The interest in the role of the solar wind termination shock (TS) and heliosheath in cosmic ray modulation studies has increased significantly as the Voyager 1 and 2 spacecraft approach the estimated ...position of the TS. For this work the modulation of cosmic ray protons (p) and antiprotons (), and the consequent charge‐sign dependence, is studied with a numerical model including a TS with diffusive shock acceleration, a heliosheath, and drifts. The model allows a comparison of modulation with and without a TS. A more fundamental and comprehensive set of diffusion coefficients is used, applicable to a number of cosmic ray species during both magnetic polarity cycles of the Sun. Newly computed and improved local interstellar spectra for p and are used. The modulation of p with an anomalous component is also done to establish charge‐sign dependence at low energies. The modulation effects of the heliosheath and TS are illustrated for the different species and how they affect the computed /p. We found that the computed modulation for is surprisingly different from p and that the heliosheath is important for cosmic ray modulation. The local proton interstellar spectrum may not be known at energies <∼1 GeV until a spacecraft actually approaches the heliopause because of the strong modulation that occurs in the heliosheath, the effect of the TS, and the presence of anomalous protons. For antiprotons, in contrast, these effects are less pronounced.
Solar cycle variations and cosmic rays Potgieter, M.S.
Journal of atmospheric and solar-terrestrial physics,
02/2008, Letnik:
70, Številka:
2
Journal Article
Recenzirano
Cosmic rays are excellent indicators of the various solar cycle variations. Galactic and anomalous cosmic rays encounter an outward moving solar wind with cyclic magnetic-field fluctuations and ...turbulence, which constitute the convection and diffusion processes in the heliosphere. They also lose energy as they propagate inwards to Earth, and experience current sheet, global curvature and gradient drifts in the heliospheric magnetic field. As a result, the intensity of cosmic rays directly reflects the various solar cycle variations, from the well-known 11- and 22-year cycles, with the reversal of the solar magnetic field at extreme solar maximum, to highly temporal variations like proton flares, Forbush decreases, corotating interaction regions and a variety of propagating diffusion barriers. All these features contribute and influence space climate and weather at Earth. Recently, the time-dependent extent and the dynamics of the heliosphere, in particular the role of the heliosheath and the location of the heliopause, have been emphasized as important to very long-term space climate. Long-term modulation over 11–22 years is breifly reviewed with emphasis on the compound time-dependent approach in modeling the solar cycle variations of cosmic rays in the heliosphere.
The heliosphere is defned as the plasmatic inuence sphere of the Sun and stretches far beyond the solar system. Cosmic rays, as charged particles with energy between about 1 MeV and millions of GeV, ...arriving from our own Galaxy and beyond, penetrate the heliosphere and encounter the solar wind and embedded magnetic field so that when observed they contain useful information about the basic features of the heliosphere. In order to interpret these observations, obtained on and near the Earth and farther away by several space missions, and to gain understanding of the underlying physics, called heliophysics, we need to simulate the heliosphere and the acceleration, propagation and transport of these astroparticles with numerical models. These types of models vary from magnetohydrodynamic based approaches for simulating the heliosphere to using standard finite-difference numerical schemes to solve transport-type partial differential equations with varying complexity. A large number of these models have been developed locally to do internationally competitive research and have become as such an important training tool for human capacity development in computational physics in South Africa. How these models are applied to various aspects of heliospheric space physics, with illustrative examples, is discussed in this overview.
A comprehensive three-dimensional numerical model for the modulation of cosmic rays in the heliosphere is applied to investigate the relative roles of the time dependence of the elements of the ...diffusion tensor on the proton to total Helium (p/He) and Helium-3 to Helium-4 (3He2/4He2) ratios at rigidities below 3 GV. At these rigidities the ratios have been observed by both PAMELA and AMS-02 detectors to have a time variation in response to changing solar activity. We found that the contribution of the time dependence of the perpendicular diffusion in the radial direction of the heliosphere is the dominant cause of this observed time variation, especially in the A < 0 magnetic field cycle, and not any fundamental difference between the solar modulation of galactic protons and He-isotopes. It follows that neglecting this time dependence from numerical models, both in value and its rigidity dependence, would produce time trends in the mentioned ratios that are incompatible with observed trends at the Earth. Furthermore, we found differences in the computed time trends of p/He and 3He2/4He2 ratios at rigidities below 1.5 GV. This is mainly a consequence of an interplay between perpendicular diffusion in the radial direction and adiabatic energy losses which begin to influence modulated spectra at a higher rigidity for 3He2 than for 4He2, and for total He than for protons.
The modulation of cosmic ray electrons in the heliosphere plays an important role in improving our understanding and assessment of the processes applicable to low-energy galactic electrons. A full ...three-dimensional numerical model based on Parker’s transport equation is used to study the modulation of 10MeV galactic electrons, in particular inside the heliosheath. The emphasis is placed on the role that perpendicular diffusion plays in causing the extraordinary large increase in the observed intensities of these electrons in the heliosheath. The modelling is compared with observations of 6–14MeV electrons from the Voyager 1 mission. Results are shown for the radial intensity profiles of these electrons, as well as the modulation effects of varying the extent of the heliosheath by changing the location of the termination shock and the heliopause and the value of the local interstellar spectrum. We confirm that the heliosheath acts as a modulation ‘barrier’ for low-energy galactic electrons. The significance of this result depends on how wide the inner heliosheath is; on how high the very local interstellar spectrum is at these low energies (E<100MeV) and on how small perpendicular diffusion is inside the inner heliosheath.
