Coincidence of heliospheric current sheet and streaminterface: Implications for the origin and evolutionof the solar windJia Huang1,2, Yong C.-M. Liu1, Berndt Klecker3, and Yao Chen4

1State Key Laboratory of SpaceWeather, National Space Science Center, Chinese Academy of Sciences, Beijing, China, 2Collegeof Earth Sciences, University of Chinese Academy of Sciences, Beijing, China, 3Max-Planck-Institut für extraterrestrische Physik,Garching, Germany, 4School of Space Science and Physics, Shandong University at Weihai, Weihai, China

Abstract In general, the heliospheric current sheet (HCS), which defines the boundary of sunward andantisunward magnetic field, is encased by the slow solar wind. The stream interface (SI) represents theboundary between the solar wind plasmas of different origin and/or characteristics. According to earlierstudies using data of low time resolution, the SI and HCS get closer further away from the Sun, and thetwo structures coincide with each other around 5AU. In this study, we use STEREO data of a much highertime resolution to reveal an unusual case where the SI and HCS are coincident near 1 AU and separatedfrom the so-called true sector boundary (TSB) at which the suprathermal electrons change their relativepropagation directions. Preliminary analysis suggests that the closed loops in pseudostreamers continuallyhave interchange reconnection with the open-field lines that lead them, resulting not only in the coincidenceof HCS and SI but also in the separation of the TSB from the HCS/SI. We therefore conclude that the interchangereconnection plays an important role in the evolution of slow solar wind.

1. Introduction

Solar wind is generally classified as “fast” (v ≥ 550 km s�1) and “slow” solar wind (v ≤ 450 km s�1), which differin their physical nature and regions of origin [e.g.,Wang et al., 2000]. In addition to differences in speed, theirproton densities, proton temperatures, helium abundances, and “freeze-in” temperatures are also distinctfrom each other [Bame et al., 1977; Borrini et al., 1981; Freeman and Lopez, 1985; Geiss et al., 1995]. Fastsolar wind is believed to originate from coronal holes, the open-field regions and highly diverging coronalstructures [Zirker, 1977; Zhao, 2011]. However, the origin of slow solar wind is still not well understood[Edmondson et al., 2009; Zhao, 2011]. Closed field regions, especially the helmet streamers, are onemost likelyorigin of the slow solar wind [e.g., Hundhausen, 1977; Feldman et al., 1981]. Recent studies show that outwardmoving plasma “blobs” and flows from pseudostreamers could be important portions of slow solar wind[Wang et al., 2000, 2007; Liu et al., 2010; Owens et al., 2014; Crooker et al., 2014].

Compression interaction regions will form when fast solar wind overtakes slow solar wind. If these structuresare roughly time stationary and can be recurrently observed, these regions are called corotating interactionregions (CIRs) [e.g., Gosling and Pizzo, 1999; Lee, 2000]. Inside a CIR, there lies at least one stream interface (SI),which separates the slow and fast solar wind [Crooker et al., 1999]. Composition analysis of Ulysses data byWimmer-Schweingruber et al. [1997] shows multiple SIs in a single CIR. Crooker et al. [1999] summarized ninecriteria to identify SIs, including both the compositional characteristics and other signatures, such as a drop indensity, a rise in temperature, and a flow shear. A pair of shocks also forms to surround a CIR with a forwardshock propagating into the slow solar wind in the leading edge and a reverse shock propagating back intothe fast solar wind in the trailing edge [Gosling and Pizzo, 1999]. CIR shocks are usually well formed beyond1AU, and pressure waves are frequently observed within 1 AU [Gosling et al., 1976; Hundhausen and Gosling,1976; Steinberg et al., 2005]. Nevertheless, Jian et al. [2006] found CIR shocks near 0.72 AU. A recent studyshows that shocks of a CIR pair could accelerate particles between the two CIRs near 1 AU [Wu et al., 2014].

