JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. A10, PAGES 20,979-20,983, OCTOBER 1, 2001

Forecasting the arrival of shock-accelerated energetic particles at Earth

C. M. S. Cohen, R. A. Mewaldt, A. C. Cummings, R. A. Leske, and E. C. Stone Califomia Instituteof Technology,Pasadena, California

P. L. Slocum and M. E. Wiedenbeck Jet PropulsionLaboratory, Califomia Instituteof Technology.Pasadena, California

E. R. Christianand T. T. von Rosenvinge NASA GoddardSpace Flight Center,Greenbelt, Maryland

Abstract. Energeticparticles accelerated at interplanetaryshocks can result in an increased radiation dose for astronauts as well as an increased risk to hardware. These shocks are known to occasionallyaccelerate protons to energiesgreater than 100 MeV and can resultin very high fluxes. Advancedwarning of the arrival of stronginterplanetary shocks can enablesteps to be takento minimize the potentialrisks. In an effort to monitor and assessthe radiationrisk to astronauts,two real-time countrate monitorswere implemented in the Solar IsotopeSpectrometer (SIS) on the ACE spacecraft.These ratesmeasure protonswith energies>10 and >30 MeV. Since ACE is locatedat the L1 Lagrangianpoint, theserates provide information and warning up to 1 hour beforean interplanetaryshock reachesEarth. Using ACE and GOES, data we have examinedexamples of shocksand associatedenergetic particles that have been detectedat ACE and were later observednear Earth in an effort to developmore fully the forecastingcapability of ACE. Becauseof the limited amountof solaractivity during 1997-1999, this currentwork is primarily a proof of concept.

1. Introduction magnetosphere[Shea et al., 1999]. The sameshocks which Recently, the Committee on Solar and Space Physics compressthe magnetospherein these geomagneticstorms (CSSP) and the Committee on Solar-Terrestrial Relations may acceleratesubstantial numbers of protons(and heavier ions) to energies greater than 100 MeV. Such local (CSTR) examined the risk of increased radiation hazards to acceleration(as well as turbulencenear the shock,which can astronautsworking on the International Space Station ([SS). confine the particlesto the shock region) can result in The results of the study were published in the report intensities at the shock which temporarily dominate an "Radiation and the International Space Station: Recommendations to Reduce Risk" and indicated that there is ongoingsolar energetic particle (SEP) event,as wasthe case duringthe October19, 1989,SEP event. Duringthe decline a significantprobability that severalconstruction flights of the of this event,the passageof the associatedshock resulted in a ISS could be impacted by penetrating particle radiation greater than fivefold increase in >100 MeV protons as events. Of primary concernto the astronautsare events with measuredby the GOES 7 spacecraft(Figure 1). Since the significant fluxes of >10 MeV/nucleon particles. During time profile of particle intensities in a typical SEP event these events, astronauts could experience a significantly decaysrelatively smoothly(especially at E > 100 MeV), such increasedradiation dose (especially if they are perforlning an increase during the decay phase is unexpected and extravehicularactivity), possiblyaffecting their future health and flight schedules.Additionally, increasedexposure could therefore would not be predictedwithout information about requiremore frequentrotation of the crews,thereby impacting the arriving shock. Forehandknowledge of such a situation spacestation schedules and costs. would allow astronautsto move to more protectedareas of the The radiation hazard can be especially high when a ISS or to rescheduleplanned extravehicular activities. suppressionof the geomagneticcutoff is coincidentwith high A large effort was made in the 1980sto understandshocks fluxes of energeticparticles [Leske et al., this issue]. The and their accelerationof particles to MeV energies. It was found that shockscan be divided into two broad categories, geomagneticcutoff is suppressedduring geomagneticstorms when a favorable orientation of the interplanetary magnetic parallel (where the angle betweenthe shocknormal and the field is combinedwith increasedsolar pressureon the magneticfield, O•, is 00-45ø) and perpendicular(where Ot• is 45ø-90ø). At parallel shocks,ions gain energythrough Fermi Copyright2001 by the AmericanGeophysical Union. acceleration,a processwhere the ions are reflectedby waves upstreamand downstream of the shock. Sincethe wave fields Papernumber 2000JA000216. are converging,the particles are being reflected by moving 0148-0227/01/2000JA000216509.00 barriers and thus gain energy. The more times an ion is

20,979 20,980 COHEN ET AL.: BRIEF REPORT

2. Instrumentation and Data Selection The Solar IsotopeSpectrometer (SIS) on ACE measures

o• energeticions from -10 to 100MeV/nucleon [Stone et al., 1998b]. A stackof large-areasilicon detectors is usedto determinethe kinetic energy,nuclear charge, and massof incomingparticles. The largegeometry factor of SIS allows 02 measurementsof low fluxes of heavy ions to be made with goodtemporal resolution. These measurements require trajectoryinformation which is obtainedfrom the top two 0 ø detectors,which are positionsensitive. While energetic ß protonstypically do not depositenough energy in these detectorsto exceed the set thresholds(which are optimized

