Extending back in time the Kp-like, open-ended, high-cadence geomagnetic Hp60 and Hp30 indices to cover the period starting from 1985

Geomagnetic indices are commonly used for various purposes, including the characterization of geomagnetic disturbance levels, parametrization of physical and empirical models of the near-Earth space environment, and data (re)analysis. The Kp (and ap) index, which is derived and disseminated by the GFZ German Research Centre for Geosciences, is one of the most extensively used such indices. The Kp index has been available since 1932 and therefore is particularly useful for studying long-term space climate trends. However, the Kp temporal resolution is limited to a three-hourly interval, which means that it cannot correctly capture rapid changes in geomagnetic activity that occur on shorter timescales. Secondly, the Kp index has an upper limit of 9, which means that all events of extremely disturbed conditions are described with one single number, which makes it difficult to differentiate between different levels of extreme geomagnetic activity. We developed a new family of geomagnetic indices, called Hpo (“H” stands for half-hourly or hourly, “p” for planetary, and “o” for open-ended). This open-ended, high-cadence index family is similar to the Kp index in its representation of planetary geomagnetic activity, but with higher time resolution and without an upper limit. The Hpo index family consists of the half-hourly Hp30 and the hourly Hp60 indices, as well as their linear versions, the ap30 and ap60 indices. The Hpo index family is based on the same 13 geomagnetic observatory data as the Kp index. Previously, the Hpo index values were only available back to 1995. However, we have recently derived Hpo indices for the period from 1985 to 1994. This period includes several strong geomagnetic storms in 1989-1992 that have been analysed using the newly derived Hpo indices. The Hpo index family provides a more comprehensive view of the geomagnetic activity, allowing for better analysis and understanding of space weather.