arXiv subjects

M. S. Potgieter

Publications and source records attributed to M. S. Potgieter.

32 records · Page 2Linked to original sources

arXiv2015

Time dependence of the e^- flux measured by PAMELA during the July 2006 - December 2009 solar minimum

Precision measurements of the electron component in the cosmic radiation provide important information about the origin and propagation of cosmic rays in the Galaxy not accessible from the study of the cosmic-ray nuclear components due to their differing diffusion and energy-loss processes. However, when measured near Earth, the effects of propagation and modulation of galactic cosmic rays in the heliosphere, particularly significant for energies up to at least 30 GeV, must be properly taken into account. In this paper the electron (e^-) spectra measured by PAMELA down to 70 MeV from July 2006 to December 2009 over six-months time intervals are presented. Fluxes are compared with a state-of-the-art three-dimensional model of solar modulation that reproduces the observations remarkably well.

astro-ph.SR↗

arXiv2015

On the causes and mechanisms of the long-term variations in the GCR characteristics

We argue that the degree of understanding the causes and mechanisms of the long-term variations (11-year and 22-year) in the galactic cosmic rays (GCR) characteristics is still insufficient and to improve it we need new approaches and methods. For the time being there is a long-lasting controversy on how these long-term variations, observed for more than 50 years in the inner heliosphere, are formed. It is widely believed that the 11-year variation is due entirely to the toroidal branch of solar activity (the area and number of sunspots, the strength of the heliospheric magnetic field etc) because of the diffusion, convection and adiabatic energy loss, while the much smaller 22-year variation is caused by the particle drifts connected with the poloidal branch of solar activity (the high-latitude solar magnetic fields). At the same time, both past and more recent numerical simulations indicate that the contribution of particle drifts could be significant for both 22- and 11-year variations in the GCR intensities. However, even those who agree on the significant influence of drifts appear to have different perceptions on the mechanisms of this influence. In this paper, we present an analysis of the possible causes of the first point of view (small role of drifts in the 11-year GCR variation) and the reasons why one can expect the significant contri- bution of the processes connected with the poloidal branch of solar activity in both types of the long-term variations of the GCR characteristics. Then we briefly discuss some numerical methods suggested in the past and recently and the approaches and perspectives for the sought-for methods are considered.

astro-ph.SR↗

arXiv2015

The effect of magnetic field modifications on the modulation of cosmic rays in the heliosphere

A numerical model for the solar modulation of cosmic rays, based on the solution of a set of stochastic differential equations, is used to illustrate the effects of modifying the heliospheric magnetic field, particularly in the polar regions of the heliosphere. To this end, the dfferences in the modulation brought about by each of three choices for the heliospheric magnetic field, i.e. the unmodified Parker field, the Smith-Bieber modified field, and the Jokipii-Kota modified field, are studied. It is illustrated that both the Jokipii-Kota and Smith-Bieber modifications are effective in modifying the Parker field in the polar regions. In addition, it is argued that the modification of Smith and Bieber is based on observational evidence and has a firm physical basis, while these motivations are lacking in the case of the Jokipii-Kota modification. From a cosmic ray modulation point of view, we found the Smith-Bieber modification to be the most suitable choice for modifying the heliospheric magnetic field. The features and effects of these three modifications are illustrated both qualitatively and quantitatively. It is also shown how the Smith-Bieber modified field can be applied in cosmic ray modulation models to reproduce observational cosmic ray proton spectra from the PAMELA mission during the solar minimum of 2006 - 2009. These results are compared with those obtained in previous studies of this unusual solar minimum activity period and found to be in good qualitative agreement.

physics.space-ph↗

arXiv2013

The Discrepancy in Modelling of Galactic Cosmic Ray Primaries and Secondaries

Given that the bulk of the galactic cosmic rays (CRs) originates from transient point-like sources, such as supernova remnants, the flux of the CR primary component measured at Earth depends on the local source history. Whereas the secondary omponent shows little to no variation due to nearby sources, and instead, should depend on the global distribution of CRs. Steady-state, rotational symmetric models (2D) of CR propagation assume smeared-out CR sources in the Galaxy and cannot properly take into account the influence of nearby point sources. In this paper, we infer evidence of nearby sources by illustrating that a 2D propagation model will not describe CR primaries and secondaries equally well. We adapted the 2D version of the GALPROP code to a compute-cluster environment and perform parameter studies comparing CR spectra with primary and secondary CR data separately. Doing a parameter study, one may expect different best fit values looking at the primary and secondary CR components separately, as it is unlikely that the source history mimicked by the 2D models coincides with the real local source history. We find that the primaries and secondaries are fitted with differing best fit parameters, with the results indicating that the 2D model being more suited to modelling secondary CRs. The insufficient modelling of primary CRs can be contributed to the possible presence of local CR sources.

astro-ph.HE↗

arXiv2013

A Heliopause Spectrum for Electrons

We present a heliopause spectrum for galactic electrons over an energy range from 1 MeV to 50 GeV at 122 AU from the Sun. This spectrum can be considered the lowest possible local interstellar spectrum (LIS). This is obtained by using a comprehensive numerical model for solar modulation in comparison with Voyager 1 observations during 2010 and PAMELA data at Earth. Below about 1 GeV, this LIS exhibits a power law with E^(-(1.5 plus minus 0.1), where E is the kinetic energy of these electrons. Reproducing the PAMELA electron spectrum for 2009 requires a LIS with a different power law above about 5 GeV of the form E^(-3.15 plus minus 0.05). The heliopause spectrum thus consists of two power laws with a break between about 800 MeV and about 2 GeV.

astro-ph.SR↗

arXiv2013

At what Rigidity does the Solar Modulation of Galactic Cosmic Rays begin?

