287 research outputs found
Asymmetric Solar Polar Field Reversals
The solar polar fields reverse because magnetic flux from decaying sunspots
moves towards the poles, with a preponderance of flux from the trailing spots.
Let us assume that there is a strong asymmetry in the sense that all activity
is in the Northern Hemisphere, then that excess flux will move to the North
Pole and reverse that pole, while nothing happens in the South. If later on,
there is a lot of activity in the South, then that flux will help reverse the
South Pole. In this way, we get two humps in solar activity and a corresponding
difference in time of reversals. Such difference was first noted by Babcock
(1959) from the very first observation of polar field reversal just after the
maximum of the strongly asymmetric solar cycle 19. At that time, the Southern
Hemisphere was most active before sunspot maximum and the South Pole duly
reversed first, followed by the Northern Hemisphere more than a year later,
when that hemisphere was most active. Solar cycles since then have had the
opposite asymmetry, with the Northern Hemisphere being most active early in the
cycle. Polar field reversals for these cycles have as expected happened first
in the North. This is especially noteworthy for the present solar cycle 24. We
suggest that the association of two peaks of solar activity when separated by
hemispheres with correspondingly different times of polar field reversals is a
general feature of the cycle
Areas of Polar Coronal Holes from 1996 Through 2010
Polar coronal holes (PCHs) trace the magnetic variability of the Sun throughout the solar cycle. Their size and evolution have been studied as proxies for the global magnetic field. We present measurements of the PCH areas from 1996 through 2010, derived from an updated perimeter-tracing method and two synoptic-map methods. The perimeter tracing method detects PCH boundaries along the solar limb, using full-disk images from the SOlar and Heliospheric Observatory/Extreme ultraviolet Imaging Telescope (SOHO/EIT). One synoptic-map method uses the line-of-sight magnetic field from the SOHO/Michelson Doppler Imager (MDI) to determine the unipolarity boundaries near the poles. The other method applies thresholding techniques to synoptic maps created from EUV image data from EIT. The results from all three methods suggest that the solar maxima and minima of the two hemispheres are out of phase. The maximum PCH area, averaged over the methods in each hemisphere, is approximately 6 % during both solar minima spanned by the data (between Solar Cycles 22/23 and 23/24). The northern PCH area began a declining trend in 2010, suggesting a downturn toward the maximum of Solar Cycle 24 in that hemisphere, while the southern hole remained large throughout 2010
Inferring Maps of the Sun's Far-side Unsigned Magnetic Flux from Far-side Helioseismic Images using Machine Learning Techniques
Accurate modeling of the Sun's coronal magnetic field and solar wind
structures require inputs of the solar global magnetic field, including both
the near and far sides, but the Sun's far-side magnetic field cannot be
directly observed. However, the Sun's far-side active regions are routinely
monitored by helioseismic imaging methods, which only require continuous
near-side observations. It is therefore both feasible and useful to estimate
the far-side magnetic-flux maps using the far-side helioseismic images despite
their relatively low spatial resolution and large uncertainties. In this work,
we train two machine-learning models to achieve this goal. The first
machine-learning training pairs simultaneous SDO/HMI-observed magnetic-flux
maps and SDO/AIA-observed EUV 304 images, and the resulting model can
convert 304 images into magnetic-flux maps. This model is then applied
on the STEREO/EUVI-observed far-side 304 images, available for about 4.3
years, for the far-side magnetic-flux maps. These EUV-converted magnetic-flux
maps are then paired with simultaneous far-side helioseismic images for a
second machine-learning training, and the resulting model can convert far-side
helioseismic images into magnetic-flux maps. These helioseismically derived
far-side magnetic-flux maps, despite their limitations in spatial resolution
and accuracy, can be routinely available on a daily basis, providing useful
magnetic information on the Sun's far side using only the near-side
observations.Comment: Accepted by Ap
On the knee in the energy spectrum of cosmic rays
The knee in the all-particle energy spectrum is scrutinized with a
phenomenological model, named poly-gonato model, linking results from direct
and indirect measurements. For this purpose, recent results from direct and
indirect measurements of cosmic rays in the energy range from 10 GeV up to 1
EeV are examined. The energy spectra of individual elements, as obtained by
direct observations, are extrapolated to high energies using power laws and
compared to all-particle spectra from air shower measurements. A cut-off for
each element proportional to its charge Z is assumed. The model describes the
knee in the all-particle energy spectrum as a result of subsequent cut-offs for
individual elements, starting with the proton component at 4.5 PeV, and the
second change of the spectral index around 0.4 EeV as due to the end of stable
elements (Z=92). The mass composition, extrapolated from direct measurements to
high energies, using the poly-gonato model, is compatible with results from air
shower experiments measuring the electromagnetic, muonic, and hadronic
components. But it disagrees with the mass composition derived from X_max
measurements using Cerenkov and fluorescence light detectors.Comment: 30 pages, 21 figures, 9 tables, accepted by Astroparticle Physic
Optimal Strouhal number for swimming animals
To evaluate the swimming performances of aquatic animals, an important
dimensionless quantity is the Strouhal number, St = fA/U, with f the tail-beat
frequency, A the peak-to-peak tail amplitude, and U the swimming velocity.
