STEREO quadrature observations of coronal dimming at the onset of mini-CMEs

Context. Using unique quadrature observations with the two STEREO spacecraft, we investigate coronal dimmings at the onset of small-scale eruptions. In CMEs they are believed to indicate the opening up of the coronal magnetic fields at the start of the eruption. Aims. It is to determine whether coronal dimming seen in small-scale eruptions starts before or after chromospheric plasma ejection. Methods. One STEREO spacecraft obtained high cadence, 75 s, images in the $\ion{He}{ii}$ 304 Å channel, and the other simultaneous images in the $\ion{Fe}{ix}$/$\ion{Fe}{x}$ 171 Å channel. We concentrate on two well-positioned chromospheric eruptions that occurred at disk center in the 171 Å images, and on the limb in 304 Å. One was in the quiet Sun and the other was in an equatorial coronal hole. We compare the timing of chromospheric eruption seen in the 304 Å limb images with the brightenings and dimmings seen on disk in the 171 Å images. Further we use off-limb images of the low frequency 171 Å power to infer the coronal structure near the eruptions. Results. In both the quiet Sun and the coronal hole eruption, on disk 171 Å dimming was seen before the chromospheric eruption, and in both cases it extends beyond the site of the chromospheric eruption. The quiet Sun eruption occurred on the outer edge of the enclosing magnetic field of a prominence and may be related to a small disruption of the prominence just before the 171 Å dimming. Conclusions. These small-scale chromospheric eruptions started with a dimming in coronal emission just like their larger counterparts. We therefore suggest that a fundamental step in triggering them was the removal of overlying coronal field