Solar Forecast

Solar Forecast:

How much damage and disruption might the next solar AHEAD cycle bring? On January 20, 2005, Earth was pummeled by a barrage of energetic protons from the Sun that resulted in the most intense ground-level radiation in a half century. Eight months later, our star belched several powerful ﬂ ares, one of them the fourth BY C. RENÉE JAMES largest in the past 20 years. Had its associated torrent of charged particles been directed earthward, we would have likely expe- rienced widespread power outages, disruptions in communica- tions, and, for any astronauts who happened to be out on a spacewalk, enough exposure to cause radiation sickness. All this during the Sun’s “quiet time.” For 250 years observers have kept counts of spots on the Sun, tallies that allow us to track how much commotion is taking place on our star’s surface as it waxes and wanes throughout a roughly 11-year-long cycle of activity. Far from being curious blemishes, sunspots are the most obvious clue of the Sun’s state. More sunspots mean more active cycles. The Sun is never really completely quiescent, but during each cycle’s minimum the solar disk can be blank for long stretches (as was the case earlier this year). Right now we are witnessing the dying gasps of the 23rd recorded solar cycle, one that even at its maximum was less active than the previous two cycles. Cycle 24, which should begin later this year, might be far more impressive, perhaps 50% stronger than the one just ending. On the other hand, it might be 40% weaker. The problem is that solar scientists are getting mixed signals from our star.

NASA / TRACE TEAM NASA / TRACE Playing the Odds TIP OF THE ICEBERG This active region on More than a bet is riding on the strength of the upcoming activity the Sun, recorded in extreme-ultraviolet light cycle. Despite the Sun’s seemingly remote distance of 150 million by the TRACE spacecraft, triggered a solar kilometers (93 million miles), what happens on its face is con- ﬂ are on January 20, 2005, that one scientist nected intimately with what happens here and throughout the called “in many ways one of the most spec- solar system. Activities that we take for granted — uninterrupted tacular of the Space Age.” Its rapid onset electrical power, polar-routed airline ﬂ ights, satellite-based com- and the torrent of high-energy protons it munications, and GPS navigation, to name just a few — can be unleashed have underscored Earth’s vulner- profoundly affected by inclement space weather. And if the fore- ability to solar outbursts. cast for a more severe cycle holds true, we’ll face a higher prob-

24 July 2007 Sky & Telescope ©2007 New Track Media LLC. All rights reserved. IC OBSERVATORY IC PHOTO-ILLUSTRATION: PATRICIA GILLIS-COPPOLA; AURORA: MICHAEL KLENSCH / WWW.AGLOWPHOTO.COM; SUN COMPOSITE: SOLAR AND HELIOSPHER SUN COMPOSITE: KLENSCH / WWW.AGLOWPHOTO.COM; MICHAEL AURORA: GILLIS-COPPOLA; PATRICIA PHOTO-ILLUSTRATION: S&T

©2007 New Track Media LLC. All rights reserved. Sky & Telescope July 2007 25 p solar forecast TROUBLE BREWING When a Jupiter-size Sun (masked) Sun (masked) sunspot group rolled into view in October 2003, scientists knew something big might happen. And it did: on October 28th, this region spawned the fourth most intense X-ray outburst ever recorded — with Earth directly in the line of ﬁ re. Visible Disk X-ray Flares Coronal Mass Ejections DODGED BULLET Six days later, the same active region exploded with an even more powerful ﬂ are, the strongest on record. Fortunately, its pulse of radiation — and the fast-moving coronal mass ejection (CME) that it spawned — struck Earth with

NASA / SOHO CONSORTIUM (8) NASA / SOHO CONSORTIUM only a glancing blow.

