Surreal illustration of a solar flare emitting ultraviolet light.

Decoding Solar Flares: What the First MUV Detection Means for Our Understanding of Space Weather

Elliot Brynn in Science & Nature July 2025 • 4 min read.

"New research unveils the detection of solar flare emission in the mid-ultraviolet Balmer continuum, offering critical insights into flare energy and space weather forecasting."

Solar flares, among the most powerful events in our solar system, release vast amounts of energy. Understanding how this energy is distributed across the solar spectrum has been a long-standing challenge. While X-ray and extreme-ultraviolet (EUV) emissions are routinely measured, they only capture a fraction of the total energy released during a flare.

Most of the flare's radiation is emitted at longer wavelengths, including the far-ultraviolet (FUV), mid-ultraviolet (MUV), and near-ultraviolet (NUV) ranges. Despite their importance, observations in these spectral ranges have been limited. The region between 1000 and 3000 Å is particularly significant but poorly understood, representing a critical gap in our knowledge of solar flare energy distribution.

Recent studies have begun to shed light on this area, with measurements taken by rocket-borne and space-borne experiments. These investigations aim to fill the gaps in our understanding of how flares emit energy across different wavelengths. The detection of solar flare emission in the MUV Balmer continuum, as measured by the Large-Yield RAdiometer (LYRA) aboard the PRoject for OnBoard Autonomy 2 (PROBA2) mission, marks a significant step forward.

What Does the Detection of MUV Balmer Continuum Tell Us About Solar Flares?

Surreal illustration of a solar flare emitting ultraviolet light.

The Large-Yield RAdiometer (LYRA), onboard the PROBA2 mission, made a groundbreaking observation during a solar flare (SOL20170906). For the first time, scientists detected flare emission in the mid-ultraviolet (MUV) wavelengths around 2000 Å. This event was also observed in the first channel of LYRA, which is centered on the HI Lya line at 1216 Å. The data revealed a non-thermal profile in both channels, indicating significant energy release.

The flare radiation detected in channel 2 is consistent with hydrogen Balmer continuum emission. This emission suggests that the chromosphere, a layer of the solar atmosphere, is heated up to approximately 10,000 K. Simultaneous observations in channels 1 and 2 allowed scientists to differentiate between line emission (primarily from the Lya line) and the Balmer continuum emission.

Balmer Continuum Emission: The detection confirms the presence of hydrogen recombination continua originating in the chromosphere.
Energy Distribution: Understanding the emission helps map how energy is distributed during solar flares.
Instrumentation: The Large-Yield RAdiometer (LYRA) played a crucial role in this discovery.
Chromospheric Heating: Indicates significant heating of the chromosphere during flares.

This new data, combined with recent detections of Balmer continuum emission in the near-ultraviolet by the Interface Region Imaging Spectrometer (IRIS), reinforces the interpretation of broadband flare emission as hydrogen recombination continua originating in the chromosphere. These findings offer valuable insights into the physics of solar flares and their impact on space weather.

The Implications for Space Weather and Future Research

The detection of MUV Balmer continuum emission provides a new avenue for understanding solar flares and their effects on space weather. By accurately measuring the energy distribution across different wavelengths, scientists can improve models and predictions of space weather events. Further research, including spatially and spectrally resolved observations, will be crucial to fully constrain the physics of broadband emission in flares and to develop effective forecasting methods.

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This article is based on research published under:

DOI-LINK: 10.3847/2041-8213/aaeace, Alternate LINK

Title: First Detection Of Solar Flare Emission In Mid-Ultraviolet Balmer Continuum

Subject: Space and Planetary Science

Journal: The Astrophysical Journal

Publisher: American Astronomical Society

Authors: Marie Dominique, Andrei N. Zhukov, Petr Heinzel, Ingolf E. Dammasch, Laurence Wauters, Laurent Dolla, Sergei Shestov, Matthieu Kretzschmar, Janet Machol, Giovanni Lapenta, Werner Schmutz

Published: 2018-11-05

Everything You Need To Know

What did the Large-Yield RAdiometer (LYRA) detect during the solar flare SOL20170906, and what does this tell us about the solar flare?

The Large-Yield RAdiometer (LYRA) detected flare emission in the mid-ultraviolet (MUV) wavelengths around 2000 Å and in the HI Lya line at 1216 Å during a solar flare (SOL20170906). This suggests significant energy release and chromospheric heating up to approximately 10,000 K. The detection of the MUV Balmer continuum, along with observations from the Interface Region Imaging Spectrometer (IRIS) in the near-ultraviolet, supports that broadband flare emission originates from hydrogen recombination continua in the chromosphere. It also helps map how energy is distributed during solar flares, which is vital for understanding their physics.

Why is the detection of the mid-ultraviolet (MUV) Balmer continuum so important for understanding solar flares?

Detecting the MUV Balmer continuum is significant because it fills a critical gap in our understanding of how solar flares distribute energy across different wavelengths. Most of a flare's radiation is emitted in the far-ultraviolet (FUV), mid-ultraviolet (MUV), and near-ultraviolet (NUV) ranges, but these spectral ranges have been poorly understood. Understanding the energy distribution across different wavelengths, scientists can improve space weather event models and predictions.

How did the Large-Yield RAdiometer (LYRA) contribute to the discovery of MUV Balmer continuum emission from solar flares?

The Large-Yield RAdiometer (LYRA), onboard the PRoject for OnBoard Autonomy 2 (PROBA2) mission, made the groundbreaking observation of solar flare emission in the mid-ultraviolet (MUV) wavelengths around 2000 Å. LYRA has multiple channels, including one centered on the HI Lya line at 1216 Å. LYRA is a radiometer with the primary goal of monitoring the solar irradiance, that helped scientists differentiate between line emission (primarily from the Lya line) and the Balmer continuum emission.

What exactly is Balmer continuum emission, and what does its detection tell us about the solar flare?

Balmer continuum emission refers to the radiation emitted when free electrons recombine with hydrogen ions in the solar atmosphere. The detection of Balmer continuum confirms the presence of hydrogen recombination continua originating in the chromosphere. Combining this with the recent detections of Balmer continuum emission in the near-ultraviolet by the Interface Region Imaging Spectrometer (IRIS) reinforces the interpretation of broadband flare emission as hydrogen recombination continua originating in the chromosphere.

How can the detection of mid-ultraviolet (MUV) Balmer continuum emission improve space weather forecasting?

Scientists can improve models and predictions of space weather events by accurately measuring the energy distribution across different wavelengths using the detection of MUV Balmer continuum emission. Further research, including spatially and spectrally resolved observations, will be crucial to fully constrain the physics of broadband emission in flares and to develop effective forecasting methods.