POLAR CORONAL PLUMES AS TORNADO-LIKE JETS E. TAVABI, 1 S. KOUTCHMY, 2 AND L. GOLUB3 1Payame Noor University (PNU), 19395-3697, Tehran, I. R. of Iran, 2Institut d’Astrophysique de Paris, UMR 7095, CNRS and UPMC, 98 Bis Bd. Arago, 75014 Paris, France. 3Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge MA 02138, USA. (Received Oct 17, 2017; Revised **************** Accepted **************) Accepted in ApJ ABSTRACT We examine the dynamical behavior of white light polar plume structures in the inner corona that are observed from the ground during total solar eclipses, based on their EUV hot and cool emission line counterparts observed from space. EUV observations from SDO/AIA of a sequence of rapidly varying coronal hole structures are analyzed. Evidence of events showing acceleration in the 1.25 Mk line of Fe XII at 193 A is given. The structures along the plume show an outward velocity of about 140 kms - 1 that can be interpreted as an upwards propagating wave in the 304 A and 171 A lines; higher speeds are seen in 193 A (up to 1000 kms - 1 ). The ejection of the cold He II plasma is delayed by about 4 min in the lowest layer and more than 12 min in the highest level compared to the hot 193 A behavior. A study of the dynamics using time-slice diagrams reveals that a large amount of fast ejected material originates from below the plume, at the footpoints. The release of plasma material appears to come from a cylinder with quasi-parallel edge-enhanced walls. After the initial phase of a longitudinal acceleration, the speed substantially reduces and the ejecta disperse into the environment. Finally, the detailed temporal and spatial relationships between the cool and hot components were studied with simultaneous multi-wavelength observations, using more AIA data. The outward-propagating perturbation of the presumably magnetic walls of polar plumes supports the suggestion that Alfven waves propagate outwardly along these radially extended walls. Keywords: Sun: chromosphere Sun: corona Sun: UV radiation Sun: solar wind Corresponding author: Ehsan Tavabi [email protected] 2 TAVABI, KOUTCHMY AND GOLUB. 1. INTRODUCTION 1.0.1. Coronal Holes Coronal holes (CH) are regions of low-density plasma in the solar atmosphere associated with magnetic field lines that open up freely towards interplanetary space (Zirker 1977). During solar minimum, such holes cover both the north and the south polar caps. In more active periods, narrow CHs can also be seen at all solar latitudes as first suggested in the late 1950s, when M. Waldmeier noticed long-lived low intensity regions in coronagraphic records made with the Fe XIV green line (e.g. Koutchmy 1977; Koutchmy and Bocchialini 1998; Wilhelm et al. 2011). Their more definite identification was done from soft X-rays disk images taken from sounding rockets (Vaiana et al. 1973a) and especially from the Skylab Apollo Telescope Mount (ATM) grazing incidence photographic X-ray telescope (Vaiana et al. 1973b). Long-lived bright points were extensively studied over the whole quiet Sun (Golub et al. 1974; Habbal & Withbroe 1981) and transient brightenings were analyzed using the soft X-ray data from the Yohkoh mission (Koutchmy et al. 1997). More specifically, CHs were seen to contain a large number of small, rapidly varying bright points often coinciding with the boundaries of supergranular cells that in turn may be related to the primary launching sites for the fast solar wind, and flares in bright points observed at the limb were seen to correspond to so-called surges and to eruptive ejections in Ha (Koutchmy et al. 1997, 2012; Moore et al. 1977; DeForest et al. 1998). The plasma in the transition region and in the corona is in a dynamical state where the details of the temperature minimum network rosettes (Beckers 1968; Sterling 2000; Lorrain et al. 2006; Tavabi 2018) and the magnetic field are measured, and the network patches higher up are observed in Ha or in chromospheric lines (Baudin et al. 1996; Georgakilas et al. 2001; De Pontieu et al. 2014). The plasma flows successively along various magnetic field tubes in the form of spicules and jets, and finally along polar plumes. The coronal material extends towards the outer corona where the solar wind actually evolves but this behavior does not take into account the intermittence of the evolution. It is accepted that the fast solar wind emerges from magnetically open CHs, which are representative of a predominantly unipolar magnetic region (e.g. Koutchmy 1977; Zirker 1977; Bravo & Stewart 1996), of a rather quiet Sun; the slow solar wind originates from above the more magnetically active part of the Sun. To establish the CH origin of the fast solar wind it is necessary to find the source regions of the high velocity solar wind stream at the surface of the Sun (Krieger et al. 1973). 