Streaming Anisotropy Research Articles

On the Anisotropy of Galactic Cosmic Rays

Abstract In the interstellar medium at rest, containing low-frequency magnetohydrodynamic linearly polarized slab Alfvén waves, the anisotropy of relativistic galactic cosmic rays consists of two parts: the streaming anisotropy g s (z, p,μ), caused by the spatial gradient of the isotropic part of the cosmic ray distribution function, and the interstellar Compton–Getting anisotropy , caused by the momentum gradient of the isotropic part of the cosmic ray distribution function. Both anisotropies depend differently on the cosmic ray pitch-angle cosine μ, cosmic ray momentum p, and cross-helicity state H c of the Alfvenic slab turbulence. First, the streaming anisotropy is independent from H c and varies as with η = 2 − s, where s denotes the power-law spectral index of interstellar turbulence. Second, the interstellar Compton–Getting anisotropy is independent of momentum and linearly proportional to . These different pitch-angle dependencies can be tested by the Liouville mapping technique to infer the pristine interstellar cosmic ray anisotropy from measurements inside the solar system. For cosmic rays with energy of 4 TeV the derived pristine interstellar cosmic ray anisotropy suggest the linear ( ) pitch-angle dependence. This is well explained by the interstellar Compton–Getting anisotropy, provided the Alfvén speed in the local interstellar medium is about 62 km s−1.

Using proton radiation from the moon to search for diurnal variation of regolith hydrogenation

With the help of a new observing technique, we have detected diurnal variations in both lunar albedo protons and incident galactic cosmic rays (GCRs) at the Moon. Using the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument on the Lunar Reconnaissance Orbiter (LRO), we have combined specific, targeted observations of the lunar horizon with our nominal nadir-viewing data to find a tentative enhancement in the yield of lunar albedo protons at the local sunrise terminator of the Moon relative to the local sunset terminator. A diurnal variation in hydrogenation in the top few cm of regolith is one possible interpretation of this result. We have also measured an unanticipated AM/PM difference in the arriving GCRs at the Moon which can be explained by a known streaming anisotropy. The relatively small data set used here (compared to previous mapping studies of CRaTER data) and heretofore untried horizon observations required us to develop improved data analysis techniques, which we will use in the future to revisit previous data sets and to analyze planned new horizon observations of lunar albedo protons.

Interplanetary Plasma Scattering Diagnostics from Anisotropy-time Profiles of Solar Energetic Particles

The anisotropy-time profile of solar particle events provides a powerful diagnostics tool to the interplanetary plasma scattering parameters of energetic charged particles. In the weak focusing limit of the transport of solar particles in axisymmetric MHD turbulence, the particle anisotropy consists of two contributions, the streaming and the Compton- Getting contribution, resulting from the parallel spatial gradient and the momentum gradient of the isotropic part of the particles' phase space density, respectively. These gradients can be calculated from the appropriate solution to the time- dependent focused transport equation of solar particles. For the illustrative case of the solution of the one-dimensional time-dependent focused transport equation with a constant focusing length and a point-like instantaneous injection of particles the streaming and Compton-Getting contributions to the anisotropy-time profile are analytically calculated in MHD turbulence consisting of isospectral undamped slab Alfven waves for equal magnetic helicity. The Compton-Getting contribution scales proportional to the ratio of interplanetary Alfven speed to solar particle speed, and therefore is much smaller than the streaming contribution for the observed mildly relativistic solar particles. After vanishing anisotropy values at times t < tM the streaming anisotropy suddenly attains its maximum value AS,max = 3 2 + (p) 2L at tM = t 0 + (zz0)/v. At later times the streaming anisotropy decreases (tt0) � 1

Termination Shock Asymmetries as Seen by theVoyagerSpacecraft: The Role of the Interstellar Magnetic Field and Neutral Hydrogen

