Asymmetric Latitudinal Gradients of Galactic Cosmic Rays at Low and High Cutoff Rigidities in Two Negative Solar Magnetic Cycles: Solar Cycles 21/22 and 23/24

Abstract

The south–north asymmetry of the Galactic cosmic-ray (GCR) density [ $n$ ] denoted as $\delta n_{\text{S-N}}/n$ with respect to the heliospheric current sheet (HCS), observed by neutron monitors at low (≈ 1 GV) and high (≈ 13 and ≈ 17 GV) cutoff rigidities [ $P_{\text{c}}$ ], is investigated in two negative solar magnetic polarities when the dominant polar field is toward the northern hemisphere of the Sun. Using a new simple correction method for secular changes due to solar activity for the $\delta n_{\text{S-N}}/n$ reveals that it is inversely rigidity dependent. Annual values of the corrected $\delta n_{\text{S-N}}/n$ are close to zero in both solar magnetic cycles and exhibit substantial deviations due to their temporal 27-day variations. After corrections for Earth’s excursions in helio-latitude for the north–south (NS) differences of HCS tilt angle [ $\alpha$ ] as $\Delta\alpha_{ \text{N-S}}^{\text{E}}$ , the number of 27.3-day Carrington rotations that possess correspondences between the corrected $\delta n _{\text{S-N}}/n$ and $\Delta\alpha_{\text{N-S}}^{\text{E}}$ in both solar cycles (SCs) is significantly increased. Correlations of the two parameters are considerably increased in both SCs, particularly for SC 21/22. The asymmetric or unidirectional latitudinal gradients [ $G _{\perp}$ ] derived from the consistencies between corrected $\delta n_{\text{S-N}}/n$ and $\Delta\alpha_{\text{N-S}}^{ \text{E}}$ for the low and high $P_{\text{c}}$ in the SC 21/22 are 1.38% AU−1 and 0.69% AU−1, respectively, which are a few times larger than of SC 23/24. This suggests that asymmetric cosmic-ray modulation by the asymmetric HCS is more pronounced in SC 21/22 than in SC 23/24. Although the corrections also provide results that are fairly consistent with the drift model for the symmetric or bidirectional latitudinal gradient [ $G_{z}$ ], the effects of the SN asymmetric modulation of the GCR density are important as well. It is found that $G_{\perp}$ , $G_{z}$ , and SN asymmetric GCR modulation by NS asymmetric solar-wind speed are also inversely dependent on rigidity. The interplay between SN asymmetric modulation of the GCR density by the asymmetries in HCS and solar-wind speed, together with the effects of diffusion, are discussed for both of the negative solar magnetic cycles.