129 research outputs found
Search for Short-Term Periodicities in the Sun's Surface Rotation: A Revisit
The power spectral analyses of the Sun's surface equatorial rotation rate
determined from the Mt. Wilson daily Doppler velocity measurements during the
period 3 December 1985 to 5 March 2007 suggests the existence of 7.6 year, 2.8
year, 1.47 year, 245 day, 182 day and 158 day periodicities in the surface
equatorial rotation rate during the period before 1996.
However, there is no variation of any kind in the more accurately measured
data during the period after 1995. That is, the aforementioned periodicities in
the data during the period before the year 1996 may be artifacts of the
uncertainties of those data due to the frequent changes in the instrumentation
of the Mt. Wilson spectrograph. On the other hand, the temporal behavior of
most of the activity phenomena during cycles 22 (1986-1996) and 23 (after 1997)
is considerably different. Therefore, the presence of the aforementioned
short-term periodicities during the last cycle and absence of them in the
current cycle may, in principle, be real temporal behavior of the solar
rotation during these cycles.Comment: 11 pages, 6 figures, accepted for publication in Solar Physic
Sun's retrograde motion and violation of even-odd cycle rule in sunspot activity
The sum of sunspots number over an odd numbered 11 yr sunspot cycle exceeds
that of its preceding even numbered cycle, and it is well known as Gnevyshev
and Ohl rule (or G--O rule) after the names of the authors who discovered it in
1948. The G--O rule can be used to predict the sum of sunspot numbers of a
forthcoming odd cycle from that of its preceding even cycle. But this is not
always possible because occasionally the G--O rule is violated. So far no
plausible reason is known either for the G--O rule or the violation of this
rule. Here we showed the epochs of the violation of the G--O rule are close to
the epochs of the Sun's retrograde orbital motion about the centre of mass of
the solar system (i.e., the epochs at which the orbital angular momentum of the
Sun is weakly negative). Using this result easy to predict the epochs of
violation of the G--O rule well in advance. We also showed that the solar
equatorial rotation rate determined from sunspot group data during the period
1879--2004 is correlated/anti-correlated to the Sun's orbital torque during
before/after 1945. We have found the existence of a statistically significant
17 yr periodicity in the solar equatorial rotation rate. The
implications of these findings for understanding the mechanism behind the solar
cycle and the solar-terrestrial relationship are discussed.Comment: 13 pages, 4 figures, accepted by MNRA
Long-Term Variations in the Growth and Decay Rates of Sunspot Groups
Using the combined Greenwich (1874-1976) and Solar Optical Observatories
Network (1977-2009) data on sunspot groups, we study the long-term variations
in the mean daily rates of growth and decay of sunspot groups. We find that the
minimum and the maximum values of the annually averaged daily mean growth rates
are ~52% per day and ~183% per day, respectively, whereas the corresponding
values of the annually averaged daily mean decay rates are ~21% per day and
~44% per day, respectively. The average value (over the period 1874-2009) of
the growth rate is about 70% more than that of the decay rate. The growth and
the decay rates vary by about 35% and 13%, respectively, on a 60-year
time-scale. From the beginning of Cycle 23 the growth rate is substantially
decreased and near the end (2007-2008) the growth rate is lowest in the past
about 100 years.Comment: 1 table, 13 figures, accepted by Solar Physic
A Comparison of Solar Cycle Variations in the Equatorial Rotation Rates of the Sun's Subsurface, Surface, Corona, and Sunspot Groups
Using the Solar Optical Observing Network (SOON) sunspot-group data for the
period 1985-2010, the variations in the annual mean equatorial-rotation rates
of the sunspot groups are determined and compared with the known variations in
the solar equatorial-rotation rates determined from the following data: i) the
plasma rotation rates at 0.94Rsun, 0.95Rsun,...,1.0Rsun measured by Global
Oscillation Network Group (GONG) during the period 1995-2010, ii) the data on
the soft X-ray corona determined from Yohkoh/SXT full disk images for the years
1992-2001, iii) the data on small bright coronal structures (SBCS) which were
traced in Solar and Heliospheric Observatory (SOHO)/EIT images during the
period 1998-2006, and iv) the Mount Wilson Doppler-velocity measurements during
the period 1986-2007. A large portion (up to approximate 30 deg latitude) of
the mean differential-rotation profile of the sunspot groups lies between those
of the internal differential-rotation rates at 0.94Rsun and 0.98Rsun.The
variation in the yearly mean equatorial-rotation rate of the sunspot groups
seems to be lagging that of the equatorial-rotation rate determined from the
GONG measurements by one to two years.The amplitude of the latter is very
small.The solar-cycle variation in the equatorial-rotation rate of the solar
corona closely matches that determined from the sunspot-group data.The
variation in the equatorial-rotation rate determined from the Mount Wilson
Doppler-velocity data closely resembles the corresponding variation in the
equatorial-rotation rate determined from the sunspot-group data that included
the values of the abnormal angular motions (> 3 deg per day) of the sunspot
groups. Implications of these results are pointed out.Comment: 22 pages, 10 figures, accepted by Solar Physic
The G-O Rule and Waldmeier Effect in the Variations of the Numbers of Large and Small Sunspot Groups
We have analysed the combined Greenwich and Solar Optical Observing Network
(SOON) sunspot group data during the period of 1874-2011 and determined
variations in the annual numbers (counts) of the small, large and big sunspot
groups (these classifications are made on the basis of the maximum areas of the
sunspot groups). We found that the amplitude of an even-numbered cycle of the
number of large groups is smaller than that of its immediately following
odd-numbered cycle. This is consistent with the well known Gnevyshev and Ohl
rule or G-O rule of solar cycles, generally described by using the Zurich
sunspot number (Rz). During cycles 12-21 the G-O rule holds good for the
variation in the number of small groups also, but it is violated by cycle pair
(22, 23) as in the case of Rz. This behaviour of the variations in the small
groups is largely responsible for the anomalous behaviour of Rz in cycle pair
(22, 23). It is also found that the amplitude of an odd-numbered cycle of the
number of small groups is larger than that of its immediately following
even-numbered cycle. This can be called as `reverse G-O rule'. In the case of
the number of the big groups, both cycle pairs (12, 13) and (22, 23) violated
the G-O rule. In many cycles the positions of the peaks of the small, large,
and big groups are different and considerably differ with respect to the
corresponding positions of the Rz peaks. In the case of cycle 23, the
corresponding cycles of the small and large groups are largely symmetric/less
asymmetric (Waldmeier effect is weak/absent) with their maxima taking place two
years later than that of Rz. The corresponding cycle of the big groups is more
asymmetric (strong Waldmeier effect) with its maximum epoch taking place at the
same time as that of Rz.Comment: 13 pages, 5 figures, 1 table, accepted by Solar Physic
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