Preferential Acceleration Of Heavy Ions Research Articles

Preferential acceleration of heavy ions in magnetic reconnection: Hybrid-kinetic simulations with electron inertia

Solar energetic particles (SEPs) in the energy range 10s KeV/nucleon - 100s MeV/nucleon originate from the Sun. Their high flux near Earth may damage the space-borne electronics and generate secondary radiation that is harmful for life on Earth. Thus, understanding their energization on the Sun is important for space weather prediction. Impulsive (or $ $He-rich) SEP events are associated with the acceleration of charge particles in solar flares by magnetic reconnection and related processes. The preferential acceleration of heavy ions and the extraordinary abundance enhancement of $ ^3$He in the impulsive SEP events are not understood yet. In this paper we study the acceleration of heavy ions and its consequences for their abundance enhancements by magnetic reconnection, an established acceleration source for impulsive SEP events in which heavy-ion enhancement is observed We employed a two-dimensional hybrid-kinetic plasma model (kinetic ions and inertial electron fluid) to simulate magnetic reconnection. All the ion species are treated self-consistently in our simulations. We find that heavy ions are preferentially accelerated to energies many times higher than their initial thermal energies by a variety of acceleration mechanisms operating in reconnection. The most efficient acceleration takes place in the flux pileup regions of magnetic reconnection. Heavy ions with sufficiently low values of charge-to-mass ratio ($Q/M$) can be accelerated by pickup mechanism in outflow regions even before any magnetic flux is piled up. The energy spectra of heavy ions develop a shoulder-like region, a nonthermal feature, as a result of the acceleration. The spectral index of the power-law fit to the shoulder region of the spectra varies approximately as $(Q/M)^ $. The abundance enhancement factor, defined as the number of particles above a threshold energy normalized to the total number of particles, scales as $(Q/M)^ alpha $, where alpha increases with the energy threshold. We discuss our simulation results in the light of the SEP observations.

Preferential Acceleration of Heavy Ions in a Spontaneously Fragmenting Flare Current Sheet

Abstract We study the ion acceleration in a mesoscale, spontaneously fragmenting flare current sheet (SFCS) characterized by the presence of a plasmoid cascade. The main subject of our investigation is to determine whether and how plasmoid cascades at intermediate scales in a fragmented current sheet of a solar flare can impact the (preferential) acceleration of specific ions. The time evolution of the SFCS is obtained from high-resolution 2.5D MHD simulations. The ion trajectories (in the background fields resulting from the MHD model), energies, and pitch angles are calculated using a relativistic test-particle code based on the half-acceleration–rotation–half-acceleration method. For light ions, the main acceleration effects of electromagnetic fields within the SFCS are analyzed using the guiding center approximation. We identify regions with the most-efficient ion acceleration within the SFCS, the accelerator efficiency, and spectra of the accelerated ions. The influence of the charge-to-mass ratio on ion behavior is also studied and resulting ion abundances are compared with observational data. The main ion acceleration takes place in the regions with a strong polarization term, which is part of the first-order Fermi acceleration. Because the term is mass dependent, heavier ions undergo preferential acceleration. The ion energy spectra, abundance-enhancement factors, and differential fluxes, obtained from the model, exhibit power-law profiles, in agreement with observed solar energetic particle events. Nonetheless, the obtained slopes for the abundance-enhancement factor do not exactly match the observed data. The computed slopes and profiles are not sensitive to changes in the initial plasma temperature.

Parametric instabilities in a two ion species plasma as a driver of super Alfvénic waves

Linear dispersion relation of parametric instabilities of a left hand circularly polarized wave in a three species plasma consisting electrons, protons, and α particles is derived and the obtained dispersion relation is numerically solved. It is shown that the so-called super Alfvénic waves, which are thought to play important roles in preferential acceleration of heavy ions, can get excited through a parametric decay instability when the parent wave is on the proton cyclotron branch and the plasma beta is sufficiently low. A variety of other parametric instabilities are also generated.

Preferential acceleration of heavy ions in the reconnection outflow region

Context. Many observations show that heating in the solar corona should be more effective for heavy ions than for protons. Moreover, the efficiency of particle heating also seems to be larger for a larger particle electric charge. The transient magnetic reconnection is one of the most natural mechanisms of charged particle acceleration in the solar corona. However, the role of this process in preferential acceleration of heavy ions has still yet to be investigated. Aims. In this paper, we consider charged particle acceleration in the reconnection outflow region. We investigate the dependence of efficiency of various mechanisms of particle acceleration on particle charge and mass. Methods. We take into account recent in situ spacecraft observations of the nonlinear magnetic waves that have originated in the magnetic reconnection. We use analytical estimates and test-particle trajectories to study resonant and nonresonant particle acceleration by these nonlinear waves. Results. We show that resonant acceleration of heavy ions by nonlinear magnetic waves in the reconnection outflow region is more effective for heavy ions and/or for ions with a larger electric charge. Nonresonant acceleration can be considered as a combination of particle reflections from the front of the nonlinear waves. Energy gain for a single reflection is proportional to the particle mass, while the maximum possible gain of energy corresponds to the classical betatron heating. Conclusions. Small-scale transient magnetic reconnections produce nonlinear magnetic waves propagating away from the reconnection region. These waves can effectively accelerate heavy ions in the solar corona via resonant and nonresonnat regimes of interactions. This mechanism of acceleration is more effective for ions with a larger mass and/or with a larger electric charge.