The heliospheric current sheet (HCS) is defined as a current layer that forms where the interplanetarymagnetic field (IMF) changes polarity [Crooker et al., 2004; Liu et al., 2014], usually encompassed by slow solarwind. Suprathermal electrons emitted from the Sun travel along the magnetic field lines due to their largescattering mean free paths, forming strahls around pitch angle 0° (180°) when the magnetic field lines aredirected antisunward (sunward) [Lavraud et al., 2010]. The place where the suprathermal electron strahls

HUANG ET AL. CASE STUDY OF HCS COINCIDES WITH SI 1

PUBLICATIONSJournal of Geophysical Research: Space Physics

RESEARCH ARTICLE10.1002/2015JA021729

Key Points:• The HCS coincides with the SI butmismatches with the TSB

• Interchange reconnection plays acrucial role in forming the coincidentHCS and SI

• The in situ signature of pseudostreamercould be changed by the evolution ofsolar wind

Correspondence to:Y. C.-M. Liu,liuyong@spaceweather.ac.cn

Citation:Huang, J., Y. C.-M. Liu, B. Klecker, andY. Chen (2016), Coincidence ofheliospheric current sheet and streaminterface: Implications for the origin andevolution of the solar wind, J. Geophys.Res. Space Physics, 121, doi:10.1002/2015JA021729.

Received 29 JUL 2015Accepted 7 DEC 2015Accepted article online 12 DEC 2015

©2015. American Geophysical Union.All Rights Reserved.

change pitch angle is called true sector boundary (TSB). Ideally, the HCS coincides with the TSB, forming theboundary that separates two solar sectors of different IMF polarities [e.g., Kahler et al., 1996, 2003; Foullonet al., 2009; Liu et al., 2014]; however, there are complications. The study of Foullon et al. [2009] revealedthe multilayer features of some HCSs. McComas et al. [1989] first put forward the concept of heat fluxdropouts (HFDs), where suprathermal electron strahls disappeared. They suggested that HFDs are a signaturefor magnetic reconnection, while more recent studies indicate that HFDs could be the result of pitch anglescattering, possibly caused by particle-wave interactions [Crooker et al., 2003; Pagel et al., 2005]. In addition,in some cases a mismatch between HCSs and TSBs has been found [Crooker et al., 1996, 2004]. These casesare special because the suprathermal electron strahls flow sunward, which is not the expected direction,during these intervals. Kahler et al. [1996, 2003] suggested that the turned back flux tubes could show suchreversal polarity signature. Crooker et al. [2004] analyzed eight cases observed during 1994–1995 and foundthat interchange reconnection may play a crucial role in forming the inverted magnetic field lines, whichresult in the sunward flowing strahls, while some cases have a signature of interplanetary coronal massejections (ICMEs). Multispacecraft observations by Foullon et al. [2009] confirmed their results.

Obviously, CIRs would have an influence on the leading sector boundaries. In most CIRs, forward shocksor pressure waves could catch up with sector boundaries [Borrini et al., 1981; Thomas and Smith, 1981;Crooker et al., 1999]. Besides, the angular separation between SI and the leading sector boundary decreasessignificantly as the heliocentric distance increases [Schwenn, 1990]. Although sector boundaries shouldalways lead the SIs [Gosling et al., 1978; Bame et al., 1993; Crooker et al., 1999], observations by the Pioneerand IMP spacecraft showed that SI coincided to within 1 or 2 h with HCS [e.g., Siscoe and Intriligator, 1993,1994]. However, HCSs and SIs usually have a scale of 104 km [Winterhalter et al., 1994; Forsyth and Marsch,1999], with a crossing time of only about 1min, so the time resolution of 1 h is too low to reveal their relativelocation. More recent conceptual studies suggest that interchange reconnection may be responsible for suchcoincidence [Crooker and Owens, 2012], but the suprathermal electron data are required to reveal themagnetic configuration related to this coincidence.

