C48

Presentation Title:

Solar Orbiter/EUI Observations and a Bifrost MHD Simulation of Fine-scale Bright Dots in Emerging Flux Regions

Author(s):

Sanjiv K. Tiwari1,2, Viggo H. Hansteen1,2,3,4, Bart De Pontieu1,3,4, Navdeep K. Panesar1,2, David Berghmans5 1Lockheed Martin Solar Astrophysics Laboratory, 3251 Hanover Street Building 203, Palo Alto, CA 94306, USA; tiwari@lmsal.com 2Bay Area Environmental Research Institute, NASA Research Park, Moffett Field, CA 94035, USA 3Rosseland Centre for Solar Physics, University of Oslo, P.O. Box 1029 Blindern, NO–0315 Oslo, Norway 4Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, NO–0315 Oslo, Norway 5Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium, Ringlaan -3- Av. Circulaire, B-1180 Brussels, Belgium

Abstract:

Observations with Solar Orbiter's EUI/HRI of an emerging flux region (a coronal bright point) in 174Å, emitted by the coronal plasma at ~1 MK, reveals the presence of numerous tiny bright dots 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). The other half show extensions, appearing as a small loop or surge/jet, with intensity propagations below 30km/s.
Many of the bigger and brighter HRI 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 is followed by a blueshift of similar or higher magnitude. We synthesized O V/VI lines [because these lines are covered in EUI/HRI passband] and Si IV line [to see how these dots would appear in IRIS observations], which both form in the transition region. These synthetic images 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) 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.

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