A statistical analysis of heliospheric plasma sheets,heliospheric current sheets, and sector boundariesobserved in situ by STEREOY. C.-M. Liu1, J. Huang1,2, C. Wang1, B. Klecker3, A. B. Galvin4, K. D. C. Simunac4, M. A. Popecki4,L. Kistler4, C. Farrugia4, M. A. Lee4, H Kucharek4, A. Opitz5, J. G. Luhmann6, and Lan Jian7,8

1State Key Laboratory of Space Weather, Center for the Space Science and Applied Research, Chinese Academy of Science,Beijing, China, 2College of Earth Sciences, University of Chinese Academy of Sciences, Beijing, China, 3Max-Planck-Institutfür extraterrestrische Physik, Garching, Germany, 4Institute for the Study of Earth Oceans and Space, University of NewHampshire, Durham, New Hampshire, USA, 5UMR 5187, Centre National de la Recherche Scientifique, Toulouse, France,6Space Science Laboratory, University of California, Berkeley, California, USA, 7Department of Astronomy, University ofMaryland, College Park, Maryland, USA, 8Heliophyics Science Division, NASA Goddard Space Flight Center, Greenbelt,Maryland, USA

Abstract The heliocentric orbits of STEREO A and B with a separation in longitude increasing by about45° per year provide the unique opportunity to study the evolution of the heliospheric plasma sheet (HPS)on a time scale of up to ~2days and to investigate the relative locations of HPSs and heliospheric current sheets(HCSs). Previous work usually determined the HCS locations based only on the interplanetary magnetic field.A recent study showed that a HCS can be taken as a global structure only when it matches with a sectorboundary (SB). Usingmagnetic field and suprathermal electron data, it was also shown that the relative locationof HCS and SB can be classified into five different types of configurations. However, only for two out of these fiveconfigurations, the HCS and SB are located at the same position and only these will therefore be used for ourstudy of the HCS/HPS relative location. We find that out of 37 SBs in our data set, there are 10 suitable HPS/HCSevent pairs. We find that an HPS can either straddle or border the related HCS. Comparing the correspondingHPS observations between STEREO A and B, we find that the relative HCS/HPS locations are mostly similar. Inaddition, the time difference of the HPSs observations between STEREO A and B match well with the predictedtime delay for the solar wind coming out of a similar region of the Sun. We therefore conclude that HPSs arestationary structures originating at the Sun.

1. Introduction

Early studies have shown that there exists a layer of plasma of enhanced proton density and decreased He/Hratio in the vicinity of the heliospheric current sheet (HCS) [Borrini et al., 1981; Gosling et al., 1981]. The layer ofplasma, later termed the heliospheric plasma sheet (HPS), is also characterized by enhanced plasma β andslightly lower temperature. Winterhalter et al. [1994] used high plasma β as a characteristic feature andconcluded that HPSs straddle HCSs. They also found that HPSs are not symmetric around the HCS. Althoughmost early studies suggested that HPSs encase HCSs, Crooker et al. [2004] noticed that some of the HPSs“border rather than encase the sector boundaries.” Recently statistical studies by Suess et al. [2009] foundindeed that most of the HPSs border the HCSs. In their case-by-case studies using ACE data, most of the HPSs,characterized by enhanced proton density and depressed He/H, usually appear on the edge of the HCSs witha few exceptions of HPSs encasing the HCSs. They also explained that the earlier finding by Borrini et al.[1981], where the HPS appeared to straddle the HCS, was due to superposed epoch analysis of their study.Using data from STEREO A, Liu et al. [2010] presented three HPSs characterized by low O6+/H+, and each ofthem was different, preceding, trailing, and encasing the HCS, respectively.

The HCS defines where the interplanetary magnetic field (IMF) changes polarity. The suprathermal electronsfrom the Sun flow antisunward along the magnetic field lines, they form strahls concentrated on pitch angle0° (180°) when IMF is pointing antisunward (sunward). The sector boundary (SB) is defined as the boundarywhere the pitch angle of the suprathermal electron strahl changes. The HCS is usually considered to be theboundary of the sectors connecting to the Sun’s surface of different magnetic polarity [Foullon et al., 2011;McComas et al., 1989; Anderson et al., 2012]. Ideally the HCSs should always coincide with the SB; however,

LIU ET AL. ©2014. American Geophysical Union. All Rights Reserved. 1

PUBLICATIONSJournal of Geophysical Research: Space Physics

RESEARCH ARTICLE10.1002/2014JA019956

Key Points:• The HPSs can either straddle or borderthe HCSs

• STEREO A and B usually observedsimilar types of HPSs

• HPSs are continuous flow from the Sun

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

Citation:Liu, Y. C.-M., et al. (2014), A statisticalanalysis of heliospheric plasma sheets,heliospheric current sheets, and sectorboundaries observed in situ by STEREO,J. Geophys. Res. Space Physics, 119,doi:10.1002/2014JA019956.

