Damping of Longitudinal Magneto–Acoustic Oscillations in Slowly Varying Coronal Plasma

We investigate the propagation of MHD waves in a magnetised plasma in a weakly stratified atmosphere, representative of hot coronal loops. In most earlier studies, a time-independent equilibrium was considered. Here we abandon this restriction and allow the equilibrium to develop as a function of time. In particular, the background plasma is assumed to be cooling due to thermal conduction. The cooling is assumed to occur on a time scale greater than the characteristic travel times of the perturbations. We investigate the influence of cooling of the background plasma on the properties of magneto–acoustic waves. The MHD equations are reduced to a 1D system modelling magneto–acoustic modes propagating along a dynamically cooling coronal loop. A time-dependent dispersion relation that describes the propagation of the magneto–acoustic waves is derived using the WKB theory. An analytic solution for the time-dependent amplitude of waves is obtained, and the method of characteristics is used to find an approximate analytical solution. Numerical calculations of the analytically derived solutions are obtained to give further insight into the behaviour of the MHD waves in a system with a variable, time-dependent background. The results show that there is a strong damping of MHD waves and the damping also appears to be independent of the position along the loop. Studies of MHD wave behaviour in a time-dependent backgrounds seem to be a fundamental and very important next step in the development of MHD wave theory that is applicable to a wide range of situations in solar physics.     

Sunspot Catalogue of the Valencia Observatory (1920 – 1928)

A sunspot catalogue was maintained by the Astronomical Observatory of Valencia University (Spain) from 1920 to 1928. Here we present a machine-readable version of this catalogue (OV catalogue or OVc), including a quality-control analysis. Sunspot number (total and hemispheric) and sunspot area series are constructed using this catalogue. The OV catalogue data are compared with other available solar data, demonstrating that the present contribution provides the scientific community with a reliable catalogue of sunspot data.     

A Simple Method to Check the Reliability of Annual Sunspot Number in the Historical Period 1610 – 1847

A simple method to detect inconsistencies in low annual sunspot numbers based on the relationship between these values and the annual number of active days is described. The analysis allowed for the detection of problems in the annual sunspot number series clustered in a few explicit periods, namely: i) before Maunder minimum, ii) the year 1652 during the Maunder minimum, iii) the year 1741 in Solar Cycle −1, and iv) the so-called “lost” solar cycle in the 1790s and the subsequent onset of the Dalton Minimum.     

Sunspot Catalogue of the Observatory of the University of Coimbra (1929 – 1941)

A sunspot catalogue was published by the Coimbra Astronomical Observatory (Portugal), which is now called the Geophysical and Astronomical Observatory of the University of Coimbra, for the period 1929?–?1941. We digitalised data included in that catalogue and provide a machine-readable version. We show the reconstructions for the (total and hemispheric) sunspot number index and sunspot area according to this catalogue and compare it with the sunspot number index (version 2) and the Balmaceda sunspot area series (Balmaceda et al. in J. Geophys. Res.114, A07104, 2009). Moreover, we also compared the Coimbra catalogue with records made at the Royal Greenwich Observatory. The results demonstrate that the historical catalogue compiled by the Coimbra Astronomical Observatory contains reliable sunspot data and can therefore be considered for studies about solar activity.     

Latitude Dependence of the Variations of Sunspot Group Numbers (SGN) and Coronal Mass Ejections (CMEs) in Cycle 23

