Cliver, E W

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29th International Cosmic Ray Conference Pune (2005) 1, 121124The Solar Energetic Particle Event of 16 August 2001:   400 MeVProtons Following an Eruption at   W180E.W. Cliver, B.J. Thompson, G.R. Lawrence, A.N. Zhukov, A.J. Tylka, W.F. Dietrich,D.V. Reames, M.J. Reiner , R.J. MacDowall, A.G. Kosovichev 	 and A.G. Ling(a) Air Force Research Laboratory, AFRL/VSBXS, 29 Randolph Rd, Hanscom AFB, MA 01731-3010 USA(b) NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA(c) Royal Observatory of Belgium, Avenue Circulaire 3, B-1180, Brussels, Belgium(d) E.O. Hulburt Center for Space Research, Naval Research Laboratory, Code 7652, Washington, DC 20375 USA(e) Consultant, Code 7652, NRL, 13 South Parkway, Prospect Heights, IL 60070 USA(f) Center for Solar Physics and Space Weather, The Catholic University of America, Washington, DC 20064 USA(g) Stanford University, HEPL A204, 445 Via Palou, Stanford, CA 94305-4085 USA(h) Radex Inc, 131 Hartwell Ave, Lexington, MA 02421 USAPresenter: E.W. Cliver (edward.cliver@hanscom.af.mil), usa-cliver-E-abs2-sh13-oralA fast (  1500 km s  ) full-halo coronal mass ejection (CME) was observed late on 15 August 2001 by thewhite-light coronagraph on SOHO, and was followed within an hour by an intense 400 MeV proton event at1 AU. The absence of significant front side activity at the inferred launch time of the CME indicates a sourceregion on the far side of the Sun. The most promising active region to produce a major CME on 15-16 Augustwas AR 9557, located near the far side disk center. Supporting evidence for this extreme back side CME originis provided by helioseismology analysis of Michelson Doppler Imager (MDI) data and from analysis of thetrajectories of low-frequency (  1 MHz) radio bursts observed by experiments on Ulysses and Wind. Extremeultra-violet Imaging Telescope (EIT) images reveal the onset of activity at the west limb of the Sun, suggestingthe arrival of a wave front near the time that the first solar energetic particles (SEPs) to arrive at Earth wereinjected into space. We interpret these observations in terms of widespread acceleration of SEPs at a coronalshock wave that is manifested by a Wind/WAVES decametric/hectometric (DH) type II radio burst.1. IntroductionPrompt SEP events that originate in solar eruptions well removed from the nominal  W60 footpoint of theinterplanetary magnetic field (IMF) spiral to Earth provide an opportunity to probe the means by which so-lar particles become rapidly distributed in solar longitude. Generally such “fast propagation” of particles isattributed to a coronal shock [1], but recently Cane and Erickson [2] have suggested cross-field transport ofparticles as an alternative. In this paper we study the  400 MeV proton event of 16 August 2001 that appearsto have originated in an eruptive flare located at  W180, i.e., on the exact opposite side of the Sun from Earth.Previous examples of prompt high-energy SEP events from behind the limb sources, including ground levelevents (indicating  500 MeV protons), have been reported by several authors (e.g., 28 January 1967,  W155,[3]; 1 September 1971,  W120, [4]; 16 February 1984,  W130, [5]). In those cases, the association waslargely made on the basis of flare histories (while on the visible disk) of candidate back side active regions. Inthe present study, we use such evidence to support a back side association for the 16 August 2001 event but alsoemploy the modern tools of helioseismology and low-frequency radio tracing that allow us to look through andbehind the Sun, respectively. We then investigate the nature of acceleration/propagation in this unusual eventusing: (1) SEP anisotropy, composition, and charge state measurements from Wind, ACE, and SAMPEX; and(2) a comparison of SEP injection timing with wave effects observed by EIT on SOHO and with radio burstsobserved with the low-frequency radio experiments on Wind and Ulysses.122 E.W. Cliver et al.Figure 1. (a) SOHO/LASCO C3 back side CME on 16 August 2001, 0142 UT. (b) Associated GOES SEP event.2. Event OverviewA CME with speed  1500 km s  was first observed off the southwest limb of the Sun at 2354 UT