SOLAR WIND, MAGNETOSPHERE, AND IONOSPHERE
国际空间科学研究所 - 北京太空 TAIKONGISSI-BJ MagazineNo. 9 October 2016LINK BETWEENSOLAR WIND, MAGNETOSPHERE,AND IONOSPHERE
FOREWORDIMPRINT太空 TAIKONGISSI-BJ MagazineAddress: No.1 Nanertiao,Zhongguancun,Haidian District,Beijing, ChinaPostcode: 100190Telephone: ella Branduardi Raymont (MSSL-UCL, UK),C. Philippe Escoubet (ESA/ESTEC, The Netherlands),Kip Kuntz (JHU/APL, USA),Tony Lui (JHU/APL, USA),Andy Read (Leicester U.,UK), David Sibeck (NASA/GSFC, USA), Tianran Sun(NSSC/CAS, China), BrianWalsh (Boston U., USA),Chi Wang (NSSC/CAS,China)EditorsAnna YangMaurizio FalangaFront CoverAn artist's impression of theSolar-wind MagnetosphereIonosphere Link Explorer(SMILE) mission.(Credit: CAS;Special thanks to:Chi Wang)2太空 TAIKONGOn July 6-7, 2016, the J)successfullyorganized a two-day Forum on“The Link between Solar Wind,Magnetosphere,Ionosphere”.ISSI-BJ Forums are informal,free debates, and brainstormingmeetingsamonghigh-levelparticipants on open questions ofscientific nature. In total, 28 leadingscientists from eight countriesparticipated in this Forum, whichwas convened by Chi Wang(NSSC, CAS), Graziella BranduardiRaymont (MMSL-UCL, UK), BenoitLavraud (CNRS, France), Tony Lui(APL, USA), and Maurizio Falanga(ISSI-BJ, China).The Forum’s main aims divided themeeting into 4 sessions: Overview ofthe Solar Wind Magnetosphere andIonosphere Coupling; Key Scienceof the Solar wind, Magnetosphere,Ionosphere Coupling; Instrumentsand Capability Required; SynergiesComplementaryMissionsandInternational Collaborations. Inthis context, the European SpaceAgency (ESA) and the ChineseAcademy of Sciences (CAS)selected a joint small mission(SMILE, to be launched in 2021)to trace these processes frombeginning (the Sun) to end (theEarth's aurora), and investigate – ina way unmatched so far – how thesolar wind interacts with the Earth'smagnetic environment.The Forum started with anoverview and goals of theSMILE mission. The participantsdiscussed the interaction betweenthe Earth's protective shield –the magnetosphere – and thesupersonic solar wind. SMILE isexpected to give an importantcontribution to our understandingof space weather and, in particular,the physical processes taking placeduring the continuous interactionbetween the solar wind and themagnetosphere. The participantsrecognized the very high scientificvalue of the mission, and raisedconstructivecommentsandsuggestions on the mission concept,payloads key techniques, and dataproduct. They concluded that theSMILE mission has complementaryobjectives to existing or futuresolar space plasma missions.Therefore, the SMILE mission isyet another excellent example ofhow the Chinese Space Scienceinstitutions can work together withthe European Space Agency oninnovative and challenging, andcomplementary to the existing,missions. This offers oncoordinationand scientific analysis that placesSMILE and China-Europe in acentral position, due to its uniqueobjectives and technology.This TAIKONG magazine providesan overview of the scientificobjectives and the overall designof the SMILE project, includingspacecraft and instrumentationdiscussed during the Forum.I wish to thank the conveners andorganizers of the Forum, as wellas the ISSI-BJ staff, Lijuan En,Anna Yang, and Xiaolong Dong, foractively and cheerfully supportingthe organization of the Forum. Inparticular, I wish to thank the authors,who, with dedication, enthusiasm,and seriousness, conducted thewhole Forum and the editing of thisreport. Let me also thank all thosewho participated actively in thisstimulating Forum.Prof. Dr. Maurizio FalangaBeijingOctober 2016
INTRODUCTIONForum OverviewThe interaction between thesolar wind and Earth’s magnetosphere, and the geospacedynamics that result, comprisea fundamental driver of spaceweather, the conditions on theSun, in the solar wind, andin the magnetosphere, ionosphere and thermosphere, thatcan influence the performanceand reliability of technologicalsystems and endanger humanlife and health. Understandinghow this vast system worksrequires knowledge of energyand mass transport, and of thecoupling both between regionsand between plasma and neutral populations.The Forum concentrated onthe main scientific drivers forthe Solar-wind MagnetosphereIonosphereLinkExplorer(SMILE) mission, and how theydefine the mission specifications, reviewed lessons learnedfrom the previous in situ andimaging missions, discussedthe SMILE mission for softX-ray magnetospheric imagingand UV auroral imaging, compared the soft X-ray simulatedresults from different numerical models, and examined opportunities for synergies withcomplementary observationsfrom other space missions andground-based facilities. TheForum also reviewed the current status and future plans forSMILE, the primary scientificgoals, the needed technologies, and how to best optimizeinternational collaborations.The forum was sponsored byISSI-BJ, with partial supportfrom the State Key Laboratoryof Space Weather, NationalSpace Science Center (NSSC),and the Chinese Academy ofSciences (CAS).Background