Intensity Conserving Spectral Fitting

TL;DR

This paper introduces the Intensity Conserving Spline Interpolation (ICSI), an iterative method that accurately fits spectral line profiles by conserving intensity within wavelength bins, reducing errors in plasma diagnostics.

Contribution

The paper presents a novel iterative fitting routine, ICSI, that accounts for spectral bin averaging effects, improving the accuracy of spectral line profile analysis.

Findings

ICSI converges rapidly and is computationally efficient.

It improves the accuracy of detecting line asymmetries and removing blends.

ICSI can be integrated into existing spectroscopic data processing pipelines.

Abstract

The detailed shapes of spectral line profiles provide valuable information about the emitting plasma, especially when the plasma contains an unresolved mixture of velocities, temperatures, and densities. As a result of finite spectral resolution, the intensity measured by a spectrometer is the average intensity across a wavelength bin of non-zero size. It is assigned to the wavelength position at the center of the bin. However, the actual intensity at that discrete position will be different if the profile is curved, as it invariably is. Standard fitting routines (spline, Gaussian, etc.) do not account for this difference, and this can result in significant errors when making sensitive measurements. Detection of asymmetries in solar coronal emission lines is one example. Removal of line blends is another. We have developed an iterative procedure that corrects for this effect. It can be used with any fitting function, but we employ a cubic spline in a new analysis routine called Intensity Conserving Spline Interpolation (ICSI). As the name implies, it conserves the observed intensity within each wavelength bin, which ordinary fits do not. Given the rapid convergence, speed of computation, and ease of use, we suggest that ICSI be made a standard component of the processing pipeline for spectroscopic data.