FPI

Fabry-Perot Interferometer

The Fabry-Perot interferometer (FPI) is designed to make routine observations of thermospheric and mesospheric winds and temperatures. This is done from observations of the Doppler shift and Doppler broadening, respectively, of suitable airglow and auroral optical emissions. The instrument started recordings in 1996. The EISCAT radar can measure ion velocities and temperatures with high height resolution. The close proximity of the two experiments allows ionospheric-thermospheric interactions to be studied in the auroral zone over northern Scandinavia. Unmanned measurements are automatically made for all dark periods.

The following wavelengths are currently used in different altitude ranges where Un, Tn, Vi and Ti are the neutral wind, neutral temperature, ion velocity and ion temperature respectively.

Emission	Wavelength (nm)	Height (km)	Quantity
OIS	557.7	~ 97 and > 100	Un + Tn
OID	630.0	~ 260 +- 20	Un + Tn
OIIP	732.0	~ 280 +- 80	Vi + Ti
OH(6,2)	843.0	87 +- 3	Un + Tn

The experiment is equipped with a steerable mirror system permitting any part of the sky to be viewed. The time resolution per measurement is 1-5 minutes depending on wavelength and actual geophysical conditions. The actual height of the emission observed is always uncertain as this is a passive experiment.