The Heliospheric Plasma Sheet Observed in situ by Three Spacecraft over Four Solar Rotations
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In this paper we present in situ observations of the heliospheric plasma sheet (HPS) from STEREO-A, Wind, and STEREO-B over four solar rotations in the declining phase of Solar Cycle 23, covering late March through late June 2007. During this time period the three spacecraft were located in the ecliptic plane, and were gradually separating in heliographic longitude from about 3 degrees to 14 degrees. Crossings of the HPS were identified using the following criteria: reversal of the interplanetary magnetic field sector, enhanced proton density, and local minima in both the proton specific entropy argument and in the alpha particle-to-proton number density ratio (Na/Np). Two interplanetary coronal mass ejections (ICMEs) were observed during the third solar rotation of our study period, which disrupted the HPS from its quasi-stationary state. We find differences in the in situ proton parameters at the HPS between the three spacecraft despite temporal separations of less than one day. We attribute these differences to both small separations in heliographic latitude and radial evolution of the solar wind leading to the development of compression regions associated with stream interaction regions (SIRs). We also observed a modest enhancement in the density of iron ions at the HPS.
Since the 1960s, in situ spacecraft measurements of the interplanetary magnetic field (IMF) have shown the direction of the IMF in the ecliptic plane alternates between “towards” and “away” sectors as a quasi-stationary structure that co-rotates with the Sun (Wilcox and Ness 1965). Threading through interplanetary space between the regions of opposite polarity is the heliospheric current sheet (HCS), which was found to correlate with the streamer belt at the Sun (Gosling et al.1981; Borrini et al.1981). More recently, Winterhalter et al. (1994), Crooker et al. (2004), and Suess et al. (2009) found that the HCS is embedded in, or adjacent to, a slow solar wind with enhanced proton density: the heliospheric plasma sheet (HPS).
Enhanced proton density and a change in magnetic sector are not the only in situ identifiers of the HPS. Burlaga, Mish, and Whang (1990), and Siscoe and Intriligator (1993) noted a drop in entropy correlated with the HPS. Winterhalter et al. (1994) found enhanced plasma β to be another criterion for HPS identification. A local minimum in alpha particle-to-proton number density ratio (Na/Np) was found in the studies of Borrini et al. (1981), Geiss, Gloeckler, and von Steiger (1995), Bavassano, Woo, and Bruno (1997), and Suess et al. (2009). Suess et al. (2009) and Liu et al. (2010a) found a local depletion in the oxygen-to-proton number density ratio at the HPS. Suess et al. (2009) also found a possible increase in the average iron charge state at the HPS, noting that it could be a remnant of interplanetary coronal mass ejection (ICME) material.
The observations presented here are from ST-A, Wind, and ST-B. Wind plasma and electron pitch angle data are from the Solar Wind Experiment (SWE, Ogilvie et al.1995). Wind magnetic field data are from the Magnetic Field Investigation (MFI, Lepping et al.1995). STEREO magnetic field data are from the Magnetic Field Experiment (MAG, Acuña et al.2008). Electron pitch angle distributions (PADs) are from the Solar Wind Electron Analyzer (SWEA, Sauvaud et al.2008). MAG and SWEA are both part of the In-SituMeasurements of Particles and CME Transients (IMPACT) suite (Luhmann et al.2008). Plasma data are from the Plasma and Suprathermal Ion Composition (PLASTIC) experiment (Galvin et al.2008). The process by which the PLASTIC solar wind plasma data were obtained is described below.
The PLASTIC investigation is novel in that each flight unit functions like three separate instruments: a solar wind proton/alpha monitor, a solar wind heavy ion composition experiment, and a plasma and suprathermal composition sensor away from the Sun–spacecraft line. Multi-functionality gives rise to a need for multiple geometric factors and fields-of-view within the same instrument. The first major subdivision within the instrument is based upon the in-ecliptic (azimuth) field-of-view. The region that covers the Sun-spacecraft line ± 22.5 degrees is the Solar Wind Sector (SWS). The remaining portion is called the Wide Angle Partition (WAP). All the data presented here are from the SWS. The SWS can be further subdivided into two sections based upon entrance aperture: the larger geometric factor “Main” channel intended for heavy ions, and the smaller geometric factor “S” channel for more abundant solar wind species such as protons. The PLASTIC electro-static analyzer (ESA) steps through logarithmically spaced energy-per-charge (E/Q) steps from about 80 keV e−1, down to about 0.2 keV e−1. At the start of each ESA sweep the small channel is gated closed with an electric field, and the Main channel is open. Upon reaching a set (but command-able) count threshold, the Main channel is closed and the S channel is opened. The larger geometric factor in the Main channel allows for sufficient counting statistics of heavy solar wind ions, and the switch to the smaller geometric factor prevents the electronics and solid-state detectors from being inundated with the high-flux solar wind protons. In-flight data show evidence that when the S channel entrance is in use a higher than expected fluence of low E/Q ions is obtained (based on pre-launch testing and calibration). The distribution of counts appears to be bifurcated in azimuth angle when the S channel is engaged. This behavior has been linked to insufficient fringe-field control in the upper section of the entrance system, which results in only partial suppression of ions through the main channel gate. Proton data have been recalibrated after an extensive in-flight characterization of the S channel behavior. (See Opitz (2007) and Simunac (2009).) Iron data were acquired in the main channel, where this is not an issue and pre-launch calibration is applicable.
