Observations of a Quickly Flapping Interplanetary Magnetic Reconnection Exhaust

On the basis of the Petschek reconnection model and the characteristics of reconnection, hundreds of reconnection exhausts were reported in the solar wind. Many multi-spacecraft observations also indicated that interplanetary magnetic reconnection is a quasi–steady-state plasma process and the reconnection X-line can extend hundreds of Earth radii. In this study, we report an interplanetary flapping reconnection exhaust observed by Wind on April 1, 2003 at one AU. The magnetic reconnection event has two adjacent accelerated flows. We compared the plasma and magnetic characteristics of the two accelerated flows and found that the second accelerated flow was due to the back-and-forth movement of the reconnection exhaust. Our observations reveal that not all interplanetary reconnections operate in a quasi–steady-state manner; some reconnection current sheets can move rapidly back and forth.


INTRODUCTION
Magnetic reconnection is a universal plasma process that not only converts magnetic field energy into plasma kinetic and thermal energies but also breaks and reconnects oppositely directed frozen-in field lines from different plasma regions. This process is implicated in various contexts, including space physics, solar physics, and laboratory plasma physics. The authors in reference [1,2] first established the Sweet-Parker model to describe magnetic reconnection using observations from solar flare activity. One of the model's limitations however is that the reconnection rate is much slower than necessary for flare eruption. In 1964, the authors in reference [3] proposed a new reconnection model and pointed out that the magnetic reconnection is a topological process, and the field lines need not reconnect resistively along the entire length of the boundary layer but could merge over a short length. The key feature of the Petschek model is its assumed separation between the small length scale of the non-ideal electric field, which is responsible for breaking magnetic field lines, and the large length scales on which energy is released and dissipated [4]. The Petschek model yields a sufficiently fast reconnection rate by virtue of the localized diffusion region and the extended slow shocks. Reconnection is also characterized by the acceleration of plasma away from the reconnection site in a pair of oppositely directed exhaust regions. The authors in reference [5] proposed a quasi-stationary reconnection exhaust criteria based on the Petschek reconnection model and the characteristic feature of the reconnection process. The requirements to identify reconnection exhausts mainly include 1) a roughly Alfvénic accelerated flow, 2) the enhancements of proton density and temperature, and 3) depression of the magnetic field strength. In accordance with the reconnection exhaust criteria, a large member of observed solar wind reconnection events has been identified and reported (e.g., [5][6][7][8][9][10][11][12][13][14][15][16][17][18]). Multi-spacecraft observations also indicated that the magnetic reconnections in the solar wind are quasi-steady plasma processes, reconnecting current sheets are roughly planar, and the reconnection X-lines can extend hundreds of Earth radii [6,12,[19][20][21]. Although most reconnection observations revealed that exhausts have quasi-steady, roughly planar structures, several reported exhausts were re-entered one or two times by the same spacecraft after the main exhaust encounter [11,[22][23][24][25][26]. Since these re-entry durations were all short, the reported narrow re-entry was interpreted as that exhaust boundaries were non-planar due to their local smallscale folds, even the folds can be orthogonal to the reconnection plane [26]. In addition, the quick flapping of reconnection exhaust may also cause that exhausts were re-entered by one spacecraft. Here, a possible swinging reconnection exhaust observed by Wind on April 1, 2003 was presented. Frontiers in Physics | www.frontiersin.org September 2021 | Volume 9 | Article 736319 OBSERVATIONS Figure 1 illustrates the suprathermal electron pitch angle spectrogram, magnetic field, and plasma data of the magnetic reconnection event, and the following short, fast magnetic structure, which has been identified as a small flux rope in a small-scale magnetic flux rope database in reference [27], measured by Wind at (228.06, 83.01, 10.58) R E (R E 6,378 km) in the GSE coordinate system. The top plot displays the suprathermal electron pitch angle spectrograms of 165.0 eV. The subsequent plots show the strength of the magnetic field (B); the x, y, and z components of the magnetic field (B x , B y , B z) ; the x, y, and z components of the proton speed (V x , V y , V z ); proton density (N); and the proton temperature (T p ), respectively. The suprathermal electron data were obtained by using a 3D plasma (3DP) analyzer, and the time resolution was approximately 97 s (Lin et al. 1995). The resolution of the plasma and magnetic field data is 3 s. Figure 1 also indicates that the magnetic reconnection event occurred at the interface between a faster magnetic structure and its slower frontal background solar wind. The faster magnetic structure lasts less than 3 h, and it exhibits smooth rotations along the magnetic field directions. Similar to a magnetic cloud, the faster magnetic structure may have a closed magnetic configuration because of its smooth field rotations. Magnetic reconnection may occur because of the compression of the following faster closed magnetic structure. Figure 2 provides the magnified images of the suprathermal electron spectrogram, magnetic field, and plasma curves of the magnetic reconnection event.
