W. H. Chen

Ionospheric data assimilation with thermosphere-ionosphere-electrodynamics general circulation model and GPS-TEC during geomagnetic storm conditions

The main purpose of this paper is to investigate the effects of rapid assimilation-forecast cycling on the performance of ionospheric data assimilation during geomagnetic storm conditions. An ensemble Kalman filter software developed by the National Center for Atmospheric Research (NCAR), called Data Assimilation Research Testbed, is applied to assimilate ground-based GPS total electron content (TEC) observations into a theoretical numerical model of the thermosphere and ionosphere (NCAR thermosphere-ionosphere-electrodynamics general circulation model) during the 26 September 2011 geomagnetic storm period. Effects of various assimilation-forecast cycle lengths: 60, 30, and 10 min on the ionospheric forecast are examined by using the global root-mean-squared observation-minus-forecast (OmF) TEC residuals. Substantial reduction in the global OmF for the 10 min assimilation-forecast cycling suggests that a rapid cycling ionospheric data assimilation system can greatly improve the quality of the model forecast during geomagnetic storm conditions. Furthermore, updating the thermospheric state variables in the coupled thermosphere-ionosphere forecast model in the assimilation step is an important factor in improving the trajectory of model forecasting. The shorter assimilation-forecast cycling (10 min in this paper) helps to restrain unrealistic model error growth during the forecast step due to the imbalance among model state variables resulting from an inadequate state update, which in turn leads to a greater forecast accuracy.

Ionosphere data assimilation modeling of 2015 St. Patrick's Day geomagnetic storm

The ionospheric plasma disturbances during a severe storm can affect human activities and systems, such as navigation and HF communication systems. Therefore, the forecast of ionospheric electron density is becoming an important topic recently. This study is conducted with the ionospheric assimilation model by assimilating the total electron content observations into the thermosphere-ionosphere coupling model with different high-latitude ionospheric convection models, Heelis and Weimer, and further to forecast the variations of ionospheric electron density during the 2015 St. Patricks Day geomagnetic storm. The forecast capabilities of these two assimilation models are evaluated by the root-mean-square error values in different regions to discuss its latitudinal effects. Results show the better forecast in the electron density at the low-latitude region during the storm main phase and the recovery phase. The well reproduced eastward electric field at the low-latitude region by the assimilation model reveals that the electric fields may be an important factor to have the contributions on the accuracy of ionospheric forecast.

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.