Statistical study of auroral spirals

We present results of a statistical study of 189 auroral spiral forms. The data set was collected by a systematic examination of the images recorded at five all-sky camera (ASC) stations in northern Fennoscandia and Svalbard during the winters 1996-1997 and 1997-1998. For every spiral we defined the time of occurrence, diameter, direction of motion, and distance between the adjacent spirals (wavelength) in cases of spiral streets. The magnetic activity level was determined by using the magnetic north component (B-x) recorded at the ASC station and the local auroral electrojet index (IL index) computed by using the data from the IMAGE magnetometer network. The typical values of the diameter, wavelength, and ratio between diameter and wavelength were 25-75 km, 125-175 km, and 2-6, respectively. According to our ASC data set, spirals are more common in the magnetic postmidnight sector than previous satellite observations suggest. Spirals drift predominantly in the same direction as the large-scale ionospheric convection. Our observations of the average drift speed of spirals of 4 km/s are comparable to the corresponding values of other auroral small-scale structures. Statistical analysis revealed that spirals occur most frequently under magnetically quiet (IL > -200 nT) conditions, and thus they cannot always be considered as precursors of substorm activity.