Onset of Magnetic Reconnection and Turbulence at Thick Current Sheets

Parker Solar Probe (PSP) reported frequent magnetic reconnection at the heliospheric current sheet (HCS), despite its thickness exceeding the ion inertial length by orders of magnitude. On the other hand, reconnection is relatively rare when HCS is observed near 1 au. We propose an explanation based on magnetohydrodynamic instabilities, as supported by evidence from PSP and ACE spacecraft. An aspect ratio (between current sheet width and length) smaller than about 0.01 and a growing time longer than about 1 Alfven crossing time is typically required for reconnection onset. In addition, MHD simulations suggest that turbulence may play an important role in the onset of magnetic reconnection.