A Numerical Study of the Solar Modulation of Galactic Protons and Helium from 2006 to 2017

With continuous measurements from space-borne cosmic-ray detectors such as AMS-02 and PAMELA, precise spectra of galactic cosmic rays over the 11 yr solar cycle have become available. For this study, we utilize proton and helium spectra below 10 GV from these missions from 2006 to 2017 to construct a cosmic-ray transport model for a quantitative study of the processes of solar modulation. This numerical model is based on Parker’s transport equation, which includes four major transport processes. The Markov Chain Monte Carlo method is utilized to search the relevant parameter space related to the drift and the diffusion coefficients by reproducing and fitting the mentioned observed spectra. The resulting best-fit normalized χ 2 is mainly less than 1. It is found that (1) when reproducing these observations the parameters required for the drift and diffusion coefficients exhibit a clear time dependence, with the magnitude of the diffusion coefficients anticorrelated with solar activity; (2) the rigidity dependence of the resulting mean free paths varies with time, and their rigidity dependence at lower rigidity can even have a larger slope than at higher rigidity; (3) using a single set of modulation parameters for each pair of observed proton and helium spectra, most spectra are reproduced within observational uncertainty; and (4) the simulated proton-to-helium flux ratio agrees with the observed values in terms of its long-term time dependence, although some discrepancy exists, and the difference is mostly coming from the underestimation of proton flux.

Energetic Solar Particle Access to the Near-Equatorial Inner Magnetosphere

<p>Solar proton events are comprised of energetic protons of solar and interplanetary origin. Such energetic particles are able to access the magnetosphere at various locations according to their cutoff rigidity. The specific properties of solar proton access are of great interest for space weather prediction purposes. Using Van Allen Probes/Relativistic Electron-Proton Telescope (REPT) 20-200 MeV proton data we examine four of the strongest solar proton events over the lifetime of the mission. We present evidence of the direct magnetospheric access of these energetic solar protons and find strong flux increases at L<4. Results indicate that small and sudden flux changes measured by ACE spacecraft sensors upstream of Earth are also seen in the near-equatorial inner magnetosphere. Using the East-West asymmetry of solar protons as a proxy for cutoffs we examine the highly dynamic cutoff rigidity. We find there is evidence for: (1) cutoff rigidity dependence on MLT; (2) suppressed cutoffs with rapid Dst changes; and (3) rapid evolution of cutoffs even during quiet magnetospheric conditions.</p>

Tandem tension sensor reveals substrate rigidity-dependence of integrin molecular tensions in live cells

Response of integrin tensions to substrate rigidity is important in cell rigidity sensing but has not been confirmed. Current fluorescent tension sensors produce cellular force signals collectively resulted from integrin tension magnitude, tension dwell time, integrin density and activity, ligand density and accessibility, etc., making it challenging to monitor the absolute molecular force level of integrin tensions in live cells. Here we developed a tandem tension sensor (TTS) consisting of two coupled tension sensing units which are subject to the same tension and respond with different activation probabilities to the tension. Reported by fluorescence, the activation probability ratio of these two units solely responds to the force level of local integrin tensions, excluding the bias from other non-force factors. We verified the feasibility of TTS in detecting integrin tensions and applied it to study cells on elastic substrates. TTS unambiguously reported that integrin tensions in platelets decrease monotonically with the substrate rigidity, verifying the rigidity-dependence of integrin tensions in live cells.

Properties of Elementary Particle Fluxes in Primary Cosmic Rays Measured with the Alpha Magnetic Spectrometer on the International Space Station

Precision measurements by AMS of the fluxes of cosmic ray positrons, electrons, antiprotons, protons as well as their rations reveal several unexpected and intriguing features. The presented measurements extend the energy range of the previous observations with much increased precision. The new results show that the behavior of positron flux at around 300 GeV is consistent with a new source that produce equal amount of high energy electrons and positrons. In addition, in the absolute rigidity range 60–500 GV, the antiproton, proton, and positron fluxes are found to have nearly identical rigidity dependence and the electron flux exhibits different rigidity dependence.

Observation of Properties of Primary and Secondary Cosmic Rays by the Alpha Magnetic Spectrometer on the International Space Station

The Alpha Magnetic Spectrometer (AMS-02) is a wide acceptance high-energy physics experiment installed on the International Space Station in May 2011 and operating continuously since then. With a collection rate of approximately 1.7 × 1010 events/year, and the combined identification capabilities of 5 independent detectors, AMS-02 is able to precisely separate cosmic rays light nuclei (1 ≤ Z ≤ 8). Knowledge of the precise rigidity dependence of the light nuclei fluxes is important in understanding the origin, acceleration, and propagation of cosmic rays. AMS-02 collaboration has recently released the precise measurements of the fluxes of light nuclei as a function of rigidity (momentum/charge) in the range between 2 GV and 3 TV. Based on the observed spectral behaviour, the light nuclei can be separated in three distinct families: primaries (hydrogen, helium, carbon, and oxygen), secondaries (lithium, beryllium, and boron), and mixed (nitrogen). Spectral indices of all light nuclei fluxes progressively harden above 100 GV. Primary cosmic ray fluxes have an identical hardening above 60 GV, of about γ = 0.12 ± 0.04. While helium, carbon and oxygen have identical spectral index magnitude, the hydrogen spectral index shows a different magnitude, i.e. the primary-to-primary H/He ratio is well described by a single power law above 45 GV with index -0.077 ± 0.007. Secondary cosmic ray fluxes have identical rigidity dependence above 30 GV. Secondary cosmic rays all harden more than primary species, and together all secondary-to-primary ratios show a hardening difference of 0.13 ± 0.03. Remarkably, the nitrogen flux is well described over the entire rigidity range by the sum of the primary flux equal to 9% of the oxygen flux and the secondary flux equal to 62% of the boron flux.

Substrate-rigidity dependent migration of an idealized twitching bacterium

An analytical model reveals generic physical mechanisms for substrate-rigidity dependence of cellular motion. Key ingredients are a tight surface adhesion and forced adhesion rupture.