The 12-month running means of the conventional sunspot number Rz, the sunspot group numbers (SGN) and the frequency of occurrence of Coronal Mass Ejections (CMEs) were examined for cycle 23 (1996 – 2006). For the whole disc, the SGN and Rz plots were almost identical. Hence, SGN could be used as a proxy for Rz, for which latitude data are not available. SGN values were used for 5° latitude belts 0° – 5°, 5° – 10°, 10° – 15°, 15° – 20°, 20° – 25°, 25° – 30° and > 30°, separately in each hemisphere north and south. Roughly, from latitudes 25° – 30° N to 20° – 25° N, the peaks seem to have occurred later for lower latitudes, from latitudes 20° – 25° N to 15° – 20° N, the peaks are stagnant or occur slightly earlier, and then from latitudes 15° – 20° N to 0° – 5° N, the peaks seem to have occurred again later for lower latitudes. Thus, some latitudinal migration is suggested, clearly in the northern hemisphere, not very clearly in the southern hemisphere, first to the equator in 1998, stagnant or slightly poleward in 1999, and then to the equator again from 2000 onwards, the latter reminiscent of the Maunder butterfly diagrams. Similar plots for CME occurrence frequency also showed multiple peaks (two or three) in almost all latitude belts, but the peaks were almost simultaneous at all latitudes, indicating no latitudinal migration. For similar latitude belts, SGN and CME plots were dissimilar in almost all latitude belts except 10° – 20° S. The CME plots had in general more peaks than the SGN plots, and the peaks of SGN often did not match with those of CME. In the CME data, it was noticed that whereas the values declined from 2002 to 2003, there was no further decline during 2003 – 2006 as one would have expected to occur during the declining phase of sunspots, where 2007 is almost a year of sunspot minimum. An inquiry at GSFC-NASA revealed that the person who creates the preliminary list was changed in 2004 and the new person picks out more weak CMEs. Thus a subjectivity (overestimates after 2002) seems to be involved and hence, values obtained before and during 2002 are not directly comparable to values recorded after 2002, except for CMEs with widths exceeding 60°.     

Photographic Observations of the Minor Planet (1) Ceres in Tashkent in 1951–1994

A procedure is described for remeasuring photographic plates with images of the minor planet (1) Ceres obtained in 1951–1994 with the Tashkent normal astrograph. To determine the observed spherical coordinates, the PPM catalog, based on the FK5 system, was used as a reference catalog. The calculation of the orbit of the minor planet (1) Ceres and the determination of the PPM zero-points and periodic errors on the basis of these observations yielded a unit weight error of 0.270. The derived coordinates and velocity components of (1) Ceres for J.D. = 2441600.5 are presented together with PPM zero-point and periodic-error-coefficient corrections. One of the results of the work is the high precision of the remeasured images, which makes it possible to use these observations, along with no less accurate observations of other selected minor planets, for determining corrections to the stellar coordinate system.     

A study of generalized second law of thermodynamics in modified f(R) Horava–Lifshitz gravity

This work investigates the validity of the generalized second law of thermodynamics in modified f(R) Horava–Lifshitz gravity proposed by Chaichian et al. (Class. Quantum Grav. 27: 185021, 2010), which is invariant under foliation-preserving diffeomorphisms. It has been observed that the equation of state parameter behaves like quintessence (w>−1). We study the thermodynamics of the apparent, event and particle horizons in this modified gravity. We observe that under this gravity, the time derivative of total entropy stays at positive level and hence the generalized second law is validated.     

