Radial evolution of magnetic helicity and characteristic scale scaling. (Top) Color map of the reduced normalized magnetic helicity density σ_m in the frequency–distance plane. Black regions indicate data gaps due to low sampling rates. The yellow line is ﹩f={f}_{0}\exp (-\lambda r)﹩, where f₀ = 20 Hz and λ = 9 au⁻¹ serve as a rough guide for the observed spectral migration. The right-hand axis provides an indicative wavenumber scale based on a constant solar wind speed V_sw = 441.4 km s⁻¹ (the average of the moment data); this axis serves as an order-of-magnitude reference. (Middle) Spectra normalized by the local proton cyclotron frequency, f/f_ci. The inset shows an enlarged view of the green-outlined area. (Bottom) Spectra normalized by the ion inertial length, kd_i. Despite the scatter induced by variable local conditions, the helicity peaks in the normalized plots do not collapse onto a fixed scale (e.g., f/f_ci ∼ const. or kd_i ∼ const.). Instead, they exhibit a systematic migration toward larger scales relative to the local ion scales as distance increases.