1.0.2. Polar Plumes Genuine polar plumes are linear, rather straight structures seen in the EUV line at 171 A up to 1.3RQ and in white-light during eclipses to much larger distances (see Figure 1 for a convincing correspondence). They should not be confused with the edge of streamer sheets seen in projection above the polar limbs (Bravo & Stewart 1996). In Figure 1 we illustrate the correlation between the classical white-light plumes from eclipse observations and the plumes seen in the excellent 171 A AIA/SDO filtergrams after using a summing of 80 consecutive images with cadence of 12 sec. Outwardly propagating fast features are visible in hot coronal lines along the plumes as illustrated in Figure 1, EUV 171 A structures with W-L structures making the ray-like features in the polar region and usually called polar plumes or polar jets. Further, we looked at the relationship with He II 304 macrospicules and polar erupting small prominence structures, as was suggested from simultaneous ground based observation performed at the feet of the plume in the cool Ha line (Georgakilas et al. 2001). Macrospicules were broadly defined in He II 304 A observations as long, usually pointed jets ranging in length from 5 to over 50 arcsec (Bohlin et al. 1975). The ions in high-speed solar wind accelerate to terminal speeds of about 800 kms-1 above the CH (Wilhelm et al. 2000, 2011). This rapid acceleration cannot be explained by hydrodynamic forces (plasma gas pressure) alone. Some other form of momentum transfer, such as forces appearing in the region due to self-excited dynamo must be at work (Lorrain et al. 2006), or other mechanisms, such as Alfven-cyclotron resonance (Kasper et al. 2008). Coronal plumes, together with other dynamic structures in the solar atmosphere, such as macrospicules and X-ray jets, are potential sources of solar wind (spicules provide 100 times more mass than what is needed to fill the corona Pneuman and Kopp (1978)) and moreover, recent studies indicate that plumes are a plausible source of the fast solar wind streams (Gabriel et al. 1994, 2009). Other investigations led to different results (e.g. Wang et al. 1998; Habbal et al. 1995; Wilhelm et al. 2000). Wang et al. (1998) reported speeds of 400-1100 kms-1 for the outermost part of the W-L jets, whereas their centroidal velocities for the bulk of material are much lower. The contribution of polar plumes to the fast solar wind rising from the polar CH is thus still a subject of debate and controversy (e.g. Habbal 1992; Gabriel et al. 2003). More details about plumes and inter-plume regions were listed in the review of Wilhelm et al. (2011) and of Poletto (2015). Del Zanna et al. (2003) found that bright points are seen near the plume footpoints only in the early phase of their formation. Wilhelm et al. (2010) found a strong association of plumes and jets from XRT/Hinode images and Raouafi et al. (2008) showed that X-ray jets are precursors of coronal plume formation. Studies of morphology, kinematics, dynamics and the production AASTEX POLAR CORONAL PLUMES AS TORNADO-LIKE JETS 3 mechanism of a rather small-scale tornado have been recently pursued (Wedemeyer-Bohm et al. 2012) at photospheric granulation scale. The evolution as a function of height towards the corona of this type of event is still unknown. Kitiashvili et al. (2013) provided a realistic numerical simulation showing that the small jets due to the vortex at the foot of a plume could capture and stretch the background magnetic field, generate shocks and push plasma to the upper atmosphere. The magnetic field captured in the vortex causes an initial perturbation and accelerates the plasma along the helical medium to produce a spontaneous-seeming ejection in the solar atmosphere (Lorrain et al. 2006). A great number of observations and studies show that tornado-type eruptions (including surges) are composed of fine, long threads (Tavabi et al. 2015b; Chen et al. 2012). The AIA observations of polar CHs suggest that the region may be finely structured at arcsec scales in width with multiple threads of both hot and cool dynamical tornados. 1.0.3. EUV Jets X-ray jets were discovered with the X-ray telescope onboard the Japanese Yohkoh mission (Yokoyama and Shibata 1995; Shibata et al. 1992). They are associated with small flares and occur above X-ray bright points (Golub et al. 1974). Moore et al. (1976, 1977) also showed that X-ray bright points could be associated with Ha macrospicules. Shibata et al. (1992) and Canfield et al. (1996) have reported that X-ray jets are observed to be associated with cool ejections or plasmoids, nearly simultaneously and in nearly the same direction.