Understanding and Predicting Space Weather

18 The Industrial Physicist DECEMBER 2003/JANUARY 2004 American Institute of Physics

TECHNOLOGY by Dawn Lenz

TThe consistency on Earth of visiblesolar radiation belies the suns dynam-ic and turbulent state. Just beneath thesolar surface, or photosphere, a layer ofionized hydrogen (along with a little heli-um and traces of heavier elements) churnsand mixes to a depth of about 200,000 km,convecting heat from the 15-million-kelvincore to the 5,800-K surface. The churningcharged particles generate electromagneticfields that blossom from the suns surfacein spectacular patterns, which are observedin the tenuous, 1-million-kelvin plasma of

the solar corona. The corona forms thebase of the solar wind, the continuous,even-more-tenuous stream of charged par-ticles that flows outward from the sun intointerplanetary space. The effects of theinteraction of solar charged particles withEarths magnetic field are referred to asspace weather.

Like terrestrial weather, space weather ischaracterized by an average state of relativecalm punctuated by bursts of activity.These solar storms vary in strength and fre-quency with the 11-year solar-activity cycleand cause disruptions of various magni-tudes on Earth. During calm periods, theonly manifestation of solar weather may bethe auroras (Northern or Southern Lights),caused by the excitation of atmosphericoxygen and nitrogen by the solar winds

energetic electrons. Flares and coronalmass ejections (CMEs) are two types ofsolar eruptions that can spew vast quanti-ties of radiation and charged particles intospace, potentially causing geomagneticstorms. If a large flux of charged particlesfrom the sun intersects the Earth, it canhave serious consequences for modernsupport systems, including electrical powergrids, communications networks, andsatellite operations.

Flares and CMEs differ spatially and tem-porally. Flares are strong transient outbursts

of radiation, released near the solar surface,that extend tens or hundreds of thousandsof kilometers into the outer solar atmos-phere (Figure 1). They are highly localizedon the sun. Flares typically last for a fewminutes to a few hours, and they emit radia-tion across most of the electromagneticspectrum. Most of a flares energy is releasedas radiation in the corona, but some energycontributes to forcing electrons and ionsthrough the outer solar atmosphere and intothe interplanetary solar wind.

CMEs are slower to develop (they emergefrom the sun over the course of a few hours)and have spatial extents many times that offlares. Most of their energy is expended indriving ionized particles into interplanetaryspace rather than in radiation (Figure 2).The angular size of a CME can range from a

few degrees up to half a solar hemisphere ormore. If a flare is analogous to an interplan-etary thunderstorm, a CME initiates aninterplanetary tsunamia flood of billionsof tons of protons and electrons burstingfrom the sun that is capable of massiveinterference with any flux-sensitive appara-tus it happens to encounter.

Solar storms on EarthAs technology advances, populations

grow, and urban industrialized areas sprawl,Earth becomes more dependent upon sys-tems that are vulnerable to damage fromsolar storms, including electrical grids andthe swarm of satellites in orbit above Earthsprotective atmosphere. Todays electricalgrids are more susceptible to solar-stormdisruption than their more localized prede-cessors because of the large geographicalareas they cover and their interconnectednature (see The Industrial Physicist, Octo-ber/November 2003, pp. 813). Communi-cations systems and networks have devel-oped beyond ground-based lines tosatellite-based transmissions. Humans andtheir support systems venturing moreextensively beyond the safety of Earthsatmosphere and into orbit, to the moon, orone day to the planets are largely unshield-ed from the solar storms that Earths mag-netosphere deflects at home (Figure 3).

Satellite-based activities and operationsare also vulnerable to the direct impact of aflux of solar energetic particles. About 150satellites currently orbit Earth hundreds tothousands of kilometers above the top ofthe atmosphere for the purpose of relayingtelevision and telephone signals at veryhigh to ultrahigh frequencies (VHF/UHF).Both frequency ranges are used becausetheir short wavelengths can penetrateEarths ionosphere with minimal reflectanceand interference. However, VHF and UHFwavelengths are not short enough to affordthem complete immunity to atmosphericinteraction, and they are susceptible to dis-ruption from significant modulations in theionosphere, which can occur during solarstorms. One such storm occurred on July14, 2000, when a large flare bombarded

TRA

CE/

Stan

ford

-Loc

khee

d In

stitu

te fo

r Sp

ace

Rese

arch

Figure 1. This false-color ultraviolet image of a flare shows a 50,000-km-wide arcade ofmagnetic structures released near the solar surface.

Earth with energetic particles that disrupt-ed communications and associated supportsystems. Weather satellites returned pic-tures blurred by static, commercial fishingboats lost radio communication, and powercompanies in the northeastern UnitedStates had to reroute electricity in responseto voltage disruptions.