We show that asymmetries of the termination shock due to the influence of the interstellar magnetic field (ISMF) are considerably smaller in the presence of neutral hydrogen atoms, which tend to symmetrize the heliopause, the termination shock, and the bow shock due to charge exchange with charged particles. This leads to a much stronger restriction on the ISMF direction and its strength. We demonstrate that in the presence of the interplanetary magnetic field the plane defined by the local interstellar medium (LISM) velocity and magnetic field vectors does not exactly coincide with the plane defined by the interstellar neutral helium and hydrogen velocity vectors in the supersonic solar wind region, which limits the accuracy of the inferred direction of the ISMF. We take into account the tilt of the LISM velocity vector with respect to the ecliptic plane and show that magnetic fields as strong as 3 μG or greater may be necessary to account for the observed asymmetry. Estimates are made of the longitudinal streaming anisotropy of energetic charged particles at the termination shock caused by the nonalignment of the interplanetary magnetic field with its surface. By investigating the behavior of interplanetary magnetic field lines that cross the Voyager 1 trajectory in the inner heliosheath, we estimate the length of the trajectory segment that is directly connected by these lines to the termination shock. A possible effect of the ISMF draping over the heliopause is discussed in connection with radio emission generated in the outer heliosheath.

A New Type of Cosmic‐Ray Anisotropy from Perpendicular Diffusion. I. Modification of the Spatial Diffusion Tensor and the Diffusion‐Convection Cosmic‐Ray Transport Equation

Perpendicular spatial diffusion of cosmic rays is particularly important in highly flattened astrophysical objects such as the Galactic disk and in the outer heliosphere. By maintaining perpendicular Fokker-Planck coefficients in the Fokker-Planck equation for the gyrotropic cosmic-ray phase-space density, it is shown that the diffusion approximation gives rise to a new, third contribution to the cosmic-ray anisotropy due to perpendicular spatial diffusion, adding to the well-known streaming anisotropy due to pitch-angle scattering and parallel spatial gradients and the Compton-Getting anisotropy due to momentum gradients. This new anisotropy modifies the transport parameters of the diffusion-convection transport equation for the isotropic gyrotropic part of the cosmic-ray phase-space density; in general, one obtains two new perpendicular rates of adiabatic deceleration and nine nonzero elements of the full cosmic-ray spatial diffusion tensor.

Radial Streaming Anisotropies of Charged Particles Accelerated at the Solar Wind Termination Shock

Recent observations of an increase in energetic particle intensities on the Voyager 1 spacecraft, for several months in late 2002 to early 2003, suggest new phenomena associated with its approach to the termination shock of the solar wind. An important diagnostic used by the experimenters to interpret the event has been the radial anisotropy of the energetic particle distribution. In this Letter we consider the transport of energetic charged particles accelerated at the termination shock, both in the diffusion approximation and by directly integrating test particle trajectories in a turbulent magnetic field. We determine the radial anisotropy in a number of models. In general, we find that the radial anisotropy of the particles is complicated and that it cannot reliably be used to determine the radial velocity of the background plasma. In all cases considered, we find that the average radial anisotropy in the region upstream of the shock is directed toward the shock and is less than or equal to the Compton-Getting anisotropy in the slower wind downstream. We attribute the difference from the Compton-Getting anisotropy to the diffusive anisotropy, which is directed upstream away from the shock and which is part of the acceleration process. We suggest that the small radial anisotropy observed on Voyager 1 at low energies may actually be an indication that Voyager 1 was located upstream of the shock during the entire event. In addition, we find large fluctuating anisotropies along the magnetic field (nearly transverse to the radial direction).

Excitation of MHD waves upstream of Jupiter by energetic sulfur or oxygen ions

Large fluxes of heavy ions have been reported upstream of Jupiter's bow shock as Voyager 1 approached the planet (Zwickl et al., 1981; Krimigis et al., 1985). Enhanced low‐frequency magnetic wave activity was also observed during the particle events. The fluctuations are left‐hand eliptically polarized in the plasma frame. The spectrum of these fluctuations contains a peak close to the Doppler‐shifted resonance frequency of a sulfur or oxygen beam with streaming energy of approximately 30 keV. These fluctuations are also present in the spectrum of the magnitude of the field. We conclude that the observations result from an instability driven by an energetic beam of either sulfur or oxygen. The wave observations can be described by a heavy ion distribution with both a streaming anisotropy and a temperature anisotropy (T⊥/T∥ ≃ 2). This class of heavy ion streaming instabilities may also play a role in wave‐particle interactions in the vicinity of comets.