-• ..... • I I for higher-Zparticles), the energythresholds of single 292 294 296 298 300 detectorsdeeper in the stack are low enoughthat the Doy of 1989 associatedsingle-detector rates are dominated by protons. Figure 1. GOES 7 fluxes of >10, >30, and >100 MeV protons Shortlybefore launch, provisions were made to include during the eventsof October 1989. two such rates from SIS among those telemetered continuouslyas partof theReal Time Solar Wind monitoring systemon ACE [Zwicklet al., 1998]. Thesetwo ratesare the reflected,the more energy it gains. At perpendicularshocks, countingrates of the singleT4 detectorand the coincidence betweenthe T6 and T7 detectors(Figure 2). The T4 and shock drift accelerationis the primary processinvolved. In T6oT7 ratesrespond primarily to >10 and>30 MeV protons, this case the particlesdrift accordingto v x B forces in the direction of the induced electric field at the shock and are respectively,similar to the I3 and I4 rateson the GOES accelerated.The more time the particlespends near the shock spacecratl.Increases in theseSIS rates due to a passingshock shouldbe followedby correspondingincreases in the GOES (under the influence of the electric field), the more energy it rates when the shock reaches GOES. gains. In both casesthe amount of energy a particle gains In an effort to study this correlationand develop the dependson Ot• and the speed of the shock. In shock drift accelerationthe energy gain also dependson the pitch angle capabilityfor near-real-timeutilization of the SIS ratesin a warningsystem, we have examinedthe dataobtained since of the ion. Comprehensiveoverviews of both types of shock launchin August1997 throughDecember 1999 for shock- accelerationhave been presentedby Armstronget al. [ 1985] related increases in the T4 rate. An indication that the and Scholer [ 1985]. increases are a result of local shock acceleration is that both Unfortunately, althoughthe general accelerationprocesses have been established, the details of shock acceleration of energetic particles and their subsequenttransport involve a number of interplanetaryparameters, and it is hard for particle acceleration events such as that of October 19, 1989, to be '% II ', entrance ,' II accurately forecast from observations of the coronal mass ejections(CMEs) typically driving the shocks[Feynman and o,. 75 ", ,'• M1 Gabriel 2000]. It is also difficult to predictfrom observations of eruptingCMEs whether a shockwill developthat will still be capable of acceleratingsignificant numbersof particles when it arrives at Earth. Kahler et al. [1984] found a significantcorrelation between the speedof CMEs observed with the Solwind and the associatedproton fluxes measured by the IMP 8 and ISEE 3 spacecraft. However, for a given CME speedthe observedproton fluxes 75" "M2 varied from event to event by as much as 4 orders of 100 T1 magnitude[see Kahler et al., 1984; Figure 5]. 100 T2 250 13 The Advanced CompositionExplorer (ACE) [Stone et al., 5OO T4 1998a], at the L 1 Lagrangianpoint, is far enoughupstream of 75O T5 the Earth that it can provide advancedwarnings (---1 hour) of 2650• incoming shocks. Since few shocks are strong enough to accelerate particles to high energies, it is not adequate to merely observethe shock at L1. A measureof the energetic particleslocally acceleratedby the shockwhen it passesACE 1 ooo , T8 is needed for a reasonable indication of what will occur-when I I the shock reaches Earth. The Real-Time Solar Wind i cm monitoring system [Zwickl et al., 1998], 'which is a Figure 2. Schematicof the Solar IsotopeSpectrometer (SIS; one compilation of ACE measurementstelemetered in near real of two telescopes). Two real-time rates are derived from the time, providesthe necessaryinformation for monitoringshock single T4 detector and the coincidence of the T6 and T7 events. detectors. COHEN ET AL ß BRIEF REPORT 20,981 the T4 and T6.T? rates rise togetherwith negligible energy plottedin Figure3. Shocksidentified by the MAG and dispersion.Energy dispersion is typicalof SEPevents with SWEPAM teams (C. Smith, private communication,2000) accelerationoccurring near the , sincethe higher-energy are indicatedby vertical lines. particlestravel at a greatervelocity and thus arrive first. In addition,we have examinedthe solar wind proton speed 3. Observations profilesfrom the SolarWind Electron,Proton, and Alpha Monitor(SWEPAM) level 2 data(see McComas et al. [1998] As can be seenfrom Figure 3, the time profilesof the rate for the instrumentdescription) for abruptincreases typical of increasesobserved by SIS and GOES can be remarkably a shock. An increase in the I3 rate of GOES was then' similar. Duringthe 1998 day 267 event,many of the small searchedfor, and if found, taken as an indication that the variationsin the ratesare apparentin both data sets. This is shockreached Earth and was still acceleratingparticles. an indicationof how little the propertiesof the shockhad Five timeperiods were identified in theSIS data,but solar changedduring the propagationfrom L I to the Earth's windplasma and magnetic field data were only available for magnetosphere.Although GOES is rarely outsidethe three of them. The SIS T4, GOES I3, SWEPAM proton magnetosphere,the bow shockand magnetopause have little velocityand density, and magnetic field strength profiles from effecton the shockparticles at theseenergies. the Magnetometer(MAG) (seeSmith et al. [1998] for the The timedelays between the particle increases seen at ACE instrumentdescription) for the threeselected time periods are and GOES were determinedby temporally shifting the two

