Brief Introduce of Planetary K-index: An Indicator of Geomagnetic Storms

The planetary K-index (Kp index) was a geomagnetic index in the H-component field. This index was calculated from data collected by a network of 13 ground-based magnetometer stations at mid-latitude locations from the International Real-time Magnetic Observatory Network (INTERMAGNET). The magnitudes of Kp index could indicate geomagnetic activity using the integer K-scale from 0 to 9 without dimension because the K-scale was estimated using the quasi-logarithms algorithm. The Kp index indicated geomagnetic storms under the condition of K-scale＞4. The three-hourly Kp index has been commonly used. The three-hourly Kp index was relatively stable for low‐variability geomagnetic activity. The hourly Kp index represented the level of auroral absorption with a more accurate characterization. For future research, the Kp index with a high sampling rate (e.g., ＜1 hour) could be considered, so that a very accurate characterization was characterized the more detailed geomagnetic activity of global region.