Extreme Space Weather Events: from Cradle to Grave

Space Sci Rev (2018) 214:21 https://doi.org/10.1007/s11214-017-0456-3

Extreme Space Weather Events: From Cradle to Grave

Pete Riley1 · Dan Baker2 · Ying D. Liu3,4 · Pekka Verronen5 · Howard Singer6 · Manuel Güdel7

Received: 20 July 2017 / Accepted: 5 December 2017 © Springer Science+Business Media B.V., part of Springer Nature 2017

Abstract Extreme space weather events, while rare, can have a substantial impact on our technologically-dependent society. And, although such events have only occasionally been observed, through careful analysis of a wealth of space-based and ground-based observations, historical records, and extrapolations from more moderate events, we have developed a basic picture of the components required to produce them. Several key issues, however, remain unresolved. For example, what limits are imposed on the maximum size of such events? What are the likely societal consequences of a so-called “100-year” solar storm? In this review, we summarize our current scientiﬁc understanding about extreme space weather events as we follow several examples from the Sun, through the solar corona and inner heliosphere, across the magnetospheric boundary, into the ionosphere and atmosphere, into the Earth’s lithosphere, and, ﬁnally, its impact on man-made structures and activities, such as spacecraft, GPS signals, radio communication, and the electric power grid. We describe pre- liminary attempts to provide probabilistic forecasts of extreme space weather phenomena, and we conclude by identifying several key areas that must be addressed if we are better able to understand, and, ultimately, predict extreme space weather events.

The Scientiﬁc Foundation of Space Weather Edited by Rudolf von Steiger, Daniel Baker, André Balogh, Tamás Gombosi, Hannu Koskinen and Astrid Veronig

B P. Riley [email protected]

1 Predictive Science Inc., 9990 Mesa Rim Rd., Suite 170, San Diego, CA 92121, USA 2 LASP, University of Colorado, Boulder, CO, USA 3 State Key Laboratory of Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing 100190, China 4 University of Chinese Academy of Sciences, Beijing 100049, China 5 Finnish Meteorological Institute, Erik Palménin aukio 1, 00560 Helsinki, Finland 6 NOAA, Space Weather Prediction Center, Boulder, CO, USA 7 Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17., 1180 Vienna, Austria 21 Page 2 of 24 P. Riley et al.

Keywords Extreme space weather events · Coronal mass ejections · Geomagnetic storms · Carrington event

1 Introduction

Extreme space weather events include a range of solar, heliospheric, magnetospheric, ionospheric, atmospheric, and ground phenomena, all driven by coronal mass ejections (CMEs). While rare, such events can have a disproportionately large impact on our technology-reliant society. In this review, we summarize the current state of our knowledge with respect to these extreme phenomena. In particular, we: (1) provide a brief historical review; (2) highlight several speciﬁc events that have been well studied; (3) discuss extreme stellar phenomena and their possible relationship to the Sun; (4) summarize the societal consequences of extreme events; (5) describe forecasting techniques that can be used to estimate the occurrence rate of these events; and (6) speculate on the future path that research into extreme events may take.

1.1 What Is an Extreme Event?

An extreme event refers to some phenomenon that is out of the ordinary, i.e., it is rare, or unusual. We could create an arbitrary threshold for such an event, but there are no such values that are agreed upon by the scientific community (although there is a National Space Weather Strategy and Action Plan that calls for establishing benchmarks for space weather events that can have “detrimental effects on the Nation’s economic and social well-being” Jonas et al. 2017). Instead, extreme events are identified subjectively, the threshold primarily driven by the specific study being undertaken. Moreover, extreme space weather events can be extreme with respect to one parameter, but only moderate or even weak with respect to another. Thus, any definition of an “extreme” event must include which parameter or parameters make it extreme. As we move from the Sun to the Earth, the quintessential example of an extreme space weather would manifest itself in terms of: (1) massive soft and hard X-rays; (2) a CME traveling extremely rapidly away from the Sun; (3) driving a strong fast-forward shock; (4) the creation of large fluxes of accelerated particles; (5) substantial variations in a range of geomagnetic parameters (e.g., Dst, Kp, and AE), sudden ionospheric disturbances; (6) strong ground-induced currents in the lithosphere; and (7) possibly, the deposition of nitrates in the polar ice. Figure 1 summarizes some of these phenomena. From a societal perspective, the same event may be viewed in terms of adverse technological consequences: satellite failures, health problems for astronauts and long-haul air passengers, communication blackouts, power outages, and any range of cascading problems from these primary hazards, including food and water shortages, all the way to a breakdown in society (NRC 2008). Given their rarity, it is reasonable to ask whether extreme solar events are merely “black swans” such as crashes in the stock market (Taleb 2007). Such phenomena are defined as highly improbable events with the following characteristics: (1) unpredictable; (2) massive impact; and (3) an inherently random event for which we retrospectively concoct an expla- nation to explain it, making it apparently more predictable. Extreme space weather events are not unpredictable, at least once their signatures have been observed at the Sun. They undoubtedly have a massive impact, ranging from failures in the power grid, navigation assets, and communication. However, they do not need us to invent reasons for their occurrence. Given the substantial knowledge we have about them, preparing for them is within our grasp; that is, this is a solvable policy and investment problem. Extreme solar events are not “black swans.” Extreme Space Weather Events: From Cradle to Grave Page 3 of 24 21

Fig. 1 Extreme space weather events include a broad range of phenomena all the way from the Sun’s surface to the Earth’s atmosphere. Note that features in this image are not to scale. Image courtesy of the Washington Post

1.2 Why Do We Care?

From a scientific perspective, extreme solar phenomena, leading to extreme space weather events, are vital for understanding the dynamic behavior and variability of the Sun. They represent the outliers of a wide spectrum of transient phenomena. And, while it could be argued that because these events are rare, they are more of a curiosity than fundamental process, it is only by understanding how these events are produced that we can gain a comprehensive understanding of their underlying physical mechanisms. But, more than this, extreme solar events are truly spectacular to contemplate. How is the Sun capable of producing them? Why are they so infrequent? How frequently do they occur? Are there unique physical processes required to produce them? How extreme can they be? From a societal perspective, the reasons we care about extreme solar events are even clearer. Even moderately severe events have resulted in significant damage to a wide range of technologies. During the March 1989 storm, for example, power to many residents in Quebec was knocked out. Similarly, the $200M Lockheed-Martin built Telstar 401 satellite suffered a massive—and terminal—power failure, likely the result of the January 7, 1997 CME. In January 1994 the ANIK E-1 tumbled out of control, and, later, in May, 1998 the Galaxy IV satellite was lost. The latter event was attributed to a failure of the satellite’s primary control processor; a result of a hole developing in the wax coating that allowed whiskers to develop, according to engineers. Solar physicists, however, pointed out that a more likely (but disputed) reason was the high levels of high-energy, solar storm produced electrons (the so-called “killer” electrons), in which the spacecraft was bathed for several weeks, and which slowly fried the electronics (Baker et al. 1994, 1998). More extreme space weather events will have more extreme societal consequences. A report commissioned by the National Research Council, relying on earlier studies, stated that 21 Page 4 of 24 P. Riley et al. if the 1921 geomagnetic storm (which was inferred to be ten times stronger than the 1989 event) occurred today, it could result in 130 million people without power and damages in the range $1–2 trillion during the first year after the event (NRC 2008). Other reports have arrived at similar estimates (McMorrow 2011; Kappenman 2012; Cannon et al. 2013). Most recently, Oughton et al. (2017) estimated the daily economic impact of extreme space weather, concluding that only 49% of the cost of an extreme solar event was due to direct causes within the so-called ‘blackout zone” (i.e., disruption of electricity), highlighting the significance of supply chain linkages. The total daily economic loss to the US economy associated with an extreme geomagnetic storm was estimated to be ∼ $40B, or ∼ 90% of daily US GDP, assuming that 66% of the US population lost electricity.