The authors in reference [13] first identified and reported the magnetic reconnection event, and they also determined the leading boundary of the exhaust at around 02:14:19 UT and found that the duration of the exhaust is 34 min, that is, their identified rear boundary is at around 02:48:19 UT. They also obtained a field shear angle of 153°across the exhaust. Figure 2 also reveals that the accelerated plasma flow and enhanced proton temperature and proton density regions are mainly at 02:14:19-02:27:52 UT, where the magnetic strength exhibits an apparent depression. In addition, the phase space densities of suprathermal electrons display an apparent enhancement in the region, and these suprathermal electrons may be heated by the reconnection event. In Figure 2, it seems that the reconnection event involves two adjacent exhausts, whose boundaries were denoted by four magnetic field directional discontinuities (DDs). The first possible exhaust event is about within 02:14: 19-02:21:34 UT, and the second possible exhaust event is from 02:23:31 UT to 02:27:52 UT. Both events have apparent exhaust signatures: the accelerated plasma flow, the enhanced density and temperature, and the depressed magnetic field strength. However, for the first event, the changes between velocity and magnetic field components are correlated with one another at the front boundary of the event and are anticorrelated at the rear boundary, but the changes in velocity and magnetic field are anticorrelated at both sides of the second one. That is to say, only the first event can be identified as a reconnection exhaust according to the reconnection exhaust criteria of Gosling et al. (2005a). Comparing observed features of the two accelerated flow events, we can find that the two events have some similar characteristics. This may indicate that the two accelerated flow events were just one exhaust, which was detected twice in a short time by the Wind spacecraft. As mentioned before, both folds of exhaust boundaries and flapping of reconnection exhausts can cause that exhausts were re-entered by one spacecraft. In the following, we will analyze and discuss the cause of the re-entered reconnection exhaust.
The spacecraft will detect different signals under different scenarios. For the scenario of flapping reconnection exhaust, the spacecraft detects the rear partial exhaust repeatedly, namely, the spacecraft passes through the same plasma environment twice. Therefore, the observed plasma and magnetic features of the partial re-entered exhaust should be similar to that of the rear part of the main exhaust. In addition, the rear boundary of the main exhaust and the both boundaries of the partial re-entered exhaust should have the same DD, which was crossed three times by the spacecraft. Although the DD moved back and forth with the exhaust, it may keep the roughly planar structure. For the scenario of folds in the rear boundary, the trajectory of the spacecraft should be close to the warping rear boundary so that it can cross its rear boundary more than two times. The spacecraft detects different plasma environments at every turn. Therefore, partial of observed plasma features of two accelerated (or decelerated) plasma flows may be similar, such as proton density and the proton temperature for the two scenarios. But the observations of the magnetic field directions and the proton velocities should be different for the two scenarios [26]. Especially, the rear boundary of the first accelerated flow and both boundaries of the second accelerated flow should be not planar in the fold scenario. The main differences between the two scenarios are illustrated in Figure 3. Now, we examine the two adjacent accelerated flows observed by Wind in more detail. Figure 2 reveals that the plasma and magnetic characteristics of the two intervals 02:14:19-02:21:34 UT and 02:23:31-02:27:52 UT are approximately consistent, and these characteristics include magnetic field strength, the magnetic field direction, the proton velocity, proton density, and the proton temperature. Especially their profiles of velocity components are almost unanimous. In addition, the profiles of the magnetic field and solar wind speed are symmetric at about 02:22:28 UT. Therefore, we infer that two adjacent accelerated flows only represent one exhaust detected twice simply due to its back-andforth movement. If so, all the directional discontinuities D2, D3, and D4 are the rear boundary of the exhaust, but D3 is crossed by Wind from the outside. All the three directional discontinuities should have consistent variation characteristics. Figure 4 gives the overlay of time-shifted magnetic field data around the three directional discontinuities. The data of D3 and D4 were shifted forward in time by 135 and 378 s, respectively, the time series of D3 are reversed. Figure 4 demonstrates that three sets of curves have a similar changing trend, but jump curves of D2 and D4 are steeper than those of D3. This indicates that the Wind spacecraft took more time to cross D3 than to cross D2 and D4. The delayed effect of D3 can be explained with back-and-forth movement of the exhaust. After Wind passed through the rear boundary the first time, flapping reconnection exhaust with the solar wind departs from the Wind satellite quickly. For the second encounter, the rear boundary needs to come back to scan the Wind satellite. Thus, the second traversing speed must be lower, and correspondingly, its traversing duration was longer. Now that D2, D3, and D4 represent that the rear boundary was crossed at different time, the three DDs should have similar normal directions. A minimum variance analysis (MVA) technique is frequently used to estimate DD normal; here, we also estimated the normals of the three DDs with the MVA technique, and Table 1 lists their normal vectors in the GSE coordinate. From Table 1, one can find that the three normal directions are coincident, and the angles between two of the three normal vectors are in the range of 2.72°and 6.06°. In conclusion, our findings suggest that the reconnection current sheet was quickly moving back and forth when the Wind spacecraft crossed the reconnection exhaust.

