Probing the energetic particle environment near the Sun
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The document reports observations from NASA's Parker Solar Probe, which recently entered the inner heliosphere approximately 24 million kilometers from the Sun, providing insights into the near-Sun energetic particle environment. It identifies various energetic particle acceleration processes, including corotating interaction regions and impulsive events often associated with solar flares, while also detailing the instruments used for measurement and the significant findings from the probe's initial orbits. The study highlights how energy particle events vary at different distances and how these particles are linked to solar wind conditions and magnetic structures.
Probing the energetic particle environment near the Sun
- 1. Nature | Vol 576 | 12 December 2019 | 223 Article Probingtheenergeticparticleenvironment neartheSun D. J. McComas1 *, E. R. Christian2 , C. M. S. Cohen3 , A. C. Cummings3 , A. J. Davis3 , M. I. Desai4,5 , J. Giacalone6 , M. E. Hill7 , C. J. Joyce1 , S. M. Krimigis7 , A. W. Labrador3 , R. A. Leske3 , O. Malandraki8 , W. H. Matthaeus9 , R. L. McNutt Jr7 , R. A. Mewaldt3 , D. G. Mitchell7 , A. Posner10 , J. S. Rankin1 , E. C. Roelof7 , N. A. Schwadron1,11 , E. C. Stone3 , J. R. Szalay1 , M. E. Wiedenbeck12 , S. D. Bale13,14 , J. C. Kasper15 , A. W. Case16 , K. E. Korreck16 , R. J. MacDowall2 , M. Pulupa13 , M. L. Stevens16 & A. P. Rouillard17 NASA’sParkerSolarProbemission1 recentlyplungedthroughtheinnerheliosphereof theSuntoitsperihelia,about24millionkilometresfromtheSun.Previousstudies fartherfromtheSun(performedmostlyatadistanceof1astronomicalunit)indicate thatsolarenergeticparticlesareacceleratedfromafewkiloelectronvoltsuptonear- relativisticenergiesviaatleasttwoprocesses:‘impulsive’events,whichareusually associatedwithmagneticreconnectioninsolarflaresandaretypicallyenrichedin electrons,helium-3andheavierions2 ,and‘gradual’events3,4 ,whicharetypically associatedwithlargecoronal-mass-ejection-drivenshocksandcompressionsmoving throughthecoronaandinnersolarwind andare the dominant sourceofprotonswith energiesbetween 1and10megaelectronvolts.However,someeventsshowaspectsof bothprocessesandtheelectron–protonratioisnotbimodallydistributed,aswould beexpectediftherewereonlytwopossibleprocesses5 .Theseprocesseshavebeen verydifficulttoresolvefrompriorobservations,owingtothevarioustransporteffects thataffecttheenergeticparticlepopulationenroutetomoredistantspacecraft6 . Herewereportobservationsofthenear-Sunenergeticparticleradiationenvironment overthefirsttwoorbitsoftheprobe.Wefindavarietyofenergeticparticleevents acceleratedbothlocallyandremotelyincludingbycorotatinginteractionregions, impulsiveeventsdrivenbyaccelerationneartheSun,andaneventrelatedtoacoronal massejection.Weprovidedirectobservationsoftheenergeticparticleradiation environmentintheregionjustabovethecoronaoftheSunanddirectlyexplorethe physicsofparticleaccelerationandtransport. OnboardtheParkerSolarProbe(PSP),theinstrumentsuiteoftheInte- gratedScienceInvestigationoftheSun(IS☉IS)7 hasmadethefirstnear- Sunmeasurementsofsolarenergeticparticles(SEPs).IS☉IScomprises twoenergeticparticleinstrumentswithoverlappingcoverage,EPI-Hi and EPI-Lo, measuring higher- and lower-energy particles7 . Together they enable IS☉IS to explore the near-Sun environment by measuring thefluxes,energyspectra,anisotropy,andcompositionofsuprather- malandenergeticionswithenergiesfromabout0.02to200 