Modeling of electrons in the heliosphere
Nndanganeni, Rendani Rejoyce
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The propagation and modulation of electrons in the heliosphere play an important part in improving our understanding and assessment of the processes of solar modulation. A locally developed, full three-dimensional, numerical model is used to study the modulation of Jovian and galactic electrons from 1 MeV to 50 GeV, and from the Earth into the heliosheath. Modeling results are compared with Voyager 1 observations in the outer heliosphere, including the heliosheath, as well as observations at or near the Earth, in particular the 2009 spectrum from the PAMELA space mission and from Voyager 1 for 1977. First, new heliopause spectra for galactic electrons are established. The issue of the intensity levels and the spectral shape of the local interstellar spectrum at lower energies is specifically addressed. The heliopause is assumed at 122 AU in the model. The extra-ordinary large increase of galactic electrons in the 5 MeV to 50 MeV range in the heliosheath is investigated. The modeling confirms that the heliosheath acts as a most effective modulation barrier for these low energy electrons. This is followed by establishing new Jovian electron source functions, also addressing its level of intensity and its spectral shape. Modulated electron spectra from the inner to the outer heliosphere are computed, first separately for Jovian and galactic electrons and then for the combined modulation. After combining these results, the intensity levels of galactic and Jovian electrons at the Earth below 50 MeV could be estimated. The energy range could be established over which the Jovian electrons dominate over galactic electrons in the inner heliosphere, a process which depends strongly on what is assumed for the Jovian electron source function. Conclusions are made about diffusion and drift theory as applicable to electrons in the heliosphere. Ways of reducing particle drifts explicitly and implicitly are investigated with interesting results. Increasing the rigidity dependence of the drift coefficient at lower energies very effectively reduces the extent to which particle drifts play a role in the modulation process. Concerning diffusion, a general result is that the rigidity dependence of both the parallel and perpendicular diffusion coefficients needs to be constant below ~0.4 GV, but to increase above this rigidity to assure compatibility between the modeling and observations at the Earth, and especially in the outer heliosphere. A modification in the radial dependence of the diffusion coefficients in the inner heliosheath is required to compute modulation that is compatible with observations in this region.