What Determines When Reconnection Turns On? Chromosphere,Corona, Solar Wind, Magnetopause, and Magnetotail
SHINE co-convener: Spiro Antiochus <spiro.k.antiochos @ nasa.gov>
GEM co-convener: Mike Shay <shay @ UDel.Edu>
This session presented a forum for the discussion of what determines when reconnection turns on in the Sun, the Solar Wind, and the Magnetosphere. The focus of this session was on the more general aspects of onset as they relate to the particular physical systems, and especially on determining the physical processes that may be common to many observed forms of activity. We invited contributions from all solar, heliospheric, and geomagnetic physicists with observations, theories, or models, that address the question posed in the title to this Session.
Amitava Bhattacharjee opened the session with a review presentation on reconnection onset in the magnetosphere. He noted that both CMEs and substorms are highly impulsive, with reconnection growth rates that change extremely quickly. He emphasized that any successful reconnection onset model must show these timescales of change. He showed that including the Hall term in fluid simulations of reconnection greatly increases the speed of onset, but it is still too slow to match observations. He discussed the ballooning instability as an onset mechanism, but pointed out that its growth is not explosive enough. He finished with the question that if Hall onset is not sufficient to explain onset in the Earth's magnetosphere, would it be sufficient for the sun?
Chris Russell presented observations of magnetic reconnection in the Earth's magnetosphere as well as that of Jupiter and Saturn's. He stated that onset in the magnetotail primarily occurs because the high density current sheet becomes eroded, leading to extremely fast reconnection of the lobe magnetic field due to the low plasma density. Michael Hesse presented simulations of reconnection onset in the magnetotail. His main point was that compression of the magnetotail eventually leads to a strong reduction in Bz, which allows the electrons to demagnetize at that point, leading to reconnection onset. In the case with the guide field, the current sheet must compress to a thickness comparable to an electron larmor radius, which requires considerable driving.
Ron Moore presented an overview of observations of CME eruptions with a focus on reconnection onset. His bottom line was that the configuration of the preeruption field is critical for determining where reconnection onset will occur. The explosive growth of a CME occurs when reconnection releases a flux rope, which drives yet more reconnection.
Misha Sitnov presented his kinetic onset theory in which passing electrons due to mirroring in the Earth's strong dipole field allows reconnection ton onset even when the electrons are still magnetized. Simulations were shown using kinetic PIC with open boundary conditions. When particles are reflected at the boundaries, there are no secondary islands formed. When the boundaries are open, there are secondary islands even though the electrons are magnetized.
Paul Cassak presented a theory that solar flare reconnection onset is caused by a catastrophe in which the slow reconnection solution ceases to exist (due to bifurcation), leading to an explosive onset of reconnection. If this theory is playing a critical role, it is postulated that the average coronal conditions will tend towards values at the critical bifurcation point. Data from the solar corona and extrasolar flares was shown which provides support for this theory.
Illa Roussev presented magnetograms from the most complicated flare events studied to date. A total of 10 flux systems were playing a role in the active region dynamics. The erupting field had a much larger separation than originally though, and they are still trying to understand the complex dynamics.
