Temporal occurrence frequency of BG events. Top: temporal evolution of temperature at log τ = −3.6 for one pixel along the slit. The red-dash-dotted line shows the intensity at λ = 396.83 nm in arbitrary units for comparison. The vertical dashed lines indicate local maxima. Middle: temporal average of number of maxima along the slit. The red-dash-dotted line shows the unsigned integrated Stokes V signal in arbitrary units. Bottom: average number of local maxima during 3150 s as a function of log τ.