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ManuscriptpreparedforJ.Name withversion4.2oftheLATEXclasscopernicus.cls. Date: 31January2017 Planar magnetic structures in coronal mass ejection-driven sheath regions 7 1 ErikaPalmerio1,EmiliaK.J.Kilpua1,andNeelP.Savani2,3 0 1DepartmentofPhysics,P.O.Box64,UniversityofHelsinki,Finland 2 2GoddardPlanetaryHeliophysicsInstitute(GPHI),UniversityofMaryland,BaltimoreCounty,Maryland,USA n 3NASAGoddardSpaceFlightCenter,GreenbeltMD20771,USA a J 0 Abstract. Planar magnetic structures (PMSs) are periods 1 Introduction 3 inthesolarwindduringwhichinterplanetarymagneticfield ] vectorsarenearlyparalleltoasingleplane. Oneofthespe- CoronalMassEjections (CMEs)aregiantclouds ofplasma R cific regions where PMSs have been reported are coronal and magnetic field that are expelled from the Sun into the S mass ejection (CME)-driven sheaths. We use here an auto- heliosphere. CMEs often travel faster than the ambient so- . h mated method to identify PMSs in 95 CME sheath regions lar wind and if their speeds exceed the local magnetosonic p observed in-situ by the Wind and ACE spacecraft between speed,ashockwavewilldevelop.Aturbulentregionofcom- - 1997 and 2015. The occurrence and location of the PMSs pressed and heated plasma between the shock front and the o r arerelatedtovariousshock,sheathandCMEproperties. We leading edge of the CME is called a sheath region. CME- t s find that PMSs are ubiquitous in CME sheaths; 85% of the drivensheathshaveanimportantroleinsolar-terrestrialstud- a studiedsheathregionshadPMSswiththemeandurationof ies; the parts in the immediate downstream of the shock [ 6.0hours. Inaboutone-thirdofthecasesthemagneticfield may contribute in the acceleration of solar energetic parti- 1 vectors followed a single PMS plane that covered a signif- cles (Manchester et al., 2005), and when interacting with v icant part (at least 67%) of the sheath region. Our analysis the Earth’s magnetosphere, sheaths are capable of driving 9 givesstrongsupportfortwosuggestedPMSformationmech- significant geomagnetic activity. In fact, a large number of 3 7 anisms: the amplification and alignment of solar wind dis- CME-driven storms are pure-sheath induced storms (Tsu- 8 continuities near the CME-driven shock and the draping of rutani et al., 1988; Huttunen et al., 2002; Siscoe et al., 0 themagneticfieldlinesaroundtheCMEejecta.Forexample, 2007). Large amplitude magnetic field and ram pressure 1. we found that the shock and PMS plane normals generally variationswithinthesheathdrivestrongactivityinparticular 0 coincided for the events where the PMSs occurred near the inthehigh-latitudemagnetosphere(HuttunenandKoskinen, 7 shock (68% of the PMS plane normals near the shock were 2004),whichmayleadtolargegeomagneticallyinducedcur- 1 separatedbylessthan20◦fromtheshocknormal),whilede- rents(GICs)(Huttunenetal.,2008;Savanietal.,2013). : v viationswereclearlylargerwhenPMSsoccurredclosetothe CME-driven sheaths are combinations of a propagation Xi ejectaleadingedge. Inaddition, PMSsneartheshockwere sheath, where the solar wind flows around the obstacle in generally associated with lower upstream plasma beta than a quasi-stationary regime, and an expansion sheath, where r a thecaseswherePMSsoccurredneartheleadingedgeofthe theobstacleisexpandingbutnotpropagatingwithrespectto CME.Wealsodemonstratethattheplanarpartsofthesheath the solar wind (Siscoe and Odstrcil, 2008). The sheath re- containahigheramountofstronglysouthwardmagneticfield gionformsgraduallyasthelayersofinterplanetarymagnetic thanthenon-planarparts,suggestingthatplanarsheathsare field (IMF) and solar wind plasma accumulate over several morelikelytodrivemagnetosphericactivity. daysittakesforaCMEtotravelfromtheSuntotheEarth. Hence,CMEsheathsoftenexhibitacomplexinternalstruc- ture,whichmakesunderstandingtheirformationandpredict- ingtheirgeoeffectivityparticularlydifficult. However, planar