IRIS views on how the low solar atmosphere is energized

Transcription

1 IRIS views on how the low solar atmosphere is energized Bart De Pontieu Lockheed Martin Solar & Astrophysics Laboratory Thanks to Juan Martinez-Sykora, Luc Rouppe van der Voort, Mats Carlsson, Viggo Hansteen, Tiago Pereira, Ineke De Moortel, Scott McIntosh, Patrick Antolin

2 What is IRIS? Interface Region Imaging Spectrograph High resolution, far/near UV imaging spectrograph with slit-jaw imaging Resolution: 0.33 arcsec (250 km), 2 seconds, 3 km/s Guide Telescope Covers photosphere, chromosphere, transition region and low corona Launch: June 2013 First light: July 2013 Data publicly available: October Science Telescope Spectrograph 200+ refereed publications since launch 2

3 The solar atmosphere in a nutshell Photosphere (~6,000K) Source of magneto-convective energy Corona (~1-10 MK) 1-10% of mechanical energy deposited in corona Photosphere Corona

4 The solar atmosphere in a nutshell Chromosphere (~10,000 K, but very dense) Most mechanical energy deposited in chromosphere Source of magneto-convective energy 1-10% of mechanical energy deposited in corona Chromosphere Chromosphere-Corona interface Photosphere Chromosphere-Corona mass cycle Corona

5 IRIS data products Mg II wing 2830Å Mg II k 2796Å C II 1330Å Si IV 1400Å SLITJAW IMAGES (SJI) 6,000 K 15,000 K 30,000 K 65,000 K FUV1: Å FUV2: Å C II 1335/1336 Si IV 1394 Si IV 1403 SPECTRA Mg II k 2796 Mg II h 2803 De Pontieu et al., 2014

6 IRIS data search

7 Full-disk spectroheliograms taken monthly

8 Long-term studies of active regions

9 Chromospheric Spicules Hinode - Solar Optical Telescope - Dynamic jets at the interface between the chromosphere and corona - Fast spicules ( type II ): upward flows of order 50 km/s, with counterpart at TR temperatures, much faster than previously reported - Have been associated with providing hot plasma to the million-degree corona (De Pontieu et al., 2009, 2010) - Connection to coronal heating has been contested (Klimchuk et al., 2013, 2014, 2015) based on theoretical arguments - Previous models not able to simultaneously reproduce very high speeds, realistic chromospheric and TR observables, and impact on the corona courtesy Mats Carlsson

10 Can IRIS help address these issues? IRIS SJI 2796 (Mg II k, 10,000 K) IRIS SJI 1400 (Si IV, 80,000 K)

11 Spicules associated with strong Alfvenic waves Hinode/SOT Ca II H (~10,000 K) shows multitude of transverse Alfvenic waves with amplitudes of ~20 km/s De Pontieu et al. 2007a, 2007b, McIntosh et al., 2011 IRIS Mg II spectra show ubiquitous twisting motions in chromosphere with associated heating to transition region temperatures De Pontieu et al., Science, 2014

12 Heating of spicules to TR (0.1 MK) temperatures IRIS SJI ,000 K IRIS SJI ,000 K RRE RBE SST Halpha Doppler ±46 km/s SST Halpha -46 km/s RRE: Rapid Redshifted Event RBE: Rapid Blueshifted Event De Pontieu et al. 2014, Rouppe van der Voort et al. 2015

13 What does IRIS tell us about heating of spicules? Ca II (SST) Mg II k Si IV He II (AIA) Ca II (SST) C II Si IV He II (AIA) 8,000 K 15,000 K 80,000 K 100,000 K 8,000 K 30,000 K 80,000 K 100,000 K Height Height Time Ca II (SST) C II Si IV He II (AIA) 8,000 K 30,000 K 80,000 K 100,000 K Time Spicules are multi-threaded with different threads at different temperatures Height Time Heating timescales of order ~1 minute Ca II H spicules are the initial, rapid phase of violent upward motions Some threads appear to show rapid heating over the whole length to TR temperatures (Si IV, C II) Skogsrud et al., 2015 (see also Pereira et al., 2014)