of the Solar Wind, Magnetosphere, IonosphereThe solar wind is a stream ofcharged particles (protons,electrons, and heavier ionizedatoms) released from the upper atmosphere of the Sun.The solar wind is divided intotwo components, respectivelytermed the slow solar wind andthe fast solar wind. The slowsolar wind has a velocity ofabout 400 km/s, a temperatureof 1.4–1.6 106 K and a composition that is a close match tothe solar corona. By contrast,the fast solar wind has a typical velocity of 750 km/s, a temperature of 8 105 K and it nearlymatches the composition of theSun's photosphere. Near theEarth, the solar wind encounters the Earth’s magnetic fieldand the particles are deflectedby the Lorentz force. The solarwind compressesthe sunward sideof the magnetosphere but dragsthe nightside outinto a long magnetotail.The interactionof the solar windwith Earth leadsto the formationof the magnetosphere, includingthe bow shock,magnetosheath,cusps,magnetopause and Fig. 1: The dayside magnetosphere. The magnethe magnetotail topause represents the outer boundary of the magnetosphere, and is compressed on the dayside.(Figure 1).The bow shock compresses and deflects the solarwind so that it may flow around the magnetopause.太空 TAIKONG3
As shown in Figure 1, a collisionless bow shock standsupstream from the magnetopause in the supersonic solar wind. The shocked solarwind plasma flows around themagnetosphere through themagnetosheath. A relativelysharp transition from dense,shocked, highly ionized solarwind plasmas to tenuous, lesshighly ionized magnetospheric plasmas marks the magnetopause. High latitude cuspsdenote locations where fieldlines divide to close either inthe opposite hemisphere or fardown the magnetotail. Weakfield strengths within the cuspsprovide an opportunity for solarwind plasma to penetrate deepinto the magnetosphere, all theway to the ionosphere. The ionosphere is a region of Earth'supper atmosphere, from about60 km to 1,000 km altitude. It isionized by solar radiation, playsan important part in atmospheric electrical activity and formsthe inner edge of the magnetosphere.The position and shape of themagnetopause change constantly as the Earth’s magnetosphere responds to varyingsolar wind dynamic pressuresand interplanetary magneticfield orientations. Both the fastand slow solar wind can be interrupted by large, fast-moving bursts of plasma calledinterplanetary coronal massejections, or CMEs. When aCME impacts the Earth's magnetosphere, it temporarily deforms the Earth's magneticfield, changing its direction andstrength, and inducing largeelectrical currents; this is calleda geomagnetic storm and it isa global phenomenon. CMEimpacts can induce magnetic reconnection in the Earth'smagnetotail; this launches protons and electrons downwardtoward the Earth's atmosphere,where they form the aurora.GLOBAL MEASUREMENTS AND THE SOLAR WINDMAGNETOSPHERE INTERACTIONHeliophysicists seek to understand, and model, the processes governing the flow ofsolar wind mass, energy, andmomentum through the Sun Solar Wind - Magnetosphere Ionosphere system. With thisknowledge in hand, they willbe able to forecast geomagnetic storms, the most hazardous space weather eventsin the near-Earth environment.Storms enhance the fluxes ofenergetic particles within themagnetosphere to levels capable of harming spacecraft electronics, drive powerful currentsinto the ionosphere that causesurges in electrical power linetransmission, enhance exospheric densities and thereforedrag on low-latitude spacecraft, and modify ionosphericdensities in ways that severelyimpact GPS navigation and satellite communication.4太空 TAIKONGA host of mechanisms havebeen proposed to explain thenature of the solar wind-magnetosphere interaction, and inparticular the entry into, storage within, and release from themagnetosphere of solar windmass, energy, and momentum(Figure 2).Proposed magnetopause entry mechanismsinclude solar wind pressurevariations battering the magnetosphere, the Kelvin-Helmholtz(wind-over-water) instability onthe magnetopause, diffusiondriven by wave-particle interactions, and magnetic reconnection. Proposed magnetotailrelease mechanisms include ahost of plasma instabilities, e.g.ballooning or cross-tail currentdriven instabilities, and magnetic reconnection.In contrast to all the other mechanisms, reconnection predictsenhanced interactions duringintervals of southward interplanetary magnetic field (IMF)orientation. Therefore statisticalstudies of remote observationsdemonstrating that ionospheric convection, the strength offield-aligned currents into andout of the ionosphere, the likelihood of geomagnetic substorms, and the magnitudeof geomagnetic storms all increase for southward IMF orientations, point to reconnectionas the dominant mode of solarwind-magnetosphere interaction. Reconnection may bethe cause or consequence ofvarious plasma instabilities proposed to occur within the nearEarth magnetotail.Reconnection is a microphysical process with macrophysical consequences. The needto understand the microscale
