http://hdl.handle.net/2248/2 2026-10-11T00:07:58Z http://hdl.handle.net/2248/9066 Application of coherent structure tracking to solar Doppler maps to determine horizontal velocity fields at the sun's surface Sampoorna, M; Roudier, T; Paletou, F Context. Coherent structure tracking (CST) is a technique for determining solar surface horizontal flows at high spatial and temporal resolution by tracking the proper motion of granules. This technique has traditionally been applied to solar intensity images in the continuum, which clearly depict granular patterns. However, solar granulation is also visible in Dopplergrams. Aims. We show that CST can be applied to solar Dopplergrams to derive solar surface horizontal velocity fields with the same level of confidence as those determined by CST on intensity images. Methods. We applied CST to continuum intensity images and Dopplergrams obtained from SDO/HMI and from a numerical simulation of granulation. We compared the resulting solar surface horizontal velocity fields and their derivatives (namely, the horizontal divergence and the vertical component of the vorticity) for different time windows. Results. Pearson's linear global correlation coefficient (GCC) between horizontal velocity fields determined from CST on Doppler and on intensity images of a relatively less active Sun is about 73% for a 30-minute time average, while the corresponding local correlation coefficient (LCC) near the disk center is about 80%. For the divergence of the horizontal velocity field, we obtain a GCC of 72% and a near-disk-center LCC of 84%. The curl of the horizontal velocity field is noisier and exhibits somewhat reduced GCC and LCC. These coefficients increase with an increasing time window. We observe a similar trend for Spearman's and Kendall's rank-order correlation coefficients, although their values are somewhat smaller. The different correlation coefficients slightly decrease for a magnetically more active Sun with sunspots or emerging pores in plage regions. We obtain a high correlation between the horizontal flows derived by applying CST to intensity and vertical velocity maps from a numerical simulation. Open Access; Open Access article published by EDP Sciences, under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 2026-09-01T00:00:00Z http://hdl.handle.net/2248/9065 Discovery of an unbound flyby companion of UBC 63: In the immediate aftermath of a close encounter Biswas, Samrat; Medhi, B. J; Messina, S; Zhu, Zhanpeng; Qin, Songmei; Deb, Sukanta; Sheikh, A. H; Das, H. S; Tamura, Motohide; Maheswar, G; Sagar, R We reinvestigate the open cluster UBC 63 using the Gaia Data Release 3 and show that, rather than being a single cluster as previously classified, it is a compelling candidate for a double cluster undergoing an unbound flyby interaction. A Gaussian mixture model decomposition performed in the 5D astrometric space reveals the two statistically distinct components, namely UBC 63A (98 members; Age = 21 ± 4 Myr) and UBC 63B (148 members, Age = 562 ± 43 Myr). A significant age difference of ∆Age = 541 ± 43 Myr between the clusters, rules out coeval formation. Their 3D separation of 60 ± 29 pc at the birth-epoch of the younger cluster, hints at a possible common environment origin . At present, the system exhibits a 3D separation of 26 ± 8 pc, with a relative velocity of 3.60 ± 1.80 km s−1. Orbital integrations and N-body simulations of the pair suggest that the systems had a close encounter, reaching a separation of 7 ± 2 pc only ∼6 Myr ago and predict a rapid divergence to a separation of 491 ± 213 pc within the next ∼100 Myr. The low escape velocity (Vesc = 0.51 ± 0.12 km s−1) of the system compared to the relative 3D velocity indicates that they are gravitationally unbound. Their low tidal factors, elongated structures and populations extending beyond the Jacobi radii may reflect a strong transient tidal interaction between the clusters. Open Access; Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 2026-09-01T00:00:00Z http://hdl.handle.net/2248/9064 Understanding the travel-time asymmetry of acoustic waves in sunspots with time─distance helioseismology Li, Haiyu; Felipe, T; Khomenko, E; Tian, H; Rajaguru, S. P; Gao, Yuhang Mapping the subsurface structure and flow field of sunspots has been a challenging task for helioseismology. In this work, we investigate the propagation of acoustic waves in a sunspot in NOAA active region 11312 using time─distance helioseismology. Travel-times of waves traveling into and out of the sunspot are measured as functions of travel distance and azimuthal angle relative to the local radial direction. The same time─distance analysis is also applied to simulated data based on a magnetohydrostatic (MHS) model of the sunspot, and forward modeling of travel-times is performed using ray tracing based on both the MHS sunspot model and a magnetohydrodynamic (MHD) simulation. We find that both ingoing (traveling from the quiet area into the sunspot) and outgoing waves (traveling from the sunspot into the quiet area) have shorter travel-times than in the quiet Sun, with travel-time reductions of up to ∼40 s. The magnitude of the mean time shift is largest for waves traveling along the radial direction at small travel distances. A clear asymmetry is detected between ingoing and outgoing waves: outgoing waves generally exhibit shorter travel-times. This asymmetry is strongest for the radial direction and small travel distances, with differences exceeding 1 minute for 3.5 and 4.5 mHz waves. From the results of both observations and models, our analysis indicates that the overall reduction in travel-time could be primarily caused by the combined effects of Wilson depression, magnetic field, and wave-speed perturbations, while the ingoing─outgoing asymmetry could be partly attributable to subsurface flows. Although the forward-modeling results reproduce several qualitative features of the observations, quantitative discrepancies remain, highlighting limitations of current sunspot models and ray-theoretical approximations. Open Access; Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 2026-09-01T00:00:00Z http://hdl.handle.net/2248/9063 Solar spicule rotation driven by plasma curtain-vortex interactions Dey, Sahel; Chatterjee, Piyali; Erdelyi, Robertus Rotational and transverse motions are frequently observed in solar spicules, yet the physical origin of these motions remains debated. We investigate how apparent spicule rotation arises in a stratified, magnetized solar atmosphere using three-dimensional radiative magnetohydrodynamic (rMHD) simulations. We show that the observed spinning signatures can arise without intrinsic rotation of individual jets. Instead, spicules organize into fluted, curtain-like plasma structures. The evolving emission from these three-dimensional curtains, when projected onto the plane of the sky through line-of-sight integration, produces clustered spicule-like features with lifetimes, heights, and apparent speeds comparable to observed ranges. The synthetic spicules develop strong density gradients at their periphery that generate (i) baroclinic vorticity and also interact with (ii) vortical flows driven by magnetic tension in the surrounding plasma. The associated vortical flows in our simulation are organized as vertically extended rotating plasma columns that reach coronal heights---in some cases, the spicules directly feeding the swirling columns. As a result, recurrent spinning signatures emerge in projected jet clusters, consistent with commonly observed solar limb dynamics, including high-cadence observations from Hinode and IRIS as also shown here for comparison. These results link spicule rotation to jet--vortex coupling and show that rotating spicule clusters may trace vortical plasma structures that transport energy and momentum into the solar corona. Open Access; Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 2026-09-01T00:00:00Z