Abstract
The opening of closed magnetic loops via reconnection with open solar flux, so called “interchange reconnection”, is invoked in a number of models of slow solar wind release. In the heliosphere, this is expected to result in local switchbacks or inversions in heliospheric magnetic flux (HMF). When observed at 1 AU, inverted HMF has previously been shown to exhibit high ion charge states, suggestive of hot coronal loops, and to map to the locations of coronal magnetic separatrices. However, simulations show that inverted HMF produced directly by reconnection in the low corona is unlikely to survive to 1 AU without the amplification by solar wind speed shear. By considering the surrounding solar wind, we show that inverted HMF is preferably associated with regions of solar wind shear at 1 AU. Compared with the surrounding solar wind, inverted HMF intervals have lower magnetic field intensity and show intermediate speed and density values between the faster, more tenuous wind ahead and the slower, denser wind behind. There is no coherent signature in iron charge states, but oxygen and carbon charge states within the inverted HMF are in agreement with the higher values in the slow wind behind. Conversely, the iron-to-oxygen abundance ratio is in better agreement with the lower values in the solar wind ahead, while the alpha-to-proton abundance ratio shows no variation. One possible explanation for these observations is that the interchange reconnection (and subsequent solar wind shear) that is responsible for generation of inverted HMF involves very small, quiet-Sun loops of approximately photospheric composition, which are impulsively heated in the low corona, rather than large-scale active region loops with enhanced first-ionisation potential elements. Whether signatures of such small loops could be detected in situ at 1 AU still remains to be determined.
Highlights
Fast solar wind originates in coronal holes that contain unipolar, open magnetic flux, while slow wind generally maps to the vicinity of closed loops within the streamer belt (e.g. McComas et al, 2003)
We find that for the inverted heliospheric magnetic flux (HMF) seen in near-Earth space to be the direct result of interchange reconnection, the loops must first erupt into the solar wind and travel a significant fraction of 1 astronomical unit (AU) before they are opened up by interchange reconnection
We have investigated the possibility of observing coronal loop opening signatures in the solar wind at 1 AU
Summary
Fast solar wind originates in coronal holes that contain unipolar, open magnetic flux, while slow wind generally maps to the vicinity of closed loops within the streamer belt (e.g. McComas et al, 2003). Inversions or folds in heliospheric magnetic flux (HMF), sometimes referred to as “switchbacks”, have long been observed in the fast wind on the basis of wave propagation and differential alpha-particle streaming direction (Balogh et al, 1999; Yamauchi et al, 2004) These structures have been associated with coronal plumes within the fast wind (Velli et al, 2011). In the first part of this study, we use simple scaling arguments to demonstrate the principle of rapid decay of HMF inversions generated in the low corona and to investigate the effect of loop apex size at the time of interchange reconnection This supports the previous findings that solar wind shear is necessary for inversions to survive to 1 AU. These observations provide hints for the features that should be more readily identifiable in Parker Solar Probe (Fox et al, 2016) and Solar Orbiter (Muller et al, 2013) observations of the inner heliosphere
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