Specialissueeditorial-PlasmaInteractionswithSolarSystemObjects:AnticipatingRosetta,MavenandMarsOrbiterMission

A.J.Coates1.2,A.Wellbrock1,2andM.Yamauchi3

1.MullardSpaceScienceLaboratory,UniversityCollegeLondon,HolmburyStMary,Dorking,SurreyRH56NT,UK

2.CentreforPlanetaryScienceatUCL-Birkbeck,GowerSt,LondonWC1E6BT

3.SwedishInstituteofSpacePhysics,POBox812,SE-98128Kiruna,Sweden

Abstract

Withinoursolarsystem,theplanets,moons,cometsandasteroidsallhaveplasmainteractions.Theinteractiondependsonthenatureoftheobject,particularlythepresenceofanatmosphereandamagneticfield.Eventhesizeoftheobjectmattersthroughthefinitegyroradiuseffectandthescaleheightofcoldionsofexosphericorigin.Italsodependsontheupstreamconditions,includingpositionwithinthesolarwindorthepresencewithinaplanetarymagnetosphere.SoonafterESA'sRosettareachedcometChuryumov-Gerasimenko,NASA'sMavenandISRO'sMarsOrbiterMission(MOM)reachedMars,andESA'sVenusExpressmissionwascompleted,thisissueexploresourunderstandingofplasmainteractionswithcomets,Mars,Venus,andmoonsinthesolarsystem.Weexploretheprocesseswhichcharacterisetheinteractions,suchasionpickupandfielddraping,andtheireffectssuchasplasmaescape.Papersarebasedondatafromcurrentandrecentspacemissions,modellingandtheory,asweexploreourlocalpartofthe'plasmauniverse'.

Introduction

Theissuerepresentspredictionsformissionswhichhaverecentlyreachedtheirtargets.Someofthepaperswerepresentedatthe6thAlfvenConferenceonPlasmaInteractionswithSolarSystemObjects:AnticipatingRosetta,MAVENandMarsOrbiterMission,heldatUniversityCollegeLondonon7-11July2014(seeFigure1).Theconferenceincluded65oral(including19invited)papersand19posterpresentations.Thestructureoftheconferenceincludedsectionsoncomets,Mars,Venus,moons,missionsandcomparisonsbetweentheinteractions.Thespecialissuewasopentopaperpresentersandtothewiderscientificcommunity,andhasresultedinthe12papersincludedhere.

Summaryofpapers

Comets:

Ionpickupisoneessentialelementofacomet’sinteractionwiththesolarwind.AsacometnearstheSun,waterandothermoleculessublimeawayfromthenucleus.Theseareeventuallyionisedandthenareacceleratedbythesolarwindconvectiveelectricfield,andtheythengyratearoundthefieldtoformaringdistributioninvelocityspace.Thisisunstableandashelldistribution,andplasmawaves,result.Severalboundariesoccurincludingabowshockandacontactsurface.

Matteinietal.(2015)usehybridsimulationstostudytheionpickupprocess.Bisphericalandnon-gyrotropicdistributionsarestudied,andtheresultingwavesserveaspredictionsforcomet67P(Churyumov-Gerasimenko,Rosetta’starget).Gunelletal.(2015)usehybridsimulationsbutalsoincludeachargeddustpopulation,andmakepredictionsforthemorphology,effectivenessandtimingofdustaccelerationat67P.Nordheimetal.(2015)modelthesurfacepotentialandelectricfieldatthesurfaceof67Pat3.5and3AU,findingpositivepotentialsonthesunlitsideandthepossibilityofsubstantialcharginginthewake.

Venus:

Venushasadenseatmosphereandanionosphere,butnomagneticfield.Thesolarwindinteractionregionincludestheionopause,inducedmagnetosphere,drapedmagneticfield,asheathregionwiththermalisedanddeceleratedsolarwind,andabowshock.

Luhmannetal.(2015)re-examinePVOandVenusExpressdataandcomparewithMHDsimulationstoexploreapreviouslydiscoveredhemisphericbiasinthemagneticfieldlowinthewakeregion.Theyfindthatalthoughsomeaspectsoftheresultsareconsistentwithaweakglobalmagneticfield,thiscanberuledoutinpreferenceforothereffects,suchashemisphericasymmetriesintheinteractionorthepossibilityofexternalfielddiffusionthroughtheplanet.

Mars:

TheMarsinteractionwiththesolarwindissomewhatsimilartoVenusexceptthattheratiooftheiongyroradiustotheobstaclesizeislarger,andMarshascrustalmagneticfieldswhichdistorttheinteractiontosomeextent.Atmosphericescapeisakeyissueoverthe3.8billionyearssinceMarslostitsglobalmagneticfield,studiedoverthelastsolarcyclebyMarsExpressandstudiesarebeginningwiththecomprehensively-instrumentedMAVENmission.

Holmstrometal.(2015)useahybridmodeltostudytheMars-solarwindinteractiononalargescale,findingapolarplumeofionsandescapealongthetail.Theescaperateincreasesastheassumedproductionrateincreases,andcomparisonsaremadewithacomet.