Infrared photometry of Sakurai’s object (V4334 Sgr) in 1996–1999

The infrared photometric observations of V4334 Sgr in 1996–1999 are presented. Together with optical data, they have allowed us to accurately estimate the bolometric flux from this star and to investigate the structure of its dust envelope over the above period. The star is shown to have passed through four well-defined stages in these four years as it moved backward along the post-AGB track, and it now appears to have started moving forward after a halt. At the first stage (1996), there was no dust in the star’s envelope. Its visual brightness slightly increased, and it reddened in the entire observed spectral range. The bolometric flux also gradually rose. At the second stage (1997), an optically thick dust envelope condensed around the star, which, however, essentially did not manifest itself at optical wavelengths. The bolometric flux continued to rise through an increase in the star’s infrared brightness alone; the rate of its rise also increased. At the third stage (1998–March 1999), V4334 Sgr entered the R CrB phase. First two shallow minima and then two deep minima were observed at optical wavelengths. The star appreciably reddened during the deep minima. The bolometric flux ceased to rise and began to gradually fall in the second half of 1998. At the fourth stage (since March 1999 up until now), V4334 Sgr has been at a protracted deep minimum, which is atypical of the R CrB stars. The bolometric flux between March and October underwent no significant variations. We describe the structure of the dust envelope around V4334 Sgr since its formation. From June 1997 until July 1998, the optical depth of the dust shell, its inner and outer radii, and its mass increased by factors of ～2.2, ～2.0, 2.3, and ～10, respectively. In July 1998, τ(V)≈2.3, R _{d, in}≈7.4×10¹⁴ cm, R _{d, in}/R _{d, out}≈0.7(R _{d, in}/R _*≈47), and M _dust≈1.6×10^?7 M _⊙.     

A synoptic investigation of particle precipitation dynamics for 60 substorms in IQSY (1964–1965) and IASY (1969)

Sixty auroral absorption substorms (30 in IQSY and 30 in IASY) have been analysed on the basis of riometer-recordings taken at some 40 stations distributed over auroral, subauroral and polar cap latitudes. Synoptic maps showing isoabsorption curves have been produced every 15 min (sometimes every 5 min) of the 60 substorms; 705 maps altogether.Some of the results of the analysis are as follows.Initiation of a substorm most frequently occurs near midnight but may occur anywhere between early evening and late morning. The time of onset becomes earlier and the latitude of onset moves equatorward as the level of magnetic activity increases.The longitude expansion velocities are contained in the range 0.7–7 km/sec except for a few extreme values which exceed 20 km/sec.The auroral absorption eastward expansion velocity is smaller than the corresponding velocity of the boundary of the region of activation of the visual aurora after break up by a factor $\text{14}\text{?12}$.The expansion velocity corresponds, in general, to drift velocities of electrons of energies in the range 50–300 keV but, for the extreme speeds, electron energies around 1 MeV are needed.Expansion of the absorption in the westward direction was seen in about half of the substorms studied. In about half of these, expansion along the auroral oval could be indentified, but in almost all of these cases some expansion in the auroral zone latitudes was also seen. In about an equal number of events, expansion was confined primarily to the auroral zone.The velocity of the westward expansion was about 1 km/sec along the auroral oval (i.e. approximately equal with the speed of the westward travelling surge) but about 2 km/sec along the auroral zone.The meridional expansion velocities found agree well with those measured for visual aurora (? 1 km/sec).The variability of the behaviour of different substorms is very large. To illuminate this the following may be mentioned, in addition to what has been stated above about the statistics.Although the absorption maximum practically always moves eastward from the initiation region, exceptions have been seen in which the maximum started moving west and in a later phase went eastward.Sometimes the absorption maximum stays in the injection area or very close to it, although in most cases it moves eastward into the dayside. In extreme eases it has been found to move more than 270° in the eastward direction.There are auroral absorption substorms in which injection seems to take place in more than one area simultaneously.The observations cannot all be understood in terms of gradient and curvature drift of electrons from a small area of injection only. A broad intrusion of hot plasma from the tail into the inner magnetosphere seems to be needed.No strong dependence of particle precipitation on the illumination of the upper ionosphere by sunlight was seen. The results do, therefore, not support the hypothesis of Brice and Lucas (1971) that cold plasma density increases, originating in the ionosphere, significantly increase the precipitation rate of energetic trapped particles.     

