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Abstract In the present study we investigate the response of the dayside ground magnetic field to the sequence of interplanetary magnetic field (IMF)BYchanges during the May 2024 geomagnetic storm. We pay particular attention to its extraordinarily large (>120 nT) and abrupt flip, and use GOES‐18 (G18) magnetic field measurements in the dayside magnetosheath as a time reference. In the dayside auroral zone, the northward magnetic component changed by as much as 4,300 nT from negative to positive indicating that the direction of the auroral electrojet changed from westward to eastward. The overall sequence was consistent with the conventional understanding of the IMFBYdriving of zonal ionospheric flows and Hall currents, which is also confirmed by a global simulation conducted for this storm. Surprisingly, however, the time delay from G18 to the ground increased significantly in time. The delay was 2–3 min for a sharpBYreduction ∼30 min prior to theBYflip, but it became as long as 10 min for the zero‐crossing of theBYflip. It is suggested that the prolonged time delay reflected the travel time from G18 to the reconnection site, which sensitively depends on the final velocity at the magnetopause, that is, the inflow velocity of the magnetic reconnection. Around theBYflip, the solar wind number density transiently exceeded 100 cm−3, and should have increased further through the bow shock crossing. It is suggested that this unusually dense plasma reduced the reconnection rate, and therefore, the solar wind‐magnetosphere energy coupling due to the extraordinary IMF.
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The Growth of Ring Current/SYM‐H Under Northward IMF B z Conditions Present During the 21–22 January 2005 Geomagnetic Storm
Abstract The total energy transfer from the solar wind to the magnetosphere is governed by the reconnection rate at the magnetosphere edges as the Z‐component of interplanetary magnetic field (IMFBz) turns southward. The geomagnetic storm on 21–22 January 2005 is considered to be anomalous as the SYM‐H index that signifies the strength of ring current, decreases and had a sustained trough value of −101 nT lasting more than 6 hr under northward IMFBzconditions. In this work, the standard WINDMI model is utilized to estimate the growth and decay of magnetospheric currents by using several solar wind‐magnetosphere coupling functions. However, it is found that the WINDMI model driven by any of these coupling functions is not fully able to explain the decrease of SYM‐H under northward IMFBz. A dense plasma sheet along with signatures of a highly stretched magnetosphere was observed during this storm. The SYM‐H variations during the entire duration of the storm were only reproduced after modifying the WINDMI model to account for the effects of the dense plasma sheet. The limitations of directly driven models relying purely on the solar wind parameters and not accounting for the state of the magnetosphere are highlighted by this work.
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- PAR ID:
- 10490449
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Space Weather
- Volume:
- 21
- Issue:
- 10
- ISSN:
- 1542-7390
- 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/10.1029/2023SW003489