Received 12 MAR 2014Accepted 2 OCT 2014Accepted article online 7 OCT 2014

there are complications. It has been documented that HCSs sometimes have multilayered structures[Wang et al., 1997; Foullon et al., 2009]. There are also cases when the strahl of suprathermal electronsdisappears, and these events were defined as heat flux dropouts byMcComas et al. [1989]. They also suggestedthat the heat flux dropout is a signature for magnetic reconnection. However, other studies showed that theheat flux dropout can also result from scattering of the electrons by waves [Pagel et al., 2005; Crooker and Pagel,2008; Crooker et al., 2010].

It has been found that in some cases the HCS does not coincide with the SB probably caused by localdisturbances in the solar wind [Crooker et al., 1996]. For such cases, the HCSs are not related to the sectorboundary near the Sun. Owens et al. [2013] listed five types of magnetic configurations and theircorresponding HCS and SB relative locations. Based on their study, only two types correspond to global andundisturbed HCS/SB structures originating from the sector boundary near the Sun’s surface. We focustherefore our study on these two types of events for investigating the relative position of the HCS and the HPS.

The origin of the HPS has been an interesting question for over 30 years. Since the plasma density in thehelmet streamer in the solar corona is also enhanced compared to the rest of the solar corona and thestreamer is also in the vicinity of the sector boundary, the origin of the HPS is usually traced back to thecoronal streamers near the sector boundary [Gosling et al., 1981]. Later studies showed that the HPSs areflows from the core of the streamer belt in the solar corona, while the rest of the slow solar wind flows out ofthe halo of the streamer belt [von Steiger et al., 1995; Bavassano et al., 1997].

Although all these studies treated the HPS as a stationary structure, a later study by Wang et al. [1998, 2000]suggested that coronal streamers, which correspond to HPSs near the Sun, are composed of discontinuous“blobs” flowing outward. They also proposed that the blobs resulted from interchange reconnection of theopen magnetic field lines with closed magnetic field lines. They then concluded that HPSs are dynamicstructures. This conclusion was reaffirmed by Crooker et al. [1996, 2004], who also developed the concept ofinterchange reconnection and suggested that interchange reconnection is responsible for the initiation notonly of HPS but also some of the coronal mass ejections (CMEs). Later analysis of “blobs” using STEREOobservations showed that they are in fact flux ropes [Sheeley et al., 2009; Sheeley and Rouillard, 2010]. A recentstudy by Simunac et al. [2012] compared the HPS observations on STEREO A, B, andWind over four CarringtonRotations, and they concluded that the HPS is quasi-stationary for at least one day. Thus, the questions of theHPS stationarity and the origin of the HPS remain open. Answers to these questions will help to solve thepuzzle of the slow solar wind origin.

In this paper, we study the HCS and HPS using the data obtained by the Plasma and Suprathermal IonComposition (PLASTIC) instrument, [Galvin et al., 2008] and in situ measurement of Particles and CMETransients (IMPACT) [Luhmann et al., 2008] onboard STEREO, focusing on undisturbed HCS/SB events. Sincewe select undisturbed HCS/SB events by the signature of the magnetic field direction and the pitch angledistribution of suprathermal electrons, we are able to characterize the relative HCS/HPS location better. Inaddition, through the comparison of the observation on STEREO A and B, we have a better understanding ofHPS stationarity. The paper is organized as follows. In section 2, we describe the instruments and the data.Section 3 lists all the HCS and SB events for the study of the relative positions of the HCS and HPS. In section 4,we present the HPS observations on each spacecraft. In section 5, we compare the HCS/SB and HPSobservations on STEREO A and B. Discussion and conclusions are in section 6.