The Anti-Tail of Comet C/1995 O1 (Hale–Bopp) in 1997/1998

The large-scale post-perihelion evolution of the dust tail in Comet C/1995 O1 (Hale–Bopp) was monitored with Schmidt telescope exposures at the European Southern Observatory (ESO) La Silla in Chile. In early October 1997 signatures of a peculiar dust-tail appeared as straight, but diffuse extensions in the northern and southern coma hemisphere. With the approach of the Earth to the orbital plane of the comet a needle-like anti-tail with a similar, but much longer counterpart in the region of the normal dust tail formed. In early January 1998, i.e., close to the plane crossing, the anti-tail pointed towards position angle 190° and was at least 0.4°long, its counter part extended over more than 5° into oppositedirection. During February and March 1998 the position angles of both features increased by more than 40$°– while their appearance became shorter, wider and more diffuse again. Thepeculiar dust tail was last observed in April 1998.Modeling of the dust tail dynamics shows that the anti-tail andpeculiar tail phenomenon observed is formed by a neck-line structureof heavy dust grains released around perihelion passage. Two scenariiare introduced to explain the extension of the anti-tail feature:(1) grains ejected about 20 days post-perihelion with a initial velocity of 170 m/s in radial direction towards the Sun and(2) grains released about 60–80 days before perihelion at a speed of70m/s into the orbital plane and against the motion directionof the comet. The out-of-plane velocity component of the dust wasless than 25 m/s. Both scenarios require dust of < 0.001to be involved, i.e., grains of at least mm-size. The rather symmetric shading of the neck-line structure before and after passage of theEarth through the orbital plane of the comet in early January 1998 supports the idea that dust of both scenarii may have contributed to the phenomenon.     

Preface: Frontiers in Astrophysics(Ⅱ)——A special issue dedicated to the 20th anniversary of RAA(2001–2020)

On the arrival of the 20 th anniversary of the journal, Research in Astronomy and Astrophysics(RAA), we see rapid progress in the frontiers of astronomy and astrophysics. To celebrate the birth and growth of RAA, a special issue consisting of 11 invited reviews from more than 30 authors, mainly from China, has been organized. This is the second volume of the special issues entitled Frontiers in Astrophysics published in RAA. The publication aims at evaluating the current status and key progress in some frontier areas of astronomy and astrophysics with a spirit of guiding future studies.     

Detection of Solar Rotational Variability in the Large Yield RAdiometer (LYRA) 190 – 222 nm Spectral Band

We analyze the variability of the spectral solar irradiance during the period from 7 January 2010 until 20 January 2010 as measured by the Herzberg channel (190?–?222 nm) of the Large Yield RAdiometer (LYRA) onboard PROBA2. In this period of time, observations by the LYRA nominal unit experienced degradation and the signal produced by the Herzberg channel frequently jumped from one level to another. Both factors significantly complicate the analysis. We present the algorithm that allowed us to extract the solar variability from the LYRA data and compare the results with SORCE/SOLSTICE measurements and with modeling based on the Code for the Solar Irradiance (COSI).     

Nova Cygni 1975 (V1500 Cyg) in 2000–2009 and the nature of its synodic period

Multicolor photometric data are presented for the asynchronous polar V1500 Cyg during 2000–2009, i.e., 23–35 years after its outburst. Some examples of individual light curves of the system are shown. An analysis of these reveals large variations in its brightness and color with the phase of the orbital period owing to a “reflection effect” caused by reradiation from the side of the red dwarf facing the hot white dwarf and heated by its hard ultraviolet radiation. The variations in the O-C residuals and in the maximum intensity with the phase of the synodic period are illustrated. It is found that the amplitude A of the orbital fluctuations increases in proportion to the intensity I at a rate dA/dI=0.64. This behavior of V1500 Cyg is most likely caused by periodic shading of the illuminated part of the red dwarf, where the degree of shading depends on the phase of the synodic cycle.     