magnetic structures (PMSs) are fre- quently reported in CME sheaths (e.g., Nakagawa et al., Correspondenceto: ErikaPalmerio 1989; Neugebauer et al., 1993; Savani et al., 2011). The (erika.palmerio@helsinki.fi) magneticfieldvectorsinaPMSarecharacterizedbyabrupt 2 Palmerioetal.: PMSsinCMEsheaths changes,buttheyallremainnearlyparalleltoasingleplane Section2wepresentthedatasetsandthemethods. InSec- overtimeintervalsfromseveralhourstoaboutoneday.Such tion 3 we present statistical results on the PMS distribution aplaneincludestheParkerspiraldirection,butisinclinedto and dependence on driver and sheath properties, on the re- the ecliptic plane from 30◦ to 85◦ (Nakagawa et al., 1989). lationship between PMS and shock normal orientations and Hence,PMSscanbeconsideredaslaminarstructures,com- on the geoeffectivity. Finally, in Section 4 we discuss and posedofparallelplaneshavingdifferentorientationswithre- summarizeourresults. specttotheinterplanetarymagneticfield(IMF).DuringPMS events, ion densities and temperatures and plasma beta are usuallyhigherthanintheambientplasmaandthemagnetic 2 Dataandmethods fieldishighlyvariablebothinmagnitudeanddirection(Nak- agawa,1993). 2.1 Data It has been suggested that PMSs are caused primarily by compressionalprocesses(e.g.,JonesandBalogh,2000;Far- Our analysis involves 95 CME-driven sheath regions. The rugia et al., 1990). Hence, CME sheaths provide a natural list of sheath regions is given in Table S1, Supplementary environment for the formation of PMSs. Two basic mech- information. The investigated period covers the years from anisms are proposed to explain how PMSs are generated in 1997 through 2015, i.e., solar cycle 23 and the rising phase CME–sheaths. Firstly,themagneticfieldlinesdrapearound andmaximumofcycle24. theCMEejectaandforcesolarwindmicrostructuresanddis- The sheath region list was compiled with the help of the continuitiestoalignthemselvesparalleltothesurfaceofthe online UCLA interplanetary CME catalog (http://www-ssc. ejecta (Farrugia et al., 1990). As the magnetic field piles igpp.ucla.edu/∼jlan/ACE/Level3/),whichcoversthetimein- upattheejectaleadingedge,thecompressionoftheIMFre- tervalof1995–2009.Forthe2010–2015periodweidentified ducesthefieldvariationsinthedirectionperpendiculartothe interplanetaryCMEsusingasimilarapproachusedtocom- surfaceoftheejecta,resultinginastructurethatisnearlypar- piletheUCLAcatalog(Jianetal.,2006). Weselectedonly alleltotheCMEsurface(Neugebaueretal.,1993). Thesec- thecaseswherethetimeoftheleadingedgeoftheejectawas ond mechanism is related to the compression of solar wind well-defined,i.e.therewasaclearandrelativelysharptransi- discontinuitiesattheCMEshock,andtheiralignmentinthe tionfromtheturbulentsheathtotheejecta.Fordiscussionon downstream region parallel with the plane of the shock. In thetypicalsheathandejectaproperties,andthedifficultiesin particular, PMSs are observed in the downstream regions determiningtheCMEleadingedgetimes,seee.g.,Richard- of quasi-perpendicular shocks (Jones et al., 2002) when the sonandCane(2010)andKilpuaetal.(2013),andreferences AlfveńMachnumberM >2andtheupstreamplasmabeta A therein. The shock times and properties are obtained from 0.05<β<0.5(Kataokaetal.,2005). theHeliosphericShockDatabase,developedandmaintained AsstronglysouthwardperiodsofIMFarerequiredforef- attheUniversityofHelsinki(http://ipshocks.fi/). ficient energy transfer from the solar wind to the magnetosphere(e.g.,Dungey,1961;Akasofu,1981),thelargeampli- Solar wind and IMF measurements are taken primarily tudeout-of-eclipticmagneticfieldvariationsrelatedtoPMSs fromtheWindspacecraft. WindwaslaunchedinNovember areexpectedtocausesignificantspaceweathereffectsatthe 