Declaration (Stauning, 2022) on “invalid PCS index” is based on the following arguments: PCS index is calculated with use of incorrect “unified” PC derivation method; PCS index used in analyses is a preliminary index, which was not approved by IAGA and, therefore, it cannot be regarded as a correct index; PCN and PCS indices demonstrate, intermittently, large difference in value, which should be treated as evidence of the PCS index invalidity. The paper presents comments to these arguments. Conclusion is made that criticism of the PCS index, presented in Stauning (2022), is based on groundless arguments.

The paper examines the essence of discrepancies between the IAGA approved PC index and the approach put forward by Stauning (2020). The IAGA endorsed PC index was designed to serve as a proxy for energy that enters into the magnetosphere during solar wind-magnetosphere coupling. It means that the ground-based PC index should be indicative of the magnetosphere state in the real time. With this aim a special “running QDC derivation” procedure has been elaborated (Troshichev et al., 2006) to determine a proper quiet daily curve (QDC) as a level of reference for evaluation of magnetic disturbances generated by solar wind. The most significant evidence of validity of the IAGA endorsed PC index is the statistically justified correspondence between the PC index changes and development of disturbances in the magnetosphere (with correlation as high as R=0.94). The alternative procedure (Stauning, 2011) also used QDC as level of reference, but this QDC does not consist with the PC index designation. The announcement (Stauning, 2020) on invalid IAGA-endorsed PC index series seems be devoid of any background.