共查询到20条相似文献,搜索用时 62 毫秒
1.
We believe the Babcock-Leighton process of poloidal field generation to be the main source of irregularity in the solar cycle. The random nature of this process may make the poloidal field in one hemisphere stronger than that in the other hemisphere at the end of a cycle. We expect this to induce an asymmetry in the next sunspot cycle. We look for evidence of this in the observational data and then model it theoretically with our dynamo code. Since actual polar field measurements exist only from the 1970s, we use the polar faculae number data recorded by Sheeley (1991, 2008) as a proxy of the polar field and estimate the hemispheric asymmetry of the polar field in different solar minima during the major part of the twentieth century. This asymmetry is found to have a reasonable correlation with the asymmetry of the next cycle. We then run our dynamo code by feeding information about this asymmetry at the successive minima and compare the results with observational data. We find that the theoretically computed asymmetries of different cycles compare favorably with the observational data, with the correlation co-efficient being 0.73. Due to the coupling between the two hemispheres, any hemispheric asymmetry tends to get attenuated with time. The hemispheric asymmetry of a cycle ei-ther from observational data or from theoretical calculations statistically tends to be less than the asymmetry in the polar field (as inferred from the faculae data) in the preceding minimum. This reduction factor turns out to be 0.43 and 0.51 respectively in observational data and theoretical simulations. 相似文献
2.
In this work we use an already-published method to infer a variation profile for the solar meridional circulation over the
last 250 years. We feed this variation profile into a numerical dynamo code, and we reconstruct a sunspot time series that
acts as a proxy for solar cycle activity. We perform three simulations with slightly different parameters, and the results
are compared with the observational data. The medium and large correlation coefficients between reconstructed and observational
time series seem to indicate that variations in meridional circulation play an important role in the modulation of solar activity. 相似文献
3.
D. Sokoloff H. Zhang K. M. Kuzanyan V. N. Obridko D. N. Tomin V. N. Tutubalin 《Solar physics》2008,248(1):17-28
A comparison between the two tracers of magnetic field mirror asymmetry in solar active regions – twist and current helicity
– is presented. It is shown that for individual active regions these tracers do not possess visible similarity but averaging
by time over the solar cycle, or by latitude, reveals similarities in their behavior. The main property of the data set is
antisymmetry over the solar equator. Considering the evolution of helical properties over the solar cycle we find signatures
of a possible sign change at the beginning of the cycle, though more systematic observational data are required for a definite
confirmation. We discuss the role of both tracers in the context of solar dynamo theory. 相似文献
4.
In this paper, the monthly counts of flare index in the northern and southern hemispheres are used to investigate the hemispheric variation of the flare index in each of solar cycles 20–23. It is found that, (1) the flare index is asymmetrically distributed in each solar cycle and its asymmetry is a real phenomenon; (2) the flare index in the northern hemisphere begins earlier than that in the southern hemisphere in each of solar cycles 20–23, and the phase shifts between the two hemispheres show an odd‐even pattern; (3) although the flare index dominating in a hemisphere does not mean that it leads in phase in this hemisphere in individual solar cycle, these two features have an intrinsic relationship. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
5.
Jie Jiang Piyali Chatterjee Arnab Rai Choudhuri 《Monthly notices of the Royal Astronomical Society》2007,381(4):1527-1542
Although systematic measurements of the Sun's polar magnetic field exist only from mid-1970s, other proxies can be used to infer the polar field at earlier times. The observational data indicate a strong correlation between the polar field at a sunspot minimum and the strength of the next cycle, although the strength of the cycle is not correlated well with the polar field produced at its end. This suggests that the Babcock–Leighton mechanism of poloidal field generation from decaying sunspots involves randomness, whereas the other aspects of the dynamo process must be reasonably ordered and deterministic. Only if the magnetic diffusivity within the convection zone is assumed to be high (of order 1012 cm2 s−1 ), we can explain the correlation between the polar field at a minimum and the next cycle. We give several independent arguments that the diffusivity must be of this order. In a dynamo model with diffusivity like this, the poloidal field generated at the mid-latitudes is advected toward the poles by the meridional circulation and simultaneously diffuses towards the tachocline, where the toroidal field for the next cycle is produced. To model actual solar cycles with a dynamo model having such high diffusivity, we have to feed the observational data of the poloidal field at the minimum into the theoretical model. We develop a method of doing this in a systematic way. Our model predicts that cycle 24 will be a very weak cycle. Hemispheric asymmetry of solar activity is also calculated with our model and compared with observational data. 相似文献
6.
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
7.
K. J. Li 《Solar physics》2009,255(1):169-177
Five solar-activity indices – the monthly-mean sunspot numbers from January 1945 to March 2008, the monthly-mean sunspot areas
during the period of May 1874 to March 2008, the monthly numbers of sunspot groups from May 1874 to May 2008, the monthly-mean
flare indices from January 1966 to December 2006, and the numbers of solar filaments per Carrington rotation in the time interval
of solar rotations 876 to 1823 – have been used to show a systematic time delay between northern and southern hemispheric
solar activities in a cycle. It is found that solar activity does not occur synchronously in the northern and southern hemispheres,
and there is a systematic time lag or lead (phase shift) between northern and southern hemispheric solar activity in a cycle.
