6 Key Points 7 • Variability of diurnal and semidiurnal internal tides within a mesoscale warm eddy is 8 detected via virtual-moored single-glider observations. 9 • Diurnal internal tides’ vertical structure changes noticeably for vertical displacement and 10 available potential energy when moving away from the eddy center. 11 • The vertical structure of diurnal internal tides within a mesoscale eddy varies with Abstract 15 Internal waves are ubiquitous ocean features that significantly contribute to diapycnal mixing, 16 and their modulation by mesoscale eddies is crucial to understanding their propagation and 17 dissipation. From an experiment as a pilot program of the Slocum glider for virtual-moored 18 profiling in a mesoscale eddy, a unique conductivity-temperature-depth (CTD) dataset was 19 obtained and used in this study for examining modulated internal waves within the eddy center. 20 Internal tide variability is detected within the eddy, where diurnal internal tides (DITs) 21 overwhelm other frequency internal waves. DITs’ vertical structures change dramatically for 22 vertical displacements and available potential energy (APE), depending on horizontal positions 23 near the eddy center. The observed behavior of DITs’ low vertical wavenumbers indicates a 24 cascade of energy from low to high modes, likely due to the hierarchy of wave-eddy interactions. 25 Especially, distinct behavior near the eddy’s inner and outer centers indicate different interaction 26 strengths on each regime. 27 Plain Language Summary 28 In this work, actively targeted CTD measurements within a mesoscale eddy were conducted 29 using a single Slocum glider. First, the virtual-mooring mode of a glider was successfully 30 controlled within a 720 m root-mean-square (RMS) around the waypoint and yielded a reliable 31 dataset to capture the internal tide variations. Second, based on the gridded version of raw data in 32 a 1 h and 1 m vertical resolution, diurnal and semidiurnal internal tides are observed in the 33 bandpassed time-depth isothermal displacement maps. Third, the vertical structure of vertical 34 isothermal displacements, as well as the vertical distribution of available potential energies, 35 shows a varying pattern depending on horizontal positions near the eddy center. It is more 36 variable in the inner center and less variable in the outer center. Lastly, there appears a weak clue 37 of energy-cascading behavior from low to higher modes for spectral behavior of low vertical 38 manuscript submitted to Geophysical Research Letters wavenumbers computed during the observation, indicating that the nonlinear interactions among 39 the waves and the eddy have different strengths in the inner and outer centers: strong in the inner 40 center and weak in the outer center. 41