JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. A10, PAGES 20,979-20,983, OCTOBER 1, 2001 Forecasting the arrival of shock-accelerated solar 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 satellite 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 wind 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 coronagraph 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 Sun, sincethe higher-energy