M. Y. Chou

Equatorial plasma bubble generation / inhibition during 2015 St. Patrick's Day storm

All sky camera observations carried out over Taiwan showed intense equatorial plasma bubbles (EPB) in 630.0 nm airglow images on consecutive nights of 13-16 March, 2015, but was absent in the following night of 17 March when St. Patricks Day magnetic storm occurred. Rate of total electron content (TEC) index by using Global Positioning System (GPS) network data also confirmed the absence of irregularities on the night 17 March. The results however revealed strong irregularities over Indian sector on the same night. Flux tube integrated Rayleigh-Taylor instability growth rates computed using the prior (forecast) state of Thermosphere-Ionosphere Electrodynamics General Circulation Model output after assimilating the GPS-TEC measurements also agree with the observations, showing smaller values over Taiwan and larger values over India on the night of 17 March. The ionospheric response to the storm over Taiwan that resulted in the apparent inhibition of EPB is investigated in this study by using the data assimilation output. Results indicate that on the night of the magnetic storm, pre-reversal enhancement of zonal electric field over Taiwan was weaker when compared to that over India. Further analysis suggests that the absence of enhancement in the zonal electric field could be due to westward penetration electric field in response to rapid northward turning of interplanetary magnetic field that occurred during the dusk period over Taiwan.

Gigantic Circular Shock Acoustic Waves in the Ionosphere Triggered by the Launch of FORMOSAT‐5 Satellite

The launch of SpaceX Falcon 9 rocket delivered Taiwan’s FORMOSAT-5 satellite to orbit from Vandenberg Air Force Base in California at 18:51:00 UT on 24 August 2017. To facilitate the delivery of FORMOSAT-5 to its mission orbit altitude of ~720 km, the Falcon 9 made a steep initial ascent. During the launch, the supersonic rocket induced gigantic circular shock acoustic waves (SAWs) in total electron content (TEC) over the western United States beginning approximately 5 min after the liftoff. The circular SAWs emanated outward with ~20 min duration, horizontal phase velocities of ~629–726 m/s, horizontal wavelengths of ~390–450 km, and period of ~10.28 ± 1 min. This is the largest rocket-induced circular SAWs on record, extending approximately 114–128°W in longitude and 26–39°N in latitude (~1,500 km in diameter), and was due to the unique, nearly vertical attitude of the rocket during orbit insertion. The rocket-exhaust plume subsequently created a large-scale ionospheric plasma hole (~900 km in diameter) with 10–70% TEC depletions in comparison with the reference days. While the circular SAWs, with a relatively small amplitude of TEC fluctuations, likely did not introduce range errors into the Global Navigation Satellite Systems navigation and positioning system, the subsequent ionospheric plasma hole, on the other hand, could have caused spatial gradients in the ionospheric plasma potentially leading to a range error of ~1 m. Plain Language Summary On 24 August 2017, a SpaceX Falcon 9 rocket departed from Vandenberg Air Force Base in California, carrying Taiwan’s FORMOSAT-5 Earth observation satellite into orbit. The lightly weighted solo payload enables the rocket to fly a lofted trajectory for direct insertion at the mission altitude of 720 km. This unique nearly vertical trajectory is different from the usual satellite launches that the rockets fly over horizontal trajectory and insert satellites at 200 km altitude followed by orbit maneuvers to its mission altitudes. Consequently, the rocket launch generated a gigantic circular shock wave in the ionosphere covering a wide area four times greater than California. It is followed by ionospheric hole (plasma depletions) due to rapid chemical reactions of rocket exhaust plumes and ionospheric plasma. The ionospheric hole causing large spatial gradients could lead to ~1 m range errors into GPS navigation and positioning system. Understanding how the rocket launches affect our upper atmosphere and space environment is important as these anthropogenic space weather events are expected to increase at an enormous rate in the near future.