2025ApJ...991...92T

The Milky Way's disk-halo interface mediates energy and mass exchange between the interstellar thin disk and the halo. In the first detailed study of the Perseus arm's disk-halo interface, we combine Hubble Space Telescope/Space Telescope Imaging Spectrograph and Cosmic Origins Spectrograph absorption spectra toward six stars and 23 active galactic nuclei (AGNs) projected behind a narrow section (95° < l < 145°, -46° < b < 0°). This provides a unique data set that bridges the disk and its extended vertical structure in these directions. We measure S II, Si IV, and C IV absorption, along with H I 21 cm emission, at heights -70 pc to -3.3 kpc from the midplane. The arm's southern vertical structure shows distinct height-dependent behaviors: H I and S II column densities sharply decline with height up to 1.5 kpc, then continue declining at a much shallower rate at greater heights. In contrast, high ion (Si IV and C IV) column densities remain relatively constant throughout the entire height range. In the disk-halo interface, where warm neutral medium dominates, S II and the high ions show similar kinematics, and we find a remarkably uniform C IV/Si IV ratio (〈N_C IV_/N_Si IV_〉 = 2.5 ± 0.5) within -0.9 to -3.25 kpc. Both the kinematic correspondence and high-ion ratio are consistent with the high ions probing turbulent mixing layers at the interfaces between warm/cool and hot gas phases. The AGN sight lines reveal minimal circumgalactic medium contribution in the low-velocity gas at ∣v_LSR∣ < 100 km s^–1. The extraplanar absorbing gas may trace material ejected from previous Galactic fountain activity.