Scale-dependent coupling between galactic cosmic rays and trace gases revealed by multifractal analysis

Abstract

Galactic cosmic rays (GCR) modulate atmospheric ionisation and may influence reactive greenhouse gases, yet linear correlations have proved inconclusive. We analyse 74 328 hourly observations (2016–2024) of pressure-corrected neutron-monitor counts and co-located CH and O mixing ratios from the high-alpine Jungfraujoch station using Multifractal Detrended Fluctuation Analysis (MFDFA) and its bivariate extension (MFDCCA). Cosmic rays exhibit a narrow, quasi-monofractal singularity spectrum (), consistent with heliospheric modulation as a single dominant driver, whereas O and CH display progressively broader spectra ( and 0.84). Cross-Hurst exponents exceed unity for small-to-moderate fluctuations, indicating super-persistent joint variability on 1–7 d (O) and 7–30 d (CH) horizons—time-scales compatible with HO/NO chemistry triggered by GCR ionisation. Quadratic fits to the cross-singularity spectra yield half-maximum widths of 0.39 (CH–GCR) and (O–GCR), quantifying a broader amplitude hierarchy for methane. Extreme gas anomalies, by contrast, show weak cross persistence, implicating dynamical intrusions rather than ionisation. Multifractal metrics thus expose a scale-selective GCR imprint masked in Pearson and Spearman statistics and suggest that incorporating GCR flux as a multiscale covariate could improve sub-monthly O/CH predictions. The approach provides a transferable framework for disentangling cosmic-ray forcing from chemical and dynamical controls in other trace-gas records.