Radio Burst Phenomenology of AD Leonis and Associated Signatures of Propagation Effects

Abstract

We present the high-resolution radio dynamic spectra of AD Leonis (AD Leo) between 1.0 and 1.5 GHz taken by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) on Dec. 1st, 2023. Over a 15-minute period, we identify complex, superimposed spectro-temporal structures, including: (1) broadband, second-long modulation lanes with downward frequency drifts, (2) narrowband ($\approx$ 50 MHz), short-duration S-burst envelopes with upward drifts, and (3) even narrower ($\approx$ 10 MHz), millisecond-scale S-burst striae within these envelopes. Using the discrete Fourier transform and auto-correlation function, we identify two dominant periodic emission patterns, corresponding to the periodicities of the S-bursts ($\approx0.1$ s) and the striae ($\approx0.01$ s). The complex superposition of diverse time-frequency structures poses a challenge to interpreting all the emission variability as intrinsic to the source. We propose that the modulation lanes could be a propagation effect as the radio waves traverse an inhomogeneous, regularly structured plasma region in the AD Leo's magnetosphere. By modelling a plasma screen with sinusoidal phase variation in one dimension, we show that we could qualitatively reconstruct the observed modulation lanes. The origin of the finest structures, the striae, remains unclear. Our work highlights that propagation effects in the stellar magnetosphere can potentially probe kilometre-scale structures in the emission regions and provide novel constraints on density inhomogeneities caused by magnetohydrodynamic waves that are difficult to access by other means.