obtained previously in other studies. Some ways to resolve the observed discrepancies are discussed.     

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.     

Cyclic variations in the differential rotation of the solar corona

The rotation of the solar corona is analyzed using the original database on the brightness of the FeXIV 530.3 nm coronal green line covering six recent activity cycles. The rate of the differential rotation of the corona depends on the cycle phase. In decay phases, there are only small differences in the rotation, which are similar to that of a rigid body. The differences are more significant (though less pronounced than in the photosphere) during rise phases, just before maxima, and sometimes at maxima. The total rate of the coronal rotation is represented as a superposition of two, i.e., fast and slow modes. The synodic period of the fast mode is approximately 27 days at the equator and varies slightly with time. This mode displays weak differences in rotation and is most pronounced in the middle of decay phases. The slow mode is manifested only at high latitudes during the rise phases of activity, and displays a mean period of 31 days. The relative contribution of each mode to the total rotational rate is determined as a function of time and heliographic latitude. These results indicate that the structure of the velocity field in the convective zone must also vary with time. This conclusion can be verified by helioseismology measurements in the near future.     

Some properties of the latitude brightness distributions in the K and F coronae according to SOHO/LASCO data

We have obtained continuous latitude distributions of the K and F corona brightnesses at various distances for the first time, using both the Hayes—Vourlidas—Howard method and our own recently proposed, simple technique for separating the emission of the K and F coronas. Data from the LASCO C2 and C3 coronagraphs are analyzed. Variations of the angular size of the brightness distribution of the F corona with latitude and distance are estimated, as well as the ratio of the maximum F-corona brightness to the F-corona brightness at the pole. The variations in the F-corona brightness at large distances (R = 25 R _⊙, where R _⊙ is the solar radius) are studied on various time scales—a month, a year, and 11 years (the solar cycle). The latitude distribution of the F-corona brightness varies most appreciably over a year, and only weakly over one solar revolution and one solar-activity cycle (as considered on a fixed day of the year).     

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.     

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.     

Formation of several current sheets preceding a series of flares above the active region AR 0365

We have carried out 3D MHD modeling of the solar corona above the active region AR 0365 before a series of flares observed on May 26–27, 2003. Maps of the evolving photospheric magnetic fields preceding the flares were used as boundary conditions. An emergence of new flux equal to ～1.5 × 10²² Maxwell preceded the observed series of X-ray flares. Modeling a large region 4 × 10¹⁰ cm in size demonstrates the formation of several current sheets in the vicinities of coronal Xlines, both already existing in the initial potential field and arising due to the emergence of the new magnetic flux. Each current sheet could be responsible for an elementary flare.     

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.     

The microwave radiation of the corona above a large single sunspot in right- and left-circular polarization

Results of a study of the corona above a large sunspot in the active region NOAA 10105 with a penumbra size of ～70″ observed in September 2002 are reported. Maps of the active region and emission spectra were constructed using observational data from the NoRH, SSRT, and RATAN-600 telescopes. The sizes and brightness temperatures of the microwave emission above the sunspot are determined. SOHO/MDI and Kitt Peak magnetograms, as well as CaII K line images obtained at the Meudon Observatory, are compared. The derived characteristics are interpreted as cyclotron emission of thermal plasma, assuming a dipole structure for themagnetic field. A stable darkening at the sunspot center observed at short wavelengths and only in the ordinary emission mode was detected. A jump-like change was observed in the structure of the sunspot source in the ordinary emission mode, due to an increase in the size and spectral flux density. These results demand a fundamental correction of model concepts about cyclotron emission sources above sunspots, since they are at variance with the initial assumptions. It is suggested that, at the top of the transition region, the cyclotron emission source may be represented only by the third gyrolevel, but is observed in the extraordinary and ordinary emission modes (in contrast to the generally accepted model, which has a combination of the second and third gyrolevels). Taking into account the new observational data may allow us to refine model distributions of the main parameters of the coronal plasma above sunspots (the electron temperature and density) and information about the character of the magnetic field.     

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.     

The proportion of hot and cold regions in the polar solar corona during 1957–2002

Occultation observations of the intensity of the FeXIV 530.3 nm and FeX 637.4 nm forbidden lines detected at the Kislovodsk Mountain Station during 1957–2002, indicate long-term changes in the structure of the solar corona. The monthly average intensities of green (KI_530.3) and red (KI_637.4) spectral lines are calculated for all latitudes (0°–90°) and for a high-latitude zone (45°–90°). A strong correlation (r = 0.91) between the green KI_530.3 line intensity and the Wolf numbers is found and used to fill gaps in the observations. The ratio KI_637.4/KI_530.3 takes on its maximum value at the solar minimum. The KI _637.4 ^p /KI _530.3 ^p ratio in the high-latitude solar zone (45°–90°) increased by more than a factor of two during 1957–2002. This means that the fraction of cool regions in the polar corona has more than doubled over these years. We suggest that this increase in the number of cool regions is related to an increase in the area of the polar solar zones occupied by magnetic field of a single polarity at the solar minimum, and possibly to an increase in the area occupied by polar coronal holes. This is associated with long-term variations in the internal structure of the Sun.     

