Observations of energetic X-rays and solar cosmic rays associated with the 23 May 1967 solar flare event

Abstract

On 23 May 1967 energetic (10–50 keV) solar flare X-rays were observed by the OGO-III ion chamber during the period 1808–2100 UT. The time-intensity profile for the X-ray event showed three distinct peaks at ≈1810, 1841 and 1942 UT. The second peak, which is equivalent to ≈2.9 × 10⁻³ ergs cm⁻²sec⁻¹ above 20 keV, is the largest X-ray burst observed so far by the OGO-I and OGO-III ion chambers. The soft (2–12 Å) X-ray observations reported by Van Allen (1968) also show similar peaks, roughly proportional in magnitude to the energetic X-ray peaks. However, the intensity of energetic X-rays peaked in each case 5–10 min earlier than the soft X-ray intensity indicating a relatively hard photon energy spectrum near the peak of the energetic X-ray emission. The corresponding time-intensity profile for the solar radio emission also showed three peaks in the microwave region nearly coincident with the energetic X-ray peaks. The third radio peak was relatively rich in the metric emission. Beyond this peak both the energetic X-rays and the microwave emission decayed with a time constant of ≈8 min while the corresponding time constant for the soft X-rays was ≈43 min. In view of the earlier findings about the energetic X-rays it is indicated that the 23 May solar X-ray event was similar to those observed earlier. During the 23 May event the integral energy flux spectrum at the time of peak intensity is found to be consistent with the form ∼e ^−E/E ₀, E ₀ being about 3.4 and 3.7 keV for the peaks at 1841 and 1942 UT, respectively. Assumption of a similar spectrum during the decay phase indicates that the spectral index E ₀ decreased nearly exponentially with time.

The OGO-III ion chamber, which is also sensitive to protons ⩾12 MeV, observed a solar particle event starting at ≈2100 UT on 23 May. It could not be determined uniquely which of the two principal X-ray peaks was associated with the particle event, and in fact both may have contributed. The particle intensity reached its maximum value at ≈1003 UT on 25 May 1967. The equivalent peak radiation dosage was ≈24 R/hour behind the 0.22 g cm⁻² thick aluminum wall of the chamber. This peak radiation dosage was considerably smaller than the maximum dosage (≈60 R/hour) during the 2 September 1966 solar particle event, the largest event observed so far by the OGO-I and OGO-III satellites. The temporal relationship between the solar X-ray and particle events on 23 May 1967 was similar to that observed in the solar flare events on 7 July 1966, 28 August 1966 and 27 February 1967.