[Paper Review] Propagating Intensity Disturbances in Fan-like Coronal Loops: Flows or Waves?

Tongjiang Wang, L. Ofman+1 more|arXiv (Cornell University)|Jan 31, 2011

Solar and Space Plasma Dynamics3 citations

TL;DR

This study uses Hinode/EIS spectroscopic sit-and-stare observations to distinguish between propagating intensity disturbances in fan-like coronal loops as slow magnetoacoustic waves or intermittent upflows. By analyzing line profile shapes via skewness and multi-component Gaussian fitting, it finds that disturbances in warm coronal lines (e.g., Fe xii 195 Å) show no significant blue-wing asymmetry and small Doppler shifts, supporting the slow wave interpretation. In hot lines (e.g., Fe xv 284 Å), a persistent high-velocity component exists, but intensity and Doppler variations are primarily due to core component modulation, not upflow changes, reinforcing the wave model.

ABSTRACT

Quasi-periodic intensity disturbances propagating upward along the coronal structure have been extensively studied using EUV imaging observations from SOHO/EIT and TRACE. They were interpreted as either slow mode magnetoacoustic waves or intermittent upflows. In this study we aim at demonstrating that time series of spectroscopic observations are critical to solve this puzzle. Propagating intensity and Doppler shift disturbances in fanlike coronal loops are analyzed in multiple wavelengths using the sit-and-stare observations from Hinode/EIS. We find that the disturbances did not cause the blue-wing asymmetry of spectral profiles in the warm (~1.5 MK) coronal lines. The estimated small line-of-sight velocities also did not support the intermittent upflow interpretation. In the hot (~2 MK) coronal lines the disturbances did cause the blue-wing asymmetry, but the double fits revealed that a high-velocity minor component is steady and persistent, while the propagating intensity and Doppler shift disturbances are mainly due to variations of the core component, therefore, supporting the slow wave interpretation. However, the cause for blueward line asymmetries remains unclear.

Motivation & Objective

• To resolve the long-standing debate on whether upwardly propagating intensity disturbances in coronal loops are slow magnetoacoustic waves or intermittent upflows.
• To determine the physical origin of observed quasi-periodic disturbances in intensity and Doppler shift using high-resolution spectroscopic time series.
• To assess whether blue-wing spectral asymmetries are caused by transient upflows or wave-related dynamics.
• To examine the role of different temperature plasma components (core and minor high-velocity components) in shaping observed disturbances.
• To evaluate whether the observed disturbances are consistent with wave propagation or steady upflows, using multi-wavelength, time-resolved spectral fitting.

Proposed method

• Utilized Hinode/EIS sit-and-stare observations of fan-like coronal loops in Fe xii 195.12 Å, Fe xiii 202.04 Å, and Fe xv 284 Å lines.
• Applied skewness analysis to quantify spectral line asymmetry, with background subtraction to isolate disturbance-related features.
• Performed single and multiple Gaussian fits (double and triple) to spectral profiles to separate core emission from minor high-velocity components.
• Calibrated Doppler velocities using reference lines (Si x 258.37 Å and Fe xii 195.12 Å) to ensure accurate velocity measurements.
• Compared normalized line profiles on and off disturbances to assess changes in profile shape and intensity.
• Used forward modeling and R-B asymmetry analysis to cross-validate findings on line profile evolution.

Experimental results

Research questions

• RQ1 Are the observed propagating intensity and Doppler shift disturbances in coronal loops best explained by slow magnetoacoustic waves or intermittent upflows?
• RQ2 To what extent do the disturbances cause blue-wing asymmetries in spectral lines, and is this due to transient upflows or wave dynamics?
• RQ3 How do variations in the core component and minor high-velocity component contribute to the observed intensity and Doppler shifts?
• RQ4 Are the observed disturbances in warm and hot coronal lines correlated, or do they propagate independently in different temperature structures?
• RQ5 What is the physical origin of the persistent high-velocity component observed in hot lines, and how does it relate to the quasi-periodic disturbances?

Key findings

• In warm coronal lines (e.g., Fe xii 195 Å), no significant blue-wing asymmetry was found during disturbances, and Doppler shifts were small (1–2 km s⁻¹), inconsistent with intermittent upflows.
• The skewness time series showed no propagating features, indicating that any associated upflows are steady and not modulated by the disturbances.
• In hot coronal lines (e.g., Fe xv 284 Å), a persistent high-velocity component (100–120 km s⁻¹) was observed in the blue wing, but it did not vary with the disturbances.
• Intensity and Doppler shift variations in hot lines were primarily due to changes in the core component, not the minor high-velocity component, supporting a wave interpretation.
• The core component showed a 4 km s⁻¹ blueshift and intensity increase of up to 75% of the total enhancement during disturbances, consistent with slow-mode wave propagation.
• The observed excess in the blue wing of hot lines could not be explained by simple superposition of wave oscillations and steady upflows, indicating a need for further investigation into the underlying physics.