First-time measurement of ACR O gradients during an A<0 period
in the inner heliosphere
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Anomalous cosmic rays (ACRs) are a sample of the interstellar medium, having
originated as interstellar neutral gas, entered the heliosphere, become
ionized and picked up by the solar wind, and carried to the outer heliosphere
where they are accelerated to MeV energies. The site and mechanism of the
acceleration is still to be determined. The particles reach the inner
heliosphere, here defined to be <5 AU, by a combination of diffusing and
drifting in the large-scale interplanetary magnetic field. The drift motions
reverse with the change in polarity of the Sun's magnetic dipole, which
reverses approximately every 11 years. In the A>0 part of the 22-year solar
magnetic cycle, when the field northward of the heliospheric current sheet
(HCS) is directed outward, positively-charged particles originating in the
outer heliosphere tend to drift towards the heliographic equator from the
polar regions (see Jokipii et al., ApJ, 213, 861, 1977). Many measurements
during A>0 periods have confirmed that the latitudinal gradients of ACRs are
positive in both the inner and outer heliosphere during such times. In the A<0
part of the cycle, which began in ~2001, the drift patterns are reversed and
positive particles drift inward along the HCS. In this case, the particles
should exhibit a negative latitudinal gradient, which has been observed in the
outer heliosphere but has not yet been established in the inner heliosphere
due to a lack of spacecraft positioned in the appropriate locations.
In a recent publication [Cummings et al., GRL, 36, L18103,
doi:10.1029/2009GL039851, 2009], this issue was addressed using data from the
Ulysses, Advanced Composition Explorer (ACE), and Solar Terrestrial Relations
Observatory (STEREO) spacecraft to determine the radial and latitudinal
gradients of ACR oxygen in the inner heliosphere for the first time during an
A<0 part of the solar cycle. Panel a) of the figure shows 27-day averaged
intensities of 7.3-15.6 MeV/nuc O from Ulysses (open circle) and ACE (solid
circle) vs. time. The units are (cm2 s sr MeV/nuc)-1. The radial position of
Ulysses (solid line) and ACE (dotted line) vs. time are shown in Panel b).
Panel c) shows the heliographic latitude of Ulysses (solid line) and ACE
(dotted line) vs. time. Finally, Panel d) shows the ratio of intensities (open
circle) vs. time. The solid line connects best-fit points (X) to the equation
ln(IU/IA) =
grΔr
+ gΘΔΘ + C
where gr is the radial gradient,
Δr is the radial separation,
gΘ is the latitudinal gradient,
ΔΘ is
|ΘU| - |ΘA|,
and C is a
normalization constant. A similar fit was done using STEREO and Ulysses O with
4.5-15 MeV/nuc. The combined results from the two energy intervals give
gr = 48±13 %/AU,
consistent with previous inferences for A<0 and larger than that
determined in the inner heliosphere for A>0. The latitude gradient,
gΘ, is
consistent with zero and in the range -0.3 to +0.4 %/degree. During the time
period of the fits, the tilt of the HCS was in the range 30-35 degrees. This
low value of the latitudinal gradient suggests that during A<0 these particles
are not able to propagate into the inner heliosphere by rapid drift along the
heliospheric current sheet if the tilt of the sheet is as large as 30 degrees.
This is perhaps because the pathlength along the sheet is large enough that
diffusion is the dominant transport mechanism. In the outer heliosphere during
A<0, Cummings et al. [21st ICRC, 6, 190, 1990] found that the gradients were
strong functions of the tilt angle when the tilt dropped below ~25 degrees.
Recently, the tilt has decreased to near 10 degrees and we expect that the
intensity of ACR O in the inner heliosphere will continue to grow and that
negative latitudinal gradients are likely to be established [see, e.g.,
Jokipii, Adv. Space Res., 9, 105, doi:10.1016/0273-1177(89)90317-7, 1989].
This item was contributed by
Alan Cummings of Caltech.
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Last modified 29 Dec 2009.