Abstract
Variability in the near-Earth solar wind conditions can adversely affect a number of ground- and space-based technologies. Such space-weather impacts on ground infrastructure are expected to increase primarily with geomagnetic storm intensity, but also storm duration, through time-integrated effects. Forecasting storm duration is also necessary for scheduling the resumption of safe operating of affected infrastructure. It is therefore important to understand the degree to which storm intensity and duration are correlated. The long-running, global geomagnetic disturbance index, mathit{aa}, has recently been recalibrated to account for the geographic distribution of the component stations. We use this mathit{aa}_{H} index to analyse the relationship between geomagnetic storm intensity and storm duration over the past 150 years, further adding to our understanding of the climatology of geomagnetic activity. Defining storms using a peak-above-threshold approach, we find that more intense storms have longer durations, as expected, though the relationship is nonlinear. The distribution of durations for a given intensity is found to be approximately log-normal. On this basis, we provide a method to probabilistically predict storm duration given peak intensity, and test this against the mathit{aa}_{H} dataset. By considering the average profile of storms with a superposed-epoch analysis, we show that activity becomes less recurrent on the 27-day timescale with increasing intensity. This change in the dominant physical driver, and hence average profile, of geomagnetic activity with increasing threshold is likely the reason for the nonlinear behaviour of storm duration.
Highlights
A geomagnetic storm is a significant disturbance in the Earth’s magnetic field (e.g. Gonzalez et al, 1994) due to specific sets of conditions in the near-Earth solar wind
This substorm cycle of tail lobe energy storage and release can result in enhanced ionospheric currents which can in turn lead to geomagnetically induced currents (GICs), a quasi-DC signal, flowing through ground infrastructure such as power grids and pipelines (Patel et al, 2016; Cannon et al, 2016)
The sharp drop off associated with a power law can be seen in the probabilities of storm peak intensity revealed by the complementary cumulative distribution function (CCDF) in Figure 3 (Left)
Summary
A geomagnetic storm is a significant disturbance in the Earth’s magnetic field (e.g. Gonzalez et al, 1994) due to specific sets of conditions in the near-Earth solar wind. Changes in magnetospheric current systems can result in magnetic fluctuations which can be measured at a number of stations around the world with ground-based magnetometers These measurements are compiled into a variety of indices, including aa, Kp, Dst and AE, which measure different properties of global geomagnetic activity. Lockwood et al (2018a) and Lockwood et al (2018b) corrected aa for a number of factors They made allowance for the changing geographic location of the midnight sector auroral oval, due to the secular change in Earth’s intrinsic geomagnetic field (giving different drifts of the geomagnetic poles in the two hemispheres, as observed), which influences the proximity of the stations to the most relevant current system, the nightside auroral electrojet of the substorm current wedge. Am gives a more accurate representation of the instantaneous state of global geomagnetic activity (Lockwood et al, 2019), the advantage of aaH is that it provides an extra century of observations which means that better statistics on large events are obtained
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