The so-called anomalous cosmic ray component, which occurs at energies of about 10 MeV/nucleon and consists only of He, N, O, and Ne, has been a subject of interest for more than a decade. The origin of this component is generally considered to be interstellar neutral gas that is ionized and accelerated in the solar wind. The mechanism and the location for the acceleration, however, remains an unsolved problem. A model is used which includes the effects of gradient and curvature drifts and considers the implications of observed spatial gradients of the anomalous component for the location of the acceleration region. It is concluded that if drifts are important the acceleration region cannot lie at the solar poles. It is also concluded that there is no single region for the acceleration which can account for both the observed intensities and gradients in models which include drift effects

Fisk, L. A.

Lee, M. A.

Potgieter, M. S.

English

NASA Technical Reports Server

IBO
SH 4.6-6
ON THE ANOMALOUS COMPONENT
M. S. Potgieter, L. A. Fisk and M. A. Lee
Space Science Center, University of New Hampshire
Durham, NH 03824, USA
1. Introduction. The so-called anomalous cosmic ray component, which
occurs at energies of about 10 MeV/nucleon and consists only of He, N, O,
and Ne, has been a subject of interest for more than a decade. The ori-
gin of this component is generally considered to be interstellar neutral
gas that is ionized and accelerated in the solar wind, as was proposed by
Fisk et al. (1974). The mechanism and the location for the acceleration,
however, remains an unsolved problem.
In this paper, we use a model which includes the effects of gradient
and curvature drifts and consider the implicationsof observed spatial
gradients of the anomalous component for the location of the acceleration
region. We conclude that if drifts are important the acceleration region
cannot lie at the solar poles. We also conclude that there is no single
region for the acceleration which can account for both the observed
intensities and gradients in models which include drift effects.
2. Results and Discussion. Pesses et al. (1981) proposed a mechanism
for the acceleration of the anomalous component based on the acceleration
of particles at the solar wind termination shock over the poles. They
argue that once the particles are accelerated, gradient and curvature
drifts will bring the particles downward from the heliospheric polar
regions onto the equatorial plane. After a polarity reversal of the
solar magnetic field, the oppositely directed drifts will restrict the
passage of particles onto the equatorial plane. They therefore predict a
dramatic change in the intensity of the anomalous component from cycle to
cycle.
A complicatingfactor for drift models, however, is the small
radial gradients these models predict for periods when the interplanetary
magnetic field (IMF) is directed outwards in the northern heliosphere, as
in 1970-80 (Potgieter, 1985). During this period the downward drifts of
particles from the polar regions onto the equatorial plane tend to make
the radial gradients in the equatorial plane small. The gradients also
prove to be rather insensitive to changes in modulation parameters, for
instance, the parallel diffusion coefficient. The question therefore
arises whether a polar source for the anomalous component and strong
drift effects can be compatible with the observed radial gradients of
10%-15%/AU for the anomalous component (Webber et al., 1981, 1985).
In this preliminary study we assumed a source spectrum for the
anomalous oxygen which yields approximatelythe observed 1976-77 energy
spectrum at Earth using a drift model for the modulation of cosmic rays
(Potgieter and Moraal, 1985). To obtain a source which can be placed at
various O-(polar angle) values at a radial distance of 50 AU, we used
Gaussian spectra of the form
JT = JT exp {-0.028(0-@ )2} ,O
with a half-width of I0° about 0o and JT the assumed O-independent anoma-
lous oxygen spectrum. We then solved the cosmic-ray Lransport equation,
https://ntrs.nasa.gov/search.jsp?R=19850026731 2020-03-20T16:51:39+00:00Z
181
SH 4.6-6
' ' ' ' ' ' ' ' _ ' ' ' I ' I ' I ' I '
20 I0 MeV Anomalous Oxygen f" ....././ 12 \\\ I0 MeV Anomalous Oxygen
.I ?C,.
16 90°'t" t" 8 I ,, " .." ..
•_' ..... I_ \ "- "-_ 90°
< 12 S" ...........8'0%.......-. .,, 4 '_,\",,.""- .,-8()°---. ...
