Omega Bands as a Source of Large d <i>B</i> /d <i>t</i> in the Dawn Sector

Abstract Omega band activity has the potential to cause geomagnetically induced currents as they can cause large values of . We have undertaken a multi‐instrument study of an omega event which occurred from 13 to 14 November 2012, during a period of southward IMF. Measurements from the European Incoherent SCATter (EISCAT) radars in Troms show increases in electron density as the luminous aurora pass over head and the nearby optical all‐sky camera data confirms that there was omega band activity occurring during in the dawn sector. Enhancements at low altitude in the electron density profiles are observed in addition to the usual auroral precipitation, suggesting acceleration of electrons to high energies. Enhancements in cosmic noise absorption measured by a nearby riometer support this. These electron density profiles lead to large Hall conductances, which in turn, lead to large magnetic perturbations observed on the ground and large values of . The Y component of the magnetic field had a peak of 915 nT and there were corresponding large values of greater than 10 nT . The ground signatures of omega band activity in the dawn sector result in fluctuations in the AL index, and hence care must be taken when using substorm lists which use the AL index to identify substorm onset times.