4.9 Bright points, jets, and plumes

In this section we summarize stereoscopy of small-scale phenomena in the lower corona, such as point-like features (also called bright points), EUV and soft X-ray jets (collimated plasma outflows mostly along straight channels), and polar plumes (long-lived plasma structures along open field lines in polar coronal holes). These small-scale phenomena may all be triggered by small-scale magnetic reconnection processes in the lower corona that produce plasma heating in small confined loops (EUV bright points) or at the footpoints of open field lines (jets and plumes; Figure 42View Image).

Stereoscopic triangulation of 210 EUV bright points yielded the following heights above the photosphere: 4.4 ± 1.7 Mm (304 Å), 5.1 ± 1.9 Mm (171 Å), 6.1 ± 1.9 Mm (284 Å), and 6.7 ± 2.0 Mm (195 Å) (Kwon et al., 2010). The length of EUV bright points was found to be roughly twice the average height. The findings are consistent with small-scale flaring loops that have a temperature stratification and are heated by magnetic reconnection.

A statistical study of 79 polar jet events observed with STEREO/EUVI and COR-1 demonstrated that the coronal jets are always anchored in underlying small-scale chromospheric bright points, but their morphology revealed symmetric (“Eiffel tower-type”), asymmetric (“lambda-type”), and helical geometries (Nisticò et al., 2009), consistent with magnetic reconnection in tripolar configuration (Figure 42View Image). More than 10,000 jets were identified also in white light with STEREO COR-1 during 2007 – 2008 (Paraschiv et al., 2010). Some jets are recurring, triggered by a repetitive cycle of magnetic flux cancellation, reconnection, and chromospheric evaporation (Chifor et al., 2008). However, the source of the steady fast solar wind emanating from coronal holes is not all supplied by jets, but also by erupting small-scale loops (He et al., 2010), with temperatures up to T = 12 MK (Madjarska, 2011). Some recurrent, quasi-homologous jets exhibit a helical geometry and the dynamics of untwisting field lines, which has been modeled (Figure 44View Image) with both a helical rotating current sheet that generates jets and a quasi-steady mode that occurs in a 2D-like current sheet located along the fan between the sheared spines (Pariat et al., 2010Jump To The Next Citation Point). Stereoscopic triangulation of such a polar jet during the initiation phase confirmed the helical geometry and the untwisting mechanism (Patsourakos et al., 2008). Similarly, the unfolding motion of a twisted magnetic flux rope was inferred in a macrospicule, while the associated X-ray jet consisted of a radial outflow (Kamio et al., 2010). A dichotomy of standard jets (Figure 42View Image) and blowout jets (Figure 43View Image) was proposed by Moore et al. (2010Jump To The Next Citation Point), where the non-standard jets undergo a miniature version of the blowout eruption known in major CMEs.

Soft X-ray and EUV observations with Hinode/XRT and STEREO/EUVI demonstrated that 90% of jets observed in a polar hole were associated with plume haze and 70% of the jets are followed by polar plumes with a time delay of a few minutes, which suggests that polar jets are precursors of polar plume formations (Raouafi et al., 2008). A time-evolving 3D tomographic reconstruction of polar plumes was attempted with SOHO/EIT images, using the solar rotation and relying on their long lifetime (Barbey et al., 2008). The first stereoscopic triangulation of polar plumes was performed on 10 polar plumes, which showed the superradial expansion in the coronal holes, a density scale height of 1.6 – 1.8 times the hydrostatic scale height, and outflows of − 1 <∼ 10 km s, insufficient to drive the fast solar wind (Feng et al., 2009; Curdt et al., 2008). In contrast, time series analysis of STEREO/EUVI image sequences revealed quasi-periodically driven high-velocity outflows in polar plumes with mean velocities of 135 km s−1 at temperatures of T ≈ 0.5 − 1.5 MK, which makes them to an efficient source of heated mass originating in the upper chromosphere and feeding the fast solar wind (McIntosh et al., 2010). Alternative interpretations in terms of acoustic waves, however, cannot be ruled out according to recent debates.

View Image

Figure 42: Schematic depiction of tripolar reconnection between a closed-field loop and an open-field line (left), which channels the heated plasma after reconnection (marked with X) upward along the open field line (red line). This tripolar configuration became the standard scenario for coronal jets (from Moore et al., 2010Jump To The Next Citation Point).
View Image

Figure 43: Schematic depiction of the topology, eruption, and reconnection of the magnetic field for a blowout jet. The red field lines demarcate the already reconnected field that contains hot plasma, while the blue field lines indicate the pre-reconnection of not-reconnecting field (from Moore et al., 2010).
View Image

Figure 44: Side view (left) and perspective view (right) of the formation of the current sheet before onset of the first jet (at three different times). The field lines map out the fan surface and fan separatrix (from Pariat et al., 2010).

  Go to previous page Go up Go to next page