From AAS NOVA: “Heating Up a Solar Flare”



19 July 2019
Kerry Hensley

NASA’s Solar Dynamics Observatory captures a solar flare in the act. [NASA/SDO]


Powerful solar flares are dazzlingly bright in ultraviolet and X-ray images of the Sun. Despite their demands for attention, there’s still a lot that we don’t know about these unpredictable eruptions.

Clues from 121.6 nm

These Solar Dynamics Observatory images of the Sun show a solar flare in three extreme ultraviolet wavelengths. From left to right: 17.1, 30.4, and 13.1 nanometers. [NASA/GSFC/SDO]

Solar flares shoot energetic particles and photons from across the electromagnetic spectrum into interplanetary space. In order to understand how energy is released in solar flares, we need to first know how energy is injected.

To explore where flares get their energy, a team led by Jie Hong (Nanjing University, China) focused on a familiar feature of the ultraviolet solar spectrum: the 121.6-nm hydrogen Lyman-α emission line, produced by the roiling, turbulent hydrogen gas in the Sun’s atmosphere. The shape and behavior of the Lyman-α emission line can be used to learn about many different types of activity in the Sun’s chromosphere and corona — including solar flares.

Evolution of Lyman-α profiles over time. The top row shows the time evolution of the profiles in the non-thermal heating case. The asymmetry of the peaks transitions from long to short wavelengths as time and energy increase. The bottom row shows the thermal heating case (left) and the thermal heating plus a soft electron beam case (right). In the thermal heating case, the double peak morphs into a single peak.[Hong et al. 2019]

Modeling Flare Emission

Hong and collaborators used radiative hydrodynamics to model solar flares heated by different mechanisms. Their goal was to explore how the type of heating might change the shape of the Lyman-α line we observe.

In particular, they examined two means of heating the flares: a thermal mechanism where the energy comes from conduction from nearby plasma, and a non-thermal mechanism where the heat is provided by a beam of energetic electrons generated by magnetic reconnection. In the non-thermal case, they also varied the strength of the heating by an order of magnitude.

After allowing the modeled flares to evolve for eight or ten seconds, the researchers looked for subtle changes in the shape of the Lyman-α profile that could be linked to the underlying heating mechanism.

The asymmetries and peaks of the modeled emission lines showed distinctive patterns and behavior over time — fingerprints, Hong and collaborators argue, that could help identify the source of heat for an observed flare.

Flare Photography

Hong and collaborators note that their modeling efforts will complement future solar observations, helping to clarify the complex picture of flare evolution.

In particular, they look forward to the joint NASA-ESA Solar Orbiter mission, set to launch in 2020, which will be the first spacecraft to snap extreme-ultraviolet pictures of the Sun from out of the ecliptic plane, and China’s Advanced Space-Based Solar Observatory (ASO-S), which is scheduled for launch in 2022. ASO-S will carry a dedicated Lyman-α imager.

ESA/NASA Solar Orbiter depiction


After decades of observations, it looks like the field of flare research is still heating up!


“The Response of the Lyα Line in Different Flare Heating Models,” Jie Hong et al 2019 ApJ 879 128.

See the full article here .


Please help promote STEM in your local schools.

Stem Education Coalition


AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009