This study reveals for the first time the existence of bi-directional jets, which are a signature of magnetic reconnection, occurring along coronal hole (CH) boundaries. The SUMER spectrometer observations obtained in the NIV 765.15Å ( K) and NeVIII 770.42Å (K) emission lines in an equatorial extension of a polar coronal hole, known as the ``Elephant's Trunk'' coronal hole, show small regions of a few arc-seconds size with strong blue- and red-shifted emission reaching Doppler shifts of up to 150km s, i.e. bi-directional jets. The jets' number density along coronal hole boundaries was found to be about 4-5 times higher with respect to the quiet Sun (Madjarska et al. 2004).
In a follow-up study, we use a high spatial resolution raster acquired on-disk with SUMER in a polar CH region. We analyse two EUV emission lines, representing the properties of solar plasma in the low transition region (TR), OIII 703.87Å (maximum electron temperature, K), as well as in the corona, MgIX 706.02Å (K). For MgIX, we find that low CH intensities correspond to negative Doppler velocities (outflows) of 5km s, Along the quiet Sun (QS)/CH boundaries, the coronal plasma begins to be more red-shifted. A coronal bright point (BP) located within the CH is blue-shifted in the coronal line. In the TR line, the outer region of the BP is red-shifted at 5km s, but, towards its middle, the shift is around zero. The OIII line, although it shows predominant downward motion of 5.5km s in the CH and 6km s in the QS, also has blue-shifts arranged in a small-scale network pattern with average negative values of 3.5km s in CH and 3km s in the QS. The blue-shifts are caused either by plasma outflows of a few km s, or by transient events such as bi-directional jets which dislocate plasma to upward velocities even higher than 100km s. The outflows originate predominantly from the intersection between the magnetic network and the inter-network cells (network boundaries). The bi-directional jets are found along the CH/QS boundaries, and, moreover, in locations where the plasma seen in the MgIX line is blue-shifted, but very close to small red-shifted regions. Another interesting change in behaviour is observed at the QS/CH boundaries, in the OIII line, where plasma from the network changes its velocity sign, becoming red-shifted. Our results constitute the lowest-altitude observed signature of plasma outflows from the chromospheric network boundaries inside a CH. We have derived this conclusion from direct correlation between Doppler velocity and the intensity of the OIII 702.87Å line (Popescu, Doyle & Xia 2004).