Heavy Ion Abundances in Impulsive Solar Flares: Influence of Pre-Acceleration in a Current Sheet

Competition between stochastic energy gains and collisional energy losses is known to lead to preferential acceleration of heavy ions in flare loops. Ion acceleration in a reconnecting current sheet is shown to mitigate the influence of collisional energy losses on stochastic particle acceleration in impulsive solar flares. This effect decreases the sensitivity of the resulting abundance ratios on initial ion charge states. The resulting abundances are determined by the fact that the energy loss rate falls off rapidly with increasing energy. As an example, the expected Fe/O enhancement ratios are computed and shown to be comparable with those observed with ACE SEPICA in several impulsive flares in 1998. One consequence of the model is that the preferential acceleration of heavy ions can occur only when the plasma gas pressure is large enough, β≈me/mp, which may explain the observed correlation between the heavy ion enrichment and selective 3He acceleration in impulsive flares.

Preferential acceleration of heavy ions in multi‐component plasmas

We discuss preferential, perpendicular acceleration of α particles in large amplitude magnetohydrodynamic (MHD) waves by using a test particle simulation. In a 3 component plasma that consists of electrons, protons and α particles, one of the wave branches has phase velocity larger than the Alfvén velocity in a long wave length regime. We find that the wave‐particle interaction between this wave and the other Alfvén wave leads to strong anisotropic acceleration in perpendicular direction to the ambient magnetic field. We examine the wave‐particle interaction for a 3 component plasma, and obtain good agreement with the test particle simulation.

Medium‐energy electrons and heavy ions in Jupiter's magnetosphere: Effects of lower hybrid wave‐particle interactions

A theory of medium‐energy (∼ keV) electrons and heavy ions in Jupiter's magnetosphere is presented. Lower hybrid waves are generated by the combined effects of a ring instability of neutral wind pickup ions and the modified two‐stream instability associated with transport of cool Iogenic plasma. The quasi‐linear energy diffusion coefficient for lower hybrid wave‐particle interactions is evaluated, and several solutions to the diffusion equation are given. Calculations based on measured wave properties show that the noise modifies substantially the particle distribution functions. The effects are to accelerate superthermal ions and electrons to keV energies and to thermalize the pickup ions on time scales comparable to the particle residence time. The [S2+]/[S+] ratio at medium energies is a measure of the relative contribution from Iogenic thermal plasma and neutral wind ions, and this important quantity should be determined from future measurements. The theory also predicts a preferential acceleration of heavy ions with an acceleration time that scales inversely with the root of the ion mass. Electrons accelerated by the process contribute to further reionization of the neutral wind by electron impact, thus providing a possible confirmation of Alfvén's critical velocity effect in the Jovian magnetosphere.

On the preferential acceleration and heating of solar wind heavy ions

We investigate the preferential heating and acceleration of solar wind heavy ions by the resonant cyclotron interaction with parallel‐propagating left‐polarized hydromagnetic waves. We set up a scenario whereby the energy for this interaction is taken from saturated low‐frequency Alfven waves via a cascade to the higher, resonant frequencies. In order to utilize the existing theoretical work, the particles are taken to be thermally isotropic, and the waves are taken to be dispersionless. This scenario is incorporated into a numerical solar wind code describing the flow from an inner radius (taken to be 10 solar radii) to 1 AU. Thus we present the first model of a wave‐driven, three‐fluid, supersonic solar wind. By varying the model parameters we test the ability of the resonant interaction in this model to produce the excess speeds and temperatures of heavy ions that are observed. We find that unrealistically steep wave spectra are required to produce differential speeds of the order of the Alfven speed at 0.3 AU. In the case of He2+, the power spectrum of waves must be steeper than γ ≃ 4.7, where γ is the power law spectral index. Ions of oxygen or iron, with larger mass‐per‐charge ratios, are accelerated more readily than helium, but still require steeper spectra than are observed. This model is also unable to produce the mass‐proportional heavy ion temperatures that are observed. We show that the production of mass‐proportional temperatures is inconsistent with preferential acceleration of heavy ions by this mechanism. The model also produces a heavy ion‐to‐proton temperature ratio at 1 AU which is anticorrelated with solar wind speed, in contradiction to the observed behavior.

Preferential acceleration of heavy ions from thermal velocities

The acceleration of ions from thermal velocities is analyzed to determine conditions under which heavy ions can be preferentially accelerated. Two accelerating mechanisms involving high-and low-frequency hydromagnetic waves respectively are considered. Preferential acceleration of heavy ions occurs for high-frequency waves if the frequency spectrum falls off faster than (frequency)−1. For the low-frequency waves heavy ions are less effectively accelerated than lighter ions. However, very heavy ions can be preferentially accelerated, the abundances of the very heavy ions being enhanced by a factor Ai over the thermal abundances. Acceleration of ions in the envelope of the Crab nebula is considered as an example.