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Fine-scale, Dot-like, Brightenings in an Emerging Flux Region: SolO/EUI Observations, and Bifrost MHD Simulations

Presentation #102.03 in the session Coronal Heating: Present Understanding and Future Progress I.

Published onOct 20, 2022
Fine-scale, Dot-like, Brightenings in an Emerging Flux Region: SolO/EUI Observations, and Bifrost MHD Simulations

Numerous tiny bright dots are observed in SolO’s EUI/HRIEUV data of an emerging flux region (a coronal bright point) in 174 Å, emitted by the coronal plasma at 1 MK. These dots are roundish, with a diameter of 675±300 km, a lifetime of 50±35 seconds, and an intensity enhancement of 30%±10% from their immediate surroundings. About half of the dots remain isolated during their evolution and move randomly and slowly (<10 km s-1). The other half show extensions, appearing as a small loop or surge/jet, with intensity propagations below 30 km s-1. Some dots form at the end of a fine-scale explosion. Many of the bigger and brighter HRIEUV dots are discernible in SDO/AIA 171 Å channel, have significant EM in the temperature range of 1–2 MK, and are often located at polarity inversion lines observed in HMI LOS magnetograms. Bifrost MHD simulations of an emerging flux region do show dots in synthetic Fe IX/X images, although dots in simulations are not as pervasive as in observations. The dots in simulations show distinct Doppler signatures — blueshifts and redshifts coexist, or a redshift of the order of 10 km s-1 is followed by a blueshift of similar or higher magnitude. The synthetic images of O V/VI and Si IV lines, which form in the transition region, also show the dots that are observed in Fe IX/X images, often expanded in size, or extended as a loop, and always with stronger Doppler velocities (up to 100 km s-1) than that in Fe IX/X lines. Our results, together with the field geometry of dots in the simulations, suggest that most dots in emerging flux regions form in the lower solar atmosphere (at 1 Mm) by magnetic reconnection between emerging and pre-existing/emerged magnetic field. The dots are smaller in Fe IX/X images (than in O V/VI, and Si IV lines) most likely because only the hottest counterpart of the magnetic reconnection events is visible in the hotter emission. Some dots might be manifestations of magneto-acoustic shocks (driven from the lower atmosphere) through the line formation region of Fe IX/X emission. A small number of dots could also be a response of supersonic downflows impacting transition-region/chromospheric density.

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