Examples of NWC electron precipitation wisp structures after background effect is extracted from signal data obtained from IDP on board the DEMETER satellite. (a) 14 April 2007, (b) 18 April 2007, (c) 20 April 2007, (d) 21 April 2007, (e) 23 April 2007, and (f) 27 April 2007. Six examples of absolute flux measurements are given in units of electrons·cm⁻²·s⁻¹·sr⁻¹·keV⁻¹ during wisp events. The signal is adopted from data recorded in April 2007, when the NWC transmitter was operating, whereas the background data were obtained in July 2007, when the NWC transmitter was deactivated. For example, the first plot is the effect of the signal data in the NWC electron precipitation area on 14 April. Thus, the background data recorded in the same area on 14 July have been subtracted.