Statistical Analysis of Asymmetric Sunspot Decay Observed by Hinode
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Statistical Analysis of Asymmetric Sunspot Decay Observed by Hinode Shinsuke Imada1
· Shota Kato1 · Masashi Fujiyama1
Received: 27 June 2020 / Accepted: 20 October 2020 © Springer Nature B.V. 2020
Abstract We statistically studied the transport of magnetic flux in and around sunspots using a magnetic-element tracking technique to investigate whether sunspot-decay processes are isotropic. Using this method, we detected moving magnetic features (MMFs). The observed radius of an MMFs region was approximately 1.7 times the sunspot radius; furthermore, the average apparent velocity of MMFs was statistically estimated to be approximately 350 m s−1 . We determined that the leading sunspots transport approximately 5% more magnetic flux to the Equator side than to the Pole side of the sunspots. In addition, the leading sunspots transport approximately 3% more magnetic flux to the back (East) than to the front (West) of the sunspots. On the other hand, the following sunspots do not show the magnetic-flux transport asymmetry. The statistics might not be sufficient for the analysis of the following sunspots. These asymmetries of magnetic flux transport might contribute to the cross-equatorial transport of net magnetic flux, which is an important physical quantity of polar magnetic-field reversal. Keywords Sun: activity · Sun: photosphere · Sunspots
1. Introduction Sunspots have been observed for a long time, and their magnetic properties have been known for more than a century. The number of sunspots varies every 11 years (e.g. Clette et al., 2014). Sunspots are considered to be an energy source that affects the solar-terrestrial environment. For example, a sudden brightening in almost all wavelengths from radio waves to gamma rays has often been observed in the solar atmosphere. This sudden brightening, now called a solar flare (e.g. Tsuneta et al., 1992; Imada et al., 2013), is believed to be the result of a rapid release of the magnetic energy stored in the sunspot. The energy released by a flare is extremely large; the total amount of this energy often reaches 1032 ergs within an hour.
B S. Imada
[email protected]
1
Institute for Space–Earth Environmental Research (ISEE), Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan
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Solar flares are often associated with coronal mass ejections, which can trigger geomagnetic storms (e.g. Svestka and Cliver, 1992; Yashiro et al., 2006; Imada et al., 2007, 2011). Therefore, it is crucial to understand the sunspot formation and decay process in terms of solar physics and space weather. Another important role of sunspots is to change the entire magnetic field on the solar surface. Similar to the sunspot numbers, the polar magnetic field, which is the seed of the magnetic field for the next cycle (Cameron and Schüssler, 2015), changes approximately every 11 years. The sunspots provide small magnetic elements during the decay. These elements are transported by solar surface flows, such as meridional flow, differential rotatio
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