318 research outputs found
Coronal Fe XIV Emission During the Whole Heliosphere Interval Campaign
Solar Cycle 24 is having a historically long and weak start. Observations of
the Fe XIV corona from the Sacramento Peak site of the National Solar
Observatory show an abnormal pattern of emission compared to observations of
Cycles 21, 22, and 23 from the same instrument. The previous three cycles have
shown a strong, rapid "Rush to the Poles" (previously observed in polar crown
prominences and earlier coronal observations) in the parameter N(t,l,dt)
(average number of Fe XIV emission features per day over dt days at time t and
latitude l). Cycle 24 displays a weak, intermittent, and slow "Rush" that is
apparent only in the northern hemisphere. If the northern Rush persists at its
current rate, evidence from the Rushes in previous cycles indicates that solar
maximum will occur in early 2013 or late 2012, at least in the northern
hemisphere. At lower latitudes, solar maximum previously occurred when the time
maximum of N(t,l,365) reached approximately 20{\deg} latitude. Currently, this
parameter is at or below 30{\deg}and decreasing in latitude. Unfortunately, it
is difficult at this time to calculate the rate of decrease in N(t,l,365).
However, the southern hemisphere could reach 20{\deg} in 2011. Nonetheless,
considering the levels of activity so far, there is a possibility that the
maximum could be indiscernibleComment: 8 pages, 4 figures; Solar Physics Online First, 2011
http://www.springerlink.com/content/b5kl4040k0626647
Photospheric Magnetic Field: Relationship Between North-South Asymmetry and Flux Imbalance
Photospheric magnetic fields were studied using the Kitt Peak synoptic maps
for 1976-2003. Only strong magnetic fields (B>100 G) of the equatorial region
were taken into account. The north-south asymmetry of the magnetic fluxes was
considered as well as the imbalance between positive and negative fluxes. The
north-south asymmetry displays a regular alternation of the dominant hemisphere
during the solar cycle: the northern hemisphere dominated in the ascending
phase, the southern one in the descending phase during Solar Cycles 21-23. The
sign of the imbalance did not change during the 11 years from one polar-field
reversal to the next and always coincided with the sign of the Sun's polar
magnetic field in the northern hemisphere. The dominant sign of leading
sunspots in one of the hemispheres determines the sign of the magnetic-flux
imbalance. The sign of the north-south asymmetry of the magnetic fluxes and the
sign of the imbalance of the positive and the negative fluxes are related to
the quarter of the 22-year magnetic cycle where the magnetic configuration of
the Sun remains constant (from the minimum where the sunspot sign changes
according to Hale's law to the magnetic-field reversal and from the reversal to
the minimum). The sign of the north-south asymmetry for the time interval
considered was determined by the phase of the 11-year cycle (before or after
the reversal); the sign of the imbalance of the positive and the negative
fluxes depends on both the phase of the 11-year cycle and on the parity of the
solar cycle. The results obtained demonstrate the connection of the magnetic
fields in active regions with the Sun's polar magnetic field in the northern
hemisphere.Comment: 24 pages, 12 figures, 2 table
Solar cycle prediction using precursors and flux transport models
We study the origin of the predictive skill of some methods to forecast the
strength of solar activity cycles. A simple flux transport model for the
azimuthally averaged radial magnetic field at the solar surface is used, which
contains a source term describing the emergence of new flux based on
observational sunspot data. We consider the magnetic flux diffusing over the
equator as a predictor, since this quantity is directly related to the global
dipole field from which a Babcock-Leighton dynamo generates the toroidal field
for the next activity cycle. If the source is represented schematically by a
narrow activity belt drifting with constant speed over a fixed range of
latitudes between activity minima, our predictor shows considerable predictive
skill with correlation coefficients up to 0.95 for past cycles. However, the
predictive skill is completely lost when the actually observed emergence
latitudes are used. This result originates from the fact that the precursor
amplitude is determined by the sunspot activity a few years before solar
minimum. Since stronger cycles tend to rise faster to their maximum activity
(known as the Waldmeier effect), the temporal overlapping of cycles leads to a
shift of the minimum epochs that depends on the strength of the following
