MIST
Magnetosphere, Ionosphere and Solar-Terrestrial
The Importance of Sudden Commencements in Causing Elevated Ground Magnetic Field Variability
By Andy Smith (Mullard Space Science Laboratory - University College London)
Large variability in the Earth’s magnetic field can induce anomalous and damaging currents in power systems and pipelines. It is crucial that we understand and can predict the processes responsible. In this work we quantified how Sudden Commencements (SCs) contribute to creating large rates of change of the surface magnetic field. SCs are caused by the impact of solar wind pressure pulses, e.g. interplanetary shocks, on the Earth’s magnetosphere. They represent one of the more reliably forecastable forms of space weather, where the driving solar wind structure can be observed upstream of the Earth prior to its arrival.
We found that SCs are related to enhanced rates of change of the ground magnetic field (R). The Figure below shows the fraction of R in excess of 50 nTmin-1 that is related to SCs, as a function of latitude. The top panel shows the percentage observed during the SCs themselves. This maximises at around 20 – 30% at low latitudes, but drops to <1% by around 55° as other processes become fractionally more important at generating large R.
SCs often precede further magnetospheric activity, such as geomagnetic storms and substorms. The lower panel below shows the statistics for the time period during SCs, while also including the 24 hours that follow. This extended period can be seen to account for around 75% of large R (in excess of 50 nTmin-1) at locations below ~55°.
This work has shown that SCs are an important source of potentially hazardous magnetic field perturbations, and proportionally they are more important at mid-to-low latitudes. Usefully, SCs also provide a 24 hour window within which the majority (~75%) of large rates of change of the field are observed, below ~55 degrees latitude.
Figure 1: The percentage of observations of R ≥ 50 nT min−1 (1996 - 2016) that can be related to SCs as a function of magnetic latitude. The rows represent the data obtained during the SCs themselves (i),
and the data inclusive of 24 hours following the SC (ii).
Please see the paper for full details:
Smith, A. W., Forsyth, C., Rae, I. J., Rodger, C. J., & Freeman, M. P. (2021). The Impact of Sudden Commencements on Ground Magnetic Field Variability: Immediate and Delayed Consequences. Space Weather, 19, e2021SW002764. https://doi.org/10.1029/2021SW002764