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Introduction to Spectroscopy of Hot Coronal Loops

Introduction to Spectroscopy of Hot Coronal Loops. Dana Tothova, MPS Supervisors: Davina Innes, Sami Solanki and Franz Kneer Thanks to Tongjiang Wang. Outline. 1. Introduction - physical conditions and processes found in the corona 2. Spectrum and line spectroscopy

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Introduction to Spectroscopy of Hot Coronal Loops

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  1. Introduction to Spectroscopy of Hot Coronal Loops Dana Tothova, MPS Supervisors: Davina Innes, Sami Solanki and Franz Kneer Thanks to Tongjiang Wang

  2. Outline 1. Introduction - physical conditions and processes found in the corona 2. Spectrum and line spectroscopy 3. Oscillations and waves in loops 4. Slow magnetoacoustic mode 5. Outlook

  3. Physical Conditions in the Corona • very low density - ~ 106 (@ 1 SR) – 109 cm-3 (@ the base of QS), ~ 1011 cm-3 (loops) • high temperature (>1 MK) – coronal heating - fundamental problem in astrophysics • inhomogeneous, transient, collisionless environment with strong magnetic fields • low plasma β (thermal pressure is generally << magnetic pressure) – frozen magnetic field lines – plasma transport occurs only in 1D along the field lines (along the loops)

  4. Atomic Processes in the Corona

  5. Spectrum of the Solar Corona White light of Corona – million times weaker than the photosphere Gamma rays (~10-3-10-1 Ǻ, 100 keV – 10 MeV) during large flares, when particles are sufficiently accelerated in the collisionless corona and interact with nuclei in chromosphere (continuum emission by primary electron bremsstrahlung, nuclear deexcitation, neutron capture, positron annihilation, pion decay) Hard X rays (~0.1-1 Ǻ, 10 – 100 keV) – relativistic electrons (~0.2-0.5 c) entering the high density TR and chromosphere – thick target bremmstrahlung Soft X rays (~1-100 Ǻ, 0.1 – 10 keV, 1.5 - 150 MK) – produced by collisional line excitation and free-free emission of electrons scattered of ions (SXT/Yohkoh) – AR(1.5 – 10 MK), Loops EUV (~100-1000 Ǻ, 0.15-1.5 MK) – produced by collisional line excitation (“cooler” ions) – QS (1-3 MK) and free-free continuum, EUV imagers – EIT/SoHO, TRACE, spectrometer SUMER/SoHO

  6. Spectrum of the Solar Corona

  7. Line Spectroscopy of the Corona XUV lines are optically thin – radiative processes such as absorption and stimulated emission are negligible, collisional excitation is in equilibrium with the spontaneous emission: LOS Intensity of an optically thin line:

  8. Doppler Shift We estimate the intensity of the line, its shift and width from the 3 moments: Line of sight Doppler velocity:

  9. Coronal Loop Oscillations Motivation. Why do we study coronal loop oscillations? • Coronal seismology - useful tool in diagnostics of the coronal parameters such as magnetic field strength, dissipation parameters,... (Roberts et al. 1984) • Search for coronal loop oscillations is motivated by coronal heating theories based on dissipation of MHD waves • MHD waves – possible driver of the solar wind

  10. Coronal MHD Modes Fast Slow longitudinal kink-assymetric (5min) sausage - symmetric >20min transversal sausage - symmetric (seconds) torsional

  11. Recent observations High resolution imaging observations with SOHO and TRACE have detected: 1. fast global kink-mode oscillations in cool (T approx 1 MK) coronal loops – lateral displacements, very small density perturbation (0 in the 1st order) 2. propagating slow magneto-acoustic waves in polar plumes and long fan-like coronal loops (TRACE and SOHO/EIT) 3.standing slow mode oscillations in hot (T approx 6-8 MK) coronal loops (SOHO/SUMER) first harmonic second harmonic hasn’t been observed yet

  12. Recent observations High resolution imaging observations with SOHO and TRACE have detected: 1. fast global kink-mode oscillations in cool (T approx 1 MK) coronal loops – lateral displacements, very small density perturbation (0 in the 1st order) 2. propagating slow magneto-acoustic waves in polar plumes and long fan-like coronal loops (TRACE and SOHO/EIT) 3.standing slow mode oscillations in hot (T approx 6-8 MK) coronal loops (SOHO/SUMER) first harmonic second harmonic hasn’t been observed yet

