首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
We consider a solar dynamo mechanism that generates large-scale magnetic fields due to the combined action of cyclonic flows (the α effect), differential rotation (the Θ effect), and the non-uniformity of large-scale magnetic fields (the Θ × J effect). Our results are based on numerical model which takes into account currently available data on the differential rotation of the convection zone and the intensity of convective flows in the solar interior. A reasonable choice of parameters characterizing the intensity of magnetic-field generation by the α and Θ × J mechanisms can account for an oscillatory dynamo regime with properties similar to the 22-year magnetic-activity cycle of the Sun. We analyze the nonlinear saturation of the generation effects in the large-scale magnetic field, due to either magnetic stresses or the conservation of magnetic helicity. Allowance for the helicity of the small-scale magnetic fields is of crucial importance in limiting the energy of the generated large-scale magnetic field.  相似文献   

2.
We analyze the X-ray emission and chromospheric activity of late-type F, G, and K stars studied in the framework of the HK project. More powerful coronas are possessed by stars displaying irregular variations of their chromospheric emission, while stars with cyclic activity are characterized by comparatively modest X-ray luminosities and ratios of the X-ray to bolometric luminosity L X/L bol. This indicates that the nature of processes associated with magnetic-field amplification in the convective envelope changes appreciably in the transition from small to large dynamo numbers, directly affecting the character of the (α-Ω) dynamo. Due to the strong dependence of both the dynamo number and the Rossby number on the speed of axial rotation, earlier correlations found between various activity parameters and the Rossby number are consistent with our conclusions. Our analysis makes it possible to draw the first firm conclusions about the place of solar activity among analogous processes developing in active late-type stars.  相似文献   

3.
Recent data from the Kepler mission has revealed the occurrence of superflares in Sun-like stars which exceed by far any observed solar flares in released energy. Radionuclide data do not provide evidence for occurrence of superflares on the Sun over the past eleven millennia. Stellar data for a subgroup of superflaring Kepler stars are analysed in an attempt to find possible progenitors of their abnormal magnetic activity. A natural idea is that the dynamo mechanism in superflaring stars differs in some respect from that in the Sun. We search for a difference in the dynamo-related parameters between superflaring stars and the Sun to suggest a dynamo mechanism as close as possible to the conventional solar/stellar dynamo but capable of providing much higher magnetic energy. Dynamo based on joint action of differential rotation and mirror asymmetric motions can in principle result in excitation of two types of magnetic fields. First of all, it is well-known in solar physics dynamo waves. The point is that another magnetic configuration with initial growth and further stabilisation can also be excited. For comparable conditions, magnetic field of second configuration is much stronger than that of the first one just because dynamo does not spend its energy for periodic magnetic field inversions but uses it for magnetic field growth. We analysed available data from the Kepler mission concerning the superflaring stars in order to find tracers of anomalous magnetic activity. As suggested in a recent paper [1], we find that anti-solar differential rotation or anti-solar sign of the mirror-asymmetry of stellar convection can provide the desired strong magnetic field in dynamo models. We confirm this concept by numerical models of stellar dynamos with corresponding governing parameters. We conclude that the proposed mechanism can plausibly explain the superflaring events at least for some cool stars, including binaries, subgiants and, possibly, low-mass stars and young rapid rotators.  相似文献   

4.
A mixing-length approximation is used to calculate Kλ for a Parker dynamo wave excited by the dynamo mechanism near the base of the solar convection zone (K is the wave number of the dynamo wave and λ the extent of the dynamo region). In a turbulent-dynamo model, this number characterizes the modes of the global magnetic field generated by a mechanism based on the joint action of the mean helical turbulence and solar differential rotation. Estimates are obtained for the helicity and radial angular-velocity gradient using the most recent helioseismological measurements at the growth phase of solar cycle 23. These estimates indicate that the dynamo mechanism most efficiently excites the fundamental antisymmetric (odd), dipole, mode of the poloidal field (Kλ≈?7) at low latitudes, while the conditions at latitudes above 50° are more favorable for the excitation of the lowest symmetric (even), quadrupole, mode (Kλ≈+8). The resulting north-south asymmetry of the poloidal field can explain the magnetic anomaly (“monopole” structure) of the polar fields observed near solar-cycle maxima. The effect of α quenching increases the calculated period of the dynamo-wave propagation from middle latitudes to the equator to about seven years, in rough agreement with the observed duration of the solar cycle.  相似文献   

