What can we tell about global auroral electrojet activity from a single meridional magnetometer chain?

The AE indices are generally used for monitoring the level of magnetic activity in the auroral oval region. In some cases, however, the oval is either so expanded or contracted that the latitudinal coverage of the AE magnetometer chain is not adequate. Then, a longitudinal chain in the key region would give more information of the real situation but, of course, only during some limited UT-period. In order to find out the UT coverage of a single meridional chain, we have compared the global AL and AU indices with corresponding local indices determined using data from the meridional part of the EISCAT Magnetometer Cross during the years 1985-1987. A statistical study shows that the local indices are close (within relative error of 0.2) to the global AU and AL during periods 1500-2000 UT (similar to 1730-2230 MLT) and 2130-0130 UT (similar to 0000-0400 MLT), respectively. In the middle of these optimal MLT-sectors the EISCAT Cross sees more than 70% of the cases when the global AE chain records activity. Then, also the correlation between the local and global indices is generally good (>0.7). Thus we conclude that five to six evenly located meridional chains are needed for covering all the UT-periods. On the other hand, already the combination of IMAGE, CANOPUS, and the Greenland chains catches similar to 50% of the substorms. Case-studies show that usually during 2130-1100 UT the AL achieved from these chains reproduces the real AL with good timing, although it does not follow all transient variations.