1 Circumstances and Consequences of the Early August 1972 Solar Storms

1.1 Flares and Ejecta Between 2 and 4 August 1972 McMath Region 11976 (Figure 1a) produced a series of brilliant flares, energetic particle enhancements, and Earth‐directed ejecta (note the terminology of the day was plasma cloud). The first two ejecta generated two impulses in the geomagnetic field and a magnetic storm shortly after 01 UT on 4 August. More importantly, they likely cleared the interplanetary (IP) path for the subsequent ultrafast 4 August shock/IP coronal mass ejection (ICME) that reached Earth in record time—14.6 hr. The ICME has been linked to the H‐α, Level‐3 Brilliant flare that peaked at 0621 UT on 4 August (Cliver et al., 1990; Srivastava, 1973). Figure 1b shows the flare at 0648 UT. In concert, a 76,000 sfu radio burst at 1 GHz peaked at 0636 UT (Bhonsle et al., 1976). At the other end of the solar spectrum, the SOLRAD‐9 satellite X‐ray detector saturated at what would be an X5.1 magnitude in the current National Oceanographic and Atmospheric Administration (NOAA) solar X‐ray classification (but clearly exceeded that level, see Dere et al., 1973, p 309). Ohshio (1974), using ionospheric radio wave phase propagation disturbances, estimated the flare class as ~X20. Dayside radio blackouts at Earth developed within minutes (Odintsova et al., 1973). X‐ray emissions from the long duration flare remained above background for >16 hr. For the first time, a space‐based detector observed gamma rays during this solar flare (Chupp et al., 1973). Dodson and Hedeman (1973) rated the flare at Comprehensive Flare Index level 17—the highest level, and one assigned to only the most extreme and broad‐spectrum flares. Figure 1 Open in figure viewer PowerPoint α solar spectroheliogram of flaring region at 0648 UT on 4 August 1972. Copyright BASS2000, Paris Observatory, PSL (used with permission). The east limb is at the left of both images. (c) Hourly‐average solar wind plasma data 4–11 August 1972 (Zastenker et al., 1978 1977 X component variations 4 August 17 UT to 5 August 7 UT (Salcedo, 1973 (a) Calcium spectroheliogram of McMath Region 11976 on 3 August 1972. (b) Hydrogen‐solar spectroheliogram of flaring region at 0648 UT on 4 August 1972. Copyright BASS2000, Paris Observatory, PSL (used with permission). The east limb is at the left of both images. (c) Hourly‐average solar wind plasma data 4–11 August 1972 (Zastenker et al.,). (d) Magnetosheath magnetic measurements for 2120 – 2230 UT from ATS 5 geosynchronous spacecraft at 15 LT (Cahill & Skillman,); (e) Manila Observatory magneticcomponent variations 4 August 17 UT to 5 August 7 UT (Salcedo,). The vertical jump at 2240 UT represents a 168‐nT/min increase. After the second sudden commencement giant geomagnetic pulsations were present in the magnetosphere. The sequence of propagating structures produced one of the largest galactic cosmic ray dropouts (Forbush decreases) of the space age (Levy et al., 1976). The ICME‐associated shock (Figure 1c) arrived at Earth at 2054 UT (e.g., Intrilligator, 1976). Vaisberg and Zastenker (1976) and Cliver et al. (1990) estimated the average transit speed as 2,850 km/s. Freed and Russell (2014) reported the transit time was an outlier, even for the family of the extreme events they studied. We believe the extraordinary speed of this event had a direct bearing on the events we discuss below.

