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. In the study of ionospheric dynamics, it is often assumed that the ionospheric electric field is a potential field. This means the contribution from induction is neglected. The induction electric field is described by Faraday's law and relates to temporal changes in the magnetic field. This assumption only holds when the ionospheric dynamics change slowly. In this study, we present a technique for calculating the ionospheric induction electric field using measurements of the magnetic field on the ground. We demonstrate the technique on real and simulated data of a dynamic event, that is, a sudden commencement. We find that the induction electric field, on a global scale, is small compared to the potential electric field. Inclusion of the induction electric field increased the total energy dissipation, that is, Joule heating, by only a couple of percent but resulted in local variations of tens of percent. Furthermore, we quantified and visualized the compression flow which is the compression and expansion of the magnetic field related to the temporal evolution of a dynamic ionospheric event. A method for estimating the induction electric field using ground magnetometer measurements is presented Locally, the estimated induction electric field can constitute tens of percent of the total electric field The spatial pattern of ionospheric Joule heating is shown to be highly affected by the induction electric field, even during weak induction