The relationship between coronal fan structures and oscillations above faculae regions

The power spectra of radial-velocity and intensity oscillations are analyzed using ground-based (the Si I 10 827 Å and He I 10 830 Å lines) and Solar Dynamics Observatory (the Fe I 6173, 1700 Å, He II 304 Å, and Fe IX 171 Å lines) data, with the aim of searching for frequency modes that most efficiently penetrate into the solar corona from the lower layers of solar faculae. Analysis of the spatial distribution of the oscillation power at various heights indicates that fan structures in the corona (at the height of the 171 Å emission) are better reproduced at frequencies of 1–1.5 mHz. This means that oscillations with periods of 10–15 min dominate in coronal loops above faculae regions. The five-minute oscillations that universally dominate in radial-velocity measurements in low layers of faculae are appreciable in coronal loops only in individual compact fragments.     

Blast-wave and piston shocks connected with the formation and propagation of a coronal mass ejection

The solar coronal mass ejection observed on November 3, 2010 is analyzed using AIA/SDO data (images in the 193 and 211 Å channels) and white-corona images obtained with the SOHO LASCO C2 and C3 coronographs. We have succeeded in revealing both piston and blast-wave shocks attributed to the formation and propagation of a coronal mass ejection. Both of these types of shocks could be responsible for type II radio bursts propagating in front of each shock.     

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 flare-related eruptions followed by long-lasting occultation of the emission in the He II 304 ? line and in microwaves

Plasma with a temperature close to the chromospheric one is ejected in solar eruptions. Such plasma can occult some part of emission of compact sources in active regions as well as quiet solar areas. Absorption phenomena can be observed in the microwave range as the so-called “negative bursts” and also in the He II 304 ? line. The paper considers three eruptive events associated with rather powerful flares. Parameters of absorbing material of an eruption are estimated from multi-frequency records of a “negative burst” in one event. “Destruction” of an eruptive filament and its dispersion like a cloud over a huge area observed as a giant depression of the 304 ? line emission has been revealed in a few events. One of the three currently known events is considered in this paper. One more of the events considered here is a possible candidate for such events.     

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.     

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.     

Some properties of the joining of solar filaments

Some possibilities for the reconnection of magnetic-field lines of solar filaments that approach when the photospheric polarity inversion lines change their positions, are discussed. The interaction between filaments depends on their internal properties, which are determined by the filament chirality, or the sign of the helicity of the filament magnetic field. In quadrupolar magnetic configurations, filaments with the same chirality can exchange their halves. Filaments with opposite chirality rupture after the reconnection of the polarity inversion lines, since the two fragments of the different filaments cannot be connected continuously. The morphology and connectivity of the filaments are analyzed using daily Hα filtergrams obtained over the period of maximum activity of the 23rd solar cycle. Examples of alterations of the filament connectivity occuring during the evolution of photospheric fields are presented.     

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%.     

Solar Photospheric Magnetic Fields,Coronal Mass Ejections,and Type II Radio Bursts in Cycles 23 and 24

Astronomy Reports - We consider the events of type II radio bursts (RBII) in the decametric and hectometric ranges from 1 to 16 MHz and the dependence of the parameters of associated coronal mass...     

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.     

The detection of coronal mass ejections in the interplanetary medium using scintillation observations

Daily observations of scintillating radio sources obtained from July 2011 through June 2012 on the Big Scanning Antenna of the P.N. Lebedev Physical Institute at 111 MHz using a 16 beams system are analyzed. Variations in the observed scintillation indices are compared with data on solar X-ray flares and geomagnetic disturbances. Comparison of the observed scintillation indices on successive days enables the detection of most propagating disturbances associated with coronal events of class M5.0 and higher.     

On the correlation between spectra of solar microwave bursts and proton fluxes near the Earth

Studies of the extreme solar proton event of January 20, 2005 intensified the contest over of a long-standing problem: are solar cosmic rays arriving at the Earth accelerated by solar flares or by shocks preceding rapidly moving coronal mass ejections? Among the most important questions is the relationship between the energy spectra of the solar cosmic rays and the frequency spectra of flare microwave bursts. Some studies of previous solar-activity cycles have shown that such a relationship does exist, in particular, for protons with energies of tens of MeV. The present work analyzes this relation using data for 1987–2008. For flare events observed in the western half of the disk, there is a significant correlation between the index δ, which is equivalent to the power-law index of the integrated energy spectrum of 10–100 MeV protons detected near the Earth’s orbit, and radio burst parameters such as a ratio of peak fluxes S at two frequencies (for example, at 9 and 15 GHz) and a microwave peak frequency f _m . Proton fluxes with hard (flat) energy spectra (δ ≤ 1.5) correspond to hard microwave frequency spectra (S ₉/S ₁₅ ≤ 1 and f _m ≥ 15 GHz), while flares with soft radio spectra (S ₉/S ₁₅ ≥ 1.5 and f _m ≤ 5 GHz) result in proton fluxes with soft (steep) energy spectra (δ ≥ 1.5–2). It is also shown that powerful high-frequency bursts with the hardest radio spectra (f _m ≈ 30 GHz) can point at acceleration of significant proton fluxes in flares occurring in strong magnetic fields. These results argue that solar cosmic rays (or at least their initial impulses) are mainly accelerated in flares associated with impulsive and post-eruptive energy release, rather than in shocks driven by coronal mass ejections.     

Quiet-Filament Eruptions and Coronal Jets as Causes of Depressions in Microwave Radio Emission

Astronomy Reports - Several solar events with different types of negative microwave bursts have been studied using data from different spectral ranges. The total radio flux data obtained at the...     

