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High-resolution Observations of the Shock Wave Behavior for Sunspot Oscillations with the Interface Region Imaging Spectrograph

H. Tian, E. DeLuca, K. K. Reeves, S. McKillop, B. De Pontieu, J. Martínez-Sykora, M. Carlsson, V. Hansteen, L. Kleint, M. Cheung, L. Golub, S. Saar, P. Testa, M. Weber, J. Lemen, A. Title, P. Boerner, N. Hurlburt, T. D. Tarbell, J. P. Wuelser, C. Kankelborg, S. Jaeggli, S. W. McIntosh

TL;DR

This study uses high-cadence IRIS spectroscopy of Mg II, C II, and Si IV lines in a sunspot to investigate nonlinear sunspot oscillations and their association with magneto-acoustic shocks. Through single-Gaussian fits to line profiles and context imaging, the authors document a pronounced sawtooth Doppler pattern, ~30% intensity fluctuations, inter-line time lags consistent with upward propagation, and line-width enhancements arising from overlapping shock components. They identify 67 shocks, quantify a strong deceleration-velocity correlation, and estimate a slow-wave energy flux of about 1.8e5 erg cm^-2 s^-1, indicating partial energy transport to the transition region. The results support shock-based models of sunspot dynamics and highlight the need to account for multi-component line formation when interpreting chromospheric and TR observations.

Abstract

We present the first results of sunspot oscillations from observations by the Interface Region Imaging Spectrograph. The strongly nonlinear oscillation is identified in both the slit-jaw images and the spectra of several emission lines formed in the transition region and chromosphere. We first apply a single Gaussian fit to the profiles of the Mgii 2796.35 Å, Cii 1335.71 Å, and Si iv 1393.76 Å lines in the sunspot. The intensity change is about 30%. The Doppler shift oscillation reveals a sawtooth pattern with an amplitude of about 10 km/s in Si iv. In the umbra the Si iv oscillation lags those of Cii and Mgii by about 3 and 12 s, respectively. The line width suddenly increases as the Doppler shift changes from redshift to blueshift. However, we demonstrate that this increase is caused by the superposition of two emission components. We then perform detailed analysis of the line profiles at a few selected locations on the slit. The temporal evolution of the line core is dominated by the following behavior: a rapid excursion to the blue side, accompanied by an intensity increase, followed by a linear decrease of the velocity to the red side. The maximum intensity slightly lags the maximum blueshift in Si iv, whereas the intensity enhancement slightly precedes the maximum blueshift in Mgii. We find a positive correlation between the maximum velocity and deceleration, a result that is consistent with numerical simulations of upward propagating magnetoacoustic shock waves.

High-resolution Observations of the Shock Wave Behavior for Sunspot Oscillations with the Interface Region Imaging Spectrograph

TL;DR

This study uses high-cadence IRIS spectroscopy of Mg II, C II, and Si IV lines in a sunspot to investigate nonlinear sunspot oscillations and their association with magneto-acoustic shocks. Through single-Gaussian fits to line profiles and context imaging, the authors document a pronounced sawtooth Doppler pattern, ~30% intensity fluctuations, inter-line time lags consistent with upward propagation, and line-width enhancements arising from overlapping shock components. They identify 67 shocks, quantify a strong deceleration-velocity correlation, and estimate a slow-wave energy flux of about 1.8e5 erg cm^-2 s^-1, indicating partial energy transport to the transition region. The results support shock-based models of sunspot dynamics and highlight the need to account for multi-component line formation when interpreting chromospheric and TR observations.

Abstract

We present the first results of sunspot oscillations from observations by the Interface Region Imaging Spectrograph. The strongly nonlinear oscillation is identified in both the slit-jaw images and the spectra of several emission lines formed in the transition region and chromosphere. We first apply a single Gaussian fit to the profiles of the Mgii 2796.35 Å, Cii 1335.71 Å, and Si iv 1393.76 Å lines in the sunspot. The intensity change is about 30%. The Doppler shift oscillation reveals a sawtooth pattern with an amplitude of about 10 km/s in Si iv. In the umbra the Si iv oscillation lags those of Cii and Mgii by about 3 and 12 s, respectively. The line width suddenly increases as the Doppler shift changes from redshift to blueshift. However, we demonstrate that this increase is caused by the superposition of two emission components. We then perform detailed analysis of the line profiles at a few selected locations on the slit. The temporal evolution of the line core is dominated by the following behavior: a rapid excursion to the blue side, accompanied by an intensity increase, followed by a linear decrease of the velocity to the red side. The maximum intensity slightly lags the maximum blueshift in Si iv, whereas the intensity enhancement slightly precedes the maximum blueshift in Mgii. We find a positive correlation between the maximum velocity and deceleration, a result that is consistent with numerical simulations of upward propagating magnetoacoustic shock waves.

Paper Structure

This paper contains 3 sections, 1 equation, 5 figures.

Table of Contents

  1. Introduction
  2. Data analysis
  3. Conclusion

Figures (5)

  • Figure 1: Images and spectra around 16:39 UT on 2013 September 2. (A)-(C) Si iv 1393.76Å, C ii 1335.71Å and Mg ii 2796.35Å spectra along the slit. The length along the slit (vertical dimension) is 47$^{\prime\prime}$. The wavelength range (horizontal dimension) is about 1.2Å for the FUV lines and 2.4Å for Mg ii. (D)-(F): IRIS SJI images in different filters. (G)-(I): AIA 193Å, 171Å & 1600Å images. The field of view of the SJI and AIA images has a size of 47$^{\prime\prime}$$\times$47$^{\prime\prime}$. In (D)-(I) the vertical white line marks the location of the IRIS slit and the two long horizontal lines indicate the spatial range shown in Figure \ref{['fig.2']}. The three short horizontal lines in (D)-(I) mark the slit locations (labeled 1, 2 and 3) where we perform detailed analysis of the line profiles. The corresponding spectral features are pointed by the three arrows in (A)-(C). Two online movies are associated with this figure.
  • Figure 2: Temporal evolution of the SGF parameters in the spatial range indicated by the two long horizontal lines in Figure \ref{['fig.1']}. From top to bottom: Si iv, C ii and Mg ii. The length along the slit (vertical dimension) is 22$^{\prime\prime}$. The total duration (horizontal dimension) is 80 minutes. The interval between two vertical dotted lines is 3 minutes. The three horizontal lines in each panel indicate the slit locations 1, 2, & 3 in Figure \ref{['fig.1']}. Note that data points with low signal to noise ratio are shown in white and black in the images of Doppler shift and line width (for Si iv and C ii), respectively.
  • Figure 3: (A)-(C) Temporal evolution of the SGF peak intensity, Doppler shift, and line width at the slit location 3. (D) Fourier power spectrum of the Si iv Doppler shift. (E) Correlation coefficient between the intensity and Doppler shift of Si iv as a function of time lag. (F) Correlation coefficient between the Si iv Doppler shift and the Doppler shift of Mg ii/C ii as a function of time lag.
  • Figure 4: (A)-(C) Wavelength-time plots for Si iv, C ii and Mg ii at the slit location 3 in part of the time range. (D)-(F) Same as (A)-(C) but for a shorter time range. (G)-(I): Profiles of the three lines at several times. Typical profiles in the plage are shown as the dashed lines.
  • Figure 5: (A)-(C) Examples of linear fit to the velocity-time relationship in the deceleration phase of shocks. The duration (Dur), maximum velocity (Vmax) and deceleration (Dec) are shown in each panel. (D)-(F) Scatter plots of the relationship between shock parameters. The correlation coefficients are also shown in each panel.