2. Data and Instrument

The twin STEREO spacecraft circle around the Sun at about 1 AU, with STEREO A ahead of the Earth andSTEREO B lagging behind. The longitudinal separation between STEREO A and B increases by approximately45° per year [Kaiser et al., 2007]. During the early stage of the mission, which is the time period chosen for thisstudy, the separation increases from 3° to about 40°. The data we used in this study are obtained by the in situinstruments onboard STEREO, including IMPACT and PLASTIC. The IMPACT instrument suite consists of sixinstruments to measure magnetic fields and energetic particles. The suprathermal electron pitch angledistribution functions are obtained by the Solar Wind Electron Analyzer [Sauvaud et al., 2008; Fedorov et al., 2011]and the magnetic field by the Magnetometer (MAG) [Acuña et al., 2008]. The kinetic properties of the solar windused in this study are calculated from 1-D Maxwellian fits to the measured distribution functions provided byPLASTIC [Simunac, 2009]. Both the electron and proton data and magnetic field data have 1min time resolution.

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3. Sector Boundary and Heliospheric Current Sheet

The HCS events suitable for the study of relative HCS/HPS locations need to be global structures originatingfrom the Sun that have not been significantly distorted during their propagation to ~1 AU. Owens et al. [2013]listed five types of configurations of HCS and SB (see their Figure 1), namely, (a), (b), (c), (d), and (e). Ideallythe IMF is frozen in the solar wind so that the HCS is accompanied by a SB and such an event is brandedas a type (a) event. The situation is different when reconnection happens or when a field line folds backon itself as a result of a disturbance in the solar wind.

Type (a) events are characterized by a single strahl with an anisotropy switching from parallel to antiparallelwhen going frommagnetic sectors with antisunward to sunward pointing magnetic field (or vice versa). In thiscase, HCS and SB are located at the same position. Such an event clearly defines the flow from the vicinity of theSun’s sector boundary; therefore, it is ideally suited for the study of the relative locations of HPS and HCS.

A type (b) event has a well-defined HCS, but the SB is amidst a period of double-streaming electrons and cannotbe identified. Therefore, type (b) events are not suitable for our study since HCS and SB are separate or eitherof their locations cannot be determined. The double-streaming electrons usually imply closed IMF loops,magnetically connecting the region to the Sun. However, sometimes magnetic connection to shocks includingboth interplanetary shocks and Earth’s bow shock may also result in double streaming [Gosling et al., 1993;

a b

c

Figure 1. Three cases representing different configurations for the heliospheric current sheet and sector boundary. (a) The HCS and SB are located at the same placewith no local current sheet in the vicinity; (b) the HCS and SB are located at the same place but followed by some local current sheet; (c) the HCS and SB are separate.For each case the two panels show (from top to bottom) the pitch angle distribution of suprathermal electrons and the phi angle of the interplanetary magnetic field.

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Crooker et al., 1996]. In the data we analyzed, well-defined HCSs and type (b) events are rare because of usuallyhighly fluctuating magnetic fields. The magnetic fields change polarity more often than the pitch angle ofthe suprathermal electrons. It should be noted here that if double-streaming electrons are related to aninterplanetary coronal mass ejection, the magnetic fields are usually stable. However, there are very few CMEsduring the time period we study. Sometimes the HCS is in the middle of a heat flux dropout for which the exactlocation of SB cannot be determined either. Such events should also be classified as type (b).

Similar to type (a) events, type (c) events show a single strahl in a bipolar region; however, embedded withinis a strahl in the opposite sense to that expected. This indicates, for example, that the magnetic field is foldedback or inverted. Thus, similar to type (a) events, type (c) events have a current sheet located at the SBand are therefore suitable for the study of the relative location of the HCS and HPS.

A type (d) event has a current sheet without SB in the vicinity. The current sheet is usually local and should beexcluded from the study of the relative position of HCS and HPS. A type (e) event has a separated currentsheet and SB due to interchange reconnection near the Sun. The actual boundary may be located somewherebetween the SB and CS, but the exact location cannot be determined due to a complicated 3-D structure.Therefore, all events with types (b), (d), and (e) HCSs are excluded from the study of the relative positionbetween HCS and HPS.

