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Abstract Pc5 (2–7 mHz) ultralow frequency (ULF) waves play a significant role in resonating with particles and transferring energy in the coupled magnetospheric and ionospheric system. Recent studies found that Pc5 ULF waves can be triggered by foreshock transients which can perturb the magnetopause through dynamic pressure variation. However, whether foreshock transient‐driven Pc5 ULF waves are geoeffective and can propagate globally is still poorly understood. In this study, we take advantage of the conjunction between in situ (by the THEMIS probes, Geotail satellite, GOES satellites, and Van Allen probes) and ground‐based (by the all‐sky imager at South Pole and ground‐based magnetometers) observations to simultaneously analyze the waves from the foreshock region to the dayside and nightside magnetosphere. Both of our two events show that the Pc5 ULF waves are generated by foreshock transients in the dayside magnetosphere. The in situ observations by THEMIS A and D and the 2‐D auroral signatures show that the compressional mode waves are likely broadband and coupled to the FLRs with different frequencies and different azimuthal phase speeds. This is the first report that foreshock transients can drive both low‐ and high‐m FLRs, with the azimuthal wave numbers varying from ~5 to ~23. Moreover, the Pc5 ULF waves propagated antisunward to midnight, this can potentially modulate magnetospheric and ionospheric dynamics globally.
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Estimating the Azimuthal Mode Structure of ULF Waves Based on Multiple GOES Satellite Observations
Abstract Characterizing the azimuthal mode number,m, of ultralow‐frequency (ULF) waves is necessary for calculating radial diffusion of radiation belt electrons. A cross‐spectral technique is applied to the compressional Pc5 ULF waves observed by multiple pairs of GOES satellites to estimate the azimuthal mode structure during the 28‐31 May 2010 storm. We find that allowing for both positive and negativemis important to achieve a more realistic distribution of mode numbers and to resolve wave propagation direction. During the storm commencement when the solar wind dynamic pressure is high, ULF wave power is found to dominate at low‐mode numbers. An interesting change of sign inmoccurred around noon, which is consistent with the driving of ULF waves by solar wind buffeting around noon, creating antisunward wave propagation. The low‐mode ULF waves are also found to have a less global coverage in magnetic local time than previously assumed. In contrast, during the storm main phase and early recovery phase when the solar wind dynamic pressure is low and the auroral electrojet index is high, wave power is shown to be distributed over all modes from low to high. The high‐mode waves are found to cover a wider range of magnetic local time than what was previously assumed. Furthermore, to reduce the 2nπambiguity in resolvingm, a cross‐pair analysis is performed on satellite field measurements for the first time, which is demonstrated to be effective in generating more reliable mode structure of ULF waves during high auroral electrojet periods.
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- Award ID(s):
- 1752736
- PAR ID:
- 10375095
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 124
- Issue:
- 7
- ISSN:
- 2169-9380
- Page Range / eLocation ID:
- p. 5009-5026
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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