Recent observations of supernova remnant W44 by the Fermi spacecraft observatory support the idea that the bulk of galactic cosmic rays is accelerated in such remnants by a Fermi mechanism, also ...known as diffusive shock acceleration. However, the W44 expands into weakly ionized dense gas, and so a significant revision of the mechanism is required. Here, we provide the necessary modifications and demonstrate that strong ion-neutral collisions in the remnant surrounding lead to the steepening of the energy spectrum of accelerated particles by exactly one power. The spectral break is caused by Alfven wave evanescence leading to the fractional particle losses. The gamma-ray spectrum generated in collisions of the accelerated protons with the ambient gas is calculated and successfully fitted to the Fermi Observatory data. The parent proton spectrum is best represented by a classical test particle power law ∝E(-2), steepening to E(-3) at E(br)≈7 GeV due to deteriorated particle confinement.
The unexpected energy spectrum of the positron/electron ratio is interpreted astrophysically, with a possible exception of the 100–300 GeV range. The data indicate that this ratio, after a decline ...between 0.5 and 8 GeV, rises steadily with a trend towards saturation at 200–400 GeV. These observations (except for the trend) appear to be in conflict with the diffusive shock acceleration (DSA) mechanism, operating in a single supernova remnant (SNR) shock. We argue that e+/e− ratio can still be explained by the diffusive shock acceleration if positrons are accelerated in a subset of SNR shocks which (i) propagate in clumpy gas media and (ii) are modified by accelerated cosmic ray protons. The protons penetrate into the dense gas clumps upstream to produce positrons and charge the clumps positively. The induced electric field expels positrons into the upstream plasma where they are shock accelerated. Since the shock is modified, these positrons develop a harder spectrum than that of the cosmic ray electrons accelerated in other SNRs. Mixing these populations explains the increase in the e+/e− ratio at E>8 GeV. It decreases at E<8 GeV because of a subshock weakening which also results from the shock modification. Contrary to the expelled positrons, most of the antiprotons, electrons, and heavier nuclei, are left unaccelerated inside the clumps. Scenarios for the 100–300 GeV AMS-02 fraction exceeding the model prediction, including, but not limited to, possible dark matter contribution, are also discussed.
The much-anticipated proof of cosmic ray (CR) acceleration in supernova remnants must hinge on the full consistency of acceleration theory with the observations; direct proof is impossible because of ...CR-orbit scrambling. Recent observations indicate deviations between helium and proton CR rigidity spectra inconsistent with the theory. By considering an initial (injection) phase of the diffusive shock acceleration, where elemental similarity does not apply, we demonstrate that the spectral difference is, in fact, a unique signature of the acceleration mechanism. Collisionless shocks inject more He(2+) when they are stronger and so produce harder He(2+) spectra. The injection bias is due to Alfvén waves driven by the more abundant protons, so the He(2+) ions are harder to trap by these waves. By fitting the p/He ratio to the PAMELA data, we bolster the diffusive shock acceleration case for resolving the century-old mystery of CR origin.
Supernova remnants (SNRs), as the major contributors to the galactic cosmic rays (CRs), are believed to maintain an average CR spectrum by diffusive shock acceleration regardless of the way they ...release CRs into the interstellar medium (ISM). However, the interaction of the CRs with nearby gas clouds crucially depends on the release mechanism. We call into question two aspects of a popular paradigm of the CR injection into the ISM, according to which they passively and isotropically diffuse in the prescribed magnetic fluctuations as test particles. First, we treat the escaping CR and the Alfven waves excited by them on an equal footing. Second, we adopt field-aligned CR escape outside the source, where the waves become weak. An exact analytic self-similar solution for a CR "cloud" released by a dimmed accelerator strongly deviates from the test-particle result. The normalized CR partial pressure may be approximated as (ProQuest: Formulae and/or non-USASCII text omitted), where p is the momentum of CR particle, and z is directed along the field. The core of the cloud expands as z sub(dif) is proportional to radicalD sub(NL) (p)t and decays in time as (ProQuest: Formulae and/or non-USASCII text omitted). The diffusion coefficient D sub(NL) is strongly suppressed compared to its background ISM value D sub(ISM): D sub(NL) ~ D sub(ISM) exp (-Pi) << D sub(ISM) for sufficiently high field-line-integrated CR partial pressure, Pi. When Pi >> 1, the CRs drive Alfven waves efficiently enough to build a transport barrier (scriptP approximate 2/|z|-"pedestal") that strongly reduces the leakage. The solution has a spectral break at p = p sub(br) where p sub(br) satisfies the equation D sub(NL) (p sub(br)) Asymptotically = to z super(2)/t.
