Introduction

Traditionally, it was thought that the only highly active portions of the terrestrial ionosphere lay at high and low latitudes. In the polar region, the magnetic field is coupled to the magnetosphere and interstellar medium, allowing energy to flow directly into the ionosphere. This coupling results in many phenomena, most familiarly the aurora, and can create a very turbulent state leading to intense scintillations on trans- ionospheric radio signals. At low latitudes, the nearly horizontal magnetic field is conducive to the generation of irregularities by the generalized Rayleigh-Taylor instability. The requirements for this mechanism are met in the post-sunset equatorial ionosphere, resulting in the explosive release of the stored gravitational energy in the ionosphere. Irregularities across many decades of scale sizes (centimeters to kilometers) develop and an intense scintillation environment is created, leading to communication and navigational outages.

In contrast to these highly active regions, the mid-latitude ionosphere has generally been considered benign and quiescent. However, as more technology and infrastructure located in this latitude range becomes dependent on satellite-based systems, it is important to study this region in greater detail. Recent experiments indicate that severe space weather can indeed occur at mid-latitudes. For example:

• A campaign carried out in the Caribbean during the later portion of the 1990s showed conclusive evidence that severe gradients in electron density, rivaling those seen at low latitudes, can exist in the nighttime mid-latitude ionosphere (Makela et al., 2000).
• During the major magnetic storms that occurred near the end of 2003, localized “plumes” of enhanced total electron content (TEC) were observed to develop over the contiguous United States (CONUS), surging from the south. These plumes severely degraded the accuracy of the GPS system (Doherty et al., 2004) and led to scintillations over CONUS (Basu et al., 2005). Similar structures have been observed during other major magnetic storms (e.g., Foster et al., 2002, 2004) extending from the Caribbean, through CONUS, to the polar region.
• An extreme event of the Rayleigh-Taylor instability was observed at the beginning of October 2002 using the incoherent scatter radar (ISR) at the Arecibo Observatory in Puerto Rico, where the instability is generally believed not to occur (Nicolls and Kelley, 2005).
These case studies, as well as several others over the years, indicate without a doubt that the mid-latitude ionosphere can awaken from its typical dormancy and behave as violently as that at high and low latitudes.

Only recently has the space science community begun to operate enough equipment at mid-latitudes to systematically observe these phenomena. This has primarily been driven by the explosion of satellite- based technologies, both in civilian and military settings, over the past decade. For example, the Federal Aviation Administration (FAA) has implemented, and declared operational, the Wide Area Augmentation System (WAAS), used to aid commercial airline navigation. During times of intense geomagnetic activity,the severe gradients that occur in the ionosphere over CONUS can force the WAAS system to shut down (e.g., Doherty et al., 2004). Similarly, with the increased use of HF and higher frequencies for communications, an uncharacterized and unexpected development of ionospheric structure can lead to a temporary loss of that communication channel (e.g., Groves et al., 1997).

What is proposed is to deploy a suite of complimentary instruments to two sites in the Caribbean that will observe and characterize the space weather that occurs at the transition from low- to mid-latitudes. The instruments will augment those already in place at the Arecibo Observatory in Puerto Rico, vastly increasing the amount of the ionosphere that can be simultaneously observed. Measurements will be made over several years to gain a better understanding of the background conditions conducive to local irregularity growth and coupling of the low- and mid-latitude ionosphere. Equipment will be characterized, deployed, and maintained with the assistance of students, providing valuable hands-on engineering experience.

The specific scientific questions to be addressed under this proposal are:

1. What are the physical extent, seasonal properties, and lifetimes of nighttime F-region structures observed over the Caribbean? This will be accomplished using the extended fields of view provided by the two ionospheric imaging systems to be deployed in conjunction with the Arecibo facility imaging system. A long-term database needs to be constructed to make progress on detailing the morphology of these structures. Thus, the equipment will remain in the field for several years.
2. What is the genesis region and mechanism for the different types of structures present in the nighttime F-region ionosphere? Do they grow locally, or are they coupled from low latitudes? What effect on trans-ionospheric radio wave propagation do these irregularities have? Progress on this front will be made possible by combining the extended fields of view of the various imaging systems (to study the genesis region) with quantitative data on the background ionospheric conditions provided by the incoherent scatter radar and Fabry-Perot interferometer at the Arecibo Observatory.
3. Are the enhancements in electron density commonly seen in the American sector during severe geomagnetic storms effective in creating scintillations on critical trans-ionospheric radio links? Information gathered from the GPS receivers deployed under this proposal will be used to study these effects. The receivers will also serve to bridge the gap in GPS coverage between South America and CONUS. When storms occur in the local night sector, the imaging systems will be used to study any resultant structuring in the ionosphere.

