## 3 Results
### Ionospheric Storms--Morphology and Background Mechanisms  
The main drivers of the ionospheric disturbances during a geomagnetic storm can be roughly categorized by four different methods: (i) enhanced high-energy particle precipitation; (ii) enhanced ionospheric electric currents and resulting Joule heating; (iii) enhanced electric fields; (iv) frictional heating at high latitudes, primarily induced by enhanced magnetospheric convection. While the direct effects of these mechanisms are mainly important at high and maybe at low (see for instance electric fields) latitudes, it has been argued that Joule and particle heating is globally the most critical factor causing the F region effects of geomagnetic storms [20; 21; 22; 23]. Such effects are still activated in high latitudes, but they are transported to lower latitudes through wind circulation and waves [21]. Long-lasting negative storm effects are the most typical feature of the ionospheric storm time response. Driven by the initial dependence of the ground-based communications on HF radio, the negative storms were the first to receive the community's attention. Today, it is believed that the physical mechanisms of the negative storms are more or less understood, and negative ionospheric storm effects are mainly attributed to changes in neutral gas composition. In this scenario, the Joule and particle heating in the high-latitude region causes thermal expansion. In this way, large temperature and pressure gradients drive horizontal winds that produce global changes in the thermospheric circulation, with the storm-induced winds blowing from high to low latitudes. The divergence of the winds at subauroral latitudes causes the upwelling of neutral gas in the thermosphere, and by transporting neutral molecules into much higher altitudes, produces a neutral composition disturbance zone (also known as \"composition bulge\") and decreases the ionospheric ionization at F region heights [24; 21; 25; 26]. Neutral composition changes are present even during geomagnetically quiet conditions, but confined in the polar region. They are relatively small and restricted to subauroral latitudes, even during moderately disturbed geomagnetic conditions, especially in the daytime sector. Both the amplitude and the extent of the composition changes increase with increasing geomagnetic activity, and they are transported to lower latitudes by both the storm-induced and regular wind circulation [24; 21]. Contrary to the case of negative ionospheric storms, positive storms have proven to be more challenging in their explanation. Even today, the research community has not arrived at a generally accepted explanation for their origin. Possible mechanisms proposed in the literature for the explanation of the observed positive storm effects mainly include [21; 22; 23; 25; 27; 28]:1. Neutral composition changes through the downwelling of neutral gas due to the convergence of neutral winds; ii. F layer uplifting due to vertical drifts. An increase in layer height leads to an increase in ionization density due to the reduced rate of ionization loss at higher altitudes. The F layer drift may be caused by two principal mechanisms: * Equatorward-directed winds: in this case, the ions and electrons feel a frictional force. The geomagnetic field-aligned component of this force pushes the ionization up to the inclined magnetic field lines, resulting in an uplifting of the F layer. Such winds may be caused either by large-scale wind circulation or as part of so-called traveling atmospheric disturbances (TADs). TADs are the superimposed result of impulse-like travelling disturbances formed by a wide spectrum of atmospheric gravity waves. These are launched by the injection of solar wind energy to the polar upper atmosphere. TADs propagate with high velocity towards lower latitudes carrying along equatorward-directed winds of moderate magnitude.