Distribution of 15-minute averaged IMF cone angle (a) and intensity (b) upstream of the bow shock during the whole MESSENGER mission. The overall histogram of the cone angle is separated into two histograms according to the sign of BY (blue: negative; red: positive), and the magnetic field intensity is separated into two histograms according to the cone angle (blue: |90° − θ_CA| > 45°; red: |90° − θ_CA| < 45°).