1.3 A Brief Historical Perspective

Richard Carrington’s observations of the September 1, 1859 solar flare probably mark the first documented record of an extreme space weather event (Carrington 1859). A massive CME hit the Earth’s magnetosphere and caused one of the largest geomagnetic storms on record (Siscoe et al. 2006). However, there are indirect records, both observer accounts and evidence in natural records, of events stretching more than a millennium back in time. The carbon-14 spikes dated to 774/775 (Miyake et al. 2012) and 993 (Miyake et al. 2013, 2014), for example, suggest much larger extreme solar events occurred in the recent past. In spite of these, as well as more recent events in the 19th century, the significance of extreme events did not become widely appreciated until much later (i.e., the space era). This significance has grown as society has become more dependent on technology that is sensitive to such events.

1.4 Extreme Events in the Corona

While it could be argued that extreme space weather events, and more specifically, the extreme CMEs that give rise to them, are ‘born’ below the Sun’s surface, at least currently, our first observations come from disk and limb observations of the photosphere and corona. What separates extreme CMEs from the vast majority of otherwise spectacular solar activity? At the least, the following factors are important: (1) They are massive; (2) They are propelled with substantially higher speeds and drive strong fast-forward shocks ahead of them; (3) They carry large internal magnetic fields strengths within them; and (4) They are directed towards the Earth. This last factor is what distinguishes an extreme solar event from an extreme space weather event (at least from the Earth’s perspective). Just how large, fast, and magnetic must the event be? Although arguable, an event with a mass in excess of 5 × 1013 Kg, a speed in excess of 3,000 km/s, and internal field strength (as measured at 1 AU) of greater than 100 nT would certainly qualify it as being extreme (Riley 2012). It does not need all three criteria to be met, and which are necessary is application dependent. However, generally, the strength and orientation of the embedded magnetic field will be a major factor in whether or not an event is geo-effective, as will its speed as it approaches 1 AU. Thus, a CME initially traveling at > 3000 km s−1, but which slows down to 1200 km/s by the time it reaches 1 AU will have much less impact than the same event that through the presence of favorable conditions ahead of it (e.g., a high-speed stream or an earlier CME that swept up ambient plasma prior to the passage of the second CME, i.e., a preconditioning CME) maintains a high speed all the way to 1 AU. To produce an extreme CME requires lots of magnetic flux. The magnetic field in the low corona provides both the energy to drive the eruption of the CME as well as the internal Extreme Space Weather Events: From Cradle to Grave Page 5 of 24 21

field that it carries away with it. There are several studies that have described empirical relationships between the properties of CMEs and the regions from which they erupt. Vršnak et al. (2007), for example, showed that CMEs from more spatially compact source regions tended to be faster and accelerated more impulsively, presumably due to the higher fields associated with the source regions. Thus, we anticipate that the most extreme CMEs will require high magnetic flux contained within a presumably large enough active region to fuel the eruption. The Carrington event was associated with a large and complex sunspot region, as drawn by Lord Carrington. It also resulted in a white-light flare lasting 5 minutes—the first such case, at least in recorded history—suggesting the conversion of significant amounts of magnetic energy through magnetic reconnection into kinetic motion. The eruption of an extreme CME also produces solar proton events (SPEs), where particles (primarily protons) are accelerated—either close to the Sun during the flaring process or further away by the CME-driven shock. These particles can pose a significant radiation hazard to both astronauts and spacecraft. Additionally, the can penetrate the geomagnetic field, ionizing particles in the atmosphere (as discussed in more detail in Sect. 1.8).

1.5 Extreme Events in the Solar Wind

In some sense, extreme space weather events in the solar wind are just extensions of what occurred in the corona back at the Sun. However, it is becoming increasingly clear that the preexisting conditions in the solar wind, into which the CME propagates, can have a profound effect on the evolution of the ejecta, and, in turn dictate whether the event will remain “extreme” by the time it reaches the Earth’s location (Liu et al. 2014). Consider the following scenarios. First, a CME that is launched into slow and dense ambient solar wind. The interaction will generate a strong magnetosonic shock ahead of the ejecta, which in turn will produce a sheath region between the shock and the leading edge of the ejecta. This will likely contain large fluctuations in the magnetic field that are compressed and amplified. On the other hand, the exchange of momentum from the CME to the wind ahead will cause the speed of the CME to decrease, which in turn will reduce the dawn-dusk electric field (a driver of magnetospheric activity, and estimated by Bz × vx ,whereBz is z-component of the interplanetary magnetic field and vx is the x-component of the solar wind velocity) that is imposed across the magnetosphere when the CME sweeps over it. Second, consider the same fast CME propagating into fast ambient solar wind. In this case, the shock will not likely be as strong, and the sheath region too will be more modest. In contrast, the CME will retain more of its momentum, and input more energy into the magnetosphere. Finally, consider an example involving the eruption of multiple CMEs from the same active region in relatively rapid succession, such as was inferred for the July 23, 2012 extreme event (discussed in more detail in Sect. 2.3), which missed Earth but was observed by a spacecraft (STEREO A) far from Earth but at a similar distance from the Sun (Liu et al. 2014). The initially fast CME propagates through predominantly slow solar wind, sweeping it up, and generating a shock and sheath in front of it. Behind, it outruns the slower ambient solar wind creating an expansion wave, or rarefaction region; a volume of space with low density and an accelerating velocity profile (Riley et al. 1997; Riley 1999). A second CME launched into this background has even less difficulty maintaining its high speed as it propagates through a tenuous medium. Such conditions, which are optimal for maintaining, or even increasing the ejecta’s extreme properties, can lead to a perfect storm scenario that retains the CME’s extreme properties far from the Sun. Moreover, these conditions, where multiple CMEs are launched from large, energetic and complex active regions are not that rare. Ruzmaikin et al. (2011), for example, showed that fast CMEs, in particular, tend to arrive (and hence are produced) in clusters. 21 Page 6 of 24 P. Riley et al.

