We investigate the properties of electromagnetic fields in isobaric $_{44}^{96}textrm{Ru}+,_{44}^{96}textrm{Ru}$ and $_{40}^{96}textrm{Zr}+,_{40}^{96}textrm{Zr}$ collisions at $sqrt{s}$ = 200 GeV by using a multiphase transport model, with special emphasis on the correlation between magnetic field direction and participant plane angle $Psi_{2}$ (or spectator plane angle $Psi_{2}^{rm SP}$), i.e. $langle{rm cos} 2(Psi_B - Psi_{2})rangle$ [or $langle{rm cos} 2(Psi_B - Psi_{2}^{rm SP})rangle$]. We confirm that the magnetic fields of $_{44}^{96}textrm{Ru}+,_{44}^{96}textrm{Ru}$ collisions are stronger than those of $_{40}^{96}textrm{Zr}+,_{40}^{96}textrm{Zr}$ collisions due to their larger proton fraction. We find that the deformation of nuclei has a non-negligible effect on $langle{rm cos} 2(Psi_B - Psi_{2})rangle$ especially in peripheral events. Because the magnetic-field direction is more strongly correlated with $Psi_{2}^{rm SP}$ than with $Psi_{2}$, the relative difference of the chiral magnetic effect observable with respect to $Psi_{2}^{rm SP}$ is expected to be able to reflect much cleaner information about the chiral magnetic effect with less influences of deformation.
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