Three-Dimensional Reconstruction of Active Regions

The STEREO mission provides an unprecedented opportunity to reconstruct the 3D configuration of solar features. In this work, we combine SECCHI/EUVI data from both spacecraft by means of a local correlation tracking method. The technique allows an automatic (without user intervention) matching of pixels in both images. This information is then used to triangulate the 3D coordinates of each pixel. We use the method in order to reconstruct and analyze the 3D structure of active regions. In particular, we focus on the extraction of coronal loop heights, observed nearly simultaneously in the 171, 195 and 284 Å passbands. We compare the properties of loops in the different wavelengths and extract valuable information regarding their geometry. In particular, we demonstrate that some loops that look co-spatial in the 171 Å and 195 Å images have in fact different heights and thus occupy different volumes. Our results have important implications for multi-wavelength studies of coronal loops, especially for calculations using filter-ratio techniques.     

SYNTHESIS OF IMAGING SPECTROMETER AND MULTISPECTRAL SCANNER DATA OF DIFFERENT SPATIAL RESOLUTIONS

The results of modeling the procedure of synthesizing data from an imaging spectrometer (IS) and a multispectral scanner (MSS) with low and high spatial resolutions, respectively, are presented. The synthesis method makes it possible to develop a classification and determine the spatial distribution of the sensed features at the high spatial resolution of MSS imagery and to retrieve their spectra with the spectroradiometric detail of IS measurements. It is shown that with an IS signal-to-noise ratio of no less than 50, a geometric matching of IS and MSS data with an accuracy of no less than 0.1 of an IS pixel along both axes, and with allowance for the IS real point-spread function, the spectral nonuniformity of the classes and the errors in retrieving the spectra fall within the range of several percentage points, with a ratio of the IS and MSS spatial resolutions of 4-8 and ～10% with a ratio of resolutions of 16-32.     

Observations of Solar EUV Radiation with the CORONAS-F/SPIRIT and SOHO/EIT Instruments

The SPIRIT complex onboard the CORONAS-F satellite has routinely imaged the Sun in the 171, 175, 195, 284, and 304 Å spectral bands since August 2001. The complex incorporates two telescopes. The Ritchey-Chretien telescope operates in the 171, 195, 284, and 304 Å bands and has an objective similar to that of the SOHO/EIT instrument. The Herschel telescope obtains solar images synchronously in the 175 and 304 Å bands with two multilayer-coated parabolic mirrors. The SPIRIT program includes synoptic observations, studies of the dynamics of various structures on the solar disk and in the corona up to 5 solar radii, and coordinated observations with other spaceborne and ground-based telescopes. In particular, in the period 2002–2003, synoptic observations with the SPIRIT Ritchey-Chretien telescope were coordinated with regular 6-hour SOHO/EIT observations. Since June 2003, when EIT data were temporarily absent (SOHO keyholes), the SPIRIT telescope has performed synoptic observations at a wavelength of 175 A. These data were used by the Solar Influence Data Analysis Center (SIDC) at the Royal Observatory of Belgium for an early space weather forecast. We analyze the photometric and spectral parameters of the SPIRIT and EIT instruments and compare the integrated (over the solar disk) EUV fluxes using solar images obtained with these instruments during the CORONAS-F flight from August 2001 through December 2003.     

Solar Sources of Interplanetary Coronal Mass Ejections During the Solar Cycle 23/24 Minimum