ability of being hit by powerful outbursts like those in 2005. Sun is not completely clear. Many scientists assert that solar These are the sorts of events that scientists really wish fl ares provide their acceleration, while others point to the they could predict. Given the potential impact on our shock waves associated with coronal mass ejections. health and societal infrastructure, the more time we have Regardless, all those X-rays and SEPs take very little to prepare for them, the better. Unlike many terrestrial- time to reach Earth. “By the time you see them happening weather disasters, however, individual space-weather there,” comments Thomas Bogdan, director of the SEC, events are currently unpredictable. “they’re happening here.” Scientists can only guess, not “It’s fair to say that space-weather forecasting has only predict, when they’ll occur, by tracking the confi gurations just reached the level that terrestrial-weather forecasting of solar magnetic fi elds most likely to result in fl ares or SEP was at in the late 1950s,” comments Douglas Biesecker of events. Frighteningly, this kind of probabilistic risk analysis NOAA’s Space Environment Center in Boulder, Colorado. would not have deterred astronauts from venturing outside Scientists currently wield predictive power only for the safety of the International Space Station on January 20, the slowest of space-weather events, coronal mass ejections. 2005. Fortunately, they stayed in that day. CMEs are great bubbles of plasma containing billions of tons of charged particles that the Sun sporadically spews Like a Hurricane into space. They can reach Earth in as little as 18 hours, So what exactly is at stake when the Sun gets agitated? The sometimes “attacking” Earth’s magnetosphere with enough most obvious and widespread problems arise from the mas- ferocity to cause severe geomagnetic storms. sive CMEs. These can create geomagnetic storms, which By contrast, fl ares in the solar atmosphere can release in- are every bit as powerful as their name implies. According tense bursts of ultraviolet and X-ray energy with speed-of- to Biesecker, “Geomagnetic storms are to space weather light consequences. X-class fl ares, the most extreme cases, what hurricanes are to terrestrial weather. Pretty much can cause power-grid failures at high latitudes, communica- everything that could be affected is.” tion blackouts, satellite failures, and signifi cant radiation The good news is that CMEs — like hurricanes — give us enhancements throughout near-Earth space. This energy plenty of warning before plowing into us. The bad news is can also energize our atmosphere, causing it to expand and that CMEs’ charged particles come in dense swarms, and so create more drag on satellites in low-altitude orbits. the list of “everything that could be affected” is quite long. Also dangerous are abrupt releases of solar energetic particles For example, the frenzied electromagnetic activity dur- (SEPs), which travel at nearly the speed of light and arrive at ing a severe geomagnetic storm can induce strong electrical Earth in just minutes. What accelerates them away from the currents in long pipelines and cables (as illustrated on page 29). Currents coursing through oil pipelines can prema- EBB AND FLOW Since the late 1700s astronomers have used counts turely corrode the metal and ultimately cause leaks that are of sunspots (below) to gauge the Sun’s state during its 11-year activity costly to locate and repair. For terrestrial power supplies, cycle. The upcoming interval, Cycle 24, may be either considerably more this added rapidly varying power can destroy transformers 300 or less active than usual (see page 30), depending on which of two and create widespread blackouts at higher latitudes. leading models proves correct. But fast-moving charged particles aren’t the cause for 200

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0 17701780 1790 1800 1810SOURCE: D. HATHAWAY / NASA 1820

26 July 2007 Sky & Telescope ©2007 New Track Media LLC. All rights reserved. Sun-Earth Science Goes Global Fifty years ago, the world’s Earth works. A global scien- off on February 19th during According to executive scientists combined forces tifi c eff ort is once again being ceremonies in Vienna, Austria. director Joseph Davila (NASA/ during the International Geo- mustered for the International Sponsored by the United GSFC), the IHY is trying to physical Year, a campaign to Heliophysical Year (www Nations, the 24-month-long forge lasting partnerships learn more about how Planet .ihy2007.org), which kicked IHY will explore how the Sun among investigators in many infl uences the electromag- disciplines and in geographic netic environments around locales, such as Africa, where Earth and other planets. One few studies have been done major thrust will be to set up before. “We want the science magnetometers and other in- to continue after IHY has struments in as many diff erent ended,” he notes. countries as possible. — J. Kelly Beatty

concern. The ultraviolet light and X-rays associated with an intense fl are can ionize our upper atmosphere, sometimes crippling radio communication for hours. Since a burst of light is our fi rst clue that a fl are has occurred, we have ab- solutely no way to prepare for these radio blackouts. Another concern is the satellite-based Global Positioning System, which is usually accurate to within meters but can be rendered unable to pinpoint positions to within 50 meters following a strong solar fl are. The reason is that fl ares generate prodigious radio energy in addition to their X-ray and ultraviolet outbursts. Once these radio waves reach Earth, they act as noise that can degrade the signals trans- mitted by GPS and other navigational systems. Only recently have researchers realized how susceptible GPS receivers are to stormy space weather. A solar fl are last December 6th created an outburst of radio noise 10 times stronger than any previously known. “At its peak,” notes Dale Gary, a physicist at the New Jersey Institute of Tech- nology, “the burst produced 20,000 times more radio emis- sion than the entire rest of the Sun. This was enough to