5.
Observations of the solar butterfly diagram from sunspot records suggest persistent fluctuations in parity, away from the overall, approximately dipolar pattern. A simple mean-field dynamo model is used with a solar-like rotation law and perturbed α effect. The parity of the magnetic field relative to the rotational equator can demonstrate can be described as resonance behavior, while the magnetic energy behaves in a more or less expected way. Possible applications of this effect are discussed in the context of various deviations of the solar magnetic field from dipolar symmetry, as reported from analyses of archival sunspot data. The model produces fluctuations in field parity, and hence in the butterfly diagram, that are consistent with observed fluctuaions in solar behavior.  相似文献   

6.
A series of yearly data on the concentration of radioactive carbon 14C in tree rings measured at the Tbilisi State University in 1983–1986 and covering the time interval 1600–1940 is statistically analyzed. We find evidence for a 22-year cyclicity in the intensity of Galactic cosmic rays (GCRs) during the Maunder minimum of the solar activity (1645–1715), testifying that the solar dynamo mechanism continued to operate during this epoch. Variations of Δ14C on timescales of tens and hundreds of years correlate well with the corresponding variations of the GCR intensity and solar activity, making radiocarbon a reliable source of information on long-timescale variations of solar activity in the past. Short-timescale (<30 years) fluctuations of Δ14C may be appreciably distorted by time variations not associated directly with solar activity; probable origins of this distortion are discussed.  相似文献   

7.
We study the twist properties of photospheric magnetic fields in solar active regions using magnetographic data on 422 active regions obtained at the Huairou Solar Observing Station in 1988–1997. We calculate the mean twist (force-free field αf) of the active regions and compare it with the mean current-helicity density of these same active regions, h c =B ∥·(?×B)∥. The latitude and longitude distributions and time dependence of these quantities is analyzed. These parameters represent two different tracers of the α effect in dynamo theory, so we might expect them to possess similar properties. However, apart from differences in their definitions, they also display differences associated with the technique used to recalculate the magnetographic data and with their different physical meanings. The distributions of the mean αf and h c both show hemispherical asymmetry—negative (positive) values in the northern (southern) hemisphere—although this tendency is stronger for h c. One reason for these differences may be the averaging procedure, when twists of opposite sign in regions with weak fields make a small contribution to the mean current-helicity density. Such transequatorial regularity is in agreement with the expectations of dynamo theory. In some active regions, the average αf and h c do not obey this transequatorial rule. As a whole, the mean twist of the magnetic fields αf of active regions does not vary significantly with the solar cycle. Active regions that do not follow the general behavior for αf do not show any appreciable tendency to cluster at certain longitudes, in contrast to results for h c noted in previous studies. We analyze similarities and differences in the distributions of these two quantities. We conclude that using only one of these tracers, such as αf, to search for signatures of the α effect can have disadvantages, which should be taken into account in future studies.  相似文献   

8.
  相似文献   

9.
We have determined activity cycles for coolest M dwarfs using photometry from the ASAS survey. The time scales of brightness variations were determined for the program stars using calculated amplitude power spectra and wavelet spectra. Most of ther program stars display periodicities in their light-curve variations, with periods from hundreds of days to years. Analysis of diagrams plotting P cyc/P rot versus 1/P rot in logarithmic coordinates shows that the data for all our program objects fit the general relation quite well. No differences in the activity cycles are found for our sample stars, which have different masses and thus internal structures, some having convective envelopes and others being totally convective. Our analysis indicates that the slope i of this relation is close to unity, regardless of whether it is determined from all data, from data for the shortest cycles, or from data for the longest cycles. This value of i differs from values in the literature for stars of other spectral types. Our analysis of the P cyc-P rot relation indicates that the activity cycles for the studied sample of M dwarfs do not depend on the rotation periods of these objects. The data for the studied objects do not agree with any of the relations for relatively young (active) stars or older (less active) stars. The studied M dwarfs probably form another branch of low-mass stars that display more random, irregular magnetic activity on their surfaces, which is generated and supported by the distributed dynamo mechanism or a small-scale dynamo mechanism.  相似文献   

