INCOHERENT-SCATTER RADAR SPECTRUM DISTORTIONS FROM INTENSE AURORAL TURBULENCE

We present EISCAT (European Incoherent Scatter Radar Facility) measure ments of > 2000 K enhancements of the apparent ion temperature which o ccurred simultaneously over a latitude range of at least 100 km for br ief periods (less than 1 min) in the auroral F region. One event occur red during a substorm onset and a second during passage of a westward- traveling surge. The apparent T(i) increases showed significant anisot ropy, with measurements oriented less parallel to Bo exhibiting the la rgest amount of apparent T(i) increase. ln these examples the vector e lectric fields measured by EISCAT were much too low to account for the temperature increases via frictional heating, and also too low to gen erate non-Maxwellian ion velocity distributions, which can cause error s in ion temperature estimates. We argue that the measured T(i) increa ses are not real, and that both their magnitude and their anisotropy w ith respect to B0 can be satisfactorily explained by turbulent plasma flows with peak amplitudes of approximately 2 km/s but which could not be directly resolved by EISCAT, because they varied with a time scale less than the 10-s integration period, or possibly because their scal e size was smaller than the approximately 3 - 5 km antenna beam width. While such unresolved but inferred turbulent flows can themselves cau se ion frictional heating, we show that an equally important cause of high T(i) estimates in our case was the distortion of the measured spe ctra by strongly varying Doppler shifts. We also present a counter exa mple which shows a bright auroral arc in the radar common volume but w ith no large increases in the radar-measured T(i), indicating that not all auroral structures are associated with electric fields which vary with sufficient intensity to distort incoherent scatter radar spectra .