109 research outputs found
A Model of the Double Magnetic Cycle of the Sun
It has been argued that the solar magnetic cycle consists of two main
periodic components: a low-frequency component (Hale's 22-year cycle) and a
high-frequency component (quasi-biennial cycle). The existence of the double
magnetic cycle on the Sun is confirmed using Stanford, Mount Wilson and Kitt
Peak magnetograph data from 1976 to 1996 (solar cycles 21 and 22). In the frame
of the Parker's dynamo theory a model of the double magnetic cycle is
presented. This model is based on the idea of two dynamo sources separated in
space. The first source of the dynamo action is located near the bottom of the
convection zone, and the second operates near the top. The model is formulated
in terms of two coupled systems of non-linear differential equations. It is
demonstrated that in the case of weak interaction between the two dynamo
sources the basic features of the double magnetic cycle such as existence of
two component and observed temporal variations of high-frequency component can
be reproduced.Comment: 6 pages, 2 figure
The Interaction of New and Old Magnetic Fluxes at the Beginning of Solar Cycle 23
The 11-year cycle of solar activity follows Hale's law by reversing the
magnetic polarity of leading and following sunspots in bipolar regions during
the minima of activity. In the 1996-97 solar minimum, most solar activity
emerged in narrow longitudinal zones - `active longitudes' but over a range in
latitude. Investigating the distribution of solar magnetic flux, we have found
that the Hale sunspot polarity reversal first occurred in these active zones.
We have estimated the rotation rates of the magnetic flux in the active zones
before and after the polarity reversal. Comparing these rotation rates with the
internal rotation inferred by helioseismology, we suggest that both `old' and
`new' magnetic fluxes were probably generated in a low-latitude zone near the
base of the solar convection zone. The reversal of active region polarity
observed in certain longitudes at the beginning of a new solar cycle suggests
that the phenomenon of active longitudes may give fundamental information about
the mechanism of the solar cycle. The non-random distribution of old-cycle and
new-cycle fluxes presents a challenge for dynamo theories, most of which assume
a uniform longitudinal distribution of solar magnetic fields.Comment: 4 pages, 5 figures; accepted for publication in ApJ Letter
The solar differential rotation in the 18th century
The sunspot drawings of Johann Staudacher of 1749--1799 were used to
determine the solar differential rotation in that period. These drawings of the
full disk lack any indication of their orientation. We used a Bayesian
estimator to obtain the position angles of the drawings, the corresponding
heliographic spot positions, a time offset between the drawings and the
differential rotation parameter \delta\Omega, assuming the equatorial rotation
period is the same as today. The drawings are grouped in pairs, and the
resulting marginal distributions for \delta\Omega were multiplied. We obtain
\delta\Omega=-0.048 \pm 0.025 d^-1 (-2.75^o/d) for the entire period. There is
no significant difference to the value of the present Sun. We find an
(insignificant) indication for a change of \delta\Omega throughout the
observing period from strong differential rotation, \delta\Omega\approx -0.07
d^-1, to weaker differential rotation, \delta\Omega\approx-0.04 d^-1.Comment: 6 pages, 6 figures, accepted for Astronomy and Astrophysic
On Quasi-biennial oscillations in chromospheric macrospicules and their potential relation to global solar magnetic field
This study aims to provide further evidence for the potential influence of the global solar magnetic field on localised chromospheric jets, the macrospicules (MS). To find a connection between the long-term variation of properties of MS and other solar activity proxies, including e.g. the temporal variation of the frequency shift of solar global oscillations, sunspot area, etc., a database overarching seven years of observations was built up. This database contains 362 MS, based on observations at the 30.4 nm of the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO). Three of the five investigated physical properties of MS show a clear long-term temporal variation after smoothing the raw data. Wavelet analysis of the temporal variation of maximum length, maximum area and average velocity is carried out. The results reveal a strong pattern of periodicities at around 2-year (also referred to as Quasi-Biennial Oscillations -- QBOs). Comparison to solar activity proxies, that also possess the properties of QBOs, provides some interesting features: the minima and maxima of QBOs of MS properties occur at around the same epoch as the minima and maxima of these activity proxies. For most of the time span investigated, the oscillations are out-of-phase. This out-of-phase behaviour was also corroborated by a cross-correlation analysis. These results suggest that the physical processes, that generate and drive the long-term evolution of the global solar activity proxies, may be coupled to the short-term local physical processes driving the macrospicules, and, therefore modulate the properties of local dynamics
Structure and properties of aqueous dispersions of sodium dodecyl sulfate with carbon nanotubes
© 2016, Springer Science+Business Media New York.The dispersing action of the surfactant (sodium dodecyl sulfate, SDS) on the carbon nanotubes (CNT) in aqueous medium has been studied. Electron microscopy, molecular docking, NMR and IR spectroscopies were applied to determine the physical-chemical properties of CNT dispersions in SDS—water solutions. It was established that micellar adsorption of the surfactant on the surface of carbon material and solubilization of SDS in aqueous medium contribute to improving CNT dispersing in water solutions. It was shown that the non-polar hydrocarbon radicals of a single surfactant molecule form the highest possible number of contacts with the graphene surface. Upon increase of the SDS in solution these radicals form micelles connected with the surface of the nanotubes. At the sufficiently high SDS concentration the nanotube surface becomes covered with an adsorbed layer of surfactant micelles. Water molecules and sodium cations are concentrated in spaces between micelles. The observed pattern of micellar adsorption is somewhat similar to a loose bilayer of surfactant molecules
Non-homogeneous Behaviour of the Spatial Distribution of Macrospicules
In this paper the longitudinal and latitudinal spatial
distribution of macrospicules is examined. We found a statistical
relationship between the active longitude determined by
sunspot groups and the longitudinal distribution of macrospicules.
This distribution of macrospicules shows an inhomogeneity and
non-axysimmetrical behaviour in the time interval from June
2010 until December 2012 covered by observations of the Solar
Dynamic Observatory (SDO) satellite. The enhanced positions
of the activity and its time variation has been calculated. The
migration of the longitudinal distribution of macrospicules shows
a similar behaviour as that of the sunspot groups
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
Astrophysical turbulence modeling
The role of turbulence in various astrophysical settings is reviewed. Among
the differences to laboratory and atmospheric turbulence we highlight the
ubiquitous presence of magnetic fields that are generally produced and
maintained by dynamo action. The extreme temperature and density contrasts and
stratifications are emphasized in connection with turbulence in the
interstellar medium and in stars with outer convection zones, respectively. In
many cases turbulence plays an essential role in facilitating enhanced
transport of mass, momentum, energy, and magnetic fields in terms of the
corresponding coarse-grained mean fields. Those transport properties are
usually strongly modified by anisotropies and often completely new effects
emerge in such a description that have no correspondence in terms of the
original (non coarse-grained) fields.Comment: 88 pages, 26 figures, published in Reports on Progress in Physic
Stem cells in ectodermal development
Tissue-specific stem cells sustain organs for a lifetime through self-renewal and generating differentiated progeny. Although tissue stem cells are established during organogenesis, the precise origin of most adult stem cells in the developing embryo is unclear. Mammalian skin is one of the best-studied epithelial systems containing stem cells to date, however the origin of most of the stem cell populations found in the adult epidermis is unknown. Here, we try to recapitulate the emergence and genesis of an ectodermal stem cell during development until the formation of an adult skin. We ask whether skin stem cells share key transcriptional regulators with their embryonic counterparts and discuss whether embryonic-like stem cells may persist through to adulthood in vivo
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