About an eight-cycle period is inferred to exist in such phase shifts. The activity on the Sun may be governed by two different
and coupled processes, not by a single process. 相似文献
8.
Ingrid Mann Melanie Köhler Hiroshi Kimura Andrzej Cechowski Tetsunori Minato 《Astronomy and Astrophysics Review》2006,13(3):159-228
Among the observed circumstellar dust envelopes a certain population, planetary debris disks, is ascribed to systems with
optically thin dust disks and low gas content. These systems contain planetesimals and possibly planets and are believed to
be systems that are most similar to our solar system in an early evolutionary stage. Planetary debris disks have been identified
in large numbers by a brightness excess in the near-infrared, mid-infrared and/or submillimetre range of their stellar spectral
energy distributions. In some cases, spatially resolved observations are possible and reveal complex spatial structures. Acting
forces and physical processes are similar to those in the solar system dust cloud, but the observational approach is obviously
quite different: overall spatial distributions for systems of different ages for the planetary debris disks, as opposed to
detailed local information in the case of the solar system. Comparison with the processes of dust formation and evolution
observed in the solar system therefore helps understand the planetary debris disks. In this paper, we review our present knowledge
of observations, acting forces, and major physical interactions of the dust in the solar system and in similar extra-solar
planetary systems. 相似文献
9.
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. 相似文献
10.
Liang Xi-long Zhao Jing-kun Chen Yu-qin Zhao Gang 《Chinese Astronomy and Astrophysics》2018,42(1):1-19
The stellar streams originated from the Galactic halo may be detected when they pass by the solar neighborhood, and they still keep some information at their birth times. Thus, the investigation of halo streams in the solar neighborhood is very important for understanding the formation and evolution of our Galaxy. In this paper, the researches of halo streams in the solar neighborhood are briefly reviewed. We have introduced the methods how to detect the halo streams and identify their member stars, summarized the progresses in the observation of member stars of halo streams and in the study of their origins, introduced in detail how to analyze the origins of halo streams in the solar neighborhood by means of numerical simulation and chemical abundance, and finally discussed the prospects of the LAMOST and GAIA in the research of halo streams in the solar neighborhood. 相似文献
11.
According to research results from solar-dynamo models, the northern and southern hemispheres may evolve separately throughout
the solar cycle. The observed phase lag between the northern and southern hemispheres provides information regarding how strongly
the hemispheres are coupled. Using hemispheric sunspot-area and sunspot-number data from Cycles 12 – 23, we determine how
out of phase the separate hemispheres are during the rising, maximum, and declining period of each solar cycle. Hemispheric
phase differences range from 0 – 11, 0 – 14, and 2 – 19 months for the rising, maximum, and declining periods, respectively.
The phases appear randomly distributed between zero months (in phase) and half of the rise (or decline) time of the solar
cycle. An analysis of the sunspot cycle double peak, or Gnevyshev gap, is conducted to determine if the double-peak is caused
by the averaging of two hemispheres that are out of phase. We confirm previous findings that the Gnevyshev gap is a phenomenon
that occurs in the separate hemispheres and is not due to a superposition of sunspot indices from hemispheres slightly out
of phase. Cross hemispheric coupling could be strongest at solar minimum, when there are large quantities of magnetic flux
at the Equator. We search for a correlation between the hemispheric phase difference near the end of the solar cycle and the
length of solar-cycle minimum, but found none. Because magnetic flux diffusion across the Equator is a mechanism by which
the hemispheres couple, we measured the magnetic flux crossing the Equator by examining Kitt Peak Vacuum Telescope and SOLIS
magnetograms for Solar Cycles 21 – 23. We find, on average, a surplus of northern hemisphere magnetic flux crossing during
the mid-declining phase of each solar cycle. However, we find no correlation between magnitude of magnetic flux crossing the
Equator, length of solar minima, and phase lag between the hemispheres. 相似文献
12.
The running cross-correlation coefficient between solar-cycle amplitudes and rise times at a certain cycle lag is found to
vary in time, when using the smoothed monthly-mean sunspot group numbers available for 1610 – 1995. It may be negative or
positive for different periods of time. The Waldmeier effect (in which the rise times decrease with amplitude) is also found
to be very weak for some cycles. This result represents an observational constraint on solar-dynamo models and can help us
better understand the long-term evolution of solar activity. 相似文献
13.
Variability studies are an important tool to investigate key properties of stars and brown dwarfs. From photometric monitoring we are able to obtain information about rotation and magnetic activity, which are expected to change in the mass range below 0.3 solar masses, since these fully convective objects cannot host a solar‐type dynamo. On the other hand, spectroscopic variability information can be used to obtain a detailed view on the accretion process in very young objects. In this paper, we report about our observational efforts to analyse the variability and rotational evolution of young brown dwarfs and very low‐mass stars. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
14.
