The Siberian Solar Radio Telescope: the current state of the instrument,observations, and data

The Siberian Solar Radio Telescope (SSRT) is one of the world's largest solar radio heliographs. It commenced operation in 1983, and since then has undergone several upgrades. The operating frequency of the SSRT is 5.7 GHz. Since 1992 the instrument has had the capability to make one-dimensional scans with a high time resolution of 56 ms and an angular resolution of 15 arc sec. Making one of these scans now takes 14 ms. In 1996 the capability was added to make full, two-dimensional images of the solar disk. The SSRT is now capable of obtaining images with an angular resolution of 21 arc sec every 2 min. In this paper we describe the main features and operation of the instrument, particularly emphasizing issues pertaining to the imaging process and factors limiting data quality. Some of the data processing and analysis techniques are discussed. We present examples of full-disk solar images of the quiet Sun, recorded near solar activity minimum, and images of specific structures: plages, coronal bright points, filaments and prominences, and coronal holes. We also present some observations of dynamic phenomena, such as eruptive prominences and solar flares, which illustrate the high-time-resolution observations that can be done with this instrument. We compare SSRT observations at 5.7 GHz, including computed `light curves', both morphologically and quantatively, with observations made in other spectral domains, such as 17 GHz radio images, Hα filtergrams and magnetograms, extreme-ultraviolet and X-ray observations, and dynamic radio spectra.     

The 26 December 2001 Solar Eruptive Event Responsible for GLE63: III. CME,Shock Waves,and Energetic Particles

The SOL2001-12-26 moderate solar eruptive event (GOES importance M7.1, microwaves up to 4000 sfu at 9.4 GHz, coronal mass ejection (CME) speed 1446 km?s^?1) produced strong fluxes of solar energetic particles and ground-level enhancement (GLE) of cosmic-ray intensity (GLE63). To find a possible reason for the atypically high proton outcome of this event, we study multi-wavelength images and dynamic radio spectra and quantitatively reconcile the findings with each other. An additional eruption probably occurred in the same active region about half an hour before the main eruption. The latter produced two blast-wave-like shocks during the impulsive phase. The two shock waves eventually merged around the radial direction into a single shock traced up to \(25~\mathrm{R}_{\odot}\) as a halo ahead of the expanding CME body, in agreement with an interplanetary Type II event recorded by the Radio and Plasma Wave Investigation (WAVES) experiment on the Wind spacecraft. The shape and kinematics of the halo indicate an intermediate regime of the shock between the blast wave and bow shock at these distances. The results show that i) the shock wave appeared during the flare rise and could accelerate particles earlier than usually assumed; ii) the particle event could be amplified by the preceding eruption, which stretched closed structures above the developing CME, facilitated its lift-off and escape of flare-accelerated particles, enabled a higher CME speed and stronger shock ahead; iii) escape of flare-accelerated particles could be additionally facilitated by reconnection of the flux rope, where they were trapped, with a large coronal hole; and iv) the first eruption supplied a rich seed population accelerated by a trailing shock wave.     

A Challenging Solar Eruptive Event of 18 November 2003 and the Causes of the 20 November Geomagnetic Superstorm. IV. Unusual Magnetic Cloud and Overall Scenario

The geomagnetic superstorm of 20 November 2003 with Dst=?422 nT, one of the most intense in history, is not well understood. The superstorm was caused by a moderate solar eruptive event on 18 November, comprehensively studied in our preceding Papers I?–?III. The analysis has shown a number of unusual and extremely complex features, which presumably led to the formation of an isolated right-handed magnetic-field configuration. Here we analyze the interplanetary disturbance responsible for the 20 November superstorm, compare some of its properties with the extreme 28?–?29 October event, and reveal a compact size of the magnetic cloud (MC) and its disconnection from the Sun. Most likely, the MC had a spheromak configuration and expanded in a narrow angle of ≤?14^°. A very strong magnetic field in the MC up to 56 nT was due to the unusually weak expansion of the disconnected spheromak in an enhanced-density environment constituted by the tails of the preceding ICMEs. Additional circumstances favoring the superstorm were i) the exact impact of the spheromak on the Earth’s magnetosphere and ii) the almost exact southward orientation of the magnetic field, corresponding to the original orientation in its probable source region near the solar disk center.     