1.6 Extreme Events in the Magnetosphere

Energy is transferred from the solar wind into the magnetosphere by magnetic reconnection, with the southward component of the interplanetary magnetic field and bulk solar wind flow speed being the prime properties of the solar wind driving the reconnection rate (Dungey 1961). Extreme events in the magnetosphere are often known as extreme geomagnetic storms. The societal consequences of such events could be severe, with our increasing reliance on a large number of commercial and military satellites currently orbiting within magnetospheric boundaries. Scientifically, the effects of an extreme event have not been well established. With our quintessential space-era event occurring almost 30 years ago (March 1989), only limited data were available. Thus we must rely primarily on models to help us understand what the potential or probable effects of an extreme geomagnetic storm would be. Current global, coupled MHD magnetospheric (and ionospheric) models are extremely sophisticated. An example, which is used operationally by NOAA’s SWPC, is the Univer- sity of Michigan’s BatsRus model. However, they have, for the most part, only been tested against recently observed non-extreme events. Ngwira et al. (2014) performed a global MHD simulation that suggested that the ground electric fields produced by the Carrington event would be twice as large as those measured during the 1989 ‘extreme’ storm. However, the magnetospheric system is incredibly complex, and driving it at such extremes could produce phenomena that are not captured in current models. Saturation effects, in particular, may become increasingly important. A related question concerns the most extreme value that Dst could attain during an extreme solar event. While, in principle, it could reach −32,000 nT (resulting in the complete cancellation of the Earth’s magnetic field), it is quite possible that it is constrained to a much lower limit of a few thousand nT (Toffoletto et al. 2016). As we have noted, ‘extreme’ space weather events may be extreme with respect to some parameter or property, but almost normal with respect to another. Based on an analysis of the aa index for all storms from 1868 through 2010, Vennerstrom et al. (2016) inferred that the May 1921 and March 1989 events were the most extreme. Moreover, they argued that neither Kp nor Dst were as good an indicator of the extremity of the storms. In addition to suggesting that extreme geomagnetic storms were often associated with interacting interplanetary CMEs (ICMEs), they also noted that a necessary precondition for extreme geomagnetic events was that geomagnetic activity was already high.

1.7 Extreme Events in the Ionosphere

At least indirectly, it could be argued that the first space weather effects to be systematically analyzed occurred in the Earth’s ionosphere. In 1722, George Graham first reported deflec- tions in a compass needle, shown later in 1822 by Balfour Stewart to be due to currents flowing in the ionosphere (Whitten and Poppoff 1965). And of course, the long-observed aurora point to the influence of the Sun’s variability on the ionosphere. During severe space weather events the ionosphere can be disturbed substantially. This can have a significant impact on the propagation of radio waves, which is of both scientific and societal interest. The scintillation (or distortion) of these waves may be so drastic as to render the signals unrecognizable. More specifically, Global Positioning System (GPS) signals can be impacted by space weather, to the extent that the GPS-based navigation tool for commercial aviation in the US (Wide Area Augmentation System, WAAS) can be disabled during very severe geomagnetic storms (Webb and Allen 2004). Additionally, transpolar Extreme Space Weather Events: From Cradle to Grave Page 7 of 24 21

flights are also impacted during severe space weather events. In addition to the poorly un- derstood radiation effects that may be suffered by passengers, crew, and equipment, solar energetic protons affect ionospheric conditions in the polar regions that degrade reliable radio communication with the aircraft and can sometimes result in the diversion of these flights. Space weather also affects shortwave (high-frequency, HF) communications. Today, their use is limited to ships and aircraft that do not have more modern capabilities (such as Iridium), but they also act as the backup for others. Shortwave transmissions, which are generally reflected by the ionosphere and ground (allowing beyond-the-line of-sight communication), may be scattered by the ionospheric irregularities produced by major solar events, causing disruptions to communications. At least three mechanisms contribute to this: (1) solar radio bursts; (2) X-rays produced from solar flares, which disturb the ionospheric D-layer; and (3) Total Electron Content (TEC) enhancements resulting from a geomagnetic storm. How severe would these impacts be? Cannon et al. (2013) estimated that an extreme space weather event would result in the complete loss of GPS service for at least one day, and possibly up to three days, although they anticipated that it might be possible to receive signals from at least one spacecraft, potentially reducing the effects. The “spill-over” effects of a disruption to the GPS system would be significant too. In the US, in particular, where cellular service relies on GPS, mobile communications would likely be vulnerable to outages (Cannon et al. 2013).

1.8 Extreme Events in the Atmosphere

In the atmospheric context, extreme events are most closely related with SPEs, which can lead to substantial increase of high-energy (> 1 MeV) proton fluxes in the polar atmosphere for several days. Although SPEs are typically identified and sorted based on > 10 MeV proton observations by the GOES satellites in geosynchronous orbit (US Dept. of Commerce 2010), another way to rank them is to look directly at the magnitude of their atmospheric effect, e.g., the amount of odd nitrogen (NOx) produced in the middle atmosphere (Vitt and Jackman 1996; Jackman et al. 2008). In the space age, very intense SPEs have occurred in 1972 (August), 1989 (October), 1991 (March), 2000 (July), 2001 (November), and 2003 (October). Such strong events are infrequent and most likely occur during the maximum or declining phase of solar cycle. Although SPE protons can have sufficient rigidity (momentum per charge) to pass through the Earth’s magnetosphere and precipitate directly into the atmosphere (indeed for some events, the most energetic particles can penetrate to the ground—as so-called “ground level enhancements” Shea and Smart 2012), in practice this only takes place in the polar regions where the Earth’s magnetic shielding is weakest. Typically, geomagnetic latitudes poleward of ∼ 60◦, in both hemispheres, experience a more-or-less homogeneous SPE forcing (Rodger et al. 2006; Verronen et al. 2007). The region between 90 and 30 km is significantly affected (e.g. Verronen and Lehmann 2013), corresponding to proton energies between 1 and 200 MeV, respectively. At higher and lower altitudes, the impact of SPEs is negligible compared to the continuous and dominant forcing from precipitating magnetospheric electrons and galactic cosmic rays, respectively. Due the their strong impact over a rather extensive latitudinal region, SPEs have played an important role in understanding the atmospheric effects of energetic particle precipita- tion (Jackman and McPeters 2004; Verronen and Lehmann 2013) and references therein. In the 1970s, it was theoretically proposed that atmospheric ionization during SPEs leads to production of odd hydrogen HOx (H, OH, HO2) and odd nitrogen NOx (N, NO, NO2), and 21 Page 8 of 24 P. Riley et al. enhanced catalytic depletion of ozone (Swider and Keneshea 1973; Crutzen et al. 1975). Later, satellite observations confirmed that ozone concentration can decrease by tens of percent in the mesosphere and upper stratosphere during large SPEs (e.g. Jackman et al. 2001; Verronen et al. 2006). In the mesosphere, the ozone response to large SPEs may lead to a cooling by a few Kelvins, which modulates zonal, meridional, and vertical winds by up to tens of percent over a period of 4–6 weeks after a event (Jackman et al. 2007). Because NOx is chemically long-lived in the upper stratosphere, a longer-term ozone decrease lasting months beyond the end of an SPE has also been predicted and observed (Seppälä et al. 2004;Jackmanetal.2008, 2009). The effect on total ozone column (most of ozone lies at altitudes below 30 km in the so-called ozone layer) is estimated to be just a few percent even after very large SPEs (Jackman et al. 1996, 2011). Thus, the overall societal impact through modulation of solar UV radiation affecting flora and fauna can be expected to be moderate (although even small increases in UV-B radiation can be harmful for many life forms). However, if the structure and strength of the geomagnetic field changed significantly, e.g. during a polarity transition (which tends to occur at irregular intervals of ∼ 200,000 years), SPEs could also affect mid- and low-latitudes. Thereby reducing the total ozone column by tens of percent (Sinnhuber et al. 2003).

