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Abstract Magnetospheric ultralow frequency (ULF) waves are fundamental for coupling energy across the magnetosphere and ionosphere. Pc3‐4 waves (7–100 mHz) are associated with small interplanetary magnetic field (IMF) cone angles, suggesting that foreshock ULF waves are the source. To understand the relationship between foreshock and magnetospheric ULF waves, we perform a statistical study using the THEMIS spacecraft to observe foreshock ULF waves and ground magnetometers (GMAG) to infer magnetospheric ULF waves. We observe a power law correlation between foreshock and ground‐based ULF wave power, with correlation coefficients exceeding 0.5 around the typical period of foreshock “30s waves” in contrast to correlation coefficients ∼0.3 between solar wind and ground‐based ULF wave power. The power law parameters decrease with lower magnetic latitudes, indicating weaker power transport deeper in the magnetosphere. Additionally, the correlation coefficients decrease with larger magnetic local time (MLT) separation between the THEMIS spacecraft and GMAG. We also find that ground‐based Pc3‐4 wave power exhibits a stronger dependence on the local bow shock θBn(angle between the shock normal and the IMF) than on the IMF cone angle. These findings indicate that foreshock ULF waves are more likely to be transmitted from the local MLT than across the subsolar point. We also find that ground‐based Pc3‐4 waves show a stronger dependence on the solar wind speed than do foreshock ULF waves, suggesting a possible contribution from Kelvin‐Helmholtz instability and shock‐generated transients. Our results provide new insights into the connection between foreshock ULF waves and ground‐based Pc3‐4 waves.
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On the Contribution of Latitude‐Dependent ULF Waves to the Radial Transport of Off‐Equatorial Relativistic Electrons in the Radiation Belts
Abstract Ultra‐low frequency (ULF) waves radially diffuse hundreds‐keV to few‐MeV electrons in the magnetosphere, as the range of drift frequencies of such electrons overlaps with the wave frequencies, leading to resonant interactions. Theoretically this process is described by analytic expressions of the resonant interactions between electrons and ULF wave modes in a background magnetic field. However, most expressions of the radial diffusion rates are derived for equatorially mirroring electrons and are based on estimates of the power of ULF waves that are obtained either from spacecraft close to the equatorial plane or from the ground but mapped to the equatorial plane. Based on recent statistical in situ observations, it was found that the wave power of magnetic fluctuations is significantly enhanced away from the magnetic equator. In this study, the distribution of the wave amplitudes as a function of magnetic latitude is compared against models simulating the natural modes of oscillation of magnetospheric field lines, with which they are found to be consistent. Energetic electrons are subsequently traced in 3D model fields that include a latitudinal dependence that is similar to measurements and to the natural modes of oscillation. Particle tracing simulations show a significant dependence of the radial transport of relativistic electrons on pitch angle, with off‐equatorial electrons experiencing considerably higher radial transport, as they interact with ULF wave fluctuations of higher amplitude than equatorial electrons. These findings point to the need for incorporating pitch‐angle‐dependent radial diffusion coefficients in global radiation belt models.
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- PAR ID:
- 10580490
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 129
- Issue:
- 11
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2024JA032905
