Doppler shift oscillations from a hot line observed by IRIS

1. ISSI-BJ 2017 October 18 Doppler shift oscillations from a hot line observed by IRIS Dong Li (lidong@pmo.ac.cn) Co-authors: Zongjun Ning, Yu Huang, N.-H. Chen, Qingmin Zhang, Yingna Su, and Wei Su Purple Mountain Observatory, CAS

2. Outlines • Backgrounds • Imaging observations • Spectroscopic observations • Observations and results • Discussions • Summary

3. Imaging observations • Coronal loop often displays the spatial displacement oscillations in EUV images AIA 171,211, and 304 Å

4. Imaging observations • The displacement amplitude of coronal loop oscillations has two types: The displacement amplitude is large but decaying quickly with time. The displacement amplitude is small and decayless. Su et al. 2017 Anfinogentov et al. 2015 Kink oscillations

5. Spectroscopic observations • The coronal loop oscillations can be detected in Doppler shifts of the emission lines, such as, the coronal line of Fe XIX Wang et al. 2002 Standing wave

6. Spectroscopic observations • Doppler shift oscillations can be observed in warm (1.3–2.0 MK) coronal lines: long-lasting Kink oscillations Coronal loop Tian et al., 2012

7. Spectroscopic observations Global fast sausage mode Tian et al., 2012

8. Observations • M7.1 Flare • AR 12192 • 2014-Oct-27 • Hot loop • IRIS slit • sit-and-stare

9. IRIS spectral fitting • Fe XXI is a broad and hot (~11 MK) emission line, it is always blended with other emission lines, such as C I, Si II, Fe II et al. • We used multi-Gaussian fitting (purple) to obtain Fe XXI (sky blue) from these blended lines (Li et al., 2015). • The line profiles and line center of Fe XXI from the fitting results.

10. Fitting results • The Doppler shifts between two green lines display oscillatory behaviors, and the oscillations change from red shift to blue shift. • The line intensities do not display the similar oscillations.

11. Time series • The time series of Doppler velocity display 6 peaks during the flare impulsive phase. • 5-min running average. • The time series of line intensity from Fe XXI (sky blue), SXR and microwave emissions do not display the similar peaks during the flare impulsive phase.

12. Wavelet analysis • (a): Detrended time series of Doppler velocity • (b) and (c): Wavelet power spectrum and global wavelet P=3.1±0.6 minutes (Torrence & Compo 1998; Yuan et al. 2011; Tian et al. 2012)

13. Flare loop • A broad loop along the IRIS slit, two green lines mark the loop top region. • Loop length (purple) L ≈ 81 Mm • Loop width W ≈ 9.8 Mm • DEM analysis positions (po & pi, Cheng et al., 2012)

14. DEM analysis • Black profile: The best-fitted DEM solution of the observations. • Color rectangles: the Monte Carlo (MC) realizations of the observations. • The density ratio between outside (po) and inside (pi) flare loop: n0/ne≈0.03 (Zucca et al. 2014)

15. Light curves • When the Doppler shift starts to oscillate, the temperature happens to increasing, but not oscillating within the same period. • AIA EUV fluxes do not display the oscillations with the same period. • Sine function fitting (e.g., Nakariakov et al. 1999; Wang et al. 2002; Anfinogentov et al. 2015)

16. Detrend time series • It is applied to the time series by removing the 12-min running average.

17. FFT spectra • We perform a Fourier analysis (Scargle 1982; Horne & Baliunas 1986) for the detrend time series of Doppler velocity, line intensity of Fe XXI, SXR and EUV fluxes. • P1=~3.1±0.6 min • P2=~10 ± 4 min

18. Discussions • The shorter period (P1) is most probably a fast kink wave (Roberts et al. 1984; Nakariakov & Ofman 2001; Tian et al. 2012) • The Alfv´en speed can be estimated to be about 630 km s−1, and the magnetic filed strength in this flare loop can be estimated to be about 68 G. • The longer period (P2) might be explained as a slow wave. 1. The local sound speed: vs=152(T/MK)0.5≈500 km s-1 (Kumar et al. 2013) 2. The phase speed: vh=2L/P2≈200~450 km s-1 (Mandal et al. 2016)

19. Summary • The Doppler shift oscillations within double periods are observed in a hot flare line, i.e., Fe XXI 1354.09 Å. One is a shorter period of ~3.1 minutes, another is a longer period of ~10 minutes. • There are no apparent corresponding oscillations within the shorter period of ~3.1 minutes in the line-integrated intensity, AIA EUV fluxes, or microwave emissions, suggesting that the oscillations of flare loop is weakly compressible. • The 3.1-min period of Doppler shift oscillations at Fe XXI 1354.09 Å might be explained by the MHD wave in a standing kink mode. • The 10-min period oscillations can also be observed in GOES SXR fluxes, SDO/AIA EUV light curves and the time series of line intensity of Fe XXI, which might be caused by a slow wave.

20. Thanks for your attention!