MeV per nucleon and electrons with energies from about 0.05 to 6 MeV. Here weexaminethisenergeticparticleenvironmentinthecontextofin situ solar wind8 and magnetic field9 conditions and surrounding density structures10 measured by other instruments onboard PSP. Figure 1 summarizes IS☉IS observations of energetic particles over PSP’sfirsttwoorbits.Higher-(1–2 MeV)andlower-energy(30–200 keV) H+ ion count rates are plotted on the outside and inside of the orbital trajectory, respectively. Intensifications indicate energetic particle events, with some seen only at higher energies, some only at lower energies,andotherssimultaneouslyacrossthecombinedenergyrange. Figure 1indicateshowrichtheIS☉ISobservationsare:abroadarrayof different types of particle events are seen at all distances. The first large intensification occurred during orbit 1 at higher energies with PSP inbound (during interval a, 2018-287 18:00 to 2018-297 08:20 universal time (ut)) at about 0.5 astronomical units (au). Although not obvious from Fig. 1, this is a corotational event also seen when PSP was outbound at about 0.65 au (during interval b, 2018-330 23:20 to 2018-341 15:00 ut). Corotating interaction regions (CIRs) form as faster solar wind piles up behind slower wind, forming a compression11,12 . Because these faster solar wind streams https://doi.org/10.1038/s41586-019-1811-1 Received: 28 June 2019 Accepted: 5 September 2019 Published online: 4 December 2019 1 Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA. 2 Goddard Space Flight Center, Greenbelt, MD, USA. 3 California Institute of Technology, Pasadena, CA, USA. 4 Southwest Research Institute, San Antonio, TX, USA. 5 University of Texas at San Antonio, San Antonio, TX, USA. 6 University of Arizona, Tucson, AZ, USA. 7 Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA. 8 National Observatory of Athens, IAASARS, Athens, Greece. 9 University of Delaware, Newark, DE, USA. 10 NASA HQ, Washington, DC, USA. 11 University of New Hampshire, Durham, NH, USA. 12 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. 13 University of California at Berkeley, Berkeley, CA, USA. 14 The Blackett Laboratory, Imperial College London, London, UK. 15 University of Michigan, Ann Arbor, MI, USA. 16 Smithsonian Astrophysical Observatory, Cambridge, MA, USA. 17 CNRS, Toulouse, France. *e-mail: dmccomas@princeton.edu
- 2. 224 | Nature | Vol 576 | 12 December 2019 Article emanate from coronal holes at the Sun, CIRs map to nearly fixed solar longitudes. Figure 2showsintervalsaandbasafunctionofthelongitudeofthe solar surface ‘foot point’ magnetically connected to the spacecraft, calculated for a nominal Parker spiral with a fixed solar wind speed of 350 km s−1 .ThiscalculationcombinestherotationoftheSunandspace- craftlocationtoshowthatbotheventsarisefromthesame,singleCIR structure. These events are ‘dispersionless’—all ions arrive at roughly the same time and fluctuations in intensity are consistent across ion speeds.SucheventsindicatethatPSPpassedacrossmagneticfluxtubes thatwerealreadyfilledwithhigh-energy(>1 MeV)particlesthatmove quicklyalongmagneticfieldlines.Theintensitiesofsunward-andanti- sunward-moving particles in intervals a and b were similar (Fig. 2a), consistent with a corotating structure that traps particles between a 0.2 AU 0.4 AU 0.6 AU 0.8 AU –0.2 0.0 0.2 0.4 0.6 0.8 0.2 AU 0.4 AU 0.6 AU 0.8 AU –0.2 0.0 0.2 0.4 0.6 0.8 –0.6 –0.4 –0.2 0.0 0.2 X (AU) Y(AU) 2018-280 (7 Oct) 