M. Dikpati 《Astronomische Nachrichten》2007,328(10):1092-1095
Observational and theoretical knowledge about global-scale solar dynamo ingredients have reached the stage that it is possible to calibrate a flux-transport dynamo for the Sun by adjusting only a few tunable parameters. The important ingredients in this class of model are differential rotation (Omega-effect), helical turbulence (alpha-effect), meridional circulation and turbulent diffusion. The meridional circulation works as a conveyor belt and governs the dynamo cycle period. Meridional circulation and magnetic diffusivity together govern the memory of the Sun's past magnetic fields. After describing the physical processes involved in a flux-transport dynamo, we will show that a predictive tool can be built from it to predict mean solar cycle features by assimilating magnetic field data from previous cycles. We will discuss the theoretical and observational connections among various predictors, such as dynamo-generated toroidal flux integral, cross-equatorial flux, polar fields and geomagnetic indices. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
15.
C. Briand 《Astronomische Nachrichten》2003,324(4):357-361
It is now accepted that the solar activity has direct impact on the Earth climate, but is also responsible for the geomagnetic storms. It is thus fundamental to understand the mechanisms responsible for this activity. We present here first some aspects of the solar activity at the different atmospheric layers of the sun: active region at photospheric levels, filaments (prominences) and flares at chromospheric level and CME's at coronal level. A quick sum‐up of the principal characteristics of each is given as well as the key questions still under investigation. In the second part, two principal parameters are presented to describe these features: helicity and topology. Finally, we sum‐up the observational challenges for new solar telescopes. 相似文献
16.
17.
J. Hirzberger 《Astronomische Nachrichten》2003,324(4):344-348
A summary of the most recent observational results on the solar granulation phenomenon is given and its physical interpretation with respect to theoretical and numerical models is briefly discussed. Special attention is paid to unsolved questions concerning the relation between convective motions and related solar features. 相似文献
18.
S.C. Marsden 《Astronomische Nachrichten》2010,331(6):577-580
Today the Sun has a regular magnetic cycle driven by a dynamo action. But how did this regular cycle develop? How do basic parameters such as rotation rate, age, and differential rotation affect the generation of magnetic fields? Zeeman Doppler imaging (ZDI) is a technique that uses high‐resolution observations in circularly polarised light to map the surface magnetic topology on stars. Utilising the spectropolarimetric capabilities of future large solar telescopes it will be possible to study the evolution and morphology of the magnetic fields on a range of Sun‐like stars from solar twins through to rapidly‐rotating active young Suns and thus study the solar magnetic dynamo through time. In this article I discuss recent results from ZDI of Sun‐like stars and how we can use night‐time observations from future solar telescopes to solve unanswered questions about the origin and evolution of the Sun's magnetic dynamo (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
19.
We demonstrate that a simple solar dynamo model, in the form of a Parker migratory dynamo with random fluctuations of the
dynamo governing parameters and algebraic saturation of dynamo action, can at least qualitatively reproduce all the basic
features of solar Grand Minima as they are known from direct and indirect data. In particular, the model successfully reproduces
such features as an abrupt transition into a Grand Minimum and the subsequent gradual recovery of solar activity, as well
as mixed-parity butterfly diagrams during the epoch of the Grand Minimum. The model predicts that the cycle survives in some
form during a Grand Minimum, as well as the relative stability of the cycle inside and outside of a Grand Minimum. The long-term
statistics of simulated Grand Minima appears compatible with the phenomenology of the Grand Minima inferred from the cosmogenic
isotope data. We demonstrate that such ability to reproduce the Grand Minima phenomenology is not a general feature of the
dynamo models but requires some specific assumption, such as random fluctuations in dynamo governing parameters. In general,
we conclude that a relatively simple and straightforward model is able to reproduce the Grand Minima phenomenology remarkably
well, in principle providing us with a possibility of studying the physical nature of Grand Minima. 相似文献
20.
Prediction of the exact date of the maximum of the 11-year solar activity cycle is a matter of disagreement among solar scientists
and of some importance to satellite operators, space-system designers, etc. Most predictions are based on physical conditions
occurring at or before the solar-cycle minimum preceding the maximum in question. However, another indicator of the timing
of the maximum occurs early in the rise phase of the solar cycle. A study of the variation over two previous solar cycles
of coronal emission features in Fe xiv from the National Solar Observatory at Sacramento Peak has shown that, prior to solar
maximum, emission features appear above 50° latitude in both hemispheres and begin to move towards the poles at a rate of
8° to 11° of latitude per year. This motion is maintained for a period of 3 or 4 years, at which time the emission features
disappear near the poles. This phenomenon has been referred to as the `Rush to the Poles'. These observations show that the
maximum of solar activity, as seen in the sunspot number, occurs approximately 19 ± 2 months before the features reach the
poles. In 1997, Fe xiv emission features appeared near 55° latitude, and began to move towards the poles. Using the above
historical data from cycles 21 and 22, we will see how the use of progressively more data from cycle 23 affects the prediction
of the date of solar maximum. The principal conclusion is that the date of solar maximum for cycle 23 could be predicted to
within 6 months as early as 1997. For solar cycle 24, when this phenomenon first becomes apparent later this decade, the average
parameters for cycles 21–23 can be used to predict the date of solar maximum. 相似文献