Tearing instability and periodic density perturbations in the slow solar wind

The slow wind properties, its composition and dynamic nature are tracers to understand its origin. Reconnection with closed structures such as helmet streamers could explain both the enrichment in first ionization potential elements and the periodic density perturbations observed in the slow wind. Tip of helmet streamers are of particular interest for this problem as they have been shown to easily trigger reconnection in numerical studies. Imaging of the inner heliosphere have been furthermore following density structures seemingly escaping from helmet streamers for more than 20 years.

Using the newly developed code of Réville et al. 2020 (ApJS), we investigated the behavior of the heliospheric current sheet at the tip of helmet streamers, focusing on the effect of finite resistivity on the reconnection process. The code is based on Alfvén wave turbulence and creates self-consistently fast and slow wind flows. The heliospheric current sheet is in our set of 2.5D simulation located at the equator and surrounded by slow wind streams.

The solar wind plasma is expected to have a very low resistivity (eta), or a very high Lundquist number (S=LvA/eta), a regime impossible to reach in numerical simulations. Our approach was thus to study the reconnection at more reasonable Lundquist numbers and compare these behaviours with the predictions of theoretical calculations. We found that the system was triggering a tearing instability, and that its properties were close to the ideal tearing scenario. The tearing mode is creating simultaneously magnetic islands or flux ropes along the current sheet. The characteristic size of these islands is controlled by the Lundquist number and should decrease for higher S.

Because our simulations comply well with the ideal tearing scenario, we were able to predict the typical length scale of the tearing induced reconnection in the real solar corona. We found a periodicity of some 80 minutes for density enhancements in between flux ropes, which is very close to the observations. This process could be directly confirmed through measurements of the WISPR instrument onboard the Parker Solar Probe.