The natural environment of the Earth can act as a sensitive detector for dark matter in ultralight axions. When axions with masses between 110^-15\, eV and 110^-13\, eV pass through the Earth, they interact with the global geomagnetic field, generating electromagnetic (EM) waves in the extremely low-frequency range (0. 3--30\, Hz) through axion-photon coupling. This paper is one of a series of companion papers for~Taruya: 2025zql, focusing on the data analysis method and search results for an axion signal. Utilizing the theoretical predictions of axion-induced EM spectra from a companion study, we analyzed long-term observational data of terrestrial magnetic fields in this frequency band to search for axion-induced signals. Our analysis identified 65 persistent signal candidates with a signal-to-noise ratio (SNR) greater than 3. Aside from these candidates, we placed a new upper bound on the axion-photon coupling parameter, significantly refining the previous constraint from CAST by at most two orders of magnitude down to g₀ 410^-13 \, GeV^-1 for the axion mass around 3 10^-14\, eV.
Nishizawa et al. (Thu,) studied this question.