10.
A method for estimating the spottedness parameter S (the spotted area as a fraction of the surface of an active star) proposed earlier is applied to an analysis of activity in 1570 M dwarf stars. The analysis is based on observational material obtained with the Kepler Space Telescope, as well as data on the fluxes of the studied objects in the near and far ultraviolet (NUV and FUV) based on data from the GALEX space telescope. The variations of S with the ages of the stars are studied (four groups with different ages are distinguished), as well as variations of S with their rotational periods. A diagram characterizing the relationship between S and the Rossby number Ro resembles the classical dependence of the X-ray luminosities of active stars on Ro, and a saturation regime is attained at the same value, Ro = 0.13. Moreover, objects with ages of more than 100 million years do not form a single sequence (and stars older than 900 million years possess surface spottednesses of order 1%). The S?Ro dependence obtained could expand possibilities for analyzing the dependence of the X-ray luminosities of active stars on their Rossby numbers, and could also be applied to refine parameters characterizing the action of dynamo mechanisms, such as the dynamo number N D . A comparison of the GALEX NUV and FUV brightness estimates with the activity parameters of the stars suggests that younger, more rapidly rotating active stars are brighter in the NUV, and that the FUV flux grows and the difference of the FUV and NUV brightnesses decreases with increasing spottedness S.  相似文献   

11.
The contribution of thermal bremsstrahlung to the total microwave flux of a solar flare loop is considered. The total-flux data were obtained on the Nobeyama Radio Heliograph. The calculation of the thermal bremsstrahlung radio flux was based on determining the integrated temperature and number density of the hot flare-loop plasma from its soft X-ray flux, obtained using data from the GOES-10 and GOES-12 satellites. The effect of thermal bremsstrahlung on the total flux and the spectral index of the microwave radiation is insignificant at the burst maximum (F th/F tot < 3%, Δα < 0.2), while the contribution of bremsstrahlung can be substantial during the decay phase of the burst (up to 80%). This results in an appreciable decrease in the observed spectral index (to Δα ～ 1.5). Therefore, when diagnosing the parameters of the accelerated electrons based on the characteristics of their gyrosynchrotron radiation, the most accurate results can be obtained using the emission characteristics obtained near the burst maximum.  相似文献   

12.
It is shown that, when all components of the large-scale solar magnetic field are longitudinally averaged, the N polarity and the eastward transverse component of the B φ field associated with both local and large-scale fields over the Northern hemisphere are somewhat stronger and occupy a smaller area during odd cycles than does the field of opposite polarity. This behavior is reversed for even cycles or the Southern hemisphere. The regular Hale law is a particular form of the above rule. The nature of this asymmetry seems to be rooted in the dynamo mechanism itself, and should be important for fields on any scale.  相似文献   

13.
We have determined the physical (T eff, logg, ζ) and kinematic (V e sini,V r ) parameters and abundances for 14 chemical elements in the atmosphere of the “antiflare” variable RZ Psc, using medium-resolution spectra obtained with the Coudé spectrographs of the 6-m telescope of the Special Astrophysical Observatory and the Crimean Astrophysical Observatory 2.6-m Shain telescope. The chemical composition of the star is characterized by a slight metal deficiency; however, the iron and calcium abundances are consistent with the solar values within the errors. We also detected a peculiar dip (depression) of the continuum level near the Hα line. Assuming that this depression and the photospheric Hα line have independent origins, we calculated the hydrogen abundance X in the atmosphere of RZ Psc. The resulting value X=0.70 (of the solar value) implies a relative deficiency of hydrogen. Together with the spatial location of the star, these properties provide evidence that RZ Psc is an evolved star, most likely belonging to population II.  相似文献   

14.
The solar cycle can be described as a complex interaction of large-scale/global and local magnetic fields. In general, this approach agrees with the traditional dynamo scheme, although there are numerous discrepancies in the details. Integrated magnetic indices introduced earlier are studied over long time intervals, and the epochs of the main reference points of the solar cycles are refined. A hypothesis proposed earlier concerning global magnetometry and the natural scale of the cycles is verified. Variations of the heliospheric magnetic field are determined by both the integrated photospheric i(B r )ph and source surface i(B r )ss indices, however, their roles are different. Local fields contribute significantly to the photospheric index determining the total increase in the heliospheric magnetic field. The i(B r )ss index (especially the partial index ZO, which is related to the quasi-dipolar field) determines narrow extrema. These integrated indices supply us with a “passport” for reference points, making it possible to identify them precisely. A prominent dip in the integrated indices is clearly visible at the cycle maximum, resulting in the typical double-peak form (the Gnevyshev dip), with the succeeding maximum always being higher than the preceding maximum. At the source surface, this secondary maximum significantly exceeds the primary maximum. Using these index data, we can estimate the progression expected for the 23rd cycle and predict the dates of the ends of the 23rd and 24th cycles (the middle of 2007 and December 2018, respectively).  相似文献   

