University of Oulu

Holappa, L., Asikainen, T., & Mursula, K. ( 2020). Explicit IMF By Modulation of precipitating electrons. Geophysical Research Letters, 47, e2019GL086676.

Explicit IMF \(B_y\) dependence in geomagnetic activity : modulation of precipitating electrons

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Author: Holappa, L.1; Asikainen, T.1; Mursula, K.1
Organizations: 1ReSoLVE Centre of Excellence, Space Physics and Astronomy Research Unit, University of Oulu, Oulu, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 1 MB)
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Language: English
Published: American Geophysical Union, 2020
Publish Date: 2020-08-06


The most important driver of geomagnetic activity is the north–south (\(B_z\)) component of the interplanetary magnetic field (IMF), which dominates the solar wind-magnetosphere coupling and all solar wind coupling functions. While the east–west (\(B_y\)) IMF component is also included in most coupling functions, its effect is always assumed to be symmetric with respect of its sign. However, recent studies have shown that, for a fixed value of any coupling function, geomagnetic activity is stronger for \(B_y > 0\) than for \(B_y < 0\) in Northern Hemisphere winter. In Southern Hemisphere winter, the dependence on the \(B_y\) sign is reversed. In this paper, we use measurements of National Oceanic and Atmospheric Administration Polar-Orbiting Operational Environmental Satellites to show that the flux of magnetospheric electrons precipitating into the atmosphere also exhibits an explicit \(B_y\) dependence. This \(B_y\) dependence is strong in the midnight and dawn sectors where it causes a related \(B_y\) effect in the westward electrojet and geomagnetic activity by modulating ionospheric conductivity.

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Series: Geophysical research letters
ISSN: 0094-8276
ISSN-E: 1944-8007
ISSN-L: 0094-8276
Volume: 47
Issue: 4
Article number: e2019GL086676
DOI: 10.1029/2019GL086676
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: We acknowledge the financial support by the Academy of Finland to the ReSoLVE Centre of Excellence (project 272157), to the postdoctoral researcher project of LH (322459), and to the PROSPECT project (321440).
Academy of Finland Grant Number: 272157
Detailed Information: 272157 (Academy of Finland Funding decision)
322459 (Academy of Finland Funding decision)
321440 (Academy of Finland Funding decision)
Dataset Reference: The solar wind data were downloaded from the OMNI2 database ( All the original POES/MEPED energetic particle data used here are archived in the NOAA/NGDC dataserver (
Copyright information: © 2020. American Geophysical Union.