Key features of intense geospace storms—A comparative study of a solar maximum and a solar minimum storm

This paper addresses the question of particular causes and dynamical characteristics of intense geospace storms through the comparative study of two specific examples: a solar maximum storm (4–6 June 1991) and a solar minimum storm (24–26 September 1998). We concentrate on issues pertaining to the origin, development, dynamical evolution and recovery of intense storms. The comparative study addresses the solar and interplanetary drivers, the ring current composition and its implications, the storm–substorm relationship, and the ring current decay. Our aim has been to identify features assumed common or repeatable and to elaborate on similarities or differences between two intense storms that are separated by 7 years and which occurred in different phases of the solar cycle. In view of planetary exploration, which is becoming the focus of space science both in the US and in Europe, classifying the drivers and parameters of solar-planetary connection is of particular interest. Solar system exploration will undoubtedly benefit from detailed knowledge of the conditions leading to intense geospace storms. The main conclusions of this paper can be listed as follows. More intense solar events do not necessarily result in more intense geospace storms. Storm development is driven by the appropriate interplanetary conditions; however, storm evolution is not defined by interplanetary conditions alone, but also by internal magnetospheric conditions—namely plasma sheet density and ion composition. Storm dynamics may also be modified by solar wind pressure. Magnetosphere–ionosphere coupling in the form of ionospheric outflow during storms presumably is the key to substorm influence on storm dynamics; i.e., substorms influence storm development substantially, whenever they drive intense outflow of ionospheric-origin O + ions into the magnetosphere. In particular, substorm-driven O + outflow is one of the main features of intense storms.