A global climatological model of extreme geomagnetic field fluctuations

Abstract

This paper presents a multi-parameter global statistical model of extreme horizontal geomagnetic field fluctuations (dBH/dt), which are a useful input to models assessing the risk of geomagnetically induced currents in ground infrastructure. Generalised Pareto (GP) distributions were fitted to 1-min measurements of |dBH/dt| from 125 magnetometers (with an average of 28 years of data per site) and return levels (RL) predicted for return periods (RP) between 5 and 500 years. Analytical functions characterise the profiles of maximum-likelihood GP model parameters and the derived RLs as a function of corrected geomagnetic latitude, λ. A sharp peak in both the GP shape parameter and the RLs is observed at |λ| = 53° in both hemispheres, indicating a sharp equatorward limit of the auroral electrojet region. RLs also increase strongly in the dayside region poleward of the polar cusp (|λ| > 75°) for RPs > 100 years. We describe how the GP model may be further refined by modelling the probability of occurrences of |dBH/dt| exceeding the 99.97th percentile as a function of month, magnetic local time, and the direction of the field fluctuation, dBH, and demonstrate that these patterns of occurrence align closely to known patterns of auroral substorm onsets, ULF Pc5 wave activity, and (storm) sudden commencement impacts. Changes in the occurrence probability profiles with the interplanetary magnetic field (IMF) orientation reveal further details of the nature of the ionospheric currents driving extreme |dBH/dt| fluctuations, such as the changing location of the polar cusp and seasonal variations explained by the Russell-McPherron effect.

The Impact of Sudden Commencements on Ground Magnetic Field Variability: Immediate and Delayed Consequences

“…An SC that is followed by a geomagnetic storm is referred to as a Storm Sudden Commencement (SSC). Rogers et al (2020) recently used extreme value theory to show that the distribution of return rates of extreme surface magnetic field variability vary significantly with local time and latitude, with evidence SMITH ET AL.…”

The distribution and direction of extreme geomagnetic fluctuations over 1-60 minute periods

“…Occurrence patterns match the known patterns of substorm expansions, Pc5 ULF waves, geomagnetic sudden commencements, and highlatitude lobe reconnection events. We have extended an earlier climatological study of 1-minute geomagnetic fluctuations [3] to cover both the ramp changes and RMS variations over periods from 1 to 60 minutes. The occurrence patterns depend strongly on the period of the disturbance and the direction of the field fluctuation.…”

“…These may be associated with intense auroral substorm expansions -intensifications of westward ionospheric currents in auroral arcs near the equatorward edge of a greatly expanded auroral electrojet region. A secondary increase in RLs towards the geomagnetic poles is observed for return periods of 100 years or more, associated with long-period oscillations during the summer months around noon for northward interplanetary magnetic field conditions, when the strong geomagnetic dipole tilt is favourable to magnetic reconnection in the magnetospheric tail lobe [3]. The strong dependence of (| / | > P99.97) on covariates (direction, MLT, month, etc.)…”

“…From these measurements we determined both the absolute rate of change, | / | , and the compass direction of the fluctuations. Details of magnetometer locations and the data processing and quality checking are given in [3]. We define | / | as…”

“…In this paper we extend our global climatological statistical model of 1-minute fluctuations [3] to include the magnitude and frequency of occurrence of extreme | / |, over periods at 1, 10, 30 and 60 minutes, both as ramp changes and as a root-mean-square (RMS) variations. This range of periods is important for GIC modelling but different ionospheric and magnetospheric processes will dominate at each end of the range.…”