swamp GPS receivers over the entire sunlit side of Earth.” KLENSCH / WWW.AGLOWPHOTO.COM MICHAEL DANGEROUS BEAUTY Whenever a surge of auroras dance in the sky, Russian Roulette Earth is under attack from an electromagnetic blitz launched by the Sun. Possibly the most alarming byproducts of our active Sun, though, are the solar energetic particles. The problem with mrem — comes from extraterrestrial sources. SEPs is their energy. Consisting of bare atomic nuclei accel- However, with higher altitudes comes greater radiation erated to relativistic speeds, the most potent ones can easily exposure. A passenger on a polar-crossing airliner can ex- penetrate and disable the delicate electronics in satellites. perience the equivalent of a chest X-ray — about 10 mrem SEPs can also have a profound biological impact. A large — during an SEP event like the one in January 2005. For enough dose of them will damage tissues and organs, caus- most people this level of exposure is not a problem. But it ing radiation sickness or even death. can pose a serious threat to a developing fetus, and it can Various natural and artifi cial sources expose us to low certainly add up for frequent fl yers and fl ight crews. levels of radiation every day, with a total dose of about 300 More disturbingly, with NASA planning to return astro- millirems per year. Thanks to the shielding provided by nauts to the Moon and, ultimately, send them to Mars, failing our atmosphere, only a fraction of that — about 45 to 50 to predict an SEP or X-ray fl are event could prove lethal to

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Earth radii Earth radii

100,000 km J. LUHMANN (UC BERKELEY) & M. WILTBERGER (NCAR) (UC BERKELEY) & M. WILTBERGER J. LUHMANN SQUEEZE PLAY When a high-density blast of solar wind reaches Earth, some of its plasma leaks into our planet’s magnetosphere (dark areas) and causes it to become distorted and compressed. Magnetic reconnections in the tail cause ﬁ eld lines to snap like stretched rubber bands that are sud- denly released. This propels high-energy electrons into the ionosphere, causing auroras and (in extreme cases) electromagnetic disruptions.

crewmembers caught outside their vehicles. To put the risk very little time to recognize the danger and shield against it, in perspective, fi refi ghters responding to the 1986 Chernobyl astronauts are playing Russian roulette with the Sun. disaster received 70,000 to 1,340,000 mrem of radiation. Of Should future crews undertake many-months-long trips the 134 fi refi ghters involved, 28 later died from this exposure. to Mars, they’ll face yet another hazard: galactic cosmic rays By comparison, a major SEP event could expose an astronaut (GCRs). These byproducts of distant supernovae carry far to a potentially fatal dose of 600,000 mrem. more energy than SEPs and, consequently, have a greater “There were events during the Apollo era that would capacity to damage tissues as they rip through the body. have killed an unshielded astronaut,” states John Bieber Here on Earth, cosmic rays are effectively stopped by our (University of Delaware). Although some scientists don’t magnetic fi eld and atmosphere. But Mars has no such mag- believe an SEP event could have lethal effects, everyone netic cocoon, and its thin atmosphere is no match for these agrees that the most potent outbursts can induce acute ra- energetic intruders. Moreover, GCRs rain down constantly diation sickness. “Fortunately,” Bieber continues, “none of from all directions. Consequently, an astronaut visiting Mars these extreme events actually occurred during a mission.” would absorb 6,000 to 20,000 mrem per year. Months on Fortunate, indeed, because right now we have no way to the Martian surface, along with the roundtrip spacefl ight, predict exactly when the Sun has us in its crosshairs. With would likely top out the crewmembers’ acceptable lifetime radiation dosage and bring their careers to an end. NASA had planned to study the solar and galactic radia- 1. Reconnection of fi eld 2. Solar-wind particles lines creates opening on enter magnetosphere tion doses at the Red Planet using an instrument called Earth’s magnetosphere MARIE (Mars Radiation Environment Experiment), car- Magnetopause ried aboard the 2001 Mars Odyssey orbiter. Ironically, the instrument was itself put out of commission by radiation from a strong solar storm in October 2003.

Solar The Sun’s Long-term Forecast: Stormy . . . or Maybe Fair wind Earthr Getting safely to the Moon or Mars and back is going to Magnetotail require more than a functional spacecraft — we’ll also Bow shock need a much deeper understanding of the Sun. “We have only one more solar cycle to learn what we must learn,” 3. Reconnection of ﬁ eld lines warns Ron Turner, lead physicist at the public service causes particle acceleration research institute ANSER (Advancing National Strategies GREGG DINDERMAN GREGG DINDERMAN toward Earth