Typical examples for types (a), (c), and (e) events are shown in Figure 1. The top panels show the pitch angledistribution of electrons with energy ~246.6 eV and the bottom panels show the azimuthal angle ϕ in theecliptic plane of the IMF for each event. The energy band is selected because it is in the suprathermal range.Figure 1a shows Event 6 observed by STEREO A from 21 April 2007 to the next day. The strahl at a pitch angle180° was very distinct before the end of 21 April 2007, then turning into pitch angle 0° at the point as markedby the solid black line. The strahl after the SB is more intense than that ahead of it. The magnetic fieldazimuthal angleϕ fluctuates; however, values are either centered around 135° or 315° as usually observed forantisunward or sunward IMF [Borovsky, 2010]. The dashed line marks the angle 225°, which shows that themagnetic field changes polarity exactly at the time of the SB crossing. Therefore, this event is suitable for theinvestigation as to whether a HPS borders or straddles HCS.

Figure 1b shows a type (c) event (Event 13) observed by STEREO B on 8 June 2007. There was a flip of theazimuthal angle ϕ from near 300° to 100° around 04:00 UT, which is almost at the same time as the SBcrossing (marked by the solid line). Contrary to type (a), for which the ϕ angle stays on one side, the ϕ angleflipped back to over 225° and turned back to below 225° between 08:00 UT and 10:00 UT (marked by thevertical dashed line). During this flip, the magnetic field stays at the antisunward direction for over 1 h, whilethe strahl stays at 180°, which suggests that the magnetic field is pointing toward the Sun. A magnetic fieldpointing against the expected direction usually implies that a flux rope folds back [Crooker et al., 1996;Owens et al., 2013]. For this kind of events, the location of the HCS can be determined at the SB as determinedby the major change of the magnetic field ϕ angle and we are confident that such HCSs are originating from theSun’s sector boundary. The folding back of the flux rope that happens at another location may not affect thepositions of the HCS and the characteristics of the HPS in the vicinity. On the other hand, the two currentsheets, caused by the folding back of a flux rope, are local current sheets and should be excluded from thestudy of HPS/HCS relative positions. We also noticed that there are very short periods of flips both priorto and after the HCS crossing. Since the flips last shorter than an hour, we take them as variations of thelocal magnetic field.

A type (e) SB event (Event 4) is shown in Figure 1c. On STEREO A, the SB crossing was at 15:00 UT (black line) on8 April 2007 and the HCS crossing was about 6 h later (red line). We also noticed that the strahl ahead of the SBand between the SB and HCS are of similar intensity, and the strahl behind the HCS is more intense. Morediscussion about this event will be presented in a later section.

4. Heliospheric Plasma Sheet Relative to the Heliospheric Current Sheet

We selected all types (a) and (c) HCS/SB events from Table 1 to study the HPS/HCS relative location fromMarch 2007 to February 2008. The HPSs are determined in the vicinity of HCSs based on a proton densityenhancement above the background values. Table 2 lists all types (a) and (c) HCS/SB events, including 20events observed by STEREO A and 13 by STEREO B from March 2007 to February 2008. The features listed in

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Table 2 include the type of SB, the start and end times of HPS, the crossing time of HCS, and the “overlap”value measuring the relative position between HPS and HCS. The “overlap” is defined as T1/T2, with

T1 ¼ MIN THCS � T start; Tend � THCSð Þ; T2 ¼ MAX THCS � T start; Tend � THCSð Þ;

in which THCS is the crossing time of the HCS, Tstart and Tend are the start and end times of the HPS,respectively. The overlap value maximizes at 1 for an event with HCS right at the center of an HPS andminimizes at 0 for a HCS on the edge. The overlap value is defined as “NO” if there is no HPS in the vicinity.In most cases, the overlap value varies between 0 and 1. The smaller the overlap value is, the closer theHCS is to the boundary of the HPS. Here we consider cases that have overlap less than 0.1 as HCS on the edgeof HPS. The other HPS types in the last column of Table 2 refer to HPS straddling HCS (“S”), HPS leadingHCS (“L”), and HPS following HCS (“F”).