In this paper, we report an observation of HCS and SI coincidence using high time resolution in situ data fromSTEREO A, which is rarely reported for such a short heliocentric distance. We also present the observations ofspacecraft nearby including Wind and STEREO B. A schematic model is proposed that could explain thecoincidence. The instrument and data are described in section 2. In section 3, we present the multispacecraftobservations. Section 4 presents the discussion, and the main results are summarized in section 5.

2. Data

The twin STEREO spacecraft were launched on 26 December 2006. After a few lunar swing-bys, both space-craft orbit around the sun at a radial distance of approximately 1 AU, with STEREO A leading ahead andSTEREO B trailing behind the Earth. The longitudinal distance between STEREO A and STEREO B increasesat approximately 44° to 45° per year when viewing from the Sun [Kaiser et al., 2008]. The event we will presentoccurred on 8 April 2007 when the longitudinal distance between STEREO A and STEREO B was about 3.7°.The observations by the Wind spacecraft are also presented because it was located near L1, right in betweenSTEREO A and STEREO B. Their positions in GSE coordinates on 8 April 2007 are presented in Figure 1(courtesy of SSCWeb). The STEREO data used in this study are from the In-Situ Measurements of Particlesand CME Transients (IMPACT) suite [Luhmann et al., 2008] and the Plasma and Suprathermal IonComposition (PLASTIC) experiment [Galvin et al., 2008]. The Magnetic Field Experiment [Acuña et al., 2008]of IMPACT provides magnetic field data with a 1min time resolution, and the Solar Wind Electron Analyzer[Sauvaud et al., 2008] provides the suprathermal electron pitch angle distributions (PADs) with 30 s timeresolution. The plasma data provided by the PLASTIC experiment also have 1min time resolution. The dataof Wind used here consist of the plasma and electron pitch angle data from the Solar Wind Experiment[Ogilvie et al., 1995] and the magnetic field data from the Magnetic Field Investigation [Lepping et al.,1995]. They have a much higher time resolution of 3 s.

3. Observations

During the days ahead and following April 2007, the solar activity was quiet in the extended solar minimumof solar cycle 23 [Russell et al., 2010]. There was no ICME documented around this time; therefore, it was ideal

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to study undisturbed solar wind.Figure 2 shows the in situ observa-tions by STEREO A, including (fromtop to bottom) magnetic fieldRTN components, suprathermal elec-tron PADs, the magnetic field azi-muthal angle ϕB, proton specificentropy S= Tp/np

2/3 [Borovsky, 2008],proton number density, bulk speed,and proton temperature. The solidvertical line marks the sharp changeof proton density and proton tem-perature at 21:00 UT. The proton den-sity decreased from a peak value near60 cm�3 to ~15 cm�3 and stayed lowfrom then on. The proton tempera-ture increased from 2× 104 K to over105 K. The solar wind speed increasesslowly from 300 km/s to 528 km/s at11:00 UT the next day. All of thesesignatures fit well to the criteria ofSI. This stream interface was alsodocumented by Lavraud et al. [2010]and Jian et al. [2013]. Although ϕB

shows some fluctuations during theday, the jump from ~120° to more than 225° is obvious at 21:00UT. Before the jump, the phi angle fluctuatesaround 120°; after the jump, the phi angle stays at nearly 300° with some small variations. The characteristicsof the magnetic field suggest an ideal heliospheric current sheet passing exactly the same time as the SI,which is rarely reported at 1 AU.

The dashed line in Figure 2 marks the TSB, when the PADs of suprathermal electrons in the second panelflipped from predominately 180° to 0°. The TSB preceded the HCS by about 6 h, revealing a typical mismatchcase. During this interval, the magnetic field lines are directed to the Sun, and the suprathermal electronstrahls concentrated on pitch angle 0°, which means they apparently flow toward the Sun. However, thesuprathermal electrons always stream antisunward [Crooker et al., 2004; Lavraud et al., 2010]. Previous studiesshowed that the interchange reconnection may roll back the magnetic field lines and in turn lead to sunwardflowing strahls [Crooker et al., 1996, 2004; Foullon et al., 2009]. We also note that the TSB coincides with apressure wave characterized by a small increase in magnetic field magnitude and solar wind speed, whilethe proton density shows a very slight drop.

