GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO.14, PAGES 2525-2528, JULY 15, 1998 Energetic I to 50 MeV) protons associated with Earth-directed coronal mass ejections J. Torsti, A. Anttile, L. Kocharov P. M/ikelii, E. Riihonen,T. Sahla M. Teittinen, E. Valtonen, T. Laitinen, and R. Vainio Space Research Laboratory, University of Turku, Finland Observations Abstract. During the period from January through midMay, 1997, four large Earth-directed CMEs were observed by the Large Angle Spectroscopic Coronograph(LASCO). These CMEs were associated with long-lasting fluxes of > 1.6 MeV protons detected by the Energetic and Relativis- tic Nuclei and Electron instrument(ERNE). However,the magnitudesof energeticproton eventsdiffered dramatically on different occasions. In strong proton events, production of 10-50 MeV protons started during expansionof the coronal Moreton wave in the western hemisphere of the Sun. The new SOHO observationssuggestthat potentialities of CMEs to produce energetic particles in the interplanetary medium crucially depend on the previous evolution of the of CME Associated Events Figures 1 and 2 present proton intensities observed by ERNE during five-day periods for the CME associated eventslistedin Table 1. In Figure 3, the intensitiesare given for the first ten hours for two of the events. Then, in Figure 4, we show cumulative-sumlevel-linesin the 1/•-time plane, i.e., level-linesof the normalizedtime-integratedex- cessof protoncountrate abovethe backgroundlevel(v = •c is the averagevelocity for a channel). We calculatethe cumulative sum of proton countsin different energy channels to reducefluctuations and to determine more preciselythe proton-eventonset-time and the velocity dispersion. In Figexplosion below•- 2//6). Forecasting of the near-Earth> 10 ure 5, we alsoshowproton energyspectrafor severalselected MeV proton intensityrequiresmultiwavelengthobservations periods. We averagedthe data over the time intervals, and of the early phase of an event, particularly the Extreme- then the spectra were determined. A "halo" CME wasrecordedby the LASCO coronograph ultravioletImaging Telescope(EIT) observations. on January 6, 1997. This event developed into a giant magnetic cloud which traveled to the Earth, where it caused a strong magnetic storm. Arrival of the interplanetary shockwave was recorded by the Charge, Element and Introduction IsotopeAnalysisSystem(CELIAS/MTOF) proton monitor is not completelyunderstood[Kallenrode,1997]. At E _• 1 on board SOHO on January 10, at 00:22 UT. However, the ERNE instrument recordeda surprisinglyweak increasesof proton flux, hardly visible even in the lowest, 1.6-3 MeV, energy channel. The only traditional event of solar activity which may be associatedwith this CME was the filament disappearanceat S23W03 observedon January 6 about 3 Travelinginterplanetary shocksare frequentlyassociated with enhancementsin energeticparticles from a few tens of keV up to a few tens of MeV. While theseobservationsindicate that shocksare capable of efficient proton acceleration, the acceleration mechanism responsiblefor these increases MeV, interplanetary shockaccelerationmay not be efficient hoursprior to the CME onset[$GD, 1997]. enoughto accelerateprotonsfrom the solarwind [Lira et al., 1995;Boberget al., 1996]and to generatea powerlaw energyspectrum[Savopulos et el., 1995]. In MeV protons, the February 7, 1997 event was essentially stronger than the January event, but still very weak During the period, i January- 15 May 1997, four Earthdirected coronal mass ejections were