In general, we can distinguish two main classes of mass bulk motions inside coronal loops: siphon flows, due to a pressure difference between the footpoints, and loop filling or draining, due to transient heating and subsequent cooling, respectively. Some other evidence of bulk motions, such as systematic redshifts in UV lines, has been difficult to interpret.
Siphon flows have been mainly invoked to explain motions in cool loops. The existence of cold loops has been known for a long time (Foukal, 1976) (see Section 2) and SoHO has collected high-quality data showing the presence of dynamic cool loops (Brekke et al., 1997). A well-identified detection was found in SoHO/SUMER data, i.e., a small loop showing a supersonic siphon-like flow (Teriaca et al., 2004) and in SoHO/CDS data (Di Giorgio et al., 2003).
Redshifts in transition region UV lines have been extensively observed on the solar disk (e.g., Doschek et al., 1976; Gebbie et al., 1981; Dere, 1982; Feldman et al., 1982; Klimchuk, 1987; Rottman et al., 1990; Brekke, 1993; Peter, 1999). Some mechanisms have been proposed to explain these redshifts: downward propagating acoustic waves (Hansteen, 1993), downdrafts driven by radiatively-cooling condensations in the solar transition region (Reale et al., 1996, 1997b), nanoflares (Teriaca et al., 1999b); a conclusive word is still to be given with the better and better definition of the observational framework.
Blue-shifts in the transition region are also studied but not necessarily associated with coronal loops (e.g., Dere et al., 1986). More localized and transient episodes of high velocity outflows, named explosive events, have been observed in the transition lines such as C iv, formed at 100 000 K (e.g., Dere et al., 1989; Chae et al., 1998b; Winebarger et al., 1999, 2002a,b). However, Teriaca et al. (2002) found indications that such EUV explosive events are not directly relevant in heating the corona, are characteristic of structures not obviously connected with the upper corona, and have a chromospheric origin.
Chae et al. (1998a) found Doppler shifts increasing and then decreasing with increasing temperature and explained them by the dominance of emission from plasma flowing downward from the upper hot region to the lower cool region along flux tubes with varying cross section (a factor about 30). Teriaca et al. (1999a) confirmed these results with the exception of blueshifts at higher temperature.
EUV spectra of coronal loops above an active region show clear evidence of strong dynamical activity. In the O v 629 Å line, formed at 240 000 K, line-of-sight velocities greater than 50 km s–1 have been measured with the shift extending over a large fraction of a loop (Brekke et al., 1997). Active region loop structures appear to be extremely time variable and dynamic at transition region temperatures, with large Doppler-shifts (Brekke, 1999). Di Giorgio et al. (2003) showed the direct observation and identification of the birth, evolution, and cooling of one of such transient cool loops, and measured a blue-shifted up-flow all along the loop, probably a one-direction siphon flow.
Winebarger et al. (2002c) analyzed co-aligned TRACE and the SoHO/SUMER observations of warm active region loops. Although these loops appear static in the TRACE images, SUMER detects line-of-sight flows along the loops of up to 40 km s–1. Apparent motions were also detected in TRACE images (Winebarger et al., 2001).
In SoHO/EIT high-cadence 304 Å images, De Groof et al. (2004) analyzed systematic intensity variations along an off-limb half loop structure propagating from the top towards the footpoint and reported several arguments supporting that these intensity variations are due to flowing/falling plasma blobs and not to slow magneto-acoustic waves (Section 4.4). This evidence has been the object of modeling studies (see Section 4.3).
The high spectral resolution of Hinode/EIS is allowing for very detailed studies of persistent loop plasma motions, which are very important to assess whether the loops are to be treated more as static or dynamic structures as a whole. More specifically, nonthermal velocities were detected in solar active regions (Doschek et al., 2007a). The largest widths seem to be located more in relatively faint zones, some of which also show Doppler outflows. Doppler flows in active region loops observed by Hinode EIS were explicitly addressed by Del Zanna (2008), who found a multifaceted scenario (Figure 10). Persistent redshifts, stronger in cooler lines (about 5 – 10 km s–1 in Fe xii and 20 – 30 km s–1 in Fe viii), were observed in most loop structures. Persistent blueshifts, stronger in the hotter lines (typically 5 – 20 km s–1 in Fe xii and 10 – 30 km s–1 in Fe xv), were present in areas of weak emission, in a sharp boundary between the low-lying “hot” 3 MK loops and the higher “warm” 1 MK loops. Strong localized outflows ( 50 km s–1) in a widespread downflow region were clearly visible in Doppler-shifts maps obtained with EIS (Doschek et al., 2008). The outflows might be tracers of long loops and/or open magnetic fields.
Ofman and Wang (2008) used the high resolution Hinode SOT observations and detected cool plasma flowing in multi-threaded coronal loops with speeds in the range 74 – 123 km s–1. In addition to flows, the loops exhibited transverse oscillations.
Further analysis of coronal plasma motions near footpoints of active region loops showed again a strong correlation between Doppler velocity and nonthermal velocity (Hara et al., 2008). Significant deviations from a single Gaussian profile were found in the blue wing of the line profiles for the upflows. These may suggest that there are unresolved high-speed upflows. Tripathi et al. (2009) found that an active region was comprised of redshifted emissions (downflows) in the core and blueshifted emissions (upflows) at the boundary. All these results have to be matched with the recent finding of extensive blueshifts correlated with spicules upflows and with coronal emission intensity (De Pontieu et al., 2009) (Section 4.4).
Living Rev. Solar Phys. 7, (2010), 5
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