The
solar storms of August 1972 were a historically powerful series of solar storms with intense to extreme
solar flare,
solar particle event, and
geomagnetic storm components in early August 1972, during
solar cycle 20. The storm caused widespread electric- and communication-grid disturbances through large portions of North America as well as satellite disruptions. On 4 August 1972 the storm caused the accidental detonation of numerous U.S.
naval mines near
Haiphong,
North Vietnam. The
coronal mass ejection (CME)'s transit time from the
Sun to the
Earth is the fastest ever recorded.
The most significant detected solar flare activity occurred from 2 to 11 August. Most of the significant solar activity emanated from
active sunspot region McMath 11976 (MR 11976; active regions being clusters of
sunspot pairs). McMath 11976 was extraordinarily magnetically complex. Its size was large although not exceptionally so. McMath 11976 produced 67 solar flares (4 of these
X-class) during the time it was facing Earth, from 29 July to 11 August. It also produced multiple relatively rare white light flares over multiple days. The same active area was long-lived. It persisted through five
solar rotation cycles, first receiving the designation as Region 11947 as it faced Earth, going unseen as it rotated past the far side of the Sun, then returning Earthside as Region 11976, before cycling as Regions 12007, 12045, and 12088, respectively.
The 4 August flare was among the largest since records began. It saturated the
Solrad 9 X-ray sensor at approximately X5.3 but was estimated to be in the vicinity of X20, the threshold of the very rarely reached R5 on the NOAA radio blackout space weather scale. A radio burst of 76,000
sfu was measured at 1
GHz. This was an exceptionally long duration flare, generating
X-ray emissions above background level for more than 16 hours. Rare emissions in the
gamma ray (
-ray) spectrum were detected for the first time, on both 4 and 7 August, by the Orbiting Solar Observatory (
OSO 7). The broad spectrum electromagnetic emissions of the largest flare are estimated to total 1-5 x 1032
ergs in energy released.
The arrival time of the associated coronal mass ejection (CME) and its coronal cloud, 14.6 hours, remains the record shortest duration as of November 2023, indicating an exceptionally fast and typically an exceptionally geoeffective event (normal transit time is two to three days). A preceding series of solar flares and CMEs cleared the
interplanetary medium of particles, enabling the rapid arrival in a process similar to the
July 2012 solar storm.
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