New indices of geomagnetic activity at test: Comparing the correlation of the analogue ak index with the digital A sub(h) and IHV indices at the Sodankyla station

We test here two recently proposed indices of geomagnetic activity, the A sub(h) index and the IHV index, which are based on digitally available hourly geomagnetic measurements. We study their correlation at different temporal scales with the Sodankylaak index that is based on analogue registrations and exists since 1914. Even at the highest temporal resolution, the three-hourly A sub(h) indices correlate extremely well with ak (correlation coefficient=0.873 for 243,374 points). Correlation is excellent at all local times, with correlation coefficients varying from dayside minimum of 0.81 to nightside maximum of 0.89, indicating that A sub(h) describes the local time variability of the ak index very reliably. Using daily averages the correlation between ak and A sub(h) is even higher (cc=0.94) and considerably better than between ak and IHV (cc=0.80). The standard deviation between daily averages of ak and the correlated A sub(h) is only 6.9nT but as large as 11.9nT for IHV. At the monthly resolution and longer, the IHV indices correlate with ak almost equally well as A sub(h) with ak. However, while the slopes of the best fit relations between ak and A sub(h) are nearly the same at all temporal resolutions (yearly, monthly, daily, three-hourly), reflecting a highly linear relation between these two indices, the slopes between ak and IHV get consistently and significantly smaller with increasing resolution, indicating a fundamentally nonlinear relation between ak and IHV. These facts verify that IHV index is very different from all K based indices, including ak and A sub(h), and strongly suggest that A sub(h) rather than IHV should be used as a long-term proxy or extension of local and global ak/K indices.