We report the first two-spacecraft (Wind and ACE) detection of oppositely directed plasma jets within a bifurcated current sheet in the solar wind. The event occurred on January 3, 2003 and provides further direct evidence that such jets result from reconnection. The magnetic shear across the bifurcated current sheet at both Wind and ACE was ∼150°, indicating that the magnetic shear must have been the same at the reconnection site located between the two spacecraft. These observations thus provide strong evidence for component merging with a guide field ∼ 30% of the antiparallel field. The dimensionless reconnection rate based on the measured inflow was 0.03, implying fast reconnection.
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 Reconnection is a universal plasma process that converts magnetic energy into particle energy. It is believed to have a pivotal role in a variety of astrophysical contexts including solar flares, coronal mass ejections and solar coronal heating; planetary magnetopauses and magnetotails; cometary tails; and accretion disks.
 Recently, it was pointed out that reconnection occurs in solar wind current sheets [Gosling et al., 2005a, 2005b]. The key evidence for reconnection reported by Gosling et al. [2005a] included observations of (1) Alfvénic accelerated flows confined to field reversal regions characterized as bifurcated current sheets and (2) associated field-aligned, inter-penetrating ions beams that demonstrated magnetic connection across the current sheets. They interpreted these events as encounters with Petschek-like exhausts emanating from reconnection sites.
 Solar wind current sheets are ideal for studying the large-scale properties of reconnection because these sheets are very large scale and do not have the limiting and fluctuating boundary conditions that are often found in the laboratory, at the magnetopause or in the geomagnetic tail. They are also ideal because the solar wind rapidly convects them past an observing spacecraft. Phan et al.  reported multi-spacecraft detections of reconnection exhausts from which it could be inferred that extended reconnection X-lines are common in the solar wind; in one case the X-line extended more than 2.5 × 106 km (390 Earth radii, RE). To date solar wind reconnection events have been reported spanning heliocentric distances ranging from 1 to 5.4 AU and heliographic latitudes ranging from 79°S to 65°N [Gosling et al., 2006].
 In every solar wind reconnection event reported so far all observing spacecraft have been on one side of the reconnection site detecting a single jet. The existence of the oppositely directed jet has always been implicitly assumed but not yet confirmed. The single jet observations did not reveal how far away a reconnection site was from the spacecraft. Furthermore, with single jet observations, although the local magnetic shear at an exhaust may be substantially less than 180° (the local magnetic shear across the exhaust for the large number of events reported by Phan et al.  and Gosling et al. [2005a, 2006] ranged from ∼90° to ∼180°) one can not rule out the possibility that the current sheet is warped or that field lines are not straight and thus that the magnetic shear at a distant reconnection site may be significantly different from the local shear.
 Here we report in-situ dual spacecraft detection, by Wind and ACE, of oppositely directed plasma jets emanating from a common reconnection X-line in the solar wind located between the two spacecraft, thus verifying an essential element in all reconnection scenarios. These observations also provide direct evidence for component reconnection.
Figure 1 shows ACE and Wind proton density, temperature, velocity and magnetic field measurements in GSE coordinates for the 08:00UT–10:00 UT interval on January 3, 2003 during which time both spacecraft crossed the same bifurcated solar wind current sheet.
ACE: Figures 1a–1e show that ACE crossed a bifurcated current sheet between 08:24:30–08:28:30 UT. At the time of the crossing ACE was located at [241.6, −14.7, 19.8] RE GSE. Figure 1c shows that during the crossing there was a significant negative enhancement of the x-component of the plasma velocity and smaller positive enhancements in the y and z-components. Figures 1a, 1d, and 1e show that there was a drop in the magnetic field strength together with enhancements of ∼50% in the proton density and temperature in the current sheet (relative to the ambient solar wind). The shear of the magnetic field across the exhaust was ∼152°.
Wind: Approximately one hour after ACE encountered this jetting plasma, between 09:25:30–09:29:00 UT (Figures 1f–1j), Wind crossed this same current sheet and encountered an oppositely directed jet (Figure 1h) with a positive enhancement in the x-component of the velocity and negative enhancements in the y and z-components. The angle between the flow enhancement detected at ACE, [−46, +19, +8] km/s, and the flow enhancement detected at Wind, [+42, −16, −11] km/s, was ∼175°, indicating that the jets were nearly oppositely directed. At the time of the crossing Wind was located at [178.2, −96.5, −3.3] RE GSE downstream and dawnward of ACE (see Figure 2). The magnetic shear across the exhaust at Wind was ∼145°. The similarities between the magnetic field profiles of the bifurcated current sheets detected at Wind and ACE and the related magnetic shears indicate that both spacecraft crossed the same current sheet.
