173 research outputs found
The origin of grand minima in the sunspot cycle
One of the most striking aspects of the 11-year sunspot cycle is that there
have been times in the past when some cycles went missing, a most well-known
example of this being the Maunder minimum during 1645-1715. Analyses of
cosmogenic isotopes (C14 and Be10) indicated that there were about 27 grand
minima in the last 11,000 yr, implying that about 2.7% of the solar cycles had
conditions appropriate for forcing the Sun into grand minima. We address the
question how grand minima are produced and specifically calculate the frequency
of occurrence of grand minima from a theoretical dynamo model. We assume that
fluctuations in the poloidal field generation mechanism and the meridional
circulation produce irregularities of sunspot cycles. Taking these fluctuations
to be Gaussian and estimating the values of important parameters from the data
of last 28 solar cycles, we show from our flux transport dynamo model that
about 1-4% of the sunspot cycles may have conditions suitable for inducing
grand minima.Comment: Accepted for publication in Physical Review Letter
The Origin of Solar Activity in the Tachocline
Solar active regions, produced by the emergence of tubes of strong magnetic
field in the photosphere, are restricted to within 35 degrees of the solar
equator. The nature of the dynamo processes that create and renew these fields,
and are therefore responsible for solar magnetic phenomena, are not well
understood. We analyze the magneto-rotational stability of the solar tachocline
for general field geometry. This thin region of strong radial and latitudinal
differential rotation, between the radiative and convective zones, is unstable
at latitudes above 37 degrees, yet is stable closer to the equator. We propose
that small-scale magneto-rotational turbulence prevents coherent magnetic
dynamo action in the tachocline except in the vicinity of the equator, thus
explaining the latitudinal restriction of active regions. Tying the magnetic
dynamo to the tachocline elucidates the physical conditions and processes
relevant to solar magnetism.Comment: 10 pages, 1 figure, accepted for publication in ApJ
Interpolation Properties and SAT-based Model Checking
Craig interpolation is a widespread method in verification, with important
applications such as Predicate Abstraction, CounterExample Guided Abstraction
Refinement and Lazy Abstraction With Interpolants. Most state-of-the-art model
checking techniques based on interpolation require collections of interpolants
to satisfy particular properties, to which we refer as "collectives"; they do
not hold in general for all interpolation systems and have to be established
for each particular system and verification environment. Nevertheless, no
systematic approach exists that correlates the individual interpolation systems
and compares the necessary collectives. This paper proposes a uniform
framework, which encompasses (and generalizes) the most common collectives
exploited in verification. We use it for a systematic study of the collectives
and of the constraints they pose on propositional interpolation systems used in
SAT-based model checking
Lifting CDCL to template-based abstract domains for program verification
The success of Conflict Driven Clause Learning (CDCL) for Boolean satisfiability has inspired adoption in other domains. We present a novel lifting of CDCL to program analysis called Abstract Conflict Driven Learning for Programs (ACDLP). ACDLP alternates between model search, which performs over-approximate deduction with constraint propagation, and conflict analysis, which performs under-approximate abduction with heuristic choice. We instantiate the model search and conflict analysis algorithms with an abstract domain of template polyhedra, strictly generalizing CDCL from the Boolean lattice to a richer lattice structure. Our template polyhedra can express intervals, octagons and restricted polyhedral constraints over program variables. We have implemented ACDLP for automatic bounded safety verification of C programs. We evaluate the performance of our analyser by comparing with CBMC, which uses Boolean CDCL, and Astrée, a commercial abstract interpretation tool. We observe two orders of magnitude reduction in the number of decisions, propagations, and conflicts as well as a 1.5x speedup in runtime compared to CBMC. Compared to Astrée, ACDLP solves twice as many benchmarks and has much higher precision. This is the first instantiation of CDCL with a template polyhedra abstract domain
The Magnetic Sun: Reversals and Long-Term Variations
A didactic introduction to current thinking on some aspects of the solar
dynamo is given for geophysicists and planetary scientists.Comment: 17 pages, 9 figures; Space Science Rev., in pres
A Comparison of Solar Cycle Variations in the Equatorial Rotation Rates of the Sun's Subsurface, Surface, Corona, and Sunspot Groups
Using the Solar Optical Observing Network (SOON) sunspot-group data for the
period 1985-2010, the variations in the annual mean equatorial-rotation rates
of the sunspot groups are determined and compared with the known variations in
the solar equatorial-rotation rates determined from the following data: i) the
plasma rotation rates at 0.94Rsun, 0.95Rsun,...,1.0Rsun measured by Global
Oscillation Network Group (GONG) during the period 1995-2010, ii) the data on
the soft X-ray corona determined from Yohkoh/SXT full disk images for the years
1992-2001, iii) the data on small bright coronal structures (SBCS) which were
