Source Of Anomalous Cosmic Rays Research Articles

Pickup ion acceleration in the successive appearance of corotating interaction regions

AbstractAcceleration of pickup ions (PUIs) in an environment surrounded by a pair of corotating interaction regions (CIRs) was investigated by numerical simulations using a hybrid code. Energetic particles associated with CIRs have been considered to be a result of the acceleration at their shock boundaries, but recent observations identified the ion flux peaks in the sub‐MeV to MeV energy range in the rarefaction region, where two separate CIRs were likely connected by the magnetic field. Our simulation results confirmed these observational features. As the accelerated PUIs repeatedly bounce back and forth along the field lines between the reverse shock of the first CIR and the forward shock of the second one, the energetic population is accumulated in the rarefaction region. It was also verified that PUI acceleration in the dual CIR system had two different stages. First, because PUIs have large gyroradii, multiple shock crossing is possible for several tens of gyroperiods, and there is an energy gain in the component parallel to the magnetic field via shock drift acceleration. Second, as the field rarefaction evolves and the radial magnetic field becomes dominant, Fermi‐type reflection takes place at the shock. The converging nature of two shocks results in a net energy gain. The PUI energy acquired through these processes is close to 0.5 MeV, which may be large enough for further acceleration, possibly resulting in the source of anomalous cosmic rays.

THE ROLE OF PRESSURE ANISOTROPY ON PARTICLE ACCELERATION DURING MAGNETIC RECONNECTION

Voyager spacecraft observations have revealed that contrary to expectations, the source of anomalous cosmic rays (ACRs) is not at the local termination shock. A possible mechanism of ACR acceleration is magnetic reconnection in the heliosheath. Using a particle-in-cell code, we investigate the effects of β on reconnection-driven particle acceleration by studying island growth in multiple interacting Harris current sheets. Many islands are generated, and particles are dominantly heated through Fermi reflection in contracting islands during island growth and merging. There is a striking difference between the heating of electrons versus the heating of ions. There is a strong dependence of β on electron heating, while the ion heating is insensitive to β. Anisotropies develop with T ∥ ≠ T ⊥ for both electrons and ions. The electron anisotropies support the development of a Weibel instability that suppresses the Fermi acceleration of the electrons. Since the Weibel instability develops at a larger T ∥/T ⊥ in lower β systems, electrons are able to accelerate more efficiently by the Fermi mechanism at low β. The variance in anisotropy implies less electron acceleration in higher β systems, and thus less heating. This study sheds light on particle acceleration mechanisms within the sectored magnetic field regions of the heliosheath and the dissipation of turbulence such as that produced by the magnetorotational instability in accreting systems.

On the possibility of existence of an additional source of anomalous cosmic rays outside the solar system

On the basis of experimental data obtained at exposure of solid-state track detectors in the magnetosphere of the Earth during solar flares and in quiet Sun periods, an estimate of possible contribution of singly charged oxygen ions to the flare particle fluxes is made. A possibility is considered of the appearance in the vicinity of the solar system of singly ionized oxygen ions generated on stars.

An asymmetric solar wind termination shock

Voyager 2 crossed the solar wind termination shock at 83.7 au in the southern hemisphere, approximately 10 au closer to the Sun than found by Voyager 1 in the north. This asymmetry could indicate an asymmetric pressure from an interstellar magnetic field, from transient-induced shock motion, or from the solar wind dynamic pressure. Here we report that the intensity of 4-5 MeV protons accelerated by the shock near Voyager 2 was three times that observed concurrently by Voyager 1, indicating differences in the shock at the two locations. (Companion papers report on the plasma, magnetic field, plasma-wave and lower energy particle observations at the shock.) Voyager 2 did not find the source of anomalous cosmic rays at the shock, suggesting that the source is elsewhere on the shock or in the heliosheath. The small intensity gradient of Galactic cosmic ray helium indicates that either the gradient is further out in the heliosheath or the local interstellar Galactic cosmic ray intensity is lower than expected.