SUMMARY AND DISCUSSION
In this study, we report an interplanetary magnetic reconnection event, which was detected by the Wind spacecraft on April 1, 2003.
Wind's observations show that the magnetic reconnection event has two discrete accelerated flows, and there is an about 2-min interval between the accelerated flows. The second accelerated flow  indicates that Wind re-entered the main exhaust. But the cause of the re-entered exhaust is different from previously reported reentered exhaust [11,[22][23][24][25][26]. A previous work suggests that the reentered parts result from the folded local exhaust boundaries. However, the analysis of the reconnection event presented here indicated that the re-entered parts resulted from quickly flapping of reconnection exhaust. The traversing times of D2 and D3 were about 30 and 60°s, respectively. The speed of the solar wind in the normal direction of the rear boundary was about 313 km/s. Taking this speed as the traversing speed of D2, the traversing speed of D3 should be 156.5 km/s. Such a traversing speed was very likely to be overestimated since D2 was probably moving in a slower speed due to the flapping motion. However, the flapping speed could not be smaller than 313 km/s; otherwise, the observations by spacecraft may not be able to reveal the flapping movement. The duration between D2 and D4 was ∼330°s. Therefore, the amplitude of the flapping motion was at least 16 R E (taking the flapping speed as 313 km/s). This reconnection exhaust was also detected by ACE at (−5, −29, −65) R E relative to the position of Wind 22 min later [13]. The observations of ACE are shown in red curves in Figure 2 (shifted toward 22 min in time for clarity). The rear boundary of the reconnection exhaust at ACE was thought to be at ∼02:18:20 (corresponding to a steep and strong change in Bz). Therefore, the duration of the reconnection exhaust at ACE was 260°s, which was less than that of 450°s at Wind. The normal direction of the reconnection exhaust was estimated to be (−0.32, −0.93, 0.20) at ACE and (−0.45, −0.85, 0.27) at Wind by applying the hybrid minimum variance analysis [28] method to the whole exhaust. The angle between the two normal was ∼8°. Therefore, the scale of the reconnection exhaust in the normal direction was ∼4.5 R E at ACE and 10.5 R E at Wind. At ∼02:22:50, ACE detected another steep and strong change in Bz. However, it is hard to decide if this steep variation in Bz was caused by re-enter the rear boundary of the reconnection exhaust, due to the low time resolution of the plasma data from ACE. Besides, one can see that the magnetic field was more fluctuating at ACE, which indicates that some explosive event was ongoing. This event may be the cause of the flapping motion of the reconnection current. In conclusion, for many reported reconnection exhausts, their exhaust geometries, the wedge angles, and dimensionless reconnection rates were calculated based on the quasi-stationary reconnection hypothesis (e.g., [28][29][30][31]). If reconnection current sheets were flapping when the spacecraft crossed reconnection exhausts, all the estimated values will be unreliable.

DATA AVAILABILITY STATEMENT
Publicly available datasets were analyzed in this study. This data can be found here: http://cdaweb.gsfc.nasa.gov/cdaweb/istp_ public/.

AUTHOR CONTRIBUTIONS
JW drafted the manuscript and led the observational analysis. YZ provided heuristic advice and revised the manuscript. All authors contributed to the interpretation of the results and helped draft the manuscript.