1994.ItspentitsfirstyearsattheLagrangianpointL1andin Earth. For example, strongly southward IMF periods dur- 1999itacquiredacomplexpetal-shapedorbitthatbroughtit ing the PMS-related periods in a sheath drove a significant furtherawayfromtheSun-Earthline. Since2004,Windhas partoftheintensestormonMarch17,2015(Kataokaetal., been operating at L1. The data from the Advanced Com- 2015).However,itisstillunclearhowplanarityinthesheath position Explorer (ACE) spacecraft is used during Wind’s affects geoeffectivity, how PMSs are distributed within the magnetospheric visits and datagaps. ACE was launched in sheathandwhatistheiroriginindifferentpartsofthesheath. August1997andithasbeenoperatingclosetoL1. AbetterknowledgeofPMSdistributionandpropertiesmay WeusedatafromtheWindMagneticFieldsInvestigation help to develop early space weather forecast techniques. In (MFI)(Leppingetal.,1995)andtheWindSolarWindExper- addition, PMS distribution and properties may also give in- iment (SWE) (Ogilvie et al., 1995). MFI data are available formationontheevolutionofCMEs. at 60-second resolution and SWE data are registered about In this paper we investigate how PMSs are distributed every 90 seconds. From ACE, we use Magnetic Field In- withinCME-drivensheathregionsandcorrelatetheiroccur- vestigation (MFI) (Smith et al., 1998) data, available from renceandextensionwiththeshock,sheathandejectacharac- September 1997, and ACE Solar Wind Electron Proton Al- teristics. Wealsostudytherelationshipbetweentheorienta- phaMonitor(SWEPAM)(McComasetal.,1998)data,avail- tionsofthePMSplaneandtheCMEshocknormals,tocheck able from February 1998. MFI and SWEPAM data are 16- whethertheplaneoftheshockfrontisdrivingthePMSori- secondand64-secondlevel2data,respectively. BothWind entation,asassumedinSavanietal.(2011),andcomparethe andACEdataareobtainedfrom theNASAGoddardSpace amount of significantly southward IMF in planar and non- Flight Center Coordinated Data Analysis Web (CDAWeb, planar cases. The structure of this paper is as follows: in http://cdaweb.gsfc.nasa.gov/). Palmerioetal.: PMSsinCMEsheaths 3 We apply the MVA and detect PMS candidates through an automated procedure. The aim of the method is to scan, within the sheath, all the possible combinations of PMS intervals with 5 minutes time steps, starting from the largest duration(fullsheath)totheminimumduration(1hour).This algorithmismoreeffectivethanasimpleslidingwindow,as twoadjacentintervalsthatsatisfytheMVAcriteriamayactu- allycorrespondtotwoseparatePMSswithdifferentplanes. Hence, our method avoids mixing two different PMS intervals. The algorithm is implemented as follows. First, the MVAisperformedoverthewholesheathandtheeigenvalue Fig.1: Anexampleofaθ-φdiagram. Thedirectionnormaltothe ratio and |Bn|/B are stored for final comparison. There- PMSplaneisdisplayedbytheblue“∗”andtheplaneperpendicular afterafive-minuteintervaliscutfromtheleadingedge(LE) to it is represented by the red curve. The black scatter points are side, anewMVAisappliedanditsresultisagainrecorded. the magnetic field vectors as observed by Wind within the sheath This reduced window is then shifted five minutes towards regionfollowinganinterplanetaryshockonJanuary10,1997. the LE (at the first time interfacing the edge) and MVA is applied again. Then the initial full-sheath window is short- ened by additional five minutes from the LE side, MVA is 2.2 PMSidentificationmethod applied,andthereafterthe10-minreducedwindowisshifted again by subsequent five-minute steps until it reaches again We describe next our approach to identify PMSs within the theLEinterface. Theentireprocedureisrepeated, decreas- sheath regions. As discussed in Introduction, a PMS is de- ingeach timetheinitial full-sheathwindowlength byaddi- fined as an interval where the IMF vectors remain paral- tional five minutes until the