102[•...... I ...... 10 2 :- •.•'•10'•- ) • 10'

102

•', 10' ½t,o' - ,- lOO I 00 I-I'IP'I I I '! 50'

50' 40 •o •o 30 20 • •20 lO 10 ooo' 000

800 800

400 400

200 2OO

30 60'

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E 15 E 20 I 5 pt•¾•' •"'• •'•"• • 0 ß ß ß • . , , • , , , ß ß ß ß 0 267.2 267.4 267.6 267.8 268.0 268.2 268.4 268.6 268.8 123.4 123.6 123.8 124.0 124.2 124.4 124.6 Doy of 1998 Doy of 1998 Figure 3. (a-c) Threeselected time periodswhere shock-accelerated >10 MeV protonswere observedfirst by SIS (first panel) and then later by GOES (secondpanel). The third throughfifth panelspresent the solarwind magneticfield magnitude,proton velocity, and densityas measuredby the MAG and SWEPAM sensorson ACE during the events. Shocksidentified by the MAG and SWEPAM teamsare indicatedby the verticallines. The list of shocksdid not include 1999 data, so none are indicatedin Figure 3c. 20,982 COHEN ET AL.: BRIEF REPORT

10 2 flux and momentumflux conservation).The time requiredfor the shock to traverse the distance between L1 and Earth at the 0,-- calculatedradial speedis given in Table 1 for time periods >• •'•o' 1998 day 267 and 1999 day 022. It was not possibleto obtain a shock speed for time period 1998 day 124 since the simplified equationsused here were not applicable to the conditionsat that shock. The given uncertaintiesindicate the 102' variation in the calculated shock normal vector from the

o differentequations given by Abraham-Shraunerand Yun. Althoughthere are only two examples,and the uncertainty >• o' is large on the calculatedtime delay for the 1999 day 022 0 m shock, it appearsthat the time delays observedbetween the ^ E SIS and GOES rates are consistent with the calculated radial U') v 0 shockspeeds. This indicatesthat shock-relatedparticle fluxes observed by SIS are a reasonable indication of what GOES 25' will measure30-60 min later and that the time delay can be ß'='- 20 determined from the shock speed as calculated from the

:• 1,5 measuredplasma parameters.

17'-"10 .•_ 4. Summary c ,5 o With the construction of the International Space Station 800 (ISS), the radiation environment to which astronauts are exposed is of particular concern. The assembly of the ISS 600 will require numerousextravehicular activities during which

400 astronauts are more susceptible to the increased radiation possiblefi'om high intensitiesof solarenergetic particles. The

200 ability to provide advancedwarning of such conditionscan greatly help to mitigate this risk. In examining time periods in which shock-relatedparticle increases were observed initially by SIS on ACE and subsequentlyby GOES, we have shown that warnings of 30- 60 min are possible. Since the vast majority of shocks passing ACE are not strong enough to accelerate energetic particlesto >10 MeV and causea significantincrease in their intensities, it is not sufficient to merely identify the

o occurrence of a shock at ACE. Measurements of energetic 22.4 22.6 22.8 2,3.0 2,3.2 2,3.4 2,3.6 particlesin real time are also required. SIS providestwo real- Doy of 1999 time count-rate monitors which respondto protons at energies Figure 3. (continued) of >10 and >30 MeV. These data are available via the Internet at http'//sec.noaa.gov/ace/SIS_3d.html and http://www. srl. c altech. edu/AC E/A SC. profiles relative to each other until distinguishingfeatures The work presentedhere is primarily a "proof of concept." were best matched by eye. These time delays are given in Additional examples are needed to evaluate the usefulness Table I along w•th uncertaintieswhich reflectthe variationof and accuracy of this technique. It is also important to the determined time delays fi'om the different featuresof the determine how many shock-accelerated increases are rate profiles. observedby SIS but not by GOES' how many increasesare Using the proton velocity componentsand densitiesfrom measuredby GOES and not identifiedwith the proposed the SWEPAM data and the magneticfield componentsfrom technique;as well as how many incidentsare forecast the MAG data, the shock speedand normal were calculated correctly. using the equations given by Abraham-Shraunerand Yun [1976]. The normal plane is definedas the planethat contains the magnetic field vectors (ahead of and behind the shock) and the vector difference of the flow velocities on both sides Table 1. ShockTime Delays'• of the shock. Equationsinvolving five differentcombinations TimePeriod SIS-GOESAT. ShockDerived AT, Year Day Time, rain min of these vectors can be used to define this plane, and a UT consistencycheck between the different expressionsprovides an indication of the reliability of the normal vector 1998 267 2330 43 = 7 38 ñ 3 1998 124 0209 36- 10 determination. The shock speed is then calculatedusing the reduced Rankine-Hugoniot conservationequations (which 1999 022 2120 66 = 5 69 z=30 %IS, SolarIsotope Spectrometer. combine Maxwell's equationswith the assumptionof mass COHEN ET AL.: BRIEF REPORT 20,983