In addition to operational interference,satellites and power grids can suffer physi-cal damage from solar storms. Satellitesdraw power from solar cells, which consistof semiconductor materials that are sensi-tive to energetic ions. The continual flux ofsolar particles gradually degrades the effec-tiveness of solar cells, eventually cripplingthe satellites when the cells can no longergenerate the required power. Solar stormssignificantly accelerate such degeneration.A single strong solar storm can decrease thelifetime of a satellites solar-cell system byseveral years.

Electromagnetic systems are vulnerableto electromagnetic-field fluctuationsinduced by a rapid influx of charged parti-cles. Within power grids, geomagneticstorms can cause large-scale fluctuationsand outages. Perhaps the most notorioussolar-induced power outage occurred onMarch 13, 1989, in Quebec, when 6 millionpeople experienced a 9-h electrical blackoutcaused by a CME. In addition to causing aloss of power, such events can damagepower-grid hardware as abnormal-ly large currents and voltages over-load the system. Widespread poweroutages and communicationsbreakdowns can cost millions ofdollars; the 1989 Quebec outagecost an estimated $300 million.On a national scale, the economicimpact of such an event can be inthe billions of dollars.

Eruption physicsOur observational picture of

solar flares and CMEs has improveddramatically over the last decadewith the inception of state-of-the-art solar telescopes and satellite-borne instruments, such as the

Solar and Heliospheric Observatory, theTransition Region and Coronal Explorer,and, most recently, the Ramaty High EnergySolar Spectroscopic Imager. However, thedetailed underlying physical causes of solarstorms largely remain a mystery.

Theories of how flares and CMEs devel-op and erupt, the conditions in the solaratmosphere required for the generation ofsuch phenomena, and the mechanisms bywhich the energy is expelled and the parti-cle flux is propelled outward into interplan-etary space are areas of active investigationin solar physics. The foundation of almostall such theories involves the twisting andtangling of magnetic-field lines in the solaratmosphere as a result of the underlyingfluid motions in the convective layer justbeneath the solar photosphere. Accordingto the theory of magnetic reconnection,developed by Eugene Parker of the Univer-

sity of Chicago and Peter Sweetof the University of Glasgow(Scotland) in the 1950s, solarmagnetic-field lines progres-sively become more chaoticallyintertwined, increasing thestresses between them (Figure4). When the stresses become

severe enough, the field lines reconnectwith an associated release of energy.

Flares and CMEs are sometimes observedto occur together. Until recently, this obser-vation compelled researchers to look for acausal relationship between the two.Although both types of eruptions are believedto have physical roots in magnetic reconnec-tion, solar physicists generally no longerenvision a causal relationship and treat eachseparately in doing phenomenological mod-eling. Similarly, solar physicists believed fordecades that flares caused geomagneticstorms. Such a correlation seemed plausible,given the enormous energy fluxes observedin flares, and solar-terrestrial storms dosometimes appear to be correlated with solarflares. However, explaining the physical cor-relation proved to be a challenge becausethere are both temporal and spatial inconsis-tencies between flares and geomagnetic

storms. Flares typically lastfor at most a few hours andare highly localized. Stormscan last for days and covermany times the area of flares.

The key player in majorsolar-terrestrial events is nowthought to be the CME ratherthan the flare. CMEs wentunrecognized as significantsolar phenomena for manydecades after flares firstreceived close attention, inpart because CMEs produceless radiation than flares and

require more sensitive andcareful observation. Ratherthan expelling energy predom-

19 The Industrial Physicist

Figure 2. This white-lightimage shows a coronal massejection exploding from thesun and extending about10 million kilometers (thesmall white circle repre-sents the sun, which is cen-tered on the instrumentsblocking disk).

SOH

O/L

ASC

O (E

SA a

nd N

ASA

)

Figure 3. The flux of charged particles in the solar wind interactswith Earths magnetic field and can cause operational and physicaldamage to orbiting satellites.

Tom

Moo

re

Earth

Orbitingsatellite

Solarwind

Earths magneticfield interactswith solar wind

20 The Industrial Physicist

inantly in the form of radiation and localizedparticle acceleration, a CME uses its energyto propel ions and electrons into interplane-tary space.

Currently, the generally accepted modelof the largest solar-terrestrial eventsis that they are caused by the accel-eration of interplanetary chargedparticles ahead of a CME-inducedshock. The triggers of CMEs, how-ever, remain under debate as scien-tists pursue observational data totest various theories. Two compet-ing views are (1) CMEs are trig-gered by the twisting and subse-quent reconnecting ofmagnetic-flux ropes beneath thesolar surface, with the releasedenergy forcing particles out frominside the sun, and (2) CMEs, likeflares, are triggered by the releaseof magnetic energy in the corona,above the solar surface (Figure 5).