observedby EIT and LASCO, on January 6, February 7, April 7 and May 12. In this paper we analyze properties of associatedenergetic proton events observedby the ERNE instrument. ERNE images,a very large event, involvinga brighteningarcade, consists of two detector telescopes, the Low Energy Detec- abovem 10 MeV (Figures1, 3). In the SOHOEIT 195,t, was seenduring severalhours starting shortly before 00 UT, February 7. The CME onset time was reported to be 2:30 UT (LASCO observations),while the earliest onset of the proton count rates was observed in the 3-6 MeV channel much later, at m 10 UT, February 7. At that time, accord- tor (LED) and the High EnergyDetector(HED). Proton ing to the Magnetic Field Instrument (WIND/MFI) energyrangesare 1.6-12 MeV and 12-100MeV for LED and HED, respectively. Both LED and HED are pointed along the nominal direction of the interplanetary magnetic field, •b - 315ø and 0 = 0ø in GSE coordinates.The view cones of LED and HED are 64ø and 120ø, respectively.A full descriptionof the ERNE experiment has been given by Torsti et al. [1995]. Copyright1998 by the AmericanGeophysicalUnion. mea- surements, the interplanetary magnetic field was pointing at q• • 250ø and 0 • 30ø in GSE coordinates, i.e. the direction was outside the LED view cone. The only classical solar-activity event during the first hours of the day was the filament disappearanceobservedat S49W02 [$GD, 1997]. Accordingto the CELIAS/MTOF proton monitor data, the interplanetary shock wave passed the SOHO spacecraft at • 12 ß45 UT on February 9, but no correspondingincreases in the > 1.6 MeV proton count rates were observed. During the April 7, 1997 event, in the 24-48 MeV en- Papernumber98GL50062. ergyband,the mainrisein protonintensitywasdetected 0094-8534/98/98GL-50062505.00 at • 15' 15 UT, April 7 (Figures2, 4). At that time, the 2525 2526 TORSTI ET AL.: CME ASSOCIATED ENERGETIC PROTONS Table 1. Characteristics of the CME AssociatedEvents Event Location X-raya Type IIb 6 Jan. 7 Feb. 7 Apr. 12 May S23W03 S49W02 S30E19 N21W07 no no 14:07 4:55 no no 13:58 4:54 Moretonwave no no 14:00-15:06 4:34-5:41 CMEc 6-12 MeV d 17:34 2:30 14:27 7:35 < 0.0003 0.007 0.23 0.27 Utr ekm , s-1 524 709 585 631 aMaximumsoftX-ray fluxtime, UT [SGD,1997]. bStarttime,UT [SGD,1997]. CStarttimeasobserved byLASCOat 2P• dMaximum 1-hour-average intensity of6-12MeVprotons, cm-•'sr-Xs -xMeV-x. eAverage transitspeed ofinterplanetary shock from2R©to i AU. interplanetary magnetic field was pointing in the direction •b• 270ø and t) ..• 15ø, which is inside the HED view cone. The event was associatedwith a gradual soft X-ray flare (classC6.8), and type II, IV, III and V radiobursts[SGD, 1997].An opticalflarewasobserved at S30E19[SGD,1997]. ,""'1""" ' I""" I" 1" I""" I ' I 100; : In theSOHOEIT 195]kmovieof theevent,a Moretonwave was clearly seen. The extrapolated start time of the shockis • 14 ß 00 UT. It took the Moreton wave about 10'1 one hour to arrive at the west limb. The CME was observedby LASCO simultaneouslywith the Moreton wave expansion. On April 10, at 12:58 UT, the interplanetary shock wave arrived at the SOHO site. The arrival of the shockcauseda sharp peak in the 1.6-3 MeV proton count rate observedby LED. In association with the May 12, 1997 CME, the first 10'2 10'3 I 10.4 ,'''1'''1'''1'''1''' 10 ø 7 10'1 8 9 10 April1997 11 12 12 13 14 15 May1997 16 17 Figure 2. The sameas in Figure i but for the periods April 7-11 and M 12-16, 1997. 