ABSTRACT Cosmic rays (CR), constrained by scattering on magnetic irregularities, are believed to propagate diffusively. However, a well-known defect of diffusive approximation, whereby some of the ...particles propagate unrealistically fast, has directed interest toward an alternative CR transport model based on the "telegraph" equation. Though, its derivations often lack rigor and transparency leading to inconsistent results. We apply the classic Chapman-Enskog method to the CR transport problem. We show that no "telegraph" (second order time derivative) term emerges in any order of a proper asymptotic expansion with systematically eliminated short timescales. Nevertheless, this term may formally be converted from the fourth order hyper-diffusive term of the expansion. However, both the telegraph and hyperdiffusive terms may only be important for a short relaxation period associated with either strong pitch-angle anisotropy or spatial inhomogeneity of the initial CR distribution. Beyond this period the system evolves diffusively in both cases. The term conversion, that makes the telegraph and Chapman-Enskog approaches reasonably equivalent, is possible only after this relaxation period. During this period, the telegraph solution is argued to be unphysical. Unlike the hyperdiffusion correction, it is not uniformly valid and introduces implausible singular components to the solution. These dominate the solution during the relaxation period. Because they are shown not to be inherent in the underlying scattering problem, we argue that the telegraph term is involuntarily acquired in an asymptotic reduction of the problem.
•LEND/LRO instrument neutron counting data sets have been analyzed to create high resolution maps of epithermal neutron flux at polar regions of the Moon.•The conversion from epithermal neutron flux ...to the H/H2O abundances is presented based on the simple subsurface models.•The polar lunar maps (poleward 70S/70N) of homogeneous hydrogen distribution are derived and discussed.
We present a method of conversion of the lunar neutron counting rate measured by the Lunar Reconnaissance Orbiter (LRO) Lunar Exploration Neutron Detector (LEND) instrument collimated neutron detectors, to water equivalent hydrogen (WEH) in the top ∼1m layer of lunar regolith. Polar maps of the Moon's inferred hydrogen abundance are presented and discussed.
Both the acceleration of cosmic rays (CRs) in supernova remnant shocks and their subsequent propagation through the random magnetic field of the Galaxy are deemed to result in an almost isotropic CR ...spectrum. However, the MILAGRO TeV observatory discovered sharp (~10°) arrival anisotropies of CR nuclei. We suggest a mechanism for producing a weak and narrow CR beam which operates en route to the observer. The key assumption is that CRs are scattered by a strongly anisotropic Alfvén wave spectrum formed by the turbulent cascade across the local field direction. The strongest pitch-angle scattering occurs for particles moving almost precisely along the field line. Partly because this direction is also the direction of the minimum of the large-scale CR angular distribution, the enhanced scattering results in a weak but narrow particle excess. The width, the fractional excess, and the maximum momentum of the beam are calculated from a systematic transport theory depending on a single scale l which can be associated with the longest Alfvén wave, which efficiently scatters the beam. The best match to all three characteristics of the beam is achieved at l ~ 1 pc. The distance to a possible source of the beam is estimated to be within a few 100 pc. Possible approaches to the determination of the scale l from the characteristics of the source are discussed. Alternative scenarios of drawing the beam from the galactic CR background are considered. Finally, the beam-related large-scale anisotropic CR component is found to be energy independent, which is also consistent with the observations.
Nonpropagating mirror‐mode structures are commonly observed in many regions of natural plasma such as solar wind, planetary magnetosheaths, in cometary plasma, Io wake, terrestrial ring current and ...even on the outskirts of solar system. Mirror structures are typically observed in the shape of magnetic holes or peaks. Fast survey mode plasma data from the THEMIS satellites are used to solve the puzzle of how mirror structures in the form of dips can be observed in the regions of mirror stable plasma. THEMIS data also show that for mirror structures with spatial scales that considerably exceed ion Larmor radius the perpendicular temperature anticorrelates with the strength of the magnetic field. This contradiction with the conservation of adiabatic invariants is explained by the role of trapped particles.
A fully kinetic theory of the magnetic mirror instability in high‐β space plasmas accounting for arbitrary ion‐Larmor radius effects is developed. It is shown that incorporation of ion‐Larmor radius ...effects leads to a substantial modification of both the instability growth rate and the instability threshold. For wavelengths of the order of the ion‐Larmor radius the effective elasticity of the magnetic field lines is substantially enhanced, yielding an increase in the instability threshold. We derive a compact expression for the growth rate of the fastest‐growing mode in the fully kinetic limit. Furthermore, it is shown that in the presence of finite ion‐Larmor radius effects a noncoplanar component of the magnetic field perturbations appears. Such a component is usually present in satellite measurements of mirror modes. The relevance of these results in understanding observations of mirror instability–generated signals in space plasmas is outlined.
Nitroxide radicals, encapsulated into the cavities of metal–organic frameworks (MOFs), have recently drawn attention as multifunctional spin probes. In this work, we investigate the mobility of TEMPO ...nitroxide in ZIF-8 framework in the presence of various solvent molecules located in the same cavity. We employ simulations of continuous wave (CW) EPR spectra to reveal anisotropic rotation of radical. It is shown that the largest anisotropy of rotation occurs for ZIF-8 impregnated with alcohols: it is most pronounced at low temperatures, but also manifested at room temperatures. Linear alkanes do not indicate any significant effect onto the rotation anisotropy of TEMPO, while various benzene derivatives show more complicated trends caused by their bulkier shapes. With a number of examples, we demonstrate the ability of CW EPR analysis to disclose effects of solvent ordering in MOFs via anisotropic rotation of encapsulated spin probe.
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