Network boundary origins of fast solar wind seen in the low transition region?

Abstract

We present a study of a high spatial resolution raster acquired on-disk with the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) grating spectrograph on SoHO (the Solar and Heliospheric Observatory) in a polar Coronal Hole (CH) region. We analyse two EUV emission lines, representing the properties of solar plasma in the low transition re- gion (TR), O  703.87 A (maximum electron temperature, Te ≈ 8 × 10 4 K), as well as in the corona, Mg  706.02 A (Te ≈ 10 6 K). For Mg , we find that low CH intensities correspond to negative Doppler velocities (outflows) of ≈ 5k m s −1 . 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 ≈ 5k m s −1 ,b ut, towards its middle, the shift is around zero. The O  line, although it shows predominant downward motion of ≈5.5 km s −1 in the CH and ≈ 6k m s −1 in the QS, it also has blue-shifts arranged in a small-scale network pattern with average negative values of 3.5 km s −1 in CH and 3 km s −1 in the QS. The blue-shifts are caused either by plasma outflows of a few km s −1 , or by transient events such as bi-directional jets which dislocate plasma to upward velocities even higher than 100 km s −1 . The outflows originate predominantly from the intersection between the magnetic network and the inter-network cells (net- work boundaries). The bi-directional jets are found along the CH/QS boundaries, and, moreover, in locations where the plasma seen in the Mg  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 O  line, where plasma from the network changes its velocity sign, becoming red- shifted. Our results constitute the lowest-in-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 O  702.87 A line.