Latitudinal and Solar-Cycle Variations of the White-Light Corona from SOHO/LASCO Observations

SOHO/LASCO data were used to obtain the latitudinal and radial distributions of the brightness of the K- and F-corona in the period of 1996 – 2007, and their solar-cycle variations were studied. Then an inversion method was employed to obtain the radial distributions of the electron density N _e(R,θ) for various latitude values on the coronal images. Our values of N _e(R,θ) are in good agreement with the findings of other authors. We found that in an edge-on streamer belt the electron density, like the K-corona brightness, varies with distance more slowly in the near-equatorial rays than in near-polar regions. We have developed a method for assessing the maximum values of the electron density at the center of the face-on streamer belt in its bright rays and depressions between them. Not all bright rays observed in the face-on streamer belt are found to be associated with an increased electron density in them. Mechanisms for forming such rays have been suggested.     

Periodicity and Hemispheric Phase Relationship in High-Latitude Solar Activity

The counts of the monthly averaged polar faculae, from observations of the National Astronomical Observatory of Japan (NAOJ), are examined by using linear and nonlinear approaches to find the periodicity characteristics of the polar faculae in the northern and southern hemispheres and the phase relationship between them. Both the cross-wavelet transform (XWT) and wavelet coherence (WTC) indicate the prominent period with 95% confidence level, namely the Schwabe cycle of about 11 years. The Schwabe cycle is in phase in the two hemispheres. Within the 11-year frequency band, there is a small phase difference during the period of 1966 – 1975 when the activity of polar faculae in the northern hemisphere slightly leads the one in the southern hemisphere. A cross-recurrence plot analysis and the line of synchronization (LOS) extracted from the cross-recurrence plot show further the phase difference between the two hemispheres. The LOS deviates significantly from the main diagonal during the period of 1965 – 1970 and LOS >0, showing that the activity of polar faculae in the northern hemisphere leads in phase, which is in accordance with XWT and WTC analyses. Moreover, asynchronization is highest (about 30 months) during this period.     

Dependence of the Characteristics of Solar Wind Discontinuities and Shocks with Heliolatitude and Radial Distance

The distribution of the shocks in the heliosphere and their characteristic variations are investigated using Ulysses observations. The jumps in solar wind velocity, IMF magnitude, and proton density across the shocks and discontinuities are evaluated and used to characterize them. The distribution of these discontinuities with respect to heliolatitude ± 80° and with radial distance 1 to 5 AU are analyzed during solar minimum and solar maximum to understand their global behavior. It is noticed that the jumps in solar wind parameters associated with shocks and discontinuities are more prominent during the second orbit of Ulysses, which coincided with the maximum phase of solar activity.     

On the Origin, 3D Structure and Dynamic Evolution of CMEs Near Solar Minimum

We have conducted a statistical study 27 coronal mass ejections (CMEs) from January 2007 – June 2008, using the stereoscopic views of STEREO SECCHI A and B combined with SOHO LASCO observations. A flux-rope model, in conjunction with 3D triangulations, has been used to reconstruct the 3D structures and determine the actual speeds of CMEs. The origin and the dynamic evolution of the CMEs are investigated using COR1, COR2 and EUVI images. We have identified four types of solar surface activities associated with CMEs: i) total eruptive prominence (totEP), ii) partially eruptive prominence (PEP), iii) X-ray flare, and iv) X-type magnetic structure (X-line). Among the 27 CMEs, 18.5% (5 of 27) are associated with totEPs, 29.6% (8 of 27) are associated with PEPs, 26% (7 of 27) are flare related, and 26% (7 of 27) are associated with X-line structures, and 43% (3 of 7) are associated with both X-line structures and PEPs. Three (11%) could not be associated with any detectable activity. The mean actual speeds for totEP-CMEs, PEP-CMEs, flare-CMEs, and X-line-CMEs are 404 km?s^?1,247 km?s^?1,909 km?s^?1, and 276 km?s^?1, respectively; the average mean values of edge-on and broadside widths for the 27 CMEs are 52 and 85 degrees, respectively. We found that slow CMEs (V≤400 km?s^?1) tend to deflect towards and propagate along the streamer belts due to the deflections by the strong polar magnetic fields of corona holes, while some faster CMEs show opposite deflections away from the streamer belts.     

Organization of Energetic Particles by the Solar Wind Structure During the Declining to Minimum Phase of Solar Cycle 23

We investigate the organization of the low energy energetic particles (≤1 MeV) by solar wind structures, in particular corotating interaction regions (CIRs) and shocks driven by interplanetary coronal mass ejections, during the declining-to-minimum phase of Solar Cycle 23 from Carrington rotation 1999 to 2088 (January 2003 to October 2009). Because CIR-associated particles are very prominent during the solar minimum, the unusually long solar minimum period of this current cycle provides an opportunity to examine the overall organization of CIR energetic particles for a much longer period than during any other minimum since the dawn of the Space Age. We find that the particle enhancements associated with CIRs this minimum period recurred for many solar rotations, up to 30 at times, due to several high-speed solar wind streams that persisted. However, very few significant CIR-related energetic particle enhancements were observed towards the end of our study period, reflecting the overall weak high-speed streams that occurred at this time. We also contrast the solar minimum observations with the declining phase when a number of solar energetic particle events occurred, producing a mixed particle population. In addition, we compare the observations from this minimum period with those from the previous solar cycle. One of the main differences we find is the shorter recurrence rate of the high-speed solar wind streams (～10 solar rotations) and the related CIR energetic particle enhancements for the Solar Cycle 22 minimum period. Overall our study provides insight into the coexistence of different populations of energetic particles, as well as an overview of the large-scale organization of the energetic particle populations approaching the beginning of Solar Cycle 24.     

Solar Wind Sources in the Late Declining Phase of Cycle 23: Effects of the Weak Solar Polar Field on High Speed Streams

The declining phases of solar cycles are known for their high speed solar wind streams that dominate the geomagnetic responses during this period. Outstanding questions about these streams, which can provide the fastest winds of the solar cycle, concern their solar origins, persistence, and predictability. The declining phase of cycle 23 has lasted significantly longer than the corresponding phases of the previous two cycles. Solar magnetograph observations suggest that the solar polar magnetic field is also ～?2?–?3 times weaker. The launch of STEREO in late 2006 provided additional incentive to examine the origins of what is observed at 1 AU in the recent cycle, with the OMNI data base at the NSSDC available as an Earth/L1 baseline for comparisons. Here we focus on the year 2007 when the solar corona exhibited large, long-lived mid-to-low latitude coronal holes and polar hole extensions observed by both SOHO and STEREO imagers. STEREO provides in situ measurements consistent with rigidly corotating solar wind stream structure at up to ～?45° heliolongitude separation by late 2007. This stability justifies the use of magnetogram-based steady 3D solar wind models to map the observed high speed winds back to their coronal sources. We apply the WSA solar wind model currently running at the NOAA Space Weather Prediction Center with the expectation that it should perform its best at this quiet time. The model comparisons confirm the origins of the observed high speed streams expected from the solar images, but also reveal uncertainties in the solar wind source mapping associated with this cycle’s weaker solar polar fields. Overall, the results illustrate the importance of having accurate polar fields in synoptic maps used in solar wind forecast models. At the most fundamental level, they demonstrate the control of the solar polar fields over the high speed wind sources, and thus one specific connection between the solar dynamo and the solar wind character.     

Short-Term Periodicities in Sunspot Activities During the Descending Phase of Solar Cycle 23

In the present study, the short-term periodicities in the daily data of the sunspot numbers and areas are investigated separately for the full disk, northern, and southern hemispheres during Solar Cycle 23 for a time interval from 1 January 2003 to 30 November 2007 corresponding to the descending and minimum phase of the cycle. The wavelet power spectrum technique exhibited a number of quasi-periodic oscillations in all the datasets. In the high frequency range, we find a prominent period of 22 – 35 days in both sunspot indicators. Other quasi-periods in the range of 40 – 60, 70 – 90, 110 – 130, 140 – 160, and 220 – 240 days are detected in the sunspot number time series in different hemispheres at different time intervals. In the sunspot area data, quasi-periods in the range of 50 – 80, 90 – 110, 115 – 130, 140 – 155, 160 – 190, and about 230 days were noted in different hemispheres within the time period of analysis. The present investigation shows that the well-known “Rieger periodicity” of 150 – 160 days reappears during the descending phase of Solar Cycle 23, but this is prominent mainly in the southern part of the Sun. Possible explanations of these observed periodicities are delivered on the basis of earlier results detected in photospheric magnetic field time series (Knaack, Stenflo, and Berdyugina in Astron. Astrophys. 438, 1067, 2005) and solar r-mode oscillations.     

Combined effects of perturbations,radiation, and oblateness on the nonlinear stability of triangular points in the restricted three-body problem

This paper investigates the nonlinear stability of the triangular equilibrium points under the influence of small perturbations in the Coriolis and centrifugal forces together with the effect of oblateness and radiation pressures of the primaries. It is found that the triangular points are stable for all mass ratios in the range of linear stability except for three mass ratios depending upon above perturbations, oblateness coefficients and mass reduction factors.     

The structure and dynamics of young star clusters: King 16, NGC 1931, NGC 637 and NGC 189

In this paper, using 2MASS photometry, we study the structural and dynamical properties of four young star clusters viz. King 16, NGC 1931, NGC 637 and NGC 189. For the clusters King 16, NGC 1931, NGC 637 and NGC 189, we obtain the limiting radii of 7′, 12′, 6′ and 5′ which correspond to linear radii of 3.6 pc, 8.85 pc, 3.96 pc and 2.8 pc respectively. The reddening values E(B?V) obtained for the clusters are 0.85, 0.65–0.85, 0.6 and 0.53 and their true distances are 1786 pc, 3062 pc, 2270 pc and 912 pc respectively. Ages of the clusters are 6 Myr, 4 Myr, 4 Myr and 10 Myr respectively. We compare their structures, luminosity functions and mass functions (φ(M)=dN/dM ∝ M ^?(1+χ)) to the parameter τ=t _age /t _relax to study the star formation process and the dynamical evolution of these clusters. We find that, for our sample, mass seggregation is observed in clusters or their cores only when the ages of the clusters are comparable to their relaxation times (τ≥1). These results suggest mass seggregation due to dynamical effects. The values of χ, which characterize the overall mass functions for the clusters are 0.96±0.11, 1.16±0.18, 0.55±0.14 and 0.66±0.31 respectively. The change in χ as a function of radius is a good indicator of the dynamical state of clusters.     

Inclusion of sdBs in evolutionary population synthesis for binary stellar populations and the application: the determination of photo-z and galaxy morphology

Subdwarf B stars (sdBs) can significantly change the ultraviolet spectra of populations at age t～1 Gyr, and have been even included in the evolutionary population synthesis (EPS) models by Han et al. (Mon. Not. R. Astron. Soc. 380:1098, 2007). In this study we present the spectral energy distributions (SEDs) of binary stellar populations (BSPs) by combining the EPS models of Han et al. (Mon. Not. R. Astron. Soc. 380:1098, 2007) and those of the Yunnan group (Zhang et al. in Astron. Astrophys. 415:117, 2004; Mon. Not. R. Astron. Soc. 357:1088, 2005), which have included various binary interactions (except sdBs) in EPS models. This set of SEDs is available upon request from the authors. Using this set of SEDs of BSPs we build the spectra of Burst, E, S0–Sd and Irr types of galaxies by using the package of Bruzual and Charlot (Mon. Not. R. Astron. Soc. 344:1000, 2003). Combined with the photometric data (filters and magnitudes), we obtain the photometric redshifts and morphologies of 1502 galaxies by using the Hyperz code of Bolzonella et al. (Astron. Astrophys. 363:476, 2000). This sample of galaxies is obtained by removing those objects, mismatched with the SDSS/DR7 and GALEX/DR4, from the catalogue of Fukugita et al. (Astron. J. 134:579, 2007). By comparison the results with the SDSS spectroscopic redshifts and the morphological index of Fukugita et al. (Astron. J. 134:579, 2007), we find that the photo-z fluctuate with the SDSS spectroscopic redshifts, while the Sa–Sc galaxies in the catalogue of Fukugita et al. (Astron. J. 134:579, 2007) are classified earlier as Burst-E galaxies.     

