Near solar maximum, a series of large radial solar wind shocks in June and July 1982 provided a unique opportunity to study the solar modulation of galactic cosmic rays with an array of spacecraft widely separated both in heliocentric radius and longitude. By eliminating hysteresis effects it is possible to begin to separate radial and azimuthal effects in the outer heliosphere. On the large scale, changes in modulation (both the increasing and recovery phases) propagate outward at close to the solar wind velocity, except for the near-term effects of solar wind shocks, which may propagate at a significantly higher velocity. In the outer heliosphere, azimuthal effects are small in comparison with radial effects for large-scale modulation at solar maximum

Pyle, K. R.

Simpson, J. A.

English

NASA Technical Reports Server

2 1 0 SH 4.7-9
THE LARGE-SCALE MODULATION OF COSMIC RAYS IN MID-1982:
ITS DEPENDENCE ON HELIOSPHERIC LONGITUDE AND RADIUS.
K. R. Pyle and J. A. Simpson
Enrico Fermi Institute, The University of Chicago,
Chicago, Illinois 60637 (USA)
ABSTRACT. Near solar maximum, a series of large radial
solar wind shocks in June and July 1982 provided a unique oppor-
tunity to study the solar modulation of galactic cosmic rays with
an array of spacecraft widely separated both in heliocentric
radius and longitude. By eliminating hysteresis effects we can
begin to separate radial and azimuthal effects in the outer heli-
osphere. We have been able to show that, on the large scale,
changes in modulation (both the increasing and recovery phases)
propagate outwards at close to the solar wind velocity, except
for the near-term effects of solar wind shocks, which may pro-
pagate at a significantly higher velocity. We show that, in the
outer heliosphere, azimuthal effects are small in comparison with
radial effects for large-scale modulation at solar maximum.
INTRODUCTION. The radially-propagating shock events of mid-1982
provide an exceptional opportunity to examine the dynamic response of the
heliosphere to a large, sudden increase in modulation and to study the
modulation onset and recovery mechanism as a function of magnetic rigi-
dity and heliospheric radius and longitude. This large series of solar
events has been studied intensively over the past several years, both in
its modulation effects (e.g. Lockwood and Webber, 1984) and in the shock
acceleration of energetic charged nucleons and electrons as far as 28 AU
from the sun (Pyle et al. 1984). Observations from a variety of instru-
ments aboard the Pioneer and Voyager spacecraft demonstrate that modula-
tion increases propagate outwards at approximately the solar wind velo-
city (Pyle et al. 1979; Lockwood and Webber, 1984; McDonald et al. 1981).
Fillius and Axford (1985) have reported an outward propagation of
recovery from modulation in the ll-year solar cycle, based on a com-
parison of neutron monitor intensity at 1AU with Pioneer relativistic
cosmic ray measurements. The outward progression of recovery could be
due to off-ecliptic fluxes or, as in the model of Perko and Fisk (1983),
due to the longer intervals between shocks during the declining phase of
solar activitT. In a companion paper at this Conference (McKibben et al.
1985) we also discuss the large-scale propagation of solar modulation,
showing that both modulation increases and decreases propagate outwards.
In this paper we examine the 1982 large-scale modulation event and
its recovery, using a counting rate responding to CNO-Fe nuclei in the
approximate energy range 200-1000 MeV/n, measured by identical instru-
ments on Pioneers I0 and 11. We show that for both the onset and
recovery of this large modulation event the dominant effect is one of
outward propagation at approximately the solar wind velocity, and is
relatively independent of heliospheric longitude.
INSTRUMENTATION AND TRAJECTORY. It is well-established that during
relatively rapid changes in modulation there is a hysteresis effect whose
magnitude is dependent on magnetic rigidity (Simpson 1963; Cooper and
https://ntrs.nasa.gov/search.jsp?R=19850026740 2020-03-20T16:51:51+00:00Z
2 1 1 8H 4.7-9
Simpson 1979). Therefore, to eliminate this rigidity-dependent effect,
we have used the F2 counting rate from the two identical Fission Cells
