Anomalous temporal behaviour of broadband Ly alpha observations during solar flares from SDO/EVE

作者：Milligan Ryan O^*; Chamberlin Phillip C

来源：Astronomy & Astrophysics, 2016, 587: A123.

DOI：10.1051/0004-6361/201526682

• Sun: activity
• Sun: chromosphere
• Sun: flares
• Sun: UV radiation

Although it is 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 he a substantial radiator of the total energy budget of solar flares (of the order of 10%). It is also a known driver of fluctuations in the Earth's ionosphere. The EUV Variability Experiment (EVE) on hoard 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 and not during the impulsive phase when energy deposition in the chromosphere (often assumed to be in the form of non-thermal electrons) is greatest. The time derivative of Ly alpha lightcurves also appears to resemble that of the time derivative of soft X-rays, reminiscent of the Neupert effect. 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.

• 出版日期2016-3