  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 He II 304A channel, and the other simultaneous images in the Fe IX/FeX 171A
channel. We concentrate on two well-positioned chromospheric eruptions that
occurred at disk center in the 171A images, and on the limb in 304A. 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 304A limb images with the
brightenings and dimmings seen on disk in the 171A images. Further we use
off-limb images of the low frequency 171A power to infer the coronal structure
near the eruptions. Results: In both the quiet Sun and the coronal hole
eruption, on disk 171A 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 171A 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.
