FAI observations with MU radar using spaced antenna interferometry 1990 - 2001

*Gernot Hassenpflug[1] ,Mamoru Yamamoto [1]
Shoichiro Fukao [1]
Radio Science Center for Space and Atmosphere[1]

The MU radar has been used to obtain data on field-aligned plasma irregularities known as FAIs since the late 1980s. Primarily to study aspect sensitivity, spaced antenna interferometry has been used since 1990 to observe the in-beamwidth extent. In this paper we compare results of the years 1990-2001, and show how improved radar capabilities allow us to observe at much higher spatial resolution than before. It appears thatin some cases, where the height extent of echoes is extremely large, refractivity due to the atmosphere may need to be taken into account if the echoes are to come from the direction along the geomagnetic field lines. This also allows us to possibly classify FAI echoes according to their aspect sensitivity signature.

The occurrence of plasma irregularities in the ionosphere at an altitude of approximately 100km, in a direction apparently along the geomagnetic field lines, has been known for more than two decades. The detailed observation of this phenomenon, known as Field-Aligned Irregularities, has been limited by the available instruments. Along the equatorial latitude, the Jicamara radar has provided years of important data for analysis, while in the mid-latitude region, the MU radar has been used to obtain data on FAIs since the late 1980s. Primarily to study the angular aspect sensitivity of the FAIs, spaced antenna interferometry was used since 1990 to observe the in-beamwidth extent of the echoes. Several papers describing these experiments were published, but no major work has been undertaken since 1997. In 2000 and 2001 interferometry experiments using the MU radar's improved resolution and data processing capabilities were carried out and the results of these experiments compared to previous data. In this paper we present the comparative results of the years 1990-2001, and show how the improved radar capabilities allow us to observe at a much higher spatial resolution than before. The main focus of the analysis, namely the angular aspect sensitivity, remains unchanged. It is evident from the results that in some cases, where the height extent of echoes is extremely large, refractivity due to the atmosphere may need to be taken into account if the echoes are to come from the direction along the geomagnetic field lines as postulated in theory. The modeling of this refractivity will be the subject of future research. Using the available data it may be possible to classify FAI echoes by their observed aspect sensitivity. The mechanism by which these echoes occur is not yet explained, and a classification may help in our understanding of the different mechanisms involved in the process.