Radial gradients for relativistic galactic cosmic rays (E 70 MeV) remained nearly constant at approx. 2.5%/AU from 1978-84, which includes the period of maximum solar modulation in 1981-82. For energies 30-70 MeV/n, gradients decreased at solar maximum to values of 1%/AU (protons) and 4%/AU (helium), and appear to be increasing again in 1983-84 toward the values found for solar minimum. The anomalous helium component has not reappeared, either at 1 AU or at Pioneer 10 at R  34 AU

Mckibben, R. B.

Pyle, K. R.

Simpson, J. A.

English

NASA Technical Reports Server

SH 4.7-5
198
GALACTIC COSMIC RAY RADIAL GRADIENTS AND THE ANOMALOUS He COMPONENT
NEAR MAXIMUM SOLAR MODULATION AND TO RADII BEYOND 34 AU FROM THE SUN
R. B. McKibben, K. R. Pyle, and J. A. Simpson
Enrico Fermi Institute, The University of Chicago
Chicago, Illinois 60637 (U. S. A.)
ABSTRACT: Radial gradients for relativistic galactic cosmic rays
(E > 70 MeV) remained nearly constant at N2.5 %/AU from 1978-84,
which includes the period of maximum solar modulation in 1981-82.
For energies 30-70 MeV/n, gradients decreased at solar maximum to
values of 1%/AU (protons) and 4 %/AU (helium), and appear to be
increasing again in 1983-84 toward the values found for solar
minimum. The anomalous helium component has not reappeared,
either at 1 AU or at Pioneer I0 at R > 34 AU.
I. INTRODUCTION: Our observations of the radial and temporal variations
of the modulated cosmic radiation and anomalous helium to the time of
maximum solar modulation in 1981-82 (McKibben et al., 1983, 1985) showed
that, at solar maximum, radial gradients for low energy (E < I00 MeV/n)
cosmic rays measured between 1 and 31 AU were sharply reduced from their
values at solar minimum. At relativistic energies, however, the radial
gradient was not strongly affected by the level of solar modulation. We
now extend our observations to the end of 1984, by which time the cosmic
ray intensity had begun to recover from maximum modulation both at I AU
and in the outer heliosphere. The Pioneer and IMP instruments used in
this study have been described by Simpson et al. (1980), and by Garcia-
Munoz et al. (1977). By late 1984_ Pioneer i0 had reached a radial
distance of > 34 AU fro_ the sun, as shown in Figure iC.
o.sV_r---_ , , , , --_-T---
I THE UNIVERSITY
I OrCH,CAGO 2. INTEGRAL GRADIENTS (E _ 70
o7'- _nA (^_ _ Figure IA shows simul-
o _ ' " _! taneous 27 day averages of
_ZT //,_,\/__ _/_L_ p_, integral counting rates from
. _°boos o / .J- Pioneer i0 (protons _ 70>~MeV)
i_ _ and IMP 8 (protons 106/ MeV), selected to exclude
o o._ IAo \_ - contributions from solar
__ 27 DAY AVERAGES _" _ /"_ particles These countingZ P-IO PROTONS> 67 MeV ",,...o-_ _j_
0.1 IMPSPROTONS>tO6MeV N rates are dominated by
s [__ ] relativistic particles with a
_ me n energy of the order of 2
• _ (_ GeV. The time variations
C__ 4 " :'* "
_'_2 4 °._.,,. ../'.'.%._Vv._,..'4,_.%,_,i observed at Pioneer I0 remain -
._:..,.. : .] well correlated with varia-o tions at 1 AU after a time
0'_'40[- - _ shift corresponding to out-
_ _ / ward propagation at near the "
_:D 20 / (C) solar wind speed (McKibben et
o_ _ / al. 1982). In late 1984, the
_ o[.-fr-, J , J J l I , , I ,_. flux at Pioneer i0 at 34 AU
1972 1974 1976 1978 1980 1982 t984
was increasing from the
FIGURE 1 maximum modulation observed
https://ntrs.nasa.gov/search.jsp?R=19850026737 2020-03-20T16:51:53+00:00Z
1 9 9 SH 4.7-5
in 1981-82, but was still not significantly greater than that observed at
1 AU at solar minimum in 1972. Therefore, the flux at Pioneer i0
remained heavily modulated in 1984.
Figure IB shows the radial gradient of the integral intensity as a
function of time. As in McKibben et al. (1983), a 400 km/s propagation
delay from 1 AU to Pioneer i0 has been assumed. Although there are
significant variations during the near-solar-minimum period (1972-1978),
since 1978 the gradient has remained nearly constant at -v2.5 %/AU
despite large changes in the intensity at both Pioneer and IMP. The
transient decrease in the gradient in 1982 is an artifact of the high
velocity [ _ 800 km/s (Lockwood and Webber, 1984)] of the shocks which
