![]() ‘∧’-shaped) and in some areas they are positive (or ‘∨’-shaped). We have used the MEME2016 model as well as observatory time series to make maps of the jerks. Jerks appear to happen every few years - the most recently accepted jerk was around March 2014 but there may have been a new jerk as recently as June 2015. Using global models of the magnetic field rather than just single points on the surface can help us understand their source within the outer core and their temporal evolution. ![]() Jerks are still not fully understood, and appear sporadically and apparently randomly in the magnetic field record. Jerks represent the most rapid observed internally generated magnetic features known and are associated (we think) with fast flows at the surface of the outer core, which can give us clues about the kinds of processes going on in a region we can never visit. Jerk amplitude □=□2-□1 is defined in both the SV and SA. Columns show successive time derivatives from left to right: the main field (MF), secular variation (SV), secular acceleration (SA) and third time derivative (impulse). The upper panel shows an idealised version of a jerk, while the lower panel shows what they look like in real observatory data, using the 1969 jerk recorded in the East (Y) component at Eskdalemuir.įigure 1: Idealised form of a jerk (at vertical line) (top row) and in monthly mean observations of 1969 jerk in East (Y) component at Eskdalemuir (ESK) (bottom row). Geomagnetic jerks are most commonly defined as ‘∨’ or ‘∧’ shaped features in the SV as illustrated in Figure 1. However, SV itself is not constant and changes on decadal timescales, with occasional periods of rapid variations in the second time derivative of the field (or secular acceleration (SA)) which are called geomagnetic jerks. Changes in the motion of the fluid cause variations of the shape and intensity of the magnetic field measured at the surface - this change over time is known as secular variation (SV). The Earth's internal magnetic field is generated by the motion of the conductive metallic fluid in the outer core. The spatial variations are modelled using spherical harmonics extending to the maximum degree and order that can be supported globally by the input data. The time variations are modelled with high order B-splines for the internal field and by linear and periodic functions with dependence on magnetic activity for the external field. MEME captures both the time-varying and the spatially-varying components of the magnetic field. These data are selected for two purposes: (1) to cover the period of time which we want to model the magnetic field and (2) to remove as much noise in the data as possible for example, by excluding data collected during geomagnetic storms. It is used as a basis for a number of models - BGS Global Geomagnetic Model (BGGM), World Magnetic Model (WMM), International Geomagnetic Reference Field (IGRF).īGS uses data from magnetic survey satellites, for example Ørsted, CHAMP and the ESA Swarm satellites, and from observatories around the world ( including the nine observatories BGS operate). ![]() This is used for scientific study and is continually improved to accommodate new geophysical understanding and computational techniques. ![]() Each year BGS produce a ‘parent’ model of the magnetic field termed the 'Model of the Earth's Magnetic Environment' (MEME).
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