Browsing by Author "Chhiber, R."
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Item Cosmic-Ray Diffusion Coefficients throughout the Inner Heliosphere from a Global Solar Wind Simulation(IOP, 2017-06-16) Chhiber, R.; Subedi, P.; Usmanov, A. V.; Matthaeus, W. H.; Ruffolo, D.; Goldstein, Melvyn; Parashar, T. N.We use a three-dimensional magnetohydrodynamic simulation of the solar wind to calculate cosmic-ray diffusion coefficients throughout the inner heliosphere (2R⊙ - 3au). The simulation resolves large-scale solar wind flow, which is coupled to small-scale fluctuations through a turbulence model. Simulation results specify background solar wind fields and turbulence parameters, which are used to compute diffusion coefficients and study their behavior in the inner heliosphere. The parallel mean free path (mfp) is evaluated using quasi-linear theory, while the perpendicular mfp is determined from nonlinear guiding center theory with the random ballistic interpretation. Several runs examine varying turbulent energy and different solar source dipole tilts. We find that for most of the inner heliosphere, the radial mfp is dominated by diffusion parallel to the mean magnetic field; the parallel mfp remains at least an order of magnitude larger than the perpendicular mfp, except in the heliospheric current sheet, where the perpendicular mfp may be a few times larger than the parallel mfp. In the ecliptic region, the perpendicular mfp may influence the radial mfp at heliocentric distances larger than 1.5 au; our estimations of the parallel mfp in the ecliptic region at 1 au agree well with the Palmer "consensus" range of 0.08–0.3 au. Solar activity increases perpendicular diffusion and reduces parallel diffusion. The parallel mfp mostly varies with rigidity (P) as P.³³ and the perpendicular mfp is weakly dependent on P. The mfps are weakly influenced by the choice of long-wavelength power spectra.Item Magnetic Field Line Random Walk and Solar Energetic Particle Path Lengths: Stochastic Theory and PSP/ISoIS Observation(EDP Sciences) Chhiber, R.; Matthaeus, W. H.; Cohen, C.M.S.; Ruffolo, D.; Goldstein, Melvyn; et alContext:In 2020 May-June, six solar energetic ion events were observed by the Parker Solar Probe/ISoIS instrument suite at 0.35 AU from the Sun. From standard velocity-dispersion analysis, the apparent ion path length is 0.625 AU at the onset of each event. Aims:We develop a formalism for estimating the path length of random-walking magnetic field lines, to explain why the apparent ion pathlength at event onset greatly exceeds the radial distance from the Sun for these events. Methods:We developed analytical estimates of the average increase in pathlength of random-walking magnetic field lines, relative to the unperturbed mean field. Monte Carlo simulations of fieldline and particle trajectories in a model of solar wind turbulence are used to validate the formalism and study the path lengths of particle guiding-center and full-orbital trajectories. The formalism is implemented in a global solar wind model, and results are compared with ion pathlengths inferred from ISoIS observations. Results:Both a simple estimate and a rigorous theoretical formulation are obtained for fieldlines' pathlength increase as a function of pathlength along the large-scale field. From simulated fieldline and particle trajectories, we find that particle guiding centers can have pathlengths somewhat shorter than the average fieldline pathlength, while particle orbits can have substantially larger pathlengths due to their gyromotion with a nonzero effective pitch angle. Conclusions:The long apparent path length during these solar energetic ion events can be explained by 1) a magnetic field line path length increase due to the field line random walk, and 2) particle transport about the guiding center with a nonzero effective pitch angle. Our formalism for computing the magnetic field line path length, accounting for turbulent fluctuations, may be useful for application to solar particle transport in general.Item Observations of Energetic-particle Population Enhancements along Intermittent Structures near the Sun from the Parker Solar Probe(AAS, 2020-02-03) Bandyopadhyay, Riddhi; Matthaeus, W. H.; Parashar, T. N.; Chhiber, R.; Goldstein, Melvyn; et alObservations at 1 au have confirmed that enhancements in measured energetic-particle (EP) fluxes are statistically associated with "rough" magnetic fields, i.e., fields with atypically large spatial derivatives or increments, as measured by the Partial Variance of Increments (PVI) method. One way to interpret this observation is as an association of the EPs with trapping or channeling within magnetic flux tubes, possibly near their boundaries. However, it remains unclear whether this association is a transport or local effect; i.e., the particles might have been energized at a distant location, perhaps by shocks or reconnection, or they might experience local energization or re-acceleration. The Parker Solar Probe (PSP), even in its first two orbits, offers a unique opportunity to study this statistical correlation closer to the corona. As a first step, we analyze the separate correlation properties of the EPs measured by the Integrated Science Investigation of the Sun (IS⊙IS) instruments during the first solar encounter. The distribution of time intervals between a specific type of event, i.e., the waiting time, can indicate the nature of the underlying process. We find that the IS⊙IS observations show a power-law distribution of waiting times, indicating a correlated (non-Poisson) distribution. Analysis of low-energy (∼15 – 200 keV/nuc) IS⊙IS data suggests that the results are consistent with the 1 au studies, although we find hints of some unexpected behavior. A more complete understanding of these statistical distributions will provide valuable insights into the origin and propagation of solar EPs, a picture that should become clear with future PSP orbits.Item On the validity of the Taylor Hypothesis in the inner heliosphere as observed by the Parker Solar Probe(2020-12-23) Chasapis, Alexandros; Bandyopadhyay, R.; Chhiber, R.; Qudsi, R.; Goldstein, Melvyn; et alWe study the validity of the Taylor “frozen-in” hypothesis in the inner heliosphere during the orbit of Parker Solar Probe. We examine the ratio of the Alfv´en velocity to the apparent solar wind velocity, and the magnitude of the turbulent fluctuations of the velocity of the solar wind, as observed by the spacecraft in its own reference frame. The necessary conditions appear to be satisfied for most of the orbit, with both these ratios being far below unity. However, at heliocentric distances smaller than ∼ 50 solar radii, these ratios are observed to rise above 0.1, and can consistently exceed 0.3, leading to the conclusion that the Taylor hypothesis may begin to break down in these inner regions. At larger distances, both ratios remain generally low. However, we observe some periods where the plasma conditions change significantly, either due to a lower plasma density or much stronger turbulent fluctuations, leading to much higher values, suggesting that the Taylor hypothesis may break down in such transient regions. An alternative formulation of the frozen-in hypothesis, which could be valid for outward-propagating dominant fluctuations, is also examined. Its conditions, namely that the Els¨asser variable corresponding to inward propagating fluctuations is much smaller than both the perpendicular spacecraft velocity, and the outward propagating fluctuation, were found to be satisfied near perihelion for encounters 1 and 2 and for parts of the encounters 4 and 5. We conclude that although the basic conditions for the validity of the Taylor hypothesis may cease to be satisfied in the inner heliosphere at distances below ∼ 50 solar radii, alternative frozen-in hypotheses may be successfully employed.Item Shear-Driven Transition to Isotropically Turbulent Solar Wind Outside the Alfv´en Critical Zone(AAS, 2020-09-15) Ruffolo, D.; Matthaeus, W. H.; Chhiber, R.; Usmanov, A. V.; Yang, Y.; Bandyopadhyay, R.; Parashar, T. N.; Goldstein, Melvyn; DeForest, C. E.; Wan, M.; Chasapis, A.; Maruca, B. A.; Velli, M.; Kasper, J. C.Motivated by prior remote observations of a transition from striated solar coronal structures to more isotropic ``flocculated'' fluctuations, we propose that the dynamics of the inner solar wind just outside the Alfvén critical zone, and in the vicinity of the first β=1 surface, is powered by the relative velocities of adjacent coronal magnetic flux tubes. We suggest that large amplitude flow contrasts are magnetically constrained at lower altitude but shear-driven dynamics are triggered as such constraints are released above the Alfvén critical zone, as suggested by global magnetohydrodynamic (MHD) simulations that include self-consistent turbulence transport. We argue that this dynamical evolution accounts for features observed by {\it Parker Solar Probe} ({\it PSP}) near initial perihelia, including magnetic ``switchbacks'', and large transverse velocities that are partially corotational and saturate near the local Alfvén speed. Large-scale magnetic increments are more longitudinal than latitudinal, a state unlikely to originate in or below the lower corona. We attribute this to preferentially longitudinal velocity shear from varying degrees of corotation. Supporting evidence includes comparison with a high Mach number three-dimensional compressible MHD simulation of nonlinear shear-driven turbulence, reproducing several observed diagnostics, including characteristic distributions of fluctuations that are qualitatively similar to {\it PSP} observations near the first perihelion. The concurrence of evidence from remote sensing observations, {\it in situ} measurements, and both global and local simulations supports the idea that the dynamics just above the Alfvén critical zone boost low-frequency plasma turbulence to the level routinely observed throughout the explored solar system.