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- ItemEvolution of the flow field in decaying active regions II. Converging flows at the periphery of naked spots(Les Ulis : EDP Sciences, 2022) Strecker, H.; Bello González, N.Context. In a previous work, we investigated the evolution of the flow field around sunspots during sunspot decay and compared it with the flow field of supergranular cells. The decay of a sunspot proceeds as it interacts with its surroundings. This is manifested by the changes observed in the flow field surrounding the decaying spot. Aims. We now investigate in detail the evolution of the flow field in the direct periphery of the sunspots of the same sample and aim to provide a complete picture of the role of large-scale flows present in sunspot cells. Methods. We analyse the horizontal velocity profiles of sunspots obtained from observations by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). We follow their evolution across the solar disc from their stable phase to their decay and their final disappearance. Results.We find two different scenarios for the evolution of the flow region surrounding a spot in the final stage of its decay: (i) either the flow cell implodes and disappears under the action of the surrounding supergranules or (ii) it outlives the spot. In the later case, an inwards flow towards the remaining naked spot develops in the vicinity closest to the spot followed by an outflow further out. These findings provide observational evidence to theoretical predictions by realistic magnetohydrodynamic (MHD) sunspot and moat region simulations. Conclusions. The Evershed flow and the moat flow, both connected to the presence of fully fledged sunspots in a spot cell, vanish when penumbrae decay. Moat flows decline into supergranular flows. The final fate of a spot cell depends on its interaction with the surrounding supergranular cells. In the case of non-imploding spot cells, the remaining naked spot develops a converging inflow driven by radiative cooling and a geometrical alignment of granules in its periphery which is similar to that observed in pores.
- ItemComparison of telescopic and naked-eye sunspots for the very small spots on February 15, 1900 and January 30, 1911(Berlin : Wiley-VCH Verl., 2020) Neuhäuser, Ralph; Geymeier, Michael; Arlt, Rainer; Chapman, JesseIn a recent discussion of the Maunder Minimum, two sunspot observations by Chinese court astronomers on February 15, 1900 and January 30, 1911 (±1 day) – presumably made with the unaided eye – were considered false detections because the spot areas of the largest spot on those days (±1 day) as recorded by the Royal Greenwich Observatory, would be too small for naked-eye detection, namely 11 and 13 millionths of a solar disk (msd), respectively (Usoskin et al. 2015). We revisit this issue here. First, we review theoretical and empirical considerations of the lower limit for the sunspot area detectable by the naked eye: under optimal conditions, very good observers can detect spots as small as ∼100 msd (and we present one example, where an observer reported a spot, when the largest spot on that day was only 65 msd, but being part of a longish group facilitating the detection). Then, we review all known sunspot observations on and around February 15, 1900 and January 30, 1911, including full-disk drawings. For February 15, 1900, Kalocsa observatory, Hungary, shows a feature close to the western limb with an area of 134 msd, but it is not clear whether it was a spot or faculae or pores (as spot, it could have been detectable even by naked-eye). The two spot groups detected in Kodaikanal, India, on January 31, 1911 and February 1 with 18.5 to 33.0 msd area would be too small for detection by the naked eye. However, the Chinese records for February 15, 1900 and January 30, 1911 do not even mention whether the observations were performed with a telescope or by the unaided eye. We conclude that there is no convincing evidence that these two – or even all – Chinese sunspot records are unreliable.
- ItemComplex network approach to characterize the statistical features of the sunspot series(Bristol : Institute of Physics Publishing, 2014) Zou, Y.; Small, M.; Liu, Z.; Kurths, J.Complex network approaches have been recently developed as an alternative framework to study the statistical features of time-series data. We perform a visibility-graph analysis on both the daily and monthly sunspot series. Based on the data, we propose two ways to construct the network: one is from the original observable measurements and the other is from a negative-inverse- transformed series. The degree distribution of the derived networks for the strong maxima has clear non-Gaussian properties, while the degree distribution for minima is bimodal. The long-term variation of the cycles is reflected by hubs in the network that span relatively large time intervals. Based on standard network structural measures, we propose to characterize the long-term correlations by waiting times between two subsequent events. The persistence range of the solar cycles has been identified over 15-1000 days by a power-law regime with scaling exponent γ = 2.04 of the occurrence time of two subsequent strong minima. In contrast, a persistent trend is not present in the maximal numbers, although maxima do have significant deviations from an exponential form. Our results suggest some new insights for evaluating existing models.