o_ 8 v,,. ""-6.0o " ,, 0 _ "" " "60_ " " ""
. ,.......... 40o '"-. ._ -. "-- ... ..
2 4 '\ "'- " 0 --. - \ \
"o 0 \ "_.._, 20 ° x. \ -81 "-,, ,, "\
£ .... _ \ \. \q, ,
(.9 ........ 0"°' _ " " "\ _ -12 \\ '
:15 '_% \ D(-) :~(1959-1970) \\
o -8 '\\. \ -16 ~(1980-1991) ?
n.," D (+) : ~ ( 1970-1980 ) \\ '_ ,,',
-12 '. I -20
L I I : I I I q I I I J I
o Ib ' io' 4o' o ,o 2o 40 5o
RadialDistance(AU) RadialDistance(AU)
(a) (b)
Fig. 1: Calculated radial gradients in the equatorial plane as
a function of radial distance with a source at 50 AU and various
polar angles; (a) for a D(+) cycle with drifts downward onto the
equatorial plane at 0 = 90°, and (b) for a D(-) cycle that is
with the polarity of the IMF reversed.
assuming a steady-state and azimuthal symmetry, shifted the source in
10° intervals from the pole to the equator atO=90 °, and calculated the
radial gradients in the equatorial plane for each location of the source.
• We repeated the calculationswith the IMF polarity reversed, but other-
wise unchanged modulation parameters. The diffusion coefficients used
correspond to solar-minimum conditions (Potgieter and Moraal, 1985).
To distinguish between the two configurationsof the IMF, we use
the notation D(+) for _1970-80 and D(-) for the 11 years before and after
this period. Starting with a D(+) period, that is with drifts directed
downward onto the equatorial plane, we show in Fig. la the radial gra-
dients in the equatorial plane for 10 MeV anomalous oxygen as a function
of radial distance with the source at 50 AU and various polar angles.
, The striking feature of this figure is the dramatic decrease of the
radial gradient with increasing radial distance, from %10%/AU at Earth to
a negative gradient,_withthe source placed at 50 AU over the pole. With
the source at the equatorial plane, the radial gradient steadily
- increases from 12%/AU at Earth to about 22%/AU at 50 AU. Fig. lb shows
the situation with the polarity of the IMF reversed, that is with drifts
directed from the equatorial plane toward the polar regions. The radial
gradients, with the source at the pole, again rapidly decrease with
increasing radial distance. With the source at the equator, the radial
gradients compared to Fig. la are smaller for most of the heliosphere out
to 50 AU.
182 SH 4.6-6
The results of Fig. 1 indicate the following. The radial gradi-
ents in the equatorial plane, with a source at the pole, are inconsistent
with the observed radial gradients for the anomalous component, indepen-
dent of the polarity of the solar magnetic field. By moving the source
towards the equatorial regions, the calculated radial gradients become
comparable to those observed. The calculated values, however; are sig-
nificantly smaller, except in the inner heliosphere, for the present
cycle as compared to the previous cycle, whereas the observed values seem
to remain constant before and after the polarity change in 1980 (Webber
et at., 1985).
24 i i i i i i i i i
'10 lVieV Anomalous Oxygen , , , --
20 Source at Equator _-...---' ..... 1.0 "'-.. I0 Me'_ Anomalous Oxygen
A ._.. %.
.t \ D('_) : ~ ( 1970 -1980)
.:_ . .i ½W:lO ° D(+) _ 0.8 ',,
16 ........... _ \ D(-)_ ~(1959-1970)
Q" > _""_ ..... /"" m '\ ~ ( 1980-1991 )
o_ 12 _ ½W:50 o ./' _ 0.6 \
"'_<......................... " '\ D(+) D(-) //_-
.. _ ',, /
(_ _,_ :N 0,4 '\. /
8 "_..__ _-W: I0 ° No-drift '\ //
(.9 _ ":-............... ;_ 02 ................. ,.
"_ 4 D(+) : ~(1970_1980)D(.):( 1959 - 1970 ) ½W: 5-0_" - -- "D("--') "_'"--'-='--- '_'_ ...............
- (1980-1991) 0 ................
_" o 6 ' i_ ' _'o ' ._o ' _o ' ;o o ia 2'0 _5 4 _o 6'o _'o 8'o 9o
Radial Distance (AU) Position of Source (in degrees) at 50 AU
Fig. 2: The effect of increasing Fig. 3: The normalized intensity
the half-width (W) of the source at Earth of 10 MeV anomalous oxygen
spectrum, from 10° to 50°, on the as a function of various polar
radial gradients in the equatorial angle positions of the source at
plane with a source at the equator 50 AU for periods before and after
at 50 AU. a polarity reversal.
We also investigatedthe effect of increasing the half-width of
the source spectra from 10°, in steps of 10°, to 50° to simulate an
extended, less localized source. For both the D(+) and D(-) cycle of the
magnetic field the radial gradients in the equatorial plane change insig-