10 '2 risc in p] )ton inte •iy use in proton intensity above the backgroundin the 24-48 Me renc ,gy band band was • .s observed MeV energy at • 6 : 05 UT on May 12,1997 (Figures 3,4). This precursorwas followedby 10.3 the main event which started to rise about 30 min later. In the beginning of the event,the interplanetary magneticfield directionwasin the vicinityof the HED viewcone,but well 10'4[ i apart from the LED view cone,•b• 15ø and 0 ..• -30 ø. The CME wasobservedabout 1.5 hour after the protonevent onset (Table1). Therewasan associated gradualsoftX6 7 8 9 10 11 6 7 8 9 10 11 rayflare(C1.3),andtypeIII, II andIV radiobursts[SGD, January 1997 February1997 1997].A smallopticalflare,class1N, wasobserved in AR Figure 1. Intensity-timeprofilesof protonsin different 8038[$GD,1997].It isseenin theSOHOEIT 195]imovie, i . . . I... I... I... l.. I ,.. I... I... I. energychannels,consecutive thin and thick linesfromtop to bottom, 1.6-3 MeV, 3-6 MeV, 6-12 MeV, 12-24MeV, and 2448 MeV, respectively.The intensitiesare 1-houraverages. For the period January6-10, only three first energychannels are shown. The vertical dashedlinesrepresentthe time of that AR 8038 produceda Moreton wave. About 50 min after the flare,this wavetraversedthe westernpart of the solar disc. At • I UT, May 15, the arrivalof a hugemagnetic structureat the L1 pointwasobserved by WIND/MFI and the CELIAS/MTOF protonmonitor. Simultaneously, the the shockpassageas observedby CELIAS/MTOF proton secondmaximumwasdetectedby ERNE in the 1.6-12 MeV monitor. proton channels. TORSTI 'I ' ' ' ET AL.: CME ASSOCIATED •'I ...... ['=1 ENERGETIC PROTONS 2527 law. Astheshockarrives,spectraseemto approach a power law in energy.The approachto a powerlaw occurswith a lO0 spectralsoftening. Astimeelapses, theshockexpands away fromthe Sun,and the Earth-connected point on the interplanetaryshockfront sweepseastward,towardsthe center of the shock.For thesereasons,the compression ratio and 10'1 the angle between the field direction and the shock normal are functionsof time, functionsgoverning the dynamicsof protonspectra. An attemptto separatetheseeffectsis, however, beyondthe scopeof the presentpaper. 10'2 _ We have studied high-energyproton events associated with the Earth-directedcoronalmassejectionsobserved during January-May,1997. No selectionregardinga maximum protonintensityhasbeenapplied.The two strongesthigh- 10'3 ß energyproton eventsare associatedwith gradual soft X-ray 10'4 flares,and with type II and IV radio bursts,and with a Moreton wave observedby EIT. The two weakest events are not associated with suchphenomena (Table 1). Kahler 10'5 [1994]studiedfivestronghigh-energy protonevents,and I 10 Hours from 08:30 UT I 10 Hours from 04:47 UT foundthat the peaksof the high-energy protoninjectionoc- curwhentheCMEis far fromthe Sun,at 5- 15R©. On Figure 3. Intensityprofilesin the sameenergychannels the otherhand, Cliveret al. [1995]studiedchromospheric as in Figure 1. The intensitiesrepresent7-minutemoving Moretonwavesand foundthat they might be involvedin the in a widerangeof solarlonaveragesfor the first ten hours of the events of Feb. 7 and solarprotonacceleration/release gitudes.A new multiwavelengthanalysisof the famous22 May 12, 1997. Precursor is marked 'P'. October1989event[Shedand Smart,1997]demonstrates that the first injection of solar particlesoccurredwhen the inneredgeof the CME wasbetween2 and 2.5R©. The Discussion recent study of Fe charge states in shock-associatedevents For the February7, April 7 and May 12 events,the timing [Boberg et al., 1996]indicates that the interplanetary CMEof the first arriving particles demonstratesa plain velocity drivenshocksmainly accelerateseedparticlesoriginating dispersion(Figure 4, for morediscussion of similarplotssee from solarcorona. Our analysissuggests that the magni- [Lockwood et al., 1990]and [Debrunneret al., 1997]).In the tudesof the > 10 MeV protonproductionin the interplancaseof the May 12 event, if the proton precursor at 6:05-6:35 UT is excluded, the distance traveled by the first particles is etary medium strongly depend on the energy releaseand particleacceleration processes below• 2R©, andthey are estimatedto be ..