1.9 Extreme Events in the Lithosphere

Space weather effects have been followed all the way to the Earth’s conduction crust and upper mantle, known as the lithosphere. The March 1989 magnetic storm, for example, resulted in large electric fields being induced into the lithosphere, the generation of un- controllable ground-induced currents (GICs) within the Quebec electric-power grid, and an ensuing blackout affecting nine million people (Allen et al. 1989). Additional effects may range from the disruption of telecommunication systems to the corrosion of buried pipelines (Pirjola 2000). In spite of the potential for a massive storm to cause the failure of the electric power grid over an entire continent (Cannon et al. 2013), and all the consequences that such an event might spawn, the effects of extreme space weather in the lithosphere are the least well known in the entire chain from the Sun to the Earth. Recognizing this, Love et al. (2016) developed geoelectric hazard maps for the continental US showing the likely extreme values for electric field amplitudes of much of the United States. They found that the size of these fields depended sensitively on location, so-called ‘once in a century’ storms producing electric fields that varied by as much as two orders of magnitude. An added complication for modeling the response of the lithosphere to extreme events is that GICs are driven not only by large-scale solar events, but also small-scale turbulent features in the solar wind (Huttunen et al. 2008; Pulkkinen 2015). Thus, a robust prediction scheme must account for a wide range of spatial and temporal scale drivers, which, at least currently, cannot be accurately forecasted (Riley et al. 2017). Moreover, small- scale ionospheric features may also be implicated in the generation of GICs (Pulkkinen et al. 2015). Since their origin has not yet been well established, current global magnetospheric/ionospheric models are unlikely to be able to produce them.

2 The Science of Extreme Space Weather Events: Case Studies

2.1 Introduction

Case studies allow us to investigate the complex processes that bring the CME from the solar surface to the Earth. And, for extreme phenomena, in particular, they are crucial. However, Extreme Space Weather Events: From Cradle to Grave Page 9 of 24 21

Fig. 2 Lord Carrington’s drawing of the sunspots associated with the “Carrington” event illustrate the remarkable detail he captured by hand (Carrington 1859). The two points, A and B mark the initial positions of an intense, bright event, which, over approximately five minutes, moved to points C and D, before disappear- ing two caveats must be made. First, as with all case study research, we must take care that our findings hold in general, or at least recognize any biases that might be present. Second, our supposed “case studies” for truly extreme events, may not be sufficiently good fiduciaries. If there are fundamental differences between the events that we can analyze in detail and strictly extreme events, our extrapolations may not be valid. With that in mind, here, we review two undoubtedly extreme solar events, the so-called Carrington event of September 1, 1859, and the July 23, 2012 event. Additionally, we discuss two events much further back in time, one in 774/775 and the other in 993 AD. Finally, we summarize the limited information available on Sun-like stars, as determined by the remarkable observations by the Kepler spacecraft and their implications for understanding extreme space weather events.

2.2 The “Carrington” Event

Perhaps the most (in)famous extreme solar event is the “Carrington” event, so named because Richard C. Carrington witnessed and reported it. This event is particularly interesting—and frustrating—because it straddles a period in science where many advances were being made both in observational techniques and our understanding of the Sun. Fig- ure 2 shows Lord Carrington’s drawing of the Sunspot that produced the September 1st, 1859 solar eruption. Even though this was the first direct report of what would later become to be known as an extreme event, the richness and complexity of the drawing foretells some of the latest advances in understanding some of the key features that make such events “extreme”. Occurring when it did, there is a tantalizing collection of data associated with the Car- rington event, but many crucial measurements are missing, relegating it to an event with 21 Page 10 of 24 P. Riley et al. considerable uncertainty. Inferences of Dst, for example, range from −850 nT to more than −1700 nT (Siscoe et al. 2006). In spite of this, at least until recently, it likely held the record as the largest space weather event observed in over 400 years (Shea et al. 2006). (Only the July 23, 2012 event, observed at the STEREO A spacecraft, and to be discussed below, might have been larger if it had encountered the Earth.) Additionally, the shock arrival time was estimated to be 16.5 hours (from the initial flare observation), making the CME one of, but not the fastest events (Siscoe et al. 2006). The event also occurred during the beginning of the electronic revolution. At the time, the telegraph had been invented and was in regular use (although communications from them during the storm were disrupted), but many other technologies, which we now appreciate are extremely sensitive to geomagnetic activity, remained to be discovered. Thus, it is difficult to extrapolate, with any degree of certainty, the specific effects the Carrington event would have had on our current technological society. The assessment of atmospheric impacts of the Carrington event, as well as those of any other SPE before the space age, is challenging due to the obvious lack of reliable proton flux measurements. On longer time scales, ice core records have been used to determine the spectral shape and fluence of very large SPEs (McCracken et al. 2001), assuming that the nitrate deposition and sedimentation reflects the amount of stratospheric NOx production during SPEs. However, there are concerns about the proposed connection between SPE-NOx and nitrate deposition, such that ice core records have been deemed unusable for SPE studies (Duderstadt et al. 2016). In particular, many ice core data do not show a nitrate spike related to the Carrington event (Wolff et al. 2008). Although atmospheric observations during the Carrington event are lacking, there are a few modeling studies that compare its implications to those of the space-age SPEs (Thomas et al. 2007; Calisto et al. 2012). Using SPE characteristics (fluence and spectrum) derived (at least partly) from the ice core records, the applied SPE forcing was set to be 4–6.5 times larger than observed for the strongest events of the space age. Reported effects on HOx, NOx and ozone are qualitatively very similar to smaller SPEs, while the magnitude of the response increases less than the SPE forcing. However, consistently in all studies, an unusu- ally strong and long-lasting ozone depletion develops in the middle-to-upper stratosphere due to larger NOx production. This leads to an estimated 4% decrease in globally-averaged ozone column, maximizing two months after the event, followed by a full recovery after several years. In the polar upper stratosphere, ozone is depleted by 20–30% for months. These results seem to be weakly dependent on the assumed SPE spectral shape. Climate simulations indicate that ozone changes may lead to cooling in the polar mesosphere and upper stratosphere, which would impact zonal winds (polar jets) and surface air temperatures by up to 7 K. However, these effects seem to be most pronounced shortly after the event, and the sign of the response as well as its statistical significance seem to be dependent, e.g., on the SPE spectral shape and seasonality. In summary, although the atmospheric effects of the Carrington event are estimated to be larger than for a typical space-age SPE, they are not drastic in comparison.