S&T: and Enabling Results). TWO-PRONGED ASSAULT Earth is most vulnerable to a strong gust of The problem with forecasting solar activity is that, with 1 solar wind when the entrained solar magnetic-ﬁ eld lines point south- the exception of 2 /2 centuries of sunspot counts, we have ward. Those connect with ﬁ eld lines in our magnetosphere, dragging only 50 years of data about the physical conditions on them downstream and allowing solar-wind plasma to leak into the mag- the Sun’s face. Traditionally, astronomers predicted the 300 netosphere’s tail. This extra plasma leads to further reconnection within strength of future cycles solely from looking at the behav- the tail, which accelerates the plasma toward Earth at high speed. ior of previous cycles. This method seemed to work rela- 200

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28 July 2007 Sky & Telescope ©2007 New Track Media LLC. All rights reserved. tively well, statistically speaking. Strong cycles with large numbers of sunspots generally followed short cycles, and high solar maxima generally followed high minima. Substorm current wedge But Cycle 23 — which was predicted to be strong but R1 R1 turned out rather average — threw everyone. Clearly, a better understanding of the physical mechanism behind the cycles is necessary. Yet we have only four solar cycles of magnetic-fi eld measurements to work with. Solar physicists R2 R2 have been hard-pressed to come up a reliable date for when a cycle should begin, let alone how strong it will be. Mixing statistical extrapolations with magnetic-fi eld data from the last four solar cycles, a team led by Leif Svalgaard of ETK (a Houston-based consulting fi rm) reported in 2005 that the strength of the Sun’s polar magnetic fi eld during a cycle’s decline is directly related to the strength of the next A urora rojjet cycle’s peak. And because the Sun’s polar fi eld during the all ellectr waning of Cycle 23 was incredibly weak, Svalgaard’s team predicts that Cycle 24 should be the weakest in a century. “We are not sure that the polar fi elds really are what do the trick,” cautions Svalgaard. “This is supposition only, but a very plausible one.” Further complicating the matter is that the polar fi elds are extremely weak by nature and,

consequently, diffi cult to measure accurately. RESEARCH INST. / SOUTHWEST & E.SOURCE: G. CROWLEY MAYFIELD The predictive method used by Mausumi Dikpati, Peter Gilman, and Giuliana de Toma (National Center for Atmo- Auroral electrojet spheric Research) is quite different. Rather than look at (millions of amps) historical trends, this group has explored how the solar magnetic fi eld moves poleward, as evidenced by sunspots and by the magnetic fi ngerprints left behind after they’ve faded from view. Magnetic field from electrojet induces voltage potential on Because the Sun’s equator rotates more quickly than do surface of Earth its poles, the equatorial magnetic fi eld shears and migrates poleward. Once near the poles, the fi eld information is swept beneath the surface in deep-reaching convection cells that sink some 200,000 km (125,000 miles) before turning Large currents can be induced to flow through back toward the equator. In essence, there’s a giant conveyor Geomagnetically induced belt of plasma beneath each hemisphere. This subsurface highly conductive current (GIC) enters power seawater fl ow has been verifi ed with helioseismology, which tracks system through ground connection sound waves reverberating inside the Sun much as ultra- Coastal areas cause sound allows doctors to see inside a patient. + Voltage gradient – abrupt transition in Dikpati’s team believes its three-dimensional model conductivity between Electric potential induced on Earth’s seawater and highly surface of up to 6 volts per kilometer resistive rock. Magnetic-field lines Northern & ELECTRIC POWER SOURCE: JOHN KAPPENMAN / MINNESOTA conveyor DEEP MEMORY New research sug- POWER PLAY Electrons in Earth’s magnetosphere cascade along magnetic belt Sun gests that gigantic “conveyor belts” fi eld lines onto the polar regions (upper panel), creating powerful two-way of plasma circulate between the currents, labeled R1 and R2. The circuit is completed by an electrojet cur- Sun’s equator and its poles. The rent (yellow) that courses along the auroral ring at an altitude of roughly Core belts carry vestiges of past sunspot 100 kilometers. Under quiet solar conditions the electrojet is confi ned to levels that can be used to predict polar latitudes. But during geomagnetic storms an additional wedge of

N A the strength of future solar activity. current intensiﬁ es the electrojet and moves it toward the equator. The elec- S A / G S trojet’s current (lower panel) induces powerful magnetic ﬁ elds that, in turn, FC Southern conveyor belt induce a strong pulse of direct current in long conductors like power lines.