on 15August at a height of 3.38 R  from Sun center. By 0142 UT on 16 August, the CME had formed a symmetric(in the east-west direction) halo about the Sun (Figure 1a). A height-time plot assuming constant deceleration(-32 m s  ) indicates a disk center launch time of 2335 UT. There was no significant (  C1) GOES 1-8 A˚activity near this time, suggesting a back side CME source. The GOES and IMP-8 spacecraft recorded aprompt proton event extending beyond  400 MeV (Figure 1b).3. Location of Parent EruptionEIT images of the visible disk near the inferred 2335 UT CME launch time reveal no candidate CME source.Based on flare productivity, magnetic evolution, and latitude, AR 9557 (Carrington longitude  290; S20) wasthe most likely back side region to have produced the CME. When AR 9557 returned to the visible disk on20 August as AR 9591 (  295; S20), it was the only region associated with M-class flares from 20-25 August.Extrapolating the positions of the last visible disk M-class flare from 9557 (on 7 August) and the first such flarefrom 9591 (24 August), yields a range of longitudes from  W180-W195 for the region on 15-16 August.The east-west symmetry of the halo CME observed by the SOHO coronagraph (Figure 1) is consistent with aCME having its source in the southern solar hemisphere near back side central meridian. Similar symmetryis displayed in an EIT image at 0025 UT showing dimming around the entire limb, most pronounced in thesouth. For both coronagraph and EIT images, the first manifestations of this event were in the southwest andthe east-west symmetry only became apparent in later frames.Helioseismology allows us to construct images of the back side of the Sun on 15-16 August from SOHOMDI observations of the visible disk. In these images, AR 9557, located between  W180-200, is the largestinvisible-disk region and exhibits flux emergence near the time of the halo CME.Low frequency radio emission in this event was observed by both Wind and Ulysses. Spectra of the bursts asobserved at Wind are shown in Figure 2(a). The absence of significant high frequency (  1 MHz) emission inthe WAVES detector at the time of the inferred CME launch at 2335 UT is consistent with an occulted flareThe Solar Energetic Particle ... 123Figure 2. (a) Wind/WAVES radio data showing a DH type II burst and associated type III emission on 15-16 August 2001.(b) Trajectory of the centroid of type III emission from 0000-0040 UT on 16 August.(impulsive phase) emission source. Wind is in Earth orbit while Ulysses was located  1.65 AU above theecliptic plane at a heliocentric latitude of  64  and an angle  34  west of the Earth-Sun line. The Wind andUlysses data can be used to triangulate the position of the centroid (below 1 MHz) of the strong radio burstfrom 0000-0040 UT as it drifts to lower frequencies. The Parker spiral fit (dot-dashed curves) to the radiosource locations (stars) derived for this burst suggests a solar origin of  W150 (Figure 2(b)). This magneticspiral, corresponding to a relatively low solar wind speed of  250 km s  , might result from an apparent(westward) shift of the radio source centroids at lower frequencies due to proximity effects as the large radiosource nears the spacecraft. Much, if not all, of the low frequency radio emission during this period appears tobe emanating from the type II shock identified in Figure 2(a).4. SEP Acceleration and TransportAt the onset of the SEP event, angular distributions from Wind/EPACT show that  3 MeV/nuc H, He, and Oions have a “front-to-back” intensity ratio of  6. The preferred direction of arrival was outward from the Sun,centered on the field line to Earth. The event-integrated Fe/O (based on measurements from Wind and ACE),increases at the lowest energies until it reaches the coronal value at  1 MeV, above which energy it remainsrelatively constant. Measurements from the MAST experiment on SAMPEX give an oxygen charge state  6,indicating a source in a 1-2 MK coronal plasma [6].We obtained three estimates for the time of SEP injection: (1) For a nominal 1.2 AU path length, the observedrelativistic electron onsets imply a departure time from the Sun of 0011 UT  3 min; (2) an onset-dispersionanalysis (onset