Average properties of geomagnetic storms in 1932–2009

Abstract

We investigate the average properties of geomagnetic storms using the global and local Dxt indices at four Dst stations in 1932–2009. Imposing the condition of complete data availability during storms, our study includes 1268/362/134/59 storms with Dxt minimum less than −50/−100/−150/−200 nT, respectively. The global Dxt minima were, on an average, −94/−156/−216/−275 nT, while deepest storm‐time local Dxt minima were −137/−214/−285/−350 nT. Accordingly, the local Dxt minima are typically 25–30% stronger than the global Dxt minima. The distribution of largest storm‐time disturbances is strongly peaked at 18 local time (LT), challenging local midnight as the dominant ion source. Relative timing of local minima verifies that stations at earlier LT hour observe their minimum a couple of hours after the deepest minimum, in agreement with westward drift of ions. Storm‐time maximum asymmetries were found to increase with storm intensity level from about 70 nT to 150 nT for −50 to −200 nT storms. However, strong storms are relatively more symmetric than weak storms when compared to the typical level of local disturbance. During individual storms the asymmetry can be more than 200 nT. The rate of evolution of storm‐time asymmetry is found to be roughly twice as fast for large storms. We emphasize that the unique database of local Dxt indices proves to be very useful in studying the average spatial distribution and temporal evolution of storms.