We describe our experiences with on-orbit calibration of, and scientific observations with, the Fine Guidance Sensors (FGS), white-light interferometers aboard Hubble Space Telescope. Our original goal, 1 milliarcsecond precision parallaxes, has been exceeded on average by a factor of three, despite a mechanically noisy on-orbit environment, the necessary self-calibration of the FGS, and significant temporal changes in our instruments. To obtain accurate absolute parallaxes from these small fields of view ($3^\prime \times 15^\prime$) observations requires a significant amount of ancillary reference star information. These data also permit an independent estimate of interstellar absorption, critical in determining target absolute magnitudes, M$_V$, often the key result of a parallax program. With these techniques we and our collaborators have obtained absolute parallaxes for 21 astrophysically interesting objects. We briefly discuss a recent determination of the parallax of the Pleiades. HST routinely produces parallaxes with half the error of the best Hipparcos results, a precision that continues down to target $V = 15$. The FGS will remain a competitive astrometric tool for the generation of high-precision parallaxes until the advent of longer-baseline space-based interferometers (SIM), or the failure of some key HST component.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Mutual event observations started in the early 1970s with the Galilean satellites. These observations were needed because of the Voyager spacecraft future arrival. Since 1979, IMCCE has organized observational campaigns for the Galilean satellites (called PHEMU), and since 1995 for the Saturnian satellites (also called PHESAT). Meanwhile, the reduction techniques have been greatly improved. Mutual event observations are one of the most accurate methods for obtaining positions of natural satellites, useful for detecting tidal effects. Hence mutual events of Jovian and Saturnian natural satellites are regularly observed around the world. This paper aims to describe mutual events and the advantages of this kind of observation besides the classical astrometric ones.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Traditionally the $\alpha - \omega$-dynamo is regarded as a basic theory of solar activity (SA) cycle. Here the model of solar magnetic cycle based on the new MHD-solutions for the mean large-scale magnetic field is presented. The hypothesis of gyrotropic plasma turbulence and cyclic restoring of the poloidal magnetic field from the rising toroidal magnetic flux tubes due to the $\alpha$-effect is not used (see Solov'ev and Kiritchek (2004)).To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

From long-term multicolour photometric observations of strongly spotted stars, one concludes usually on a presence of active longitudes. We argue the latitudinal effects in distribution spots over the surface and noticeable equatorward drift of the low-latitude boundary of the spotted region during the rising phase of activity.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

An account is given of Lord Lindsay's lavishly equipped independent expedition to the Island of Mauritius in the Indian Ocean to observe the 1874 Transit of Venus. The expedition's secondary programme, the deriving of the solar parallax from observations of the minor planet Juno, is also described. This work proved a positive outcome of a generally disappointing event and brought about an important shift in the approach to the parallax challenge. The site on Mauritius where Lord Lindsay observed the Transit in 1874, now preserved as a National Monument, was the centre of celebrations during the Transit of 2004.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The new model of coronal loop in the form of the double magnetic flux tube embedded into the uniform external magnetic field is proposed to explain the coronal oscillations phenomena. We investigate the MHD-waves in the magnetic flux tube, wich consists of the dense hot cylindrical cord surrounded by the co-axial shell. The magnitudes of Alfven and sound speeds in the cord, in the shell and in the environment are $V_{Ai}$, $C_{si}$, $V_{Am}$, $C_{sm}$, $V_{Ae}$, $C_{se}$ correspondingly. Under the coronal conditions we have \[ C_{se}<C_{si}< V_{Ai}<V_{Ae},\\ C_{si}< V_{Am},\ C_{se}<C_{sm}<V_{Am},\\ C_{sm}<V_{Ai}.\]To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We have digitized and processed the daily K-CaII-line full disk spectroheliograms from the archive of the Kodaikanal Observatory during 1907-1995. The programm has been worked out to determine the boundaries of the bright areas (plages, decayed plages, enhanced network features, K-CaII bright points and so on) with contrast that exceeded a level of the quiet Sun on the given value. About the $1.2\,{\times}\,10^6$ K-CaII active regions of different scales were processed. The coordinates, areas, the tilt and latitude-time distributions of bright features were determined. At the high latitudes the K-CaII bright points form a polar branch of solar activity at the period between the polar magnetic field reversals. This polar activity shows both 11-year's and 22-year's cycles. We found that the polar K-CaII bright point cycles proceed on average 5.5 years the sunspot cycles.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Figure 1 shows the horizontal velocity pattern of photospheric features in the main part of active region NOAA 9077 on 2000 July 12 – 15, where the horizontal velocity vectors are overlapped by white light images (left) and contours represent $G=-2{\bf U}\cdot {\bf A_p}B_n$ (injective rate of magnetic helicity) inferred from the horizontal velocity vectors and longitudinal magnetic field (right). The black (white) contours with white (black) areas mark the positive (negative) change rate of magnetic helicity of $\pm$5, 20, 50, 100, 180, 300 ($\times 10^{13}G^2m^2s^{-1}$). The arrows overlapped by $G$ mark the transverse magnetic field.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

