The Voyager 1 spacecraft is now about 25 AU beyond the heliospheric termination shock and
soon it should encounter the outer boundary of the heliosphere, the heliopause. This is set to
be at 120 AU in the modulation model used for this study. This implies that Voyager 1, and
soon afterwards also Voyager 2, should be able to measure the heliopause spectrum, to be
interpreted as the lowest possible local interstellar spectrum, for low energy galactic electrons
(1 MeV to 120 MeV). This could give an answer to a long outstanding question about the
spectral shape (energy dependence) of the galactic electron spectrum at these low energies.
These in situ electron observations from Voyager 1, until the year 2010 when it was already
beyond 112 AU, are used for a comparative study with a comprehensive three dimensional
numerical model for the solar modulation of galactic electrons from the inner to the outer
heliosphere.
A locally developed steady state modulation model which numerically solves the relevant
heliospheric transport equation is used to compute and study modulated electron spectra from
Earth up to the heliopause. The issue of the spectral shape of the local interstellar spectrum at
these low energies is specifically addressed, taking into account modulation in the inner
heliosheath, up to the heliopause, including the effects of the transition of the solar wind
speed from supersonic to subsonic in the heliosheath. Modulated electron spectra from the
inner to the outer heliosphere are computed, together with radial and latitudinal profiles,
focusing on 12 MeV electrons. This is compared to Voyager 1 observations for the energy
range 6–14 MeV. A heliopause electron spectrum is computed and presented as a new
plausible local interstellar spectrum from 30 GeV down to 10 MeV.
The comparisons between model predictions and observations from Voyager 1 and at Earth
(e.g. from the PAMELA mission and from balloon flights) and in the inner heliosphere (e.g.
from the Ulysses mission) are made. This enables one to make conclusions about diffusion
theory applicable to electrons in the heliosphere, in particular the rigidity dependence of
diffusion perpendicular and parallel to the local background solar magnetic field. A general
result is that the rigidity dependence of both parallel and perpendicular diffusion coefficients
needs to be constant below P < 0.4 GV and only be allowed to increase above this rigidity to
assure compatibility between the modeling and observations at 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 realistic modulation in this region. With this study,
estimates of the intensity of low energy galactic electrons at Earth can be made. A new local
interstellar spectrum is computed for these low energies to improve understanding of the
modulation galactic electrons as compared to previous results described in the literature. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2012.
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/7034 |
| Date | January 2012 |
| Creators | Nndanganeni, Rendani Rejoyce |
| Publisher | North-West University |
| Source Sets | North-West University |
| Detected Language | English |
| Type | Thesis |
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