cycle. This information is picked up by precursor methods and also by our flux
transport model with a schematic source. Therefore, their predictive skill does
not require a memory, i.e., a physical connection between the surface
manifestations of subsequent activity cycles.Comment: Astrophys. Journal, in pres
Evolution and Flare Activity of Delta-Sunspots in Cycle 23
The emergence and magnetic evolution of solar active regions (ARs) of
beta-gamma-delta type, which are known to be highly flare-productive, were
studied with the SOHO/MDI data in Cycle 23. We selected 31 ARs that can be
observed from their birth phase, as unbiased samples for our study. From the
analysis of the magnetic topology (twist and writhe), we obtained the following
results. i) Emerging beta-gamma-delta ARs can be classified into three
topological types as "quasi-beta", "writhed" and "top-to-top". ii) Among them,
the "writhed" and "top-to-top" types tend to show high flare activity. iii) As
the signs of twist and writhe agree with each other in most cases of the
"writhed" type (12 cases out of 13), we propose a magnetic model in which the
emerging flux regions in a beta-gamma-delta AR are not separated but united as
a single structure below the solar surface. iv) Almost all the "writhed"-type
ARs have downward knotted structures in the mid portion of the magnetic flux
tube. This, we believe, is the essential property of beta-gamma-delta ARs. v)
The flare activity of beta-gamma-delta ARs is highly correlated not only with
the sunspot area but also with the magnetic complexity. vi) We suggest that
there is a possible scaling-law between the flare index and the maximum umbral
area
A Bayesian Analysis of the Correlations Among Sunspot Cycles
Sunspot numbers form a comprehensive, long-duration proxy of solar activity
and have been used numerous times to empirically investigate the properties of
the solar cycle. A number of correlations have been discovered over the 24
cycles for which observational records are available. Here we carry out a
sophisticated statistical analysis of the sunspot record that reaffirms these
correlations, and sets up an empirical predictive framework for future cycles.
An advantage of our approach is that it allows for rigorous assessment of both
the statistical significance of various cycle features and the uncertainty
associated with predictions. We summarize the data into three sequential
relations that estimate the amplitude, duration, and time of rise to maximum
for any cycle, given the values from the previous cycle. We find that there is
no indication of a persistence in predictive power beyond one cycle, and
conclude that the dynamo does not retain memory beyond one cycle. Based on
sunspot records up to October 2011, we obtain, for Cycle 24, an estimated
maximum smoothed monthly sunspot number of 97 +- 15, to occur in
January--February 2014 +- 6 months.Comment: Accepted for publication in Solar Physic
The fluctuating \alpha-effect and Waldmeier relations in the nonlinear dynamo models
We study the possibility to reproduce the statistical relations of the
sunspot activity cycle, like the so-called Waldmeier relations, the cycle
period - amplitude and the cycle rise rate - amplitude relations, by means of
the mean field dynamo models with the fluctuating \alpha-effect. The dynamo
model includes the long-term fluctuations of the \alpha-effect and two types of
the nonlinear feedback of the mean-field on the \alpha-effect including the
algebraic quenching and the dynamic quenching due to the magnetic helicity
generation. We found that the models are able to reproduce qualitatively and
quantitatively the inclination and dispersion across the Waldmeier relations
with the 20% fluctuations of the \alpha-effect. The models with the dynamic
quenching are in a better agreement with observations than the models with the
algebraic \alpha-quenching. We compare the statistical distributions of the
modeled parameters, like the amplitude, period, the rise and decay rates of the
sunspot cycles, with observations.Comment: revision, (add comment about effect of nonlinear diffusivity): 18
pages, 7 Figures, submitted to Physica Script
Response of the solar atmosphere to magnetic field evolution in a coronal hole region
Methods. We study an equatorial CH observed simultaneously by HINODE and
STEREO on July 27, 2007. The HINODE/SP maps are adopted to derive the physical
parameters of the photosphere and to research the magnetic field evolution and
distribution. The G band and Ca II H images with high tempo-spatial resolution
from HINODE/BFI and the multi-wavelength data from STEREO/EUVI are utilized to
study the corresponding atmospheric response of different overlying layers.