  13. Fast Global Kink Mode • The first spatial oscillations of coronal loops were discovered by direct imaging in EUV (171 A) with TRACE, in the temperature range of 1-1.5 MK (Aschwanden et al. 1999). The observed loop oscillations occurred during a flare which began at 1998 July 14, 12:55 UT (Bastille day), and were most prominent during the first 20 minutes. • Transverse oscillations are excited by a flare blast wave indicated by initial motions away from the flare source. • Average period Pfast-kink=2L/vA=280±30 s • Amplitude of the transverse displacement A=4100-1300 km (L=130 000 ± 30 000) • Damping - 4 oscillation periods

  14. Fast Magnetoacoustic Kink Wave Data slice of spatial coordinate across the tadpole-ray structures as a function of time at a fixed height above the solar surface. The transverse wave-trains of the tadpole tails are clearly visible Fast magnetoacoustic kink surface waves - propagating transverse waves in an open coronal structure. Post-flare (X 1.5) supra-arcade of NOAA active region 9906 on the 21st of April 2002, associated with dark tadpole-like sunward moving structures. TRACE 195Å image of AR 9906 on April 21st, 2002 at 02:00:41 UT

  15. Fast Magnetoacoustic Kink Wave

  16. Standing Slow Mode Oscillations The simplest solution of slow-mode acoustic oscillations, a standing wave in the fundamental mode, which has the two endpoints of a fluxtube as fixed nodes. Neglecting energy dissipation and the magnetic field, the linearized MHD equations yield Momentum EQ Continuity EQ solution: slow branch of the magnetoacoustic waves is only weakly dispersive in low b-plasma Tube speed

  17. Standing Slow Mode Oscillations Very Recent SOHO/SUMER spectroscopic observations on the solar limb have revealed strongly damped slow-mode oscillations in the hot coronal loops (Wang et al. 2002, 2003a, b, Kliem et al. 2002). They can be seen in the hot flare-like EUV lines (mainly lines belonging to the ion Fe XIX and Fe XXI) as intensity and Doppler shift fluctuations with large initial Doppler velocities. They are more clearly detected in the Doppler shift signal than in intensity. SUMER spectrometer- sit & stare observations above AR loops 40Mm – 1 microflare/hour, 20-40min, > 6MK these flaring events are rare at T< 5MK Fe XIX (6.6 MK) and Fe XXI show Doppler shift oscillations (fundamental slow mode standing wave)

  18. Standing Slow Mode Oscillations • 1. Standing waves are set up quickly about a half period after the onset of events. • 2. These oscillations are the fundamental mode, so far no evidence for the second harmonics • 3. Initial loop temperature is above 2−3 MK, then impulsively heated to a temperature of 6−8 MK. • 4. The duration of the flare-like brightenings is several times the oscillation period. • 5. Except for strong initial injected flows lasting for about half a wave period, no background flow is present.

  19. Standing Slow Mode Oscillations TRACE 195

  20. Summary • TRACE has discovered transverse - horizontal and vertical loop oscillations -interpreted as a global kink mode. The possible exciter - flare shock • SUMER has discovered slow-mode standing waves in hot (T>6MK) coronal loops. The possible trigger - small flare-like events at a single footpoint (fundamental mode) • EIT and TRACE have found propagating intensity oscillations in fan-like coronal loops, and interpreted as the propagating slow magneto-acoustic waves. The possible trigger - leakage of p-mode photospheric oscillations. • All these oscillations are all strongly damped.

  21. Outlook SUMER/SoHO Doppler shift oscillation studies with have been limited - we only have off limb observations. Their interpretation is difficult because we lack the necessary soft X-ray, magnetic field and Hα context images. STEREO mission will provide us with the full loop geometry and together with EIS, XRT and SOT/HINODE observations we will hopefully be able to answer the following questions: 1. What fraction of active region microflares produce Doppler shift oscillation? 2. What is the ratio - Loop Length/Oscillation Period? 3. Does oscillation occur in lines colder than 6 MK (Ca XVII)? 4. How do the damping times depend on loop parameters? 5. Is there any transverse loop oscillation associated with the Doppler oscillation? 6. Is the oscillation simultaneous with brightening in the chromosphere? 7. Is the fundamental mode or a higher harmonic excited? 8. Are the kink modes mainly transverse, vertical or distortion oscillations?

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