Yamauchietal.(2015a)presentMarsExpressASPERA-3observationsoftheMartianforeshock.Theyfindprotonsand,forthefirsttime,oxygenionsupstreamofthebowshock.Theydiscusshowthismaybeanadditionalescapemechanisminvolvingaccelerationinthebowshock-sheathregion.

Yamauchietal.(2015b)studypickupprotonsinthesolarwindnearMarsover8years,findingthattheamountofpickupionsisrelatedtotheSun-Marsdistance.ThisindicatesthattheUVfluxplaysamajorroleinpickupionproductionbutothereffectsareimportanttoo,atopicforMAVEN.

Xuetal.(2015)studytheproductionofphotoelectronsintheMartianionosphereusingthreedifferentsolarirradiancemodels.Theyfindafactor~2variationinthemodelledphotoelectronfluxes.

Vaisberg(2015)reviewsionescapemechanismsatMars,includingpickupions,tailescapeandionosphericescape.Simulationsarealsodiscussed,discrepanciesbetweenobservationsandsimulationsareidentified,andsuggestionsaremadeforfutureresearchincludingMAVENandMOM.

Fraenzetal.(2015)useASPERA-3andMARSISdatatoestimateMartianescaperates.Byincludingthepreviouslyunderestimatedcoldioncomponenttheyfindhighertotalescaperates(2.8±0.4x1025s-1)thanpreviouslyreported,andinagreementwithsomemodellingstudies.

Alhoetal.(2015)useahybridsimulationtostudytheshieldingeffectofanearlyMarsmagneticfield,startingatareasonableassumedlevelanddecliningtotoday’sinducedmagnetosphere.TheyfindsignificantshieldingispresentonearlyMarsagainstsolarenergeticparticles,causedbythesolarwindinteraction.

Moons:

Poppeetal.(2015)studythechargingenvironmentofairlessbodiesforarangeofanglesbetweentheincidentflowandthesolardirection,relevantforthemoonsofMarsandsomeouterplanetmoonsaswellasEarth’sMoon.Theyfindsomecomplexpotentialstructurescanexistnearthesurfaces.

Acknowledgments

WewishtothanktheScientificandLocalOrganisingCommitteesfortheirhelpwiththeconference,andtherefereesfortheirhelpwiththeindividualpapers.WeparticularlythanktheLOCfortheirhardworkbefore,duringandafterthemeeting.Wethankthesponsorsofthemeetingfortheirgeneroussupport–ESA(includingtheRosettaproject),RAS,STFC,UKSA,RosettaprojectandADS(UK)

References

Matteini,L.,S.J.Schwartz,andP.Hellinger,Cometaryioninstabilitiesinthesolarwind,PSSinpress2015

Gunell,H.,I.Mann,C.S.Wedlund,E.Kallio,M.Alhod,H.Nilsson,J.DeKeyser,F.Dhooghe,R.Maggiolo,Accelerationofionsandnanodustatacometinthesolarwind,PSSinpress2015

Nordheim,T.,G.H.Jones,J.S.Halekas,E.Roussos,A.J.Coates,Surfacechargingandelectrostaticdustaccelerationatthenucleusofcomet67Pduringperiodsoflowactivity,PSSinpress2015

Luhmann,J.G.Y-J.Ma,M.N.Villarreal,H-Y.Wei,andT-L.Zhang,TheVenus-SolarWindInteraction:IsitPurelyIonospheric?,PSSinpress2015

Holmstrom,M.,X.-D.Wang,Marsasacomet:Solarwindinteractiononalargescale,PSSinpress2015

YamauchiM.,R.Lundin,R.A.Frahm,J.-A.Sauvaud,M.Holmström,andS.Barabash,OxygenforeshockofMars,PSSinpress2015a

YamauchiM.,T.Hara,R.Lundin,E.Dubinin,A.Fedorov,J.-A.Sauvaud,R.A.Frahm,R.Ramstad,Y.Futaana,M.Holmstrom,andS.Barabash, SeasonalVariationofMartianPick-upIons:EvidenceofBreathingExosphere,PSSinpress2015b

Xu,S.,M.W.Liemohn,W.K.Peterson,J.Fontenla,P.Chamberlin,ComparisonofDifferentSolarIrradianceModelsfortheSuperthermalElectronTransportModelforMars,PSSinpress2015

Vaisberg,O.,Marsatmosphericlossesinducedbythesolarwind:comparisonofobservationswithmodels,PSSinpress2015

Fraenz,M.,E.Dubinin,D.Andrews,S.Barabash,H.Nilsson,A.Fedorov,ColdIonEscapefromtheMartianIonosphere,PSSinpress2015

Alho,M.,S.McKenna-Lawlor,E.Kallio,PaleoMarsEnergeticParticlePrecipitation,PSSinpress2015

Poppe,A.R.,M.I.Zimmerman,J.S.Halekas,W.M.Farrell,Theelectrostaticplasmaenvironmentofasmallairlessbodyundernon-alignedplasmaflowandUVconditions,PSSinpress2015

Figure1-Attendeesatthe6thAlfvénConference,10July2014