Predicting storms The peak of the last 11-year solar

cycle, with a corresponding peak inflare and CME events, was in 2000,when Earth was significantly moredependent on power grids and

satellite-based communication than duringthe previous peak. This dependency, cou-pled with new knowledge about the causesand effects of solar storms, spurred effortsto predict large geomagnetic storms in

hopes of mitigating their effects. As in meteorology, the tools of space-

weather forecasting include observationsand model predictions. Observational datainclude in situ measurements of radiationand energetic particles at satellite orbit alti-tudes, and ground-based magnetometerdata. In addition, solar-physics researchsatellites can provide data on current condi-tions at the sun. However, their instrumentscollect high-resolution data of just a fewpercent of the solar disk at a time, soonly events occurring in the field of viewfor a specific observation sequence are cap-tured. Space-weather modeling aims to takeobservational data as input and help fore-casters predict storms. This relatively new

field has grown significantlyin recent years; about 70%of the existing academic lit-erature on space-weathermodeling has been pub-lished since 2000. As inter-est in space-weather predic-tion increases, the modelscontinue to improve.

Satellites designed forspace-weather explorationinclude Wind (launched in1994), the Advanced Com-position Explorer (1997),and the Imager for Magne-topause-to-Aurora GlobalExploration (2000). Theirinstruments gather radia-tion and particulate data todiscover the characteristics

of the interaction betweenthe solar wind, solar ener-getic particles, and Earthsmagnetosphere. On Earth,networks of ground-based

Technology

Tom

Moo

re

Tom

Moo

re

Magnetic-field lines

Reconnection points

Solar surface

Flux rope

To the solar wind

Openmagnetic

field

Closed magnetic field

Convectionpatterns

Figure 4. Solar magnetic-field lines,anchored in the turbulent convective zonebeneath the surface, become tangled andbraided. The associated buildup of magneticstress triggers reconnection, in which thefield reverts to a topologically simpler statevia release of the stored energy.

Figure 5. The solar magnetic field consists of both closed-field andopen-field regions. Coronal mass ejections are thought to involvethe evolution of field between closed and open configurations. Fluxropestwisted bundles of magnetic-field linesare believed to bepart of the precursor field configuration.

magnetometers detect fluctuations in theplanets magnetic field. Solar storms com-monly induce fluctuations on the order of1% in the measured magnetic field; magne-tometers can detect fluctuations severalorders of magnitude smaller. Together, satel-lite and magnetometer data can provideaccurate, up-to-the-minute space weatherforecasting.

The first commercial space-weather pre-diction system was installed in England inJanuary 2000. SpaceCast/PowerCast, devel-oped by the Metatech Corp. (Goleta, CA),collects up-to-the-minute data from a groupof satellites and networks of ground-basedmagnetometers about the suns radiationand magnetic-activity levels. Predictive mod-eling is coupled with observational data tocreate specific regional forecasts. The systemprovides advance warning of an impendingsolar storm, permitting crucial or sensitivepower-grid components to be shut down orotherwise protected. Devices that blockanomalous currents are expensive to installon a large scale, however, so disabling essen-tial components is currently the most cost-effective way to prevent damage.

As with severe terrestrial storms, the

effects of solar storms can be mitigated withaccurate and expeditious forecasting. Theability to predict major solar storms cangive power companies sufficient lead timeto implement preventive measures. Likesandbagging and nailing boards over win-dows before a hurricane, contingencystrategies cannot disarm a major geomag-netic event, but they can significantly lessenits impact. Advance warning of storms canalso, in principle, allow communicationscompanies to notify their customers that alapse in service may be imminent and esti-mate how long the lapse might last.

Our understanding of both the causesand the terrestrial effects of space weather isa subject of active research. Industrial focuson geomagnetic storms has thus far beenmotivated by efforts to reduce their impact,but just as we have learned to capture solarradiation and wind energy for modernpower applications, we may one day learnto lasso and exploit the energy that reachesus in solar storms.

Further readingCarlowicz, M. J.; Lopez, R. E. Storms

from the Sun: The Emerging Science of Space

Weather; Joseph Henry Press: Washington,DC, 2002; 256 pp.

Golub, L.; Pasachoff, J. M. Nearest Star:The Surprising Science of Our Sun; HarvardUniversity Press: Cambridge, MA, 2001;267 pp.

Kappenman, J. G.; Zanetti, L. J.; Radasky,W. A. Geomagnetic Storms Can ThreatenElectric Power Grid. Earth in Space 1997, 9(7), 9.

Parker, E. N. Interplanetary DynamicalProcesses; Interscience Publishers: NewYork, 1963; 272 pp.

Space-weather forecasts and more infor-mation about space weather are availablefrom the Space Environment Center of theNational Oceanic and Atmospheric Admin-istration at http://www.sec.noaa.gov, andthe Canadian Space Weather Forecast Cen-tre at http://www.spaceweather.gc.ca.

21 The Industrial Physicist

Dawn Lenz, a solar physicist by training,is a consultant at Research Systems, Inc.(a Kodak company), a scientific softwareand services company in Boulder, Col-orado (dlenz@RSInc.com).

B I O G R A P H Y

Copyright of Industrial Physicist is the property of American Institute of Physics and its content may not becopied or emailed to multiple sites or posted to a listserv without the copyright holder's express writtenpermission. However, users may print, download, or email articles for individual use.