Estimating the ionospheric induction electric field using ground magnetometers

The ionospheric convection electric field is often assumed to be a potential field. This assumption is not always valid, especially when the ionosphere changes on short time scales T less than or similar to 5 $T\lesssim 5$ min. We present a technique for estimating the induction electric field using ground magnetometer measurements. The technique is demonstrated on real and simulated data for sudden increases in solar wind dynamic pressure of similar to ${\sim} $1 and 10 nPa, respectively. For the real data, the ionospheric induction electric field is 0.15 +/- $\pm $ 0.015 mV/m, and the corresponding compressional flow is 2.5 +/- $\pm $ 0.3 m/s. For the simulated data, the induction electric field and compressional flow reach 3 mV/m and 50 m/s, respectively. The induction electric field can locally constitute tens of percent of the total electric field. Inclusion of the induction electric field increased the total Joule heating by 2.4%. Locally the Joule heating changed by tens of percent. This corresponds to energy dissipation that is not accounted for in existing models.