[Paper Review] Spectroscopic Diagnosis of Propagating disturbances in coronal loops: Waves or flows?

T. Wang, L. Ofman+1 more|arXiv (Cornell University)|Jan 16, 2015

Solar and Space Plasma Dynamics4 citations

TL;DR

This study re-evaluates the nature of propagating disturbances (PDs) in coronal loops using Hinode/EIS spectroscopic data, challenging the prevailing interpretation of PDs as high-speed upflows. By developing a line-profile difference method to isolate transient emission and correcting for saturation in Red-Blue asymmetry analysis, the authors find that observed velocity shifts are consistent with slow magnetoacoustic waves (10–30 km s⁻¹), not the 50–150 km s⁻¹ upflows previously inferred, supporting the wave interpretation over the flow model.

ABSTRACT

The analysis of multiwavelength properties of propagating disturbances (PDs) using Hinode/EIS observations is presented. Quasi-periodic PDs were mostly interpreted as slow magnetoacoustic waves in early studies, but recently suggested to be intermittent upflows of the order of 50-150 km/s based on the Red-Blue (RB) asymmetry analysis of spectral line profiles. Using the forward models, velocities of the secondary component derived from the RB analysis are found significantly overestimated due to the saturation effect when its offset velocities are smaller than the Gaussian width. We developed a different method to examine spectral features of the PDs. This method is assuming that the excessive emission of the PD profile against the background (taken as that prior to the PD) is caused by a hypothetic upflow. The derived LOS velocities of the flow are on the order of 10-30 km/s from the warm (1-1.5 MK) coronal lines, much smaller than those inferred from the RB analysis. This result does not support the flow interpretation but favors of the early wave interpretation.

Motivation & Objective

• To resolve the long-standing debate on whether propagating disturbances (PDs) in coronal loops are slow magnetoacoustic waves or intermittent high-speed upflows.
• To investigate the reliability of Red-Blue (RB) asymmetry analysis in diagnosing upflow velocities in EIS spectral lines, especially under low signal-to-noise conditions.
• To develop and apply a new line-profile difference method to isolate transient emission features and estimate the true velocity of hypothetical upflows associated with PDs.
• To test whether the observed PDs are consistent with wave-like behavior or require high-speed flows, using forward modeling and observational constraints.

Proposed method

• Applied a line-profile difference method: subtracted the background line profile (between PDs) from the PD profile to isolate the transient component, assuming the background is slowly varying.
• Used forward modeling with synthetic spectral lines including photon noise (S/N ≈ 17) to simulate EIS observations and assess the accuracy of RB asymmetry analysis.
• Compared three RB asymmetry techniques (RB_S, RB_P, RB_D) to quantify how instrumental broadening and noise affect derived velocities and asymmetry parameters.
• Calculated the remnant emission profile after background subtraction to detect blueward asymmetry or core blueshifts indicative of wave-like compression.
• Evaluated the consistency of observed PD profiles with both wave and flow models by comparing simulated and observed line profiles.
• Used 3D MHD simulations as a reference to validate the physical plausibility of inferred flow speeds and their alignment with magnetic field geometry.

Experimental results

Research questions

• RQ1 Does the Red-Blue asymmetry analysis of EIS spectra reliably diagnose upflow velocities in coronal loop PDs, or is it biased by instrumental broadening and noise?
• RQ2 Are the observed PDs better explained by slow magnetoacoustic waves or by intermittent high-speed upflows of 50–150 km s⁻¹?
• RQ3 What is the true velocity of the transient component in PDs when the background profile is properly subtracted from the PD profile?
• RQ4 Is the observed blueward asymmetry in spectral lines consistent with wave-like compression or with high-velocity outflows?
• RQ5 Do the derived flow speeds align with the magnetic field inclination in coronal loops, or do they require implausibly high angles?

Key findings

• The RB asymmetry analysis significantly overestimates upflow velocities due to saturation effects when the actual offset is below the Gaussian line width, especially under low S/N conditions.
• For a true upflow velocity of 20 km s⁻¹, the RB method yields derived velocities of 55–75 km s⁻¹, indicating a systematic overestimation of 50–100 km s⁻¹.
• The line-profile difference method reveals only small hypothetical upflows of 10–30 km s⁻¹ in Fe x–Fe xiii and Si x lines, inconsistent with the 50–150 km s⁻¹ values from RB analysis.
• The remnant emission profiles after background subtraction show blueshifted peaks and negative intensities in the red wing, characteristic of wave-like compression, not high-speed flows.
• The apparent PD speed of ~100 km s⁻¹ implies magnetic field inclinations of 73°–84° if PDs are flows, which exceeds observed or modeled values, making the flow interpretation physically implausible.
• The results support the early wave interpretation of PDs as slow magnetoacoustic waves, not intermittent high-speed upflows, resolving a key ambiguity in coronal heating and solar wind acceleration mechanisms.