106 research outputs found
Subventricular zone stem cells are heterogeneous with respect to their embryonic origins and neurogenic fates in the adult olfactory bulb
Wedetermined the embryonic origins of adult forebrain subventricular zone (SVZ) stem cells by Cre-lox fate mapping in transgenic mice. We found that all parts of the telencephalic neuroepithelium, including the medial ganglionic eminence and lateral ganglionic eminence (LGE) and the cerebral cortex, contribute multipotent, self-renewing stem cells to the adult SVZ. Descendants of the embryonic LGE and cortex settle in ventral and dorsal aspects of the dorsolateral SVZ, respectively. Both populations contribute new (5-bromo-2(')-deoxyuridine- labeled) tyrosine hydroxylase- and calretinin-positive interneurons to the adult olfactory bulb. However, calbindin-positive interneurons in the olfactory glomeruli were generated exclusively by LGE- derived stem cells. Thus, different SVZ stem cells have different embryonic origins, colonize different parts of the SVZ, and generate different neuronal progeny, suggesting that some aspects of embryonic patterning are preserved in the adult SVZ. This could have important implications for the design of endogenous stem cell-based therapies in the future
Solar-Cycle Characteristics Examined in Separate Hemispheres: Phase, Gnevyshev Gap, and Length of Minimum
Research results from solar-dynamo models show the northern and southern
hemispheres may evolve separately throughout the solar cycle. The observed
phase lag between the hemispheres provides information regarding the strength
of hemispheric coupling. 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 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 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.Comment: 15 pages, 7 figure
Multi-timescale Solar Cycles and the Possible Implications
Based on analysis of the annual averaged relative sunspot number (ASN) during
1700 -- 2009, 3 kinds of solar cycles are confirmed: the well-known 11-yr cycle
(Schwabe cycle), 103-yr secular cycle (numbered as G1, G2, G3, and G4,
respectively since 1700); and 51.5-yr Cycle. From similarities, an
extrapolation of forthcoming solar cycles is made, and found that the solar
cycle 24 will be a relative long and weak Schwabe cycle, which may reach to its
apex around 2012-2014 in the vale between G3 and G4. Additionally, most Schwabe
cycles are asymmetric with rapidly rising-phases and slowly decay-phases. The
comparisons between ASN and the annual flare numbers with different GOES
classes (C-class, M-class, X-class, and super-flare, here super-flare is
defined as X10.0) and the annal averaged radio flux at frequency of 2.84
GHz indicate that solar flares have a tendency: the more powerful of the flare,
the later it takes place after the onset of the Schwabe cycle, and most
powerful flares take place in the decay phase of Schwabe cycle. Some
discussions on the origin of solar cycles are presented.Comment: 8 pages, 4 figure
Subdiffusive transport in intergranular lanes on the Sun. The Leighton model revisited
In this paper we consider a random motion of magnetic bright points (MBP)
associated with magnetic fields at the solar photosphere. The MBP transport in
the short time range [0-20 minutes] has a subdiffusive character as the
magnetic flux tends to accumulate at sinks of the flow field. Such a behavior
can be rigorously described in the framework of a continuous time random walk
leading to the fractional Fokker-Planck dynamics. This formalism, applied for
the analysis of the solar subdiffusion of magnetic fields, generalizes the
Leighton's model.Comment: 7 page
Evolution of active and polar photospheric magnetic fields during the rise of Cycle 24 compared to previous cycles
The evolution of the photospheric magnetic field during the declining phase
and minimum of Cycle 23 and the recent rise of Cycle 24 are compared with the
behavior during previous cycles. We used longitudinal full-disk magnetograms
from the NSO's three magnetographs at Kitt Peak, the Synoptic Optical Long-term
Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM), the
Spectromagnetograph and the 512-Channel Magnetograph instruments, and
longitudinal full-disk magnetograms from the Mt. Wilson 150-foot tower. We
analyzed 37 years of observations from these two observatories that have been
observing daily, weather permitting, since 1974, offering an opportunity to
study the evolving relationship between the active region and polar fields in
some detail over several solar cycles. It is found that the annual averages of
a proxy for the active region poloidal magnetic field strength, the magnetic