minimum duration of one hour lel to a fixed plane for an extended time period (of the or- is reached. At each step the eigenvalue ratio and |B |/B n der of hours). Let’s assume that the IMF field vectors in are recorded and finally compared. The PMS intervals are the Geocentric Solar Ecliptic (GSE) coordinates are B≡ the longest non-overlapping intervals fulfilling the require- (BX,BY,BZ)≡(Bcosθcosφ,Bcosθsinφ,Bsinθ)(φandθ ments described above. In a case of multiple windows of are the IMF longitude and latitude, respectively) and that the same length satisfying our criteria, we select the inter- they are all parallel to a fixed plane with the normal n≡ val which features the lowest |B |/B. Note that if the en- n (nX,nY,nZ). Nowtherelationbetweenθandφis tiresheathwouldbeonesinglePMS,alreadythefirstMVA wouldgivethebestresult. Wealsorequirefortheselection nxcosθcosφ+nycosθsinφ+nzsinθ=0. (1) that the longitudinal coverage of the magnetic field vectors along the PMS curve has to exceed 90◦ (Jones and Balogh, Hence, PMSs are identified as the periods where in the θ-φ 2000). spacetheIMFvectorsaredistributedinthecloseproximity ToinvestigatethedistributionofPMSswedividesheaths of the curve presented by Eq. (1), see example from Fig. intothreesub-regions(Fig. 2): Near-Shock,Mid-Sheathand 1. Moreover, in a PMS event IMF data points in a θ-φ dia- Near-Leading-Edge (Near-LE) regions. Then the sheath re- gramcoverawiderangeofφvalues(Nakagawaetal.,1989), gionsareplacedintofivegroupsaccordingtothefollowing meaningthattheycanassumealmostanydirectionwithinthe criteria: plane. TodeterminetheorientationofthePMSplaneweusethe – Group1–noPMS:NoPMSsarefound(0%PMScov- minimumvarianceanalysis(MVA)(SonnerupandScheible, erageinallthreesub-regions) 1998). TheminimumvariancedirectionoftheIMFvectors – Group2–Near-ShockPMS:PMSsstartsatthevicin- correspondstothenormalofthePMSplane. Theratioofthe ityoftheIPshock(latestonehourfromtheshock)and intermediate(λ )tominimum(λ )eigenvaluecanbeusedas 2 3 theycoveratleast67%oftheNear-Shockregion.PMSs aproxyofthequalityoftheMVAandweuseherethecon- mayextendintheMid-Sheathregion sistency requirement λ /λ ≥5 (e.g., Savani et al., 2011). 2 3 Thequalityoftheresultincreaseswiththeincreasingeigen- – Group3–Mid-SheathPMS:PMSscoveratleast67% valueratioandthesolutionisdegeneratewhenλ2(cid:39)λ3. The oftheMid-Sheathregion,butlessthan67%oftheNear- two-dimensionalityofthefieldvectorsinPMSeventscanbe ShockandNear-LEregions. confirmed through the value of |B |/B (Nakagawa, 1993), n where B is the magnetic field magnitude and B =B·n – Group4–Near-LEPMS:PMSsstartsatthevicinity n is the component of the magnetic field perpendicular to the oftheCMEleadingedge(latestonehourfromthelead- PMS plane. We impose a requirement that |B |/B<0.2 ing edge) and they cover at least 67% of the Near-LE n (e.g.,JonesandBalogh,2000andKataokaetal.,2005). region. PMSsmayextendintheMid-Sheathregion 4 Palmerioetal.: PMSsinCMEsheaths �� Fig.3: Distributionoftheinvestigatedsheathregionsaccordingto thepresenceandpositionofPMSs(fortheexplanationofdifferent groupsseeSection2.2). Fig.2: AsheathregiononMay18,2002observedbyWind. The bluelinesindicatetheCMEshockandtheejectaleadingedge(LE), eventsrangefrom1.3to20.5hours,withanaverageduration respectively,whiletheredlinesseparatethesheathinto33%-33%- of6.0hours. 33%sub-regions: Near-Shock,Mid-SheathandNear-LEdomains. InthemajorityofthecasesfeaturingPMSs(74oftotal82) Theshownparametersarefromtoptobottom: (a)magneticfield magnitude,(b)θand(c)φcomponentsinGSEangularcoordinates, wefoundonlyonePMSplane.Sevensheathregionsfeatured (d)solarwindspeedand(e)protondensity. twodistinctPMSintervalswithdifferentplaneorientations. Onlyoneevent(July17,2005)hadthreePMSperiodswith different plane orientations. The sheaths with two different – Group 5 – Full-PMS: PMSs cover at least 67% of all PMS planes are distributed in the following way: three in threeregions Group2, twoinGroup3, oneinGroup4andoneinGroup 5.TheeventwiththreedifferentPMSplanesdoesnotbelong Fromthetotalof95analyzedevents,eightsheathswerenot toanyofourgroups, sincePMSdurationsdidnotmeetour included in any of the above described groups, i.e. at least 67%-coveragecriterion. onePMSwasfound,butitscoveragewas<67%inallthree regions. 