Measurements of visual double stars with PISCO2 at the Nice 76-cm refractor in 2013–2014 (new data) and in 2015

We present relative astrometric and photometric measurements of visual double stars made in 2013–2015, with PISCO2 installed at the 76-cm refractor of Côte d'Azur Observatory in Nice (France). Our observing list contains orbital couples as well as double stars whose motion is still uncertain. Most of the observations were done in 2015, but some other observations of wide couples were done in 2013–2014 (0.3% of the total of the observations). Three different techniques were used for obtaining measurements: lucky imaging, speckle interferometry and the direct vector autocorrelation method. From our observations of 2837 multiple stars, we obtained 5182 new measurements with angular separations in the range of 0 ${}^{″}$.1–32 ${}^{″}$ and an average accuracy of $0^{″}.018$. The mean error on the position angles is 0°.8. Most of the position angles were determined without the usual 180° ambiguity with the application of the direct vector autocorrelation technique and/or by inspection of the Lucky images or the long integration files. We managed to routinely monitor faint systems ( $m_V\approx 9-11$) with large magnitude difference (up to $\mathrm{\Delta }{m}_V\approx 4.5$). We have thus been able to measure 21 systems containing red dwarf stars that had been poorly monitored since their discovery, from which we estimated the stellar masses thanks to Gaia measurements. We also measured the magnitude difference of the two components of 1079 double stars with an estimated error of 0.2 mag. Except for a few objects that are discussed, our measurements are in good agreement with the ephemerides computed with published orbital elements, even for the double stars whose separation is smaller than the diffraction limit. Thanks to good seeing images and with the use of high-contrast numerical filters, we have also been able to obtain 196 measurements with an angular separation smaller than the diffraction limit of our instrumentation, and consistent with those obtained with larger telescopes. We also report measurements of the 164 new double stars that we found in the files obtained during the observations. Finally, from a study of the DR3 Gaia release, it is shown that all the objects of the Tycho Double Star Catalog that we did not resolve in 2015 are probably false detections by Tycho.     

The spectrum of comet C/2009 R1 (McNaught) in 4140–5240 Å wavelength region

The spectroscopic observations of comet C/2009 R1 (McNaught) were carried out with the 2 m Zeiss-RCC Telescope of Pik Terskol Observatory operated by the International Center for Astronomical and Medico-Ecological Research (Ukraine, Russia). The Multi Mode Cassegrain spectrometer was used to obtain spectra of moderate spectral resolving power with a wavelength coverage from 4140 to 5240 Å. The spectrum is characterized by the extremely low continuum level and strong molecular features. 192 emission lines of C₂, CN, CH, NH₂, CO⁺, and CH⁺ have been identified by the comparison of the observed and theoretical spectra of the molecules. The ratios of the gas production rates of Q(C₂)/Q(CN)=1.32, Q(CH)/Q(CN)=0.49, and Q(NH₂)/Q(CN)=0.81 were derived using a Haser model.     

Identification of Emissions in the Spectrum of Comet C/1989 VI (Skoritchenko–George) Obtained with the 6-m SAO RAS Reflector

The emission spectrum of comet Skoritchenko–George (C/1989 VI), unusual in its information content, was obtained on February 26.7 UT, 1990, with the use of a TV scanner installed on the 6-m BTA reflector of the Special Astronomical Observatory of the Russian Academy of Sciences (SAO RAS) in Nizhni Arkhyz. Detailed identification of the emission lines of this comet was made. The observed spectrum contains 311 emission lines, including those of the molecules. Among others, the lines of the negative carbon C ₂ ^- ion and the lines corresponding to the electron transition in the neutral CO molecule are discovered. The presence of a large number of lines of the neutral CO molecule (the Asundi bands and the triplet bands) in the visible region is one of the uncommon features of the emission spectrum of this comet. The triplet lines : 15–3, 13–2, 11–2, 9–1, 8–1, 7–1, 7–0, 5–0, 4–0; : 7–0, 6–0, 5–0; and a" : 11–1 (K = 3, 4); 16–4 (K= 0, 1, 2, 4); 9-0 (K= 0, 1, 2); 8–0 (K= 0) were identified for the first time. Prior to this work, the lines of CO in the visible range were observed only in the spectrum of comet C/1979 VI (Bradfield) in 1989.