The formation of IRIS diagnostics I. A quintessential model atom of Mg II and general formation properties of the Mg II h&k lines

TL;DR

This paper develops a simplified atomic model for Mg II h&k lines to facilitate accurate forward modeling of solar atmospheric observations from IRIS, emphasizing the importance of partial redistribution and 3D effects.

Contribution

It introduces a 4-level Mg II atom model for radiative transfer, balancing accuracy and computational feasibility for IRIS data interpretation.

Findings

A 4-level Mg II atom model suffices for accurate line profile modeling.

1D PRD computations effectively model emission peaks.

Approximate 3D transfer with CRD can simulate line depression.

Abstract

NASA's Interface Region Imaging Spectrograph (IRIS) space mission will study how the solar atmosphere is energized. IRIS contains an imaging spectrograph that covers the Mg II h&k lines as well as a slit-jaw imager centered at Mg II k. Understanding the observations will require forward modeling of Mg II h&k line formation from 3D radiation-MHD models. This paper is the first in a series where we undertake this forward modeling. We discuss the atomic physics pertinent to h&k line formation, present a quintessential model atom that can be used in radiative transfer computations and discuss the effect of partial redistribution (PRD) and 3D radiative transfer on the emergent line profiles. We conclude that Mg II h&k can be modeled accurately with a 4-level plus continuum Mg II model atom. Ideally radiative transfer computations should be done in 3D including PRD effects. In practice this is currently not possible. A reasonable compromise is to use 1D PRD computations to model the line profile up to and including the central emission peaks, and use 3D transfer assuming complete redistribution to model the central depression.