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reveals what is at the heart of the solar cycle. A key im- Solar Sentinels plication is that the Sun retains a memory not just of the previous cycle’s activity but also that of the previous three Some of our most valuable information about the Sun comes from cycles. Remarkably, this method has simulated with 98% a small armada of spacecraft that observe it around the clock. Here accuracy the sunspot counts of the previous eight cycles. are some of the missions that have contributed to our understand- And it predicts an impressively strong Cycle 24. ing of space weather and some of the missions that are on the way.

Spacecraft Agency Function Wanted: A Few Hours’ Notice Ultimately, it falls to a 12-person task force called the Solar SOHO ESA/NASA Solar and Heliospheric Observatory Cycle 24 Prediction Panel to decide which prediction is Located at L1 Lagrangian point 1.5 most likely to be correct. Their recommendation will be- million km from Earth. Has provided uninterrupted view of the Sun at come the basis of forecasts issued by NOAA’s Space Envi- visible, extreme-ultraviolet, and X-ray ronment Center, which in turn will be utilized by private wavelengths since 1995. Has also and federal entities to determine both day-to-day and long- discovered 1,000+ sungrazing comets. term operations affected by solar activity. As of late April, GOES/POES NASA/NOAA Geostationary/Polar Operational the panel was still split on how Cycle 24 should play out. Environmental Satellites Meanwhile, teams of scientists at the SEC will be moni- In addition to monitoring terrestrial toring current space-weather conditions using a growing weather, these are equipped with solar armada of space-based observatories. During Cycle 23, the X-ray imagers, magnetometers, and SOHO, ACE, and GOES/POES missions (described at right) energetic-particle sensors that alert proved invaluable. But with the recent launch of NASA’s NOAA to space-weather conditions. twin STEREO spacecraft and Japan’s Hinode, and with ACE NASA Advanced Composition Explorer the Solar Dynamics Laboratory awaiting its ride into orbit, Also at the L1 point. Measures solar solar scientists stand to gain a much deeper understanding composition from elements and isotopes in the solar wind. Also gives of the mechanisms driving space weather. The coordinated forecasters about an hour’s warning of global effort taking place during the International Helio- impending geomagnetic activity physical Year (see page 27) should be a boon as well. due to solar outbursts. So what do space-weather forecasters hope to gain from RHESSI NASA Reuven Ramaty High Energy Solar all this new science? Right now their desires are modest. Spectroscopic Imager “It would be extremely valuable to be able to forecast a Launched in February 2002, RHESSI solar storm 6 to 12 hours in advance,” states John Bieber uses an X- and gamma-ray telescope (University of Delaware). Even a few hours’ warning would to study the basic physics of particle allow spacecraft operators to take protective action. Power acceleration and energy release in utilities could ready crews for trouble on the ground. And solar fl ares. would-be spacewalkers could be confi ned to their quarters. Hinode JAXA/NASA “Sunrise” in Japanese With so much riding on the seemingly capricious whims Launched September 2006. Equipped of the solar surface, one would think that the public would with X-ray, extreme-ultraviolet, and be clamoring for a better understanding of our nearest star. visible-light detectors to study intera- tions between Sun’s magnetic fi eld and “If anything there’s a frustration that our message isn’t corona. Hinode’s Solar Optical Tele- getting out,” Bogdan admits. Worse, it seems that the US scope has 0.2-arcsecond resolution. Congress is turning something of a blind eye to the Space Environment Center’s needs (as explained on page 102). STEREO NASA Solar Terrestrial Relations Observatory Launched October 2006. Twin space- Perhaps it will take another destructive solar outburst like craft — one leading Earth, one trailing a 1989 event that plunged much of Québec into darkness to — provide a 3-D view of the Sun to underscore the necessity of accurate space-weather forecasts. determine causes for coronal mass If only we knew when that was going to take place. † ejections and the acceleration mechanism(s) for solar energetic particles. C. Renée James is an associate professor of physics at Sam Hous- SDO NASA Solar Dynamics Observatory ton State University in Huntsville, Texas. She’s been interested in Due to be launched into a geosynchro- space weather ever since an unusally vivid aurora appeared over nous orbit in 2008. SDO will study how 300 Houston in 1989, which she missed because of complete ignorance. solar magnetic fi elds generate and evolve, how their magnetic energy is transferred to CMEs and SEPs, and 200 how the corona gets heated.

Dikpati Solar Sentinel NASA Four probes to be launched in 2015. prediction From four vantage points (one on the 100 Svalgaard far side of the Sun) they will study how Sunspot number prediction and when CMEs, ﬂ ares, and other pro- Cycle 23 Cycle 24 cesses accelerate particles, and will 0 study both the time and space distri- 2000 2010 2020 bution and evolution of these events. GREGG DINDERMAN S&T: