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1.
Overshooting and semiconvection are two poorly known mechanisms which affect the extent and the efficiency of chemical mixing outside classical convection zones in stars. We discuss the uncertainties and the inferences of those processes in main sequence stars burning hydrogen in a convective core. We then focus on the asteroseismic signatures of partially or fully mixed zones surrounding the convective core, through the detailed shape of the induced chemical composition profile. We emphasize the potential power of asteroseismology to determine the internal structure of stars and thus to help us understand the physical processes at work inside the stars.  相似文献   

2.
K. P. Raju 《Solar physics》2009,255(1):119-129
Relative Doppler velocities and spectral linewidths in a coronal hole and in the quiet Sun region outside have been obtained from Solar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer (CDS) observations. Five strong emission lines in the CDS wavelength range (namely, O? iii 599 Å, O?v 630 Å, Ne?vi 562.8 Å, He?ii 304 Å, and Mg?ix 368 Å), whose formation temperatures represent different heights in the solar atmosphere from the lower transition region to the inner corona, have been used in the study. As reported earlier, relative velocities in the coronal hole are generally blueshifted with respect to the quiet Sun, and the magnitude of the blueshifts increases with height. It has been found that the polar coronal hole has larger relative velocities than the equatorial extension in the inner corona. Several localized velocity contours have been found mainly on network brightenings and in the vicinity of the coronal hole boundary. The presence of velocity contours on the network may represent network outflows whereas the latter could be due to localized jets probably arising from magnetic reconnection at the boundary. All spectral lines have larger widths in the coronal hole than in the quiet Sun. In O?v 630 Å an extended low-linewidth region is seen in the coronal hole?–?quiet Sun boundary, which may indicate fresh mass transfer across the boundary. Also polar coronal holes have larger linewidths in comparison with the equatorial extension. Together with larger relative velocities, this suggests that the solar wind emanating from polar hole regions is faster than that from equatorial hole regions.  相似文献   

3.
Statistical analysis of the relationship between type II radio bursts appearing in the metric (m) and decameter-to-hectometer (DH) wavelength ranges is presented. The associated X-ray flares and coronal mass ejections (CMEs) are also reported. The sample is divided into two classes using the frequency-drift plots: Class I, representing those events where DH-type-II bursts are not continuation of m-type-II bursts and Class II, where the DH-type-II bursts are extensions of m-type-II bursts. Our study consists of three steps: i) comparison of characteristics of the Class I and II events; ii) correlation of m-type-II and DH-type-II burst characteristics with X-ray flare properties and iii) correlation of m-type-II and DH-type-II burst characteristics with CME properties. We have found no clear correlation between properties of m-type-II bursts and DH-type-II bursts. For example, there is no correlation between drift rates of m-type-II bursts and DH-type-II bursts. Similarly there is no correlation between their starting frequencies. In Class I events we found correlations between X-ray flare characteristics and properties of m-type-II bursts and there is no correlation between flare parameters and DH-type-II bursts. On the other hand, the correlation between CME parameters and m-type-II bursts is very weak, but it is good for CME parameters and DH-type-II bursts. These results indicate that Class I m-type-II bursts are related to the energy releases in flares, whereas DH-type-II bursts tend to be related to CMEs. On the contrary, for Class II events in the case of m-type-II and DH-type-II bursts we have found no clear correlation between both flare and CMEs.  相似文献   

4.
V band photometry of three RS CVn stars, II Peg, IM Peg and UX Ari, is carried out to study the physical properties of these variables. We verified the significant and regular optical photometric variability to be present in all three stars. The strong photometric variability and emission of Hα and Ca II H K using high resolution optical spectroscopy with the Hanle Echelle Spectrograph(HESP),which operates in conjunction with the Himalayan Chandra Telescope, verify the strong chromospheric activity which is present in RS CVn stars. The photometric studies of II Peg, UX Ari and IM Peg were subjected to light curve analysis for spot parameters using a two-starspot model.  相似文献   

5.
We present a statistical analysis of a sky survey of interstellar H2O and OH masers. These masers can be classified into three categories: isolated H2O masers, isolated OH masers, and simple OH/H2O maser associations. The total number of sources in each category is of the same order of magnitude, and as an evolutionary phase they can maintain -105 yr. An improved radiative pumping mechanism is proposed. This model avoids some of the deficiencies of previous radiative models, such as shortage of exciting photons. The statistical results obtained from the survey can be interpreted by the new mechanism together with the evolutionary model in which the gravitational force of the central stellar objects is responsible for the HII region.  相似文献   

6.
We present a model for the general circulation and dynamical transport in Saturn’s upper troposphere and stratosphere and derive the effective advective circulation and eddy transport coefficients required for use in two-dimensional (latitude–altitude) photochemistry–transport models. A three-dimensional Outer-Planet General Circulation Model (OPGCM) is used to generate the transport data. We find that the OPGCM adequately captures the global-scale, pole-to pole temperature contrast, but overestimates mid- and high-latitude temperatures in the summer hemisphere by ～5 K. In addition, the model reproduces the local temperature minimum seen at the equator in Cassini Composite Infrared Spectrometer (CIRS) 0.1-mbar data but not the local maximum in 1-mbar temperatures, suggesting that it is capturing the phase of Saturn’s Semiannual Oscillation associated with a temperature minimum at the equator but not the opposite phase. The meridional circulation at low latitudes is found to be dominated by a seasonally reversing Hadley circulation, characterized by upwelling near the equator, cross-equatorial flow from summer to winter hemisphere, and strong subsidence centered near 25° latitude in the winter hemisphere. The cross-equatorial flow induces an asymmetry in which the equatorial jet is found to be stronger in the winter than in the summer stratosphere. The location of the subsidence near 25°N for Ls ～ 310° coincides with local maxima in acetylene, diacetylene, and methylacetylene mixing ratios measured by Cassini/CIRS (Guerlet, S., Fouchet, T., Bézard, B., Moses, J.I., Fletcher, L.N., Simon-Miller, A.A., Flasar, F.M. [2010]. Icarus 209, 682–695). This result supports the suggestion by Guerlet et al. (2010) that the hydrocarbon abundances are enhanced at this latitude by pronounced downward transport of hydrocarbon-rich air from above. The lateral eddy diffusion coefficient is found to typically be ～105–106 m2 s?1 at mid-latitudes, implying meridional eddy transport time scales of order 100–1000 years.  相似文献   

7.
The observations with Swift X-ray telescope (XRT) challenge the conventional gamma-ray burst model in many aspects. The XRT light curves are generally composed of four consecutive segments, i.e., a steep decay segment, a shallow decay segment, a normal decay segment, a jet-like steep decay segment, and sometimes erratic flares as well. The phys-ical origin of the X-ray emission is highly debatable. We focus here on the physical origin of the X-ray emissions of GRBs 050318 and 060124. We present the XRT light curves and spectra of the two bursts. The light curve decay slopes of the two bursts are normal, and their relations to the spectral indices are consistent with the prediction of the standard forward shock model. The multi-wavelength light curves at 0.5keV, 1.0keV, 2.0keV and 4.0keV can be reproduced by this model with an isotropic kinetic energy Ek = 2.2×1052 erg, e = 0.04, B = 0.01 for GRB 050318 and Ek = 4.2×1053 erg, e = 0.05, B = 0.01 for GRB 060124. These facts suggest that the normal decay phases of the X-rays for the two bursts are of the forward shock origin.  相似文献   

8.
Solar coronal holes (CHs) are large regions of the corona magnetically open to interplanetary space. The nearly rigid north?–?south CH boundaries (CHBs) of equatorward extensions of polar CHs are maintained while the underlying photospheric fields rotate differentially, so interchange magnetic reconnection is presumed to be occurring continually at the CHBs. The time and size scales of the required reconnection events at CHBs have not been established from previous observations with soft X-ray images. We use TRACE 195 Å observations on 9 December 2000 of a long-lived equatorial extension of the negative-polarity north polar CH to look for changes of ??5 arcsec to >?20 arcsec at the western CHB. Brightenings and dimmings are observed on both short (≈?5 minutes) and long (≈?7 hours) time scales, but the CHB maintains its quasi-rigid location. The transient CHB changes do not appear associated with either magnetic field enhancements or the changes in those field enhancements observed in magnetograms from the Michelson Doppler Imager (MDI) on SOHO. In seven hours of TRACE observations we find no examples of the energetic jets similar to those observed to occur in magnetic reconnection in polar plumes. The lack of dramatic changes in the diffuse CHB implies that gradual magnetic reconnection occurs high in the corona with large (??10°) loops and/or weak coronal fields. We compare our results with recent observations of active regions at CHBs. We also discuss how the magnetic polarity symmetry surrounding quasi-rigid CHs implies an asymmetry in the interchange reconnection process and a possible asymmetry in the solar wind composition from the eastern and western CHB source regions.  相似文献   