A Challenging Solar Eruptive Event of 18 November 2003 and the Causes of the 20 November Geomagnetic Superstorm. III. Catastrophe of the Eruptive Filament at a Magnetic Null Point and Formation of an Opposite-Handedness CME

Our analysis in Papers I and II (Grechnev et al., Solar Phys. 289, 289, 2014b and Solar Phys. 289, 1279, 2014c) of the 18 November 2003 solar event responsible for the 20 November geomagnetic superstorm has revealed a complex chain of eruptions. In particular, the eruptive filament encountered a topological discontinuity located near the solar disk center at a height of about 100 Mm, bifurcated, and transformed into a large cloud, which did not leave the Sun. Concurrently, an additional CME presumably erupted close to the bifurcation region. The conjectures about the responsibility of this compact CME for the superstorm and its disconnection from the Sun are confirmed in Paper IV (Grechnev et al., Solar Phys. submitted, 2014a), which concludes about its probable spheromak-like structure. The present article confirms the presence of a magnetic null point near the bifurcation region and addresses the origin of the magnetic helicity of the interplanetary magnetic clouds and their connection to the Sun. We find that the orientation of a magnetic dipole constituted by dimmed regions with the opposite magnetic polarities away from the parent active region corresponded to the direction of the axial field in the magnetic cloud, while the pre-eruptive filament mismatched it. To combine all of the listed findings, we propose an intrinsically three-dimensional scheme, in which a spheromak-like eruption originates via the interaction of the initially unconnected magnetic fluxes of the eruptive filament and pre-existing ones in the corona. Through a chain of magnetic reconnections their positive mutual helicity was transformed into the self-helicity of the spheromak-like magnetic cloud.     

Absorption Phenomena and a Probable Blast Wave in the 13 July 2004 Eruptive Event

We present a case study of the 13 July 2004 solar event, in which disturbances caused by eruption of a filament from an active region embraced a quarter of the visible solar surface. Remarkable are the absorption phenomena observed in the SOHO/EIT 304 Å channel, which were also visible in the EIT 195 Å channel, in the Hα line, and even in total radio flux records. Coronal and Moreton waves were also observed. Multispectral data allowed reconstructing an overall picture of the event. An explosive filament eruption and related impulsive flare produced a CME and blast shock, both of which decelerated and propagated independently. Coronal and Moreton waves were kinematically close and both decelerated in accordance with an expected motion of a coronal blast shock. The CME did not resemble a classical three-component structure, probably because some part of the ejected mass fell back onto the Sun. Quantitative evaluations from different observations provide close estimates of the falling mass, ～3×10¹⁵?g, which is close to the estimated mass of the CME. The falling material was responsible for the observed large-scale absorption phenomena, in particular, shallow widespread moving dimmings observed at 195 Å. By contrast, deep quasi-stationary dimmings observed in this band near the eruption center were due to plasma density decrease in coronal structures.     

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.     

Pulsations of microwave emission as a consequence of oscillatory transients in the solar atmosphere

We offer a new interpretation of the origin of oscillations in active regions. Quasi-periodic oscillation trains with periods of 3 min, 5 min, and min arise due to slow oscillatory transients which accompany the appearance of plasma flow along the sunspot's magnetic field. In oscillatory transients of the fast type, occurring in current loop systems, the typical period of oscillations shifts towards values of the order of a second.     

Radio,Hard X-Ray,and Gamma-Ray Emissions Associated with a Far-Side Solar Event

The far-side solar eruptive event SOL2014-09-01 produced hard electromagnetic and radio emissions that were observed with detectors at near-Earth vantage points. Especially challenging was a long-duration >?100 MeV \(\gamma\)-ray burst that was probably produced by accelerated protons exceeding 300 MeV. This observation raised the question how high-energy protons could reach the Earth-facing solar surface. Some preceding studies discussed a scenario in which protons accelerated by a shock driven by a coronal mass ejection high in the corona return to the solar surface. We continue with the analysis of this challenging event, involving radio images from the Nançay Radioheliograph and hard X-ray data from the High Energy Neutron Detector (HEND) of the Gamma-Ray Spectrometer onboard the Mars Odyssey space observatory located near Mars. HEND recorded unocculted flare emission. The results indicate that the emissions observed from the Earth’s direction were generated by flare-accelerated electrons and protons trapped in static long coronal loops. They can be reaccelerated in these loops by a shock wave that was excited by the eruption, being initially not driven by a coronal mass ejection. The results highlight ways to address the remaining questions.     

An experience of radioheliographic prediction of powerful solar flares

An experience of radioheliographic prediction of powerful solar flares using active region microwave emission intensity and circular polarization distribution is briefly described.