title: Quantitative maps of geomagnetic perturbation vectors during substorm onset and recovery.
abstract: We have produced the first series of spherical harmonic, numerical maps of the time-dependent surface perturbations in the Earth's magnetic field following the onset of substorms. Data from 124 ground magnetometer stations in the Northern Hemisphere at geomagnetic latitudes above 33° were used. Ground station data averaged over 5 min intervals covering 8 years (1998-2005) were used to construct pseudo auroral upper, auroral lower, and auroral electrojet (AU*,AL*, andAE*) indices. These indices were used to generate a list of substorms that extended from 1998 to 2005, through a combination of automated processing and visual checks. Events were sorted by interplanetary magnetic field (IMF) orientation (at the Advanced Composition Explorer (ACE) satellite), dipole tilt angle, and substorm magnitude. Within each category, the events were aligned on substorm onset. A spherical cap harmonic analysis was used to obtain a least error fit of the substorm disturbance patterns at 5 min intervals up to 90 min after onset. The fits obtained at onset time were subtracted from all subsequent fits, for each group of substorm events. Maps of the three vector components of the averaged magnetic perturbations were constructed to show the effects of substorm currents. These maps are produced for several specific ranges of values for the peak |AL*| index, IMF orientation, and dipole tilt angle. We demonstrate an influence of the dipole tilt angle on the response to substorms. Our results indicate that there are downward currents poleward and upward currents just equatorward of the peak in the substorms' westward electrojet.
  
  
  CONCLUSIONS
  Show quantitative maps of ground geomagnetic perturbations due to substorms Three vector components mapped as function of time during onset and recovery Compare/contrast results for different tilt angle and sign of IMF Y-component.
authors.affiliation: Department of Atmospheric and Planetary Sciences, Hampton University Hampton, Virginia, USA ; National Institute of Aerospace Hampton, Virginia, USA ; Bradley Department of Electrical and Computer Engineering, Center for Space Science and Engineering Research, Virginia Tech Blacksburg, Virginia, USA ; Department of Atmospheric, Oceanic, and Space Science, University of Michigan Ann Arbor, Michigan, USA.
authors.forename: N M
authors.initials: NM
authors.lastname: Pothier
authors.affiliation: National Institute of Aerospace Hampton, Virginia, USA ; Bradley Department of Electrical and Computer Engineering, Center for Space Science and Engineering Research, Virginia Tech Blacksburg, Virginia, USA.
authors.forename: D R
authors.initials: DR
authors.lastname: Weimer
authors.affiliation: Department of Atmospheric and Planetary Sciences, Hampton University Hampton, Virginia, USA ; National Institute of Aerospace Hampton, Virginia, USA.
authors.forename: W B
authors.initials: WB
authors.lastname: Moore
keywords: aurora; current systems; geomagnetic effects; ionosphere; substorms