Abstract Light bridges (LBs) are bright lanes that divide one sunspot umbra into two or more parts. Though frequently observed in sunspots, their formation mechanisms have rarely been studied and thus are not well understood. Here we present results from the first statistical investigation on the formation of LBs. Using observations with the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we identified 144 LBs within 71 active regions (ARs) over the whole year of 2014. The formation processes of these LBs can be categorized into three groups: penumbral intrusion (type-A), sunspot merging (typeB) and umbral-dot emergence (type-C). The numbers of events in these three categories are 74, 57 and 13, respectively. The duration of the LB formation process is mostly less than 40 hours, with an average of ∼20 hours. Most LBs have a maximum length of less than 20′′. For type-A LBs, we found a positive correlation between the LB length and the duration of the LB formation process, suggesting a similar speed of penumbral intrusion in different sunspots.
Keywords sunspots — Sun: photosphere — Sun: activity — Sun: evolution
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