Quasibiennial oscillations of the north-south asymmetry

The north-south (N-S) asymmetry of the solar activity (A), which reflects differences in the behavior of the northern and southern hemispheres of the Sun, is studied using data on the brightness of the coronal green line, the total number and area of sunspots, and the net magnetic flux. The spatial and temporal distributions and correlations between the A values represented by these indices are considered. The characteristic time variations in A are similar for all the indices, on both long and short time scales. Quasibiennial oscillations (QBOs) can be traced in the asymmetries of all four indices. A detailed study of the QBOs is carried out based on spectral-variation and wavelet analyses. Long-term increases and decreases occur synchronously in the asymmetries of various indices and are much more pronounced in A than in the indices themselves. A negative correlation between the power of the QBOs and the asymmetry of A can be traced; it is most clearly manifest as a substantial diminishing of the QBOs during the mid-1960s, which coincided with an especially strong increase in A. Our analysis shows that the N-S asymmetry is probably a fundamental property that controls the coupling and degree of coincidence between the magnetic-field-generation mechanisms operating in the northern and southern hemispheres.     

Global complexes of activity

A new concept of “Global Complexes of Activity” on the Sun is presented, which brings together objects associated with both global and local fields in a single framework. Activity complexes have traditionally been identified purely from observations of active regions. We show here that a global complex also includes coronal holes and active regions. Our analysis is based on a large dataset on magnetic fields on various scales, SOHO/MDI observations of active regions and magnetic fields, and UV observations of coronal holes. It is shown that the evolution of coronal holes and active regions are parts of a single process. The relationships between the fields on different scales during the generation of the cycle is discussed.     

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.     

Magnetohydrostatic model for a coronal hole

A model treating a solar coronal hole as an axially symmetrical magnetic formation that is in equilibrium with the surrounding medium is proposed. The model is applicable in the lower corona (to heights of the order of several hundreds of Mm), where the influence of the solar-wind outflow on the state of the system can still be neglected. The magnetic field of the coronal hole is comprised of a relatively weak open flux that varies with height, which extends into interplanetary space, and a closed field, whose flux closes at the chromosphere near the coronal hole. Simple analytical formulas are obtained, which demonstrate for a given equilibrium configuration of the plasma and field the main effect of interest—the lowering of the temperature and density of the gas in the coronal hole compared to their values in the corona at the same geometric height. In particular, it is shown that, at heights of several tens of Mm, the temperature and density of the plasma in the coronal hole are roughly half the corresponding values at the same height in the corona, if the cross-sectional radius of the hole exceeds the scale height in the corona by roughly a factor of 1.5: R _h ≈ 1.5H(T ₀). In the special case when R _h ≈ H(T ₀), the plasma temperature in the hole is equal to the coronal temperature, and the darkening of the coronal hole is due only to an appreciable reduction of the plasma density in the hole, compared to the coronal density. An analogy of the properties of coronal holes and sunspots is discussed, based on the similarity of the magnetic structures of these formations. In spite of the fundamental difference in the mechanisms for energy transport in coronal holes and sunspots, the equilibrium distributions of the plasma parameters in these formations are determined only by the magnetic and gravitational forces, giving rise to a number of common properties, due to their similar magnetic structures.     

Long-term variations in the rotation of the solar corona

The characteristic time scales for variations in the differential rotation of the solar corona are determined using measurements of the intensity of the FeXIV 5303 Å coronal line made from 1939–2004. Drift waves of the variations in the rotational speed with an 11-year periodicity can be distinguished. Moving averages with time intervals from two to five years are used to identify torsional waves. In addition, longer-period variations in the rotational speed can be distinguished when longer averaging intervals are used. When the interval used for the moving average is increased to 8–12 years, a quasi-22-year rotational period appears. The low-latitude corona rotates more slowly in odd cycles than in even cycles. Increasing the duration of the averaging interval further shows that rapid rotation at low latitudes was observed in 1940–1950 and 1990–2000, while slow rotation was observed in 1960–1980, possibly suggesting the presence of a 55-year period in the rotational variations. Long-term variations are found in the rotation of polar regions. The rotational variations for high-latitude corona are in antiphase with those for the low-latitude corona. The origins of zones of anomalous coronal rotation and their dynamics in the global activity cycle are discussed.     

Bright points and ejections observed on the sun by the KORONAS-FOTON instrument TESIS

Five-second observations of the solar corona carried out in the FeIX 171 Å line by the KORONAS-FOTON instrument TESIS are used to study the dynamics of small-scale coronal structures emitting in and around coronal bright points. The small-scale structures of the lower corona display complex dynamics similar to those of magnetic loops located at higher levels of the solar corona. Numerous detected oscillating structures with sizes below 10 000 km display oscillation periods from 50 to 350 s. The period distributions of these structures are different for P < 150 s and P > 150 s, which implies that different oscillation modes are excited at different periods. The small-scale structures generate numerous flare-like events with energies 10²⁴–10²⁶ erg (nanoflares) and with a spatial density of one event per arcsecond or more observed over an area of 4 × 10¹¹ km². Nanoflares are not associated with coronal bright points, and almost uniformly cover the solar disk in the observation region. The ejections of solar material from the coronal bright points demonstrate velocities of 80–110 km/s.