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 region (TR), O III 703.87 Å (maximum electron temperature, T_e≈ 8× 10⁴ K), as well as in the corona, Mg IX 706.02 Å (T_e≈ 10⁶ K). For Mg IX, we find that low CH intensities correspond to negative Doppler velocities (outflows) of ≈5 km 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 ≈5 km s^-1, but, towards its middle, the shift is around zero. The O III line, although it shows predominant downward motion of ≈5.5 km s^-1 in the CH and ≈6 km 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 (network boundaries). The bi-directional jets are found along the CH/QS boundaries, and, moreover, in locations where the plasma seen in the Mg IX 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 III 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 III 702.87 Åline.