Speaker
Description
he first of a successful line of semi-empirical magnetospheric models was created by Tsyganenko (1989), henceforth referred to as the TSY models. TSY89 captured the structure of the magnetosphere well for low to mid geomagnetic disturbances and was parametrised simply using the planetary Kp index. For this reason, TSY89 is still widely used today when modelling the magnetosphere under similar conditions, despite the subsequent development of more advanced and complex TSY models. This model is particularly helpful when we model cutoff suppressions over historical geomagnetic storms before satellite measurements. TSY89 was initially parametrised for Kp values of 0, 1, 2, 3, 4, and ≥5. To extend the model to higher levels of disturbance and mitigate the problem of the Kp saturations, Boberg et al. (1995) introduced a novel method in which the ring current parameter in TSY89 was modulated based on the Dst index. This simple modification relied on a linear fit between the ring current parameter in TSY89 and the mean Dst for each Kp level. This “Boberg extension” has since seen wide application in numerous studies. The linear fit describing the Boberg extension is heavily reliant on the fitted parameters within TSY89. A reparametrisation in 1996, coinciding with the development of the TSY96 model, led to significantly different ring current parameters within the TSY89 model. This fact, however, has not been widely documented, and the original 1995 Boberg extension continues to be used despite the update to the TSY89 model.
In this work, the TSY89 model is revisited, and spacecraft from 1966 to 2025 are collected to enable a complete reparametrisation of TSY89, the most recent version of which used spacecraft data between 1966 to 1986, using the expanded dataset. The Boberg extension is re-evaluated for the 1989, 1996, and new 2026 versions of TSY89 by repeating notable prior studies that employed the Boberg extension, namely transmission function computations for the October 1989 solar energetic particle (SEP) events and cutoff latitude calculations for SAMPEX during the November 1992 SEP events. A comparison of the effectiveness of the respective Boberg extensions is then conducted. It is found that all models perform well under low geomagnetic disturbance conditions, but that the 1995 and 2026 versions perform significantly better during more extreme disturbances. We show preliminary calculations for the cutoff suppressions over the most extreme geomagnetic storms such as the Carrington storm, the February 1872 storm, and the May 1921 storm. We also try to estimate the cutoff rigidity suppressions for the theoretically greatest geomagnetic storm.