Abstract
AbstractThanks to mapping missions, like Ørsted, CHAMP, and Swarm, we have gained a detailed understanding of the geomagnetic field. High‐resolution models like POMME, GRIMM, or CHAOS are able to describe the main parts of the Earth's magnetic field reliably. These models represent well contributions from the core and crustal fields. But their validity of describing magnetospheric field effects is limited to low activity periods (Kp ~ 0–2). Here, we study the differences between CHAMP magnetic observations and the predictions from CHAOS‐6‐x9, a recent version, outside this validity range. Systematic residuals appear at times of elevated activity. Mean amplitudes at the equator grow up to 12 nT around 20 hr magnetic local time for magnetic activity around Kp = 4.7. Negative residuals are obtained in the evening to midnight sector and positive ones in the morning. A seasonal dependence of the magnetospheric currents causes more negative deflections of the residuals in the winter than in the summer hemisphere. This hemispheric asymmetry cannot be accounted for by a degree 1 spherical harmonics function. A surprising observation is that the residuals show a clear longitude dependent pattern, which changes with local time. The analysis reveals that this feature can be interpreted as a Universal Time dependence of the residuals with a peak‐to‐peak amplitude of about 8 nT and a period of 12 hr at an activity level of Kp = 4.7. All these results call for a better parameterization of the magnetospheric current effects in a geomagnetic field model that is reliable at least up to moderate activity levels.
Highlights
The Earth's magnetic field, as we observe it on ground or at low‐Earth orbit, is representing the sum of contributions from many different sources
All these results call for a better parameterization of the magnetospheric current effects in a geomagnetic field model that is reliable at least up to moderate activity levels
We have shown that the CHAOS‐6 model represents CHAMP magnetic field observations very well for magnetically quiet conditions with Kp = 0 to 1
Summary
The Earth's magnetic field, as we observe it on ground or at low‐Earth orbit, is representing the sum of contributions from many different sources. Several model families have evolved from these activities They include POMME (e.g., Maus et al, 2010), GRIMM (e.g., Lesur et al, 2008, 2015), CHAOS (e.g., Finlay et al, 2016; Olsen et al, 2014), and the comprehensive model CM (e.g., Sabaka et al, 2002, 2018). These models characterize the dominating core field, the crustal field, and the magnetic fields from large‐scale magnetospheric currents. Due to the unpredictable evolution of the Earth's magnetic field, the model parameters need regular overhauls, typically once a year, to extend their validity to actual dates
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