Observationally, it is difficult to establish at what rigidity the modulation of galactic cosmic rays (CRs) actually begins in the heliosphere. It should be possible to do if the relevant local interstellar CR spectra were known and reliable measurements were made between 10 GV and about 200 GV, inside and outside the heliosphere. Numerical models for solar modulation studies are based on simply assuming that CR modulation begins at a given spatial boundary and at rigidities between 30 to 50 GV, usually handled as an initial condition. The Stochastic Differential Equations approach to numerical modelling presents the opportunity to determine the level of modulation from high to low rigidities because an initial condition at a prescribed high rigidity is not required. We present the results of such an approach and show the percentage modulation of CR protons as a function of kinetic energy between 100 MeV and 250 GeV.

astro-ph.SR↗

arXiv2013

The Highest Recorded Proton Spectrum at Earth since the Beginning of the Space Age

The recent solar minimum activity period and the consequent minimum modulation conditions for cosmic rays were unusual compared to previous solar minimum periods. The highest spectra of galactic protons (and other cosmic rays) were recorded by the PAMELA instrument at Earth in late 2009, in contrast to expectations. The spectrum, between 100 MeV and 50 GeV, for December 2009 is compared to proton spectra observed during previous solar minimum periods, back to 1965. Corresponding numerical modeling is presented which predicts that the next solar minimum spectra could even be higher if similar modulation conditions then would occur as in 2008-2009. The reason is that incorporating gradient and curvature drifts in modulation models causes proton spectra for A > 0 solar magnetic cycles (e.g., around 1976, 1997) to always be higher than during A < 0 cycles (e.g. around 1965, 1987, 2009) at energies below a few GeV, if the same modulation conditions would prevail.

astro-ph.SR↗

arXiv2013

A Very Local Interstellar Spectrum for Galactic Electrons

We present a new local interstellar spectrum for cosmic ray electrons over an energy range from 1 MeV to 70 GeV. Below (0.8 plus minus 0.2) GeV it has a power law form, E^-(1.55 plus minus 0.05), with E kinetic energy, which is consistent with previous studies. This is derived from comparing Voyager 1 electron data observed during 2010 with a comprehensive modulation model. However, reproducing the PAMELA electron spectrum observed at Earth during late 2006 and to address an unexpected increase in the electron spectrum between about 2 GeV and up to about 20 GeV, a spectral index of -(3.3 plus minus 0.1) instead of the reported -(3.18 plus minus 0.05) was found for this energy range. This feature cannot be caused by solar modulation or any other process inside the heliosphere.

astro-ph.SR↗

arXiv2013

A local interstellar spectrum for galactic electrons

A heliopause spectrum at 122 AU from the Sun is presented for galactic electrons over an energy range from 1 MeV to 50 GeV that can be considered the lowest possible local interstellar spectrum (LIS). The focus is on the spectral shape of the LIS below 1.0 GeV. The study is done by using a comprehensive numerical model for solar modulation in comparison with Voyager 1 observations at 110 AU from the Sun and PAMELA data at Earth. Below 1.0 GeV, this LIS exhibits a power law,E to the power -(1.55+-0.05), where E is the kinetic energy. However, reproducing the PAMELA electron spectrum averaged for 2009, requires a LIS with a different power law of the form E to the power -(3.15+-0.05) above about 5 GeV. Combining the two power laws with a smooth transition from low to high energies yields a LIS over the full energy range that is relevant and applicable to the modulation of cosmic ray electrons in the heliosphere. The break occurs between 800 MeV and 2 GeV as a characteristic feature of this LIS.

astro-ph.HE↗

arXiv2013

Modulation of galactic protons in the heliosphere during the unusual solar minimum of 2006 to 2009

The last solar minimum activity period, and the consequent minimum modulation conditions for cosmic rays, was unusual. The highest levels of galactic protons were recorded at Earth in late 2009 in contrast to expectations. Proton spectra observed for 2006 to 2009 from the PAMELA cosmic ray detector on-board the Resurs-DK1 satellite are presented together with the solutions of a comprehensive numerical model for the solar modulation of cosmic rays. The model is used to determine what mechanisms were mainly responsible for the modulation of protons during this period, and why the observed spectrum for 2009 was the highest ever recorded. From mid-2006 until December 2009 we find that the spectra became significantly softer because increasingly more low energy protons had reached Earth. To simulate this effect, the rigidity dependence of the diffusion coefficients had to decrease significantly below ~3 GeV. The modulation minimum period of 2009 can thus be described as relatively more "diffusion dominated" than previous solar minima. However, we illustrate that drifts still had played a significant role but that the observable modulation effects were not as well correlated with the waviness of the heliospheric current sheet as before. Protons still experienced global gradient and curvature drifts as the heliospheric magnetic field had decreased significantly until the end of 2009, in contrast to the moderate decreases observed during previous minimum periods. We conclude that all modulation processes contributed to the observed increases in the proton spectra for this period, exhibiting an intriguing interplay of these major mechanisms.