The Observed Long- and Short-Term Phase Relation between the Toroidal and Poloidal Magnetic Fields in Cycle 23

The observed phase relations between the weak background solar magnetic (poloidal) field and strong magnetic field associated with sunspots (toroidal field) measured at different latitudes are presented. For measurements of the solar magnetic field (SMF) the low-resolution images obtained from Wilcox Solar Observatory are used and the sunspot magnetic field was taken from the Solar Feature Catalogues utilizing the SOHO/MDI full-disk magnetograms. The quasi-3D latitudinal distributions of sunspot areas and magnetic fields obtained for 30 latitudinal bands (15 in the northern hemisphere and 15 in the southern hemisphere) within fixed longitudinal strips are correlated with those of the background SMF. The sunspot areas in all latitudinal zones (averaged with a sliding one-year filter) reveal a strong positive correlation with the absolute SMF in the same zone appearing first with a zero time lag and repeating with a two- to three-year lag through the whole period of observations. The residuals of the sunspot areas averaged over one year and those over four years are also shown to have a well defined periodic structure visible in every two – three years close to one-quarter cycle with the maxima occurring at − 40° and + 40° and drifts during this period either toward the equator or the poles depending on the latitude of sunspot occurrence. This phase relation between poloidal and toroidal field throughout the whole cycle is discussed in association with both the symmetric and asymmetric components of the background SMF and relevant predictions by the solar dynamo models.     

Phase Space Analysis: The Equilibrium of the Solar Magnetic Cycle

We present the results of a statistical study of the solar cycle based on the analysis of the superficial toroidal magnetic field component phase space. The magnetic field component used to create the embedded phase space was constructed from monthly sunspot number observations since 1750. The phase space was split into 32 sections (or time instants) and the average values of the orbits on this phase space were calculated (giving the most probable cycle). In this phase space it is shown that the magnetic field on the Sun’s surface evolves through a set of orbits that go around a mean orbit (i.e., the most probable magnetic cycle that we interpret as the equilibrium solution). It follows that the most probable cycle is well represented by a van der Pol oscillator limit curve (equilibrium solution), as can be derived from mean-field dynamo theory. This analysis also retrieves the empirical Gnevyshev – Ohl’s rule between the first and second parts of the solar magnetic cycle. The sunspot number evolution corresponding to the most probable cycle (in phase space) is presented.     

MHD simulations of accretion disks and jets: strengths and limitations

Observations are providing increasingly detailed quantitative information about the accretion flows that power such high energy systems as X-ray binaries and Active Galactic Nuclei. These observations have been modeled in some detail by a variety of accretion scenarios, but such models rely on unavoidable assumptions such as regular flow geometry and a simple, parameterized stress. Global numerical simulations offer a way to investigate the basic physical dynamics of accretion flows without these assumptions. We are now carrying out fully three-dimensional general relativistic magnetohydrodynamic simulations of time-dependent inflows into Kerr black holes. The results from recent global simulations of black hole accretion disks will be reviewed, and some implications of those results for observations will be discussed.     

Exact Bianchi type-II, VIII and IX perfect fluid cosmological models in Saez-Ballester theory of gravitation

Exact Bianchi type-II, VIII and IX cosmological models are obtained in a scalar tensor theory proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986) with perfect fluid as a source. Some physical and geometrical properties of the models are studied. It is observed that the models are free from initial singularities and they are expanding with time.     

Study of CME Propagation in the Inner Heliosphere: SOHO LASCO, SMEI and STEREO HI Observations of the January 2007 Events

We are investigating the geometric and kinematic characteristics of interplanetary coronal mass ejections (ICMEs) using data obtained by the LASCO coronagraphs, the Solar Mass Ejection Imager (SMEI), and the SECCHI imaging experiments on the STEREO spacecraft. The early evolution of CMEs can be tracked by the LASCO C2 and C3 and SECCHI COR1 and COR2 coronagraphs, and the HI and SMEI instruments can track their ICME counterparts through the inner heliosphere. The HI fields of view (4?–?90°) overlap with the SMEI field of view (>?20° to all sky) and, thus, both instrument sets can observe the same ICME. In this paper we present results for ICMEs observed on 24?–?29 January 2007, when the STEREO spacecraft were still near Earth so that both the SMEI and STEREO views of large ICMEs in the inner heliosphere coincided. These results include measurements of the structural and kinematic evolution of two ICMEs and comparisons with drive/drag kinematic, 3D tomographic reconstruction, the HAFv2 kinematic, and the ENLIL MHD models. We find it encouraging that the four model runs generally were in agreement on both the kinematic evolution and appearance of the events. Because it is essential to understand the effects of projection across large distances, that are not generally crucial for events observed closer to the Sun, we discuss our analysis procedure in some detail.     