developed to measure the intense fluxes of protons during the Pioneer i0
and II encounters with
Jupiter, and later, Pio- _
neer II with Saturn. In __/_='--'_-__ _
interplanetary space -I _o_5OLAR _ _ VOYAGER 2
these counting rates are,pEx ¢ ,o ' 20 ' _o 4'OAU'
s nsitive to variety
of galactic cosmic ray -_____,_
species, (H-He 1%, C-Ne
14%, Na-V 28%, and Cr-Ni PmNECe,o
56%) (see Simpson et Pro46
al., 1974 and especially --- FIGURE I ---
McKibben,1983 for a
detailed discussion of the Fission Cell response). The energy interval
for response is N200-1000 MeV/n (N1-3 GV rigidity). These counting rates
provide relatively good statistics and permit a fairly precise measure of
modulation changes. The overall elemental response is expected to be in-
sensitive to any long-term instrumental drifts.
For comparison we have us@d the Voyager 1 and 2 counting rates for
nucleons >75 MeV (Stone et al., 1977), kindly made available by the
Voyager CRS experimenters.
In Figure 1 we have plotted the trajectories of Pioneers i0/II and
Voyagers 1/2, projected onto the eclip-
tic plane. Pioneer I0, alone among the
4 spacecraft, is moving away from the ..._,{
solar apex, in the direction of the ._I" _ (A)(~28AU)• L I _ IJ PIONEER10
heliospheric tail The time interval _ _ t_,/L ,l"_tt_ CNO-r,
studied here, 1982-3, is indicated by i . _r,the heavy lines along the trajectories• py _, '3 ~soo,,v/N
F _ L 7(.)(~_4AU)
OBSERVATIONS. In Figure 2 we have ,-LI_I_II_#I_ Jl_I P'ONEER,_
plotted 3-day averages of counting .,_'_ "_'""'_"_'"'_'1 -.0../N
rates, showingthe behavior of the event _L . .,_ _'rf_ _(c)(-,,A0_
at various radii and longitudes in the _[_f_vy i _] VOYAGERt
I/ L" _._V_NUC'EONS
heliosphere. The two large Forbush x 1 V_- _ >75MeV/Ndecreases seen at 1 AU in mid-1982 (Fig.
activityJuly, early in June and early in ' _ (D)V(yIOGEAU_:
patently caused by the large solar wind <LI V[I_I_ "%"I _ V_/_" )'V'_.] NUCLEONS>75MeV/Nt_
- on 3 June, on the back side of the sun z__ _2V , "_ V TM ](E)(__U)
as seen from the earth (Pyle et al. -_r,_: v _ / _P_
1984) • The longitude of this flare fl >_OS_.V/N
placed it almost directly "under"
oeer0eeoeeJuly), originated with a large solarflare event visible from Earth. In theFigure we have indicated (filled striangles) the outward progression of ............ ,.....
the modulation. The outward propagation 1982 1983 ero_Tc
--- FIGURE 2 ---
of the recovery is indicated by the open
SH 4.7-9
212
triangles. At 1 AU the two intensity .......... T ...........
decreases are distinct in time and THEUNWERS_TYOFCHICAGO
superimposed on a general downward ,. 27AU 29 3111,
trend of flux (Fig. IE,F). The two J " I(A) " "_
decreases are similarly resolvable at < ......_;I_ (D)._F'_
10-14 AU (Fig. 1B,C,D). However, at u_
Pioneer i0 (Fig. IA) the modulation &
increased over a ten-day period, and x 1
:) NEER 10
remained more or less constant for a __
six-month period before a relatively . 4' _ _ ,' , I ,
_,' ,' ,' _,' _JJL.J
sharp onset of recovery (open _=. _ _,, _,, _,, _j_,
triangle). At Pioneer 10 the second < _,, _,, ,'
shock has no significant effect on '.' ,' ,'
the °verall intensity level (filled z __E!:,Ii--
triangle with "?"). _j PIONEER11 "
It is clear from Figure 2 that
both the initial Forbush decreases as •
well as the subsequent rise from the , . 12Au 13 !4 15
'9 '8' 2'depressed flux levels (open tri- '1 ' ' 'J ' ' , 98 3'',/o4,'A
angles) propagate outwards. --- FIGURE 3 ---
Calculation of an accurate value for the radial velocity of the increase,
based on this Figure, is difficult owing to the possibility of hysteresis
between many of the counting rates shown. To accurately determine the
outward velocities we have isolated the two identical CNO-Fe 3-day
averaged counting rates from Pioneer I0 and Pioneer ii, between which
there are no significant energy hysteresi_ effects, and plotted them on
an expanded scale in Figure 3. The dashed arrows indicate the time
shifts necessary to best align the four intensity changes labelled A-D.
The triangles show the distance of the two spacecraft from the sun. The
initial modulation increase (event (A)) is nearly azimuthally symmetric:
we find that the modulation increase propagates outward with ap-
proximately the same velocity towards Pioneer ii and Pioneer I0.