Figure 2 shows, from top to bottom, the pitch angle distribution of the suprathermal electrons, themagnetic fieldazimuthal angle, and the proton density and speed for Event 5 observed by STEREO A from 16 April 2007 tothe next day. The sector boundary crossing was on 17 April, 02 UT, the HCS crossing was about 10 min later. This

Table 1. All Sector Boundary Crossingsa

No.Date

(mmddyyyy)

STEREO A

Date(mmddyyyy)

STEREO B

SB CrossingTime (UT)

HCS CrossingTime (UT)

HCS/SBType

SB CrossingTime (UT)

HCS CrossingTime (UT) HCS/SB Type

1 3/4/2007 01:19 01:19 a 3/4/2007 NA 05:10 b2 3/11/2007 15:14 16:30 e 3/11/2007 NA NO DATA NA3 3/31/2007 22:20 22:25 c 4/1/2007 01:26 01:26 a4 4/8/2007 15:00 20:58 e 4/9/2007 NA NA NA5 4/17/2007 02:20 02:20 a 4/17/2007 06:40 06:28 c6 4/21/2007 23:40 23:40 a 4/22/2007 NA 12:15 NA7 4/26/2007 19:45 18:40 e 4/26/2007 20:20 17:50 e8 5/7/2007 NA 09:58 NA 5/7/2007 NA 12:30 NA9 5/14/2007 09:00 09:00 a 5/14/2007 NA 09:55 NA10 5/18/2007 12:02 13:38 e 5/18/2007 06:24 06:24 a11 5/22/2007 NA 12:20 NA 5/23/2007 NA 09:35 NA12 6/2/2007 17:50 NO DATA NA 6/2/2007 NA 01:00 b13 6/8/2007 05:00 05:00 a 6/8/2007 03:56 03:56 c14 6/14/2007 NA 01:10 NA 6/13/2007 NA 14:40 NA15 6/21/2007 17:50 18:14 a 6/21/2007 03:20 03:20 c16 6/30/2007 02:58 03:30 e 6/28/2007 16:06 16:20 a17 7/3/2007 16:23 16:30 a 7/2/2007 NA 20:09 NA18 7/11/2007 10:32 10:32 c 7/10/2007 20:17 20:17 c19 7/20/2007 12:40 12:40 c 7/19/2007 NA 01:30 NA20 7/27/2007 05:03 05:03 a 7/26/2007 08:20 08:20 a21 7/29/2007 08:28 08:28 a 7/28/2007 18:00 18:00 c22 8/6/2007 04:22 8/7-04:15 e 8/5/2007 NA 23:50 b23 8/16/2007 04:58 04:58 a 8/14/2007 17:20 17:20 a24 9/1/2007 14:25 14:25 c 8/31/2007 NA 16:20 NA25 9/14/2007 NA 16:50 b 9/13/2007 NA 10:10 NA26 9/28/2007 17:38 17:38 a 9/26/2007 18:10 18:10 c27 10/12/2007 NA 20:52 NA 10/10/2007 NA 07:00 NA28 10/25/2007 21:00 18:20 e 10/23/2007 11:05 16:35 e29 11/9/2007 11:08 10:56 c 11/4/2007 NA NA NA30 11/21/2007 03:16 03:16 a 11/19/2007 15:55 15:55 c31 12/6/2007 NA 22:30 NA 11/29/2007 NA 15:05 NA32 12/18/2007 NA 12:40 b 12/15/2007 23:22 23:22 a33 1/6/2008 NA 15:10 NA 12/30/2007 07:24 11:38 e34 1/13/2008 21:20 21:40 a 1/10/2008 NA 12:10 NA35 2/2/2008 NA 08:45 NA 1/29/2008 1/30-00:27 23:28 e36 2/10/2008 07:16 07:16 a 2/7/2008 NA 10:40 b37 2/29/2008 NA 3/1-00:20 NA 2/26/2008 NA 15:25 NA

aIf the separation time between HCS and SB is shorter than 30min the event is classified as a type (a) or (c) event. Type NA means that either the HCS is notavailable, or the SB is not available because of HFDs.

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time difference was very small compared to the time scale of the HPS observed, which was several hours inthis event. Therefore, we consider this event still as a type (a) HCS/SB and included it in our study. Thebackground density of the solar wind (Figure 2, third panel) was about 5 cm�3. The enhancement of thedensity started at 23:00 UT on 19 April and ended at 07:00 UT the next day, lasting for 8 h (shaded areain the figure). Although there were significant density variations within the HPS, most of the time, thedensity was larger than 15 cm�3, which was 3 times the background solar wind density. The solar windspeed was below 350 km/s within the HPS. The HPS straddled the HCS with 3 h ahead and 5 h behind(i.e., type “S”), resulting in an overlap value of 0.67. The HPS asymmetry around a HCS has been discussedby Winterhalter et al. [1994].