2018-290 (17 Oct) 2018-300 (27 Oct) 2018-310 (6 Nov) 2018-320 (16 Nov) 2018-330 (26 Nov) 2018-340 (6 Dec) 2018-350 (16 Dec) 2019-010 (10 Jan) 2019-020 (20 Jan) Hi Lo Lo,H+~30–200keV(cps) Hi,H+~1.0–1.8MeV(cps) 10–4 10–3 10–2 10–1 100 10–2 10–1 100Perihelion 2019-040 (9 Feb) 2019-050 (19 Feb) 2019-060 (1 Mar) 2019-070 (11 Mar) 2019-080 (21 Mar) 2019-090 (31 Mar) 2019-100 (10 Apr) 2019-110 (20 Apr) 2019-120 (30 Apr) 2019-130 (10 May) X (AU) Hi Lo Perihelion a b d ba c Fig.1|Observationsofenergeticparticlesduringorbits1and2.a, b, Observationsofenergeticparticles(primarilyH+ )atlower(Lo,about30– 200 keV;insidetrack)andhigherenergies(Hi,about1–2 MeV;outsidetrack) fromPSP’sorbit1(a)andorbit2(b).Intervalswithoutdataareindicatedbythe blackorbitaltrack.Particleintensityisindicatedbyboththecolourandthe lengthofthebars.Intervalsa–dareearmarkedfordetailedstudy.Thescale indicatedbywhiterectanglesontheoutertrackismeasuredindaysut,from 2018-280(7October2018)to2019-020(20January2019). 0.2 0.3 0.4 0.5 0.6 0.7 a b c d R(AU) 10–4 10–3 10–2 10–1 H+R1rate 0 100 200 300 10–4 10–3 10–2 10–1 H+R1rate (s–1) 10–4 10–3 10–2 H+outward(s–1) 1 2 5 H+(MeV) Foot-point solar longitude (°) a b a b a b Before or after 2018-310 288 290 292 Day of year 2018 (UT) 294 296 332 334 336 Day of year 2018 (UT) 338 340 From Sun Towards Sun 10–1 10–2 1–9MeVH+ (cm–2sr–1s–1MeV–1) 10–3 (s–1) Fig.2|Recurringcorotatingenergeticparticleevents.a–d,Corotatingion eventsobservedinintervalsa(blue)andb(red)representedbysunward/anti- sunwardfluxtimeseries(a),acount-ratespectrogram(b), thefluxasa functionofthemagneticfootpointinCarringtonlongitudeandradiusfrom theSun(c),andthe1–2 MeVH+ rateversusfoot-pointlongitude(d).Inc, d, vsw = 350 km s−1 isanominalsolarwindspeed.
- 3. Nature | Vol 576 | 12 December 2019 | 225 source more distant than the spacecraft and the increasing magnetic field strength closer to the Sun. The particle acceleration probably occursatreverseshocks,whichtypicallyformbeyondabout2 aufrom compressions in such CIRs. Theinboundlegtowardsperihelion1(whichoccurredat0.17au,at 03:28 6 November 2018 ut) was extremely quiet from about 0.4 au, providinganidealopportunityforotherPSPinstruments13,14 toobserve veryquietsolarwindconditionswithessentiallynoSEP-producedpen- etrating backgrounds. IS☉IS began to observe lower-energy SEPs just beforeperihelion1,whichthenincreased.Figure 3showstheeventsin intervalc,includinglow-energyionsaheadofacoronalmassejection (CME;Fig. 3b,f,g),thepassageofacompressionwaveafterit(Fig. 3c), and a subsequent higher-energy particle event (Fig. 3a). IS☉ISobservationsshowanSEPeventstartingearlyon2018-315and extendingtoaboutwhentheCMEarrivedatPSPon2018-316.Particle anisotropies (Fig. 3f) demonstrate that these particles are streaming outwardfromtheSun.Thefasterparticlesarrivefirst,characteristicof a‘dispersive’SEPevent(Fig. 3g)withthedifferingarrivaltimesgiving an estimate of the distance along the magnetic field to the source of their acceleration. For the time–energy slope in Fig. 3g, we estimate a path length3 longer than that of the Parker spiral from PSP at about 0.25 au, which might be explained by a longer path length associated with magnetic field ‘switchbacks’ observed by PSP in situ14 . Solarobservationsfromthewhite-lightcoronagraphonthe‘A’space- craft of NASA’s Solar Terrestrial Relations Observatory (STEREO-A) indicatethattheSEP-associatedCMEstartedliftingofffromtheSunon 2018-314atabout18:00 ut(ExtendedDataFig. 