Velocity shear-related ion upflow in the low-altitude ionosphere

Strong ion upflows with field-aligned velocity above 1000 ms−1 were observed by the European Incoherent Scatter (EISCAT) UHF Radar at Tromsø, Norway in the dayside auroral region at heights between 500–600 km during the 15 May 1997 magnetic storm. Both the EISCAT observations and the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) simulation results show that this event occurred in a region with low Joule heating rate, but with strong velocity shear. During the same period, the electron density and temperature showed no sign of soft particle precipitation, which is consistent with the UVI images from the POLAR satellite, thus excluding possible ion energization through soft particle precipitation. Our simple calculation shows that the velocity shear can provide sufficient energy for the observed ion upflow, thus suggesting shear-driven instabilities as a possible heating mechanism.