• 1.7 AU (Figure4). We considerthis value closelyconnectedwith the occurrence of soft X-ray flares, as an upper limit for the interplanetary magnetic line length, becauseduring the period of observations,LED was in a position to detect only scattered particles. Thus, the estimated length of the interplanetary magnetic field line is 1.2-1.7 AU for the onset phase of the 12 May 1997 event. Correspondingly, for the first protons arriving along the magnetic field line inside the large view cone of HED, the solar injection type II and IV radio bursts and Moreton waves in solar corona. All availabledata are in agreementwith the idea that a seedpopulationfor the interplanetaryCME-driven shockacceleration is producedin the solarcoronaduringan early phaseof the explosion. time is estimated to be close to the time when the Moreton HED LED ' ?..• wave approached the west limb of the Sun. The same is valid for the April 7 event. During the May 12 event, a precursor was seenin the 24-48 MeV proton intensity, which lasted for about 30 min (Figure 3). During this first protoninjection, the energy spectrum was extremely hard, so that the 24-48 MeV proton flux exceededthe flux in the lower, 12-24 MeV, energy range. The precursormay be causedby a DC electric field accelerationduring magneticreconnectiontriggeredby the solar eruption. This first accelerationprecededa major, shock-wave I ._72 • - E •o o • HED o acceleration. The first CME associatedevent observed by ERNE in 1997, on January 10, was very weak, and no reliable proton energy spectrum can be deduced. For the other three events, time-integrated proton spectra for selected periods are shown in Figure 5. We use rather long integration periods to reduce the effect of velocity dispersion. The spectral evolution : turns out to be rather similar for the three events studied. In the beginning of an event, the shapeof the highenergy proton spectrum is very far from the classicalpower 2 0 0 5 10 15 20 Figure A. Thetimesof firstprotonarrivalvs. 1//• sho•n by tNe cumulated-sum level-lines. •o is 1A:00 U• •d U• fo• April ? •d N•7 12 e•e•ts, mspect•velT. 5:00 2528 TORSTI ET AL.: CME ASSOCIATED ENERGETIC PROTONS May Apr 1+ 3o 4z• 50 Acknowledgments. We are grateful to Jean-PierreDelaboudinierefor permissionto use the SOHO/EIT data and to Chris St.Cyr for commentson SOHO/LASCO data. Thanks are extended to the LASCO team for the coronograph data available in the SOHO archive. We especially thank F.M. Ipavich and the CELIAS team for the use of the CELIAS/MTOF data. Special thanks go to Ron Lepping for helpful comments on the manuscript and for permiSSion to useWIND/MFI magneticfield data. We thank Jim Ryan for helpful information on COMPTON GRO observations. The Academy of Finland is thanked for financial support. SOHO is an international co-operation project •o•+ • • between + ESA and NASA. References Boberg, P. R., A. J. Tylka, and J. H. Adams, Jr., Solar energetic Fe chargestate measurements: implications for acceleration by coronal mass ejection-driven shocks, Astrophys. J., 471, L65• , L68, 