Evolution of the flow field in decaying active regions

Strecker, Hanna; González, N. Bello

doi:10.1051/0004-6361/202142564

Cited by 4 publications

(2 citation statements)

References 27 publications

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“…1. INTRODUCTION Evershed flows are characteristic outflows observed in the penumbrae of sunspots (Evershed 1909) with typically subsonic velocities of ∼1-3 km s −1 in the body of the filament (e.g., Schlichenmaier & Schmidt 2000;Strecker & Bello González 2022) and 5-10 km s −1 at the endpoints (e.g. Tiwari et al 2013).…”

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confidence: 99%

How rare are counter Evershed flows?

Durán

Lagg

Solanki

2021

A&A

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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.

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confidence: 99%

How rare are counter Evershed flows?

Durán

Lagg

Solanki

2021

A&A

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“…Evershed flows are characteristic outflows observed in the penumbrae of sunspots (Evershed 1909) with typically subsonic velocities of ∼1-3 km s −1 in the body of the filament (e.g., Schlichenmaier & Schmidt 2000;Strecker & Bello González 2022) and 5-10 km s −1 at the endpoints (e.g., Tiwari et al 2013). The characteristic filamentary structure of penumbrae observed in continuum images is the result of the interaction between buoyant convective cells rising from the solar interior and inclined magnetic field (see Borrero & Ichimoto 2011, for reviews).…”

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confidence: 99%

Expulsion of Counter Evershed Flows from Sunspot Penumbrae

Durán

Solanki

2023

ApJ

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In addition to the Evershed flow directed from the umbra toward the outer boundary of a sunspot, under special circumstances a counter Evershed flow (CEF) in the opposite direction also occurs. We aim to characterize the proper motions and evolution of three CEFs observed by the Solar Optical Telescope on board the Japanese Hinode spacecraft and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We use state-of-the-art inversions of the radiative-transfer equation of polarized light applied to spectropolarimetric observations of the Fe i line pair around 630 nm. The three CEFs appeared within the penumbra. Two of the CEF structures, as part of their decay process, were found to move radially outwards through the penumbra parallel to the penumbral filaments with speeds, deduced from their proper motions, ranging between 65 and 117 m s−1. In these two cases, a new spot appeared in the moat of the main sunspot after the CEFs reached the outer part of the penumbra. Meanwhile, the CEFs moved away from the umbra, and their magnetic field strengths decreased. The expulsion of these two CEFs seems to be related to the normal Evershed flow. The third CEF appeared to be dragged by the rotation of a satellite spot. Chromospheric brightenings were found to be associated with the CEFs, and those CEFs that reached the umbra–penumbra boundary showed enhanced chromospheric activity. The two CEFs, for which line-of-sight velocity maps were available during their formation phase, appear as intrusions into the penumbra. They may be associated with magnetic flux emergence.

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