Diffusive Acceleration at the Blunt Termination Shock

Voyager 1's recent and long-anticipated passage into the heliosheath contradicted the prediction that we would observe the source of anomalous cosmic rays accelerated by the termination shock. The observed energetic protons reveal a power-law spectrum below several MeV, but above several MeV the spectrum falls more sharply, and we observe the familiar bump caused by modulation of anomalous cosmic rays. This paper develops the theoretical framework to include the motions and drift of particles during diffusive shock acceleration at a three-dimensional (3-D) termination shock, including cross-field diffusion, and curvature and gradient drifts. Our model supports the concept of McComas and Schwadron that because of the termination shock's blunt structure with a strong nose-to-tail asymmetry, there should be a strong deficit of locally accelerated anomalous cosmic rays (ACRs) near the nose. With reasonable parameters for the scattering mean free path and the ratio of perpendicular to parallel diffusion, the model produces an energy spectrum that agrees well with the Voyager 1 observations near the termination shock. These parameters also lead to an acceleration time to 10 MeV of about 1 yr, which is comparable to previous estimates derived from the observed charge states of ACRs. Thus, we provide a theory for diffusive acceleration at the blunt termination shock. The predictions of this theory are consistent with Voyager 1's observations, showing a lack of ACRs accelerated near the nose of the termination shock and the ACR acceleration timescale derived from ACR charge states.

An explanation of the Voyager paradox: Particle acceleration at a blunt termination shock

Voyager 1 recently crossed the termination shock at the edge of our heliosphere. In contrast to the expectations of essentially all prior models, however, Voyager 1 did not observe the source of Anomalous Cosmic Rays (ACRs) as had been widely anticipated. We show here that the dearth of higher energy particles near the nose of the heliosphere is a natural consequence of the magnetic geometry in the region ahead of a flattened shock. Particle energization happens primarily back along the flanks of the shock where the injection energy is lower and where the magnetic field has had progressively longer connection times to accelerate particles. In addition to explaining the most baffling aspects of the Voyager 1 observations, this paradigm makes explicit predictions about what should be observed when Voyager 2 reaches the termination shock, significantly further back from its nose.

Reforming perpendicular shocks in the presence of pickup protons: initial ion acceleration

Abstract. Acceleration processes associated with the heliospheric termination shock may provide a source of anomalous cosmic rays (ACRs). Recent kinetic simulations of supercritical, quasi-perpendicular shocks have yielded time varying shock solutions that cyclically reform on the spatio-temporal scales of the incoming protons. Whether a shock solution is stationary or reforming depends upon the plasma parameters which, for the termination shock, are ill defined but believed to be within the time-dependent regime. Here we present results from high phase space resolution particle-in-cell simulations for a three-component plasma (solar wind protons, electrons and pickup protons) appropriate for the termination shock. We find reforming shock solutions which generate suprathermal populations for both proton components, with the pickup ions reaching energies of order twenty times the solar wind inflow energy. This suprathermal "injection" population is required as a seed population for subsequent acceleration at the shock which can in turn generate ACRs.

The dynamic 3D heliosphere: Implications for and new sources of its energetic particles

In recent years, two important aspects of heliospheric physics have emerged: (1) the intrinsic dynamism of the Sun's open magnetic flux and its vast importance for energetic particle propagation and acceleration; and (2) the importance of heliospheric grain populations for the production of energetic particles and anomalous cosmic rays. These new aspects have seeded the discovery of the inner and outer pickup ion sources; the discovery of the outer source of anomalous cosmic rays; the discovery that nearly radial magnetic fields in co-rotating rarefaction regions and associated energetic particle dwells are caused by the motions of open magnetic field footpoints across coronal hole boundaries; the relevance of these footpoint motions for ubiquitous statistical acceleration in slow solar wind and for Favored Acceleration Locations at the Termination Shock (FALTS). Here, we review these emerging discoveries and discuss how their combination is leading to a vastly different and improved understanding of our intrinsically dynamic heliosphere.

The outer source of pickup ions and anomalous cosmic rays

The traditionally accepted source of Anomalous Cosmic Rays (ACRs) is neutral atoms penetrating the heliosphere from the local interstellar cloud (LIC). The ACR composition should be depleted in easily ionized atoms such as C, Si, and Fe. However, significant fluxes of these ions are observed in ACRs and their source has not been previously identified. We show that there is an “outer source” of pickup ions, and hence ACRs, caused by sputtered atoms (subsequently ionized and picked up by the solar wind) from small grains generated via collisions of objects in the Edgeworth‐Kuiper Belt. The outer source accounts for the abundance and composition of the additional population of ACRs. The discovery that ACRs are generated from material in the Edgeworth‐Kuiper Belt provides an exciting new tool for understanding the mass distribution and composition of the Edgeworth‐Kuiper Belt, and for probing the plasma‐dust interactions in stellar environments.