Small-scale, field-aligned currents at the top-side ionosphere

High-precision vector magnetic field observations from the Orsted satellite are used to investigate small-scale, field-aligned currents on spatial scales down to 600 m. Placed in a low-Earth, polar orbit, the slowly drifting orbit plane has covered all local times since launch in 1999 with more than 25000 polar passes. Small-scale currents are primarily found in the cusp with densities of 10-100 muAm(-2), reaching more than 1000 muAm(-2) during disturbed conditions. These values are 1-2 orders of magnitude larger than large-scale region 1 and 2 currents. Current densities are 2-4 times larger when the ionosphere is illuminated than when in darkness. While the location of currents follows the changes in cusp location with changing IMF, the intensity of the currents seems to depend on the level of solar wind turbulence. The data suggest that small-scale currents may be associated with significant local heating of the ionosphere and thermosphere.