However, event (B)'s transit velocity from Earth to Pioneer-ll is ap-
proximately 700 Km/sec, but from Pioneer Ii to Pioneer I0 only 545
Km/sec, indicating either a) a large deceleration or b) a significantly
higher velocity towards Pioneer II (700 Km/sec) than towards Pioneer I0
(600 Km/sec earth-Pioneer i0).
Events (C) and (D) are intensity recoveries where the propagation
velocities are near the solar wind velocity, indicating a very small, if
any, azimuthal dependence. Near each dashed line in Figure 3 we have
indicated the calculated _ 90 ............
propagation velocity for _ (A) UNSHIFTZD (B) SHIFTED(Vsw_SOOKrn/s )
the feature, assumin_ _ _
x D034~060,(84)._
azimuthal symmetry. In _ 75
Figure 4 the Barrels
solar rotation averages zo Dn4250(82)(SHOCK)/of the CNO-Fe counting _ _o75-, ....
©? .
rates for Pioneer I0 and o_ 60 \,,7o,_18,)
ii are plotted against
e ch othe . I panel (A) - _/_/\0,97 n,_)
the data, covering mid- T 4511_. . . . . .O- lt3 lt5 lt7 11 1i3 115 lt7
1982 to mid-1983, are P-IO F2 COUNTING RATE x 10 3 (S -t) PFO44A
plotted with no time-
shifting; in panel (B) --- FIGURE 4 ---
SH 4.7-9
213
the Pioneer-ll data have been shifted two solar rotations, equivalent to
a solar wind velocity of_,500 Km/sec. This time-shift has placed all of
the data points on the same regression line except the one corresponding
to the large modulation increase associated with the high-velocity,
radial shock. Thus, the assumptions of a) azimuthal symmetry and b) out-
ward propagation at near the solar wind velocity result in a near-perfect
alignment of the data at the two spacecraft, differing byNl6 AU in
heliospheric radius and by nearly 180 degrees in longitude.
SUMMARY AND CONCLUSIONS. In order to determine the outward
velocity of the onset and recovery from modulation of galactic cosmic
rays in the heliosphere it is essential to eliminate all magnetic
rigidity dependent effects so that rigidity-dependent hysteresis in the
heliosphere does not lead to erroneous conclusions.
We have, therefore, selected sets of nucleon-intensity data whose
magnetic rigidity ranges are close to each other. The solar-flare gener-
ated series of radially-propagating shocks which passed outward through
the heliosphere in 1982 provide a unique test of the dynamical processes
resulting in large-scale heliospheric modulation.
We have shown that major increases and decreases in solar modula-
tion are primarily radially dependent and not strongly azimuthally
dependent.
Not only is the increase in solar modulation an outward propagating
effect, as has been shown earlier (e.g. Lockwood and Webber, 1984;
Fillius and Axford, 1985) but the recovery from the 1982-3 enhanced
modulation period also propagates outward at the approximate solar wind
velocity.
ACKNOWLEDGEMENTS. We thank R. Vogt, Principal Investigator on the
Voyager CRS experiments, E. Stone, and A. Cummings for making available
the Voyager >75 MeV/n nucleon counting rates. This work was supported in
part by NASA/Ames Contract NAS 2-11126, by NASA Contract NAS5-28442, and
by NSF Grant ATM 84-12382.
REFERENCES:
Cooper, J.F., and Simpson, J.A., 1979, Proc. 16th ICRC, Kyoto, 12, 176.
Fillius, W., and Axford, I., 1985, J. Geophys. Res., 90, 517.
Lockwood, J.A., and Webber, W.R., 1984, J. Geophys. Res., 89, 17.
McDonald, F.B., Lal, N., Trainor, J.H., Van Hollebeke, M.A.I., and
Webber, W.R., 1981, Ap. J. (letters), 249, L71.
McKibben, B., 1983, Enrico Fermi Institute Preprint no. 83-21, The
University of Chicago.
McKibben, R.B., Pyle, K.R., and Simpson, J.A., 1985, Proc. 19th ICRC, La
Jolla, Paper SH4.7-7.
Perko, J.S., and Fisk, L.A., 1983, J. Geophys. Res., 88, 9033.
Pyle, K.R., Simpson, J.A., Mihalov, J.D., and Wolfe, J.H., Proc. 16th
. ICRC, Kyoto, _, 345.
Pyle, K.R., Simpson, J.A., Barnes, A., and Mihalov, J.D., 1984,
Astrophys. J. (letters), 282, LI07.
Simpson, J.A., 1963, Pont. Acad. Sci. Scripta Varia, (Vatican) 25, 363.
Simpson, J.A., Hamilton, D.C., McKibben, R.B., Mogro-Campero, A., Pyle,
K.R., and Tuzzolino, A.J., 1974, J. Geophys. Res., 79, 3522.
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The large-scale modulation of cosmic rays in mid-1982: Its dependence on heliospheric longitude and radius

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