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Register a new userThe Effect of Cosmic Rays on Cometary Nuclei: I Dose deposition
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Comets are small bodies thought to contain the most pristine material in the solar system. However, since their formation 4.5 Gy ago, they have been altered by different processes. While not exposed to much electromagnetic radiation, they experience intense particle radiation. Galactic cosmic rays and solar energetic particles have a broad spectrum of energies and interact with the cometary surface and subsurface; they are the main source of space weathering for a comet in the Kuiper Belt or in the Oort cloud; and also affect the ice prior to the comet agglomeration. While low energy particles interact only with the cometary surface, the most energetic ones deposit a significant amount of energy down to tens of meters. This interaction can modify the isotopic ratios in cometary ices and create secondary compounds through radiolysis, such as O2 and H2O2 (paper II: Maggiolo et al., 2020). In this paper, we model the energy deposition of energetic particles as a function of depth using a Geant4 application modified to account for the isotope creation process. We quantify the energy deposited in cometary nucleus by galactic cosmic rays and solar energetic particles. The consequences of the energy deposition on the isotopic and chemical composition of cometary ices and their implication on the interpretation of cometary observations, notably of 67P/Churyumov Gerasimenko by the ESA/Rosetta spacecraft, will be discussed in Paper II.
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Cosmic rays (CRs) are tracers of solar events when they are associated with solar flares, but also galactic events when they come from outside our solar system. SEPs are correlated with the 11-year solar cycle while GCRs are anti-correlated due to their interaction with the heliospheric magnetic field and the solar wind. Our aim is to quantify separately the impact of the amplitude and the geometry of the magnetic field on the propagation of CRs of various energies in the inner heliosphere. We focus especially on the diffusion caused by the magnetic field along and across the field lines. To do so, we use the results of 3D MHD wind simulations running from the lower corona up to 1 AU. The wind is modeled using a polytropic approximation, and fits and power laws are used to account for the turbulence. Using these results, we compute the parallel and perpendicular diffusion coefficients of the Parker CR transport equation, yielding 3D maps of the diffusion of CRs in the inner heliosphere. By varying the amplitude of the magnetic field, we change the amplitude of the diffusion by the same factor, and the radial gradients by changing the spread of the current sheet. By varying the geometry of the magnetic field, we change the latitudinal gradients of diffusion by changing the position of the current sheets. By varying the energy, we show that the distribution of values for SEPs is more peaked than GCRs. For realistic solar configurations, we show that diffusion is highly non-axisymmetric due to the configuration of the current sheets, and that the distribution varies a lot with the distance to the Sun with a drift of the peak value. This study shows that numerical simulations and theory can help quantify better the influence of the various magnetic field parameters on the propagation of CRs. This study is a first step towards generating synthetic CR rates from numerical simulations.
In the Milky Way, cosmic rays (CRs) are dynamically important in the interstellar medium, contribute to hydrostatic balance, and may help regulate star formation. However, we know far less about the importance of CRs in galaxies whose gas content or star formation rate differ significantly from those of the Milky Way. Here we construct self-consistent models for hadronic CR transport, losses, and contribution to pressure balance as a function of galaxy properties, covering a broad range of parameters from dwarfs to extreme starbursts. While the CR energy density increases from $sim 1$ eV cm$^{-3}$ to $sim 1$ keV cm$^{-3}$ over the range from sub-Milky Way dwarfs to bright starbursts, strong hadronic losses render CRs increasingly unimportant dynamically as the star formation rate surface density increases. In Milky Way-like systems, CR pressure is typically comparable to turbulent gas and magnetic pressure at the galactic midplane, but the ratio of CR pressure to gas pressure drops to $sim 10^{-3}$ in dense starbursts. Galaxies also become increasingly CR calorimetric and gamma-ray bright in this limit. The degree of calorimetry at fixed galaxy properties is sensitive to the assumed model for CR transport, and in particular to the time CRs spend interacting with neutral ISM, where they undergo strong streaming losses. We also find that in some regimes of parameter space hydrostatic equilibrium discs cannot exist, and in Paper II of this series we use this result to derive a critical surface in the plane of star formation surface density and gas surface density beyond which CRs may drive large-scale galactic winds.
Precise time-dependent measurements of the Z = 2 component in the cosmic radiation provide crucial information about the propagation of charged particles through the heliosphere. The PAMELA experiment, with its long flight duration (15th June 2006 - 23rd January 2016) and the low energy threshold (80 MeV/n) is an ideal detector for cosmic ray solar modulation studies. In this paper, the helium nuclei spectra measured by the PAMELA instrument from July 2006 to December 2009 over a Carrington rotation time basis are presented. A state-of-the-art three-dimensional model for cosmic-ray propagation inside the heliosphere was used to interpret the time-dependent measured fluxes. Proton-to-helium flux ratio time profiles at various rigidities are also presented in order to study any features which could result from the different masses and local interstellar spectra shapes.
The sidereal anisotropy of galactic cosmic ray (GCR) intensity observed with the Tibet Air Shower (AS) experiment still awaits theoretical interpretation. The observed global feature of the anisotropy is well reproduced by a superposition of the bi-directional and uni-directional flows (BDF and UDF, respectively) of GCRs. If the orientation of the deduced BDF represents the orientation of the local interstellar magnetic field (LISMF), as indicated by best-fitting a model to the data, the UDF deviating from the BDF orientation implies a significant contribution from the streaming perpendicular to the LISMF. This perpendicular streaming is probably due to the drift anisotropy, because the contribution from the perpendicular diffusion is expected to be much smaller than the drift effect. The large amplitude deduced for the UDF indicates a large spatial gradient of the GCR density. We suggest that such a density gradient can be expected at the heliosphere sitting close to the boundary of the Local Interstellar Cloud (LIC), if the LIC is expanding. The spatial distribution of GCR density in the LIC reaches a stationary state because of the balance between the inward cross-field diffusion and the adiabatic cooling due to the expansion. We derive the steady-state distribution of GCR density in the LIC based on radial transport of GCRs in a spherical LIC expanding at a constant rate. By comparing the expected gradient with the observation by Tibet experiment, we estimate the perpendicular diffusion coefficient of multi-TeV GCRs in the local interstellar space.
Simultaneous and continuous observations of galactic cosmic-ray electrons and positrons from the PAMELA and AMS02 space experiments are most suitable for numerical modeling studies of the heliospheric modulation of these particles below 50 GeV. A well-established comprehensive three-dimensional modulation model is applied to compute full spectra for electrons and positrons with the purpose of reproducing the observed ratio positrons/electrons for a period which covers the previous long and unusual deep solar minimum activity and the recent maximum activity phase including the polarity reversal of the solar magnetic field. For this purpose the very local interstellar spectra for these particles were established first. Our study is focused on how the main modulation processes, including particle drifts, and other parameters such as the three major diffusion coefficients, had evolved, and how the corresponding charge-sign dependent modulation had occurred subsequently. The end result of our effort is the detailed reproduction of positron/electrons from 2006 to 2015, displaying both qualitative and quantitative agreement with the main observed features. Particularly, we determine how much particle drifts is needed to explain the time dependence exhibited by the observed positron/electron during each solar activity phase, especially during the polarity reversal phase when no well-defined magnetic polarity was found.
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