Remote sensing of mid-latitude ionospheric magnetic field fluctuations using cosmic radio sources

This paper describes a new method for remote sensing of magnetic field fluctuations at ionospheric altitudes using a relatively long-baseline interferometer and exceptionally bright cosmic radio sources at 35 MHz. The technique uses sensitive measurements of the difference in phase between two phased array telescopes separated by about 75 km and between the right and left circular polarizations to measure the amount of differential Faraday rotation. Combined with estimates of the background magnetic field and total electron content, these can be converted to measurements of fluctuations in the differential magnetic field parallel to the line of sight, ΔΒ||. The temporal gradient in ΔΒ|| roughly follows the diurnal pattern expected for B|| due to the vertical gradient in the background electric field, but at roughly 25% the magnitude and offset by ∼50 nT hr⁻¹. This suggests that the diurnal variation in the electric fields observed by the two telescopes are similar but slightly different (|ΔE| ≤ 0.1 mV m⁻¹). Fluctuations in ΔΒ|| were typically ∼10–30 nT with wavelike fluctuations often apparent. These typically have oscillation periods of about 10–30 min, similar to traveling ionospheric disturbances (TIDs). Simultaneous observations toward two sources separated by 25.4° on the sky (∼140 km in the F-region) show a few detections of wavelike disturbances with lags of ±10–30 min between them. These imply speeds on the order of 100–200 m s⁻¹, also similar to TIDs. We estimate that gravity waves with amplitudes within the dynamo region of ∼10 m s⁻¹ could generate the observed fluctuations in ΔΒ||.