3.2 Dependenceonshock,sheathandCMEejectaprop- erties 3 Results Figure 4 shows the results of our statistical analysis on the relationbetweenthePMSdistributionandtheshock,sheath 3.1 PMScoverage andejectaproperties. Figure 3 shows that 86% of the investigated sheaths (82 of 3.2.1 Shockparameters total 95) present at least one PMS. The 67%-coverage criterion was satisfied for 43% of the Near-Shock regions (41 The averages of selected shock parameters are presented in events),66%oftheMid-Sheathregions(63events)and53% thetoprowofFig. 4. ThemagnetosonicMachnumberM ms (50 events) of the Near-LE regions. The distribution of the (panel 4a) is an indicator of the strength of the shock (e.g., events in five PMS groups (see Section 2.2) is also seen Burgess,1995),i.e. theestimateoftheenergyprocessedby from Fig. 3: for 13 sheaths no PMSs were found (Group theshock.ItshouldbenotedthatM dependsontheshock ms 1),15sheathsareintheNear-ShockPMSgroup(Group2), angleandthemagnetosonicspeed,whichmayhavelargeer- nineintheMid-Sheathgroup(Group3), 24intheNear-LE rors. Hence,weshowtheresultsalsoforthedownstream-togroup(Group4),and26eventsarethecaseswherethe67%- upstream magnetic field magnitude ratio (panel 4b), which coverage criteria was satisfied in all three sub-regions, i.e. typicallyincreaseswithincreasingM (e.g.,Burtonetal., ms “full PMS” cases (Group 5). In addition, as mentioned in 1996). Panels(a)and(b)ofFig. 4indeedshowverysimilar Section 2.2, there were eight events featuring PMS, but for characteristics. ThehighestvaluesofM andthemagnetic ms which the 67%-coverage criterion was not satisfied in any fieldratiosareattributedtoGroup2(Near-Shock)andGroup ofthethreesub-regions. ThedurationsoftheidentifiedPMS 3(Mid-Sheath). Palmerioetal.: PMSsinCMEsheaths 5 (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) Fig.4: TheinvestigatedparametersandtheirrelationtoPMSgroups. Histogramsshowtheaverages, andtheerrorbarsshowtheextent of one standard deviation. First row – shock parameters: (a) magnetosonic Mach number, (b) downstream-to-upstream magnetic field magnituderatio,(c)upstreamplasmabeta,(d)shocknormal. Secondrow–sheathparameters: (e)averagemagneticfieldmagnitude,(f) averageprotonnumberdensity,(g)averageplasmabeta,(h)sheaththickness.Thirdrow–CMEejectaparameters:(i)leadingedgemagnetic fieldmagnitude,(j)leadingedgeprotonnumberdensity,(k)differencebetweenleadingedgeandupstreamspeed,(l)velocitygradient(only forexpandingCMEejecta). Theupstreamplasmabetaβ (panel4c),i.e.,theratiobe- carryparticlesthroughitrelativelyeasily,resultinginbroad up tweenplasmaandmagneticpressure,describestheupstream andgradualtransitionsintheplasmaandmagneticfieldpa- plasma state. The histogram shows that the lowest values rameters across the shock (e.g., Burgess et al., 2005). In ofβ arefoundforGroup2(Near-Shock),whereβ ≈1, contrast,atquasi-perpendicularshocks(i.e. θ >45◦),the up up Bn whilethehighestvaluesbelongtoGroup3(Mid-Sheath)and magnetic field lines are more parallel to the surface of the Group5(Full-PMS),withβ ≈2. shock and the particles gyrating along the field lines reflect up backtotheshock, resultinginasteepchangeinplasmapa- The shock angle θ (panel 4d) between the upstream Bn rametersandinthemagneticfieldmagnitudes(e.g., Baleet magneticfield directionand the shocknormal describesthe al.,2005). Panel4dshowsthatnon-planarsheaths(Group1) natureandstructureoftheIPshock.Atquasi-parallelshocks