Space Science Reviews

, Volume 206, Issue 1–4, pp 91–122 | Cite as

The Mid-Latitude Positive Bay and the MPB Index of Substorm Activity

  • Robert L. McPherron
  • Xiangning Chu


Substorms are a major source of magnetic activity. At substorm expansion phase onset a westward current flows through the expanding aurora. This current is the ionospheric closure of the substorm current wedge produced by diversion of tail current along magnetic field lines. At low latitudes the field-aligned currents create a systematic pattern in the north (X) and east (Y) components of the surface magnetic field. The rise and decay in X is called a midlatitude positive bay whose start is a proxy for expansion onset. In this paper we describe a new index called the midlatitude positive bay index (MPB) which monitors the power in the substorm perturbations of X and Y. The index is obtained by removing the main field, storm time variations, and the solar quiet (Sq) variation from the measured field. These are estimated with spline fits and principal component analysis. The residuals of X and Y are high pass filtered to eliminate variations with period longer than 3 hours. The sum of squares of the X and Y power is determined at each of 35 midlatitude stations. The average power in night time stations is the MPB index. The index series is standardized and intervals above a fixed threshold are taken as possible bay signatures. Post processing constrains these to have reasonable values of rise time, strength, and duration. Minima in the index before and after the peak are taken as the start and end of the bay. The MPB and AL indices can be used to identify quiet intervals in the magnetic field.


Substorm Substorm current wedge Field-aligned current Quiet day variation Midlatitude positive bay index (MPB) MPB onsets 



The principal author is grateful to the International Space Science Institute (ISSI) for inviting him to take part in the Workshop on “Earth’s Magnetic Field” held in Bern in May 2015. This work was supported by ISSI, NSF GEM 1003854, and NASA NESSF NNX14AO02H. We gratefully acknowledge THEMIS (themis., INTERMAGNET (, GOES, OMNI database (, SuperMAG (, and their data providers. The authors would also like to thank the NASA NSSDC and NASA VMO centers.

Supplementary material

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Copyright information

© Springer Science+Business Media Dordrecht 2016

Authors and Affiliations

  1. 1.Department of Earth, Planetary, and Space SciencesUniversity of California Los AngelesLos AngelesUSA

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