Acknowledgments. We thank the SWEPAM and MAG teams McComas, D. J., et ai., Solar Wind Electron Proton Alpha Monitor and the ACE Science Center [or providing level 2 data for shock (SWEPAM) for the AdvancedComposition Explorer, Space Sci. analysis. We thank Adam Szabofor helpful discussionsregarding Rev., 86, 563-612, 1998. the shockcalculations. The GOES data were contributedby the U.S. Scholer, M., Diffusive acceleration, in Collisionless Shocks in the Departmentof Commerce,NOAA, SpaceEnvironment Center. This Heliosphere: Reviews of Current Research, Geophys.Monagr work was supported by NASA at the California Institute of Ser., vol. 35, editedby B. T. Tsurutaniand R. G. Stone,pp. 287- Technology (under grant NAG5-6912), the Jet Propulsion 301, AGU, Washington,D.C., 1985. Laboratory,and the GoddardSpace Flight Center. Shea, M. A., D. F. Smart, and G. Siscoe, Statistical distribution of JanetG. Luhmannthanks StephenW. Kahler and Ronald D. Zwickl geomagneticactivity levels as a functionof solarproton intensity for their assistancein evaluatingthis paper. at Earth, Proc. Con/.'. Cosmic Rays 25th. 7. 390-393, 1999. Siscoe, G. L., et al., Radiation and the h•ternational Space Station: Recommendationsto Reduce Risk, 76 pp., National Academy References Press,Washington, D.C., 2000. Smith, C. W., et al., The ACE Magnetic Fields Experiment, Space Abraham-Shrauner,B. and S. H. Yun, Interplanetaryshocks seen by Sci. Rev., 86. 613-632, 1998. Ames plasmaprobe on Pioneer6 and 7, J. Geophys.Res., 81, Stone,E. C., et al., The AdvancedComposition Explorer, SpaceSci. 2097-2102, 1976. Rev., 86. 1-22, 1998a. Armstrong,T. P, M. E. Pesses,and R. B. Decker, Shock drift Stone,E. C., et al., The Solar IsotopeSpectrometer for the Advanced acceleration,in Collisio•less Shocksin the Heliosphere: Reviews CompositionExplorer, Space Sci. Rev., 86. 357-408, 1998b. of CurrentResearch, Geopltvs Monagr Set'., vol. 35, editedby B. Zwickl, R. D., et al.. The NOAA real-time solar wind (RTSW) T. Tsurutaniand R. G. Stone, pp. 271-285, AGU, Washington, systemusing ACE data, SpaceSci. Rev.. 86. 633-648, 1998. D.C., 1985. Feynman,J. and S. B. Gabriel. On spaceweather consequences and E. R. Christian and T. T. yon Rosenvinge,Code 661.0, NASA predictions,J. Geophys.Res.. 105. 10,534-10,564,2000 GoddardSpace Flight Center,Greenbelt, MD 20771. Kahler, S. W., et al., Associationsbetween coronal mass ejections C. M. S. Cohen, A. C. Cummings, R. A. Leske, R. A. Mewaldt, and solar energeticproton events,J. Geoph?s.Res., 89, 9683- and E. C. Stone, MC 220-47, California Institute of Technology, 9693, 1984. Pasadena,CA 91125. (cohen((v•srl.caltech.edu) Leske, R. A., R. A. Mewaldt, E. C. Stone, and T.T. von Rosenvinge, P. L. Slocum, M. E. Wiedenbeck, M/S 169-327, Jet Propulsion Observationsof geomagneticcutoff variations during solar Laboratory,4800 Oak Grove Dr., Pasadena,CA 91109. energetic particle events and implications for the radiation environmentat the SpaceStation, J. Geophys.Res., this volume, (ReceivedJune 30, 2000; revisedOctober 25, 2000; 2000. acceptedNovember 17, 2000.)