15.
Over the last few years, various researches have reached the conclusion that cosmic ray variations and geomagnetic disturbances are related to the condition of the human physiological state. In this study, medical data concerning the number of incidents of different types of cardiac arrhythmias for the time period 1983–1992, which refer to 1902 patients in Tbilisi, Georgia, were used. The smoothing method and the Pearson r-coefficients were used to examine the possible effect of different solar and geomagnetic activity parameters and cosmic ray intensity variations on the different types of arrhythmias. The time interval under examination was separated into two different time periods, which coincided with the polarity reversal of the solar magnetic field occurred in the years 1989–1990, and as a result, a different behavior of all the above-mentioned parameters as well as of the different types of arrhythmias was noticed during the two time intervals. In addition, changing of polarity sign of the solar magnetic field was found to affect the sign of correlation between the incidence of arrhythmias and the aforementioned parameters. The primary and secondary maxima observed in the solar parameters during the solar cycle 22, also appeared in several types of arrhythmias with a time lag of about 5 months.  相似文献   

16.
We have obtained continuous latitude distributions of the K and F corona brightnesses at various distances for the first time, using both the Hayes—Vourlidas—Howard method and our own recently proposed, simple technique for separating the emission of the K and F coronas. Data from the LASCO C2 and C3 coronagraphs are analyzed. Variations of the angular size of the brightness distribution of the F corona with latitude and distance are estimated, as well as the ratio of the maximum F-corona brightness to the F-corona brightness at the pole. The variations in the F-corona brightness at large distances (R = 25 R ⊙, where R ⊙ is the solar radius) are studied on various time scales—a month, a year, and 11 years (the solar cycle). The latitude distribution of the F-corona brightness varies most appreciably over a year, and only weakly over one solar revolution and one solar-activity cycle (as considered on a fixed day of the year).  相似文献   

17.
  相似文献   

18.
《Tectonophysics》2001,330(1-2):93-102
The fractality of the earthquake sequence (1983–1997) of Irpinia–Basilicata (Southern Italy), one of the most seismically active regions of the Mediterranean area, has been analysed by temporal and spatial fractal tools. The fractal exponent α, estimated by the Allan Factor method, characterises the time-clustering behaviour of the set of earthquakes, while the correlation dimension DC, calculated by means of the correlation integral method, gives information on the space-clustering behaviour of the sequence of seismic events. Analysing the variations of both the parameters, we recognised the presence of a strong space–time clusterisation associated with the major events that occurred in the investigated area.  相似文献   

19.
Comparisons of the brightness distributions of the white corona observed at distances of several solar radii with solar wind velocities derived from interplanetary-scintillation observations, as well as analyses of solar wind data obtained on spacecraft from December 1994 to June 1995, indicate that the fast solar wind can contain plasma with velocities V ≈ 300–450 km/s, approaching those typical for the slow solar wind that flows in the streamer belt and chains of streamers. At the same time, certain other parameters, first and foremost the plasma density N and ratio T/N 0.5 (where T is the temperature), indicate that these two flows differ considerably. The slow solar wind flowing in the streamer belt and chains displays high densities N > 10 ± 2 cm?3 and low T/N 0.5 < 1.7 × 104 K cm3/2 at the Earth’s orbit. The number of slow solar-wind sources observed in chains can be comparable with the number observed in the belt. The fast solar wind flowing from coronal holes always displays low densities N≤ 8 cm?3 and high T/N 0.5 > 1.7 × 104 K cm3/2. These properties probably indicate different origins of the fast and slow solar winds.  相似文献   

20.
Vibrational modelling is at the present time the only known way to predict the heat capacities of the Earth's mantle minerals at high-pressure and high-temperature. To test the validity of this method for λ-type transitions, we have applied it to the α-β quartz transition (T 0=846±1 K). Raman spectra of quartz were recorded up to 900 K. Measured frequency shifts of the α-quartz Raman modes were then used in conjunction with available high-pressure Raman data to calculate intrinsic mode anharmonicity, through the parameter a i=(?Lnvi/?T)v. Vibrational modelling of the heat capacity at constant volume, Cv, and at constant pressure, Cp, including anharmonic corrections deduced from the a i parameters, are compared to experimental data. Taking into account the soft-mode associated to the α-β quartz transition, the model reproduces the excess of Cp related to the transition. Then, this study confirms that detecting a soft-mode from vibrational data allows one to predict λ-type transitions. However, when modelling the thermodynamic properties, the contribution of a soft-mode cannot be established from spectroscopic data. Therefore, one needs first to determine this contribution in order to predict the heat capacities of Earth's mantle minerals displaying λ-type transitions. In α-quartz, this contribution has been determined as 0.007% of the total number of the optic modes in the model of the density of states.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号