2025 THEMIS SCIENCE NUGGETS
Investigation of the occurrence of significant deviations in the magnetopause location: solar-wind and foreshock effects
Niklas Grimmich
Institute of Geophysics and Extraterrestrial Physics, Technical University Braunschweig,
Braunschweig, Germany
Introduction
The Earth's magnetic field acts as an obstacle to the stream of charged particles and magnetic fields ejected from the Sun, known as the solar wind. Consequently, the solar wind flows around this obstacle within a large interaction region spanning multiple Earth radii. Two important boundaries of this region are the magnetopause, where the Earth's magnetic field meets the diverted solar wind, and the bow shock, where the solar wind is initially decelerated and diverted around the planet.
Based on the accumulation of spacecraft observations of the magnetopause, empirical models assume that it has a rigid shape and attempt to determine its average location for a set of quasistatic solar wind input parameters. The focus of the input parameters is often on a pressure dependence and the effect of the orientation of the interplanetary magnetic field on the boundary. However, the scatter observed in comparisons of model predictions and observations is often larger than expected. These deviations might be caused by processes occurring in the foreshock region — a turbulent area filled with plasma waves that is located upstream of the bow shock. Of course, the models may also be missing aspects related to the solar wind input parameters, which are often viewed as having independent dependencies on the magnetopause. However, since the parameters are actually coupled to some extent, it may be beneficial to examine the influence of different types of solar wind.
In this study, we examine how different types of solar wind and the presence of an upstream foreshock might explain why the real observed magnetopause sometimes does not match model predictions, using data from spacecraft missions in near-Earth space (Cluster, THEMIS and MMS).
Results
For our magnetopause crossing (MPC) dataset, we gathered solar wind data from the OMNI dataset in order to predict the location of the magnetopause using primarily the model of Shue et al. (1998), in short SH98. Furthermore, since the bow shock can be separated into quasi-parallel and quasi-perpendicular regimes depending on a given angle, we estimate that angle between the local bow shock normal upstream of the magnetopause observation and the interplanetary magnetic field. The separation is also an indicator of the presence of a foreshock region, as it can only form in quasi-parallel bow shock regimes due to the reflection and backscattering of solar wind particles at the bow shock.
Figure 1 shows the distributions of deviations of the observed magnetopause locations from the SH98 model for different regions of near-Earth space, and how this distribution changes depending on the associated bow shock regime. We find that large model deviations are present throughout the dayside magnetosphere. Specifically, overestimated MPCs occur more frequently in the magnetospheric flanks, while underestimated MPCs occur more frequently in the nearequatorial plane. Furthermore, distributions associated with quasi-parallel bow shock conditions (angles below 45 degrees) exhibit a shift towards positive model deviations in the fitted mean, as well as broader distributions and a higher occurrence rate of deviant MPCs.
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| Figure 1. The normalised distributions of the deviation Δr⊥ between the MPC observations and the predictions of the SH98 model are shown for the subsolar magnetopause (a and b), the high-latitude magnetopause in the noon sector (c and d), and the flank magnetopause observations in the equatorial plane (e and f ). The yellow lines represent the reported uncertainty of the SH98 model. In the upper panels, the distribution of all MPCs in a given region is shown, with the read and cyan regions of the histograms representing the MPCs that clearly deviate from the selected model in the data set. The dashed black lines represent a Gaussian fit to the histograms, with the mean and full width at half maximum (FWHM) of the fits also shown. In the lower panels, the distribution of MPCs associated with quasi-parallel bow shock conditions is compared with that associated with quasi-perpendicular conditions. |
In the second part of our study, we categorise the solar wind conditions observed during the crossings according to the Xu and Borovsky (2015) classification scheme into four distinct categories called ejecta (EJC), coronal hole origin (CHO), streamer belt origin (SBO), and sector reversal region (SRR). Fig. 2 shows the effect on the occurrence rate of significantly deviating MPCs from this classification. We find that overestimated MPCs often appear alongside EJC- and CHO-type solar wind. A southward-orientated interplanetary magnetic field is common for these MPCs when they are associated with quasi-perpendicular conditions. Conversely, underestimated MPCs often occur alongside CHO-type solar wind. Additionally, radial interplanetary magnetic fields are common for deviant MPCs associated with quasi-parallel bow shock conditions.
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| Figure 2. Comparison of the occurrence of compressed MPCs (top panels) and expanded MPCs (bottom panels) deviant from the SH98 model for different solar wind plasma conditions. The solar wind conditions are grouped according to the classification scheme of Xu and Borovsky (2015), with different colours corresponding to different solar wind types: red for coronal hole origin (CHO), yellow for streamer belt origin (SBO), blue for sector reversal region (SRR) and grey for ejecta (EJC). Each solar wind type is further divided into quasi-radial IMF (r.), northward IMF (n.) and southward IMF (s.). Panels (a) and (b) show all deviant MPCs from the combined datasets, panels (c) and (d) show the occurrence of deviant MPCs associated with quasi-parallel bow shock conditions and panels (e) and (f ) show the occurrence of deviant MPCs associated with quasi-perpendicular bow shock conditions. |
Conclusion
We combined data from three space missions spanning more than 20 years in order to identify instances where common models do not align with direct observations of the Earth's magnetopause. Our extensive statistics show that misrepresented MPCs are more likely in foreshock/quasiparallel shock conditions, even if not directly caused by the foreshock. On average, this results in a deviation of 0.1 to 0.2 Earth radii anti-earthward, leading us to suspect that the magnetopause moves more frequently and with greater amplitude due to the foreshock.
Additionally, we propose that model predictions are more likely to be inaccurate during storms triggered by a southward IMF embedded in EJC-type solar wind with high magnetic fields, when foreshock activity is not a plausible explanation. New models should also take the 'fast' solar wind (CHO-type) into account, as it has been identified as a source of deviant MPCs.
Biographical Note
Niklas Grimmich is a PhD student at the Institute of Geophysics and Extraterrestrial Physics, Technical University Braunschweig, Braunschweig, Germany. His work focused on magnetospheric physics, examining the motion of magnetospheric discontinuities in order to improve our understanding of how the magnetopause responds to external and internal driving sources on a global scale.
References
Grimmich, N., Poppelwerth, A., Archer, M. O., Sibeck, D. G., Plaschke, F., Mo, W., Toy-Edens, V., Turner, D. L., Kim, H., and Nakamura, R. (2025). Investigation of the occurrence of significant deviations in the magnetopause location: solar-wind and foreshock effects. Ann. Geophys., doi: 10.5194/angeo-43-151-2025Shue et al. (1998). Magnetopause location under extreme solar wind conditions. JGR, doi: 10.1029/98JA01103
Xu and Borovsky (2015). A new four-plasma categorization scheme for the solar wind. JGR: Space Physics, doi: 10.1002/2014JA020412
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Emmanuel Masongsong / emasongsong @ igpp.ucla.edu