One of three basic types of flow lava. Aa is basaltic lava characterized by a rough or rubbly surface composed of broken lava blocks called clinker. [Wikipedia]
A daily and half-daily index of geomagnetic activity determined from the k indices scaled at two nearly antipodal stations at invariant magnetic latitude 50 degrees (Hartland, England, and Canberra, A
ustralia). The aa values are in units of 1 nT. The index is available back to 1868, and is provided by the Institut du Globe de Paris, France.