On the Temporal Variability of the “Strahl” and Its Relationship with Solar Wind Characteristics: STEREO SWEA Observations

The “strahl” is a specific population of the solar wind, constituted by strongly field aligned electrons flowing away from the Sun, with energies >60 eV. Using the Solar Wind Electron Analyzer (SWEA) onboard STEREO, we investigate the short time scale fluctuations of this population. It is shown that its phase space density (PSD) at times presents fluctuations larger than 50% at scales of the order of minutes and less. The fluctuations are particularly strong for periods of a few tens of hours in high-speed streams, following the crossing of the corotating interaction region, when the strahl is also the most collimated in pitch angle. The amplitude of the fluctuations tends to decrease in conjunction with a broadening in pitch angle. Generally, the strongly fluctuating strahl is observed when the magnetic field is also highly perturbed. That SWEA is able to perform a very rapid 3D analysis at a given energy is essential since it can be demonstrated that the observed magnetic turbulence can only marginally perturb the PSD measurements.     

Different Features of Jets and Isotropic Fireballs in Gamma-ray Burst Phase

We investigated physical quantities including the spectrum,emission lines and pulse profiles expected from a uniform jet,a spherical fireball and the 1/Γ region (the portion of the spherical fireball with opening angle 1/Γ)in the prompt emission phase,after taking into consideration of the Doppler effect under the fireball frame- work.Our study shows that:a)for these physical quantities the spherical fireball and the uniform jets do not present obvious differences,so we cannot use these to distinguish a spherical fireball from a uniform jet;b)for the spherical fireball and a uniform jet,the observed quantities mainly come from the 1/Г region,so we can simply use the 1/Г region in approximate calculations;c)broadening of emission lines is a general phenomena,which mainly comes from the curvature effect;d)the 1/Γ region plays different roles in different frequency ranges,and the radiation from the 1/Γ region is greater in the higher than in the lower frequencies.     

Erratum to: The Maunder Minimum and the Sun as the Possible Source of Particles Creating Increased Abundance of the <Superscript>14</Superscript>C Carbon Isotope

Solar flares take place in regions of strong magnetic fields and are generally accepted to be the result of a resistive instability leading to magnetic reconnection. When new flux emerges into a pre-existing active region it can act as a flare and coronal mass ejection trigger. In this study we observed active region 10955 after the emergence of small-scale additional flux at the magnetic inversion line. We found that flaring began when additional positive flux levels exceeded 1.38×10²⁰ Mx (maxwell), approximately 7 h after the initial flux emergence. We focussed on the pre-flare activity of one B-class flare that occurred on the following day. The earliest indication of activity was a rise in the non-thermal velocity one hour before the flare. 40 min before flaring began, brightenings and pre-flare flows were observed along two loop systems in the corona, involving the new flux and the pre-existing active region loops. We discuss the possibility that reconnection between the new flux and pre-existing loops before the flare drives the flows by either generating slow mode magnetoacoustic waves or a pressure gradient between the newly reconnected loops. The subsequent B-class flare originated from fast reconnection of the same loop systems as the pre-flare flows.     

Long-Term Changes in Sunspot Activity,Occurrence of Grand Minima,and Their Future Tendencies

Long-term changes in the magnetic activity of the Sun were studied in terms of the empirical mode decomposition that revealed their essential modes. The occurrence of grand minima was also studied in their relation to long-term changes in sunspot activity throughout the past 11 000 yr. Characteristic timescales of long-term changes in solar activity manifest themselves in the occurrence of grand minima. A quantitative criterion has been defined to identify epochs of grand minima. This criterion reveals the important role of secular and bicentennial activity variations in the occurrence of grand minima and relates their amplitudes with the current activity level, which is variable on a millennial timescale. We have revealed specific patterns in the magnetic activity between successive grand minima which tend to recur approximately every 2300 yr but occasionally alternate with irregular changes. Such intermittent activity behavior indicates low dimensional chaos in the solar dynamo due to the interplay of its dominant modes. The analysis showed that in order to forecast activity level in forthcoming cycles, one should take into account long-term changes in sunspot activity on a ≈2300-yr timescale. The regularities revealed suggest solar activity to decrease in the foreseeable future.