Figures 3a and 3b show two HPS events observed by STEREO A, each event has a HCS bordering a HPS. Theformat is the same as in Figure 2. The Event 6 on 21 April 2007 has a HCS crossing at 23:40 UT as marked by avertical red line. The background solar wind density is about 20 cm�3 and increased to over 30 cm�3 from19:00 UT. The enhancement lasted for about 5 h. Although there was a short enhancement to 25 cm�3 afterthe HCS crossing, both the amplitude and time duration were minor compared to the major HPS; therefore,this is an event with a HPS leading the HCS and adjacent to it (type “L”). The solar wind speed was below350 km/s and had only small variations. Figure 3b shows Event 1 which has a HPS following the HCS crossing.The background solar wind density was only 7 cm�3, increasing to 13 cm�3 inside the HPS, nearly 2 timesthe background density. The HPS lasted for only 2 h following the HCS (type “F”). The solar wind speed wasnearly constant at 350 km/s.

Table 2. HPSs Observed by STEREO A and Ba

No. Date (mmddyyy) HCS/SB Type HPS Start (UT) HPS End (UT) HCS Crossing (UT) Overlap HPS Type

STEREO A1 3/4/2007 a 01:19 02:55 01:19 0 F3 3/31/2007 c 21:22 4/1-02:42 22:25 0.25 S5 4/17/2007 a 4/16-23:02 06:52 02:20 0.67 S6 4/21/2007 a 18:58 23:58 23:40 0.06 L9 5/14/2007 a NO NO 09:00 NO NO13 6/8/2007 a 01:10 10:30 05:00 0.7 S15 6/21/2007 a 11:48 18:14 18:14 0 L17 7/3/2007 a 04:30 18:00 16:30 0.12 S18 7/11/2007 c 03:44 20:22 10:32 0.69 S19 7/20/2007 c 08:50 7/21-03:30 12:40 0.26 S20 7/27/2007 a 01:33 05:03 05:03 0 L21 7/29/2007 a 03:08 12:48 08:28 0.81 S23 8/16/2007 a 02:18 08:34 04:58 0.74 S24 9/1/2007 c 14:00 9/2-06:40 14:25 0.03 F26 9/28/2007 a 14:28 21:00 17:38 0.94 S29 11/9/2007 c 11/8-19:40 11:30 10:56 0.04 L30 11/21/2007 a 11/20-18:40 12:40 03:16 0.92 S34 1/13/2008 a 16:30 21:40 21:40 0 L36 2/10/2008 a NO NO 07:16 NO NO

STEREO B3 4/1/2007 a 3/31-23:26 14:00 01:26 0.16 S5 4/17/2007 c NO NO 06:28 NO NO10 5/18/2007 a 04:00 5/19-04:00 06:24 0.11 S13 6/8/2007 c 6/07-20:16 10:00 03:56 0.79 S15 6/21/2007 c 6/20-23:40 03:20 03:20 0 L16 6/28/2007 a 11:30 17:45 16:20 0.29 S18 7/10/2007 c 15:07 7/11-07:44 20:17 0.45 S20 7/26/2007 a 01:54 23:00 08:20 0.44 S21 7/28/2007 c 08:00 7/29-07:30 18:00 0.74 S23 8/14/2007 a 16:10 20:00 17:20 0.44 S26 9/26/2007 c 15:50 19:30 18:10 0.57 S30 11/19/2007 c 13:50 11/20-07:00 15:55 0.14 S32 12/15/2007 a 22:00 12/16-20:45 23:22 0.06 F

aS means HPS is straddling HCS; L means HPS is leading HCS; and F means HPS is following HCS. For the events with overlap value smaller than 0.1, the HCS isconsidered to be bordering HPS.

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Among the 32 types (a) and (c) SB events we investigated, 29 events have HPSs in the vicinity of thecorresponding HCS; the other three events do not. The 29 HPSs include 20 straddling, six HPS leading, andthree following the HCS.