2.3 The July 23, 2012 CME

For many years, the operational space weather community has sought a defensible, plausible “worst case” space weather scenario that was better documented than the Carrington event. The Sun provided an almost ideal active experiment on July 23, 2012, when it launched a powerful and extreme CME away from, but within sight of the Earth’s many observing platforms that fortuitously encountered the STEREO A spacecraft. Thus, we were able to obtain phenomenal measurements of the event and conclude that, had it hit the Earth, it would likely have caused catastrophic damage (Baker et al. 2013). Extreme Space Weather Events: From Cradle to Grave Page 11 of 24 21

The July 23, 2012 event demonstrated how the “perfect” storm” could arise through a combination of circumstances (Liu et al. 2014). In particular, interactions between consecu- tive CMEs led to a significantly more powerful event than it would have otherwise been. The event was discussed in detail by Russell et al. (2013), Baker et al. (2013), Liu et al. (2014), Riley and Love (2017), Zhu et al. (2016). Here, we provide a brief summary, focusing on the properties of the event within the context of understanding extreme events in general and potentially inferring what consequences such an event could have had, had it hit the Earth. At 02:08 UT on July 23, 2012 a fast CME erupted from solar active region 11520 on the Sun. It was located at S15◦ and W133◦, and, thus, 43◦ behind the west limb as viewed from the Earth. STEREO A, however, was 121◦ west of the Earth’s location at this time. Hence, from this spacecraft’s perspective, the eruption occurred 12◦ west of its central meridian, and, as best as in situ measurements and related numerical simulations suggest (see below), the spacecraft bore the full brunt of the ejecta and its associated disturbance (Baker et al. 2013). We do not, however, believe that the event was a single CME (Liu et al. 2014). Two prominence eruptions occurred from the same AR, separated in time by 10–15 minutes, which were subsequently seen as CMEs in COR1 observations by STEREO’s sister spacecraft (see Fig. 3). Figure 4 shows in situ measurements at STEREO A (∼ 1 AU) during the arrival of the structures associated with the July 23 complex CME. The solar wind speed downstream of the shock is as high as 2246 km s−1. Comparison with the CME speed near the Sun indicates a very weak deceleration during the transit from the Sun to STEREO A: The speed of the CME near the Sun was estimated to be 3050 km s−1 while the average transit speed was inferred to be 2150 km s−1 (Liu et al. 2014). The cause of the weak deceleration is interpreted as the preconditioning of the upstream solar wind by earlier CMEs including the July 19 event, which swept through the corona with an initial speed of 1900 km s−1, propagated through the inner heliosphere, and created a tenuous medium into which the July 23 CME propagated (Liu et al. 2014). The shock is quasi-parallel with its normal predominantly along the radial direction from the Sun (Riley et al. 2016), which supports the “preconditioning” idea. Two interplanetary CMEs (ICMEs) can be identified from the magnetic field measurements behind the shock, which may have already merged. The maximum magnetic field inside the ejecta is 109 nT, and the ejecta magnetic fields larger than 60 nT lasted for over 6 hours. The southward magnetic field component was larger than 20 nT for more than 5 hours with a maximum value of 52 nT inside ICME2. Liu et al. (2014) suggest that the extremely enhanced ejecta magnetic field was generated by the in-transit interaction between the two closely launched eruptions. Those magnetic field values would have driven a massive geomagnetic storm, if the event had hit the Earth. Baker et al. (2013), for example, estimated that the July 2012 storm would have driven Dst to under −1200 nT, far exceeding the record set by the March 1989 storm (−589 nT). It should be noted, however, that the storm size is dependent on the empirical scheme used to model Dst (Fig. 4), with one predicting a truly extreme event (−1154 nT) and the other a very severe one (−598 nT). The magnitude of the associated flare on the Sun was estimated (based on EUVI observations) to be at most X2.5 (Nitta et al. 2013), which is not particularly intense compared with the Carrington (> X10) and 2003 November (X28) flares. This is surprising given how extreme the solar wind disturbance was. The July 23 event indicates a “perfect storm” scenario for the formation of extreme events, i.e., a combination of circumstances results in an event of unusual magnitude. A recent study of the 2015 March 17 intense geomagnetic storm (minimum Dst =−223 nT) shows a similar scenario (Liu et al. 2015), although the storm is not “super” in the same sense as the 2012 July 23 event. This may suggest that the “perfect storm” scenario is not as rare as the phrase implies. 21 Page 12 of 24 P. Riley et al.

Fig. 3 Origin and initial evolution of the July 23, 2012 CME as viewed from STEREO B (a, b, c, d,ande), STEREO A (g and h) and SOHO (f and i). The top row show two prominence eruptions (E1 and E2) and resulting CMEs, while the middle and bottom rows show the CMEs’ evolution through the corona. Adapted from Baker et al. (2013), Liu et al. (2014)

What would have been the consequences to society had the July 23, 2012 CME hit the Earth? Answering this question brings us squarely into the realm of speculation. Some predictions are on ﬁrmer ground than others. The X-class ﬂare, for example, would have undoubtedly resulted in immediate radio blackouts. Similarly, it’s likely that aurora would have been observed at least as far south as Central America, and the polar cap would have been pushed down to 20◦ magnetic latitude. The CME and its associated shock would have pushed the magnetopause far further toward the Earth than any other event in the space era; perhaps as close as 1–2 RE altitude. At this distance, GEO spacecraft, which provide weather, communication, and surveillance, would have found themselves outside the Earth’s bow shock, in the solar wind. Similarly, GPS spacecraft would no longer have been located within the magnetosphere. Finally, it is possible that such a storm would completely eradi- cate the outer radiation belt (with it reforming at low L), and totally deplete the inner radiation belt. Further down, it has been estimated that the July 23 storm would have resulted Extreme Space Weather Events: From Cradle to Grave Page 13 of 24 21

Fig. 4 In situ measurements of the 2012 July 23 complex CME at STEREO A (after Liu et al. 2014). The panels show: (a) the proton density—as directly computed from the plasma moments (black) and as inferred from electrons at energies greater than 45 eV (red); (b) bulk speed; proton temperature—as directly computed from the plasma moments (black) and as inferred from the “expectation temperature” based on solar wind speed; (d–g) magnetic ﬁeld strength and components (BR, BT ,andBN ,whereBN is approximately the same as the z-component of the IMF, Bz); and (h) modeled Dst index using two different empirical schemes. The shaded regions indicate the two ICME intervals, and the vertical dashed line marks the associated shock. For further details, please see Liu et al. (2014) in only modest geomagnetically induced currents (GICs) in and low-latitude power grids (Zhang et al. 2016), although this work did not account for equatorial electrojet effects, which might substantially increase the risk for at least some power networks. More work remains to be done on the effects at mid- and high-latitudes.

