This project is to derive empirical estimates of the electric field-aligned (Pederson) current carried by electrons in the auroral ionosphere near 120 km (the E-region). The electron Pederson current, typically much smaller than that carried by ions, is hypothesized to be driven under conditions of strong electric fields by the dissipation of plasma instabilities known as Farley-Buneman waves. Estimates of this "anomalous" Pederson current will be inferred from discrepancies between total ground magnetic deflection as measured by an extensive network of high-latitude magnetometers and that derived from incoherent scatter radar (ISR) measurements using a multiple-beam transmission mode. A large set of combined magnetometer and ISR data will be used to generate statistical estimates of the anomalous electron Pederson current and associated turbulence heating rate of the thermosphere/ionosphere. This project will involve the participation of both graduate and undergraduate students during summer collaborations between SRI International and Worcester Polytechnic Institute.
The objective of this NSF project is to understand the electrical currents and plasma flows in the ionosphere to better predict solar-wind-driven energetic processes in the Earth's thermosphere. The solar wind-magnetosphere dynamo drives large electrical currents on the magnetosphere-ionosphere current circuit. At most thermospheric regions, electrical currents flow without anomaly. However, when electrical currents become intense, plasma instabilities lead to ionospheric turbulence at a wide range of spatial scales. Radars, in situ sounding rockets, and satellite measurements have shown that meter-scale waves in the form of plasma turbulence exist on the lower part of the magnetosphere-ionosphere circuit. However, the turbulence-driven excess electrical currents and energy dissipation are not well known. A general objective of this project is to assess turbulence-driven currents by determining the empirical relationship of geomagnetic perturbations on the ground relative to Hall and Pedersen currents in the ionosphere. With the construction of an Incoherent Scatter Radar in Resolute Bay, Canada, where the geomagnetic field is nearly perpendicular to the ground, a unique set of high spatial and temporal resolution data have become available to infer electrical currents by comparing the directions of magnetic deflections on the ground to ionospheric plasma flows measured by the radar. Our analysis of radar, magnetometer and neutral wind data show that magnetic deflections on the ground are not purely due to the ionospheric Hall currents. That is, the magnetic field due to the magnetic-field-aligned currents does not cancel the magnetic field due to the ionospheric Pedersen current, as commonly assumed according to the Fukushima theorem. In fact, the direction of the magnetic deflection can deviate from what would be caused by an equivalent Hall current by as much as 70 degrees. The NASA Global Magnetosphere model (SWMF) for two of the magnetometer stations (Resolute Bay and Thule) and for the days/times of the experimental data, show, in agreement with the radar/magnetometer dataset, that the ground-magnetic deflection is not purely due to the ionospheric Hall current. This finding is critical because the methodology of "equivalent ionospheric currents", which is often used in global modeling of ionospheric circulation, is purely based on Hall currents, and therefore, the equivalent current would significantly deviate from the actual currents. Part of the funds supported the analysis and modeling of Radio Aurora Explorer CubeSat radar data to quantify turbulence wave energy dissipation due to turbulence-driven Pedersen currents. The analysis involved fitting to the data a model wavevector distribution function of wave propagation angle. The more waves propagate off the plane perpendicular to the magnetic field, the more energy is dissipated by the waves via parallel electric fields. This wave-driven energy dissipation is supplied by the turbulence-driven or anomalous Pedersen currents. We find that the deviation of wave propagation off the plane of perpendicularity is roughly an order of magnitude lower than is commonly known. If the measured waves, which are meter-scale, are representative of Farley-Buneman turbulence, this finding corresponds to an order of magnitude lower anomalous Pedersen current than commonly thought. This means that the turbulence contribution to Joule energy dissipation is possibly smaller than originally thought. The project has contributed to the professional development of a graduate student who is now an SRI staff member managing incoherent scatter radar data for the Advanced Modular Incoherent Scatter Radar (AMISR), a community radar operated by SRI in cooperative agreement with the NSF.