time vs. inverse beta plot) using measurements of only the most energetic electrons and protonsyields a solar release time of 0005 UT  4 min (path length = 1.8  0.3 AU); and (3) an onset dispersionanalysis that includes alphas and slower protons gives an injection time at the Sun of 0000 UT  3 min (2.1 0.1 AU). To accommodate the uncertainty, we take the range of injection times to be two sigma beyond124 E.W. Cliver et al.the extremities obtained by the three methods, i.e., from 2354-0017 UT. The corresponding range of times forcomparison with electromagnetic emissions observed at 1 AU is 0002-0025 UT.What was happening on the Sun at the time that the first arriving particles were injected onto the field lineconnected to Earth? The first indication of activity at the limb due to the back side eruption can be seen in anEIT difference image at 2348 UT where dimming occurs in the southwest. By 0000 UT, the dimming is visibleat the west limb and the dimming in the southwest has intensified. By 0025 UT, the dimming has becomeprominent around the entire Sun. The dimming above the limb between 2348-0000 UT implies that an EITwave had propagated outward from the inferred CME launch site, with an average speed of  400-800 km s  .The WAVES radio data in Figure 2 show a fundamental component of a DH type II burst starting at  0010UT at  5 MHz. Extrapolating the frequency-time drift of the type II yields an initiation time  2330 UT. Thetype II onset at  0010 UT may be due to deoccultation or it may result from decreasing Alfven speed in thecorona (e.g., [7]). The radio emission arising at lower frequencies than the type II burst during the inferred0002-0025 UT injection time is type III emission caused by electrons propagating on open field lines. Becauseof their temporal association with the shock, these radio bursts would be referred to as SA (shock-accelerated)events. Given: (1) the onset of particle injection near the onset of the DH type II (and the type III emissionwhich appears to emanate from the slow-drift burst); (2) the continuation of the DH shock through the peak ofthe high-energy SEP event at  0230 UT; and (3) the essentially coronal (rather than flare) values of both theFe/O ratio and O charge state of the SEP event, it is natural to attribute the energetic protons to the shock.Based on the above observations, we propose the following picture for this extreme propagation event: Asthe CME propagates outward it drives a disturbance/shock (manifested by the EIT dimming at the limb) thatencompasses the invisible hemisphere and wraps around to the front side of the Sun where it intercepts theEarth IMF spiral sometime between 0002-0025 UT, giving shock-accelerated SEPs access to Earth.5. DiscussionWe have presented evidence that: (1)  400 MeV protons were injected onto the spiral field line connected toEarth within  40 minutes of a solar eruption at  W180; and (2) that the energetic protons observed near Earthwere accelerated and injected onto the IMF line to Earth by a coronal shock wave.Our near disk center back side location for the 15-16 August 2001 SEP event is supported by associations ofprompt energetic SEP events with front side eastern hemisphere eruptions, e.g., 7 October 1981 (E83), thatare located at longitudinal distances from the footpoint to Earth’s IMF spiral comparable to that observedin this case. Other cases of injection of high-energy SEPs in conjunction with large-scale propagating solardisturbances have been reported previously (e.g., [4, 8]).References[1] D.V. Reames, Space Sci. Rev. 90, 413 (1999).[2] H.V. Cane and W.C Erickson, J. Geophys. Res. 108, Issue A5, pp. SSH 8-1.[3] H.W. Dodson and E.R. Hedeman, Solar Phys. 9, 278 (1969).[4] E.W. Cliver, Solar Phys. 75, 341 (1982).[5] H. Debrunner et al., J. Geophys. Res. 93, 7206 (1988).[6] A.W. Labrador et al., Proc. 28th Int. Cosmic Ray Conf., 6, 3269 (2003).[7] S.A. Knock and I.H. Cairns, J. Geophys. Res., 110, Issue A1, cite ID A01101 (2005).[8] Cliver, E.W., et al., Proc. 24th Int. Cocmic Ray Conf., 4, 257 (1995).

The Solar Energetic Particle Event of 16 August 2001: ~ 400 MeV Protons Following an Eruption at ~ W180

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