2.4 The 774/775 and 993 AD Events

If the July 2012 provided a giant step forward in terms of data availability over the 1859 event, two recently discovered but nevertheless exciting events represent a commensurately 21 Page 14 of 24 P. Riley et al. large step backward. Miyake et al. (2012) described an increase in 14C concentrations from tree rings that she attributed to a cosmic ray event in AD 774/775. Later, another event was identified from AD993 (Miyake et al. 2014). The inferred flare size for these events was ∼ 1034 erg, which is some 20 times larger than the bolometric energy estimated for a Carrington-class flare (Miyake et al. 2014; Cliver and Dietrich 2013). Currently, there is no consensus on the origin of these events. Some potential candidates are: (1) the Sun (Usoskin and Kovaltsov 2012), in which case, they would represent “Super-Carrington” events; (2) a nearby supernova (Miyake et al. 2012); or (3) a gamma-ray burst (Hambaryan and Neuhäuser 2013). Thus, currently, at least some doubt exists as to whether these events are extreme solar events or not.

2.5 Kepler Superﬂares

The Kepler satellite was developed to detect exoplanets through their transit of the star around which they orbit. From 2009 through 2013 it continuously observed 160,000 stars, approximately half of which could be defined as “solar type” stars. Maehara et al. (2012) used the publicly available data (at 30 min cadence) to search for superflares on solar type stars. They identified 365 of them from 148 G-type main sequence stars. They also studied brightness variations associated with these stars, inferring that they likely resulted from the rotation of a star with a large “starspot.” Simple model calculations suggest that these starspots would have a radius of 0.16 Rstar, which is considerably larger than any spot seen on the Sun. More recently, Maehara et al. (2015) analyzed 1-min Kepler data, identifying 187 superflares on 23 solar-type stars with bolometric energy ranges from 1032 to 1036 erg. For reference, the largest solar flares recorded have a bolometric luminosity of < 1030 erg. Us- ing these results, they were able to estimate that occurrence rates of superflares with energies of 1033 or larger (i.e., ×100 solar flare) is approximately once per 500–600 years. They further estimated that the maximum energy released by these events was comparable to the amount of energy stored within the starspots. A reasonable criticism that could be leveled at these stellar extrapolations is that perhaps the underlying mechanism for solar flares and the solar-like stellar flares is different. For example, in star-disk systems the large-scale field of the star might be connected to a co-rotating disk, the interaction of which can lead to some form of disruption and the resulting superflare (Hayashi et al. 1996). Karoff et al. (2016) studied stars observed by the LAMOST telescope, 48 of which produced superflares. They argued that magnetic reconnection in the corona of these stars remained the best candidate mechanism based on the increased chromospheric emissions from these stars.

3 Societal Consequences of Extreme Space Weather Events

One of the advances—or realizations—over the last decade has been to recognize the importance of the interdependence of various components in our complex and technology reliant society. As illustrated in Fig. 5, disruptions in electric power, and in turn oil/gas supplies can have significant and cascading effect in a wide range of sectors, including, but not limited to: banking and finance, government services, and emergency response. In particular, it is worth pointing out that since 2008, when this figure was published, society now relies more heavily on other technologies, such as GPS and cell phones, suggesting that the impact may be even wider. Moreover, as society becomes evermore dependent on newer technologies, the impact will only become larger; possibly in quite unanticipated ways. Extreme Space Weather Events: From Cradle to Grave Page 15 of 24 21

Fig. 5 The interdependency of Society and the cascading effects of an extreme space weather event. Image courtesy of NRC (2008)

One of the more controversial potential consequences of an extreme geomagnetic storm concerns the widespread outage of the US power system. Kappenman (2012) projected that the worst-case scenario for a one-in-a-hundred-year storm would include: (1) 1–2 Trillion dollars of economic loss; and (2) 130 million people without electrical power for several years, based on the destruction of several hundred transformers. Figure 6 illustrates such a scenario, where the red and green circles show the size of ground induced currents (GICs) at transformers. Two areas—the Eastern Interconnect and the Pacific Northwest— are highlighted as areas of probable power system collapse, which would result in more than 2/3 of the population suffering power loss. Because of long (15 month) delivery times for replacement transformers, estimates of four years were made for full service to be re- stored. Inferring the probable consequences of an extreme space weather event, however, is fraught with uncertainty. Our experience is limited to a handful of driving events provid- ing, at best, anecdotal evidence for what might happen. The Carrington event of 1859, for example, resulted in a several reports of fires starting in telegraph offices. But how can we translate this into effects within today’s electronic society? As we have noted above, more recent storms, such as that of March 1989, provide a glimpse of how technology and the electric grid, in particular might be affected. The geomagnetic storm of October 2003 (the so-called “Halloween Storms”), for example, may have caused the destruction of five transformers and the damage of ten more in South Africa (McMorrow 2011). Nevertheless, extrapolation of these results to truly extreme events is challenging. First, current cases of transformer failure are relatively rare, making it difficult to predict how many more would be destroyed when GICs spike or due to longer-term degra- dation from overheating. Second, our ability to model and forecast the US grid remains in its infancy. Third, estimates for the replacement time for destroyed transformers may change/improve, as, more generally will happen as the grid is incrementally improved over time. 21 Page 16 of 24 P. Riley et al.

Fig. 6 Prediction of regional power grid disruptions from an extreme space weather event in the USA: ◦ 4800 nT/min geomagnetic ﬁeld disturbance at 50 geomagnetic latitude scenario. Red and green circles indicate the magnitude of GICs into and out of transformers from the ground. The areas of probable power system collapse are enclosed in thick black curves. An estimated 130 million or more people would be affected by this event. Image courtesy of J. Kappenman, Metatech Corp.

