Core-mantle boundary flows obtained purely from Swarm secular variation gradient information

Abstract

<p>The Swarm constellation provides information on both along- and across-track magnetic field gradients. Spatial changes of the magnetic vector field elements are described by a magnetic field gradient tensor, whose elements and their uncertainties can be estimated using the Virtual Observatory (VO) concept, whereby data within a cylinder centred on the VO with axis perpendicular to the Earth’s surface are reduced to a central point at satellite altitude. Recent experiments have shown that analysing data collected over a 4 month window provides the best compromise between reducing bias from the way the satellite orbits sample each VO cylinder and preserving information on temporal changes of the field, and that the data provide spatial information sufficient to resolve 300 non-overlapping VOs. We invert annual first differences of the 5 independent gradient tensor elements (providing estimates of secular variation, SV, gradients) at these 300 VOs over the Swarm era for advective velocity at the core-mantle boundary, forcing the flow to have minimal acceleration while providing an adequate fit to the data. We obtain flows similar to those from previous SV inversions but purely from the gradient information. The resolution of the SV gradients is higher than that of the SV itself, resulting in a ~30% increase in the number of effective flow parameters; this is thought to be because the gradients are less affected by long period external signals that are difficult to remove from the data, resulting in an improved signal to noise ratio. Although very little temporal change in the flow is required to reproduce even rapid changes in the magnetic field, we are able to isolate some robust flow changes, in particular regarding changes in the azimuthal flow acceleration, associated with the geomagnetic impulse in the Pacific region in around 2016.</p>

South Atlantic Anomaly Areal Extent as a Possible Indicator of Geomagnetic Jerks in the Satellite Era

“…This corresponds with a new pulse observed in recent times in the SA of Br (Figure 4), with a maximum in 2016.2 and a minimum detected in 2017.5. Both observations could indicate the occurrence of a new geomagnetic jerk as suggested by Brown and Macmillan (2018) and observed by Hammer (2018) and Whaler et al (2020). Hammer (2018) analyzed the secular variation (SV) and secular acceleration (SA) over ground and virtual observatories calculated from Swarm data and found the characteristic V-shape in the SV and the steep change in the SA in the observatory of Hawaii in the Pacific region in 2017 and in the French Guyana, where the SAA is situated, at the beginning of 2016.…”

“…This situation (Figures 6R-S) along with the new pulse of SA that was registered in 2016.2 (Figure 4) and the minimum observed in 2017.8 in S •• of the SAA (Figure 2C) and of the RFPs (Figure 3) could indicate the occurrence of a new geomagnetic jerk during these years. In fact, Whaler et al (2020) found changes in the azimuthal flow acceleration beneath the Pacific region in 2017, which they linked to the occurrence of the geomagnetic jerk that was registered by Hammer (2018) particularly well in observatories in the Pacific. These changes in the flow were accentuated in the west side of the Pacific region, closer to the Indian sector considered as the EOF #3 by Chulliat and Maus (2014).…”

Signs of a new geomagnetic jerk between 2019 and 2020 from Swarm and observatory data

“…1 (see Additional file 1: Figure S1, where the monthly means are also plotted for each observatory). The obtained mean and SV values have been compared with the estimations from the CHAOS-7 model (Finlay et al 2020, more information about this model is given in "Discussion: a global view from geomagnetic models" Section) for each observatory (red and blue lines in Fig. 1 and Additional file 1: Figure S1).…”

“…To further investigate the evidence of this new jerk around 2020, we have used what is perhaps the most recent spherical harmonic model of the main geomagnetic field based on observatory and satellite data: the 7th release of the CHAOS-7 model (Finlay et al 2020). This geomagnetic model is developed using Swarm satellite data up to 6th March 2021 and the available ground observatory data up to February 2021 (new releases of CHAOS can be found at https:// www.…”

“…The last two decades have been characterized by the occurrence of geomagnetic jerks in 2003 (Olsen and Mandea 2008), 2007(Olsen et al 2009, 2011 (Chulliat and Maus 2014), 2014 (Torta et al 2015), and the most recent one in 2017, which was more regional in character than the preceding ones (Hammer 2018;Whaler et al 2020). This succession of events points to a regular pace of about 3-4 years, suggesting that the jerks are caused by an oscillatory phenomenon within the outer core (Mandea et al 2010;Chulliat and Maus 2014).…”