areassociatedwiththemostparallelshockswhencompared (i.e. θ <45◦) the magnetic field lines crossing the shock Bn 6 Palmerioetal.: PMSsinCMEsheaths toplanarsheaths. Themostperpendicularshocksarefound ejecta, are taken into account in Fig. 4l. This histogram forGroup2(Near-Shock)andGroup3(Mid-Sheath),witha showsasimilartrendtothepreviousone(panel4k)andthe meanvalueθ ≈60◦. strongest expansion is found for events in Group 3 (Mid- Bn Sheath) and Group 4 (Near-LE), while the events in Group 3.2.2 Sheathparameters 2 (Near-Shock) have on average the weakest expansion. In total there were five events (6%) with positive velocity gra- The mid row of Fig. 4 shows the average magnetic field dients, i.e. the CME ejecta was compressed in such cases. magnitude, plasma density and plasma β within the sheath All these events are full-PMS cases. Two of the ejecta had aswellasthesheaththickness(i.e.,theaveragesheathdura- nearlyzerovelocitygradients.Theassociatedsheathsbelong tionmultipliedbytheaveragespeedwithinthesheath). The toGroup3(Mid-Sheath)andtoGroup5(Full-PMS). magnetic field magnitudes (panel 4e) are rather similar for all investigated groups. The lowest value is found for non- 3.3 Plasmaparametersinthesheathsub-regions planar cases and the highest value for full-PMS cases. The non-planarsheathshavealsothelowestaveragedensitiesand As discussed in Introduction, previous studies suggest that plasmaβ. Forplanarsheathsthelowestdensityandplasma localsolarwindconditionshaveasignificantroleinthefor- β are found for the Near-Shock group (Group 2). The av- mation of PMSs. While Fig. 4 showed selected solar wind erage plasma β is also low for the Full-PMS group (Group parameters averaged over the whole sheath, in Fig. 5 we 5). Group3(Mid-Sheath)featuresthehighestplasmaβ,but presenttheaveragesofamorecompletesetofplasmaparam- the corresponding error bar is quite large. Panel 4h shows eters in three sheath sub-regions (Near-Shock, Mid-Sheath, that Full-PMS sheaths are shortest (∼0.09 AU) when com- Near-LE regions, see Fig. 2). The white histograms show paredwiththeothergroups,whichhaveameanthicknessof thecaseswhereourPMScriterionwasfulfilled(in41Near- ∼0.13AU. Shock regions, 63 Mid-Sheath regions and 50 Near-LE regions)andtheblackhistogramsthecaseswherenoPMSwas 3.2.3 Ejectaparameters identified(in30Near-Shockregions,19Mid-Sheathregions and29Near-LEregions). The first two panels in the bottom row of Fig. 4 show Figure 5 shows that the mean magnetic field magnitude the CME leading edge magnetic field magnitude and pro- (panel5a),solarwindspeed(panel5c),andthethicknessof ton number density, respectively, calculated here as three- the sub-region (panel 5f) do not vary significantly between hour averages from the leading edge time onwards through non-planarandplanarpartsofthesheathregion. the CME ejecta. The non-planar sheaths and the events in In turn, plasma β, solar wind density and temperature the Near-Shock group are associated with weaker leading showclearvariations.Thoseregionsinwhichthe67%PMS- edge magnetic field magnitudes than the other groups. The coveragePMScriteriaisfulfilledhavehigherdensity(panel comparisonofpanels4fand4jshowsthatnon-planarcases 5b) than their non-planar counterparts. This trend is most (Group1)haveverysimilardensitiesbothattheejectalead- pronounced in the Near-LE region. For the Near-Shock re- ing edge and in the sheath, while for planar events (Groups gion temperatures (panel 5d) are clearly lower when PMSs 2-5) the densities are clearly larger in the sheath and then are present, while for PMS-related Near-LE regions the av- droptovaluescomparablewithnon-PMSeventsattheejecta erage temperature is even slightly higher than in a case of leadingedge. non-planarity. Itisinterestingtonotethataveragetempera- Panel 4k