ULF waves in the active magnetosphere

We have carried out a detailed case study of ULF waves in near-Earth space during the passage of a magnetic cloud which caused a moderate geomagnetic storm on 20 July 2016, published in the Journal of Geophysical Research [Takahashi et al., 2020]. The exceptionally good coverage of spacecraft and ground-based observations during this event, including measurements in the foreshock, the magnetosheath, the outer magnetosphere, the nightside magnetosphere and on the ground in the morning sector, provided us with a unique opportunity to study the transmission of foreshock waves from the upstream region of the bow shock to the ground, and how their properties vary from one region to another.Our observational study was complemented by the analysis of a numerical simulation performed with the Vlasiator model, which provided a global view of near-Earth space during this event.

Because of the extreme solar wind conditions caused by the magnetic clouds, the foreshock waves were characterized by higher frequencies and more complex signatures than during quiet times [Turc et al., 2019, GRL]. This allowed us to study for the first time how the wave activity inside the magnetosphere is affected by these different foreshock wave signatures.

Our main findings are that

  • The spatial scale lengths of the waves is much shorter than usual during this magnetic cloud event, both in the foreshock (consistent with the findings of Turc et al., 2019) and in the magnetosphere, where the coherence of the waves between different observation points is low.
  • The wave power inside the magnetosphere is strongly attenuated away from local noon, resulting in a very low wave power being observed in the nightside magnetosphere in this frequency range, whereas previous studies have reported significant wave power on the nightside associated with transmitted foreshock waves during quiet solar wind conditions [Takahashi et al., 2016]. We note that the wave power in the foreshock is stronger than usual during the 2016-07-20 magnetic cloud event, suggesting that the strong decrease in magnetospheric wave power may be due to the shorter spatial scale lengths of the waves rather than a weaker wave source.
  • Ground magnetometers located near local noon did not detect oscillations at frequencies matching the foreshock waves, nor fundamental field-line resonances. This is likely due to the fact that the frequency of the foreshock waves was too high to couple with the local field-line resonances. The absence of field
    line resonance driven by foreshock waves is a feature unique to times of high interplanetary magnetic field magnitudes.