1.2 Energetic Particles The chain of events led to extraordinary effects, including a solar energetic particle (SEP) event that punished spacecraft solar panels, satellite detectors, and Earth's atmosphere. The solar particle flux observed at Earth, attributed to activity in McMath Region 11976, began on 2 August after three brilliant flares in solar latitude‐longitude region N12–N14 and E26–E34. These were a 3N flare at 0316 UT (Hakura, 1976) and two rare white light flares at 1844 UT and 2058 UT, rated at 1B and 2B, respectively (Neidig & Cliver, 1983). The 19–80 MeV proton flux on National Aeronautics and Space Administration (NASA) Interplanetary Monitoring Platforms IV and V started to increase at 0515 UT on 2 August (Van Hollebeke et al., 1974). The proton flux on the Interplanetary Monitoring Platforms spacecraft dramatically increased with the 4 August 2054 UT IP‐shock arrival at Earth; the maximum particle flux was so intense that the particle detectors were saturated (see Kohl et al., 1973; Van Hollebeke et al., 1974), resulting in uncertainty as to the actual magnitude of the particle increase. The flux peak also triggered the event mode data compression algorithm on the Vela neutron counter that was monitored in real time at Air Force Global Weather Central (AFGWC) for nuclear test ban verification. This situation was swiftly dealt with by AFGWC personnel monitoring the event (D. Smart, personal experience). These energetic proton fluxes produced a ground level event (Kodama et al., 1973). Levy et al. (1976) and Smart and Shea (1992) argued that IP medium‐energy (seed) particles were accelerated between the 2 August IP structure(s) and the 4 August ultrafast shock, thus producing a swarm of SEPs. The SEPs were so intense that the ongoing Forbush decrease partially abated (see Figure 3 of Pomerantz & Duggal, 1973). Rauschenbach (1980) showed an ~5% drop in solar cell power generation capability for the INTELSAT IV F‐2 solar panel arrays during the 4 August SEP event, roughly equivalent to 2 years of magnetospheric trapped‐radiation exposure to the panels. Shortly thereafter a Defense Communications Satellite Program II satellite suffered a mission‐ending on‐orbit power failure (Shea & Smart, 1998). Lockwood and Hapgood (2007) note this as one of only a handful of events in the space age that would have posed an immediate threat to astronaut safety, had humans been in transit to the moon at the time. Reanalysis by Jiggens et al. (2014) suggests that the 10‐MeV ion flux reached 70,000 cm−2 ·s−1 ·sr−1, thus bordering on a NOAA S5 event. The SEPs interacted with the Defense Meteorological Spacecraft Program satellite optical line scanner electronics, producing anomalous dots of light in the southern polar cap imagery (A. Lee Snyder, personal communication, 2018). The energetic particle bombardment created a Northern Hemisphere polar ozone cavity—a 46% reduction at 50 km that recovered over several days; while at ~39 km the ozone cavity persisted and circulated as a semirigid structure for more than 50 days (Reagan et al., 1981).

1.3 Geomagnetic Storm and Its Effects At 2054 UT on 4 August the Guam observatory (0654 local time) reported an extraordinary 62‐s risetime for the sudden storm commencement consistent with a 3,080 km/s shock sweeping across the magnetosphere (Araki et al., 2004). The Boulder, CO, USA magnetometer traces went off scale, and bright aurora appeared in the northern United States. Another significant disturbance, a sudden impulse (SI) at 2238 UT, swept across predawn, low‐latitude India with amplitudes from 301 to 486 nT (Bhargava, 1973). The SI sent the magnetometer traces off scale at the near‐noon Honolulu, HI, observatory (Figure 3, Matsushita, 1976). Within 15 min the first glow of what would become a spectacular aurora, bright enough to cast shadows, appeared along the southern coast of the United Kingdom at ~54° MLAT (Taylor & Howarth, 1972). Within 2 hr commercial airline pilots reported aurora as far south as Bilboa, Spain, at ~46° MLAT (McKinnon, 1972). Between 2240 and 2310 UT the postnoon magnetopause compressed to within 5.2 R E (Hoffman et al., 1975). Cahill and Skillman (1977) described numerous magnetopause crossings by satellites in the noon sector (Figure 1d). Based on measurements from the Prognoz‐1 spacecraft located within the morning‐side magnetosheath, D'Uston et al. (1977) suggested