August 1972 solar storms

Sunspot region 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. Flare of 4 August Electromagnetic effects 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. 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 (\gamma-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 × 1032 ergs in energy released. CMEs 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. This corresponds to an ejecta speed of an estimated . The near Earth vicinity solar wind velocity may also be record-breaking and is calculated to have exceeded (about 0.7% of light speed). The velocity was not directly measurable as instrumentation was off-scale high. Analysis of a Guam magnetogram indicated a shockwave traversing the magnetosphere at and astonishing sudden storm commencement (SSC) time of 62 s. Estimated magnetic field strength of 73-103 nT and electric field strength of >200 mV/m was calculated at 1 AU. Fluxes at other energy levels, from soft to hard, at >1 MeV, >30 MeV, and >60 MeV, also reached extreme levels, as well as inferred for >100 MeV. The intense solar wind and particle storm associated with the CMEs led to one of the largest decreases in cosmic ray radiation from outside the Solar System, known as a Forbush decrease, ever observed. Solar energetic particle (SEP) onslaught was so strong that the Forbush decrease in fact partially abated. SEPs reached the Earth's surface, causing a ground level event (GLE). Geomagnetic storm The 4 August flare and ejecta caused significant to extreme effects on the Earth's magnetosphere, which responded in an unusually complex manner. A 2006 study found that if a favorable IMF southward orientation were present that the Dst may have surpassed −1,600 nT, comparable to the 1859 Carrington Event, and a 2024 study found that such a storm could have produced a 774–775-scale solar particle event. Magnetometers in Boulder, Colorado, Honolulu, Hawaii, and elsewhere went off-scale high. Stations in India recorded geomagnetic sudden impulses (GSIs) of 301-486 nT. Estimated AE index peaked at over 3,000 nT and Kp reached 9 at several hourly intervals (corresponding to NOAA G5 level). Solar wind dynamic pressure increased to about 100 times normal, based upon data from Prognoz 1. ==Impacts==

Spacecraft Astronomers first reported unusual flares on 2 August, later corroborated by orbiting spacecraft. On 3 August, Pioneer 9 detected a shock wave and sudden increase in solar wind speed from approximately . A shockwave passed Pioneer 10, which was 2.2 AU from the Sun at the time. The Intelsat IV F-2 communications satellite solar panel arrays power generation was degraded by 5%, about 2 years worth of wear. An on-orbit power failure ended the mission of a Defense Satellite Communications System (DSCS II) satellite. Disruptions of Defense Meteorological Satellite Program (DMSP) scanner electronics caused anomalous dots of light in the southern polar cap imagery. Extending to 5 August, intense geomagnetic storming continued with bright red (a relatively rare color associated with extreme events) and fast-moving aurora visible at midday from dark regions of the Southern Hemisphere. Radio frequency (RF) effects were rapid and intense. Radio blackouts commenced nearly instantaneously on the sunlit side of Earth on HF and other vulnerable bands. A nighttime mid-latitude E layer developed. Geomagnetically induced currents (GICs) were generated and produced significant electrical grid disturbances throughout Canada and across much of eastern and central United States, with strong anomalies reported as far south as Maryland and Ohio, moderate anomalies in Tennessee, and weak anomalies in Alabama and north Texas. The voltage collapse of 64% on the North Dakota to Manitoba interconnection would have been sufficient to cause a system breakup if occurring during high export conditions on the line, which would have precipitated a large power outage. Many U.S. utilities in these regions reported no disturbances, with the presence of igneous rock geology a suspected factor, as well as geomagnetic latitude and differences in operational characteristics of respective electrical grids. Manitoba Hydro reported that power going the other way, from Manitoba to the U.S., plummeted 120 MW within a few minutes. Protective relays were repeatedly activated in Newfoundland. AT&T also experienced a surge of 60 volts on their telephone cable between Chicago and Nebraska. that the seemingly spontaneous detonation of dozens of Destructor magnetic-influence sea mines (DSTs) within about 30 seconds in the Hon La area (magnetic latitude ≈9°) was highly likely the result of an intense solar storm. One account claims that 4,000 mines were detonated. It was known that solar storms caused terrestrial geomagnetic disturbances but it was as yet unknown to the military whether these effects could be sufficiently intense. It was confirmed as possible in a meeting of Navy investigators at the NOAA Space Environment Center (SEC) as well as by other facilities and experts. Had the most intense solar activity of early August occurred during a mission, it would have forced the crew to abort the flight and resort to contingency measures, including an emergency return and landing for medical treatment. ==Implications for heliophysics and society==

The storm was an important event in the field of heliophysics, the study of space weather, with numerous studies published in the next few years and throughout the 1970s and 1980s, as well as leading to several influential internal investigations and to significant policy changes. Almost fifty years after the fact, the storm was reexamined in an October 2018 article published in the American Geophysical Union (AGU) journal Space Weather. The initial and early studies as well as the later reanalysis studies were only possible due to initial monitoring facilities installed during the International Geophysical Year (IGY) in 1957-1958 and subsequent global scientific cooperation to maintain the data sets. That initial terrestrial data from ground stations and balloons was later combined with spaceborne observatories to form far more complete information than had been previously possible, with this storm being one of the first widely documented of the then young Space Age. It convinced both the military and NASA to take space weather seriously and accordingly devote resources to its monitoring and study. Other researchers conclude that the 1972 event could have been comparable to 1859 for geomagnetic storming if magnetic field orientation parameters were favorable, or as a "failed Carrington-type storm" based on related considerations, which is also the finding of a 2013 Royal Academy of Engineering report. ==See also==