## 4 Discussion and Conclusive Remarks  
This paper attempts to provide a brief survey of knowledge on ionospheric storms, with the emphasis on the ionospheric effects at middle latitudes and the F region. As the ionosphere merges the influence from both above (solar wind/magnetosphere) and below (neutral atmosphere), ionospheric storms act as an important link in the complex chain of solar-terrestrial relations. In this respect, their investigation establishes a wide field of research that combines interdisciplinary expertise and/or knowledge. To keep the present survey brief, but at the same time, as complete as possible, the discussion is developed in a rather simplified way. For a fully justified overview of the present understanding on the ionospheric storms morphology and background mechanisms, the reader is urged to refer to extended reviews, collections of papers or chapter books available in the literature [1, 21, 22, 23, 24, 25, 53, 54]. Following many years of investigation, the physical mechanisms for the origin and occurrence of ionospheric storm effects are more or less in place. The situation is clearer in case of negative ionospheric storm effects, which are mainly attributed to changes in the neutral gas composition that are driven by Joule, and particle heating in the high-latitude region and consequent thermal expansion. Positive storm effects are more complicated in their explanation. They can be activated either also by neutral composition changes or the F layer uplifting, due to vertical drifts that bring the F layer higher where the rate of ionization loss is lower. The F layer drift may be caused by two principal mechanisms: equatorward-directed winds and electric fields (disturbance dynamo electric fields or the penetration of electric fields originated from the solar wind/magnetosphere). The advection of high-density plasma can take place in combination with plasma uplift, or through the interaction with the plasmasphere, and further explain observed ionization increases. Among the proposed mechanisms, vertical drifts seem to be the dominant ones, but the relative importance of equatorward winds and electric fields in the generation of ionization increases, especially in the middle latitudes, is still an open issue for the research community. The occurrence of ionospheric storm effects is subject to several dependencies. Today, seasonal and local time dependencies are rather well justified and satisfactorily interpreted through the relative blow of storm-induced and regular thermospheric winds that keep the neutral composition disturbances confined to higher latitudes during daytime hours and the winter season. One of the key challenges in the investigation of the ionospheric storms in the last decade has been their solar cycle dependence. At the beginning of the 2000s, this dependence was rather an assumption based on model predictions [21, 38]. However, interest in the solar cycle effect on the ionospheric storm time response was greatly revived in the years following the deep 2008-2009 solar minimum, and probably the low activity solar cycle 24. Shortcomings in ionospheric prediction capabilities during this deep solar minimum that were identified for all types of model formulations (from empirical to physics-based ones), the investigation of the long-term behavior of the ionospheric disturbances in solar cycle 24, and the analysis of specific storm events through multi-instrument data and models simulations, revealed the significant impact of the geomagnetic storm drivers in the observed effects [39, 42, 46, 47, 48, 49, 50, 51, 52]. Although it is known that ionospheric storm effects of significant intensity are observed even during solar minimum years, there is now some evidence that the ionospheric response to CIR/HSSs-driven storms may be different than the one to CME-driven events.