1).DerivationoftheCME speedfromSTEREO-Aimaging(ExtendedDataFig. 2)revealsthatthe CMEwasmovingslowly(<400 km s−1 )fromtheSuntoPSP,verysimilar tothesurroundingsolarwindspeed.BypropagatingthisCMEfluxrope ataconstantspeedof380 km s−1 fromneartheSuntoPSP,wefindgood agreement with the in situ magnetic field observations. Preliminary analysisofthiseventusingshock-modellingtechniques15 suggeststhat there was probably no shock on the field lines well connected to PSP. However,aquasi-perpendicularsub-criticalshock(Machnumber<3) couldhaveformedoveranextendedregionofthefluxropeandperhaps accelerated the protons measured by PSP (A. Kouloumvakos, private communication). This energetic particle event was not seen at any of the1 auspacecraft,sosuchsmalleventsmayonlybeobservableclose totheSunandthereforemuchmorecommonthanpreviouslythought. At the end of 2018-318, the solar wind speed increased from about 300toabout500 km s−1 (ref.13 ),indicativeofastrongdynamicpressure 10–4 10–3 10–2 0.3 a b c d e f g h 0.4 10–2 10–1 100 101 102 103 H+flux 102 10 100 400 600 –50.0 0.0 50.0 100.0 315 316 317 318 319 320 321 322 323 324 Magneticfield vector (nT) Magneticfield vector (nT) Solarwind velocity (kms–1) Solarwind density (cm–3) Energy (keV) Energy (keV) 1–9MeVH+ (cm–2sr–1s–1MeV–1) 30–500keVH+ (cm–2sr–1s–1keV–1) Day of year 2018 (UT) H+, Lo H+, Lo Compression From Sun Towards Sun From Sun Towards Sun |B| BRBTBN 0.24 0.260.25 100 200 50 10–2 10–1 100 101 –50.0 0.0 50.0 100.0 4:00 8:00 12:00 16:00 20:00 0:00 4:00 8:00 Time + 2018-315 (UT) H+, Hi H+, Lo R (AU) (cm–2sr–1s–1keV–1) 10–2 10–1 100 101 102 103 H+flux (cm–2sr–1s–1keV–1) R (AU) Fig.3|CME-relatedlow-energyeventandsubsequenthigh-energyevent. a–e,Timeseriesofprimarilyprotonfluxat>1 MeV(a),H+ fluxaround30– 500 keV(b),solarwinddensity(c)andradialspeed13 (d),andmagneticfield vectorandmagnitude14 (e)overintervalc.f–h,Magnificationofthedispersive SEPeventandCMEforH+ fluxaround30–500 keV.
- 4. 226 | Nature | Vol 576 | 12 December 2019 Article wave in the solar wind. IS☉IS observed a small enhancement in very- low-energyparticles(<50 keV)asthiscompressionalwavepassed.This event is the first direct observation of local energization in the IS☉IS observations. Shocks are not required for particle acceleration16 and plasmacompressionscanaccelerateparticlesprovidedthattheparti- clesareabletopropagateacross,butremaincloseto,thecompression17 . Thelargetwo-stepincreaseinspeedon2018-318showsthatthiscom- pressionwavewaswellonitswaytosteepeningintoaforward–reverse- shock pair, which probably accelerates the higher-energy (>1 MeV) particles observed from 2018-320 to 2018-324. This is not a CIR as in intervals a and b, because it has a much narrower range of foot-point longitudes (see H+ count rate at about 300° in Fig. 2c, d) and does not recur, but instead indicates the interaction of a single fast solar wind stream,possiblyassociatedwithorevenmagneticallyinitiatedbythe precedingCME.Inanycase,aswithCIR-associatedparticleevents,the particle isotropy indicates that these ions are trapped on flux tubes, probably with a source more distant than PSP. In fact, at the time the secondeventwasseen,about1–6daysafterthepassageofthecompres- sion at PSP, the pressure front had expanded outward to heliocentric distances of about 0.6–2 au, where the shocks probably formed. Very near perihelion (about 35 solar radii, R☉) during PSP orbit 2 (intervald),IS☉ISobservedauniquepairofSEPevents(Fig. 4).AsPSP isnearlycorotationalwiththeSunnearperihelion,thetwoeventsare