Introduction
The Remote Equatorial Nighttime Observatory of Ionospheric Regions (RENOIR) project is a joint collaboration between researchers from several institutions, including The University of Illinois at Urbana-Champaign, Clemson University, Cornell University, the Brazilian National Institute for Space Research (INPE) and the Federal University at Campina Grande (UFCG). Through the construction and deployment of a RENOIR station, we hope to come to a better understanding of the variability in the nighttime ionosphere and the effects this variability has on critical satellite navigation and communication systems.
The instruments included in a RENOIR station allow for the study the ionospheric effects caused by equatorial plasma instabilities and thermosphere-ionosphere coupling. These two areas are critical to characterizing the ionosphere and the effects it can have on radiowave propagation. The occurrence of equatorial plasma instabilities, commonly referred to as equatorial spread-F, equatorial plasma bubbles, or depletions, can cause radio signals propagating through the disturbed region to scintillate. This results in a fade in received signal power translating to a loss of the signal. Scintillations on frequencies from several GHz and below are known to occur and are a concern to many sectors, both civilian and military. The is demonstrated in the figure to the right, which shows data collected from a collocated imaging system and GPS SCINTMON on the Haleakala Volcano on Maui, HI. Scintillations are seen (as an increase in the S4 index in the top-right panel) when the look direction from the receiver to the satellite (the green square on each image) passes through the dark region of the image.
As this system is highly dynamic, with both the satellites and the airglow depletions in motion, it is sometimes better to visualize the data using a movie. This movie shows images collected by two imaging systems on the Haleakala Volcano and tracks three different GPS satellites. As with the image above, the scintillation index (S4) increases when the look direction passes through the depleted regions of the images.
Since the first observations of these instabilities in the late 1920s, much has been learned. We have gained a general understanding of the seasons when they are most likely to occur for a given location. For example, two year’s worth of occurrence statistics is summarized in the image to the left for the data collected from the Haleakala Volcano. The enigma comes from the fact that for a given location and season conducive to the growth of the instability, on a day-to-day basis, we are not very good at predicting if it will actually occur. In other words, within the “spread-F” season of a given location, instabilities do not always occur. The converse is also true, in that we sometime will see instabilities outside of the typical season for a given location. Understanding this day-to-day variability has become a highly active area of research in the ionospheric community, and much work remains to be done before we have a complete understanding of this phenomenon.
Instrumentation
The equipment comprising a single RENOIR station will consist of:
- one wide-field ionospheric imaging system,
- two miniaturized Fabry-Perot interferometers (FPI),
- a dual-frequency GPS receiver,
- an array of five single-frequency GPS scintillation monitors.
The wide-field imaging system will be used to characterized the two-dimensional (latitude vs longitude) structure of the depletions. This will be done by measuring the natural emissions occurring in the ionosphere at wavelengths of 630.0 and 777.4 nm. The two FPI systems will be used to measure the background thermospheric neutral winds and the neutral temperature. From the FPI data, we will be able to deduce what, if any, control neutral dynamics have on the development of these irregularities. Two systems are included so we can field them at sites separated in latitude in order to study wind gradients and gravity waves known to be present in the thermosphere. The dual-frequency GPS receiver is used to characterize the electron density present in the ionosphere. This data will be used to deduce how the background electron density affects the development of irregularities. Having a GPS receiver collocated with the other equipment is crucial as the severe gradients associated with the depletions can create ambiguities when using data from instruments separated by even a relatively short distance. The array of single-frequency GPS receivers will be used to measure the drift velocities of the small-scale irregularities internal to the large-scale plasma depletions observed by the imaging system. In this way, we will be able to deduce how the internal velocities of the small-scale irregularities relate to the overall drift velocity of the depletions and the background thermospheric neutral wind. The GPS equipment will also be used to characterize the adverse effects these irregularities have on L-band transionospheric signals.
Current Deployment
In May 2009, the installation of the RENOIR equipment was completed by Prof. Makela, Prof. Meriwether, and two students (graduate students from UI and Clemson University) in collaboration with Brazilian colleagues from INPE and UFCG. The two sites, Cajazeiras (geographic: -6.89 N, -38.56 E; geomagnetic: -5.75 N, 32.97 E) and Cariri (geographic: -7.39 N, -36.53 E; geomagnetic: -6.81 N, 34.69 E), are located in eastern Brazil. This region of Brazil was selected in order to accomplish the scientific goals of RENOIR by locating the equipment under the bright equatorial anomaly region south of the magnetic equator. This will increase the optical signal measured by the FPI and imaging systems, while also maximizing the effects that the equatorial plasma bubbles will have on trans-ionospheric wave propagation. This region is arid and skies are particularly clear during the winter dry season and has prooven to be conducive to optical observations in past campaigns. Data collected can be viewed in our database.
Data from Brazil have been collected since 2009. This has encompassed both deep solar minimum conditions through the current rise towards solar maximum. As such, the results represent a unique and near-continuous characterization of the temperatures and neutral winds at low latitudes. Summaries of the collected data are found below.
Future Deployment Scenarios
As additional funding and instrumentation become available, we are interested in constructing and fielding addition RENOIR stations in collaboration with the United Nations International Space Weather Initiative (ISWI). We are particularly interested in deployment possibilities in Africa. Ideally, the RENOIR stations would be fielding in Africa at a longitude of approximately 7 degrees from the magnetic equator. Scientists who are interested in collaborating in hosting a RENOIR station are encouraged to contact Professor Jonathan J. Makela.
The instrumentation that make up a RENOIR station have all been used in the field in previous experiments and are at a moderately mature level of development. The optical systems can be housed in individual, self-contained housing units, requiring very little infrastructure. If an optical facility is available at a host institution, the optical equipment could easily be modified to interface with available optical domes. The facility should be located in a region with relatively dark skies (away from any major cities) and away from any tall structures (buildings and trees). If two Fabry-Perot interferometers are to be fielded, the second system should be located approximately 300 km away from the main site. The dual-frequency GPS receiver is quite rugged and simply requires a location to mount the antenna and minimal space to locate the control computer. The array of single-frequency GPS scintillation monitors requires a space of approximately 100 m x 100 m over which to space the 5 antenna in a cross formation. Again, minimal space is needed to locate the control computers for each receiver. The facility should be located away from any tall strucutres (buildings and trees).
This material is based upon work supported by the National Science Foundation under Grant Number (AGS-0940253). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
Collaborators
Parties interested in collaborating in the RENOIR project are encouraged to contact Professor Jonathan J. Makela. We are actively seeking partners who are interested in hosting a RENOIR station. The requirements for potential sites are described above.