shows the speed difference between the leading tures are rather similar in all three sub-regions when PMSs edge and the solar wind in front of the sheath. The CME wereidentified,whiletheaveragesshowlargevariationsfor leading edge speed is calculated as the three-hour average non-planar sub-regions. The same trend is also visible for fromtheleadingedgetimethroughtheejecta,whiletheup- the plasma β (panel 5e). In the Mid-Sheath region, and in streamspeediscalculatedasthesolarwindvelocityaverage particular in the Near-Shock region, plasma β is higher for during the three hours immediately before the IP shock. At non-planar cases, while in the Near-LE region β is higher CME flanks the shock is essentially a blast-wave type and whenasignificantPMSscoverageisfound. itsspeeddecreaseswithdistance(Pick,2002). Intotalthere were four events (4%) with negative speed differences that 3.4 Relation between PMS and shock normal orienta- were not included in the analysis. The highest speed dif- tions ferenceisfoundforgroup3(Mid-Sheath), whilethesmall- estdifferencesareassociatedwithGroup1(Non-PMS)and Figure6showshowtheanglebetweenthePMSnormaland Group2(Near-Shock). the shock normal (δ) are related to the angle that the shock Thelastpanelinthebottomrowshowstheaveragesofthe normalmakeswiththeradialdirection(α),i.e.,whenδ=0◦ CME velocity gradient, defined as the difference in the ve- theshocknormalisalignedwiththePMSnormal.Theradial locities between the CME trailing and leading edges. Only direction corresponds to the Sun-Earth direction (−xˆ ). GSE cases featuring negative velocity gradients, i.e. expanding The location angle α is used here as an approximation of Palmerioetal.: PMSsinCMEsheaths 7 (a) (b) (c) (d) (e) (f) Fig.5: DependenceofPMSoccurrenceonaveragesheathplasmaparametersineachsheathsub-regions(Near-Shock: N-SH,Mid-Sheath: MIDandNear-LE:N-LE).Thewhitebarsrepresenttheaverageswithinthesub-regionswhereourPMScriteriaweresatisfied(67%PMS coverage),whiletheblackbarsshowtheaveragesinnon-planarsub-regions(0%PMScoverage). Theblackandrederrorbarsshowthe extentofonestandarddeviation.Thestudiedparametersare:(a)magneticfieldmagnitude,(b)protonnumberdensity,(c)solarwindspeed, (d)protontemperature,(e)plasmabeta,and(f)sub-regionthickness. closetozerothespacecraftiscrossingtheejectaclosetoits apexandtheangleincreasesasthecrossingtakesplacemore ontheflankoftheCME. It is clear from Fig. 6 that there is no correlation be- tweenδandα(correlationcoefficientR=0.08). Inturn,the color-codedscatterpointspresentacleardependenceofthe PMS location on thevalue of δ. For Group2 (Near-Shock) the normal of the PMS plane is more or less aligned with the shock normal (δ does not exceed ≈32◦ for the yellow points). Group5(Full-PMS),thatalsofeaturesaPMSiniti- atingwithinonehouraftertheIPshock,showssimilarchar- acteristics(δ doesnotexceed≈48◦ forthebluepoints). In turn, Group3andGroup4haveasignificantlylargerrange of δ values (reaching nearly 90◦) and a tendency for PMS planehavingconsiderabledeviationfromtheshocknormal. 3.5 Relationship between PMSs and southward magneticfield Fig.6:Distributionoftheangleδ(betweenPMSnormalandshock normal) versus the angle α (between shock angle and radial dis- Finally, weinvestigatewhetherthepresenceofPMSscould tance, absolute value). The different colors and symbols refer to influence the geoeffectivity of the sheath region. Figure 7 thePMSgroups:Near-ShockPMS(yellowdiamonds),Mid-Sheath presentsthefractionsofdurationcoveredbyB <−5nTand PMS(greensquares),Near-LEPMS(redcircles),Full-PMSsheath z (bluetriangles). Bz<−10 nT in the three sheath sub-regions (Near-Shock, Mid-Sheath,Near-LEregions,seeFigure2). Theresultsare presentedinthesameformatasFig. 