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A Geomagnetic Estimate of Heliospheric Modulation Potential over the Last 175 Years
Mathew J Owens1, Luke A Barnard1, Raimund Muscheler2
1Department of Meteorology, University of Reading, Earley Gate, PO Box 243, Reading, RG6 6BB UK.
Solar Physics
|May 22, 2026
Summary
This study reconstructs the heliospheric modulation potential (ϕ) using geomagnetic data, extending the record back to 1845. This new reconstruction improves long-term solar activity studies by bridging gaps in previous data.
Area of Science:
- Space Physics and Aeronomy
- Solar-Terrestrial Physics
- Paleoclimatology
Background:
- Galactic cosmic rays (GCRs) interact with Earth's atmosphere, influenced by geomagnetic and solar magnetic fields.
- The heliospheric modulation potential (ϕ) quantifies solar shielding but historical records have gaps.
- Previous methods using sunspot numbers (SN) or 14C present calibration and uncertainty issues.
Purpose of the Study:
- To develop a new, extended reconstruction of the heliospheric modulation potential (ϕ) using geomagnetic data.
- To bridge the gap between neutron monitor data and older 14C-based estimates for improved solar reconstructions.
- To assess the reliability of existing instrumental and 14C-based ϕ estimates.
Main Methods:
- Modified and tested an existing model of ϕ using heliospheric parameters: open solar flux (Fs), heliospheric current sheet tilt angle (α), and global heliospheric magnetic polarity (p).
- Applied the modified model to updated geomagnetic estimates of Fs and cyclic variations of α and p.
- Validated the reconstructed ϕ against neutron monitor data (1951-2023) and compared with 14C estimates.
Main Results:
- The new geomagnetic reconstruction of ϕ shows excellent agreement with neutron monitor data (1951-2023).
- Ionization chamber estimates of ϕ (1933-1951) appear poorly calibrated.
- The long-term trend in recent 14C-based ϕ estimates may be overestimated, potentially due to the Suess effect.
Conclusions:
- The new geomagnetic reconstruction provides a robust, extended record of the heliospheric modulation potential (ϕ) back to 1845.
- This improved ϕ record offers a better basis for calibrating long-term solar activity reconstructions.
- Findings highlight potential issues with previous 14C data interpretations and instrumental ϕ estimates.
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