arXiv subjects

Rohan Eugene Louis

Publications and source records attributed to Rohan Eugene Louis.

4 recordsLinked to original sources

arXiv2026

A Modified Multi-Level Tracking Scheme for the Detection of Sunspot Umbral Dots

Umbral dots (UDs) are small-scale convective intrusions in the umbral core of sunspots and pores. Different methods have been used in the past to determine the physical properties of UDs. One of the methods typically used is multi-level tracking (MLT), which tags spatial structures at equi-spaced intensity levels from the highest level while progressing downward. A modified approach to the regular MLT is explored in this article that also uses the local intensity maxima with a change in the threshold condition to enclose a UD, such that diffuse UDs do not appear extended than they visually appear. The physical properties of UDs from these two MLT approaches are compared. The methods are implemented on high-resolution blue continuum images of four sunspots from the 50-cm Solar Optical Telescope on board Hinode. In addition, we introduce a density-based, spatial clustering routine for the first time to ascertain the differences resulting from the two tracking methods. The modified MLT approach yields an effective diameter with median values ranging from 250-310 km which is on average 70-90 km smaller than the regular MLT approach. The lower effective diameter in the modified method is associated with a reduced UD fill fraction of 12%-13% while the regular method yields 17-19%. However, these differences are still within the range of values cited by earlier works. On the other hand, the histogram of the mean intensity of UDs from both methods is nearly identical. The spatial clustering of UDs from both methods also shows very similar results. There is, however, a preferential spatial concentration of UDs, particularly at locations where the umbral core is highly irregular and in the vicinity of faint light bridges. The dependency of the localized clustering of UDs on the evolutionary phase of the sunspot and its magnetic complexity needs to be further explored.

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arXiv2024

Classification of circular polarization Stokes profiles in a sunspot using k-means clustering

The magnetic and velocity fields in sunspots are highly structured on small spatial scales which are encoded in the Stokes profiles. Our aim is to identify Stokes profiles in a sunspot which exhibit spectral characteristics that deviate from those associated with the Evershed flow and their spatial distribution. We employ a k-means clustering routine to classify Stokes V spectra in the penumbra of a sunspot. 75% of the penumbral region is dominated by profiles comprising two, nearly anti-symmetric lobes, while 21% of the area is occupied by three-lobed profiles that represent the Evershed flow returning to the photosphere. 4% of the area is dominated by four profile groups - Group 1: three-lobed profiles in which both the rest and strong downflowing component have the same polarity as the sunspot and seen exclusively in the light bridge. Group 2: single, red-lobed profiles over an area of about 2% seen at the outer penumbra in discrete patches that possibly signify the downflowing leg of an Omega-loop. Group 3: three-lobed/highly asymmetric profiles, where the rest and strong downflowing component have a polarity opposite the sunspot. These occupy 1.4% of the penumbral area over conspicuous, elongated structures or isolated patches in the outer penumbra and penumbra-QS boundary. Group 4: three lobed-profiles, in which the rest component has the same polarity as the sunspot and a weaker, upflowing component with an opposite polarity. These profiles are located near the entrance of the light bridge and are found in only 0.12% of the penumbral area. These minority groups of profiles could be related to dynamic phenomena that could affect the overlying chromosphere. The simplicity and speed of k-means can be utilized to identify such anomalous profiles in larger data sets to ascertain their temporal evolution and the physical processes responsible for these inhomogeneities.

astro-ph.SR↗

arXiv2019

Investigation of a confined C-class flare in an arch filament system close to a regular sunspot

A moderate C1.1 class confined flare is investigated here, which occurred on 2013 September 24 at 22:56~UT, in an arch filament system close to a regular, unipolar sunspot. Spectro-polarimetric observations from the Tenerife Infrared Polarimeter at the 70 cm German Vacuum Tower Telescope were combined with data from the Helioseismic Magnetic Imager and the Atmospheric Imaging Assembly to identify the processes that triggered the flare. The legs of this arch filament were anchored in the leading sunspot and the network flux region of opposite polarity. The flare was driven by small-scale, flux cancellation at the weak neutral line underlying the arch filament which resulted in two small flaring events within an hour of the C1.1 flare. Flux cancellation was facilitated by the moat flow from the leading sunspot wherein small-scale magnetic fragments stream towards patches of pre-existing flux. The cancellation of flux led to the destabilization of the arch filament which was seen as an increase in the twist along the arch filament. The horizontal fields across the weak neutral line decay faster which cannot prevent the filament from rising that results in a two-ribbon flare at the neutral line. The arch filament unwinds as it rises, but is confined by the higher, overlying fields between the two polarities of the active region that decay much more slowly.

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arXiv2009

Supersonic Downflows in a Sunspot Light Bridge

We report the discovery of supersonic downflows in a sunspot light bridge using measurements taken with the spectropolarimeter on board the Hinode satellite. The downflows occur in small patches close to regions where the vector magnetic field changes orientation rapidly, and are associated with anomalous circular polarization profiles. An inversion of the observed Stokes spectra reveals velocities of up to 10 km/s, making them the strongest photospheric flows ever measured in light bridges. Some (but not all) of the downflowing patches are cospatial and cotemporal with brightness enhancements in chromospheric Ca II H filtergrams. We suggest that these flows are due to magnetic reconnection in the upper photosphere/lower chromosphere, although other mechanisms cannot be ruled out.

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