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December 19: Flares observed in Lyman Alpha

     
  December 19: Lyman Alpha Flare  
  Lyman Alpha Flare  
 

 

Click on the above image for a link to a paper describing Lyman-α flares

 

 
 

Solar flares have been observed in a wide range of wavelengths, from radio to X-rays. We have talked about some of the features of flares on days 7, 9, and 14 of our calendar. Although the flare appears in many wavelength, its characteristics may be different in these wavleneghts. This is because different temperature plasma emits in different wavelengths, so the differences in flare characteristics tells you about how the temperature is changing at various locations in the solar atmosphere. One of these changing characteristics is the time of the peak of the flare, which can vary by several minutes. Analyzing the evolution of a flare in various spectral ranges can help scientists to understand the solar atmopshere in which the flare occured. In the plot above, we show an M2.0 flare that happened on February 8, 2010, as it is seen by three LYRA channels and the GOES X-ray instrument.

One of the LYRA channels observes in Lyman-α (the red line on the plot). Lyman-α is the strongest line in the solar spectrum and has a lot of impact on the Earth's ionosphere, where it acts not only as a source of ionization in the D-layer, but also as a major contributor to the dissociation of water vapor.

In the flare chronology, Lyman-α peaks very early compared to GOES and to the other LYRA channels. The reason is that this Lyman alpha emission is produced when energetic electrons--that have been accelerated during the process known as magnetic reconnection--impact the chromosphere of the Sun and heat the ambient plasma to temperatures of tens of millions degrees. The heated plasma is then evaporated into the solar corona where it slowly cools down. The other lines in the plot above are sensitive to the coronal plasma, so their peaks occur after the Lyman-α peak.  

Observations of flares in this spectral range have very rarely been reported. A paper discussing observations of Lyman-α flares made with LYRA can be found here.