81 research outputs found
Three-dimensional magnetohydrodynamic simulation of the solar magnetic flux emergence: Parametric study on the horizontal divergent flow
Solar active regions are formed through the emergence of magnetic flux from
the deeper convection zone. Recent satellite observations have shown that a
horizontal divergent flow (HDF) stretches out over the solar surface just
before the magnetic flux appearance. The aims of this study are to investigate
the driver of the HDF and to see the dependency of the HDF on the parameters of
the magnetic flux in the convection zone. We conduct three-dimensional
magnetohydrodynamic (3D MHD) numerical simulations of the magnetic flux
emergence and vary the parameters in the initial conditions. An analytical
approach is also taken to explain the dependency. The horizontal gas pressure
gradient is found to be the main driver of the HDF. The maximum HDF speed shows
positive correlations with the field strength and twist intensity. The HDF
duration has a weak relation with the twist, while it shows negative dependency
on the field strength only in the case of the stronger field regime. Parametric
dependencies analyzed in this study may allow us to probe the structure of the
subsurface magnetic flux by observing properties of the HDF.Comment: 7 pages, 5 figures, 1 table, accepted for publication in Astronomy &
Astrophysic
Probability Distribution Functions of Sunspot Magnetic Flux
We have investigated the probability distributions of sunspot area and
magnetic flux by using the data from Royal Greenwich Observatory and USAF/NOAA.
We have constructed a sample of 2995 regions with maximum-development areas
500 MSH (millionths of solar hemisphere), covering 146.7 years
(1874--2020). The data were fitted by a power-law distribution and four
two-parameter distributions (tapered power-law, gamma, lognormal, and Weibull
distributions). The power-law model was unfavorable compared to the four models
in terms of AIC, and was not acceptable by the classical Kolmogorov-Smirnov
test. The lognormal and Weibull distributions were excluded because their
behavior extended to smaller regions ( MSH) do not connect to the
previously published results. Therefore, our choices were tapered power-law and
gamma distributions. The power-law portion of the tapered power-law and gamma
distributions was found to have a power exponent of 1.35--1.9. Due to the
exponential fall-off of these distributions, the expected frequencies of large
sunspots are low. The largest sunspot group observed had an area of 6132 MSH,
and the frequency of sunspots larger than MSH was estimated to be every
3 -- 8 years. We also have estimated the distributions of the
Sun-as-a-star total sunspot areas. The largest total area covered by sunspots
in the record was 1.67 % of the visible disk, and can be up to 2.7 % by
artificially increasing the lifetimes of large sunspots in an area evolution
model. These values are still smaller than those found on active Sun-like
stars
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