Figures 1f, 1i, and 1j show that the magnetic field strength decreased and the proton density and temperature increased by ∼50% in the current sheet detected by Wind, similar to the changes detected by ACE. The enhanced density and temperature and depressed magnetic field strength across the boundaries of the exhaust are qualitatively consistent with slow-mode waves (in addition to rotational discontinuities) bounding the exhaust.
3. Analyses and Discussions
3.1. Geometry of the Current Sheet
 To calculate the normal to the reconnecting current sheet we performed a minimum variance analysis [Sonnerup and Cahill, 1967] at Wind. We calculated the normal, N, to be [0.42, 0.75, 0.51] GSE. We verified the normal by comparing the delay between the arrival times of the exhausts at the two spacecraft to that predicted for exhausts with the above normal orientation propagating with the solar wind. The predicted delay was accurate to within 8.5 minutes or 14% of the total delay.
 We next determined the X-line orientation, M, in the plane of the current sheet. We used the relation M = N × (BA − BB)/∣BA − BB∣ where BA and BB are the tangential (to the current sheet) magnetic field vectors on the two sides of the exhaust and N is the overall current sheet normal [Sonnerup, 1974]. The X-line orientation was found to be [0.18, 0.48, −0.86] GSE.
 We then constructed a LMN coordinate system in which the normal formed the N-component, the X-line formed the M-component and the relation M × N formed the L-component. Figure 3a shows the Wind magnetic field data in the LMN coordinate system. The normal component of the magnetic field was small (<1 nT) across the exhaust, the M-component had a steady value of ∼−3.5 nT and the field rotation occurred almost entirely in the L direction with BL changing by ∼21.5 nT. The M-component of the field was thus about 30% of the average of the L-component on opposite sides, consistent with the 145° field shear. Figures 3b and 3c show the flow velocity magnitude and components at Wind, respectively. Wind observed a maximum positive enhancement in VL of ∼47 km/s within the bifurcated current sheet. When the minimum variance analysis was performed across the exhaust at ACE, we obtained a normal direction of [0.40, 0.68, 0.62] GSE, thus within 8° of the normal at Wind. This indicates the 1-D nature of the solar wind current sheet on the scale of a few hundred Earth radii. Figure 4a shows the ACE magnetic field data in the ACE LMN coordinate system. Similar to Wind, ACE observed the normal component of the magnetic field to be small (<1 nT) across the exhaust and the field rotation occurred predominately in the L direction. ACE did, however, observe a negative enhancement to the M-component of the magnetic field during the exhaust crossing, which was not seen at Wind. Figures 4b and 4c show the proton velocity magnitude and components, respectively, and demonstrate that ACE observed a maximum negative enhancement in VL of ∼50 km/s within the exhaust.
 By projecting the intersection of both spacecraft with the current sheet on the LMN coordinate system (see Figure 2), we calculated that ACE and Wind were separated by ∼12 RE along the X-line (or M direction). This is small compared to the spacecraft separation of ∼200 RE along the exhaust (L) direction.
3.2. Further Evidence for Reconnection
 The oppositely directed plasma jets observed within the January 3, 2003 bifurcated current sheet are strong confirming evidence for reconnection since such pairs of jets are expected from reconnection. Since it is predicted that for symmetric reconnection in the presence of a guide field (of 30% in this case), a substantial part of the plasma acceleration is accomplished across a pair of rotational discontinuities bounding the exhaust (and upstream of the slow wave pair) [Lin and Lee, 1994], we further compare the observed plasma velocity enhancements in the exhaust with the prediction given by the Walen relation [Hudson, 1970; Paschmann et al., 1986]:
Here B, V, and ρ are the magnetic field vector, the proton flow velocity and the proton density, respectively. Alpha is the pressure anisotropy factor and is defined α ≡ (P∥ − P⊥)/μ0∣B∣2 where P∥ and P⊥ are the plasma pressures parallel and perpendicular to the magnetic field, respectively. The choice of sign is based on which side of the reconnection site the observer is located. Subscript “1” denotes the reference time and “2” denotes the prediction for all other times.
Figures 3b and 3c show the predicted (based on equation (1)) and observed plasma flow velocity in the LMN coordinate system for Wind. The negative (positive) sign in equation (1) is chosen for the leading (trailing) edge of the bifurcated current sheet. The left (right) vertical dashed lines in Figure 3 denote the reference time for the prediction of reconnection flow acceleration at the leading (trailing) edge of the exhaust. The leading and trailing edge predictions merge at 9:27:35 UT. The observed flow speed reached 70–80% of the predicted flow speed immediately across the leading and trailing edges of the exhaust, but only 60% at the center of the exhaust.
