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What Causes the High Apparent Speeds in Chromospheric and Transition Region Spicules on the Sun?

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What Causes the High Apparent Speeds in Chromospheric and Transition Region Spicules on the Sun? The Astrophysical Journal Letters, 849:L7 (7pp), 2017 November 1 https://doi.org/10.3847/2041-8213/aa9272 © 2017. The American Astronomical Society. All rights reserved. What Causes the High Apparent Speeds in Chromospheric and Transition Region Spicules on the Sun? Bart De Pontieu1,2 , Juan Martínez-Sykora1,3 , and Georgios Chintzoglou1,4 1 Lockheed Martin Solar and Astrophysics Laboratory, Palo Alto, CA 94304, USA; [email protected] 2 Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, NO-0315 Oslo, Norway 3 Bay Area Environmental Research Institute, Petaluma, CA 94952, USA 4 University Corporation for Atmospheric Research, Boulder, CO 80307-3000, USA Received 2017 June 22; revised 2017 October 5; accepted 2017 October 9; published 2017 October 23 Abstract Spicules are the most ubuiquitous type of jets in the solar atmosphere. The advent of high-resolution imaging and spectroscopy from the Interface Region Imaging Spectrograph (IRIS) and ground-based observatories has revealed the presence of very high apparent motions of order 100–300 km s−1 in spicules, as measured in the plane of the sky. However, line of sight measurements of such high speeds have been difficult to obtain, with values deduced from Doppler shifts in spectral lines typically of order 30–70 km s−1. In this work, we resolve this long-standing discrepancy using recent 2.5D radiative MHD simulations. This simulation has revealed a novel driving mechanism for spicules in which ambipolar diffusion resulting from ion-neutral interactions plays a key role. In our simulation, we often see that the upward propagation of magnetic waves and electrical currents from the low chromosphere into already existing spicules can lead to rapid heating when the currents are rapidly dissipated by ambipolar diffusion. The combination of rapid heating and the propagation of these currents at Alfvénic speeds in excess of 100 km s−1 leads to the very rapid apparent motions, and often wholesale appearance, of spicules at chromospheric and transition region temperatures. In our simulation, the observed fast apparent motions in such jets are actually a signature of a heating front, and much higher than the mass flows, which are of order 30–70 km s−1. Our results can explain the behavior of transition region “network jets” and the very high apparent speeds reported for some chromospheric spicules. Key words: magnetohydrodynamics (MHD) – methods: numerical – radiative transfer – Sun: atmosphere – Sun: chromosphere – Sun: transition region Supporting material: animations 1. Introduction (Skogsrud et al. 2016). The bulk of spicules observed at the solar limb are, however, type II spicules. These are much faster, Jets are very common in the solar chromosphere. They are showing apparent motions with maximum speeds of most often detected as rapidly evolving linear features in which − 30–150km s 1 (De Pontieu et al. 2007b; Pereira et al. 2012). plasma appears to be accelerated and sometimes heated, with Such speeds dominate the spicules seen in the chromospheric the bulk of the plasma flow often penetrating into the overlying Ca II H passband, which recent results indicate are the initial and much hotter corona. There is a wide variety of chromo- upward phase of longer-lived spicules that show up and spheric jets, including surges, penumbral microjets, anemone ( fi ) downward motion when viewed over a wide range of jets, macrospicules, and spicules Raoua et al. 2016 . To fully chromospheric and TR passbands (Skogsrud et al. 2015).In understand the impact of these different jets on the mass and contrast to type I spicules, type II spicules often appear to be energy balance of the low solar atmosphere, it is key to better associated with heating to at least transition region tempera- understand the mechanisms that drive these jets. In this Letter, tures (De Pontieu et al. 2014a; Rouppe van der Voort we focus on spicules, the most common of the solar jets, as et al. 2015). Their short-lived (~20– 50 s) transition region they have the largest potential to impact the overlying corona or counterparts often appear in Si IV and C II slit-jaw images (SJI) ( solar wind Beckers 1968; Athay & Holzer 1982; De Pontieu from the Interface Region Imaging Spectrograph (IRIS;De et al. 2011; McIntosh et al. 2011). During the past decade, Pontieu et al. 2014b) as linear features that emanate from significant progress has been made to better understand magnetic network with even higher apparent speeds of spicules and how they impact the solar atmosphere, but key 80–300 km s−1 (Tian et al. 2014; Narang et al. 2016). The questions remain. transition region counterparts appear to be shorter and slower in At least two different types of spicules have been reported. quiet Sun than in coronal holes, presumably because of The so-called