Polar and Magnetospheric Substorms : Springer : 9789401034630 : 940103463X : 02 Nov 2011 : It has become increasingly clear that the magnetosphere becomes intermittently unstable and explosively releases a large amount of energy into the polar upper atmos­ phere. This particular magnetospheric phenomenon is called the magnetospheric sub­ storm. It is manifested as an activity or disturbance ofvarious polar upper atmospheric phenomena, such as intense auroral displays and X-ray bursts. Highly active conditions in the polar upper atmosphere result from a successive occurrence of such an element­ ary activity, the polar substorm, which lasts typically of order one to three hours. The concept of the magnetospheric substorm and its manifestation in the polar upper atmosphere, the polar substorm, has rapidly crystallized during the last few years. We can find a hint of such a concept in the term 'polar elementary storm' introduced by Kristian Birkeland as early as 1908. However, we are great