The role of radial perpendicular diffusion and latitude dependent acceleration along the solar wind termination shock
Unit for Space Physics and School of Physics, North-West University, 2520 Potchefstroom, South AfricaAdvances in Space Research (Impact Factor: 1.36). 01/2008; 41(2):368-372. DOI: 10.1016/j.asr.2006.11.025
A numerical model, based on Parker’s transport equation, describing the modulation of anomalous cosmic rays and containing diffusive shock acceleration is applied. The role of radial perpendicular diffusion at the solar wind termination shock, and as the dominant diffusion coefficient in the outer heliosphere, is studied, in particular the role it plays in the effectiveness of the acceleration of anomalous protons and helium when its latitude dependence is changed. It is found that the latitudinal enhancement of radial perpendicular diffusion towards the heliospheric poles and along the termination shock has a prominent effect on the acceleration of these particles. It results in a ‘break’ in the energy spectrum for anomalous protons at ∼6.0 MeV, causing the spectral index to change from E−1.38 to E−2.23, but for anomalous helium at ∼3.0 MeV, changing the spectral index from E−1.38 to E−2.30. When approaching the simulated TS, the changes in the modulated spectra as they unfold to a ‘steady’ power law shape at energies below 50 MeV are much less prominent as a function of radial distances when radial perpendicular diffusion is increased with heliolatitude.
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ABSTRACT: 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 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.Journal of Geophysical Research Atmospheres 12/2010; 115(A12). DOI:10.1029/2010JA015690 · 3.43 Impact Factor
Article: Long period oscillations in sunspots[Show abstract] [Hide abstract]
ABSTRACT: Long period oscillations of the gyroresonant emission from sunspot atmospheres are studied. Time series data generated from the sequences of images obtained by the Nobeyama Radioheliograph operating at a frequency of 17 GHz for three sunspots have been analysed and are found to contain significant periods in the range of several tens of minutes.Wavelet analysis shows that these periods are persistent throughout the observation periods. The presence of the oscillations is confirmed by several methods (periodogram, wavelets, Fisher randomisation and empirical mode decomposition). Spatial analysis using the techniques of period, power, correlation and time lag mapping reveals regions of enhanced oscillatory power in the umbral regions. Also seen are two regions of coherent oscillation of about 25 pixels in size, that oscillate in anti-phase with each other. Possible interpretation of the observed periodicities is discussed, in terms of the shallow sunspot model and the leakage of the solar g-modes.Astronomy and Astrophysics 04/2010; 513(4). DOI:10.1051/0004-6361/200913683 · 4.38 Impact Factor
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ABSTRACT: Context. Since the crossings of the solar wind termination shock by the Voyager 1 and 2 spacecraft, much speculation has surrounded the acceleration mechanism and region where the anomalous cosmic ray component is accelerated. A peculiar, and mostly overlooked feature of the observed anomalous oxygen spectrum near the termination shock, is the power law form of the roll-over (cut-off) at the high energy range of this spectrum. Aims: We investigate, using a numerical model, why this deviation from the expected exponential form of the cut-off part of the anomalous oxygen spectrum occurs, and if the observed power law form can be explained in terms of the acceleration of multiple charged anomalous oxygen. Methods: Multiple charged anomalous cosmic rays are incorporated in a numerical model, based on the standard Parker transport equation, including acceleration at the solar wind termination shock. This is done by specifying an energy dependent charge state, constrained by observations. Results: Comparing computational results with spacecraft observations, it is found that the inclusion of multiply charged anomalous cosmic rays in the modulation model can explain the observed spectrum of anomalous oxygen in the energy range from 10-70 MeV per nucleon. The more effective acceleration of these multiple charge anomalous particles at the solar wind termination shock causes a significant deviation from the usual exponential cut-off spectrum to display instead a power law decrease up to 70 MeV per nucleon where galactic oxygen starts to dominate. In addition, the model reproduces the features of multiple charged oxygen at Earth so that a good comparison is obtained between computations and observations.Astronomy and Astrophysics 04/2010; 513(4). DOI:10.1051/0004-6361/200913866 · 4.38 Impact Factor
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