[Paper Review] A new view of the solar interface region from the Interface Region Imaging Spectrograph (IRIS)

Bart De Pontieu, Vanessa Polito+12 more|arXiv (Cornell University)|Mar 30, 2021

Solar and Space Plasma DynamicsPhysics and Astronomy735 references81 citations

TL;DR

This paper presents groundbreaking insights into the solar interface region—spanning the chromosphere, transition region, and corona—using high-resolution ultraviolet spectra and images from the Interface Region Imaging Spectrograph (IRIS). By combining subarcsecond spatial resolution, high-cadence observations (down to 1.3 s), and advanced modeling, IRIS reveals the dynamics of mass and energy transfer, uncovering key roles for magnetic reconnection, wave dissipation, non-thermal particles, and ion-neutral interactions in heating the solar atmosphere and driving space weather events.

ABSTRACT

The Interface Region Imaging Spectrograph (IRIS) has been obtaining near- and far-ultraviolet images and spectra of the solar atmosphere since July 2013. The unique combination of near and far-ultraviolet spectra and images at subarcsecond resolution and high cadence allows the tracing of mass and energy through the critical interface between the solar surface and the corona or solar wind. IRIS has enabled research into the fundamental physical processes thought to play a role in the low solar atmosphere such as ion-neutral interactions, magnetic reconnection, the generation, propagation, and dissipation of various types of waves, the acceleration of non-thermal particles, and various small-scale instabilities. These new findings have helped provide novel insights into a wide range of phenomena including the discovery of non-thermal particles in coronal nanoflares, the formation and impact of spicules and other jets, resonant absorption and dissipation of Alfvenic waves, energy release associated with braiding of magnetic field lines, the thermal instability in the chromosphere-corona mass and energy cycle, the contribution of waves, turbulence, and non-thermal particles in the energy deposition during flares and smaller-scale events such as UV bursts, and the role of flux ropes and other mechanisms in triggering CMEs. IRIS observations have also advanced studies of the connections between solar and stellar physics. Advances in numerical modeling, inversion codes, and machine learning techniques have played a key role in driving these new insights. With the advent of exciting new instrumentation both on the ground (e.g., DKIST, ALMA) and space-based (e.g., Parker Solar Probe, Solar Orbiter), we aim to review new insights based on IRIS observations or related modeling, and highlight some of the outstanding challenges that have been brought to the fore.

Motivation & Objective

• To understand the physical mechanisms driving energy and mass transport in the solar interface region, a critical zone between the photosphere and corona.
• To resolve long-standing questions about chromospheric and coronal heating, particularly the role of non-thermal processes and magnetic reconnection.
• To leverage IRIS’s high-resolution imaging and spectroscopy to probe small-scale dynamics such as spicules, nano-flares, and Alfvénic waves.
• To integrate observations with advanced numerical modeling and inversion techniques to interpret complex diagnostics in non-LTE conditions.
• To establish connections between solar phenomena and stellar physics, especially through the study of transition region line profiles in active stars.

Proposed method

• Utilizes IRIS’s high-resolution far- and near-ultraviolet spectroscopy (0.33–0.4 arcsec spatial, 2.7 km/s spectral resolution) and imaging at high cadence (1.3 s) to capture dynamic processes in the solar atmosphere.
• Combines IRIS data with coordinated observations from SDO/AIA, SDO/HMI, Hinode/EIS, RHESSI, and other space-based and ground-based instruments (e.g., DKIST, ALMA).
• Applies advanced radiative transfer modeling and non-LTE inversion techniques to interpret optically thick chromospheric lines and infer thermodynamic conditions.
• Employs high-performance numerical simulations (e.g., MHD, multi-fluid models) to study ion-neutral interactions, ambipolar diffusion, and magnetic reconnection dynamics.
• Incorporates machine learning techniques to enhance diagnostics, including flare prediction and inversion of chromospheric temperature and velocity structures.
• Analyzes transition region line profiles to identify narrow and broad components, linking them to physical processes like turbulence and non-thermal motions.

Experimental results

Research questions

• RQ1 What physical mechanisms drive the heating of the chromosphere and corona in the solar interface region?
• RQ2 How do magnetic reconnection, wave propagation, and ion-neutral interactions contribute to energy transport and dissipation in the low solar atmosphere?
• RQ3 What is the role of non-thermal particles and turbulence in energy deposition during nano-flares and small-scale events like Ellerman bombs?
• RQ4 How do the observed properties of transition region lines (e.g., broad and narrow components) inform our understanding of plasma dynamics and heating mechanisms?
• RQ5 To what extent can IRIS observations and modeling help bridge the gap between solar and stellar atmospheric physics?

Key findings

• IRIS observations revealed the presence of non-thermal particles in coronal nano-flares, providing direct evidence for impulsive energy release mechanisms.
• The mission captured the formation and evolution of spicules and jet-like structures with unprecedented spatial and temporal resolution, showing their role in mass and energy injection into the corona.
• Resonant absorption and dissipation of Alfvénic waves were observed, supporting their role in heating the transition region and corona.
• Magnetic braiding and reconnection events were linked to jet-like dynamics and energy release, with simulations showing the importance of tearing mode instabilities and turbulence in reconnection.
• Thermal instability was identified as a key driver in the chromosphere-corona mass and energy cycle, influencing the formation of transient structures.
• Machine learning techniques enabled improved chromospheric diagnostics and novel flare prediction models, revealing new insights into trigger mechanisms of space weather events.