Abstract
AbstractThe eigenfrequencies of standing Alfvén waves on closed magnetospheric field lines can be estimated using the cross‐phase technique. These eigenfrequencies can be used to monitor the plasma mass density distribution along the field line. So far, this has only been applied to ground‐based magnetometer data. The Super Dual Auroral Radar Network (SuperDARN) radars offer some benefits over magnetometers. They provide greater spatial resolution and coverage, as well as direct sensing above the E region ionosphere, which screens ultralow frequency (ULF) waves from the ground. However, there are significant data quality issues. These include the uncertain origin of radar backscatter, uneven sampling of data due to data gaps, and inaccurate fitting to the autocorrelation functions. Artificial backscatter from an ionospheric heater has been used to remove the uncertainty in backscatter location. We have developed a Lomb‐Scargle cross‐phase analysis for application to discontinuous radar data. The First Principles Fitting Methodology has been used to improve the fitted data products derived from the autocorrelation functions. Using these techniques, we have shown that it is possible to measure eigenfrequencies with SuperDARN data, and we have verified an example using ground‐based magnetometer data. Finally, we have demonstrated that the eigenfrequency signature in this example was caused by a broadband source of energy.
Highlights
Closed magnetospheric field lines can host standing Alfvén waves and have a natural set of frequencies, or eigenfrequencies
We have shown it is possible to apply the LS cross-phase technique to SuperDARN ionospheric radar data
Measured eigenfrequencies have been used in conjunction with suitable magnetic field models to estimate plasma mass densities with ground magnetometers in previous work (Berube et al, 2006; Sandhu et al, 2018; Waters et al, 1996), with Wharton et al (2018) showing that the higher harmonics can be routinely detected and used to determine the shape of the plasma mass density distribution
Summary
Closed magnetospheric field lines can host standing Alfvén waves and have a natural set of frequencies, or eigenfrequencies. The electric field component of ULF waves causes an oscillating ExB drift of the ionospheric plasma This can be observed in SuperDARN line-of-sight Doppler velocity measurements. We mean both the sampling rate is inconsistent and the time series contains significant data gaps, which is more likely for SuperDARN This results in errors in the spectral analysis when using the fast Fourier transform (FFT), which in turn affects the cross-phase. We use intervals of data from the Þykkvibær radar located on Iceland when the European Incoherent Scatter (EISCAT) ionospheric modification (heating) facility (Rietveld et al, 1993) was operating at Tromsø It was chosen over the Hankasalmi radar due to its eastward viewing direction, required to observe oscillations from the toroidal mode. We compare our SuperDARN eigenfrequency examples to ground magnetometer estimations of eigenfrequencies and explore the wave populations responsible for the eigenfrequency signatures
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