Mid-latitude Radio-Frequency Auroras

Radio-frequency (RF) auroras, like the beautiful example shown in Figure 1 and the movie (link here), are often dancing across the night skies of Puerto Rico. Unlike their high-latitude cousins, however, these tropical phantasms do not generate optical emission and are not associated with solar activity. These are mid-latitude RF echoes observed with coherent radar and displaying radio signatures similar to those detected from the northern lights.

The Arecibo Observatory has been key to understanding the nature of these mid-latitude RF auroras. This unique cluster of diverse and highly sensitive equipment includes both radar and optical instruments. The ionized particles are observed with Arecibo’s incoherent scatter radar (ISR) and show billowy behavior as well as unexplained changes in the density and temperature distributions. At the same time, the neutral particles are observed with Arecibo’s lidars, which also show the billowy behavior and temperature changes. An example of the neutral billows using the sodium resonance lidar is shown in Figure 2. The background ionospheric medium is observed simultaneously with Arecibo’s imagers and ionosondes.

RF auroras appear to result from the interaction between neutral particles in the thermosphere and charged particles in the ionosphere. Neutral winds produce a shear that generates Kelvin-Helmholtz rolls. Current work suggests that RF auroras are produced by neutral particles that drag ions and result in coherent echoes. This finding is motivating a new area of research that focuses on the study of ion behavior to infer neutral winds. Recent work reported convective instabilities and other neutral dynamics during the daytime, opening up new possibilities to expand our understanding of ion-neutral coupling in the lower ionosphere and adding to our understanding of RF auroras.

Figure 1. Mid-latitude RF auroras observed on the night of September 30, 2008, with a coherent scatter radar located on St. Croix and looking perpendicular to the earth's magnetic field line over the Arecibo Observatory at an altitude close to 105 km. The contour corresponds to the island of Puerto Rican and the “X” to the location of the Arecibo Observatory. The color scale indicates variations in Doppler, intensity, and spectral width.

Animation related to Figure 1 Arecibo Observatory: Mid-latitude Radio-Frequency Auroras. By Eliana Nossa, Shikha Raizada, and David Hysell

Figure 2. Altitudinal and temporal distribution of Na atoms obtained using a resonance lidar over Arecibo. The main layer between 80-100 km has been saturated to reveal the structures between 101-110 km. Sarkhel et al. (2012) found atmospheric conditions to be dynamically unstable and conducive a Kelvin-Helmholtz instability during this event.

Paper Title: Dynamic instability in the lower thermosphere inferred from irregular sporadic E layers
Paper Authors: D.L. Hysell, E. Nossa, M.F. Larsen, J. Munro, S. Smith, M.P. Sulzer, and S.A. González
Paper Reference: J. Geophys. Res., (2012), 117, A08305, doi:10.1029/2012JA017910.
Keywords: Ionosphere, Auroras, Aeronomy, Nossa, Hysell, Raizada, MLT Region, Mesosphere, Thermosphere, E Region, Radio Propagation, Space, Atmosphere, Earth