field strength of the high-latitude poleward streams, and the time derivative
of the polar field strength are all well correlated in each hemisphere. These
results are based on statistically significant cyclical patterns in the active
region fields and are consistent with the Babcock-Leighton phenomenological
model for the solar activity cycle. There was more hemispheric asymmetry in the
activity level, as measured by total and maximum active region flux, during
late Cycle 23 (after around 2004), when the southern hemisphere was more
active, and Cycle 24 up to the present, when the northern hemisphere has been
more active, than at any other time since 1974. The active region net proxy
poloidal fields effectively disappeared in both hemispheres around 2004, and
the polar fields did not become significantly stronger after this time. We see
evidence that the process of Cycle 24 field reversal has begun at both poles.Comment: Accepted for publication in Solar Physic
Recovering Joys Law as a Function of Solar Cycle, Hemisphere, and Longitude
Bipolar active regions in both hemispheres tend to be tilted with respect to
the East West equator of the Sun in accordance with Joys law that describes the
average tilt angle as a function of latitude. Mt. Wilson observatory data from
1917 to 1985 are used to analyze the active-region tilt angle as a function of
solar cycle, hemisphere, and longitude, in addition to the more common
dependence on latitude. Our main results are as follows: i) We recommend a
revision of Joys law toward a weaker dependence on latitude (slope of 0.13 to
0.26) and without forcing the tilt to zero at the Equator. ii) We determine
that the hemispheric mean tilt value of active regions varies with each solar
cycle, although the noise from a stochastic process dominates and does not
allow for a determination of the slope of Joys law on an 11-year time scale.
iii) The hemispheric difference in mean tilt angles, 1.1 degrees + 0.27, over
Cycles 16 to 21 was significant to a three-sigma level, with average tilt
angles in the northern and southern hemispheres of 4.7 degrees + 0.26 and 3.6
degrees + 0.27 respectively. iv) Area-weighted mean tilt angles normalized by
latitude for Cycles 15 to 21 anticorrelate with cycle strength for the southern
hemisphere and whole-Sun data, confirming previous results by Dasi-Espuig,
Solanki, Krivova, et al. (2010, Astron. Astrophys. 518, A7). The northern
hemispheric mean tilt angles do not show a dependence on cycle strength. vi)
Mean tilt angles do not show a dependence on longitude for any hemisphere or
cycle. In addition, the standard deviation of the mean tilt is 29 to 31 degrees
for all cycles and hemispheres indicating that the scatter is due to the same
consistent process even if the mean tilt angles vary.Comment: 13 pages, 4 figures, 3 table
Comparisons of Supergranule Characteristics During the Solar Minima of Cycles 22/23 and 23/24
Supergranulation is a component of solar convection that manifests itself on
the photosphere as a cellular network of around 35 Mm across, with a turnover
lifetime of 1-2 days. It is strongly linked to the structure of the magnetic
field. The horizontal, divergent flows within supergranule cells carry local
field lines to the cell boundaries, while the rotational properties of
supergranule upflows may contribute to the restoration of the poloidal field as
part of the dynamo mechanism that controls the solar cycle. The solar minimum
at the transition from cycle 23 to 24 was notable for its low level of activity
and its extended length. It is of interest to study whether the convective
phenomena that influences the solar magnetic field during this time differed in
character to periods of previous minima. This study investigates three
characteristics (velocity components, sizes and lifetimes) of solar
supergranulation. Comparisons of these characteristics are made between the
minima of cycles 22/23 and 23/24 using MDI Doppler data from 1996 and 2008,
respectively. It is found that whereas the lifetimes are equal during both
epochs (around 18 h), the sizes are larger in 1996 (35.9 +/- 0.3 Mm) than in
2008 (35.0 +/- 0.3 Mm), while the dominant horizontal velocity flows are weaker
(139 +/- 1 m/s in 1996; 141 +/- 1 m/s in 2008). Although numerical differences
are seen, they are not conclusive proof of the most recent minimum being
inherently unusual.Comment: 22 pages, 5 figures. Solar Physics, in pres
From GHz to mHz: A Multiwavelength Study of the Acoustically Active 14 August 2004 M7.4 Solar Flare
We carried out an electromagnetic acoustic analysis of the solar flare of 14
August 2004 in active region AR10656 from the radio to the hard X-ray spectrum.