9.
We investigate further the interpretation of dark magnetic faculae observed in previous imaging of the solar photosphere at 1.63 m. We show that their contrast at 1.63 m increases with magnetic flux beyond a threshold value of 2 × 1018 Mx and blends smoothly with the contrast vs flux relation measured at this wavelength for larger structures of sunspot size. Not all facular structures that are bright in Ca K are dark at 1.63 m, apparently because their magnetic flux is not large enough. After correction for blurring, the contrast of the dark faculae observed near the disc center at 1.63 m is approximately 4%. But our observations at 1.23 m, which probe slightly higher photospheric levels, do not show these dark faculae. These results indicate that magnetic flux tubes of diameter as small as 500 km significantly inhibit convective heat flow to the photosphere, much as do sunspot flux tubes of much larger diameter. They also suggest that, in even smaller flux tubes, the inhibition becomes rapidly less significant. Finally, we show that the sunspot-size dependence of umbral infrared contrast versus wavelength that we observe can probably be explained in terms of instrumental blurring. Observations with lower scattered light will be required to determine whether a real decrease of contrast with diameter also plays a role.  相似文献   

10.
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.  相似文献   

11.
1 INTRoDUCTIONIt is well known that solar proton flares are maiuly correlated with strong, bright H. flares,in particular, with two ribbon flares. Ellison et al. (l961) were the first to draw attelltion tothe faCt that all cosndc-ray flares had the typical two-ribbon shape. Then, Svestka and Simon(1976) drew uP the "Catalog of Solar Particle Evellts, 195ty1969", and using this Catalog,Dodson and Hedeman could safely identify the flare sources for 50 proton events. Out of the 50idellti…  相似文献   

12.
A spectrometer was built to examine the interstellar medium (ISM) using the hydrogen Balmer a line. It is called Dual Etalon Fabry-Perot Optical Spectrometer (DEFPOS). DEFPOS will be coupled to coude exit of the 150 cm telescope (RTT150) installed at TUBITAK National Observatory (TUG). DEFPOS was ready for observations about two years ago, but work was still continuing on the RTT150 coude exit alignment. So we have started observing HII regions with DEFPOS without the RTT150. We present here some characteristics of the instrument and some of the results obtained.  相似文献   

13.
A brief review of our courrent understanding of the interaction between the innermost Galilean satellite,Io,and Jupiter's magnetosphere is presented.Particular consideration is given to the neutral clouds and lo plasma torus which have been monitored by ground-based and Earth-orbiting observations for the past 25 years.Detailed investigation of observed phenomena in the neutral clouds has revealed several processes which have a direct bearing on the nature of the interaction and suggest that a revision of the existing concept is necessary.Similarly.observed variability in the torus indicates the presence of complex plasma processes which at present remain unexplained.Areas where further research is necessary are outlined.  相似文献   

14.
We study the evolution of the longitudinal asymmetry in solar activity through the wave packet technique applied to the period domain of 25 – 31 days (centered at the 27-day solar rotation period) for the sunspot number and geomagnetic aa index. We observe the occurrence of alternating smaller and larger amplitudes of the 11-year cycle, resulting in a 22-year periodicity in the 27-day signal. The evolution of the 22-year cycle shows a change of regime around the year 1912 when the 22-year period disappears from the sunspot number series and appears in the aa index. Other changes, such as a change in the correlation between solar and geomagnetic activity, took place at the same time. Splitting the 27-day frequency domain of aa index shows an 11-year cycle for higher frequencies and a pure22-year cycle for lower frequencies, which we attribute to higher latitude coronal holes. This evidence is particularly clear after 1940, which is another benchmark in the evolution of the aa index. We discuss briefly the mechanisms that could account for the observed features of the 22-year cycle evolution.  相似文献   

15.
1 INTRODUCTION The Sun is close enough to observe in some detail, and it shows that a star is more than the traditional stable self-gravitating thermonuclear body established half a celltury ago. For the fact is that out of sight beneath the visible surface the outward flow of heat from the thermonuclear core drives hydrodynamics that generates magnetic fields. It is the complicated dynamics of those magnetic fields that produces the modern mysteries of the active Sun. The…  相似文献   