4 Forecasting Extreme Space Weather Events

4.1 Event-Based Predictions

Ultimately, one of the primary goals of any operational space weather service is to predict the properties of specific CMEs long before they reach the Earth. Ideally, this prediction should be several days to more than a week in advance to give the relevant authorities sufficient time to implement their preventative strategies. In reality, such an advance warning would require a prediction of the event prior to the appearance of any reliable CME-related signatures at the Sun. Although as we discuss below, this may be a realizable goal in a probabilistic sense, no observational signatures have yet been discovered that would allow us to predict the occurrence of a specific event from a particular active region. Thus, at least for the foreseeable future, we must focus on forecasts that begin when the first signs of a CME eruption are present. This limits our prediction window to as little as 12 hours in the most extreme cases, limiting the actions that can be taken, for example, by power grid operators. In order of importance, we would like to predict the z-component of the magnetic field, Bz,thex-component of the velocity, vx , and the plasma density, ρ. These produce the dawn- dusk electric field (Bz × vx ) and the solar wind momentum flux (ρxv2). For extreme events, in particular, knowledge even of the average value of these quantities, particularly the sign (or variation in sign) of Bz, as well as their duration, would provide vital information for estimating the probable impact of an extreme event. Extreme Space Weather Events: From Cradle to Grave Page 17 of 24 21

If as Kepler data suggest, some solar-type stars are capable of producing ﬂares with energies as high as 1036 erg, we might ask what would be the ramiﬁcations for Earth and civilization? Although considerably more speculative, we might forecast the damage and loss of all Earth-orbiting satellites, loss of power and potential long-term or permanent damage to power stations, including nuclear stations, a global blackout, loss of essentially all modern modes of communication. Aircraft would be exposed to fatal doses of radiation, and depletion of the ozone layer may even occur (NRC 2008).

4.2 Probabilistic Forecasts

Probabilistic forecasts avoid, or perhaps more accurately evade the difficult issue of predicting when a particular event is going to occur, by addressing the significantly easier question of the likelihood of an event occurring. While this provides little day-to-day guidance for space weather operations, in analogy with climatology, it can be useful for longer term policy decisions. Riley et al. (2012) first addressed the likelihood of extreme space weather events occurring using a rigorous statistical model. Assuming that distributions of some space weather datasets could be approximated by a power-law distribution, they estimated the probability of another Carrington-like event occurring over the next decade. Depending on the parameter, that probability ranged from a few percent up to 12%. Love (2012) found similar estimates, but, more importantly estimated the errors associated with those forecasts, finding them to be substantial. More recently, Love (2012), Love et al. (2015) questioned the assumption that the data could be adequately represented by a power law, instead, suggesting that a log-normal distribution was more appropriate; at least for Dst. In spite of this, they concluded that a Carrington event was likely to occur approximately 1.13 times per century, with 95% CI [0.42, 2.41], consistent with the 10% per decade estimates obtained previously. We can extend these studies in several important ways. First we can ask whether the data are distributed to one of several families (exponential, log-normal, or power-law). Second, we can remove the subjectivity associated with choosing the regions of the distribution that are fitted. Third, we can include non-parametric bootstrapping to estimate the confidence intervals (i.e., the uncertainties in the forecasts). And fourth, we can apply a likelihood test to discriminate between the various models. Figure 7 summarizes geomagnetic storms where Dst exceeded −100 nT over the last 60 years. We note that only one event approached 600 nT (the March 1989 storm) and only five events exceeded 400 nT. Additionally, it is apparent that the storms tend to cluster on a timescale of 11 years, mimicking, but trailing the sunspot cycle. Bimodel peaks in these clusters are also present, matching those observed in CME rates (Riley et al. 2006). Finally, there is an apparent tendency for the strongest storms to become stronger from 1965 through 2007; while the largest five storms around 1970 were 200–300 nT, the five most intense storms around 2005 were 350–450 nT. In contrast the most recent decade shows a paucity of events, particularly severe ones. Taken together, these observations suggest that Dst displays both periodic and secular trends during this 60 year period, which attacks a basic assumption in this approach that the data are time stationary over the period of interest. With this caveat in mind, following Riley and Love (2017) we can estimate the likelihood of an extreme space weather event (as large, or larger than the Carrington event) for a set of distributions of storm sizes. Figure 8 summarizes the procedure. The left panel shows the complementary cumulative distribution function (CCDF), which is the probability that an event as large, or larger than some critical value will occur during a unit time interval. The open circles show all of the events. One of the advantages in using the CCDF, as oppose to 21 Page 18 of 24 P. Riley et al.

Fig. 7 Magnetic storms, deﬁned by events where |Dst| exceeds 100 nT, are shown as a function of time. Individual storms are identiﬁed as contiguous intervals where |Dst| exceeded 100 nT. The data were obtained from NASA’s COHOWEB, which in turn obtained the data from World Data Center for Geomagnetism at the Data Analysis Center for Geomagnetism and Space Magnetism, Kyoto University

Fig. 8 (a) Complementary cumulative distribution function (CCDF) for the geomagnetic storms shown in Fig. 7. Bootstrap fits for the three distributions (power-law, log-normal, and exponential) are superimposed. (b) Histogram and density plots showing the probability of an event as large as, or larger than the largest event in the dataset (−589 nT) over the duration of the dataset. The density curve colors follow the convention given in the legend within panel (a) Extreme Space Weather Events: From Cradle to Grave Page 19 of 24 21 the probability of an event of size x occurring is that the data are not binned, but summed from right to left. More practically, we are also not interested in events of a particular size, but events as large or larger than a particular size, since those more severe events will also cause at least as much damage. The data seem to follow a straight line (in log-log space), at least out to 280 nT. Beyond this, with the exception of the most severe storm (589 nT), they appear to fall off more rapidly. The colored curves summarize a selection of bootstrap fits for each of the three distributions considered. We infer that the power-law profiles capture the lower severity measurements well, but over-estimate the likelihood of the most severe events. In contrast, the log-normal profiles underestimate the low-severity events but capture the trends at the highest severities. Finally, the exponential profiles both over-estimate the low-severity events and underestimate the high-severity events. Figure 8(b) summarizes the likelihood of observing an event as severe as or more severe than the most severe event observed, that is, Dst < −589 nT, during the entire span of the data (∼ 57 years). The probabilities were calculated for each bootstrap iteration and the distributions derived from the probabilities for each of them. Based on these results, our best estimate for the probability of another extreme geomagnetic event comparable to the Carrington event occurring within the next 10 years is 10.3% with 95% confidence intervals (CI) in the range [0.9, 18.7] for a power-law distribution, but only 3.0% with 95% CI [0.6, 9.0] for a log-normal distribution (see also Riley and Love 2017). Our results, however, depend on: (1) how an extreme event is defined; (2) the statistical model used to describe how the events are distributed in intensity; (3) the techniques used to infer the model parameters; and (4) the data and duration used for the analysis. Ri- ley and Love (2017) tested the assumption that the data are approximately represented by a time-stationary process, arguing that while not strictly true, it could be approximately fac- tored into one’s interpretation of the forecasts. One of the key results from these studies is that both the uncertainties and the assumptions used to derive the forecasts are at least as important as the forecasts themselves. It has been argued that, extreme events may be associated with smaller solar cycles. This stems from the observation that the 1859 and 1921 storms, arguably the largest two geomagnetic storms on record, occurred during cycles where the peak sunspot number reached only 100. While suggestive, more analysis needs to be done. In fact, based on the work by Riley and Love (2017), it is more likely that extreme events will be associated with solar active conditions.