nificantly with the extended source over the polar regions. The effect
of an extended source placed at the equator is shown in Fig. 2. For the
D(+) period the radial gradients now seem compatible with those observed.
The intensity for the anomalous oxygen at Earth with a source at
various latitudes should also be taken into consideration. The intensity
of the 10 MeV anomalous oxygen at Earth, normalized to the intensity
during a D(+) period with a source at the pole, is given in Fig. 3 as a
function of the position of the source at 50 AU at various polar angles.
For the D(+) period (downward drifts) the intensity at Earth decreases
rapidly as the source is shifted from the pole to the equator. Comparing
the intensity for the two periods, with the source at the pole, the D(-)
intensity is 3ubstantially lower (a factor of 200) as was suggested by
Pesses et at. (1981). For the D(-) cycle the intensity increases as the
source is shifted toward the equator. It is interesting to compare this
behavior, dependent on the polarity of the IMF, to the no-drift case
shown in Fig. 3.
183
SH 4.6-6
Our calculations show that, although the intensity of the anoma-
lous oxygen, with a source at the pole, dramatically decreases after a
change in the IMF polarity, the radial gradients in the equatorial plane
are inconsistent to those observed. Furthermore, although the radial
gradients for a D(-) cycle (the present cycle) with a source at 50 AU at
the equator seem comparable to observations, the calculated intensity
during this cycle exceeds the intensity during a D(+) cycle (the previous
cycle), which is again contrary to what is observed.
3. Concluding Remarks. It should be noted that no attempt has been made
in this preliminary study to include acceleration at the termination
shock in a self-consistent manner. As has been pointed out recently by
Jokipii (1985), drifts along the shock front and in the IMF can affect
the spectrum and the spatial distribution of particles at the termination
shock. When drifts are downward from the poles, they are poleward along
the shock front. Both effects tend to limit the shock acceleration of
particles that are injected near the poles. Conversely, when the drifts
are towards the poles in the IMF, as they are in the current cycle, the
drifts are downward from the poles along the shock front. These effects
will increase the number of times particles interact with the shock,
thereby increasing the energy they can gain, and will spread particles
injected over the poles towards the equatorial region.
The complication of an accelerated spectrum which evolves with
changing field polarity does not affect our conclusion that the injection
and acceleration cannot occur primarily over the solar poles in models in
which drift effects are important. During the last cycle, when drifts
were downward from the poles, drifts along the termination shock front
would concentrate the acceleration at the poles. However, as we demon-
strated above, in this case the radial gradient seen in the equatorial
plane is inconsistent with observations. An evolving spectrum at the
termination shock could, however, affect the intensity seen at_arth.
In conclusion, we argued in this paper that the source for the
acceleration of the anomalous component cannot lie at the solar poles in
models that include gradient and curvature drifts. Such models will not
yield the observed spatial gradients in the equatorial plane. We argued
also that a source near the equatorial plane can yield the correct
gradients, but in drift models will not yield the observed behavior in
the intensity from cycle to cycle.
It would appear, therefore, that drift models may have significant
difficulty in accounting for the observed features of the anomalous
component.
References
Fisk, L. A., Kozlovsky, B., and Ramaty, R., 1974, Ap.J., 190, L35.
Jokipii, J. R., 1985, submitted to J. Geophys. Res.
Pesses, M. E., Jokipii, J. R., and Eichler, D., 1981, Ap. J., 246, L85.
" Potgieter, M. S., 1985, Paper SH4.2-5, this conference.
Potgieter, M. S., and Moraal, H., 1985, Ap. J., in press.
Webber, W. R., McDonald, F. B., Von Rosenvinge, T. T., and Mewaldt, R.A.,
1981, Proc. 17th ICRC (Paris),10, 92.
Webber, W. R., Cummings, A. C., and Stone, E. C., 1985, this conference.


On the anomalous component

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