Statistics of Extreme Time‐Integrated Geomagnetic Activity

Abstract

A statistical analysis of the time‐integrated Dst index is performed over 1958–2007. The tail of the probability distribution of extreme time‐integrated Dst events, which occur during strong geomagnetic storms, can be precisely fitted by a power law function with upper cutoff, apparently not exceeded even by the 1859 Carrington event. This time‐integrated Dst is expected to be a reasonable proxy for maximum densities of MeV electrons in the heart of the outer radiation belt, which are known to pose a serious threat to satellites. During such strong events, a correlation is found between the time‐integrated levels of various physical quantities, such as interplanetary magnetic field Bz, particle energy fluxes measured during injections in the magnetotail, geosynchronous ULF wave index, and geomagnetic activity in the inner magnetosphere, suggesting cumulative effects from successive disturbances.

Electron Flux Enhancements at L = 4.2 Observed by Global Positioning System Satellites: Relationship With Solar Wind and Geomagnetic Activity

“…In addition, many of these works mainly considered the geostationary orbit (i.e., the outer edge of the radiation belt) or only examined several years of electron flux data from the Van Allen Probes with a relatively long time step, ∼5–9 hr (Moya et al, ; Murphy et al, ; Tang et al, ; Turner, O'Brien, et al, ; Zhao, Baker, Jaynes, et al, ). Furthermore, recent studies have shown that the most significant correlations with electron flux increases are found when considering geomagnetic indices ( K p , D s t , or ULF wave index) integrated in time over at least 1 day (during the considered storm or periods of sustained substorms), instead of their hourly or peak value, suggesting the presence of cumulative effects (Borovsky, ; Mourenas et al, ; Simms et al, ). This kind of correlation between a progressive flux increase and an extended period of geomagnetic activity could be easily identified through a superposed epoch analysis over hundreds of events.…”

Dynamical Properties of Peak and Time‐Integrated Geomagnetic Events Inferred From Sample Entropy

“…Simultaneously, we keep over each sliding window the minimum value of and the maximum values of , , and , calculating also as the time‐integrated when 70 nT (Mourenas et al, ), as the time‐integrated when , and and as time‐integrated and over periods where , as in Mourenas et al (). However, we do not require here geomagnetic activity to remain continuously strong over the time integration window, because when considering significant time‐integrated or values 1,500 nT hr, the relative weight of one important continuous event generally largely dominates over the 10 days of the integration window.…”

“…To check possible differences between entropy variations during quiet or moderately disturbed periods and entropy variations during strong disturbances, best least‐squares fits have also been calculated separately during low and high geomagnetic activity (see dashed and dotted blue curves in Figure ), using approximate thresholds of high activity nT, 2,000 nT hr, nT, 1,500 nT hr from previous studies (Mourenas et al, , ; Tsurutani et al, , ), such that highly active periods represent 10% to 20% of the data points. The results in Figure show that the best global fits (obtained by considering all data points) generally remain close to the best fits obtained during moderately disturbed periods (comprising 90% of the data points).…”

“…Accordingly, the strongest or events should probably be more predictable than weak events. Since stronger time‐integrated and events were shown to correspond to higher time‐integrated fluxes of megaelectron‐volt electrons in the heart of the outer radiation belt (Mourenas et al, , ), it is a promising result for the development of space weather forecasts. On the downside, periods of 5–10 days of elevated 2‐MeV electron flux at 4.5–6.6 are slightly better correlated in general with or than with (Mourenas et al, ), suggesting a potentially weaker predictability of such electron fluxes at than at , where correlations are higher with , , and .…”

“…Studies aiming at providing forecasts of the radiation belts have mostly investigated the correlations between indices of geomagnetic activity and some solar wind parameters and measured electron fluxes at a given location (Borovsky, , ). However, recent works have shown that cumulative effects of geomagnetic activity can sometimes be more important for predicting relativistic electron fluxes and geomagnetically induced currents than instantaneous or peak values of geomagnetic activity (Borovsky, ; Hajra et al, ; Lotz & Danskin, ; Mourenas et al, , ). Moreover, different investigations of the 1‐hr index, of the 1‐hr index, and of the 1‐min SYM‐H index have demonstrated that quiet times and geomagnetic storms correspond in general to different dynamical regimes, with more persistence (Balasis et al, ; Uritsky & Pudovkin, ) and a reduced entropy (Balasis et al, ; Donner et al, ) during storms.…”