magnetically connected to a common solar source <5° apart in lon- gitude. First, on 2019-092 there was a low-energy dispersive event, probably associated with an impulsive source in the low corona. Two dayslater,on2019-094,therewasadifferenttypeofimpulsiveevent, marked by a substantial increase in ions with >1-MeV energy. Both eventsexhibitstrong,persistentmagnetic-field-alignedionsstream- ing away from the Sun. Thefirstevent,startingon2019-092,maybeassociatedwithdistur- bancesalsoobservedinextremeultravioletimagesfromSTEREO-Ain the vicinity of active region AR2738, as well as multiple type-III radio burstsbybothSTEREO-AandFIELDS14 ,themagneticfieldinstrument aboard PSP. This small active region was about 70° off the nominal magnetic connection of PSP to the Sun. The fluxes of high-energy protons are near background, but we observed a substantial num- ber of heavy high-energy ions and at low energies (about 30 keV per nucleon). He/H is about 20 times higher than the event on 2019-094, andincreasesinOandFeabundancesareevengreater.Theseresults suggest that this may be a ‘Z-rich’ event18 ; such events are relatively rare at 1 au. The second SEP event on 2019-094 also exhibits velocity disper- sion and outward streaming, but has many fewer ions <1 MeV and a 100 10–1 10–2 102 10–3 10–4 0.1 1.0 10.0 100.0 –150 –50 50 150 92 93 94 95 0.18 0.17 From Sun Towards Sun From Sun Towards Sun R (AU)Magneticfield vector (nT) Energy (keV) Energy (MeV) 1–9MeVH+ (cm–2sr–1s–1MeV–1) 30–500keVH+ (cm–2sr–1s–1keV–1) H+, Hi H+, Hi H+, Lo H+, Lo Day of year 2019 (UT) 10–2 10–1 100 H+flux (cm–2sr–1s–1MeV–1) 102 101 100 10–1 103 104 H+flux (cm–2sr–1s–1keV–1) |B| BN BTBR Fig.4|Pairofimpulsiveeventsnearsecondperihelion.a–e,Twoimpulsive SEPevents(duringintervald)nearthesecondperihelionofPSP(<40R☉)at higherenergies(a, b),lowerenergies(c, d),andthemagneticfieldmeasured duringtheevents(e).|B|isthemagnitudeofthemagneticfieldandBR, BT andBN aretheradial,transverseandnormalcomponentsofthemagneticfield, respectively.
- 5. Nature | Vol 576 | 12 December 2019 | 227 substantialincreaseat>1 MeV.Aswiththeeventon2019-092,thereis potentially related radio and extreme ultraviolet activity in AR2738. However, the heavy ion abundances were similar to more typical SEP events. The magnetic field observed at PSP (Fig. 4e) between the two events was stronger and considerably smoother than before or after, indicating that this was probably a single, lower β (particle pressure/ magnetic pressure) magnetic structure connecting the two events. Further, these observations indicate that processes inside 0.17 au, as suggestedbyearlymulti-spacecraftstudiesinsolarcycle20,aswellas later Helios and STEREO studies19–22 , enable fast, direct access of SEPs toawiderangeofsolarlongitudes.Laterstudiesthatcombinedin situ data with solar source region observations showed that the smaller, longitudinally distributed SEP events are associated with multiple jet-likecoronalemissions23,24 closetothesourceregionaswellaswith more spatially extended eruptions25 . IS☉ISobservedaricharrayofenergeticparticlephenomenaduring PSP’sfirsttwoorbits.Severaloftheseeventswerenotobservedby1 au spacecraft, and so small events only observable close to the Sun may bemuchmorecommonthanpreviouslythought.Withthesenewdata, we are well on the way to resolving the fundamental questions of the origin, acceleration, and transport of SEPs into the heliosphere. 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