Initial formation of an “impulsive” coronal mass ejection

An “impulsive” coronal mass ejection (CME) observed on August 24, 2014 is analyzed using ultraviolet images obtained in the SDO/AIA 193, 304, 1600, and 1700 Å channels and Hα (6562.8 Å) data obtained with the EI Teide and Big Bear telescopes. The formation of this impulsive CME was related to a magnetic tube (rope) moving with a velocity of ≈35 km/s and containing plasma that was cooler than the photospheric material. Moving in the corona, the magnetic tube collides with a quasi-stationary coronal magnetic rope, with its two bases rooted in the photosphere. This interaction results in the formation of the CME, with the surface of the coronal magnetic rope becoming the CME frontal structure. According to SDO/HMI data, no enhancements or changes in magnetic flux were detected in the vicinity of the CME bases during its formation. This may support the hypothesis that the magnetic tube starts its motion from layers in the vicinity of the temperature minimum.     

Quasi-periodic line-of-sight velocity variations at the bases of polar coronal holes

Spatial and temporal variations in the line-of-sight velocities and brightnesses measured in the Hα and FeI 6564 Å Hβ and FeI 4864 Å NiI 4857 Å lines at the bases of polar coronal holes are analyzed. Time series with durations of 43–120 min were recorded using a CCD strip (3700 pixels 200×7 µm in size) and a CCD array (256×1024 pixels 24 µm in size). Quasi-stationary upward flows (with radial velocities reaching 3 km/s in the photosphere and 12–15 km/s in the chromosphere) were observed near dark points at the boundaries of the chromospheric network. The acoustic 3-min and 5-min oscillations are amplified in the coronal hole, and reach 1 km/s in the photosphere and 3–4 km/s in the chromosphere. The spectra of fluctuations of the line-of-sight velocity exhibit significant maxima at low frequencies, clustering near 0.4, 0.75, and 1 mHz.     

Coronal mass ejection of April 27, 2003, and evolution of the active region NOAA 10338 in the radio

The results of a study of the coronal mass ejection (CME) of April 27, 2003, which was intrinsically associated with the active region NOAA 10338, are reported. Particular attention is paid to the initial stage of the event, which was accompanied by X-ray bursts of class C9.3 and C6.7, with the aim of determining the origin of CMEs. The energy source of the ejection was in the active region NOAA 10338. This region had a complicated and dynamic magnetic-field topology, and produced a series of CME-type events. The basis for the study was observations at wavelengths of 1.92–17 cm with high spatial resolution, 17″–20″, obtained on the Siberian Solar Radio Telescope (SSRT) and RATAN-600, together with simultaneous data from the Nobeyama Radio Heliograph (NoRH, wavelength 1.76 cm) and 195 Å ultraviolet data from the TRACE spacecraft. The development of the event was followed over three hours, first through observations against the disk at heights of 10,000–100,000 km from the photosphere, then in the post-limb stage to distances of the order of 10⁶ km from the solar center, i.e., in the zone inaccessible to the LASCO coronographs. According to the radio observations, ～10 min before the beginning of the event, the radio structure of the active region NOAA 10338 had an S-shaped (sigmoid) configuration. A rising, gradually expanding dark loop originated at the points where this structure was observed; according to the TRACE data, this loop initiated the event. Subsequently, the structure of the radio image drastically changed, suggesting that coronal plasma was heated and cooled at different sites of the emission region (or was shielded by the cooler material of the ejection). Profiles of the burst that accompanied the ejection are presented for four points in the region. The post-limb part of the event first had a compact (～50″) structure receding from the Sun and visible to distances ～10⁶ km. An asymmetric loop was then formed, with its material falling back onto the Sun at the end of the event. The brightness temperature of the loop was ～15 × 10³ K, and its emission was weakly polarized (P ≈16%). The mean speed of the material was 160 km/s. It is concluded that the observations of the event of April 27, 2003 are most consistent with the model of Amari et al., in which the formation of an eruptive twisted magnetic rope, taken to be responsible for CME-type events, is explained by the emergence of new magnetic flux within an old field of opposite polarity.     

Analysis of a solar eruptive event on November 4, 2001, using CORONAS-F/SPIRIT data

We analyze large-scale solar activity following the eruption of a very powerful, geoeffective coronal mass ejection in the 23rd solar cycle, observed at 175, 284, and 304 Å on November 4, 2001, using data from the CORONAS-F/SPIRIT telescope. In particular, we have shown that the restructuring of the magnetic field above the eruption center was accompanied by the formation of a multicomponent post-eruptive arcade, which was observed in all three bands over many hours and had an extent of the order of 0.5R_⊙. Two kinds of dimmings were observed, i.e., compact dimmings on either side of this arcade and channeled dimmings along some extended features beyond the active region. The intensity in the dimmings decreased by several tens of percent. The enhanced emission observed at the top of the post-eruptive arcade can be due to energy release in the course of magnetic reconnection high in the corona at the relaxation stage of the perturbed magnetic field to a new equilibrium state with a closed configuration. It can also be due to an enhanced emission measure because of the oblique direction of the line of sight crossing both loop tops and footpoint regions. The spatial coincidence of the main dimmings in lines corresponding to different temperatures indicates that a plasma outflow from the transition region and coronal structures with opened field lines are responsible for these dimmings. Variations in the plasma temperature associated with coronal mass ejections probably play an important role for some dimmings, which appear different in different lines.     

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.     

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.