A high time-resolution analysis of the Ground-Level Enhancement (GLE) of 23 February 1956 in terms of the CSHKP standard flare model

Le résumé fourni par la source

The original real-time analog heritage data from the Huancayo recording ionization chamber is differentiated to yield 15 – 60 second resolution of the first 15 minutes of the cosmic ray Ground-Level Event (GLE) of 23 February 1956. The initial High-Energy Impulsive (HEI) event of >12.5-20 GeV solar cosmic rays has a rise time of 2 minutes 15 seconds with an exponential rise time constant of 49 seconds, a fluctuating peak pulse of duration 5-7 minutes, followed by an abrupt decrease of <1 minute duration to ∼33% of the peak flux. Detailed analysis of these data shows close consistency with the Carmichael-Sturrock-Hirayama-Kopp-Pneuman (CSHKP) standard flare model. The <49-second acceleration time to >12.5 GeV is explicable in terms of any one of three separate mechanisms: super-Dreier electric acceleration, statistical (Fermi first order), and shock acceleration deep in the corona. The fluctuating solar cosmic ray flux, the abrupt cessation, and the strong cosmic ray anisotropy on arrival at Earth are consistent with the acceleration event consisting of a sequence of short-lived reconnection events within the parent active center. In addition, the HEI event had a duration and time domain profile similar to the gyro-synchrotron emission from electrons associated with the initial acceleration event in the CSHKP model, and the observation of a white light flare towards the end of the microwave pulse. The properties of the GLE-5 HEI event are similar to those accompanying ten other previously studied large GLEs to the west of 280 West on the solar disk. We conclude that the historic high-resolution data from the GLE on 23 February 1956 demonstrate previously unknown properties of the HEI event that are all consistent with cosmic ray acceleration to 25 GeV by the CSHKP and similar flare magnetic reconnection models. A slowly varying and smaller increase commenced 6 minutes after the commencement of the HEI event and rose steadily to a maximum ∼40 minutes later. This is consistent with several models of acceleration in association with the coronal mass ejection generated by the flare.

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