3 Plasma Sheet Identification
In order to confirm that we identified a true sector boundary (rather than a local field reversal), we examined both the magnetic field data and the electron PAD (cf. Crooker et al.1996). ST-A crossed the HCS on 31 March around 22:15 UT, when the electron flux distribution changed from predominantly 0 degrees to predominantly 180 degrees. This switch in the PAD from the “away” to the “towards” polarity shows that we have identified a true sector boundary. In this case the sector boundary agrees with the location of the HCS, which is shown with a vertical dashed line in Figure 2. At the HCS the radial and tangential magnetic fields both undergo and sustain a change in polarity. To identify the associated HPS we look for increased proton density (Winterhalter et al.1994), accompanied by low proton entropy (Burlaga, Mish, and Whang 1990; Siscoe and Intriligator 1993) and a drop in Na/Np (Borrini et al.1981; Geiss, Gloeckler, and von Steiger 1995; Suess et al.2009).
STB start [UT]
STB stop [UT]
STB Vp [km s−1]
STB radial width [106 km]
Wind start [UT]
Wind stop [UT]
WindVp [km s−1]
Wind radial width [106 km]
STA start [UT]
STA stop [UT]
STA Vp [km s−1]
STA radial width [106 km]
4.1 CROSSING 1, CR 2054
4.2 CROSSING 2, CR 2055
4.3 CROSSING 3, CR 2056
The sector boundary crossing during CR 2056 was modified by the passage of magnetic clouds associated with eruptions from active region 10956. Kilpua et al. (2009) identified two magnetic clouds encountered by ST-A during 22 – 23 May 2007, while ST-B and Wind encountered one magnetic cloud during this period. Based on Grad–Shafronov reconstruction, Kilpua et al. (2009) found ST-B and Wind crossed near the center of the first cloud, while ST-A received just a glancing blow. The second cloud delivered a much more direct impact to ST-A (Möstl et al.2009), and had a slight encounter with Wind, but missed ST-B. (See Figures 1 and 6 of Kilpua et al.2009.)
Assuming an equatorial, sidereal, solar angular rotation speed of 14.38 degrees per day (Newton and Nunn 1951) and taking into account the motion of the spacecraft, the third HPS crossing was expected to take place on 23 May 2007, when the STEREO observatories were separated by about 9 degrees in heliographic longitude and 0.1 AU in radial distance from the Sun.
4.4 CROSSING 4, CR 2057
5 Summary and Discussion
We have combined in situ observations from three in-ecliptic spacecraft slowly separating in heliographic longitude to study successive crossings of the HPS over four solar rotations. This study is unique in its multiple vantage points for each crossing of the HPS. We can therefore compare plasma parameters not only from one solar rotation to the next, but within the same Carrington rotation over time scales of hours. We have chosen several questions to address with this data set. First, can the HPS be better described as transient (cf. Suess et al.2009) or quasi-stationary (cf. Liu et al.2010b) in nature? Second, what is the origin of disparities in the HPS properties from one spacecraft to another when the temporal separation between observations is much less than a Carrington rotation? Third, what can we learn about the solar source region of the HPS?
Foullon et al. (2009) studied the same HPS crossing in CR 2053 and found the width along the Sun–Earth line to be 560 – 580 RE, or 3.6 – 3.7×106 km. The average width from the three observations in CR 2054 is 5.6×106 km, and in CR 2055 it is 7.7×106 km. In other words, the radial extent of the HPS is growing from one CR to another before it is disrupted by the ICME in CR 2056.
To sum up: Based on three-point observations, the undisturbed HPS was found to lie roughly parallel to the idealized Parker spiral in the ecliptic plane. The location of the HCS within the plasma sheet was similar between all three spacecraft within a Carrington rotation, indicating that the HPS is quasi-stationary over the time scale of about a day, rather than transient in nature. However, the HPS clearly evolved from one solar rotation to the next. A further study is planned to narrow the range over which the HPS can be described as quasi-stationary. Next, the kinetic solar wind parameters associated with the HPS observed at the respective spacecraft were not identical, even though the temporal separation between observations was small, less than 1 day. Separation in heliographic latitude, though small, is the most likely explanation for the disparities between observation points. Lastly, in the absence of ICME material we observed a modest enhancement in the density of iron ions with each crossing of the HPS, similar to the observations of Schmid, Bochsler, and Geiss (1988). This supports the conclusion of Wimmer-Schweingruber (1994) that gravitational stratification at the solar source region is not responsible for the well-observed Na/Np depletion associated with encounters of the HPS. This suggests the source of the HPS material sampled in this study was not streamer cores.
This work was supported at UNH under NASA contract NAS5-00132, and grant NNX10AQ29G.
K.D.C. Simunac would like to thank Claire Foullon and Robert Wimmer-Schweingruber for helpful discussions.
We thank the STEREO/SECCHI team for making their data available.
We thank the ACE SWEPAM and ACE SWICS instrument teams, and the ACE Science Center for providing the ACE data.
This work utilizes data obtained by the Global Oscillation Network Group (GONG) Program, managed by the National Solar Observatory, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. The data were acquired by instruments operated by the Big Bear Solar Observatory, High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de Astrofísica de Canarias, and Cerro Tololo Interamerican Observatory.
Simulation results have been provided by the Community Coordinated Modeling Center at Goddard Space Flight Center through their public Runs on Request system (http://ccmc.gsfc.nasa.gov). The CCMC is a multi-agency partnership between NASA, AFMC, AFOSR, AFRL, AFWA, NOAA, NSF and ONR. The Wang–Sheeley–Arge (WSA) Model was developed by Nick Arge at AFRL.
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