217 research outputs found
Maximum Coronal Mass Ejection Speed as an Indicator of Solar and Geomagnetic Activities
We investigate the relationship between the monthly averaged maximal speeds
of coronal mass ejections (CMEs), international sunspot number (ISSN), and the
geomagnetic Dst and Ap indices covering the 1996-2008 time interval (solar
cycle 23). Our new findings are as follows. (1) There is a noteworthy
relationship between monthly averaged maximum CME speeds and sunspot numbers,
Ap and Dst indices. Various peculiarities in the monthly Dst index are
correlated better with the fine structures in the CME speed profile than that
in the ISSN data. (2) Unlike the sunspot numbers, the CME speed index does not
exhibit a double peak maximum. Instead, the CME speed profile peaks during the
declining phase of solar cycle 23. Similar to the Ap index, both CME speed and
the Dst indices lag behind the sunspot numbers by several months. (3) The CME
number shows a double peak similar to that seen in the sunspot numbers. The CME
occurrence rate remained very high even near the minimum of the solar cycle 23,
when both the sunspot number and the CME average maximum speed were reaching
their minimum values. (4) A well-defined peak of the Ap index between 2002 May
and 2004 August was co-temporal with the excess of the mid-latitude coronal
holes during solar cycle 23. The above findings suggest that the CME speed
index may be a useful indicator of both solar and geomagnetic activities. It
may have advantages over the sunspot numbers, because it better reflects the
intensity of Earth-directed solar eruptions
Magnetic evolution of superactive regions. Complexity and potentially unstable magnetic discontinuities
In this work, we have studied the temporal evolution of some properties of a
sample of superactive regions with the aim to single out the most significant
for flare activity forecasting. We have investigated properties of 14
superactive regions, observed between January 1st 2000 and December 31st 2006
with MDI/SOHO instrument and characterized by a particularly intense flare
activity during their passage on the solar disk. We have analyzed the temporal
evolution of fractal and multifractal properties of photospheric magnetic
fields, namely the generalized fractal dimension and the cntribution and
dimensionality diversities, as well as the potential unstable volumes of
magnetic discontinuities above the studied ARs. Correlations of these
quantities with the flare index, which provides information about the flare
activity of a region, have also been estimated. We found that in 50 % of our
sample the generalized fractal dimension is correlated with the flare index
computed over windows of 50 hours, while the contribution diversity and the
dimensional diversity are anticorrelated with the same index. An increase of
the potential unstable volume of magnetic discontinuities in the corona is
observed before the phases characterized by more frequent and intense flares.
We also found that the free energy distribution functions of unstable volumes
of the analyzed superactive regions can be fitted with straight lines whose
slope is larger than the values found in previous works for less active
magnetic regions. The generalized fractal dimension and the potential unstable
volume of magnetic discontinuities are the most suitable for statistical
investigations of relations with flare activity over longer (50 hours) and
shorter (few hours) time intervals, respectively
Parameters of the Magnetic Flux inside Coronal Holes
Parameters of magnetic flux distribution inside low-latitude coronal holes
(CHs) were analyzed. A statistical study of 44 CHs based on Solar and
Heliospheric Observatory (SOHO)/MDI full disk magnetograms and SOHO/EIT 284\AA
images showed that the density of the net magnetic flux, , does
not correlate with the associated solar wind speeds, . Both the area and
net flux of CHs correlate with the solar wind speed and the corresponding
spatial Pearson correlation coefficients are 0.75 and 0.71, respectively. A
possible explanation for the low correlation between and
is proposed. The observed non-correlation might be rooted in the structural
complexity of the magnetic field. As a measure of complexity of the magnetic
field, the filling factor, , was calculated as a function of spatial
scales. In CHs, was found to be nearly constant at scales above 2 Mm,
which indicates a monofractal structural organization and smooth temporal
evolution. The magnitude of the filling factor is 0.04 from the Hinode SOT/SP
data and 0.07 from the MDI/HR data. The Hinode data show that at scales smaller
than 2 Mm, the filling factor decreases rapidly, which means a mutlifractal
structure and highly intermittent, burst-like energy release regime. The
absence of necessary complexity in CH magnetic fields at scales above 2 Mm
seems to be the most plausible reason why the net magnetic flux density does
not seem to be related to the solar wind speed: the energy release dynamics,
needed for solar wind acceleration, appears to occur at small scales below 1
Mm.Comment: 6 figures, approximately 23 pages. Accepted in Solar Physic
Properties of Umbral Dots as Measured from the New Solar Telescope Data and MHD Simulations
We studied bright umbral dots (UDs) detected in a moderate size sunspot and
compared their statistical properties to recent MHD models. The study is based
on high resolution data recorded by the New Solar Telescope at the Big Bear
Solar Observatory and 3D MHD simulations of sunspots. Observed UDs, living
longer than 150 s, were detected and tracked in a 46 min long data set, using
