Evaluation of extreme ionospheric total electron content gradient associated with plasma bubbles for GNSS Ground-Based Augmentation System

Associated with plasma bubbles, extreme spatial gradients in ionospheric total electron content (TEC) were observed on 8 April 2008 at Ishigaki (24.3°N, 124.2°E, +19.6° magnetic latitude), Japan. The largest gradient was 3.38 TECU km⁻¹ (total electron content unit, 1 TECU = 10¹⁶ el m⁻²), which is equivalent to an ionospheric delay gradient of 540 mm km⁻¹ at the GPS L1 frequency (1.57542 GHz). This value is confirmed by using multiple estimating methods. The observed value exceeds the maximum ionospheric gradient that has ever been observed (412 mm km⁻¹ or 2.59 TECU km⁻¹) to be associated with a severe magnetic storm. It also exceeds the assumed maximum value (500 mm km⁻¹ or 3.08 TECU km⁻¹) which was used to validate the draft international standard for Global Navigation Satellite System (GNSS) Ground-Based Augmentation Systems (GBAS) to support Category II/III approaches and landings. The steepest part of this extreme gradient had a scale size of 5.3 km, and the front-normal velocities were estimated to be 71 m s⁻¹ with a wavefront-normal direction of east-northeastward. The total width of the transition region from outside to inside the plasma bubble was estimated to be 35.3 km. The gradient of relatively small spatial scale size may fall between an aircraft and a GBAS ground subsystem and may be undetectable by both aircraft and ground.