The Anomalous Temporal Behaviour of Broadband Ly$\alpha$ Emission During Solar Flares From SDO/EVE

Abstract

Despite being the most prominent emission line in the solar spectrum, there has been a notable lack of studies devoted to variations in Ly$\alpha$ emission during solar flares in recent years. However, the few examples that do exist have shown Ly$\alpha$ emission to be a substantial radiator of the total energy budget of solar flares (on the order of 10%). It is also a known driver of fluctuations in earth's ionosphere. The EUV Variability Experiment (EVE) onboard the Solar Dynamics Observatory now provides broadband, photometric Ly$\alpha$ data at 10 s cadence with its Multiple EUV Grating Spectrograph-Photometer (MEGS-P) component, and has observed scores of solar flares in the 5 years since it was launched. However, the MEGS-P time profiles appear to display a rise time of tens of minutes around the time of the flare onset. This is in stark contrast to the rapid, impulsive increase observed in other intrinsically chromospheric features (H$\alpha$, Ly$\beta$, LyC, C III, etc.). Furthermore, the emission detected by MEGS-P peaks around the time of the peak of thermal soft X-ray emission, rather than during the impulsive phase when energy deposition in the chromosphere - often assumed to be in the form of nonthermal electrons - is greatest. Given that spectrally-resolved Ly$\alpha$ observations during flares from SORCE/SOLSTICE peak during the impulsive phase as expected, this suggests that the atypical behaviour of MEGS-P data is a manifestation of the broadband nature of the observations. This could imply that other lines and/or continuum emission that becomes enhanced during flares could be contributing to the passband. Users are hereby urged to exercise caution when interpreting broadband Ly$\alpha$ observations of solar flares. Comparisons have also been made with other broadband Ly$\alpha$ photometers such as PROBA2/LYRA and GOES/EUVS-E.

Submitted to A&A Research Notes, 5 pages 4 figures

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• Extreme Ultra-Violet Spectroscopy of the Lower Solar Atmosphere During Solar Flares
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Cited by in corpus (13)

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• On the Performance of Multi-Instrument Solar Flare Observations During Solar Cycle 24
• The Response of the Lyman-Alpha Line in Different Flare Heating Models
• Solar Irradiance Variability Due To Solar Flares Observed in Lyman-alpha Emission
• Suppression of Hydrogen Emission in an X-class White-light Solar Flare
• Hydrogen Emission in Type II White-light Solar Flares
• The Lyman-alpha Emission in Solar Flares. I. a Statistical Study on Its Relationship with the 1--8 Å Soft X-ray Emission
• Lyman Continuum Observations of Solar Flares Using SDO/EVE
• The Lyman-$\alpha$ Emission in a C1.4 Solar Flare Observed by the Extreme Ultraviolet Imager aboard Solar Orbiter
• Quasi-Periodic Pulsations Detected in Ly$\alpha$ and Nonthermal Emissions During Solar Flares