Patchy layers of metallic ions in Earth’s atmosphere, called sporadic E layers, are often observed during the night at midlatitudes across the globe. Recently, however, patchy E layers were observed at higher latitudes above Alaska and during the daytime.
Hysell et al. suggest that the high-latitude, daytime E layers they observed likely form when neutral waves in the atmosphere interact with Earth’s ionospheric layer—between 90 and 125 kilometers above the surface. During their measurement, the scientists also discovered an unusual behavior of field-aligned plasma density irregularities (FAIs) at a certain radio frequency in the layers.
Metallic E layers are between 1 and 2 kilometers thick and cause radio waves to reflect, refract, diffract, or scatter off course. This can deter radio communication. Using the High Frequency Active Auroral Research Program facility in Gakona, Alaska, the researchers generated high-frequency radio emissions to measure instabilities in the patchy E layers. The authors compare this approach to the process of injecting dye into biological tissue to highlight functions that would otherwise be invisible.
The authors discovered that when the radio frequencies matched the critical frequency of the layers, they measured a characteristic quasiperiodic FAI echo. Above the third electron gyroharmonic frequency, however, the echo strength diminishes, they note. FAI echoes have been observed before in the E and higher-altitude F layers, but those echoes are unlike the ones the researchers observed at this frequency. The authors speculate on what causes echo strengths to diminish at this frequency but remain unsure of how to interpret this strange behavior. (Geophysical Research Letters, doi:10.1002/2014GL061691, 2014)
—Jessica Orwig, Freelance Writer
Citation: Orwig, J. (2015), Unusual echo signal in atmospheric E layer, Eos, 96, doi:10.1029/2015EO025945. Published on 11 March 2015.