an automatic detection code. Total 1553 (620) UDs were detected in the
photospheric (low chromospheric) data. Our main findings are: i) none of the
analyzed UDs is precisely circular, ii) the diameter-intensity relationship
only holds in bright umbral areas, and iii) UD velocities are inversely related
to their lifetime. While nearly all photospheric UDs can be identified in the
low chromospheric images, some small closely spaced UDs appear in the low
chromosphere as a single cluster. Slow moving and long living UDs seem to exist
in both the low chromosphere and photosphere, while fast moving and short
living UDs are mainly detected in the photospheric images. Comparison to the 3D
MHD simulations showed that both types of UDs display, on average, very similar
statistical characteristics. However, i) the average number of observed UDs per
unit area is smaller than that of the model UDs, and ii) on average, the
diameter of model UDs is slightly larger than that of observed ones.Comment: Accepted by the AP
Three-dimensional Evolution of Solar Wind during Solar Cycles 22-24
This paper presents the analysis of 3D evolution of solar wind density
turbulence and speed at various levels of solar activity between solar cycles
22 and 24. The solar wind data has been obtained from interplanetary
scintillation (IPS) measurements made at the Ooty Radio Telescope. Results show
that (i) on the average, there was a downward trend in density turbulence from
the maximum of cycle 22 to the deep minimum phase of cycle 23; (2) the
scattering diameter of the corona around the Sun shrunk steadily towards the
Sun, starting from 2003 to the smallest size at the deepest minimum, and it
corresponded to a reduction of ~50% in density turbulence between maximum and
minimum phases of cycle 23; (3) The latitudinal distribution of solar wind
speed was significantly different between minima of cycles 22 and 23. At the
minimum phase of solar cycle 22, when the underlying solar magnetic field was
simple and nearly dipole in nature, the high-speed streams were observed from
poles to ~30 deg. latitudes in both hemispheres. In contrast, in the long-decay
phase of cycle 23, the sources of high-speed wind at both poles, in accordance
with the weak polar fields, occupied narrow latitude belts from poles to ~60
deg. latitudes. Moreover, in agreement with the large amplitude of heliospheric
current sheet, the low-speed wind prevailed the low- and mid-latitude regions
of the heliosphere. (4) At the transition phase between cycles 23 and 24, the
high levels of density and density turbulence were observed close to the
heliospheric equator and the low-speed speed wind extended from equatorial- to
mid-latitude regions. Results are consistent with the onset of the current
cycle 24, from middle of 2009 and it has almost reached near to the maximum
phase at the northern hemisphere of the Sun, but activity not yet developed in
the southern hemisphere.Comment: 14 pages, 9 figures, Accepted for Publication in The Astrophysical
Journa
\Lambda-buildings and base change functors
We prove an analog of the base change functor of \Lambda-trees in the setting
of generalized affine buildings. The proof is mainly based on local and global
combinatorics of the associated spherical buildings. As an application we
obtain that the class of generalized affine building is closed under ultracones
and asymptotic cones. Other applications involve a complex of groups
decompositions and fixed point theorems for certain classes of generalized
affine buildings.Comment: revised version, 29 pages, to appear in Geom. Dedicat
Magnetic Non-Potentiality of Solar Active Regions and Peak X-Ray Flux of the Associated Flares
Predicting the severity of the solar eruptive phenomena like flares and
Coronal Mass Ejections (CMEs) remains a great challenge despite concerted
efforts for several decades. The advent of high quality vector magnetograms
obtained from Hinode (SOT/SP) has increased the possibility of meeting this
challenge. In particular, the Spatially Averaged Signed Shear Angle (SASSA)
seems to be an unique parameter to quantify the non-potentiality of the active
regions. We demonstrate the usefulness of SASSA for predicting the flare
severity. For this purpose we present case studies of the evolution of magnetic
non-potentiality using 115 vector magnetograms of four active regions namely
ARs NOAA 10930, 10960, 10961 and 10963 during December 08-15, 2006, June 03-10,
2007, June 28-July 5, 2007 and July 10-17, 2007 respectively. The NOAA ARs
10930 and 10960 were very active and produced X and M class flares
respectively, along with many smaller X-ray flares. On the other hand, the NOAA
ARs 10961 and 10963 were relatively less active and produced only very small
(mostly A and B-class) flares. For this study we have used a large number of
high resolution vector magnetograms obtained from Hinode (SOT/SP). The analysis
shows that the peak X-ray flux of the most intense solar flare emanating from
the active regions depends on the magnitude of the SASSA at the time of the
flare. This finding of the existence of a lower limit of SASSA for a given
class of X-ray flare will be very useful for space weather forecasting. We have
also studied another non-potentiality parameter called mean weighted shear
angle (MWSA) of the vector magnetograms along with SASSA. We find that the MWSA
does not show such distinction as the SASSA for upper limits of GOES X-Ray flux
of solar flares, however both the quantities show similar trends during the
evolution of all active regions studied.Comment: 25 pages, 5 figures, accepted for publication in the Astrophysical
Journa
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