The following description is from the webpage of Niels Bohr Institue's Former Centre for Ice and Climate
The Earth reacts to the solar wind by increasing the strength of the shielding magnetic field. Therefore, higher solar activity results in stringer shielding and thus lower production of cosmogenic isotopes.
The first intuition is the solar wind will cancel Earth's magnetic field but it seems in reality it will make Earth's magnetic field stronger. How should we explain this phenomenon?
The conservation of magnetic flux is a simple approach. During times of higher speed solar wind it is generally the case that the associated dynamic pressure is higher, which exerts a force on the bow shock formed upstream of the Earth's magnetic field. This compresses the magnetosphere on the sunward side of Earth but stretches it on the anti-sunward side. If you decrease the area through which the magnetic field is normal, the field must increase if the flux is to be conserved. Thus, on the sunward side, the magnetic field enhances and it decreases on the anti-sunward side, which induces lots of currents (think Faraday's law).