astro-ph.SR↗

arXiv2013

Time dependence of the proton flux measured by PAMELA during the July 2006 - December 2009 solar minimum

The energy spectra of galactic cosmic rays carry fundamental information regarding their origin and propagation. These spectra, when measured near Earth, are significantly affected by the solar magnetic field. A comprehensive description of the cosmic radiation must therefore include the transport and modulation of cosmic rays inside the heliosphere. During the end of the last decade the Sun underwent a peculiarly long quiet phase well suited to study modulation processes. In this paper we present proton spectra measured from July 2006 to December 2009 by PAMELA. The large collected statistics of protons allowed the time variation to be followed on a nearly monthly basis down to 400 MV. Data are compared with a state-of-the-art three-dimensional model of solar modulation.

astro-ph.HE↗

arXiv2008

A Cosmic Ray Positron Anisotropy due to Two Middle-Aged, Nearby Pulsars?

Geminga and B0656+14 are the closest pulsars with characteristic ages in the ran ge of 100 kyr to 1 Myr. They both have spindown powers of the order 3e34 erg/s at present. The winds of these pulsars had most probably powered pulsar wind nebulae (PWNe) that broke up less than about 100 kyr after the birth of the pulsars. Assuming that leptonic particles accelerated by the pulsars were confined in th e PWNe and were released into the interstellar medium (ISM) on breakup of the PW Ne, we show that, depending on the pulsar parameters, both pulsars make a non-ne gligible contribution to the local cosmic ray (CR) positron spectrum, and they m ay be the main contributors above several GeV. The relatively small angular dist ance between Geminga and B0656+14 thus implies an anisotropy in the local CR po sitron flux at these energies. We calculate the contribution of these pulsars to the locally observed CR electr on and positron spectra depending on the pulsar birth period and the magnitude o f the local CR diffusion coefficient. We further give an estimate of the expecte d anisotropy in the local CR positron flux. Our calculations show that within the framework of our model, the local CR posit ron spectrum imposes constraints on pulsar parameters for Geminga and B0656+14, notably the pulsar period at birth, and also the local interstellar diffusion co efficient for CR leptons.

astro-ph↗

arXiv2001

Secondary antiprotons and propagation of cosmic rays in the Galaxy and heliosphere

High-energy collisions of cosmic-ray nuclei with interstellar gas are believed to be the mechanism producing the majority of cosmic ray antiprotons. Due to the kinematics of the process they are created with a nonzero momentum; the characteristic spectral shape with a maximum at ~2 GeV and a sharp decrease towards lower energies makes antiprotons a unique probe of models for particle propagation in the Galaxy and modulation in the heliosphere. On the other hand, accurate calculation of the secondary antiproton flux provides a ``background'' for searches for exotic signals from the annihilation of supersymmetric particles and primordial black hole evaporation. Recently new data with large statistics on both low and high energy antiproton fluxes have become available which allow such tests to be performed. We use our propagation code GALPROP to calculate interstellar cosmic-ray propagation for a variety of models. We show that there is no simple model capable of accurately describing the whole variety of data: boron/carbon and sub-iron/iron ratios, spectra of protons, helium, antiprotons, positrons, electrons, and diffuse gamma rays. We find that only a model with a break in the diffusion coefficient plus convection can reproduce measurements of cosmic-ray species, and the reproduction of primaries (p, He) can be further improved by introducing a break in the primary injection spectra. For our best-fit model we make predictions of proton and antiproton fluxes near the Earth for different modulation levels and magnetic polarity using a steady-state drift model of propagation in the heliosphere.

astro-ph↗

arXiv2001

Secondary Antiprotons in Cosmic Rays

High energy collisions of cosmic ray (CR) nuclei with interstellar gas are believed to be the mechanism producing the majority of CR antiprotons. The distinguishing spectral shape with a maximum at 2 GeV and a sharp decrease towards lower energies makes antiprotons a unique probe of the models of particle propagation in the Galaxy and modulation in the heliosphere. Besides, accurate calculation of the secondary antiproton flux provides a ``background'' for searches for exotic signals from the annihilation of supersymmetric particles and primordial black hole evaporation. Recently new data with large statistics on the antiproton flux have become available which allow for such tests to be performed. We use our 3D Galactic cosmic ray propagation code GALPROP to calculate interstellar propagation in several models. For our best model we make predictions of proton and antiproton fluxes near the Earth for different modulation levels and polarity using a steady-state drift model for heliospheric modulation.

astro-ph↗