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How might a weak magnetic field during a reversal allow more cosmic rays to reach Earth?

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geomagnetic field The magnetic field of a bar magnet has a simple configuration known as a dipole field. Close to Earth's surface this field is a reasonable approximation of the actual field.

During a geomagnetic polarity reversal, Earth’s primary dipole magnetic field weakens significantly, leaving only a nondipolar component that is a small fraction of the normal field's strength.1

Under normal conditions, Earth's geomagnetic field acts as a protective barrier, deflecting the solar wind and trapping charged particles within the magnetosphere. However, in the temporary absence of a strong dipole during a reversal, this protective obstacle is diminished. Consequently, the solar wind can approach much closer to the planet, and particles that are typically deflected by the field or trapped in its outer regions are able to penetrate deeper toward the surface. This reduction in magnetic shielding allows a greater flux of cosmic radiation to reach the atmosphere, which could theoretically increase the rate of genetic damage and mutations in living organisms, though evidence of such biological impacts in the fossil record remains inconclusive.1