Investigation of chromospheric evaporation in a Neupert-type solar flare

Zongjun Ning, Wenda Cao

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The Neupert effect implies a flare model in which the hard X-rays (HXRs) are produced by energetic electrons via electron-ion bremsstrahlung as they lose their energies in the chromosphere, while the soft X-rays (SXRs) are produced by thermal bremsstrahlung from "chromospheric evaporation" due to plasma being heated by those same electrons. Based on this concept, we investigate the evidence for chromospheric evaporation in a Neupert-type flare on 2004 October 30. First, we demonstrate that this event is consistent with the Neupert effect. Using the RHESSI data, both thermal and nonthermal energies are derived after the onset of this flare. The high correlation between the derivative of the SXRs and HXRs and between the derivative of the thermal energy and nonthermal energy indicates that the 2004 October 30 flare is a Neupert-type event. Second, chromospheric evaporation is necessarily expected during the flare's rising phase. We analyze RHESSI images at different energies and different times around the flare maximum. The HXR emission tends to move the footpoints close to each other, finally merging them into a single source with the same position as the loop-top source. When the projection effect (due to this event being near the disk center) is taken into account, this fact can be regarded as the signature of chromospheric evaporation in the X-ray observations. RHESSI observations show three kinds of evidence that are consistent with the evaporation model. First, at a given instant, the higher energy X-rays originate from the deeper layers of the atmosphere or further away from the loop top. Second, in a given energy band, i.e., at 20-30keV, the earlier X-rays originate from the deeper layers or further away from the loop top. Third, the X-ray footpoint sources at higher energies disappear at later times. Based on these characteristics, chromospheric evaporation took about 100 s for the 2004 October 30 flare. X-rays at all energy bands do not show evidence of evaporation probably because measurements are the most sensitive only between 12 and 30keV. After measuring the source scale as a function of time, we roughly estimate the shrinkage velocities at different energies, for instance, 238kms-1 for 12-15keV, 285kms-1 for 15-20keV, and 846kms-1 for 20-30keV. If the evaporation processes primarily contribute to the source shrinkage, these values are considered as the evaporation velocities, indicating that the evaporation flow would be faster during the latter part of flare.

Original languageEnglish (US)
Pages (from-to)1232-1242
Number of pages11
JournalAstrophysical Journal
Issue number2
StatePublished - 2010

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Sun: X-rays, gamma rays
  • Sun: flares


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