The separation of TSB and HCS suggests that the IMF may be folded back as a result of some solar winddisturbances. However, the curved magnetic field lines would not explain the coincidence of SI and HCS.For a better understanding of the structure of magnetic field and plasma in the solar wind, we present theobservations of STEREO B and Wind below.

Figure 3 shows the observations of STEREO B on 9 April 2007. The plot is nearly in the same format as Figure 2except for normalized PADs added in Figure 3 (third panel). The normalization is derived by dividing thephase space density in each pitch angle by the mean value of all pitch angles during the same time period.The original PADs of 246.6 eV electrons are also shown in Figure 3 (second panel). Normalization does notmodify the physical characteristic of the data but improves the clarity of the plot. The solid vertical line marksthe SI crossing at 10:15 UT characterized by the decreased proton density, the elevated proton temperature,and speed. The crossing was more than 13 h later than that observed by STEREO A. However, the expectedtime difference should be about 2.3 h based on solar wind speed and separation of the two STEREOspacecraft with the data presented in Figure 1 [Opitz et al., 2009; Liu et al., 2014]. The large deviation betweenobservational and theoretical time differences indicates that this boundary may have changed at the origin

Figure 1. The positions of STEREO A, STEREO B, and Wind (GSE coordinates inEarth radii (RE)) on 8 April 2007. All three spacecraft were nearly in the eclipticplane. The blue dots indicate the three spacecraft positions with their namesfollowing. The right bottom of the picture presents their detailed positions inunits of RE, using data from SSCWeb (http://sscweb.gsfc.nasa.gov).

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when the solar wind flows out of the sun, which implies that some of themagnetic field configurationmay bemodified. We note that this SI crossing is also documented by Simunac et al. [2009] and Lavraud et al. [2010].

The HCS cannot be determined on STEREO B since the observed magnetic azimuthal angle fluctuated backand forth from smaller to larger values than 225° a few times starting at 02:00UT. Therefore, STEREO B did notobserve a coincidence of HCS and SI. The magnetic fluctuations may be caused by the planar structureobserved from 06:30 UT to 08:40UT as shown by the shaded region in Figure 3. The angle between thenormal of its front and rear boundary is about 8° based on the minimum variance analysis of the magneticfield (MVAB). This planar structure is also characterized by the elevated entropy, smoothly varying magneticphi angle, enhanced magnetic magnitude, and enhanced strahl flux. Entropy is often used to distinguishdifferent plasmas since it stays unchanged during compression or other adiabatic processes [Pagel et al.,2004; Borovsky, 2008]. The density within the planar structure was significantly smaller and so was the bulkspeed. Most of these features are consistent with small flux ropes [e.g., Janvier et al., 2014]; however, the rota-tion of magnetic field is not observed in the planar structure. This planar structure may not be strong enoughto modify the structure of the solar wind; it may result from a reconfiguration of the magnetic field in thesolar corona.

Figure 2. The STEREO A observations on 8 and 9 April 2007. (first to seventh panels) Magnetic field components in RTNcoordinates, pitch angle distribution of ~250 eV suprathermal electrons, magnetic field azimuthal angle (RTN coordinates),entropy, and proton density, velocity, and temperature. The vertical dashed line shows the location of the true sector boundary(TSB), and the vertical solid line shows the location of the heliospheric current sheet (HCS) and stream interface (SI).The horizontal dashed line in the third panel marks the ϕB angle of 225°.

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The dashed vertical line in Figure 3 marks the TSB crossing near 02:00 UT, which was 11 h later than thatobserved by STEREO A, and the PADs varied several times after the crossing. This time difference of TSBcrossing is approximately equal to the time difference of the SI crossing. In addition, a typical forward shockwas observed, when proton density, magnetic field strength, temperature, and solar wind speed all had asteep increase.