Results. We explore an emerging dipole locating at the CH boundary. Mini-scale
arch filaments (AFs) accompanying the emerging dipole were observed with the Ca
II H line. During the separation of the dipolar footpoints, three AFs appeared
and expanded in turn. The first AF divided into two segments in its late stage,
while the second and third AFs erupted in their late stages. The lifetimes of
these three AFs are 4, 6, 10 minutes, and the two intervals between the three
divisions or eruptions are 18 and 12 minutes, respectively. We display an
example of mixed-polarity flux emergence of IN fields within the CH and present
the corresponding chromospheric response. With the increase of the integrated
magnetic flux, the brightness of the Ca II H images exhibits an increasing
trend. We also study magnetic flux cancellations of NT fields locating at the
CH boundary and present the obvious chromospheric and coronal response. We
notice that the brighter regions seen in the 171 A images are relevant to the
interacting magnetic elements. By examining the magnetic NT and IN elements and
the response of different atmospheric layers, we obtain good positive linear
correlations between the NT magnetic flux densities and the brightness of both
G band (correlation coefficient 0.85) and Ca II H (correlation coefficient
0.58).Comment: 9 pages, 9 figures. A&A, in pres
Predicting the solar maximum with the rising rate
The growth rate of solar activity in the early phase of a solar cycle has
been known to be well correlated with the subsequent amplitude (solar maximum).
It provides very useful information for a new solar cycle as its variation
reflects the temporal evolution of the dynamic process of solar magnetic
activities from the initial phase to the peak phase of the cycle. The
correlation coefficient between the solar maximum (Rmax) and the rising rate
({\beta}a) at {\Delta}m months after the solar minimum (Rmin) is studied and
shown to increase as the cycle progresses with an inflection point (r = 0.83)
at about {\Delta}m = 20 months. The prediction error of Rmax based on {\beta}a
is found within estimation at the 90% level of confidence and the relative
prediction error will be less than 20% when {\Delta}m \geq 20. From the above
relationship, the current cycle (24) is preliminarily predicted to peak around
October 2013 with a size of Rmax =84 \pm 33 at the 90% level of confidence.Comment: 7 pages, 3 figures, accepted for publication in SCIENCE CHINA
Physics,Mechanics & Astronom
A homogeneous database of sunspot areas covering more than 130 years
The historical record of sunspot areas is a valuable and widely used proxy of
solar activity and variability. The Royal Greenwich Observatory (RGO) regularly
measured this and other parameters between 1874 and 1976. After that time
records from a number of different observatories are available. These, however,
show systematic differences and often have significants gaps. Our goal is to
obtain a uniform and complete sunspot area time series by combining different
data sets. A homogeneus composite of sunspot areas is essential for different
applications in solar physics, among others for irradiance reconstructions.
Data recorded simultaneously at different observatories are statistically
compared in order to determine the intercalibration factors. Using these data
we compile a complete and cross-calibrated time series. The Greenwich data set
is used as a basis until 1976, the Russian data (a compilation of observations
made at stations in the former USSR) between 1977 and 1985 and data compiled by
the USAF network since 1986. Other data sets (Rome, Yunnan, Catania) are used
to fill up the remaining gaps. Using the final sunspot areas record the
Photometric Sunspot Index is calculated. We also show that the use of
uncalibrated sunspot areas data sets can seriously affect the estimate of
irradiance variations. Our analysis implies that there is no basis for the
claim that UV irradiance variations have a much smaller influence on climate
than total solar irradiance variations.Comment: 40 pages, 8 figures; JGR - Space Physics, publishe
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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