Time dependent thermospheric neutral response tothe 2–11 November 1993 storm period

Abstract

Many satellite and ground-based observations from 2–11 November 1993 werecombined in the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure toderive realistic time dependent global distributions of the auroral precipitation and ionosphericconvection. These were then used as inputs to the Thermosphere–Ionosphere–ElectrodynamicsGeneral Circulation Model (TIEGCM) to simulate the thermospheric and ionospheric responseduring the storm period. The November 1993 storm was an unusually strong storm associatedwith a recurring high speed stream of solar plasma velocity in the declining phase of the solarcycle. Significant gravity waves with phase speeds of about 700 m/s caused by Joule heating werepresent in the upper thermosphere as perturbations to the neutral temperature and wind fields,especially on 4 November. The observed gravity waves in the meridional wind and in the height ofthe electron density peak at several southern hemisphere stations were generally reproduced bythe model using the AMIE high latitude inputs. Both model and observed equatorward windswere enhanced during the peak of the storm at Millstone Hill and at Australian ionosondestations. The observed neutral temperature at Millstone Hill increased about 400 K during thenight on 4 November, returning to normal on 9 November, while the model increased 300 K thefirst night at that location but was still elevated on 11 November. Enhanced westward windsduring the storm were evident in the UARS WIND Imaging Interferometer (WINDII) data. Theenhanced westward winds in the model were largest around 40–45° magnetic latitude at night,and also tended to be largest in the longitudes containing the magnetic poles. The peak westwardwind enhancements at 0 LT reached about 250 m/s at 300 km, and about 100 m/s at 125 km thefirst day of the storm at 40° magnetic latitude. At 20° magnetic latitude, the maximum westwardwind enhancements at 125 km at 0 LT appeared 2–4 days after the major part of the storm,indicating very long time constants in the lower thermosphere. The model showed global averageneutral temperature enhancements of 188 K after the peak of the storm that decayed with time,and which correlated with variations 8 h earlier in the Dst index and in the electric potential dropinput from AMIE. The global average temperature enhancement of 188 K corresponded to apotential drop increase of only about 105 kV. The results showed that the TIEGCM usingrealistic AMIE auroral forcings were able to reproduce many of the observed time dependentfeatures of this long-lived geomagnetic storm. The overall global average exospheric temperaturevariation correlated well with the time variation of the cross-tail potential drop and the Dst indexduring the storm period. However, the enhanced westward winds at mid-latitudes were stronglyrelated to the corrected Joule heating defined by the time dependent AMIE inputs.