physics underlying reconnection has led to the launch ofmultispacecraft missions likeISEE-1/2, Cluster, THEMIS,and MMS with ever decreasing interspacecraft separations.These missions have confirmedthe presence of the accelerated plasma flows, magneticfield components normal to themagnetopause and magnetotail current sheet, streaming energetic particles, and boundarylayers containing admixtures ofthe particle populations on bothsides of reconnecting currentsheets at the magnetopauseand within the magnetotail, justas predicted by reconnectionmodels.microsatellites capable of making in situ measurements at allrelevant locations.In the absence of any plans forsuch a constellation, imagerscan supply the global measurements needed to understand the nature of the solarwind-magnetosphere interaction. The boundaries seen insoft X-ray (and low energy neutral atom) images correspond toplasma density structures likethe bow shock, magnetopause,and cusps. Thus soft X-rayimagers can be used to tractthe inward erosion of the dayside magnetopause during thegrowth phase of geomagnetic substorms and the outwardmotion of this boundary following substorm onsets. The location of the magnetopause provides information concerningthe amount of closed flux withinthe dayside magnetosphere,the rate of magnetopause erosion or recovery provides information concerning the steadiness of reconnection, while thelocation of the portion of themagnetopause that moves provides information concerningthe component or antiparallelnature of reconnection.Soft X-ray imagers can also beused to track the equatorwardmotion of the cusps during thesubstorm growth phase andtheir poleward motion followingonset. Just as in the case of themagnetopause, cusp observations can be used to determinethe amount of closed flux within the dayside magnetosphere,the rates of erosion and recovery, the steadiness of reconnection, and the equatorial orWhile isolated single or closely-spaced multipoint in situmeasurements can be used toidentify reconnection eventsand study the microphysicsof reconnection, they cannot be used to distinguishbetween models in which reconnection is predominantlypatchy or global, transient orcontinuous, triggered by solar wind features or occurringin response to intrinsic current layer instabilities, component and occuring on theequatorial magnetopause orantiparallel and occurringon the high-latitude magnetopause. Nor can isolatedmeasurements be used todetermine the global stateof the solar wind-magnetosphere interaction, as measured by the rate at whichclosed magnetic flux is Fig. 2: A snapshot of the complex plasma density structures generated byopened or open flux closed. the solar wind-magnetosphere interaction according to the Lyon-FedderFor all of these tasks, and Mobarry (LFM) global magnetohydrodynamic simulation (C. Goodrich, permany more, global observa- sonal communication). Color shading indicates the density in the noon-midtions are needed. It would, night meridional plane, while lines in the lower density inner magnetospherichowever, be a major under- cavity suggest the magnetospheric magnetic field configuration. The insettaking to launch a flotilla of in the lower right corner shows corresponding predictions for auroral activityin the northern hemisphere.太空 TAIKONG5
high-latitude location of reconnection.Global auroral images from ahigh inclination, high altitude,spacecraft provide an excellent complement to soft X-rayimages. The dimensions of theauroral oval indicate the openmagnetic flux within the Earth’smagnetotail.Poleward andequatorward motions of thedayside and nightside auroraloval provide crucial information concerning the occurrences and rates of reconnectionat the dayside magnetopauseand within the Earth’s magnetotail. Ground-based auroralimagers frequently observetransients in the dayside aurorawhich can be interpreted as evidence for bursty reconnectionand/or the Kelvin-Helmholtzinstability. Global imagers areneeded to determine the occurrence rates and extents ofthese transients, which in turndetermine their importance tothe solar wind-magnetosphereinteraction.Observations ofthe nightside auroral oval canbe used to pinpoint the time ofsubstorm onset, determine theextent of the reconnection linein the magnetotail, and distinguish between steady, bursty,and sawtooth modes of reconnection in the magnetotail.Global auroral images can beused to test the recently proposed hypotheses that plasmaflows (and aurora) originatingwithin the dayside oval andstreaming across the polar captrigger substorm onset whenthey reach the nightside oval.Finally, measurements of thesolar wind plasma and magnetic field input to the magnetosphere by a monitor locatednear Earth are essential for theabove studies, because havingsuch a monitor reduces concerns regarding the arrival timesof possible solar wind triggersfor magnetospheric events andreduces concerns regarding thedimensions of solar wind structures transverse to the SunEarth line. In situ measurements from the same plasmaand magnetic field