5. The Comparison of STEREO A and B Observations

Table 3 compares the corresponding HPSs observed by STEREO A and B for types (a) and (c) HCS events.The time delays for the HPSs observed by STEREO A and B are also listed with the expected timedelays assuming that HPSs continuously flow out of the Sun. The expected time delay Δt is the sum ofradial separation time tr and longitudinal separation time tL [Opitz et al., 2009]. The radial separation timeis tr =ΔR/Vsw, where ΔR is the heliocentric distance between STEREO A and B, and Vsw is the speed ofthe solar wind. The longitudinal separation time is tL =Δφ/ω, where Δφ is the longitudinal separation fromA to B and ω is the equatorial angular rotation speed of the Sun. As noted early, the longitude separationof the two spacecraft in this period monotonically increases from 3.03° to 40.65°. The heliocentric distancevaries in the range [0.05, 0.13] AU. With increasing longitudinal separation of STEREO A and B, the timedelay for the HPS also increases and the expected time delays match well with the observations inmost events.

Ten pairs of type (a) or (c) HCS events are observed on both STEREO A and B. Events 5 and 20 have differenttypes of HPSs. STEREO A observed a straddling HPS for Event 5, but no HPS was observed on STEREO B for thecorresponding event. For Event 20, a leading HPS was observed on STEREO A, a straddling HPS on STEREO B.The pitch angle distribution of suprathermal electrons, the magnetic field angle, the proton density, and thesolar wind speed are shown in Figure 4 for Event 2 observed by STEREO B on 17 April 2007. The HCS crossingwas between 6 UT and 7 UT. The background proton density was about 5 cm�3; there were some densityvariations around 6 UT, but the amplitude was too small to be identified as an HPS. Other eight event pairsobserved the same type HPS on STEREO A and B, with one pair (Event 15) observed leading HPSs on 21 June2007 and the other seven pairs observed straddling HPSs, both on STEREO A and B. The ratio for STEREO A andB to observe similar type of HPS is 80%.

Figure 2. STEREO A data of the type S HPS event observed on 17 April 2007. The four panels show (from top to bottom) thepitch angle distribution of suprathermal electrons, the phi angle of the interplanetary magnetic field, the proton density,and the solar wind speed. The solid vertical line marks the HCS, and the shaded region indicates the HPS.

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Small streamer blobs near 1 AU do not show signatures of expansion, since they have flat speed profiles andno significant change in temperature [Moldwin et al., 2000]. Recent studies using STEREO observationsshowed that blobs may have an angle spread of a wide range of values [Sheeley et al., 2009]. The longitudinalseparation of STEREO A and B observing the same type of HPS is as large as 40°, which might be within the

a

b

Figure 3. (a) A Type L HPS event observed by STEREO A. The format is the same as in Figure 2. (b) A Type F HPS eventobserved by STEREO A. The format is the same as in Figure 2.

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angle spread of the blobs. However, the time delays forSTEREO observing HPSs, instead of being 0, match wellwith the expected delays of plasma continuouslyejecting from the same region on the Sun’s surface(Table 3). Therefore, the HPSs are more likely to beformed by the higher density plasma continuouslyemitted from the Sun near the sector boundary. Theduration of the emitting process varies and the HPSs arequasi-stationary. Our observations show that the HPSscan last even up to 2 days.

We also note that HCSs/HPSs could be perturbed ordisrupted by transient structures in the solar wind such asshocks and CMEs [Liu et al., 2010]. In those cases, they willbe very dynamic and we will not be able to observe thesame type of identifiable HCSs. Therefore, these cases arenot studied here. When separation of the observingspacecraft increases to the scale for which the magneticconfiguration near the Sun’s surface has significant changein addition to the spin with the Sun, the HCS structure isnot expected to be coherent anymore.

6. Discussion and Conclusions

We greatly expand in this paper the study of Simunacet al. [2012] by including all events observed by STEREObetween 4 March 2007 and 29 February 2008. Inaddition, we use suprathermal electron and magneticfield data to select only those HCS events which can beconsidered to be a global structure. Limiting our studyto these events, we determined the HCS/HPS relativelocation and compared the observations on STEREO Aand B. Among the 32 HCS/SB events, 29 events haveHPSs in the vicinity, while for three events we did notidentify a HPS. The comparison of the HPSs observed onSTEREO A and B showed that, in most cases, the relativeHPS/HCS locations are similar. Thus, we draw thefollowing conclusions:

1. For the HCS events determined as a global structure,the related HPSs are observed in most cases and theseHPSs can have various relative locations to the HCS:straddling, leading, and following;

2. When the HCS stays as type (a) or (c) in interplanetaryspace, it tends to have a similar type of HPS in its vicinity.In addition, most of the time delays between STEREO Aand B match well with the prediction assuming thatHPSs are stationary. For such cases, we conclude that theHPSs are quasi-stationary for up to 48h. Stability forlonger time periods cannot be excluded, but will requirea similar analysis for events later in the mission whenthe separation in longitude is larger.

