Abstract
This paper provides insights into the possible coupling between reconnection and interchange/ballooning in the magnetotail related to substorms and flow bursts. The results presented are largely based on recent simulations of magnetotail dynamics, exploring onset and progression of reconnection. 2.5-dimensional particle-in-cell (PIC) simulations with different tail deformation demonstrate a clear boundary between stable and unstable cases depending on the amount of deformation, explored up to the real proton/electron mass ratio. The evolution prior to onset, as well as the evolution of stable cases, are governed by the conservation of integral flux tube entropy S as imposed in ideal MHD, maintaining a monotonic increase with distance downtail. This suggests that ballooning instability in the tail should not be expected prior to the onset of tearing and reconnection. 3-D MHD simulations confirm this conclusion, showing no indication of ballooning prior to reconnection, if the initial state is ballooning stable. The simulation also shows that, after imposing resistivity necessary to initiate reconnection, the reconnection rate and energy release initially remain slow. However, when S becomes reduced from plasmoid ejection and lobe reconnection, forming a negative slope in S as a function of distance from Earth, the reconnection rate and energy release increase drastically. The latter condition has been shown to be necessary for ballooning/interchange instability, and the cross-tail structures that develop subsequently in the MHD simulation are consistent with such modes. The simulations support a concept in which tail activity is initiated by tearing instability but significantly enhanced by the interaction with ballooning/interchange enabled by plasmoid loss and lobe reconnection.
Highlights
The mechanism(s) for the onset of magnetotail activity related to substorms or other dynamic events still represent an unsolved problem and a matter of controversy.Crucial elements in this context are the tearing instability, presumably the initiator of magnetic reconnection and plasmoid formation (e.g., Schindler 1974), and ballooning/interchange instability (e.g., Liu et al 2012, and references therein), which can provide cross-tail structure and wave modes that are possibly related to auroral onset arc features (“beading”) observed prior to the expansion of the auroral oval (e.g., Keiling et al 2013, and references therein).The onset and growth of collisionless tearing instability depends strongly on the magnitude of the normal magnetic field Bz (Brittnacher et al 1995; Pellat et al 1991; Schindler 2007) and on the current sheet thickness (e.g., Daughton 1999)
Summary and conclusions We have presented recent results on the stability and possible interaction of the major modes discussed in the context of substorms and other tail activity: tearing and ballooning/interchange
Since the entropy variation governs ballooning/interchange stability in 2-D configurations (Schindler and Birn 2004), this result suggests that ballooning instability in the tail should not be expected from slow adiabatic evolution prior to the onset of tearing and reconnection, it cannot be excluded that ballooning might arise in the inner more dipole-like region under suitable conditions
Summary
The mechanism(s) for the onset of magnetotail activity related to substorms or other dynamic events still represent an unsolved problem and a matter of controversy.Crucial elements in this context are the tearing instability, presumably the initiator of magnetic reconnection and plasmoid formation (e.g., Schindler 1974), and ballooning/interchange instability (e.g., Liu et al 2012, and references therein), which can provide cross-tail structure and wave modes that are possibly related to auroral onset arc features (“beading”) observed prior to the expansion of the auroral oval (e.g., Keiling et al 2013, and references therein).The onset and growth of collisionless tearing instability depends strongly on the magnitude of the normal magnetic field Bz (Brittnacher et al 1995; Pellat et al 1991; Schindler 2007) and on the current sheet thickness (e.g., Daughton 1999). The conservation of the entropy function S(A) prior to onset, and the conservation of monotonicity for closed field lines with A > Ad, indicate that the tail configuration remains stable to ballooning (Schindler and Birn 2004) prior to the onset of tearing.
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