## 1 Introduction  
The Earth's ionosphere is the ionized part of the upper atmosphere, extending from about 60 km to 1000 km altitude imbedded in the neutral atmosphere. Ionospheric ionization is produced and further stratified through solar radiation, with the maximum concentration of the ionization to occur in the ionospheric F region. As the inner edge of the magnetosphere with clear links to the neutral atmosphere, the ionosphere plays a critical role in solar-terrestrial relationships, merging the influence from both above and below. In this respect, the transient changes imposed by space weather events in the near-Earth space environment lie among most challenging topics for both scientific and operational applications, e.g., [1; 2; 3].  
During disturbed space weather conditions, the highly variable solar wind energy input in the magnetosphere significantly impacts the upper atmosphere's state through the interaction of the ionospheric plasma with atmospheric neutrals. Most dramatic changes follow the occurrence of geomagnetic storms, during which, a variety of photo-chemical and chemical reactions, as well as dynamical and electrodynamical processes that are activated in the magnetosphere-ionosphere-thermosphere system, drive the exchange and transportation of mass, momentum, and energy between the system's elements, and consequently, substantially alter the ionospheric structure. A key element of the ionospheric response to geomagnetic storm events comes in the form of disturbances in the peak electron density (NmF2) and column density (i.e., the total electron content--TEC): large-scale increases and decreases in the two parameters are observed globally to formulate the so-called positive and negative ionospheric storms, respectively.  
Both the origin and the occurrence of the ionospheric storms have been subject to many studies during recent decades. The community's interest is justified by the significant impact of ionospheric storm effects on the reliable performance of technological systems, while it is further fed by the scientific challenges that are met in their investigation along the complex chain of solar-terrestrial relations. Negative ionospheric storm effects (i.e., NmF2 and TEC decrease well below their normal levels) cause serious problems in ground-based HF radio communications, while on the other hand, positive ionospheric storm effects (i.e., NmF2 and TEC increase well above their normal levels) can cause serious problems such as time delay, range error, and scintillations in satellite communication and navigation [4]. On the other hand, a set of dependencies (e.g., seasonal, local time, latitudinal, altitudinal) compile a rather complicated scene for the appearance of ionospheric storm effects to keep the relevant topic a vivid research field.  
This paper attempts a brief survey of present knowledge on key aspects of ionospheric storms with the emphasis in the ionospheric storm effects at middle latitudes and the F region. Recent results and some additional evidence dealing with the critical role of the solar wind conditions that trigger the geomagnetic and ionospheric storm activity are also discussed. The paper is structured as follows: materials and methods used in the analysis are presented in Section 2, while the results are provided in Section 3, and are further discussed in Section 4, together with some suggestive concluding remarks.