instrumentson the observing spacecraft onan elliptical polar orbit can alsoplay a crucial role in validatingthe inferences concerning processes at the magnetopauseand in the magnetotail that aredrawn from the soft X-ray andauroral imagers.A NOVEL METHOD TO IMAGE THE MAGNETOSPHERESolar wind charge-exchange(SWCX) occurs when highly ionized species in the solar windinteract with neutral atoms.An electron from the neutral istransferred to the ion, initially ina highly excited state. On relaxation to the ground state one ormore photons are emitted, usually in the extreme ultravioletor the soft (low energy) X-ray.The energy band below 0.5 keVis extraordinarily rich in SWCXemission lines from a largenumber of ionization states of alarge number of species, whilethe 0.5-2.0 keV band is dominated by a few strong linesdue to charge-exchange byO 7, O 8, Ne 9, and Mg 11. Thereare many sources of SWCXemission in the heliosphere,including comets and the neutral interstellar medium thatflows through the solar system.Typically the brightest source of6太空 TAIKONGSWCX is that due to the Earth’sexosphere, which is primarilyhydrogen, interacting with theshocked, compressed, solarwind in the magnetosheath.SWCX emission due to themagnetosheath was first observed by ROSAT, though itssource was a mystery at thetime. ROSAT scanned great circles through the ecliptic poles,with each scan overlapping 95% of the previous scan.Comparison of successivescans revealed strong temporalvariations with scales of hoursto days that were dubbed the“Long Term Enhancements”(LTE). A large-scale minimization routine was used to isolatethe LTE, though the absoluteminimum level could not be determined. Comparison of theLTE rate during an observationof the Moon to the flux from thedark side of the Moon suggested that the bulk of the emissionwas cis-lunar. The LTE rateswere later shown to be stronglycorrelated with the solar windflux, and thus likely to be dueto SWCX.The SWCX flux is given by theintegral along the line of sightof ς(nnnpvrel) ςQ where nn isthe density of neutral particles,np is the density of solar windprotons, vrel is their relative velocities, and ς contains the information about ion abundances, interaction cross-sections,branching ratios, etc. Q can bedetermined from MHD modelsof the magnetosheath. Howeverthe value of ς for strong lines issometimes quite uncertain, andfor weak lines it is usually completely unknown. A recent comparison of the ROSAT LTE rateswith the Q determined from
models for the solar wind duringthe ROSAT observations hasled to a determination of ς forthe ROSAT ¼ keV band, whichallows one to scale any MHDmodel of the magnetosheathto X-ray emissivity. Thus, onecan feel relatively confident ofthe simulations of instrumentalviews of the magnetosheath.Three different groups havebeen simulating the X-ray emission from the magnetosheath.Although there has as yet notbeen a detailed comparison, itis clear that useful parameters,such as the magnetopausedistance, can be determinedfor a large range of observingaspects, so long as the spacecraft is sufficiently far from theEarth. Determining the magnetopause distance is particularly interesting, not only for thescience goals described above,but also given the divergentrecent results from astrophysical missions.SWCX emission from the magnetosheath has been observedby all recent astrophysicalX-ray observatories. The XMMNewton observatory is in highEarth orbit and sometimes observes through the nose of themagnetosheath. Given the expected SWCX X-ray brightnessof the dayside magnetosheathsuch observations can serve asan important check on our simulations. Discrepancies whencomparing predicted and observed emission strengths maybe due to errors in the distances to the magnetopause predicted by the MHD models. Thedifferences in the underlyingMHD codes demonstrate theneed for X-ray observations toconstrain and validate the MHDresults.It should also be noted thatstudies of the X-ray emissionfrom the magnetosheath require wide-field imagery, anarea of current interest in astrophysics. For low to mediansolar wind conditions, the signal from the magnetosheath isonly a few times stronger thanthe soft X-ray background.Thus, study of the X-ray emission from the magnetosheathwill require astrophysical techniques and expertise. In return,astrophysics is deeply interested in detecting the Warm HotIntergalactic Medium throughO 6 and O 7 emission, and thusdepends upon researches suchas these to characterize andremove the SWCX emission.Understanding the SWCX fromthe magnetosheath will necessarily require interdisciplinarystudy.AURORA AND SUBSTORMA visible manifestation of thesolar wind-mag
recognized the very high scientific value of the mission, and raised constructive comments and suggestions on the mission concept, payloads key techniques, and data product. They concluded that the SMILE mission has complementary objectives to existing or future solar space plasma missions. Therefore, the SMILE mission is
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