An analysis of some 523 F layer patches observed over Eureka, Northwest Territories, Canada (89° corrected geomagnetic latitude) during the four winters from 1990–1991 to 1993–1994 has given some definitive results on their occurrence patterns and characteristics. They are observed on average about 25% of the time and are seen at all hours of the day but with more present in the local evening hours than the morning hours. For the 182 patches for which interplanetary magnetic field data were available, 143 of the patches occurred with Bz negative and 39 with Bz positive. In most of the latter cases, Bz was either near zero or had changed to positive only 30–120 min prior to the patch observation. The average optical emission intensity of the patches over the 4-year period was 150 R of 630 nm and 55 R of 558 nm. The excitation is by dissociation recombination of the oxygen ion with a branching ratio O[1S]/O[1D] of 0.37. They were on average less bright during the midnight hours (by about 50 R) than through the midday, a fact perhaps due to the offset of the Eureka station from the magnetic pole. The average patch intensity has decreased from 190 R in 1990–1991 to 120 R in 1993–1994 as solar activity has declined. While patch sizes in the dawn-dusk direction varied up to >2000 km, the average cross-sectional dimension was 500–600 km.
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