A global model of cometary tail disconnection events triggered by solar wind magnetic variations

Abstract

Presented in this paper are the results of a time‐dependent, fully three‐dimensional self‐consistent ideal magneto‐hydrodynamics (MHD) model of disconnection events (DEs) in the cometary plasma tail. Understanding the interaction of cometary plasma with the solar wind is a very important problem in space plasma physics. This study focuses on the detailed MHD processes which occur during DEs based on typical comets with ideal solar wind conditions, as well as different experiences of DEs triggered by different forms of heliospheric current sheet (HCS). The crossing process against a typical HCS, which is interpreted as various tangential discontinuities (TDs), is simulated. Tail rays and/or disconnected tails are formed in such simulations. With our line‐of‐sight integration results, we conclude that observers from different directions relative to the IMF plane will see two different types of DE image series. The magnetic reconnection that occurs in our results confirms previous publications from different models. With additional cases, our study also provides the limitation for such DEs to be triggered: the angle of magnetic field rotation across the TD should be within 90° to 270°. The effects of various intervals of multiple HCS crossings are explored to show that a 2‐h minimum interval is needed for consecutive multiple HCS to trigger a DE. In addition, such DEs caused by multiple HCS crossings have more tail rays and appear closer to observations than those resulted by single HCS crossings. In addition to presenting the first global DE time evolution with a well‐resolved contact‐surface, our results are compared morphologically with observational images. Quasi‐quantitative agreement with observational measurements is found by investigating the recession speed during the DE. Moreover, the importance of having a solar wind condition being able to trigger a frontside magnetic reconnection is found critical for such DEs to originate. The importance of a well‐resolved contact surface to this problem is discussed regarding to the mass loading effect. On the basis of our deductions and simulations, an empirical equation and data table to estimate the minimum upwind boundary distance is derived with respect to the shock distance, which is sensitive to the cometary production rate.