Speaker
Description
Geomagnetic indices are essential tools to characterize the effect of solar activity in the Earth’s magnetosphere. The K index, in use for decades, provides a normalized local measure of geomagnetic disturbance over three-hour intervals. However, its limited temporal resolution constrains its applicability for short-term studies and real-time monitoring. To overcome this limitation, one-minute local indices such as the Local Disturbance index (LDi) have been developed. Nevertheless, the present work has revealed critical limitations in LDi: its reliance on spline-based detrending techniques tends to remove smooth trends during storm recovery phases, leading to an underestimation of the actual geomagnetic activity; in addition, its lack of standardization results in a large overlap between quiet and disturbed days, reducing its ability to discriminate activity levels when compared to the classical K index.
This study introduces the development of a new local geomagnetic index with one-minute resolution, grounded on the identification of quiet days through the K index. This approach ensures consistency with internationally accepted definitions of quiet and disturbed conditions. The objective is to construct a robust and operational index, aligned with consolidated standards, while overcoming the limitations observed in LDi.
The methodology comprised three main steps and was applied to 24 years of data from six INTERMAGNET stations. First, rigorous preprocessing was applied for the detection and removal of spurious spikes, with spectral analysis used as a supporting tool to identify anomalies. Second, the official Finnish Meteorological Institute (FMI) algorithm for K index calculation was re-implemented in Python and validated against international reference data, providing a reliable basis for the identification of quiet days and subsequent modeling. Finally, the quiet-time baseline was constructed through an empirical and exploratory approach tailored to each station. Secular and seasonal variations were modeled using an annual segmentation based on key astronomical events (perihelion, aphelion, and equinoxes). Particular attention was given to the solar regular variation (Sr) and the solar quiet variation (Sq). Sr was derived by directly linking its definition to the K-based classification of quiet days, a methodology widely validated since 1939, thus ensuring consistency with international standards. Sq was analyzed through a comparative assessment of approaches available in the state of the art —harmonic models and quiet-day based methods— and tested against observed values at the selected stations, allowing the identification of the most reliable strategy for an operational estimation of the daily quiet-time variation.
Preliminary results confirm that the new index preserves the ability to detect sustained disturbances, avoids underestimation during recovery phases, and provides a more consistent discrimination between quiet and active conditions than LDi. Furthermore, the index has been conceived from the outset to maintain consistency with international standards and to deliver reliable results in real-time operational environments.