16.
Gilman  Peter A. 《Solar physics》2000,192(1-2):27-48
We review recent progress and define unanswered scientific questions in five related topics: granulation- to supergranulation-scale convection and magnetic structures; global convection and circulation; the rise of magnetic flux tubes to the photosphere, and their injection into the base of the convection zone; tachocline fluid dynamics and MHD; and the solar dynamo. We close with a set of observational `targets' for helioseismologists to aim for.  相似文献   

17.
We report a filament eruption near the center of the solar disk on 1999 March 21, in multi-wavelength observations by the Yohkoh Soft X-Ray Telescope (SXT), the Extreme-ultraviolet Images Telescope (EIT) and the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). The eruption involved in the disappearance of an Ha filament can be clearly identified in EIT 195 A difference images. Two flare-like EUV ribbons and two obvious coronal dimming regions were formed. The two dimming regions had a similar appearance in lines formed in temperature range 6×104 K to several 106 K. They were located in regions of opposite magnetic polarities near the two ends of the eruptive filament. No significant X-ray or Ha flare was recorded associated with the eruption and no obvious photospheric magnetic activity was detected around the eruptive region, and particularly below the coronal dimming regions. The above surface activities were closely associated with a partial halo-type coronal mass ejection (CME) observed by the Large Angle and Spectrometric Coronagraphs (LASCO) on the SOHO. In terms of the magnetic flux rope model of CMEs, we explained these multiple observations as an integral process of large-scale rearrangement of coronal magnetic field initiated by the filament eruption, in which the dimming regions marked the evacuated feet of the flux rope.  相似文献   

18.
The combination of seasonal and orbital changes in Martian insolation result in complex latitude dependent surface temperature variations that effect the total radiance of the planet as seen from the earth. These surface temperature variations have been calculated, based upon a computer simulation of the thermal environment of the planet. The temperature variations are then integrated to yield the total radiance of the planet as seen from the earth as a function of time. The absolute radiance of Mars was measured on April 4, 1971, with a balloon-borne radiometer system operating in the wavelength range between 10.5 and 12.5 μm. The average brightness temperature of the Mars disk determined from these measurements was 254°K with a 1 σ error of 4°K.  相似文献   

19.
The morphology of impact craters on the icy Galilean satellites differs from craters on rocky bodies. The differences are thought due to the relative weakness of ice and the possible presence of sub-surface water layers. Digital elevation models constructed from Galileo images were used to measure a range of dimensions of craters on the dark and bright terrains of Ganymede. Measurements were made from multiple profiles across each crater, so that natural variation in crater dimensions could be assessed and averaged scaling trends constructed. The additional depth, slope and volume information reported in this work has enabled study of central peak formation and development, and allowed a quantitative assessment of the various theories for central pit formation. We note a possible difference in the size-morphology progression between small craters on icy and silicate bodies, where central peaks occur in small craters before there is any slumping of the crater rim, which is the opposite to the observed sequence on the Moon. Conversely, our crater dimension analyses suggest that the size-morphology progression of large lunar craters from central peak to peak-ring is mirrored on Ganymede, but that the peak-ring is subsequently modified to a central pit morphology. Pit formation may occur via the collapse of surface material into a void left by the gradual release of impact-induced volatiles or the drainage of impact melt into sub-crater fractures.  相似文献   

20.
Fitting observed power and cross spectra of medium-degree p modes in velocity (V) and intensity (I) has been widely used for getting information about the p-mode excitation process and, in particular, for trying to determine the type and location of the acoustic sources. Numerical simulations of solar convection allow one to “observe” velocity and temperature (T, used as proxy for I) fluctuations in different reference frames. Sampling the oscillations on planes of constant optical depth (τ-frame) closely corresponds to the observer’s point of view, whereas sampling the oscillations at constant geometrical height (z-frame) is more appropriate for comparison with predictions from theoretical models based on Eulerian hydrodynamics. The results of the analysis in the two frames show significant differences. Considering the effects introduced on oscillations by the steep temperature gradient of the photosphere and by the temperature- and pressure-dependent continuum opacity, we develop a new model for fitting the simulated V and T power and cross spectra both in the τ- and z-frames and discuss its merits and limitations.  相似文献   

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