Small-scale bright point characteristics at high-resolution with the Daniel K. Inouye Solar Telescope

TL;DR

This study presents the first analysis of bright points (BPs) at DKIST's highest spatial resolution using the VBI G-band sequence of quiet-Sun regions, identifying $12{,}486$ BPs and revealing a log-normal area distribution with a peak near $2300$ km$^2$ and an average lifetime of $95\pm29$ s. The results show a mean transverse BP velocity of $1.60\pm0.41$ km s$^{-1}$ and a super-diffusive motion with a diffusion index $\gamma\approx1.2$, consistent with prior work. By degrading the DKIST data to the PSFs of GREGOR, SST, and DST, the study demonstrates how spatial resolution and seeing can shift the observed area peak to much larger values and broaden velocity distributions, while a subset of the best frames approximates DKIST's intrinsic statistics. The authors emphasize that seeing fluctuations and boundary-detection choices strongly affect small-scale BP statistics, underscoring the need for consistent high-quality seeing and polarimetric data to fully characterize the magnetic structure and dynamics of BPs at the smallest solar scales.

Abstract

Bright points (BPs) are small-scale, dynamic features that are ubiquitous across the solar disc and are often associated with the underlying magnetic field. Using broadband photospheric images obtained with the Visible Broadband Imager at the National Science Foundation's Daniel K. Inouye Solar Telescope (DKIST), the properties of BPs have been analyzed with DKIST for the first time at the highest spatial resolutions achievable. BPs were observed to have an average lifetime of 95$\pm$29 s and a mean transverse velocity of 1.60$\pm$0.41 km/s. The BPs had a log-normal area distribution with a peak at 2300 km$^2$. Transverse velocity and lifetimes across the DKIST images were comparable and consistent with previous studies. The area distribution of the DKIST data peaked in areas significantly lower than those from the literature. This was explored further and was observed to be due to an overestimation of BP areas due to the merging of close features when the spatial resolution is reduced, in tandem with possible over-splitting of features in the DKIST images. Furthermore, the effect of variable seeing within the data was determined. This showed that the average spatial resolution of the data was around 0.''034$\pm$0.''007 in comparison to the theoretical diffraction-limit of 0.''022. Accounting for the influence of seeing, the peak of the area distribution of BPs in the DKIST data was estimated as 4800 km$^2$, which is still significantly lower than previously observed.

Figures (8)

• Figure 1: A sample image of the quiet Sun data obtained with VBI on 26 May 2022 with the G-band filter at 17:50UT. The images have a spatial sampling of $0{\,}.{\!\!}{"}011$ pixel$^{-1}$ and a cadence of around 6 s. The estimated spatial resolution of this image is $0{\,}.{\!\!}{"}032$ ($\sim 23$ km). The full field-of-view of the image can be seen in the left hand panel. The red box indicates the region shown in the zoomed in right hand panel. The zoom indicates the typical structure and configurations of the bright points under investigation at the resolution of DKIST. Examples of both elongated chains, bright point groups, as well as isolated point-like features, can be seen.
• Figure 2: Samples images of the DKIST data degraded to match the resolution of other commonly used ground-based facilities (GREGOR, SST and DST). Degradation was performed using a similar methodology as outlined in Campbell2021. The top panel shows the zoomed image from Figure \ref{['fig_fov']} of the original DKIST image. The labeled panels below show, from left to right, the DKIST data degraded to the equivalent resolution of GREGOR, the SST and the DST, respectively. This figure shows the effect of spatial resolution on the observed structure of BPs for both BP chains and isolated features.
• Figure 3: The variation of spatial resolution over the course of observations. The spatial resolution is estimated with Fourier techniques outlined in Beck2007. The zero time is the first frame of the observations obtained at 17:46UT on 2022 May 26. The plot is limited to images prior to an AO lock point jump, which occurs at about 90 minutes into the sequence. The drop in image quality prior to the loss of the AO lock is evident in the plot. The red dot-dashed line indicates the theoretical diffraction-limited resolution of the data acquired. The average spatial resolution for the sequence is $0{\,}.{\!\!}{"}034\pm0{\,}.{\!\!}{"}007$.
• Figure 4: The area distribution of BPs identified in the datasets. The solidblack line represents the original DKIST data, while the dot-dashed green, dashed orange, and dot-dot-dashed blue lines represent the area distributions for data degraded to the resolution of GREGOR, SST and DST, respectively. The dotted lines indicate the corresponding log-normal fits for each of the distributions. The peak of the distributions occurs at approximately $2300$ km$^2$, $13{\,}700$ km$^2$, $25{\,}700$ km$^2$ and $26{\,}200$ km$^2$ for the DKIST, GREGOR, SST, and DST data, respectively.
• Figure 5: The area distribution of DKIST BPs adjusted for variations in seeing across the data set. The filtered DKIST distribution is displayed with the solidblack line while the associated DKIST data degraded to the resolution of other facilities, as shown in Figure \ref{['degraded_area_dist']}, are included here again for reference. Again, the dotted lines show the associated log-normal fits for the distributions. Accounting for the variable seeing conditions across the data set shifts the peak location to around $4800$ km$^2$. The peak of the distribution occurs at smaller areas than those of the degraded data sets; however, accounting for seeing returns a distribution shape that matches closer to those previously reported in the literature for facilities with reduced spatial resolution. The effects of seeing are pronounced with the DKIST data, so care must be taken in analyzing results obtained with these data.