Unprecedented 19 Day Type IV Radio Burst as a Corotating Electron Reservoir

Abstract We report a hectometric (0.5–3 MHz) type IV continuum observed over nearly 19 days (2025 August 21–September 9) in three successive visibility windows as the source corotates with the Sun—first at Solar Orbiter, then 12 days later in the near-Earth sector where Wind and Parker Solar Probe observed overlapping intervals, and finally 1 day later at STEREO-A—indicating a long-lived source that rotates through favorable viewing geometries. During the STEREO-A interval, the continuum is strongly left-hand circularly polarized (∣ V / I ∣ ≳ 0.9) and quasiperiodic pulsations (45–60 minutes). We interpret these quasiperiodic pulsations as consistent with standing fast-mode MHD oscillations of a large coronal trap. We introduce a single-spacecraft localization technique, the wavevector-corrected ray sphere (WCRS), which applies a correction to direction-finding angles combined with a coronal density model. WCRS places the source near a helmet streamer. Independent size estimates from solar rotation geometry and magnetoseismology are consistent with a transverse diameter of W ≈ 2.5–3.0 R ⊙ at the inferred source heights ( r ≃ 6–10 R ⊙ ), with the seismology providing a lower bound. The goniopolarimetric apparent half-width at the spacecraft ( γ ≈ 20°) exceeds the geometric width by a factor of B ∼ 60 ± 18 , highlighting strong interplanetary scattering. Three fast coronal mass ejections (CMEs) plausibly helped sustain the electron population (e.g., by reorganizing the large-scale trap and/or enabling continued injection from the low corona), maintaining the continuum. This event sets a new duration benchmark for type IV emission and shows that WCRS enables single-spacecraft localization, with potential relevance for operational space weather forecasting, including single-point tracking of CME-driven shocks (type II bursts) and open field connections (type III bursts).