Coexistence of ionospheric positive and negative storm phases under northern winter conditions: A case study

The response of the thermosphere and ionosphere to the famous January 10, 1 997, geomagnetic storm is simulated using the thermosphere-ionosphere-elect rodynamics general circulation model with realistic, time-dependent distrib utions of ionospheric convection and auroral precipitation as inputs. The s imulation results show a dominant positive storm phase of increased F layer electron density over much of the northern winter hemisphere, but a negati ve storm phase with reduced electron density at middle and low latitudes is also evident in the simulation. The coexistence of both positive and negat ive storm phases is a result of the complex dynamical and chemical interact ions between charged particles and neutral gases. The impulsive magnetosphe ric energy inputs via auroral precipitation and Joule heating generate trav eling atmospheric and ionospheric disturbances (TADs and TIDs) which propag ate from the northern auroral zone to lower latitudes and penetrate well in to the Southern Hemisphere. The simulation results demonstrate that positiv e storm phases are caused primarily by enhanced auroral precipitation over high latitudes and by TIDs at middle and low latitudes. Globally speaking, composition changes in terms of enhancements in the N-2/O ratio are mainly responsible for negative storm effects. However, although there is some cor relation between increases in N-2/O and decreases in the F layer critical f requency f(o)F(2) in the winter hemisphere during the storm main phase and early recovery phase, the overall changes in f(o)F(2) are also determined b y other processes, such as the ionization production associated with enhanc ed auroral precipitation and the variations associated with TIDs. In the lo w to middle-latitude region changes in f(o)F(2) approximately anticorrelate with changes at the height of the F layer electron density peak (e.g., h(m )F(2)) at 70 degreesW during the storm main phase as well as its early reco very phase. This is attributed in part to the relation that exists between meridional wind velocity and vertical shear of that velocity for aurorally produced TADs.