The Wind spacecraft observed similar structures like STEREO B as shown in Figure 4. The SI arrived at 05:56 UTon 9 April, 4 h preceding that observed by STEREO B. Based on the solar wind speed and the separation of thespacecraft, the expected time difference is 3.2 h. Similarly, the TSB arrived at 22:40 UT on 8 April as shown bythe black dashed vertical line, 3.3 h preceding the TSB observed by STEREO B. Therefore, we concluded thatWind and STEREO B observed the solar wind of the same origin but different from that observed by STEREO A.

The planar structure is also observed by Wind from 01:00 UT to 04:00 UT on 9 April as shown by the shadedregion. The characteristics of the planar structure observed by STEREO B and Wind are similar. The anglebetween the normal of its front and rear boundary is about 17° based on the MVAB. In addition, the proton

Figure 3. The STEREO B observations on 9 April 2007. (third panel) In this figure the normalized pitch angle distribution ofsuprathermal electrons is added. The other panels have the same format as Figure 2. The vertical dashed line shows thelocation of the TSB and a forward shock, the vertical solid line shows the location of SI, and the shaded region shows aplanar structure.

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temperature was higher and the suprathermal electron flux was significantly larger compared to adjacentsolar wind. Unlike the leading edge of the CIR that overtook the TSB as observed by the twin STEREOspacecraft, Wind found a small forward shock preceding the TSB by 50min as shown by the blue dashedvertical line, which indicates that the forward shock intruded deeply into the leading slow solar wind.

The multispacecraft observations reveal some important facts. First, the coincidence of SI and HCS and theseparation of TSB and HCS are observed on STEREO A. Second, Wind and STEREO B, separated in longitudeby only ~350 RE, observe a much fluctuating magnetic field, and it is therefore not possible to determinethe HCS and its relative location with SI. Third, a planar structure which may help to form the fluctuatingmagnetic fields is observed by both Wind and STEREO B.

4. Discussion

To investigate the coincidence of SI and HCS observed by STEREO A, the trajectories of the spacecraft areprojected on the Global Oscillation Network Group’s (GONG) synoptic map along with STEREO B and Windas shown by the red, blue, and green lines Figure 5a (courtesy of GONG). Due to their very small latitudeseparations during this time period, the projected lines are nearly overlapping. The red vertical line denotes

Figure 4. The Wind observations on 8 and 9 April 2007, in the same format as Figure 3. The blue dashed line shows thelocation of a forward shock, the black dashed line shows the TSB, and the vertical solid line shows the location of the SI.

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the HCS crossing time at 04:24UT on 4 April, which is estimated based on the travel time of the solar wind withspeed of about 370 km/s. This time fits well with the crossing of the projected spacecraft trajectory (horizontalred line) and the HCS (solid black curve) on the map. Figure 5b shows the synoptic map observed by STEREO Awith SECCHI/COR1 at 2.2RS (solar radii); the brightness denotes the solar wind density. Usually, brighter partscorrespond to slow solar wind [e.g.,Wang et al., 2000; Sheeley et al., 2009]. After the crossing of the HCS (markedby the red vertical line), the slow solar wind was split into two parts as the red arrows show.

Figures 5c and 5d show the simulated solar wind speed and coronal holes (WSA model) using the magneto-grams from the Mount Wilcox Observatory as an input (download from CCMC STEREO Support, http://ccmc.gsfc.nasa.gov/stereo_support.php). The red ovals in both panels mark the HCS crossing region. The solarwind speed agrees well with the in situ data. The dark blue band denotes the slow solar wind, which tracesthe path of HCS, and is also split to a double band as the red arrows show near 5 April just after the HCScrossing. Both the observation and the simulation show that the slow wind split into two parts. The split isusually a sign of a pseudostreamer structure flowing out from the sun, since they are originating from sepa-rated coronal holes of the same polarity [e.g., Wang et al., 2007; Simunac et al., 2012; Crooker et al., 2012].