3. The HPSs flowing out from the Sun is shown schema-tically in Figure 5. Unlike intermittent “blobs,” whichmight originate from interchange reconnection, theflows are continuous up to 2 days.Ta

ble

3.Com

parison

ofHPS

sa

No.

Date

(mmdd

yyyy)

STER

EOA

Date

(mmdd

yyyy)

STER

EOB

V SW

(km/s)

HPS

Sepa

ratio

n(h)

Spacecraft

Sepa

ratio

n

Expe

cted

Time

Delay

(h)

HCS/SB

Type

HPS

Type

HCS/SB

Type

HPS

Type

ΔR(AU)

Δϕ(deg

)

33/31

/200

7C

S4/1/20

07a

S38

0�6

.68

0.05

8�3

.03

�0.9

54/17

/200

7A

S4/17

/200

7c

NO

305

NA

0.07

1�4

.49

�1.5

136/8/20

07A

S6/8/20

07c

S32

52.7

0.11

1�1

1.67

7.0

156/21

/200

7A

L6/21

/200

7c

L38

013

.52

0.11

8�1

4.11

12.7

187/11

/200

7C

S7/10

/200

7c

S37

512

.62

0.12

6�1

7.95

18.6

207/27

/200

7a

L7/26

/200

7a

S33

014

.85

0.12

9�2

1.21

22.3

217/29

/200

7a

S7/28

/200

7c

S35

012

.22

0.12

9�2

1.66

24.0

238/16

/200

7a

S8/14

/200

7a

S36

435

.35

0.12

8�2

5.40

31.5

269/28

/200

7a

S9/26

/200

7c

S41

548

.07

0.11

1�3

3.70

50.1

3011

/21/20

07a

S11

/19/20

07c

S35

529

.25

0.07

2�4

0.65

65.5

a VSW

isthesolarwindvelocity

observed

bySTER

EOB;

Spacecraftsepa

ratio

nbe

tweenAan

dBinclud

esΔR=R B

�R A

long

itudina

lsep

arationΔϕ=ϕB�ϕA.

Journal of Geophysical Research: Space Physics 10.1002/2014JA019956

LIU ET AL. ©2014. American Geophysical Union. All Rights Reserved. 9

However, the mechanism which causes the HPS near the Sun is still unknown. Nevertheless, interchangereconnection may not be able to sustain a continuous flow for such a long time period.

The HPSs near HCSs of types other than (a) or (c) may be dynamic. Figure 6 shows a type (e) HCS eventobserved by STEREO A which has a very large density enhancement to more than 80 cm�3, which is muchlarger than the proton density in the preceding slow solar wind (5–10 cm�3). The corresponding eventobserved on STEREO B does not have a similar magnitude of enhancement. It could be that the type (e) HCSsare caused by interchange reconnection and the HPS in the vicinity of such types of HCS may be related tothe “blobs” and they are dynamic. Further investigations on HCS events of such type are required in the

Figure 4. A HCS crossing event with no HPS. The format is the same as in Figure 2.

Figure 5. Schematic plots for two types of heliospheric plasma sheet flowing out of the sun. (top) Plasma sheet straddlingthe heliospheric current sheet. (bottom) Plasma sheet bordering the heliospheric current sheet.

Journal of Geophysical Research: Space Physics 10.1002/2014JA019956

LIU ET AL. ©2014. American Geophysical Union. All Rights Reserved. 10

future. Future missions like Solar Probe Plus may supply valuable data from the Sun and from interplanetaryspace that will provide a better understanding of the plasma originating from the inner solar corona.

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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 work at UNH issupported by NASA under contractNAS5-00132, grants NNX10AQ29G andNNX13AP39G. The solar wind and IMFdata are provided by the STEREOScience Center (stereo-ssc.nascom.nasa.gov). The authors are also grateful forY. D. Liu at the Center for Space Scienceand Applied Research in China forhelpful discussions.

Yuming Wang thanks Paulett Liewerand another reviewer for theirassistance in evaluating the paper.

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