Conveners:
Jonathan Makela (jmakela@uiuc.edu)
John Mathews (jdmathews@psu.edu)
John Meriwether (john.meriwether@ces.clemson.edu)
2005 Monday 27 June 0130 - 0330 PM
A workshop entitled "Nighttime midlatitude F-region structures: What are they and how should we study them?" was held at the 2005 joint CEDAR/GEM workshop in Santa Fe, NM. The purpose of this workshop was to examine the current status of our understanding of the various forms of F-region structure that have been observed over the years at midlatitudes. Approximately 70 people attended the session. Although the focus of the session was on the Caribbean region, mainly due to coordinated studies centered on the Arecibo incoherent scatter radar in Puerto Rico, significant work has been performed at other longitude sectors. As was pointed out by the introductory talk given by Jonathan Makela (University of Illinois at Urbana-Champaign), many observations of different types of structure (F-layer height bands, brightness waves, MSTIDs, to name a few) have been carried out using a variety of instruments over the past thirty years. These structures can pose a hazard to space-based communication and navigation systems that rely on trans-ionospheric propagation as they tend to be characterized by sharp gradients and small-scale structure in electron density. The observations have been carried out in the past by a variety of instruments, including both incoherent and coherent scatter radars, imaging systems, Fabry-Perot interferometers, and GPS. In a few specific cases, campaigns have been carried out bringing many of these instruments together to perform coordinated observations. However, a large portion of results to date have been gathered by one or two instruments operating independently. One of the clear results from this workshop was the recognition of the need to perform more coordinated, long-term campaigns operating multiple instruments from a variety of closely-space locations to study these structures in more detail.
Wes Swartz (Cornell University) discussed how new technologies have evolved that give us new capabilities in studying these structures that were not available during the last set of campaigns held in the Caribbean. New radar modes have been developed for the Arecibo radar which give us higher resolution data in the E region, which may play an important role in the development of these structures. In addition, new methods are being used to image these irregularities using small portable coherent scatter radars. This will prove useful in comparing the radar data to other two-dimensional data sources, such as those provided by all-sky imaging systems, and will provide new insights into the physics of midlatitude F-region structures. Many other instruments have been upgraded and new technologies have been developed, all of which present new capabilities for studying midlatitude F-region structure. Bailes Brown (Harvard University), an undergraduate student, presented information on the MiniME Fabry-Perot interferometer (FPI) that is being developed at Clemson University. The miniaturization of the FPI will allow for collecting information on the thermospheric winds from locations that were previously unaccessible due to the infrastructure required by previous FPIs. MiniME will also have a higher sensitivity than previous systems, allowing for the study of the winds and temperatures during structuring events in greater detail than before.
The importance of the E region in the development of F-region structure discussed by Dr. Swartz was also echoed by Rebecca Bishop (Aerospace Corp.) and Russell Cosgrove (SRI). Dr. Bishop presented results from a study investigating E- and F-region coupling during the passage of tropical storms. Her results suggested that pressure variations caused by the passage of these storms could result in the launching of gravity waves which could be the seed for some of the structures seen in the midlatitude F region. Dr. Cosgrove presented modeling results from recent simulations. His model results showed how wave breaking in sporadic-E layers could translate into dramatic changes in the F-region electron density. More accurate measurements of, among other quantities, the Poynting flux are needed to verify these results. It was suggested that simultaneous measurements of the magnetically connected E and F regions are needed, something that could be accomplished during future campaigns. During the discussion, it was noted by several people that the modeling of midlatitude F-region structure is still in its infancy. Several researchers have studied two-dimensional linear models, but more realistic three-dimensional non-linear models are still needed to fully understand the complex physics at play. Thus, in addition to more experimental campaigns, it was suggested that more theoretical studies are needed.
Another new development is the use of microbaragraphs to search for the origin of the waves responsible for the generation of the F-region structure. John Mathews (Penn State University) presented data from three collocated instruments: the Arecibo ISR, an all-sky imager, and a microbaragraph. The data from the imager show a clear wave structure which can also be seen in the electron density profiles obtained from the radar, but only after a high-pass filter has been applied to the data. The coincident microbaragraph data shows evidence for an oscillation in the pressure corresponding to the oscillations in the other data sets. It was suggested that an array of microbaragraphs should be deployed in the Caribbean to study the spatial characteristics of these oscillations.
Finally, the importance of geographical coupling in the development of midlatitude stricture was discussed by two graduate students. Carlos Martinis (Boston University) showed results from an imager at the Arecibo Observatory and one in El Leoncito, Argentina. Although the two imagers are not exactly magnetically conjugate, similar features are often seen by the two. Interestingly, these two sites are subject to structures due to both equatorial physics (such as equatorial plasma bubbles) and the midlatitude physics discussed above. The importance of conjugate observations was discussed and it was noted that sites with better conjugacy need to be instrumented to study these phenomena. Michael Nicolls (Cornell University) presented results from a case study showing how a large-scale traveling ionospheric disturbance launched from the auroral region generated plasma instabilities over Arecibo. The traveling ionospheric disturbance was observed in GPS total electron content data over North America and in all-sky images at both Arecibo and Hawaii. The instabilities were observed by both the Arecibo ISR and the radar at Jicamarca, Peru at the magnetic equator, and were initiated in the morning hours after the F region had been raised to an abnormally high altitude. To study these conjugate and global events, more instrumentation and future collaborative campaigns are needed.
Overall, this session demonstrated the many outstanding questions relating to nighttime midlatitude F-region structure. There seems to be enthusiasm from many of our colleagues, suggesting that the time may be ripe for putting together a multi-technique, multi-site campaign following in the tradition of many other successful CEDAR campaigns. We would suggest that the appropriate time to carry out a prototype campaign would be in the summer of 2006. We encourage anyone with interest to contact the session organizers. By combining the strengths of many different instruments and the expertise of members of the CEDAR community, we believe we can begin to come to a better understanding of the physics causing these many different types of structures.