Difference in the heliospheric modulation of cosmic-ray protons and electrons during the solar minimum period of 2006 to 2009
Abstract
Context.
The PAMELA experiment observed galactic proton and electron spectra down to 70 MV and 400 MV, respectively, from
mid-2006 to the end of 2009 during what is called an
A
<
0 solar magnetic polarity cycle. During this period, solar activity was
at its lowest level since the beginning of the space exploration era. This provides the opportunity to study charge-sign-dependent
modulation under very quiet heliospheric conditions.
Aims.
Drift theory for the solar modulation of cosmic rays predicts that the intensity of protons at the Earth is expected to show
a di
ff
erent rate of recovery towards solar minimum when compared to electrons during
A
<
0 cycles. These charge-sign related
di
ff
erences are investigated.
Methods.
The solutions of a comprehensive three-dimensional drift model are compared to PAMELA spectra to authenticate the
modelling approach and then to make predictions of how electrons and protons are di
ff
erently modulated down to 1 MeV, based on
new very local interstellar spectra.
Results.
The comparison of observations and modelling provides insight into how the rigidity dependence of the three major di
ff
usion
coe
ffi
cients changes during such quiet modulation conditions. How drift e
ff
ects dissipate above several GeV and below 100 MeV is
illustrated for both protons and electrons. The modulation that occurred at the Earth during this quiet period is shown as a function of
rigidity and time. The e
−
/
p ratio is computed from 10 MV to 50 GV for this period and a prediction is made for what may be observed
in terms of spectra during the next
A
>
0 solar minimum.
Conclusions.
The presence of drifts during this quiet period is established based on the presented modelling and PAMELA mea-
surements. Drift e
ff
ects for protons and electrons are quantified in terms of their rigidity and temporal development from 2006 to
2009