- Arecibo Celebrates National Engineers Week 06 Apr, 2022
- The Arecibo Observatory at the Upcoming 240th American Astronomical Society Meeting06 Apr, 2022
- The Arecibo Observatory Survey Salvage Committee Report06 Apr, 2022
- Facilities and Operations Update06 Apr, 2022
- PRISMA Meteor Radar Arrives at AO04 Apr, 2022
- The Grand Reopening of the Angel Ramos Science and Visitor Center at the Arecibo Observatory01 Apr, 2022
- Orbital stability analysis and photometric characterization of the second Earth Trojan asteroid 2020 XL531 Mar, 2022
- Arecibo Celebrates International Women’s Day31 Mar, 2022
- A Letter from the Director Eng. Francisco Cordova31 Mar, 2022
- The History of Arecibo’s Legacy Telescope to Impact the Future, Thanks to the AO Salvage Survey Committee31 Mar, 2022
- Announcing AO/GBT Single Dish Summer School May 16th - 20th, 2022 30 Mar, 2022
- NSF REU program at Arecibo receives funding for next 3 years23 Mar, 2022
- A Parkes "Murriyang" Search for Pulsars and Transients in the Large Magellanic Cloud23 Mar, 2022
- Noise analysis in the European Pulsar Timing Array data release 2 and its implications on the gravitational-wave background search23 Mar, 2022
- Arecibo S-band Radar Characterization of Local-scale Heterogeneities within Mercury's North Polar Deposits23 Mar, 2022
- Arecibo’s Eye on the Sun21 Mar, 2022
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