Instrumentation

Two types of instrumentation will be deployed to study the mid-latitude ionosphere under this project:

• a wide-field ionospheric imaging system,
• a dual-frequency GPS receiver capable of making 50-Hz scintillation measurements

The wide-field imaging system will be used to characterized the two-dimensional (latitude vs longitude) properties of the ionospheric structure. This will be done by measuring the natural emissions occurring at wavelengths of 630.0 and 777.4 nm. The imaging system to be used is called the Portable Ionospheric Camera and Small-Scale Observatory (PICASSO) and is a miniaturized version of systems that have been used in the past. The reduction in size allows for easier deployment and the need need for minimal infrastructure. These systems have been successfully deployed in the US, S. America, and the Pacific sectors.

The dual-frequency GPS receiver is used to characterize the electron density present in the ionosphere. The system to be deployed is a NovAtel GSV4004B, modified by GPS Silicon Valley. This data will be used to deduce how the background electron density affects the development of irregularities. Having a GPS receiver collocated with the PICASSO 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 high-rate (50-Hz) data collected by the receiver will be used to characterize the adverse effects these irregularities have on L-band transionospheric signals.

In addition to the instrumentation that will be deployed specifically for this project, we intend to
collaborate with colleagues at the Arecibo Observatory to utilize the ISR, GPS receivers, Fabry-Perot
interferometers (FPI), and all-sky imaging system located there. The ISR and FPI will be especially
useful in constructing the physical framework in which the irregularities being studied occur. They will
provide information on the background electron densities, electric fields, and neutral winds that are not
obtainable from the other instruments.

Deployment

The previous Caribbean Ionospheric Campaigns focused on fielding instrumentation near the island of Puerto Rico in order to take advantage of the Arecibo ISR. One of the limitations of these campaigns was that most of the time, structure drifted into the fields of view of the instruments fully developed. This made determining the seeding region and mechanisms impossible. In order to address this deficiency, we wish to install instrumentation at various locations throughout the Caribbean for this study. Since many of the more spectacular events are observed to surge over Puerto Rico from the south, we will concentrate on instrumenting the islands in the southern portion of the Caribbean, off the northern coast of South America. In this way, we will be able to determine the genesis region of these irregularities, as well as provide a link between the instrumentation already in place, and soon to be installed, in South America and that at Arecibo.

Ideally, we would like to have one set of instruments (PICASSO and GPS) in the southwestern Caribbean (e.g., northern Venezuela, Aruba, Curacao) and one set in the southeastern Caribbean (e.g., Trinidad and Tobago, Isla de Margarita, Grenada). The requirements for a host site are fairly minimal and include:

1. Access to stable/reliable electrical power (power requirements are approximately the same as if powering three desktop computers)
2. Removed from contaminating lights
3. Horizon relatively unobstructed down to 15 degrees elevation
4. A relatively high proportion of clear nights versus cloudy nights
5. Access to the internet (desired but not strictly required)

The PICASSO system is either installed in a building at the host site with a hole cut in the ceiling covered by an optical observing dome or in a custom-built miniature enclosure that sits on a building roof. The GPS antenna is mounted on a pole which can easily be attached to a roof. We plan to have the equipment deployed for several years to study the seasonal trends of ionospheric structure near solar minimum.

Collaborators

Parties interested in collaborating in this project are encouraged to contact Professor Jonathan J. Makela. We are actively seeking partners who are interested in hosting a station. The requirements for potential sites are described above. All data collected will be made available to any collaborators in addition to the community at large.

This project is funded through a grant from the US National Science Foundation.