Weather at the core: Defining and categorizing geomagnetic excursions and reversals

Abstract

Summary Paleomagnetic records provide us with information about the extreme geomagnetic events known as excursions and reversals, but the sparsity of available data limits detailed knowledge of the process and timing. To date there are no agreed on criteria for categorizing such events in terms of severity or longevity. In an analogy to categorizing storms in weather systems, we invoke the magnitude of the global (modified) paleosecular variation index $P_{i_D}$ to define the severity of the magnetic field state, ranging in level from 0 to 3, and defined by instantaneous values of $P_{i_D}$ with level 0 being normal ($P_{i_D}<0.5$) to extreme ($P_{i_D}\ge 15$). We denote the time of entry to an excursional (or reversal) event by when $P_{i_D}$ first exceeds 0.5, and evaluate its duration by the time at which $P_{i_D}$ first returns below its median value, termed the end of event threshold. We categorize each excursional event according to the peak level of $P_{i_D}$ during the entire event, with a range from Category-1 (Cat-1) to Cat-3. We explore an extended numerical dynamo simulation containing more than 1200 events and find that Cat-1 events are the most frequent (72%), but only rarely lead to actual field reversals where the axial dipole, $g_1^0$, has reversed sign at the end of the event. Cat-2 account for about 20% of events, with 34% of those leading to actual reversals, while Cat-3 events arise about 8% of the time but are more likely to produce reversals (43%). Higher category events take as much as 10 times longer than Cat-1 events. Two paleomagnetic field models separately cover the Laschamp excursion and Matuyama-Brunhes (M-B) reversal which are Cat-2 events with respective durations of 3.6 and 27.4 ky. It seems likely that Cat-2 may be an underestimate for M-B due to limitations in the paleomagnetic records. Our overall results suggest no distinction between excursions and reversals other than a reversal having the ending polarity state opposite to that at the start.