Experiments with flapping foils have exhibited maximum propulsive efficiency in
the interval 0.25 < St < 0.35 and it has been argued that animals likely
evolved to swim in the same narrow interval. Using Lighthill's elongated-body
theory to address undulatory propulsion, it is demonstrated here that the
optimal Strouhal number increases from 0.15 to 0.8 for animals spanning from
the largest cetaceans to the smallest tadpoles. To assess the validity of this
model, the swimming kinematics of 53 different species of aquatic animals have
been compiled from the literature and it shows that their Strouhal numbers are
consistently near the predicted optimum.Comment: 21 pages, 6 figure
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Near-Earth heliospheric magnetic field intensity since 1750. Part 2: cosmogenic radionuclide reconstructions
This is Part 2 of a study of the near-Earth heliospheric magnetic field strength, B, since 1750. Part 1 produced composite estimates of B from geomagnetic and sunspot data over the period 1750–2013. Sunspot-based reconstructions can be extended back to 1610, but the paleocosmic ray (PCR) record is the only data set capable of providing a record of solar activity on millennial timescales. The process for converting 10Be concentrations measured in ice cores to B is more complex than with geomagnetic and sunspot data, and the uncertainties in B derived from cosmogenic nuclides (~20% for any individual year) are much larger. Within this level of uncertainty, we find reasonable overall agreement between PCR-based B and the geomagnetic- and sunspot number-based series. This agreement was enhanced by excising low values in PCR-based B attributed to high-energy solar proton events. Other discordant intervals, with as yet unspecified causes remain included in our analysis. Comparison of 3 year averages centered on sunspot minimum yields reasonable agreement between the three estimates, providing a means to investigate the long-term changes in the heliospheric magnetic field into the past even without a means to remove solar proton events from the records
Short-term variations in response distribution to cortical stimulation
Patterns of responses in the cerebral cortex can vary, and are influenced by pre-existing cortical function, but it is not known how rapidly these variations can occur in humans. We investigated how rapidly response patterns to electrical stimulation can vary in intact human brain. We also investigated whether the type of functional change occurring at a given location with stimulation would help predict the distribution of responses elsewhere over the cortex to stimulation at that given location. We did this by studying cortical afterdischarges following electrical stimulation of the cortex in awake humans undergoing evaluations for brain surgery. Response occurrence and location could change within seconds, both nearby to and distant from stimulation sites. Responses might occur at a given location during one trial but not the next. They could occur at electrodes adjacent or not adjacent to those directly stimulated or to other electrodes showing afterdischarges. The likelihood of an afterdischarge at an individual site after stimulation was predicted by spontaneous electroencephalographic activity at that specific site just prior to stimulation, but not by overall cortical activity. When stimulation at a site interrupted motor, sensory or language function, afterdischarges were more likely to occur at other sites where stimulation interrupted similar functions. These results show that widespread dynamic changes in cortical responses can occur in intact cortex within short periods of time, and that the distribution of these responses depends on local brain states and functional brain architecture at the time of stimulation. Similar rapid variations may occur during normal intracortical communication and may underlie changes in the cortical organization of function. Possibly these variations, and the occurrence and distribution of responses to cortical stimulation, could be predicted. If so, interventions such as stimulation might be used to alter spread of epileptogenic activity, accelerate learning or enhance cortical reorganization after brain injury