14 How do spicules form? Observational constraints - High speeds (50-100km/s) - Lengths ~ 8 Mm - Disk counterparts: short lived RBE s in Ca II, Halpha and Mg II h/k (a few ten of seconds) - Heating: Counterpart in (at least) TR EUV lines - Associated with torsional/rotational and transversal motion/waves. - Upper chromospheric and TR lines reveal parabolic trajectories of time-scales of ~10 min. - Preferentially occur at the boundaries of plage regions and enhanced network. Pereira T. M. D. et al. (2014) see also Skogsrud et al. (2015) Multitude of models but none can reproduce all observed features: ubiquity, thermal evolution, speeds, etc

15 Advanced numerical simulations play key role in IRIS science investigation Temperature Shock Waves in Quiet Sun Type 1 spicules courtesy Juan Martinez-Sykora Density km/s 2-5 Mm TR emission at top and/or beginning Martinez-Sykora et al., 2017 Numerical simulations using Bifrost code solve for 2.5 or 3D radiative MHD equations and include scattering, non-lte approximations for radiative losses in chromosphere, optically thin radiative losses, thermal conduction Numerical domain covers convection zone to chromosphere/corona, which are self-consistently created Main free parameter is magnetic field configuration Single-fluid MHD simulations only produce type I spicules, driven by magneto-acoustic shocks from p- mode leakage, convective flows (Hansteen et al., 2006)

16 Ambipolar diffusion plays key role in forming chromospheric spicules Temperature Shock Waves in Quiet Sun Type 1 spicules km/s 2-5 Mm TR emission at top and/or beginning courtesy Juan Martinez-Sykora Density Type II spicules km/s 5-15 Mm heated to TR along whole length Martinez-Sykora et al., D radiative MHD simulations including ambipolar diffusion from ion-neutral interactions for the first time produce type II-like spicules - 2.5D with initial large scale loop magnetic field configuration (~190 G in the photosphere) - Domain: 90Mm horizontal axis, 3Mm of upper convection zone and 40 Mm of atmosphere with ~16km resolution - Open boundary conditions and constant entropy at the bottom boundary in order to have convection. - The hot corona is self-maintained in 2.5D!

17 u t Single fluid equations in a partially ionized chromosphere? t + ( u) =0 + ( uu + ) = p + j B g e t + (eu)+p u = F r + F c + j 2 + Q visc B t = r (u B)+ r2 B B t = r (u B cr B h(r B) B + a (r B) B B) Hall effect and ambipolar diffusion arise from the presence of neutrals in the partially ionized chromosphere Ambipolar diffusion: neutrals do not experience the Lorentz force, so neutrals decouple from magnetic field lines allowing magnetic field lines to diffuse through neutral gas.

18 Strong flows driven by magnetic tension that diffuses into upper chromosphere because of ambipolar diffusion Martinez-Sykora et al., 2017

19 Type II spicules are launched by release of magnetic tension Temperature Velocity Ambipolar Diffusion Perpendicular Current 1. created by interaction of weak and network/plage fields in photosphere 2. diffusion of weak fields/tension into chromosphere through ambipolar diffusion 3. ambipolar diffusion concentrates currents further amplifying tension 4. violent release of tension above β=1 layer Martinez-Sykora et al., 2017

20 Granular fields are prevalent in vicinity of network Up to 50% of the QS flux is in the form of IN patches (Wang et al. 1995) IN elements appear at a rate of 120 Mx cm -2 day -1 (Gošić et al. 2016) (the rate in ARs is 1 Mx cm -2 day -1,Thornton & Parnell 2010) IN elements flux disappears at same rate as appearance through fading, merging with network and cancellation Gosic et al., 2017

21 Ambipolar diffusion can lead to misalignment between magnetic field lines and thermal structures Martínez-Sykora et al. 2016

22 The release of magnetic tension leads to generation of transverse magnetic waves with periods similar to those observed ( s) Observations Martinez-Sykora et al., 2017 Okamoto & De Pontieu (2011)

23 Temperature Velocity Heating of spicules to at least TR temperatures occurs naturally in 3D radiative MHD simulations Ambipolar diffusion in low-density spicular environment plays a key role Density Ambipolar heating Neutrals not directly tied to magnetic field, but collisionally coupled to ions When ion-neutral collision frequency low, slippage between ions and neutrals In cold and/or low density pockets, this means magnetic field can diffuse through neutral population Martinez-Sykora et al., 2017

