Tutorial on geomagnetic storms and substorms

Geomagnetic storms have been studied for more than a century and substorms for nearly four decades. The space era which began in the late 1950s has ushered new discoveries and given new practical importance to this scientific discipline as we continue to amass technological assets in space. Geomagnetic storms and substorms pose hazards to our venture into the final frontier, much like adverse atmospheric weather does to our outdoor activity. This tutorial provides an overview of the main characteristics of these space phenomena and a brief review of the present theories concerning their initiation. For geomagnetic storms, the two prevailing views on their cause are discussed. One considers a storm to be the accumulated effects of a series of substorms while the other considers it to be the result of strong and prolonged enhancement of the global magnetospheric electric field. A synergistic model combining elements from both views is proposed as the likely explanation for the cause of geomagnetic storms. For substorms, there are generally four categories of models. The first proposes some plasma instabilities or externally-imposed reduction in the magnetospheric electric field at the near-Earth region. The second calls for magnetic reconnection occurring in the mid-tail environment. The third focuses on coupling the ionosphere to the magnetosphere while the fourth invokes abstract descriptions in nonlinear dynamics to address some statistical characteristics of substorms. An evaluation of observations and these models suggests that substorm onset may not be uniquely attributed to a single physical process.