Two nearby coronal holes with the same polarity are necessary for the formation of a pseudostreamer. Thederived coronal holes shown in Figure 5d, with the solid black lines connecting the outer coronal boundaryand its source regions at the photosphere, are possible explanations for the formation of this pseudostreamer[e.g., Neugebauer et al., 2004;Wang et al., 2012]. Before 5 April, the solar windmapped to the large positive polarhole (the sign obtained from Figure 5a) in the Southern Hemisphere. After 5 April, the source seemed to be thepositive coronal hole located between Carrington longitude 264° and 294° in the same hemisphere at midlati-tudes. Two nearby coronal holes exist during this time period; therefore, twin loops can be formed at the bound-ary of these coronal holes, leading the flow of pseudostreamer [Neugebauer et al., 2004; Wang et al., 2012].

Although the existence of a pseudostreamer is suggested by the observations and the WSA coronal holepredictions, how the pseudostreamer can explain the coincidence of SI and HCS is still not clear. Crooker andOwens [2012] suggested a schematic evolution of the solar magnetic field to explain such coincidence;however, their theory cannot explain the separation of HCS and TSB in our observation.

Interchange reconnection between closed loops from the streamer belt and open magnetic field lines withopposite polarity leads to the separation of HCS and TSB [Crooker et al., 2004]. Owens et al. [2013] pointed

Figure 5. (a) The trajectories of three spacecraft overlaid on the GONG map on CR2054 (courtesy of GONG). The red vertical line represents the estimated crossingtime of the HCS. (b) Synoptic map of the white light corona at 2.2 solar radii on the same CR observed by STEREO A/SECCHI/COR1, with the red line representing thecrossing time of the HCS. (c and d) The expected solar wind speed and derived coronal holes from the output of the WSA model (courtesy of CCMC). The red ovaldenotes the region just after the crossing of the HCS. The red arrows represent the split double band.

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out that the closed magnetic loops of pseudostreamers could be dragged out by the solar wind, reconnectwith open fields at high altitude, resulting in the inverted heliospheric magnetic field. More recent simula-tions studied the interchange reconnection around the coronal hole boundaries [Edmondson et al., 2009,2010]. They constructed a small scale bipolar or pseudostreamer-like structure and a global dipole thatformed the streamer belt. They showed that the interchange reconnection could happen continuouslybetween the pseudostreamer-like field and the open flux of coronal holes to exchange materials restrainedby the closed field.

Our observations suggest that a pseudostreamer formed near the Sun as we observed the coincidence of SIand HCS and the relative separation of TSB at 1 AU. A schematic process is illustrated in Figure 6. The blacklines are the magnetic field lines along which the suprathermal electrons flow out. Figure 6a shows the initialstate, when the HCS coincides with TSB and both precede the SI. A pseudostreamer appears adjacent to thehelmet streamer. At this time, the SI is formed to separate streamer belt flow from pseudostreamer flowrather than coronal hole flow. On average, pseudostreamer flow is slightly faster than streamer belt flow[e.g.,Wang et al., 2012]. Figure 6b shows the solar wind dragging out the rising loops to meet the open-fieldlines at higher altitude. The trailing compression on the side of fast solar wind is a magnetic field configura-tion favorable for interchange reconnection as marked by a yellow circle. Figure 6c pictures the primarytopology of magnetic field lines after reconnection. The inverted magnetic field lines marked by yellow colorappear, and the open flux between helmet streamer and pseudostreamer reduces. In Figure 6c, the magneticfield lines start to roll back; the suprathermal electrons flow sunward as the red arrow indicates, but thecurrent sheets around the region are local and the HCS still coincides with the TSB.

