exoALMA XXV: A Statistical Analysis of Gaseous Substructure in Low-Inclination Disks
Protoplanetary disks exhibit complex structure in both their gas dynamics and their temperature. Characterizing it has proven challenging, owing to the mismatch between the observed complexity and the relatively simple semi-analytical models typically used to interpret it. To augment these analyses, we apply the second-order structure function to the velocity and temperature perturbations of four low-inclination sources from the exoALMA program, each measured against a noise-only null built from the data themselves. All exhibit correlated structure. Its radial correlation length is limited by the beam in every case, which excludes non-axisymmetric structure on radial scales larger than ~1.5 pressure scale heights. In three of the four disks the structure is instead resolved and elongated in azimuth, by factors of 1.6-2.3, as expected of small-scale perturbations sheared out by Keplerian rotation. We further introduce two statistics that characterize spiral structure without tracing an arm crest: a modal power spectrum, measuring the periodicity of the azimuthal structure, and a winding contrast, measuring the radial dependence of its phase. The velocity residuals of three disks are significantly wound. MWC 758 is the only source in which the winding is coherent across the disk and shared by the temperature field. Its trailing m=1 temperature structure is consistent with the illumination of a warp, while its velocity field carries small-scale, sheared structure that could redistribute angular momentum. Because neither statistic requires a crest, both extend a quantitative description of winding to the flocculent regime, where a spiral cannot be parameterized.