The flare was a GOES soft X-ray class M7.4 and produced a detectable sun quake,
confirming earlier inferences that relatively low-energy flares may be able to
generate sun quakes. We introduce the hypothesis that the seismicity of the
active region is closely related to the heights of coronal magnetic loops that
conduct high-energy particles from the flare. In the case of relatively short
magnetic loops, chromospheric evaporation populates the loop interior with
ionized gas relatively rapidly, expediting the scattering of remaining trapped
high-energy electrons into the magnetic loss cone and their rapid precipitation
into the chromosphere. This increases both the intensity and suddenness of the
chromospheric heating, satisfying the basic conditions for an acoustic emission
that penetrates into the solar interior.Comment: Accepted in Solar Physic
Acoustic Power Absorption and its Relation with Vector Magnetic Field of a Sunspot
The distribution of acoustic power over sunspots shows an enhanced absorption
near the umbra--penumbra boundary. Earlier studies revealed that the region of
enhanced absorption coincides with the region of strongest transverse potential
field. The aim of this paper is to (i) utilize the high-resolution vector
magnetograms derived using Hinode SOT/SP observations and study the
relationship between the vector magnetic field and power absorption and (ii)
study the variation of power absorption in sunspot penumbrae due to the
presence of spine-like radial structures. It is found that (i) both potential
and observed transverse fields peak at a similar radial distance from the
center of the sunspot, and (ii) the magnitude of the transverse field, derived
from Hinode observations, is much larger than the potential transverse field
derived from SOHO/MDI longitudinal field observations. In the penumbra, the
radial structures called spines (intra-spines) have stronger (weaker) field
strength and are more vertical (horizontal). The absorption of acoustic power
in the spine and intra-spine shows different behaviour with the absorption
being larger in the spine as compared to the intra-spine.Comment: 18 pages, 7 figures, In Press Solar Physics, Topical Issue on
Helio-and-Astroseismolog
Solar Magnetic Carpet I: Simulation of Synthetic Magnetograms
This paper describes a new 2D model for the photospheric evolution of the
magnetic carpet. It is the first in a series of papers working towards
constructing a realistic 3D non-potential model for the interaction of
small-scale solar magnetic fields. In the model, the basic evolution of the
magnetic elements is governed by a supergranular flow profile. In addition,
magnetic elements may evolve through the processes of emergence, cancellation,
coalescence and fragmentation. Model parameters for the emergence of bipoles
are based upon the results of observational studies. Using this model, several
simulations are considered, where the range of flux with which bipoles may
emerge is varied. In all cases the model quickly reaches a steady state where
the rates of emergence and cancellation balance. Analysis of the resulting
magnetic field shows that we reproduce observed quantities such as the flux
distribution, mean field, cancellation rates, photospheric recycle time and a
magnetic network. As expected, the simulation matches observations more closely
when a larger, and consequently more realistic, range of emerging flux values
is allowed (4e16 - 1e19 Mx). The model best reproduces the current observed
properties of the magnetic carpet when we take the minimum absolute flux for
emerging bipoles to be 4e16 Mx. In future, this 2D model will be used as an
evolving photospheric boundary condition for 3D non-potential modeling.Comment: 33 pages, 16 figures, 5 gif movies included: movies may be viewed at
http://www-solar.mcs.st-and.ac.uk/~karen/movies_paper1
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