Recent geomagnetic secular variation from Swarm and ground observatories as estimated in the CHAOS-6 geomagnetic field model

Abstract

We use more than 2 years of magnetic data from the Swarm mission, and monthly means from 160 ground observatories as available in March 2016, to update the CHAOS time-dependent geomagnetic field model. The new model, CHAOS-6, provides information on time variations of the core-generated part of the Earth's magnetic field between 1999.0 and 2016.5. We present details of the secular variation (SV) and secular acceleration (SA) from CHAOS-6 at Earth's surface and downward continued to the core surface. At Earth's surface, we find evidence for positive acceleration of the field intensity in 2015 over a broad area around longitude 90°E that is also seen at ground observatories such as Novosibirsk. At the core surface, we are able to map the SV up to at least degree 16. The radial field SA at the core surface in 2015 is found to be largest at low latitudes under the India-South-East Asia region, under the region of northern South America, and at high northern latitudes under Alaska and Siberia. Surprisingly, there is also evidence for significant SA in the central Pacific region, for example near Hawaii where radial field SA is observed on either side of a jerk in 2014. On the other hand, little SV or SA has occurred over the past 17 years in the southern polar region. Inverting for a quasi-geostrophic core flow that accounts for this SV, we obtain a prominent planetary-scale, anticyclonic, gyre centred on the Atlantic hemisphere. We also find oscillations of non-axisymmetric, azimuthal, jets at low latitudes, for example close to 40°W, that may be responsible for localized SA oscillations. In addition to scalar data from Ørsted, CHAMP, SAC-C and Swarm, and vector data from Ørsted, CHAMP and Swarm, CHAOS-6 benefits from the inclusion of along-track differences of scalar and vector field data from both CHAMP and the three Swarm satellites, as well as east-west differences between the lower pair of Swarm satellites, Alpha and Charlie. Moreover, ground observatory SV estimates are fit to a Huber-weighted rms level of 3.1 nT/year for the eastward components and 3.8 and 3.7 nT/year for the vertical and southward components. We also present an update of the CHAOS high-degree lithospheric field, making use of along-track differences of CHAMP scalar and vector field data to produce a new static field model that agrees well with the MF7 field model out to degree 110.

An accelerating high-latitude jet in Earth’s core

“…It has secular variation at the core surface which is stable out to at least spherical harmonic degree 16 [1]. In this article, we focus on CHAOS-6 in preference to other available models because of its continuous treatment of ground and satellite data since the start of the modern era of satellite geomagnetism in 1999, its focus on high resolution secular variation, and the generally good agreement of the CHAOS family of models with other geomagnetic reference models [33].…”

“…A snapshot of the magnetic field at the CMB in 2015, alongside its time-derivative (the secular variation or SV), from the CHAOS-6 geomagnetic field model [1], is presented in Fig 1. The top row shows the radial component of the main field on the CMB for both the northern and southern high-latitudes, while the bottom row shows the associated SV. Of particular prominence within the SV is a daisy-chain of patches of alternating sign (labelled A-F), arranged around the north geographic pole at about latitude 70 • .…”

“…The most recent high resolution data from the Ørsted, CHAMP and Swarm satellite missions, along with ground-based observatory data, show that, over the past decade, the northern flux lobes over Canada and Siberia have not only been moving but accelerating [7,1]. In this article, we provide an explanation of this new observation based on an accelerating tangent cylinder jet that sheds light on the hidden internal dynamics of the core, and which adds to the body of evidence suggesting a marked difference in magnetic change between low and high latitudes [8,9,6,10,11].…”

“…Field models based on archeomagnetic data, geomagnetic observatories, historical records and satellites have been used to reconstruct the time evolution of the magnetic field at the coremantle boundary (CMB), the edge of the source region, over a range of timescales [1,2,3,4].…”

Saint Petersburg magnetic observatory: from Voeikovo subdivision to INTERMAGNET certification

“…We used freely available and widely used models, which are considered to be the most precise at the moment: CHAOS-6 (Finlay et al, 2016), SIFM (Olsen et al, 2015), and EMM2015 (Magnetic Crustal Field Model, 2017). All of them are constructed using the high-precision new-generation satellite observations.…”