Figures 4b and 4c show the predicted and observed proton velocity magnitudes and components in the LMN coordinate system at ACE, respectively. In contrast to Wind, the positive (negative) sign in equation (1) is chosen for the leading (trailing) edge of the bifurcated current sheet at ACE. The observed velocity reaches 70% of the maximum predicted magnitude.The 20–40% discrepancy between the observed and predicted flow velocity may be due in part to the fact that the Walen test was performed across both the rotational discontinuity and the slow mode wave whereas this test is valid only across a single rotational discontinuity. Additional reasons for the discrepancy may include the neglect of He++ and the effect of inter-penetrating ion beams not taken into account in the Walen test. Such inter-streaming ion beams, of the type first reported by Gosling et al. [2005a], were also observed by both ACE and Wind during this event (not shown), consistent with magnetic connection from one side of the exhaust to the other.
3.3. Reconnection Rate and the Location of the X-Line
Figure 3d shows that in the direction normal to the exhaust direction the flow velocity measured at Wind was nearly constant except for a small 4 km/s shift. This velocity shift is consistent with a reconnection inflow, in the frame of the current sheet, of vn,rec = 2 km/s. With a 10 nT magnetic field convecting into the reconnection region at 2 km/s, the reconnection electric field was 0.02 mV/m. The dimensionless reconnection rate, vn,rec/vA, was 3%, where the external Alfvén speed was vA = 62 km/s. The corresponding reconnection exhaust wedge angle was 3.7°. The 3% reconnection rate also implies a normal magnetic field of 0.3 nT, which is beyond the accuracy of the magnetic field measurements and boundary normal determination from the minimum variance analysis, but consistent with our small (<1 nT) measured BN. A 3% rate is in the range of fast reconnection.
 The distance to the stationary reconnection site can be estimated as ΔL = (vA/vn,rec) Δs, where Δs is the half width of the exhaust. For a solar wind speed of ∼200 km/s along the exhaust normal and the 190 second duration of the exhaust crossing, the exhaust width was 3.8 × 104 km or 550 ion skin depths. Thus the distance from Wind to the X-line along the exhaust is estimated to be ∼5.9 × 105 km, or ∼93 RE (8500 ion skin depths) i.e., the X-line lay approximately half way between ACE and Wind. This is consistent with the fact that the exhaust widths at ACE and Wind were roughly the same. This consistency check validates our estimate of the reconnection rate (of 3%).
 It should be pointed out that although the estimated reconnection rate of 3% falls in the range of fast reconnection, we do not find any evidence for the quadrupolar Hall magnetic field (BM in Figures 3a and 4a) 8500 ion skin depths away from the X-line. There are fluctuations in BM in the exhaust but these variations are not consistent with the Hall field pattern, and their magnitude is substantially less then the 30–40% level (of the anti-parallel field) that had previously been reported in the vicinity of the X-line in the magnetosphere [e.g., Nagai et al., 2001; Øieroset et al., 2001]. This could suggest that the Hall field diminishes in strength with distance from the X-line.
3.4. Evidence for Component Reconnection
 The magnetic shears across solar wind exhausts reported in previous studies [e.g., Gosling et al., 2005a, 2006] were often substantially less than 180°, suggesting reconnection with a guide field. However, without knowledge of the location of the X-line one could not exclude the possibility that the magnetic shear at the reconnection site may be substantially different from what a spacecraft measures far away from it.
 In this present case, since the X-line was located approximately midway between ACE and Wind, the fact that the magnetic shear was nearly identical at the two spacecraft indicates that the magnetic shear at the X-line must have been close to 150° as well. This corresponds to a guide field of ∼30% of the anti-parallel field. Thus the January 3, 2003 event must have resulted from component reconnection.
 The January 3, 2003 detection of oppositely directed jets emanating from a reconnection site located between the ACE and Wind spacecraft provides strong confirmation that this and similar accelerated flow events result from magnetic reconnection in the solar wind, as previously interpreted. The dimensionless reconnection rate based on the inflow velocity was 3%, similar to the reconnection rate obtained for the 2002-02-02 event reported by Phan et al. . Furthermore, the similar magnetic shears measured at both spacecraft for the oppositely directed exhausts provides strong evidence for a 30% guide field in this event. Finally, the detection of the oppositely directed reconnection exhausts by the 2 spacecraft one hour apart indicates that reconnection with such a guide field can be continuous for at least an hour, or 550 ion gyroperiods.
 We thank the principal investigators of Wind 3DP and MFI and ACE SWEPAM and MAG experiments for making their data available. We appreciate helpful discussions with Mike Shay and Jim Drake. This research was funded by NASA grants NAG05GD79G at UC Berkeley, NNG06GC27G at the University of Colorado, and NSF grant ATM-0613886.