5 How Extreme Can Extreme Events Become?

Having witnessed events with speeds of greater than 3,000 km/s near the Sun, with embedded fields exceeding 100 nT at 1 AU, and producing Dst values in excess of 850 nT, it is clear that the Sun is capable of producing events that would have catastrophic effects in the Earth’s environment. But, just how bad could the most extreme event be? Considering the Earth’s magnetosphere, there’s a physical limit to the most extreme value of Dst: −32,000 nT. Why? Because this would be enough to completely nullify the Earth’s geomagnetic field. What kind of CME could produce such a deviation? Something immea- surably larger than we have yet witnessed, and, possibly larger than anything inferred from Kepler observations. Instead, we might consider what the Sun is capable of producing. What would be the most extreme CME that could be generated by the largest active region we can imagine being supported by the Sun? Knowledge of the magnetic field, velocity, and mass of such an event could be used to estimate the maximum possible deviation in Dst. 21 Page 20 of 24 P. Riley et al.

The largest active region observed during the past 100+ years was the “great sunspot” of 1947, which had an approximate area of ∼ 7.5 × 10−3 of the visible area of the Sun. This of course does not tell us about whether or not the magnetic fields embedded within it will erupt, but it does provide a rough limit on the available energy that could be used to fuel an eruption. Shibata et al. (2013) estimated that to generate a flare with energy of 1034–1035 erg would require a massive sunspot housing a magnetic flux of ∼ 2 × 1023–1 × 1024 Mx. Using simple ideas related to our understanding of the mechanisms that drive the solar dynamo, he estimated that it would take approximately 40 years to produce such a region on the Sun. Thus, even if the Sun were physically able to support such a structure (and it is not clear that the mechanisms that actually produce the stellar superflares have analogous processes on the Sun) we would be able to anticipate the potential likelihood of such an event well in advance.

6 Conclusions and Future Directions

In this review, we have discussed the basic features of extreme space weather phenomena, in part, through the use of two case studies: The Carrington event of 1859 and the STEREO A event of July 23, 2012. While infrequent, extreme space weather phenomena occurring in the future will likely produce catastrophic effects on Earth. Although we have developed a basic understand of how they are produced, evolve, and interact with the Earth’s environment, many mysteries remain. What, for example, is the largest possible CME (in terms of mass, speed, and embedded magnetic field strength) that the Sun can produce? Can isolated extreme CMEs produce catastrophic effects, or must there be a “perfect storm” of conditions? What is the largest variation in the Earth’s horizontal magnetic field during an extreme event? How large are the GICs that can be produced? These questions all state the more general question: Just how bad can things get? From a scientific perspective, there are a number of avenues we could pursue, which would allow us to better understand, and, ultimately predict extreme space weather events. Observational-based studies of our limited set of events may identify further knowledge about what makes these events so extreme. The concept of “preconditioning” appears to be a necessary criterion. If further substantiated, this suggests that even rudimentary forecasts may be able to distinguish between “severe” and “extreme” based on whether a prior CME erupted from the same or nearby active region. Additionally, if the origin of the 774/775 and 993 events can be better constrained they, together with extreme events from solar-like stars, may provide further clues about just how extreme these events can become. Modeling- based studies will, at some point, be able to reproduce all the key features within extreme ICMEs as well as their likely effects in the Earth’s magnetosphere. Current modeling efforts can capture the hydrodynamic effects of such events (Baker et al. 2013), and recent work is beginning to explore the complex evolution of the magnetic field within the erupting CME and the role it plays in the structure’s evolution as it propagates to 1 AU (Linker et al. 2016). From a societal point of view, low-frequency but high-impact extreme events are clearly a danger. With an estimated $1–2 trillion in damages from the consequential harm caused by a widespread power grid blackout, and a projected recovery time of 4–10 years, some questions that naturally arise are: (1) How much will it cost to mitigate these damages? (2) How long will it take to implement hardening procedures? (3) How do the hazards from extreme space weather events compare with those from other natural disasters? We propose that policy makers and space weather professionals should adopt the July 23, 2012 solar storm to “war game” emergency preparedness planning for extreme space Extreme Space Weather Events: From Cradle to Grave Page 21 of 24 21 weather events. It is more likely than not that the Earth will be hit by such an event within the next 50 years, and the consequences could be absolutely devastating. To address these questions, however, requires that we assess what assets are most at risk, and under what types of extreme phenomena. What, for example, are the effects on the US power grid, and dependent interconnected systems, under the influence of severe space weather? What techniques or engineering solutions are required to keep GICs isolated from key infrastructure? These are not the types of questions that can be addressed by scientists. They require work by the power engineers and transformer experts. While our emphasis here has centered on a scientific discussion, the obvious societal impacts that an extreme space weather event may have require that we consider more prac- tical approaches for understanding and, ultimately, forecasting these events. For example, it appears that more extreme space weather events require more time to develop. If a truly extreme “superflare” event is possible, it is likely that it will take decades or more to develop (Shibata et al. 2013). If so, this suggests that we have time to prepare for it. More importantly, by studying more deeply the relationship between the dynamo and visible photosphere/corona, we may be able to better constrain the timescales for production of events of different sizes. Since the sunspot associated with the 1859 event developed rapidly during the month prior to its eruption, it appears that even for “modestly” extreme events, there would be little to no warning, since they could develop just past the west limb and have ∼ 14 days to develop. Observations near the east limb, which would afford us, at best one week’s advance warning serve as our long term forecast. But even then, even with a massive active region to observe, our models are currently not sophisticated enough to predict when (or, in some cases, if) an eruption will take place. Consider the July 23, 2012 event. There were multiple eruptions associated with that active region. The earlier July 12 event is the one that is currently the focus of several studies because it intercepted Earth (e.g., Hu et al. 2016). However, it was considerably more modest than the July 23 event that measured by STEREO A. Can we identify patterns in our complex active regions erupt, which will ultimately result in an estimate for the region’s propensity to erupt? In closing, these are but a few of the scientific challenges that must be addressed if we are to successfully protect our technologically-dependent society from the impacts of extreme space weather.

Acknowledgements The authors would like to thank the International Space Science Institute (ISSI) for their gracious hospitality while hosting our team for a workshop on space weather. PR would also like to acknowledge support from NASA’s Living with a Star program. Finally, we would like to extend our sincerest thanks to a reviewer who provided extremely detailed and constructive comments on a draft of the paper.

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