Science Overview

The structure of the Sun - Image: Wikimedia - User: Kelvinsong
The structure of the Sun – Image: Wikimedia – User: Kelvinsong

The IRIS mission will use UV images and spectra returned by the spacecraft and advanced computer models generated from this data to examine how matter, light and energy move from the surface of the sun through the chromosphere, to the little understood interface region (transition region) and into the corona.

The chromosphere and interface region help drive heat into the corona through poorly understood processes and IRIS will provide more insight into those as it takes on the challenge of studying these regions that were notoriously hard to examine with previous missions.

IRIS will observe the lowest part of the sun’s atmosphere, the chromosphere, which is comprised of ionized gas or plasma that lies just above the surface of the sun. The interface region is located between the chromosphere and the corona and builds the core of the science objective set for the IRIS mission. Scientists believe that the interface region holds clues as to why the and how the sun creates massive explosive events such as flares or coronal mass ejections, or how the high temperatures of several million Kelvins are reached within the corona.

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Coronal heating has its origin within the chromosphere and transition region which are highly dynamic regions that are constantly in motion with new features and phenomena forming and others disappearing. Creating thermal maps of these regions will not provide the insight needed to understand their predominate processes. A wide range of temperatures can occur at similar heights with different swaths of material propelled up- and downward as a result of the release of magnetic energy and plasma waves.

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Hinode Image of the Photosphere, Chromosphere & Corona showing plasma moving between regions of different magnetic polarity - Photo: NASA
Hinode Image of the Photosphere, Chromosphere & Corona showing plasma moving between regions of different magnetic polarity – Photo: NASA
TRACE image of superheated gas emission - Photo: NASA
TRACE image of superheated gas emission – Photo: NASA

This moving interface region covers a range of heights over which extreme temperature and density gradients are found. The interface region contains more mass than the corona and entire heliosphere – making it an important part of solar mechanisms which also influence our lives on Earth.

The central science objective of the IRIS mission is to understand the flow of mass and energy through the chromosphere and transition region in which the density, temperature and magnetic field show extreme gradients – that enable the transition region to form a critical interface through which all non-thermal energy that drives space weather is transported and ultimately affects near-Earth Geospace.

IRIS will attempt to answer the following three questions:

1. Which types of non-thermal energy dominate in the chromosphere and beyond?

2. How does the chromosphere regulate mass and energy supply to the corona and heliosphere?

3. How do magnetic flux and matter rise through the lower atmosphere and what role does flux emergence play in flares and mass ejections?

Observations from previous missions have revealed that the chromosphere is intimately connected to coronal dynamics. High resolution spectroscopic observations coupled with high spatial and temporal resolution imagery are required to address the questions and provide insight into chromosphere dynamics.

Recent missions have shown phenomena such as short-lived jet-like brightenings in the chromosphere that were connected to rapidly upflowing coronal plasma. These missions were unable to resolve and characterize these phenomena due to insufficient temporal and spatial resolutions. Although their data suggest that upper chromospheric dynamics and heating cause frequent evaporative upflows in the corona, the question of the correlation between chromosphere dynamics and coronal heating has still to be answered.

With these processes occurring at dynamic time scales of a few seconds to about 10 seconds, previous instruments with exposure times of 1 minute and cadences of 5 minutes were not able to provide quality data on these phenomena that also occur on much smaller scales of <1 arcsec with plasma velocities of 100km/s.

IRIS and its instrument provide the necessary spatial resolution to resolve small features of under 250 Kilometers and the instrument’s rapid exposures and low cadence will allow IRIS to study these dynamics that are believed to be the driver of coronal mass and energy flow.

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The Sun in 1700-angström UV - Photo: NASA SDO
The Sun in 1700-angström UV – Photo: NASA SDO

IRIS traces plasma properties during the dominant heating modes in the sun’s atmosphere and the mission will examine the mechanism that drives chromospheric jets, the effect of chromosphere dynamics on the coronal mass balance, and the general energization and dynamics of these regions.

IRIS observations will be used with data provided by other observatories such as the Solar Dynamics Observatory. SDO looks at the photosphere and the corona while IRIS will look at the regions in between – complementing SDO’s data.

The instrument will distinguish between two processes that are responsible for powering this region: magnetic field reconnection and dissipation of waves that travel through the sun’s atmosphere. IRIS will be able to determine which forms of energy cause which effects. These observations of our own star will also help deepen the insight into the atmospheres of distant stars.

Ultimately, the knowledge gained by the IRIS mission will help understand solar dynamics and provide better forecasting models for space weather that affects Earth.

IRIS Science Overview

Image: NASA/LMSAL
Image: NASA/LMSAL

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