The heliosphere was in a state of ideal solar minimum conditions for at least three years up to the end of 2009. During this period the highest ever recorded cosmic ray spectra were observed at the ...Earth. Fortunately, the PAMELA and Ulysses KET instruments simultaneously observed proton intensities for most of the period between July 2006 and June 2009, while Voyager 1 made observations in the outer heliosphere. This provides a good opportunity to compare the basic features of a comprehensive numerical model for the global modulation of cosmic rays in the heliosphere with these observations. Global gradients for protons are computed with the model for this prolonged solar minimum of cycle 23 24. This is done for both radial and latitudinal gradients, with the latter possible because Ulysses changed its position significantly in the heliocentric meridional plane during this period. The modulation model is set up for the conditions that prevailed during this unusual solar minimum period so that insight is gained on what role particle drifts played in establishing the observed gradients for this period. Good agreement is found between computed and observed gradients so that we conclude that the model gives a most reasonable representation of modulation conditions from the Earth to the heliopause for the mentioned period. These results can be used to refine the theory for diffusion, particle drifts and turbulence in the heliosphere.
Time dependent cosmic ray modulation in the outer heliosphere is calculated and results are compared to Voyager 1 and 2 observations using a two-dimensional time-dependent cosmic ray transport model. ...We predict possible future 133–242
MeV proton observations along the Voyager 1 and 2 spacecraft trajectories. Recent theoretical advances in cosmic ray transport parameters are introduced in order to provide a time-dependence for the assumed transport parameters used in the model. This leads to results that are in general compatible with the spacecraft observations in the inner and outer heliosphere over multiple solar cycles. However, for the outer heliosphere, we find that the Voyager 1 and 2 spacecraft observations cannot be fitted with an identical set of parameters along both trajectories. This indicates a possible asymmetric heliosphere or a symmetric heliosphere but with different diffusion parameters in the northern and southern hemispheres, respectively. Furthermore, results indicate that Voyager 2 observations are still under the influence of solar cycle related changes because of the large modulation volume between the heliopause and spacecraft location in contrast to Voyager 1 which shows a steady increase in cosmic ray intensities.
Cosmic ray drift directions depend both on the charge of the particle population under consideration, as well as the heliospheric magnetic field polarity which oscillates within a ∼22 year cycle. The ...differences in the cosmic ray drift patterns between successive solar cycles manifest themselves in the sign of the latitudinal cosmic ray gradient. For positively charged particles, a positive latitudinal gradient is expected in the qA > 0 polarity cycle, while a negative value is expected in the qA < 0 cycle. For the first time observed radial and latitudinal values for anomalous cosmic ray oxygen are available in both of these cycles. In the present qA < 0 cycle, the observations seem, however, inconsistent with the drift‐dominated considerations discussed above, as the latitudinal gradient is smaller than expected and perhaps even positive. This issue is addressed by using a comprehensive numerical model for anomalous cosmic ray acceleration and transport, based on new diffusion coefficients, to simulate these gradients over the previous two magnetic polarity cycles. Computed gradients and energy spectra (at Earth and at Ulysses) are compared to observations, with good agreement achieved between observations and model predictions. For the qA > 0 cycle, the model predicts a positive latitudinal gradient but for the qA < 0 cycle a latitudinal gradient that is smaller and that can be either positive or negative. It is concluded that drifts and poleward diffusion set up competing latitudinal gradients in the qA < 0 cycle, with the resulting sign of the gradient determined by the most effective of the two processes.
The PAMELA experiment with magnetic spectrometer operated almost ten years on board of the Resurs DK1 satellite. The satellite was launched on 15 June 2006 on polar orbit with an inclination of 70° ...and an altitude of 350-610 km. The spectrometer continuously measured charged cosmic ray particles in wide energy range from about 50 MeV up to several TeV. In this work the spectra of electrons and positrons averaged over one year were obtained from July of 2006 until January 2016, i.e. from the end of 23th and at the beginning of 24th solar cycle including the period of interplanetary magnetic field polarity reversal. These precise long duration time-dependent measurements of the electron and positron spectra are important to estimate possible contributions of exotic cosmic ray sources such as dark matter annihilation or decay.
Modulation of cosmic ray protons in the heliosheath Langner, U. W.; Potgieter, M. S.; Webber, W. R.
Journal of Geophysical Research - Space Physics,
October 2003, Letnik:
108, Številka:
A10
Journal Article
Recenzirano
A contemporary numerical model including a solar wind termination shock, a heliosheath, and drifts is used to study the features of cosmic ray proton modulation in the outer heliosphere. Voyager and ...Pioneer spacecraft observations over 22 years and out to ∼82 AU have shown markedly different behavior for minimum modulation conditions between the radial intensity profiles for periods of opposite magnetic polarities and that most of the residual modulation for these periods took place in the outer heliosphere, near and beyond where the termination shock is expected to be. The modeling results show that the inclusion of a termination shock in the modulation model for solar minimum conditions causes abrupt changes in the radial gradients, gr, at the termination shock, at almost all energies of interest to modulation studies. During A > 0 solar magnetic field polarity cycles this “barrier” effect contributes ∼60% to the overall modulation at 0.5 GeV. However, for A < 0 cycles the contribution at this energy is markedly smaller. For increased solar activity the modulation in the heliosheath is quite different from minimum activity. The heliosheath does not any longer play the role of a distinguished “barrier,” although abrupt changes in gr at the shock may still occur, especially for A > 0 cycles, surprisingly more so for higher energies.
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