5: thewhitehistograms show the cases where our 67%-coverage PMS criteria was the spacecraft crossing distance from the CME apex (e.g., fulfilledandtheblackhistogramsthecaseswherenoPMSs Janvier et al., 2015; Savani et al., 2015), where for values wereidentified(0%PMSoccurrence). 8 Palmerioetal.: PMSsinCMEsheaths (a) (b) Fig. 7: Dependence of PMS occurrence on southward magnetic field B (in GSM coordinates) in the same format as Figure 5. The z investigatedparameteristhefractionofsub-regioncoveredby(a)B <−5nT,(b)B <−10nT.Theerrorbarsshowtheextentofone z z standarddeviation. Bothpanelssuggestthatplanarregionsaremorelikelyto mals generally coincide for the Near-Shock group events, havestronglysouthwardmagneticfieldsthannon-planarre- whiletheNear-LEgroupeventshadclearlylargerdeviations gions. The Near-LE region has equal fraction of B <−5 (Fig. 6). WealsofoundthatinagreementwithKataokaetal. z nT for PMS and non-PMS cases, but there is a higher frac- (2005) the events in the Near-Shock group were associated tion of intense southward magnetic field (B <−10 nT) in with relatively strong (high M ) shocks with low upstream z A PMS-relatedNear-LEregions. Inparticular,theMid-Sheath β (Figs. 4a and 4c). These shocks were also highly quasi- region has significantly more B <−10 nT intervals when perpendicular, but a similar tendency was found for all in- z PMSsarefound. vestigatedPMS-relatedgroups. AccordingtoKataokaetal. (2005), downstream regions behind quasi-parallel and high upstream β shocks become highly turbulent, which inhibits 4 Discussionandconclusions the formation of PMSs. A low upstream β ahead of an IP shock would create suitable conditions for the alignment of Inthispaperwehavestudiedtheoccurrenceanddistribution themagneticdiscontinuitiesastheypasstheshock. ofplanarmagneticstructures(PMSs)within95CME-driven Inaddition,thecaseswherePMSswereobservednearthe sheath regions and their association with the shock, sheath CME leading edge had higher expansion speeds (Fig. 4l) andICMEproperties. Thesheathregionsunderstudywere andhigherdifferencebetweentheCMEleadingedgeandthe divided in three sub-regions (Near-Shock, Mid-Sheath, and ambient solar wind speeds (Fig. 4k) than the events where Near-LE;LE=leadingedge)andsubsequentlyinfivegroups PMSsoccurredclosetotheshock.Thisisconsistentwiththe according to PMS presence and location within the sheath amount of IMF draping ahead of the ejecta increasing with (seeFig. 3andSect. 2.2). theincreasingCMEspeedandexpansionrate(e.g.,Gosling Our study shows that PMSs are ubiquitous in CME andMcComas,1987). sheaths; 85% of the analyzed sheaths feature at least one Interestingly,wefoundPMSsmostoftenwithintheMid- PMS and in over one-third (35%) of the cases the PMSs Sheath region. There is no obvious (compressive) physical coveratleasttwo-thirdsofthewholesheath,consistentwith mechanism that would explain the formation of PMSs in Jones et al. (2002) and Savani et al. (2011). The PMS du- the middle of the sheath. Hence, we argue that Mid-Sheath rations (average 6.0 hours) were also in agreement with the PMSs that have formed relatively early phase of the CME previousstudies(JonesandBalogh,2000;Nakagawa,1993). travel from Sun to Earth due to shock alignment and am- In addition, we found a significant amount of short PMSs plification. The Mid-Sheath-only events also stand-out in (durations between 1-6 hours), as suggested by Jones and ourstatisticsbyhavingthelargestplasmaβ upstreamofthe Balogh(2000). shock(Fig. 4c)andinthesheath(Fig. 4g),thelargestsheath Considering the two suggested PMS formation mecha- thickness (Fig. 4h) and being associated with the strongest nisms (see Introduction), we focused on comparing differ- shocks (Fig. 4a) and the strongest CME expansion speeds ences between PMSs in the Near-Shock and Near-LE re- (Fig. 4k) and speed gradients (Fig. 4l). It should be noted gions. 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