produced the intensity decrease in 1982.
3. LOW ENERGY (E < 70 MeV/n_ DIFFERENTIAL FLUXES: Figure 2A shows the
low energy (30-70 MeV) cosmic ray proton flux as observed at Pioneer I0,
Pioneer II, and IMP 8 starting in 1977, shortly before the onset of
renewed solar activity, continuing through solar maximum in 1981-82 and
into 1984 when recovery of the cosmic ray flux had begun. After 1979,
measurements in this energy range are not available from Pioneer Ii
because a detector problem has blocked analysis of particles with energy
> 20 HeV/n. As reported earlier (McKibben et al., 1985), at solar maxi-
mum (1981-82) for low energy protons, nearly the same flux was measured
at Pioneer i0 (R > 20 AU) and at i AU. At both Pioneer I0 and IMP 8 the
proton flux remained essentially constant during 1981-82, and
the shocks that produced such strong
effects at higher energies had little ,0.........
oP-lO
effect on the low energy proton flux. As _ .P-11
the recovery began in 1983-84, the flux z o., _._ . ,IMP8
at Pioneer I0 once again increased above _ __ (A)that at 1 AN. _ _
Figures 2B-D show the 27-day-average _ _'°o -_. o_o_.lO-S
radial gradients measured for 30-70 MeV _ "8.®-_'_
protons, 11-20 MeV/n helium, and 30-70 g HEUNfVERS_Y
• _ OFCHICAGO
HeV/n helium. The accuracy of the ,o_
measurements is reflected in the scatter _ ,2 ° ]
° OAYAW
of the points. All three gradients _ '_% (o)
decrease at solar maximum. The largest =>. 4_° %_,_o_. . •
decrease is shown by the gradient of Ii- <= • _." ...+._:I
20 MeV/n helium and corresponds to the _g I
disappearance of the anomalous helium in _ -4 -P-10/IMP8 J
1980-81. The shocks in 1982 appear to _" 20 _ oP-10/P-11
° ohave produced a further decrease, lasting z ,s _mQZ _+
_ s+_...... (c)
6 months or more, in the 30-70 MeV/n o>TM ,o _O.o.'_%
helium gradient. No clear corresponding <:_ _ % °' %
effects are observed for lower energy o,
helium. After maximum modulation, the .- o _o_.o_°
-_ _ _°
gradient of 30-70 MeV protons increased <. ;_'_ % +
o 0o o
slightly, and the data also suggest an =z ..._o o (D)
increase in gradients for helium. _<_ , _+o_
The observations are summarized in -= _°#. +_.I_'_o_..._
Figures 3A-C, which contain yearly snap- o +- <,+ ,,_.o.+_..-+_._.,
shots of the convection-corrected _= 0 1977' '1979' '"19811 $119831
intensity of 30-70 MeV protons, and of
11-20 and 30-70 MeV/n helium. Open FIGURE 2
200 SH 4.7-5
10-! _ , i i i , i i ii , , i , _ , , , e ett , _ , i , i
PROTONS 30-70 MeV HELIUM 11-20 MeV/N HELIUM 30-70 MeV/N
10-z ..........
THE UNIVERSITY • IMP-8
• INCLUDES
T_ OF CHICAGO • P-I 1
_,1o -s • P-IO ANOMALOUS He
"J= . _ 77,5 _.l 77.5 $
_mto'" _-78,6 _ ,78.6"_
• /- Jt 78.6
_,o-' Q __,e_084" ,,/j_./ _/,,so.s ,,__ 082.133 _I 44• Ir'-_q" _ .82,1
_,,.,_-__..:+ + - 81.4
lo4 __" 0 83.3
(A) (B) (C)
io -1 l I I I _I; i i l I l! i I i i I
,'o _'o' +'o' 4oo 1'o 2'0' +'o' ,o o 1'o 2'0 3'o ;0
HELIOCENTRIC RADIUS (AU) L'_00,A
FIGURE 3
symbols correspond to data obtained since maximum modulation in 1981, and
the heavy dashed lines indicate estimated interstellar galactic cosmic
ray fluxes from the model of Evenson et al. (1983). For all three com-
ponents, the flux measured in 1984 at Pioneer 10 remains below that
measured at 1 AU at solar minimum in 1977. Comparing the observed fluxes
in 1984 to the interstellar fluxes, at least 95% of the modulation takes
place outside R = 34 AU. Linear extrapolation of the gradients observed
in 1984 to the interstellar fluxes suggests modulation boundaries in the
range 100-300 AU. However, modulation models suggest that the radial
gradient may not remain constant to the boundary, so that linear ex-
trapolation provides no rellable estimate of the boundary locatlon.
Furthermore, the radius of the boundary itself may vary with solar
activity. Thus, based on our observations to date, we conclude only that
the current radius of the modulation region is much larger than 35 AU.
IV. ANOMALOUS RELIUM: The anomalous helium component was present from
1972-79 at Earth and at Pioneer I0 and 11. It disappeared at I AU in
1979 and at Pioneer 10 (R > 20 AU) in 1980-81, at roughly the same time
