A Magnetic Reconnection Mechanism for Ion Acceleration and Abundance Enhancements in Impulsive Flares

Abstract

The acceleration of ions during magnetic reconnection in solar flares is explored with simulations and analytic analysis. Ions crossing into Alfvénic reconnection outflows can behave like pickup particles and gain an effective thermal velocity equal to the Alfvén speed. However, with a sufficiently strong ambient out-of-plane magnetic field, which is the relevant configuration for flares, the ions can become adiabatic and their heating is then dramatically reduced. The threshold for nonadiabatic behavior, where ions are strongly heated, becomes a condition on the ion mass-to-charge ratio, m i /m p Z i > 10 √ β 0x /2/π , where m i and Z i are the ion mass and charge state, m p is the proton mass, and β 0x = 8πnT /B 2 0x is the ratio of the plasma pressure to that of the reconnecting magnetic field B 0x . Thus, during flares high mass-to-charge particles gain energy more easily than protons and a simple model reveals that their abundances are enhanced, which is consistent with observations.

Source Regions of the Interplanetary Magnetic Field and Variability in Heavy-Ion Elemental Composition in Gradual Solar Energetic Particle Events

“…Any magnetic reconnection within areas containing such large, complex, and dynamic magnetic fields might reasonably be expected to be much more effective in producing suprathermal ions that could subsequently become seeds for shock acceleration. Moreover, if these reconnection processes are similar to those that produce impulsive SEP events (Drake et al 2009;Knizhnik et al 2011), the suprathermals may have impulsive-like enhancements in heavy ions (Reames 2000;Reames & Ng 2004;Mason et al 2004). We now explore that notion more fully by examining the complete event sample, using not only hourly averaged abundance ratios, but also interval-integrated ratios and ensemble-averages of events in the two classes.…”

“…Our approach is motivated by two considerations. First, several lines of evidence suggest that suprathermal ions have a significant advantage over thermal ions when it comes to being efficiently accelerated by shocks (Mason et al 1999;Desai et al 2006;Tylka & Lee 2006;Lee 2007); reconnection processes (Drake et al 2009;Knizhnik et al 2011) that occur at the IMF source regions are one potentially promising source of suprathermal ions.…”

The Origin of Element Abundance Variations in Solar Energetic Particles

“…In impulsive SEP events, acceleration produces a power-law dependence on A/Q (Drake et al, 2009;Reames, Cliver, and Kahler, 2014a, b) and scattering during transport contributes less (Mason et al, 1989).…”

Temperature of the Source Plasma for Impulsive Solar Energetic Particles

“…Mason et al, 1986;Reames, Meyer, and von Rosenvinge, 1994; Article 2) suggests the operation of two physical mechanisms. Recently, the heavy-element enhancements, relative to coronal abundances, and their strong power-law dependence on A/Q of the ions in impulsive SEP events, have been linked theoretically to the physics of magneticreconnection regions from which the ions escape (Drake et al, 2009(Drake et al, , 2010Knizhnik, Swizdak, and Drake 2011;Drake and Swizdak, 2014). This model is based upon particle-in-cell (PIC) calculations.…”

“…Ions must first be pre-accelerated to the Alfvén speed in the exhaust from a magnetic reconnection region where the minimum ion A/Q depends upon the plasma β which decreases with time (Drake and Swisdak, 2014). Ions are then fast enough to undergo Fermi acceleration by scattering back and forth from the ends of magnetic islands of reconnection as they approach each other, attaining power-law spectra (Drake et al 2009, Drake, Swisdak, and Fermo 2013. The highest rigidity (highest A/Q) ions escape earliest before the islands collapse.…”