Acidic microenvironment plays a key role in human melanoma progression through a sustained exosome mediated transfer of clinically relevant metastatic molecules
Background: Microenvironment cues involved in melanoma progression are largely unknown. Melanoma is highly influenced in its aggressive phenotype by the changes it determinates in its microenvironment, such as pH decrease, in turn influencing cancer cell invasiveness, progression and tissue remodelling through an abundant secretion of exosomes, dictating cancer strategy to the whole host. A role of exosomes in driving melanoma progression under microenvironmental acidity was never described. Methods: We studied four differently staged human melanoma lines, reflecting melanoma progression, under microenvironmental acidic pHs pressure ranging between pH 6.0-6.7. To estimate exosome secretion as a function of tumor stage and environmental pH, we applied a technique to generate native fluorescent exosomes characterized by vesicles integrity, size, density, markers expression, and quantifiable by direct FACS analysis. Functional roles of exosomes were tested in migration and invasion tests. Then we performed a comparative proteomic analysis of acid versus control exosomes to elucidate a specific signature involved in melanoma progression. Results: We found that metastatic melanoma secretes a higher exosome amount than primary melanoma, and that acidic pH increases exosome secretion when melanoma is in an intermediate stage, i.e. metastatic non-invasive. We were thus able to show that acidic pH influences the intercellular cross-talk mediated by exosomes. In fact when exposed to exosomes produced in an acidic medium, pH naĂŻve melanoma cells acquire migratory and invasive capacities likely due to transfer of metastatic exosomal proteins, favoring cell motility and angiogenesis. A Prognoscan-based meta-analysis study of proteins enriched in acidic exosomes, identified 11 genes (HRAS, GANAB, CFL2, HSP90B1, HSP90AB1, GSN, HSPA1L, NRAS, HSPA5, TIMP3, HYOU1), significantly correlating with poor prognosis, whose high expression was in part confirmed in bioptic samples of lymph node metastases. Conclusions: A crucial step of melanoma progression does occur at melanoma intermediate -stage, when extracellular acidic pH induces an abundant release and intra-tumoral uptake of exosomes. Such exosomes are endowed with pro-invasive molecules of clinical relevance, which may provide a signature of melanoma advancement
Automated Detection of EUV Polar Coronal Holes During Solar Cycle 23
A new method for automated detection of polar coronal holes is presented.
This method, called perimeter tracing, uses a series of 171, 195, and 304 \AA\
full disk images from the Extreme ultraviolet Imaging Telescope (EIT) on SOHO
over solar cycle 23 to measure the perimeter of polar coronal holes as they
appear on the limbs. Perimeter tracing minimizes line-of-sight obscurations
caused by the emitting plasma of the various wavelengths by taking measurements
at the solar limb. Perimeter tracing also allows for the polar rotation period
to emerge organically from the data as 33 days. We have called this the Harvey
rotation rate and count Harvey rotations starting 4 January 1900. From the
measured perimeter, we are then able to fit a curve to the data and derive an
area within the line of best fit. We observe the area of the northern polar
hole area in 1996, at the beginning of solar cycle 23, to be about 4.2% of the
total solar surface area and about 3.6% in 2007. The area of the southern polar
hole is observed to be about 4.0% in 1996 and about 3.4% in 2007. Thus, both
the north and south polar hole areas are no more than 15% smaller now than they
were at the beginning of cycle 23. This compares to the polar magnetic field
measured to be about 40% less now than it was a cycle ago.Comment: 18 pagers, 7 figures, accepted to Solar Physic
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