How rare are counter Evershed flows?

Abstract

One of the main characteristics of sunspot penumbrae is the radially outward-directed Evershed flow. Only recently have penumbral regions been reported with similar characteristics to normal penumbral filaments but with an opposite direction of the flow. Such flows directed toward the umbra are known as counter Evershed flows (CEFs). We aim to determine the occurrence frequency of CEFs in active regions (ARs) and to characterize their lifetime and the prevailing conditions in the ARs. We analyzed the continuum images, Dopplergrams, and magnetograms recorded by SDO/HMI of 97 ARs that appeared from 2011 to 2017. We followed the ARs for 9.6 ± 1.4 days on average. We found 384 CEFs in total, with a median value of six CEFs per AR. Counter Evershed flows are a rather common feature, occurring in 83.5% of all ARs regardless of the magnetic complexity of the AR. However, CEFs were only observed, on average, during 5.9% of the mean total duration of all the observations analyzed here. The lifetime of CEFs follows a log-normal distribution with a median value of 10.6−6.0+12.4 h. In addition, we report two populations of CEFs, those that are associated with light bridges and those that are not. We explain that the rarity of reports of CEFs in the literature is a result of highly incomplete coverage of ARs with spectropolarimetric data. By using the continuous observations now routinely available from space, we are able to overcome this limitation.

Strong magnetic fields and unusual flows in sunspots

“…The existing Hinode and SDO archives might already contain the answer. In this work, 19 CEFs were detected in AR 11967 and this number comes on top of the 387 CEFs already reported by Castellanos Durán, Lagg, and Solanki (2021). A statistical analysis of the known ∼400 CEFs is a unique opportunity to understand their origin and impact in the dynamics of sunspots.…”

“…Bi-directional flows within the bipolar light bridges typical are usually seen as "red-blue-red-blue" line-of-sight velocity shifts (Chapter 7). While in one scan, the flow pattern inside these regions might look similar to a CEF, the temporal evolution of these features can differ significantly from the "typical" flows inside CEFs found in the literature (Kleint 2012;Kleint and Sainz Dalda 2013;Louis et al 2014;Siu-Tapia et al 2017;Louis, Beck, and Choudhary 2020;Castellanos Durán, Lagg, and Solanki 2021). Therefore, this bipolar light bridge is not part of the current analysis.…”

“…Recently, Castellanos Durán, Lagg, and Solanki (2021) reported that CEFs appear ubiquitously in all types of sunspots. These authors found almost ∼400 CEFs on their survey and documented different types of CEFs.…”

“…These authors found almost ∼400 CEFs on their survey and documented different types of CEFs. In particular, those that appear in penumbrae with regular umbra, and those CEFs that are linked to light bridges (Castellanos Durán, Lagg, and Solanki 2021).…”

Estudio de las velocidades Doppler y su relación con las oscilaciones del campo magnético en la penumbra de las manchas solares

“…Los datos de velocidades Doppler y magnitud del campo magnético en la línea de la visión fueron calibrados para corregir los efectos orbitales del SDO. Adicionalmente, a los Doppler se les corrigió el efecto de la rotación diferencial del Sol, corrimiento al azul, entre otros efectos detallados por [8].…”