2D maps of the power of the plasma density fluctuations, over the 4−8 h periodicity band, derived in the fields of view of COR1-B (left panel) and COR1-A (right panel), on April 18, 2008 at 21:50 UT; the power of the fluctuating structures is significant with a 99% statistical confidence, for a white-noise process; the extrapolated magnetic field lines (white lines refer to closed field lines, red and blue lines to open field lines, with positive and negative polarity, respectively) and the SOHO/MDI disk magnetograms as seen from the STEREO-B and STEREO-A viewpoints on April 19, 2008 at 00:04 UT are also shown; the white dashed lines indicate the edge of the STEREO occulter.