that the solar wind dynamic pressure was 100 times its normal value. Figure 1c shows the hourly solar wind plasma values reconstructed from Prognoz, Prognoz‐2, and HELIOS data (Vaisberg & Zastenker, 1976; Zastenker et al., 1978). Lockwood et al. (1975), Simnett (1976), Smith (1976), Venkatesan et al. (1975), Lanzerotti (1992), and Tsurutani et al. (1992) have offered data and insights related to the IP structures that likely passed Earth on 4–5 August 1972. All suggest a highly variable north‐south interplanetary magnetic field (IMF) ahead of the main ejecta and a northward IMF at the ICME leading edge. Tsurutani et al. (1992) reasoned that the Dst intensity during early 5 August was due to sheath southward IMF, while the subsequent leading ICME field was northward, resulting in quieting magnetic conditions in the subsequent interval. Some of these authors note signals of multiple tangential discontinuities indicative of interacting IMF structures ahead of the ICME. Medrano et al. (1975) reported an additional square‐wave discontinuity passing Earth between 03 and 05 UT on 5 August. The IP disturbances created geomagnetically induced current effects in North American power and communications lines. Albertson and Thorson (1974) listed numerous United States and Canadian power companies that reported minor to strong power issues on 4–5 August 1972. They show strong induced current disturbances as far south as the US states of Maryland and Ohio. According to Odenwald (2015) significant voltage swings and power disruptions were reported in northern tier U.S. states. In Newfoundland, Canada, geomagnetically induced currents activated protective relays many times on 4–5 August. The Manitoba Hydro Company recorded 120 MW drops in power supplied to Minnesota in only a few minutes. Anderson et al. (1974) reported an outage on the L4 American Telephone and Telegraph cable connecting the U.S. states of Illinois and Iowa. The induced electric field of 7.0 V/km, which exceeded shutdown threshold for high current, accompanied magnetic field variations (dB/dt) of ~ 800 nT/min at 2240–2242 UT (the time of the L4 outage). In central and western Canada, Boteler and Jansen van Beek (1999) estimated that dB/dt exceeded ~2,000 nT/min coincident with the SI. On the other side of the world, coincident with the initial magnetopause compression at ~ 2240 UT, and roughly at dawn local time, there was >160 nT/min positive magnetic perturbation (Figure 1e) reported from the near‐equatorial observatory at Manila, Philippines (Salcedo, 1973, p. 762). Simultaneously, a similar dB/dt perturbation was reported at Sao Jose dos Campos, Brazil (~12.6°S magnetic latitude) by Sahai and Sales (1973). Online images of the 4 August Kakioka, Japan, magnetogram also show a midlatitude pulse (http://www.kakioka‐jma.go.jp). After 22 UT the AE index spiked to >3,000 nT (Figure 1, Tsurutani et al., 1992) as the storm asymmetry index underwent severe variations (Kawasaki et al., 1973; Akasofu, 1974; J. Love, personal communication, 2018). Giant magnetic pulsations rocked the magnetosphere. Odintsova et al. (1973) reported development of a nighttime midlatitude E layer on 4–5 August. Jachiaa and Slowey (1973) showed multi‐altitude neutral density perturbations continuing into 5 August that equaled or exceeded those from the great storm of May 1967, during which half of the North American Aerospace Defense Command satellite‐tracking catalog had been reacquired (Knipp et al., 2016). Hoffman et al. (1975) speculated that the sequence of strong cross‐tail electric field and magnetopause compression allowed ring current particles that could have produced a more substantial Dst storm to drift out the compressed/eroded dayside magnetopause. Lanzerotti (1992) argued that with the arrival of driver ejecta on 5 August, the IMF turned northward, thus cutting short the ring current development. Brace et al. (1974) reported the plasmapause to be at/inside 2 R E . Grafe et al. (1979) indicated that the energetic electrons invaded the radiation belt slot region, while Spjeldvik and Fritz (1981) reported orders of magnitude increases in the trapped energetic heavy ion population (Z ≥ 4) within the radiation belts and slot region (L ~ 2.5–5) between 4 and 5 August. Auroral disturbances continued into 5 August with reports of intense midday red aurora in the dark Southern Hemisphere (Akasofu, 1974).