Association between magnetic field fluctuations and energetic particle bursts in the Earth's magnetotail

The association between energetic protons (0.29–0.50 MeV) and simultaneous local fluctuations of magnetic field at 35 to 45 RE in the magnetotail is examined statistically with data from APL/JHU particle telescopes aboard IMP 7 and IMP 8. About four satellite years of 5.5 min averaged measurements are used in this study. In addition to confirming that the level of magnetic field fluctuations generally increases with the presence of energetic protons and their streaming anisotropy, it is found that increase in occurrence frequency of streaming of energetic protons are ordered far better by magnetic field fluctuations than by proximity to the neutral sheet. However, the presence of large magnetic field fluctuations (δB &gt; 5 nT or δB/B &gt; 50%) is neither a necessary nor a sufficient condition for the detection of large streaming in energetic protons.

Z-rich solar particle event characteristics 1972-1976

It is found in the reported investigation that Z-rich solar particle events usually have large and prolonged anisotropies in addition to an extremely variable charge composition that varies not only from event to event but also throughout the event. These observations suggest that one can no longer regard the event-averaged composition of solar particle events at low energies as providing an unbiased global sample of the solar atmospheric composition. The variability from event to event and among classes of events is just too great. However, the tendency for the Z-rich events to be associated with both the low-speed solar wind at or just before the onset of solar wind streams and with active regions located in the western hemisphere, indicates that charge composition studies of solar particle events can yield a better knowledge of the flare acceleration process as well as the inhomogeneous nature of magnetic field structure and particle composition in the solar atmosphere.

Isotropy of cosmic rays caused by magnetic discontinuities

view Abstract Citations (5) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Isotropy of cosmic rays caused by magnetic discontinuities. Wentzel, D. G. Abstract When cosmic rays stream past a bend in a magnetic field, they acquire a correlation in their phase of gyration about the magnetic field. The resulting plasma instability generates Alfven waves and simultaneously reduces the cosmic rays' mean streaming velocity along the field. The growth rate of the instability is proportional to a fractional power of the cosmic-ray density and is rapid even for cosmic rays above 10 to the 11th power eV. A few interstellar shocks or interplanetary rotational discontinuities suffice to reduce the streaming anisotropy to the local Alfven velocity. Cosmic rays of 10 to the 11th power eV may be made nearly isotropic even within the solar system. Publication: The Astrophysical Journal Pub Date: September 1977 DOI: 10.1086/182509 Bibcode: 1977ApJ...216L..59W Keywords: Cosmic Rays; Interplanetary Magnetic Fields; Magnetohydrodynamic Stability; Radiation Distribution; Gyration; Isotropy; Magnetohydrodynamic Waves; Particle Flux Density; Propagation Velocity; Shock Wave Propagation; Space Radiation full text sources ADS |

Detailed directional and temporal properties of solar energetic particles associated with propagating interplanetary shock waves

In the period May 1967 to May 1968, detailed Vela 4 observations of the properties of protons in the enhancements (‘spikes’) associated with propagating interplanetary shocks show that the proton energy spectrum does not extend beyond ∼5 Mev; the protons often possess unusually large streaming anisotropies (e.g., a pitch-angle anisotropy of >1000∶1 for the February 20, 1968, event), and the peak of the spike can occur as much as 3×105 km (∼10 min) on the sunward side of the shock front. Neither the Axford-Reid nor the Fisk model satisfactorily explains the observations: shock spikes were observed to occur when the interplanetary-field line connecting the interplanetary shock and the observer probably did not intersect the bow shock, contrary to the prediction of the Axford-Reid model; and the prediction of Fisk concerning the shape of the proton energy spectrum at the shock front was not observed at energies above 0.5 Mev. In their published form, neither model can predict an extremely large streaming anisotropy, although a recent extension of Fisk's model may be able to explain anisotropy ratios as large as 10. A single-encounter model, where particles are accelerated in a single elastic collision with the interplanetary shock, is qualitatively examined and can explain large enhancements and anisotropies only if the magnetic field and the shock normal are inclined at an angle of ≳85° to each other.