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<p>During Solar Cycle 24, several groups independently noticed that a sharp transition in coronal activity occurred early in 2011. The transition took the form of a sudden jump in the intensity of (“hot”) EUV lines formed at temperatures above about log(T) = 6.1, whereas (“cool”) lines formed below log(T) = 6.0 showed little change. This led to the suggestion of bimodal behavior in the corona, and has been linked to the timing of the “terminator” of the previous solar cycle. An obvious question is whether this behavior is typical of solar cycle onsets in the corona. In this brief article we investigate whether the corona showed similar behavior at the onset of Solar Cycle 25, using data from the EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) satellite. Previous observations have shown that hot coronal lines vary by orders of magnitude over the solar cycle while cool lines show very little variation. EVE has measurements of a number of strong coronal lines, and here we compare the onsets to Cycles 24 and 25 in the hot Si <span>xii</span> 499 Å line and the cool Ne <span>viii</span> 770 Å line. We find that, in contrast to Cycle 24, the onset of emission in the higher temperature lines during Cycle 25 is relatively gradual, with no clear indication of bimodal behavior, suggesting that sharp onsets of coronal activity are not a recurrent feature of the solar cycle.</p>

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The Bimodal Solar Corona Revisited

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年11月17日

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  • Stephen M. White,
  • Samuel J. Schonfeld

摘要

During Solar Cycle 24, several groups independently noticed that a sharp transition in coronal activity occurred early in 2011. The transition took the form of a sudden jump in the intensity of (“hot”) EUV lines formed at temperatures above about log(T) = 6.1, whereas (“cool”) lines formed below log(T) = 6.0 showed little change. This led to the suggestion of bimodal behavior in the corona, and has been linked to the timing of the “terminator” of the previous solar cycle. An obvious question is whether this behavior is typical of solar cycle onsets in the corona. In this brief article we investigate whether the corona showed similar behavior at the onset of Solar Cycle 25, using data from the EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) satellite. Previous observations have shown that hot coronal lines vary by orders of magnitude over the solar cycle while cool lines show very little variation. EVE has measurements of a number of strong coronal lines, and here we compare the onsets to Cycles 24 and 25 in the hot Si xii 499 Å line and the cool Ne viii 770 Å line. We find that, in contrast to Cycle 24, the onset of emission in the higher temperature lines during Cycle 25 is relatively gradual, with no clear indication of bimodal behavior, suggesting that sharp onsets of coronal activity are not a recurrent feature of the solar cycle.