24 Synthetic SST Synthetic observables from the simulations show great similarity with observations from IRIS and SST SST Martinez-Sykora et al., 2017

25 Can the model reproduce very high apparent speeds ( km/s) in TR counterparts? And the discrepancy with measured Doppler shifts (<100 km/s)? IRIS 1400 Å Space-time plot IRIS observations of transition region counterparts of spicules show extremely high apparent speeds Yet Doppler shifts of the same features reveal speeds < 100 km/s Tian et al., 2014, Narang et al., 2016 They often occur on previously formed spicules

26 High apparent speeds in transition region and chromospheric spicules caused by heating fronts Temperature Joule heating Si IV intensity Space Si IV intensity Current Time Heating fronts caused by propagation of electrical currents at Alfvenic speeds and subsequent rapid dissipation by ambipolar diffusion This can explain some of the very high apparent speeds in Ca II spicules Affects any estimates of kinetic energy or mass flux in spicules

27 Is there any coronal heating associated with these spicules? Si IV 1400 Å (80,000 K) AIA Fe IX 171Å (1MK) Spicules abound at footpoints of coronal loops: exploit longevity in TR observables from IRIS (5-10 min vs. 30 s) Coronal loops carry multitude of propagating disturbances Are these two types of features connected? Previously suggested by De Pontieu et al., 2005, De Pontieu & McIntosh 2010, Petralia et al., 2014 IRIS C II 1330 Å (30,000 K) Si IV 1400 Å (80,000 K) AIA Fe IX 171Å (1MK) AIA Fe IX 171Å running difference

28 Spicules often lead to propagating coronal disturbances (PCDs) Si IV 1400 Å (80,000 K) Si IV 1400 Å (80,000 K) AIA Fe IX 171Å running difference AIA Fe IX 171Å running difference See also McIntosh et al., 2009 (indirect evidence of spicules), De Pontieu et al., 2011 (at the limb), Tanmoy et al., 2016 (at the limb)

29 Formation of coronal loop strands associated with PCDs/spicules AIA 171Å AIA 171Å AIA 193Å AIA 193Å Loop strand formation appears to occur after initial PCD passes through - Propagating disturbances leads to long-lived brightenings in both 195 and 171Å channels - Previous interpretations for PCDs (transient flows, waves) would not lead to long-lived brightenings - Timing of brightenings (195Å followed by 171Å) suggests scenario in which: - heating associated with PCD/spicules forms a new coronal loop strand - followed by slow cooling over minutes

30 Model predicts heating of coronal plasma and generation of propagating coronal disturbances AIA 171 Å emission Temperature Density AIA 171 Å emission AIA 195 Å emission Joule heating Alfven speed AIA 195 Å emission Temperature Density Joule heating Spicule leads to: - propagating shock wave - strong flows at coronal temperatures - heating throughout the volume because of Joule dissipation of ambipolar currents associated with spicule formation mechanism PCD is combination of all of the above In this scenario, spicules are a signature of coronal heating De Pontieu et al., 2017

31 How would heating associated with spicules really occur on the Sun? Observations show evidence for a variety of wave modes Including resonant absorption and Kelvin Helmholtz instability pixel Hinode/SOT, pixel Antolin et Ca al., II 2017 H 420 IRIS/FG, Mg II k Evidence for resonant absorption first reported in prominences (Okamoto et al., 2015, Antolin et al., 2015)

32 Conclusions 1. IRIS observations show chromospheric spicules heated to transition region temperatures, with coronal counterparts D radiative MHD models including ambipolar diffusion lead to fast jets (100 km/s) that share many properties with spicules 3. Interaction between strong and weak, granular-scale fields combined with ambipolar diffusion drives spicules and triggers Alfven waves 4. Spicule formation process includes strong currents that help heat spicular plasma to transition region temperatures and leads to coronal counterparts and heating of coronal plasma 5. Ambipolar diffusion can also lead to misalignment between magnetic field and thermal structures in upper chromosphere Outstanding issues 1. How well does this work in 3D? With non-equilibrium H/He ionization? 2. What is the role of the variety of wave modes in dissipation/heating? 3. What is the role of spicules in coronal heating?