We examine solar sources for 20 interplanetary coronal mass ejections (ICMEs) observed in 2009 in the near-Earth solar wind. We performed a detailed analysis of coronagraph and extreme ultraviolet (EUV) observations from the Solar Terrestrial Relations Observatory (STEREO) and Solar and Heliospheric Observatory (SOHO). Our study shows that the coronagraph observations from viewpoints away from the Sun–Earth line are paramount to locate the solar sources of Earth-bound ICMEs during solar minimum. SOHO/LASCO detected only six CMEs in our sample, and only one of these CMEs was wider than 120^°. This demonstrates that observing a full or partial halo CME is not necessary to observe the ICME arrival. Although the two STEREO spacecraft had the best possible configuration for observing Earth-bound CMEs in 2009, we failed to find the associated CME for four ICMEs, and identifying the correct CME was not straightforward even for some clear ICMEs. Ten out of 16 (63 %) of the associated CMEs in our study were “stealth” CMEs, i.e. no obvious EUV on-disk activity was associated with them. Most of our stealth CMEs also lacked on-limb EUV signatures. We found that stealth CMEs generally lack the leading bright front in coronagraph images. This is in accordance with previous studies that argued that stealth CMEs form more slowly and at higher coronal altitudes than non-stealth CMEs. We suggest that at solar minimum the slow-rising CMEs do not draw enough coronal plasma around them. These CMEs are hence difficult to discern in the coronagraphic data, even when viewed close to the plane of the sky. The weak ICMEs in our study were related to both intrinsically narrow CMEs and the non-central encounters of larger CMEs. We also demonstrate that narrow CMEs (angular widths ≤?20^°) can arrive at Earth and that an unstructured CME may result in a flux rope-type ICME.     

Wave transformation at the cusp resonance in a finite conductivity isothermal atmosphere permeated by a nearly horizontal magnetic field

The paper considers wave transformation in the vicinity of the cusp resonance in an isothermic finite conductivity medium with a nearly horizontal magnetic field. It is shown that absorption of magnetogravity waves takes place when the inclination angle of the magnetic field is smaller than the critical angle. When the inclination angle is large then the critical magnetogravity waves are transformed into slow magnetoacoustic waves.     

The Recovery of CME-Related Dimmings and the ICME’s Enduring Magnetic Connection to the Sun

It is generally accepted that transient coronal holes (TCHs, dimmings) correspond to the magnetic footpoints of CMEs that remain rooted in the Sun as the CME expands out into the interplanetary space. However, the observation that the average intensity of the 12 May 1997 dimmings recover to their pre-eruption intensity in SOHO/EIT data within 48 hours, whilst suprathermal unidirectional electron heat fluxes are observed at 1 AU in the related ICME more than 70 hours after the eruption, leads us to question why and how the dimmings disappear whilst the magnetic connectivity is maintained. We also examine two other CME-related dimming events: 13 May 2005 and 6 July 2006. We study the morphology of the dimmings and how they recover. We find that, far from exhibiting a uniform intensity, dimmings observed in SOHO/EIT data have a deep central core and a more shallow extended dimming area. The dimmings recover not only by shrinking of their outer boundaries but also by internal brightenings. We quantitatively demonstrate that the model developed by Fisk and Schwadron (Astrophys. J. 560, 425, 2001) of interchange reconnections between “open” magnetic field and small coronal loops is a strong candidate for the mechanism facilitating the recovery of the dimmings. This process disperses the concentration of  “open” magnetic field (forming the dimming) out into the surrounding quiet Sun, thus recovering the intensity of the dimmings whilst still maintaining the magnetic connectivity to the Sun. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

Techniques for positional measurements of telescopic meteor TV images

We present the results of our positional reduction of the observational material obtained using a meteor patrol based on a Schmidt telescope and a TV CCD detector. More that 1000 telescopic meteors were recorded in three years of meteor patrolling. Techniques for the cataloging and positional reduction of 3050 TV images with meteor trails are described. We have developed a technique for measuring the images of reference stars to determine the rectangular coordinates in the image frame. We discuss the achieved accuracy of determining the equatorial coordinates of reference and check stars by Turner’s method (of the order of a few arcseconds). We have developed software that allows the rectangular coordinates of meteor trajectory points to be determined after the meteor image reduction. These coordinates are used to determine the equatorial coordinates of the poles of the great circles of meteor trajectories (the angular length is not less than 15′ with an accuracy of at least 4′. We consider the possibility of using Stanyukovich’s method to determine the equatorial coordinates of radiants for non-basis meteor observations. The accuracy of determining the radiant coordinates has been estimated to be 4′–5′. Prospects for obtaining the kinematic characteristics of meteor particles are discussed.