Modulation of the galactic low-energy proton spectrum in the inner heliosphereстатья
Статья опубликована в высокорейтинговом журнале
Информация о цитировании статьи получена из
Web of Science,
Scopus
Статья опубликована в журнале из списка Web of Science и/или Scopus
Дата последнего поиска статьи во внешних источниках: 18 июля 2013 г.
Аннотация:We study the energy spectra of 0.3–100 MeV protons and find that, at the lower part of the galactic particle
spectrum, they are significantly steeper than the J(E) ∼ E spectrum predicted by analytical approximations, such
as the force-field model of modulation. We select a series of low-flux periods, and approximate the spectral form
by J(E) = AE−γ + CEν , where the two terms describe solar/heliospheric and galactic components, respectively.
By determining the best fit parameters to energy spectra, correlations are sought with solar activity indices and
between the parameters themselves. In the majority of cases, ν turns out to be between 1.2 and 1.4, with an average
of 1.32 ± 0.12, significantly greater than the commonly expected ν = 1 predicted by the force-field approximation.
In modulation theories ν > 1 corresponds to a negative Compton–Getting factor, which poses a challenge. Such an
inversion may occur if the radial diffusion coefficient κrr < rV (where r is heliocentric distance and V solar wind
speed), in which case a large fraction of the 10–100 MeV protons reaching 1 AU would have been cooled down
within 1 AU and subsequently convected outward by the solar wind. We also find that the position of the intensity
minimum of the proton spectrum dividing the solar and galactic populations shifts toward higher values with
increasing solar activity. Correlations obtained with solar activity indicate that the slope of the solar/heliospheric
spectrum is practically independent of solar activity. Observations are compared with numerical solutions of the modulation equation adopting simple spherical models. Possible interpretations are discussed.