type I spicules, identified as dynamic fibrils or differing magnetic field configurations (Narang et al. 2016). mottles on the disk, show both apparent motions and mass − Such high apparent speeds have also been found in other upper- flows of maximally 10–40km s 1 and appear to be driven by chromospheric lines such as Lyα (Kubo et al. 2016). There is, magneto-acoustic shocks that form when convective motions, however, a large discrepancy between these high apparent p-modes or magnetic disturbances propagate upward into the speeds and the actual mass flows in spicules as deduced from chromosphere (Hansteen et al. 2006; De Pontieu et al. 2007a; Doppler shift measurements. For example, the on-disk Rouppe van der Voort et al. 2007; Martínez-Sykora chromospheric counterparts of type II spicules are rapid et al. 2009). The transition region response to type I spicules blueshifted events which show typical Doppler shifts of appears to be limited to brightenings at the top of the spicule 20–50 km s−1 (Sekse et al. 2012, 2013). Similarly, Doppler 1 The Astrophysical Journal Letters, 849:L7 (7pp), 2017 November 1 De Pontieu, Martínez-Sykora, & Chintzoglou Figure 1. Space–time evolution of Si IV from observations with the IRIS1400 slit-jaw channel reveals linear, jet-like, intensity brightenings (red, almost vertical features in the right panels) at t=10:41:00 UTC (bottom) and 11:22:20 UTC (top) that propagate at high speeds along an already existing spicule (which is outlined as a parabolic shape in the right panels). The left panels show maps of the SJI 1400 and the rectangles highlight the spicules. For the space–time plot (right panels),we follow the intensity (in IRIS Data Numbers per second) along the major axis of the rectangles in the left plot. The black vertical lines in the space–time plots are caused by data gaps in the timeseries. (Animations (a) and (b) of this figure are available.) shift measurements in transition region lines of spicules show constraints for any theoretical model of spicules. A multitude of velocities up to only 50–70 km s−1 (Rouppe van der Voort theoretical models have been developed in the past, but until et al. 2015). recently no single model has been able to simultaneously The viewing geometry could in principle explain some of the explain in detail the ubiquity, dynamic, and thermal evolution differences between the apparent motions, often measured at and visibility and appearance in various chromospheric and the limb, and Doppler shifts, typically measured on the disk. transition region observables (for a review, see Sterling 2000; − However, the fact is that Doppler shifts of 100–300 km s 1 are Tsiropoula et al. 2012). Here, we exploit a recent numerical extremely uncommon on the disk and they would be expected model based on 2.5D radiative MHD simulations that proposes if the plane-of-the-sky motions were real mass motions. More that type II spicules occur when magnetic tension, created importantly, the IRIS observations of the apparent and real through the interaction between strong flux concentrations and mass motions of transition region counterparts of spicules (Tian weaker, granular-scale, magnetic flux concentrations, can et al. 2014; Rouppe van der Voort et al. 2015) are both on the diffuse into the chromosphere through ambipolar diffusion. In disk, i.e., with similar viewing angles. These differences thus the chromosphere, the sudden release of tension drives strong appear to be real and not easy to explain, thus providing strict flows and ion-neutral interactions lead to heating of the spicular 2 The Astrophysical Journal Letters, 849:L7 (7pp), 2017 November 1 De Pontieu, Martínez-Sykora, & Chintzoglou The simulation analyzed here is the 2.5D radiative MHD model described in detail in Martínez-Sykora et al. (2017a, 2017b). The numerical domain ranges from the upper layers of the convection zone (3 Mm below the photosphere) to the self-consistently maintained hot corona (40 Mm above the photosphere) and 90Mm along the horizontal axis. We use this particular model because it is the first Bifrost simulation in which type II spicules occur ubiquitously. Martínez-Sykora et al. (2017a) show that ambipolar diffusion plays a dominant role in the formation of spicules. To compare our simulation with observations, we calculate the synthetic Si IV intensity assuming equilibrium ionization and the optically thin approx- imation, similar to the methods used by Hansteen et al. (2010). 4. Results Measurements of apparent motions of spicules in the plane of the sky reveal a wide range of velocities, with the highest velocities recorded in IRIS SJI. These are sensitive to emission from lower transition region lines like C II1335 Å (SJI 1330 Å) and Si IV1402 Å (SJI 1400 Å). Previous results indicate that velocities of 80–250 km s−1 (Tian et al. 2014) are common. A statistical study shows that apparent velocities of up to 350 km s−1 are sometimes seen, with clear differences between coronal hole (190 ± 60 km s−1) and quiet Sun (110 ± 40 km s−1) spicules (Narang et al.
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