traced in Solar and Heliospheric Observatory (SOHO)/EIT images during the
period 1998-2006, and iv) the Mount Wilson Doppler-velocity measurements during
the period 1986-2007. A large portion (up to approximate 30 deg latitude) of
the mean differential-rotation profile of the sunspot groups lies between those
of the internal differential-rotation rates at 0.94Rsun and 0.98Rsun.The
variation in the yearly mean equatorial-rotation rate of the sunspot groups
seems to be lagging that of the equatorial-rotation rate determined from the
GONG measurements by one to two years.The amplitude of the latter is very
small.The solar-cycle variation in the equatorial-rotation rate of the solar
corona closely matches that determined from the sunspot-group data.The
variation in the equatorial-rotation rate determined from the Mount Wilson
Doppler-velocity data closely resembles the corresponding variation in the
equatorial-rotation rate determined from the sunspot-group data that included
the values of the abnormal angular motions (> 3 deg per day) of the sunspot
groups. Implications of these results are pointed out.Comment: 22 pages, 10 figures, accepted by Solar Physic
Seismology of the Sun : Inference of Thermal, Dynamic and Magnetic Field Structures of the Interior
Recent overwhelming evidences show that the sun strongly influences the
Earth's climate and environment. Moreover existence of life on this Earth
mainly depends upon the sun's energy. Hence, understanding of physics of the
sun, especially the thermal, dynamic and magnetic field structures of its
interior, is very important. Recently, from the ground and space based
observations, it is discovered that sun oscillates near 5 min periodicity in
millions of modes. This discovery heralded a new era in solar physics and a
separate branch called helioseismology or seismology of the sun has started.
Before the advent of helioseismology, sun's thermal structure of the interior
was understood from the evolutionary solution of stellar structure equations
that mimicked the present age, mass and radius of the sun. Whereas solution of
MHD equations yielded internal dynamics and magnetic field structure of the
sun's interior. In this presentation, I review the thermal, dynamic and
magnetic field structures of the sun's interior as inferred by the
helioseismology.Comment: To be published in the proceedings of the meeting "3rd International
Conference on Current Developments in Atomic, Molecular, Optical and Nano
Physics with Applications", December 14-16, 2011, New Delhi, Indi
Active region formation through the negative effective magnetic pressure instability
The negative effective magnetic pressure instability operates on scales
encompassing many turbulent eddies and is here discussed in connection with the
formation of active regions near the surface layers of the Sun. This
instability is related to the negative contribution of turbulence to the mean
magnetic pressure that causes the formation of large-scale magnetic structures.
For an isothermal layer, direct numerical simulations and mean-field
simulations of this phenomenon are shown to agree in many details in that their
onset occurs at the same depth. This depth increases with increasing field
strength, such that the maximum growth rate of this instability is independent
of the field strength, provided the magnetic structures are fully contained
within the domain. A linear stability analysis is shown to support this
finding. The instability also leads to a redistribution of turbulent intensity
and gas pressure that could provide direct observational signatures.Comment: 19 pages, 10 figures, submitted to Solar Physic
Photospheric and Subphotospheric Dynamics of Emerging Magnetic Flux
Magnetic fields emerging from the Sun's interior carry information about
physical processes of magnetic field generation and transport in the convection
zone. Soon after appearance on the solar surface the magnetic flux gets
concentrated in sunspot regions and causes numerous active phenomena on the
Sun. This paper discusses some properties of the emerging magnetic flux
observed on the solar surface and in the interior. A statistical analysis of
variations of the tilt angle of bipolar magnetic regions during the emergence
shows that the systematic tilt with respect to the equator (the Joy's law) is
most likely established below the surface. However, no evidence of the
dependence of the tilt angle on the amount of emerging magnetic flux, predicted
by the rising magnetic flux rope theories, is found. Analysis of surface plasma
flows in a large emerging active region reveals strong localized upflows and
downflows at the initial phase of emergence but finds no evidence for
large-scale flows indicating future appearance a large-scale magnetic
structure. Local helioseismology provides important tools for mapping
perturbations of the wave speed and mass flows below the surface. Initial
results from SOHO/MDI and GONG reveal strong diverging flows during the flux
emergence, and also localized converging flows around stable sunspots. The wave
speed images obtained during the process of formation of a large active region,
NOAA 10488, indicate that the magnetic flux gets concentrated in strong field
structures just below the surface. Further studies of magnetic flux emergence
require systematic helioseismic observations from the ground and space, and
realistic MHD simulations of the subsurface dynamics.Comment: 21 pages, 15 figures, to appear in Space Science Review
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