1996. Cliver, E. W., Kahler, S. W., Neidig, D. F., et al., Extreme "prop- Figure 5. Time-integratedproton energyspectrafor the followingperiods:the February7-9 event:(1) 9:30Feb. 7 - 2•:00feb. 7, (2) 2•:00 feb. 7- 7:00feb. 8, (3) 7:00feb. 8 - 19:00Feb. 8, (4) 19:00Feb. 8- 7:00Feb. 9, (5) 7:00Feb. 9 - 19:00 Feb. 9; the April 7-11 event: (1) 15:10 Apr. 7- 3:00Apr. 8, (2) 3:00Apr. 8- 3:00Apr. 9, (3) 3:00Apr. 9 - 3:00 Apr. 10, (4) 3:00 Apr. 10- 15:00Apr. 10, (5) 15:00 Apr. 10- 24:00Apr. 11; the May 12-16event:(1) 6:00May 12- 18:00May 12, (2) 18:00May 12- 17:00May 13, (3) 17:00 May 13 - 16:00 May 14, (4) 16:00May 14 - 8:00 May 15, (5) 8:00 May 15- 8:00 May 16. agation" of solar energetic particles, Proc. œ4thInternat. Cosmic. Ray Conf., 4 257-260, 1995. Debrunner, H., Lockwood, J. A., Barat, C., et al., Energetic neutrons, protons, and gamma rays during the 1990 May 24 solar cosmic-ray event, Astrophys. J., 479, 997-1011, 1997. Kahler, S., Injection profiles of solar energetic particles as functions of coronal mass ejection heights, Astrophys. J., 4œ8, 837842, 1994. Kallenrode, M.-B., A statistical study of the spatial evolution of shock acceleration efficiency for 5 MeV protons and subsequent particle propagation, J. Geophys. Res., 102, A10, 22335-22345, 1997. Lim, T. L., J. J. Quenby, M. K. Reuss, et. al, Can Diffusive Shock Acceleration Work Fast Enough in Interplanetary Shocks? Evidence from the November 1992 Event, Proc. œ4th Internat. Cosmic. Ray Conf., 4, 353-356, 1995. Lockwood,J. A., Debrunner,H., and Fliickiger,E. O., Indications Conclusions for diffusive coronal shock acceleration of protons in selected solar cosmic ray events, J. Geophys. Res., 95, 4187-4201, 1990. Savopulos, M., J. J. Quenby, and A. R. Bell, Interplanetary diffusive shock acceleration: Exponential or power-law spectra ? Solar Phys., 157, 349-366, 1995. The observationalfactspresentedheresuggestthat (i) in strongproton eventsassociatedwith Earth-directedCMEs, productionof 10-50 MeV protonsstarted duringexpansion Shea, M. A., and Smart, D. F., Dual accelerationand/or release of the coronal Moreton wave in the western hemisphere of the Sun; (ii) a proton precursorobservedon 12 May 1997 can be consideredas the signature of a magnetic reconnec- tion that happenedbefore/during the CME launch, well after the soft X-ray flare maximum; (iii) potentialitiesof CMEs to produceacceleratedparticlesin the interplanetary mediumcruciallydependon the solarexplosionevolu- tion below-• 2R©; (iv) forecasting of the near-Earth> 10 of relativistic solar cosmic rays, Proc. œ5th Internat. Cosmic. Ray Conf., 1, 129-132, 1997. Solar-Geophysical Data, NOAA, Space Environment Center, Boulder, Colorado, 1997. Torsti, J., E. Valtonen, M. Lumme, et. al, Energetic Particle Experiment ERNE, Solar Phys., 162, 505-531, 1995. A. Anttila, L. Kocharov, T. Laitinen, P. M/ikel/i, E. Riihonen, T. Sahla, M. Teittinen, J. Torsti, R. Vainio and E. Valtonen, Space Research Laboratory, Department of Physics, Turku MeV proton intensityrequiresmultiwavelengthobservations University, FIN-20014, Finland. (e-mail: antti.anttila@utu.fi; of the beginningof the event, especiallythe EIT observations of coronal Moreton waves;(v) theoreticalmodelsof > 10 MeV proton accelerationat CME-driven interplanetary shocksshouldtake into accountinitial conditionsat the Sun. kocharov@helium.srl.utu.fi; timo.laitinen@utu.fi; pamakela@utu. fi; esa.riihonen@utu.fi; tetasa@utu.fi; matti.teittinen@utu.fi; jarmo.torsti@utu.fi;rami.vainio@utu.fi;eino.valtonen@utu.fi) (ReceivedSeptember11, 1997; revised December 18, 1997; acceptedDecember29, 1997.)