A steady-state supersonic downflow in the transition region above a sunspot umbra

Straus, T.; Fleck, B.; Andretta, V.

doi:10.1051/0004-6361/201525805

Cited by 30 publications

(83 citation statements)

References 54 publications

(105 reference statements)

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“…It should also be mentioned that a downflow is usually observed in chromospheric lines in plumes (Straus et al 2015;Chitta et al 2016), which might be associated with coronal rain or a siphon flow. The downflowing coronal material interacting with the dense lower atmosphere could result in heating and enhancement of the radio emission.…”

Section: Origin Of the Umbral Brightness Enhancementmentioning

confidence: 99%

ALMA Discovery of Solar Umbral Brightness Enhancement at λ = 3 mm

Iwai

Loukitcheva

Shimojo

et al. 2017

ApJL

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We report the discovery of a brightness enhancement in the center of a large sunspot umbra at a wavelength of 3 mm using the Atacama Large Millimeter/sub-millimeter Array (ALMA). Sunspots are amongst the most prominent features on the solar surface, but many of their aspects are surprisingly poorly understood. We analyzed a λ=3 mm (100 GHz) mosaic image obtained by ALMA, which includes a large sunspot within the active region AR12470 on December 16, 2015. The 3 mm map has a 300"×300" field-of-view and 4".9×2".2 spatial resolution, which is the highest spatial-resolution map of an entire sunspot in this frequency range. We find a gradient of 3 mm brightness from a high value in the outer penumbra to a low value in the inner penumbra/outer umbra. Within the inner umbra, there is a marked increase in 3mm brightness temperature, which we call an umbral brightness enhancement. This enhanced emission corresponds to a temperature excess of 800 K relative to the surrounding inner penumbral region and coincides with excess brightness in the 1330 and 1400 Å slitjaw images of the Interface Region Imaging Spectrograph (IRIS), adjacent to a partial lightbridge. This λ=3 mm brightness enhancement may be an intrinsic feature of the sunspot umbra at chromospheric heights, such as a manifestation of umbral flashes, or it could be related to a coronal plume since the brightness enhancement was coincident with the footpoint of a coronal loop observed at 171 Å.

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Section: Origin Of the Umbral Brightness Enhancementmentioning

confidence: 99%

ALMA Discovery of Solar Umbral Brightness Enhancement at λ = 3 mm

Iwai

Loukitcheva

Shimojo

et al. 2017

ApJL

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“…They interpreted the observations as cool material falling as coronal rain along thermally unstable loops. They observed fast apparent downflows in the 1400 Å slit-jaw time sequence, suggesting that the material is indeed cooler plasma at the TR temperatures, i.e., around 10 5 K. Straus et al (2015) investigated another supersonic downflow event in a sunspot above the lightbridge. They identified that the downflow components are steady for almost 80 min and show up as "satellite" lines of Si iv and O iv TR lines, redshifted by ≈90 km s −1 .…”

Section: Introductionmentioning

confidence: 99%

A closer look at a coronal loop rooted in a sunspot umbra

Chitta

Peter

Young

2016

A&A

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Context. Extreme UV (EUV) and X-ray loops in the solar corona connect regions of enhanced magnetic activity, but they are not usually rooted in the dark umbrae of sunspots because the strong magnetic field found there suppresses convection. This means that the Poynting flux of magnetic energy into the upper atmosphere is not significant within the umbra as long as there are no light bridges or umbral dots. Aims. Here we report a rare observation of a coronal loop rooted in the dark umbra of a sunspot without any traces of light bridges or umbral dots. This allows us to investigate the loop without much confusion from background or line-of-sight integration effects. Methods. We used the slit-jaw images and spectroscopic data from the Interface Region Imaging Spectrograph (IRIS) and concentrate on the line profiles of O iv and Si iv that show persistent strong redshifted components in the loop rooted in the umbra. Using the ratios of O iv, we can estimate the density and thus investigate the mass flux. The coronal context and temperature diagnostics of these observations is provided through the EUV channels of the Atmospheric Imaging Assembly (AIA). Results. The coronal loop, embedded within cooler downflows, hosts supersonic downflows. The speed of more than 100 km s −1 is on the same order of magnitude in the transition region lines of O iv and Si iv, and is even seen at comparable speed in the chromospheric Mg ii lines. At a projected distance of within 1 of the footpoint, we see a shock transition to smaller downflow speeds of about 15 km s −1 being consistent with mass conservation across a stationary isothermal shock. Conclusions. We see no direct evidence for energy input into the loop because the loop is rooted in the dark uniform part of the umbra with no light bridges or umbral dots near by. Thus one might conclude that we are seeing a siphon flow driven from the footpoint at the other end of the loop. However, for a final result data of similar quality at the other footpoint are needed, but this is too far away to be covered by the IRIS field of view.

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“…Recent observations from the Solar Optical Telescope (SOT) on board Hinode, the Interface Region Imaging Spectrograph (IRIS), and ground-based observatories such as the Swedish Solar Telescope have demonstrated coronal rain is a common occurrence (e.g. Antolin et al 2010;Antolin & Rouppe van der Voort 2012;Kleint et al 2014;Antolin et al 2015;Straus et al 2015). Coronal rain accelerates towards the solar surface at overall average values of around 80±30 m s −2 (Antolin & Rouppe van der Voort 2012), which is considerably smaller than solar gravitational acceleration, even when taking into account the effective gravity in the direction of the guiding magnetic field.…”

Section: Introductionmentioning

confidence: 99%

Excitation and evolution of vertically polarised transverse loop oscillations by coronal rain

Verwichte

Kohutova

2017

A&A

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Context. Coronal rain is composed of cool dense blobs that form in solar coronal loops and are a manifestation of catastrophic cooling linked to thermal instability. The nature and excitation of oscillations associated with coronal rain is not well understood. Aims. We consider observations of coronal rain in a bid to elucidate the excitation mechanism and evolution of wave characteristics. Methods. We analyse IRIS and Hinode/SOT observations of an oscillating coronal rain event on 17 Aug 2014 and determine the wave characteristics as a function of time using tried and tested time-space analysis techniques. Results. We exploit the seismological capability of the oscillation to deduce the relative rain mass from the oscillation amplitude. This is consistent with the evolution of the oscillation period showing the loop loosing a third of its mass due to falling coronal rain in a 10-15 minute time period. Conclusions. We present first evidence of the excitation of vertically polarised transverse loop oscillations triggered by a catastrophic cooling at the loop top and consistent with two thirds of the loop mass comprising of cool rain mass.

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A steady-state supersonic downflow in the transition region above a sunspot umbra

Cited by 30 publications

References 54 publications

ALMA Discovery of Solar Umbral Brightness Enhancement at λ = 3 mm

ALMA Discovery of Solar Umbral Brightness Enhancement at λ = 3 mm

A closer look at a coronal loop rooted in a sunspot umbra

Excitation and evolution of vertically polarised transverse loop oscillations by coronal rain

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