as the reversal of the sun's polar magnetic fields. Alternate models for
the acceleration and modulation of this component predict either that it
will be observed near the ecliptic only when the north pole of the sun is
magnetically positive (Pesses et al., 1981), or that it will be observed
whenever modulation is sufficiently weak (Garcia-Munoz et al., 1983).
The anomalous component helium spectrum is significantly softer than
the spectrum of modulated galactic cosmic rays. Figure 4 displays the
correlation observed at Pioneer i0 between the power law spectral index
(i.e. _' for a spectrum of the form dJ/dEo_ E+_/) for helium between Ii
and 70 MeV/n, which indicates the strength of the anomalous component,
and the flux of 29-67 MeV/n helium, which measures the level of solar
modulation. From 1972-1980, the correlation is well defined, and weaker
modulation corresponds to a larger contribution from the anomalous
component. At solar maximum in 1981-82, the spectral indices cluster
near the value of_+l.0 expected for normally modulated galactic helium.
In 1983-84, as shown by the large unfilled data points, the helium flux
began to recover, and by the end of 1984, the helium flux had reached
levels where in previous years the spectrum had shown clear evidence of
the anomalous component. As of late 1984, however, the spectrum remained
SH4.7-5
201
consistent with the presence only z.o , ._
of galactic cosmic ray helium. PlONeeR_O
presence of the anomalous com- x 1.5
ponent near the ecliptic is not a
function only of the strength of _ 1.0
modulation. Alternatives include
I) existence of a hysteresis _ 05the
effect, in which case _ i ___
anomalous helium should reappear _ +
at Pioneer i0 in the near future, m 00
2) sensitivity of the anomalous ._ 2__7DAYAVERAGES
helium to the sign of the helios- _ _ 1972-1982 +h
I -0.5
pher ic mag n e tic fie Id , as _ 1983-1984 THE UNIVERSITY
suggested by Pesses et al. (1981), oF CHICAGO
or 3) dependence of acceleration -,.o ...... , .......or modulation of the anomalous 1o-6 Io-s -4
helium on some as yet undefined 29-67 MeV/N HELIUMFLUX
characteristic of heliospheric (CM2 S SR MeV/N)-' _
structure. FIGURE 4
4. SUMMARY AND CONCLUSIONS: i. For relativistic cosmic rays, the radial
gradient measured between 1AU and Pioneer 10 at R > 30 AU was nearly
independent of the level of modulation throughout the period 1978-84,
remaining approximately constant at _ 2.5 %/AU.
2. For cosmic rays below i00 MeV/n, the radial gradientwas much smaller
at maximum modulation than at solar minimum. As the flux increased in
1982-84 after solar maximum, the gradient also began to increase.
3. The cosmic ray flux at Pioneer i0 at > 34 AU remains as heavily modu-
lated as it was at 1AU in 1972, so that Pioneer I0 is still far from the
boundary of the modulation region. Based on estimates of the interstel-
lar spectra, for energies < I00 MeV/n more than 95% of the modulation was
occuring beyond 34 AU in 1984.
4. The anomalous helium has yet to reappear at either i AU or Pioneer I0.
5. ACKNOWLEDGEMENTS: We are grateful to C. Lopate for his assistance in
preparation of the data. This work was supported in part by NASA/Ames
Contract NAS 2-11126, and by NASA Contract NAS5-28442.
REFERENCES:
Evenson, P., Garcia-Munoz, M., Meyer, P., Pyle, K.R., and Simpson, J.A.,
1983, Ap. J. (Letters), 275, LIb.
Garcia-Munoz, M., Mason, G.M., and Simpson, J.A., 1977, Ap. J., 217, 859.
Garcia-Munoz, M., Pyle, K.R., and Simpson, J.A., 1983, Ap. J. (Letters),
" 274, L93.
Lockwood, J.A., and Webher, W.R., 1984, J. Geophys. Res., 89, 17.
McKibben, R.B.j Pyle, K.R., and Simpson, J.A., 1982, Ap. J. (Letters),
. 254, L23.
....., 1983, Proc. XVIII ICRC, Bangalore_ India, 3, 63.
, 1985, Ap. J. (Letters), 289, L35.
Pesses, M.E., Jokipii, J.R., and Eichler, D., 1981, Ap. J. (Letters),
246, L85.
Simpson, J.A., Bastian, T.S., Chenette, D.L., McKibben, R.B., and Pyle,
K.R., 1980, J. Geophys. Res., 85, 5731.


Galactic cosmic ray radial gradients and the anomalous He component near maximum solar modulation and to radii beyond 34 AU from the Sun

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