Figure 6. Schematic of the evolution of the magnetic field configuration in the ecliptic plane. (a) The initial state,a pseudostreamer follows the streamer belt. The HCS coincides with the TSB, and they still lead the SI. (b) Interchangereconnection processes between closed loops from the pseudostreamer and the leading open magnetic field lines.(c) Magnetic field configuration after interchange reconnection, the inverted magnetic field lines appear. (d) Final state, theinterchange reconnection reduced the open flux, and the inverted magnetic field lines are dragged to higher altitudeswhere they can be observed. Consequently, the HCS is shifted to coincide with SI and separates from TSB.

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In Figure 6d, the continuous reconnection process reduces all the open flux in between the original TSB andSI, and the inverted magnetic field lines move to higher altitude. Then, the new SI is developed between theleading slow solar wind and the trailing fast solar wind originating from coronal holes. The slight drops inspeed after the crossing of SI as observed by STEREO A and Wind are associated with the interchangereconnection processes [Gosling and Skoug, 2002, and references therein]. It is unlikely that all of the fieldlines involved in the interchange reconnection would lie in the same plane in three dimensions. To explainthe case under consideration, it is reasonable to assume that the rollback field lines (as shown by the yellowline) lie inside the plane of the figure, but the reconnected field lines (dotted portions) lie outside.Consequently, the spacecraft flying in the plane happened to observe the HCS and the SI in the samelocation, and TSB separated from the HCS.

In addition, the interchange reconnection process could not only result in a rollback of the magnetic fieldlines but also spreads the slow solar wind to a wider space after reducing the sandwiched open flux. Thereduced open flux between TSB and SI may provide a chance for the forward shock to penetrate into slowsolar wind deeply as the Wind observation shows.

Neither STEREO B nor Wind observed the coincidence of HCS and SI. Instead, they both observed a planarstructure. In addition, the observed time differences suggested that the observed SI originated from the sameregion but different from what STEREO A observed. It is very likely that the configuration in Figure 6d isevolving, and it is likely to reconfigure, resulting in an SI that originates from a different location as observedby STERO B andWind. The planar structure may be resulting from the reconfiguration. Further analysis on theorigin and the transport of the planar structure will be conducted in our future work. Furthermore, we wouldlike to discuss the source of the streamers on either side of the SI as observed by STEREO B and Wind. Basedon the proton and alpha particle (not shown) characteristics, the streamers after the SI should be fast solarwind that originates from coronal holes. However, the situation is more complicated for the source of thesolar wind before the SI, when the SI is different from that observed by STEREO A. The source may be similarto that of STEREO A, and the difference could be caused by the evolving interchange reconnection processes.However, the actual source is difficult to pin down.

5. Summary

We first present an event in which the HCS coincides with the SI observed by STEREO A at a distance near1 AU, with time resolution of 1min. It is also found that the HCS mismatches the TSB in this event. Theorigin of such coincidence is explained by pseudostreamer and interchange reconnection near the Sun.Interchange reconnection between the magnetic loops of pseudostreamers and neighboring open fieldscan lead to the coincidence of the HCS and SI, as schematically shown by the evolution of the magnetic fieldconfiguration (Figure 6). Further analysis on STEREO B and Wind data found that they do not observe suchcoincidence but a planar structure. Thus, we draw the following conclusions:

1. Interchange reconnection between the magnetic loops of pseudostreamers and neighboring open fieldscan lead to the coincidence of the HCS and SI.

2. A planar structure was observed in the vicinity, and it may be related to the evolving interchangereconnection processes.

3. The pseudostreamer originating from the sun may evolve through reconnection and eventually have theheliospheric current sheet in the vicinity.

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AcknowledgmentsThis work is supported by the ChineseAcademy of Science “Hundred TalentedProgram” of contract Y32135A47S, theChinese National Science Foundationof contract 411774149, and theSpecialized Research Fund for State KeyLaboratory of China. The solar wind andIMF data are provided by the STEREOScience Center (stereo-ssc.nascom.nasa.gov). The authors are also grateful toL. Jian, B. Li, L. Zhao, P. B. Zuo, andQ. Hu for their thoughtful suggestionsand comments.

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