Volume opacity within the l = 30−210° vertical plane (identical to Fig. 4 (bottom) of Lallement et al. 2014). The Sun is at (0,0). The color scale is identical to the one in Fig. 1. The circles show potential locations of the NPS source region discussed in the text, assuming the NPS emission is generated within a cavity. The dashed line indicates the latitude b = +8°, above which the NPS is detected.