ARTIFICIAL PERIODIC IRREGULARITIES IN THE AURORAL IONOSPHERE

Artificial periodic irregularities (API) are produced in the ionospher ic plasma by a powerful standing electromagnetic wave reflected off th e F region. The resulting electron-density irregularities can scatter other high-frequency waves if the Bragg scattering condition is met. S uch measurements have been performed at midlatitudes for two decades a nd have been developed into a useful ionospheric diagnostic technique. We report here the first measurements from a high-latitude station, u sing the EISCAT heating facility near Tromso, Norway. Both F-region an d lower-altitude ionospheric echoes have been obtained, but the bulk o f the data has been in the E and D regions with echoes extending down to 52-km altitude. Examples of API are shown, mainly from the D region , together with simultaneous VHF incoherent-scatter-radar (ISR) data. Vertical velocities derived from the rate of phase change during the i rregularity decay are shown and compared with velocities derived from the ISR. Some of the API-derived velocities in the 75-115-km height ra nge appear consistent with vertical neutral winds as shown by their ma gnitudes and by evidence of gravity waves, while other data in the 50- 70-km range show an unrealistically large bias. For a comparison with ISR data it has proved difficult to get good quality data sets overlap ping in height and time. The initial comparisons show some agreement, but discrepancies of several metres per second do not yet allow us to conclude that the two techniques are measuring the same quantity. The irregularity decay time-constants between about 53 and 70 km are compa red with the results of an advanced ion-chemistry model, and height pr ofiles of recorded signal power are compared with model estimates in t he same altitude range. The calculated amplitude shows good agreement with the data in that the maximum occurs at about the same height as t hat of the measured amplitude. The calculated time-constant agrees ver y well with the data below 60 km but is larger above 60 km by a factor of up to 2 at 64 km. The comparisons with the model are considered to be a good basis for more refined comparisons.