Abstract
During three of its first five orbits around the Sun, Parker Solar Probe (PSP) crossed the large-scale heliospheric current sheet (HCS) multiple times and provided unprecedented detailed plasma and field observations of the near-Sun HCS. We report the common detections by PSP of reconnection exhaust signatures in the HCS at heliocentric distances of 29.5–107 solar radii during encounters 1, 4, and 5. Both sunward and antisunward-directed reconnection exhausts were observed. In the sunward reconnection exhausts, PSP detected counterstreaming strahl electrons, indicating that HCS reconnection resulted in the formation of closed magnetic field lines with both ends connected to the Sun. In the antisunward exhausts, PSP observed dropouts of strahl electrons, consistent with the reconnected HCS field lines being disconnected from the Sun. The common detection of reconnection in the HCS suggests that reconnection is almost always active in the HCS near the Sun. Furthermore, the occurrence of multiple long-duration partial crossings of the HCS suggests that HCS reconnection could produce chains of large bulges with spatial dimensions of up to several solar radii. The finding of the prevalence of reconnection in the HCS is somewhat surprising since PSP has revealed that the HCS is much thicker than the kinetic scales required for reconnection onset. The observations are also in stark contrast with the apparent absence of reconnection in most of the small-scale and much more intense current sheets encountered near perihelia, many of which are associated with “switchbacks”. Thus, the PSP findings suggest that large-scale dynamics, either locally in the solar wind or within the coronal source of the HCS (at the tip of helmet streamers), plays a critical role in triggering reconnection onset.
Highlights
Magnetic reconnection in current sheets is a universal plasma process that converts magnetic energy into plasma jetting and heating, and it is important in many laboratory, space, solar, and astrophysical contexts (e.g., Parker 1983; Priest 1984; Kronberg 2002; Yamada et al 2010; Paschmann et al 2013)
Dataset and instrumentation We studied Parker Solar Probe (PSP) heliospheric current sheet (HCS) crossings observed during encounters 1, 4, and 5, at distances of 29.5–107 solar radii (RS) from the Sun
Since the spacecraft does not sample both edges of the HCS, we examine the flows inside the HCS more quantitatively by comparing the observed flows to the Walén relation for a rotational discontinuity (e.g., Hudson 1970; Sonnerup et al 1981; Paschmann et al 1986)
Summary
Magnetic reconnection in current sheets is a universal plasma process that converts magnetic energy into plasma jetting and heating, and it is important in many laboratory, space, solar, and astrophysical contexts (e.g., Parker 1983; Priest 1984; Kronberg 2002; Yamada et al 2010; Paschmann et al 2013). Earth’s dayside magnetopause current sheet is usually thin due to the constant compression of the solar wind against the magnetosphere, but when a spacecraft crosses the magnetopause, the probability of observing ongoing reconnection locally is only about 50%, even when the magnetic shear angle across the magnetopause is large (e.g., Paschmann et al 1986; Phan & Paschmann 1996). In some events the field rotated fully or nearly fully, but the strahl electron flux level before and/or after the field rotation was not at the asymptotic levels This behavior may be related to the different ways the HCS is created near the Sun (e.g., Crooker et al 2004; Huang et al 2016), and does not affect our ability to evaluate the presence or absence of reconnection exhaust ion jets in the HCS. They have smaller magnetic field rotation than the complete crossing
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