Speaker
Description
The flux of galactic cosmic rays (GCR) traversing into and inside the heliosphere are modulated by the magnetic activity of the Sun through the heliospheric magnetic field, as the particles are deflected and slowed down by magnetic discontinuities. This modulation of GCR in the heliosphere can be parametrized by the modulation (potential) parameter ϕ, which is estimated using the force field approximation. Despite the complexity of its full physical interpretation, the parameter is convenient and powerful for studying modulation and, e.g., recreating observed GCR variations when carefully applied with relevant modulation and yield models.
Due to the force-field assumptions, the modulation parameter ϕ is usually only considered on monthly or longer timescales. However, recently results in Väisänen et al. (2023) and Väisänen et al. (2025, In revision) validated that the empirical ϕ parameter based on neutron monitor (NM) count rates does have merit to be considered also on shorter time scales. This top-down approach has many potential uses in improving our space weather, radiation and modulation monitoring capabilities, but still requires further work in validating the methodology and fully understanding the limitations.
Here we propose and discuss the 1-hour cadence modulation parameter ϕ and share preliminary results. Specifically, the shorter-than-day cadence is subject to diurnal variation and anisotropy of the local GCR flux, which need to be separated from the resulting modulation. Subsequently, this will enable us to better understand the GCR anisotropy and diurnal variations, in addition to uses in space weather and background modulation analyses. To better account for the diurnal variations, we have also rigorously computed precise 1-hour cutoff rigidities and particle traces for all the NM stations used in the computation from the year 1964 onwards using the Oulu—Open-source geomagneToSphere prOpagation (OTSO) tool, which is supersedes the previously used MAGNETOCOSMICS tool.
References:
Larsen, N., Mishev, A., & Usoskin, I. (2023). A new open-source geomagnetosphere propagation tool (OTSO) and its applications. Journal of Geophysical Research: Space Physics, 128, e2022JA031061. https://doi.org/10.1029/2022JA031061
Väisänen, P., Usoskin, I., Kähkönen, R., Koldobskiy, S., & Mursula, K. (2023). Revised reconstruction of the heliospheric modulation potential for 1964–2022. Journal of Geophysical Research: Space Physics, 128, e2023JA031352. https://doi.org/10.1029/2023JA031352
Väisänen, P., Bertucci, B., Tomassetti, N., Orcinha, M., Koldobskiy, S., Usoskin, I. 2025. Simulation of Galactic cosmic ray proton fluxes with the daily modulation potential: Validation with AMS02 data for 2011-2019. Journal of Geophysical Research: Space Physics (In Review, preprint: https://doi.org/10.22541/essoar.173532496.66800900/v2).
