The summation of growth rates of the upper hybrid instability for a variety of wavelengths. The colored regions show where the upper hybrid instability will occur in the ionosphere. The contours of w_uh/Ω are plotted as well, showing the gaps in the instability occur where w_uh≈nΩ and Landau damping by thermal electrons suppresses the instability.
Geophysical Research Letters—Nicholas Pedatella
For decades large radars have observed strong, unexplained echoes returning from altitudes of 130-170 km in the atmosphere. All radars work by reflecting radio waves off a target and measuring the returned signal. For atmospheric radars the target is free electrons within the plasma in the upper atmosphere. Since the free electrons are typically a disordered gas the radio waves are reflected in random directions. This means some process is needed to create a coherent structure in the plasma for the radio waves to strongly reflect back in the direction of the radar. In this paper we show that the region between 130-170 km in the upper atmosphere is likely to create and maintain a specific set of plasma waves that act as a coherent structure for radar measurements. We show that predictions of where the plasma waves are generated match well with the observed patterns of these “150 km echoes”. This is the first research to provide a specific explanation for what causes 150 km echoes. In understanding this cause, we learned more about the Sun’s influence on our upper atmosphere while expanding the capabilities of atmospheric radars.