This presentation has the following goals: (i) to explain, within a historical context, some of the difficulties in disentangling our Galaxy's three-dimensional structure; (ii) to summarise in broad terms what we presently know of this structure, concentrating on those features inferred from accurate distance measurements; (iii) to illustrate selected aspects in a visually stimulating manner; and (iv) to give a foretaste of the exciting results that lie ahead with future space astrometric experiments. The public lecture given in Preston, UK on 9 June 2004 employed 3-d visualisation techniques using polarised light to illustrate the talk.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Outside the solar disk cool emissions can be recorded. They are inserted into the surrounding hot corona and their properties are still not well known. Emissions from neutral hydrogen can be registred using the well known H-alpha line. With a coronagraph it is possible to record this line through an interference filter broad enough to avoid Doppler effects. In these observations we used a 0.3 nm width filter associated to a 15 cm aperture coronagraph equipped with a 16 bits 1K-CCD camera. The filter is wide enough to excude all usual Doppler shift effects produced during a prominence eruption.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

More than 300000 images of the Sun have been acquired during the SPIRIT experiment with 10 telescopic and spectroheliographic EUV and X-ray instruments aboard the CORONAS-F satellite. We have developed methods and routines for the first-level data processing to make the SPIRIT data easy for use in a way similar to that one for SOHO/EIT images.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The observations of the bursts with fine structures are one of the best remote sensing probe of the primary energy release sites in solar flares. The Siberian Solar Radiotelescope (SSRT) gives us the unique possibility of direct measurements of their positions with temporal resolution up to 14 ms and spatial resolution of a few arcsec. National Astronomical Observatories of China (NAOC) spectropolarimeters (5.2–7.7 GHz) provide us with the simultaneous spectral data around the SSRT receiving frequency (5.7 GHz). An analysis is made of the subsecond pulses observations: short wide range pulses, U-type microwave bursts, the bursts with zebra pattern.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Radio emissions from the solar active regions can be generated by nonlinear coupling of Langmuir waves with Alfvén waves. Multi-wavelength observations can be used to provide evidence for Langmuir-Alfvén turbulence in the solar atmosphere.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Magnetic coupling relates the energy sources below the photosphere to the structure and dynamics of the chromosphere and corona of the Sun. Multi-wavelength investigations of the solar activity provides the information needed to understand the acting physical mechanisms. Descriptions of magnetic coupling often start with force-free mathematical extrapolations of the photospheric magnetic field. Observations have revealed, however, perpendicular electric current sheets, reconnection and other locally non-force-free structures in chromosphere and corona. Obviously, the solar magnetic coupling includes dynamic forcing due to photospheric plasma motion. We developed a model which though it starts with force-free extrapolated observed photospheric magnetic field additionally takes into account the energy input due to the chromospheric neutral gas and plasma motion. We demonstrate the abilities of our model by applying it to SoHO multi-wavelength observations of an EUV bright point (BP).To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Geometry of the polar magnetic field is changing during the activity cycle. Tangents to polar ray structures are usually crossed near some point below the surface. This point can be named as a “magnetic focus”. The distance $q$ between the focus and the center of the solar disk changes from the maximum value about 0.7 $R_\odot$ at solar minimum activity to the minimum value about 0.5 $R_\odot$ at solar maximum. This behavior has not been understood till now. Really, all tangents to field lines near the pole of the axially symmetric potential magnetic field described by zonal spherical harmonics cross at the same point. However, the greater is the number of a harmonic, the closer to the surface is the crossing point. As far as at activity maximum high harmonics dominate, the behavior of the focuses seems to be in contradiction with the results of the harmonic analysis. We have found that the focus depth changes could be explained by complicated relationship between the harmonics of “new” and “old” cycle near the instant of the polar magnetic field reversal.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The distance to the Pleiades open cluster is of fundamental importance to many aspects of stellar astrophysics but is currently controversial. The ‘long’ distance scale of $132 \pm 3$ pc is supported by main sequence fitting analyses, ground-based parallax observations and analysis of the astrometric binary Atlas. The ‘short’ distance scale of $120 \pm 3$ pc comes from parallaxes observed by the Hipparcos satellite. Munari et al. (2004) studied the detached eclipsing binary HD 23642 and found a distance of $132 \pm 2$ pc. We reanalyse the data of Munari et al. to explore the different methods of estimating the distance of an eclipsing binary system. We use the surface brightness relations of Kervella et al. (2004) to find a distance of $139 \pm 4$ pc, which is consistent with the ‘long’ Pleiades distance and in disagreement with the Hipparcos parallax distances to the Pleiades and to HD 23642 itself. By comparing the observed masses and radii of the binary with theoretical predictions we derive a metal abundance approximately equal to or slightly greater than solar. Further photometric observations of the binary are needed to improve the analysis.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Four series of coronal images have been obtained by the expedition of Abastumani Astrophysical Observatory during the August 11, 1999 total solar eclipse with the help of the photographic mirror-lens polarimeter (D=150 mm, F=1000 mm). Each series includes three images corresponding to three positions of the polarization analyzer. The position of the solar disk center relative to the Moon's center has been determined beforehand. In addition, the background skylight polarization and intensity are calculated. All measurements are absolute given in units of the sun's average surface brightness. Equatorial electron densities and temperatures are determined. Our data are compared with previous absolute measurements.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The extended discussion of the Nonlinear MHD Perturbations of the Solar Corona and the Heliosphere has been exposed.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Here we report thermodynamical properties of a a so-called “thermal plume” observed near the solar disc center in 2001. The spectral observations of two iron lines analysed have been obtained using the Vacuum Tower Telescope (Tenerife).To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

This is a preliminary report on the observations of solar irradiance fluctuations with the DIFOS photometer aboard the Russian-Ukrainian satellite CORONAS-F launched in 2001. In addition the parallel ground-based spectral observations (VTT, Tenerife) carried out with the first 20-days observing space campaign are described.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html