A survey is of the long term temporal variations of 6.6 to 12 MeV/nucleon anomalous oxygen at 1 AU covering the period 1977 to 1984. This time interval included the recent solar maximum, with the recovery at neutron monitor energies beginning in 1982. During this time interval, 6.6 to 12 MeV/nucleon 0 fluxes decreased by at least a factor of 50, and indeed remained below the instrumental detection threshold after 1979. By late 1984, neutron monitors had recovered to roughly 1979 levels from the 1982 solar maximum, and anomalous O still remained below the detection threshold

Galvin, A. B.

Hovestadt, D.

Ipavich, F. M.

Klecker, B.

Mason, G. M.

English

NASA Technical Reports Server

SH4.6-3
168
TEMPORAL VARIATIONS OF THE ANOMALOUS OXYGEN COMPONENT, 1977 - 1984
G.M. Mason I, B. Klecker 2, A.B. Galvin I, D. Hovestadt 2, and F.M. Ipavich 1
iDept, of Physics & Astronomy, Univ. of MD, College Park, MD 20742 USA
2MPI fur Extraterrestrische Physik, 8046 Garching, FRG
ABSTRACT
We present a survey of the long term temporal variations of
6.6-12 MeV/nucleon anomalous oxygen at 1 AU covering the
period 1977-1984. This time interval included the recent
solar maximum, with the recovery at neutron monitor energies
beginning in 1982. During this time interval, 6.6-12 MeV/
nucleon 0 fluxes decreased by at least a factor of 50, and
indeed remained below our instrumental detection threshold
after 1979. By late 1984, neutron monitors had recovered to
roughly 1979 levels from the 1982 solar maximum, and
anomalous 0 still remained below our detection threshold.
i. Introduction. The 1972 discovery of the anomalous components of He
(4), N, 0 and Ne (1,5,8) added a new particle population to the set
available to probe acceleration and transport processes in the helio-
sphere. Observed over the energy range from a few to a few 10s of
MeV/nucleon during the 1972-77 solar minimum, these components display
an unusual composition resembling neither galactic cosmic rays nor solar
energetic particles. Of the many models suggested to explain the anoma-
lous components, perhaps the most attractive is that by Fisk, Kozlovsky
and Ramaty (3), who proposed that the particles begin as interstellar
neutrals which are ionized and accelerated in the heliosphere, yielding
a particle population in which high ionization potential e_nts are
favored. _
t
A key prediction of his model (3) is that the particle_ are
singly ionized. However, a direct test of this prediction r_ains
beyond the capabilities of any instruments flown to date. Temporal
variations, however, may offer clues pertinent to this question. For
example, a special temporal behavior has been predicted by Pesses,
Jokipii and Eichler (i0) in a model where the anomalous component is
accelerated at the solar wind termination shock over the solar poles,
and the particles subsequently flow down to the ecliptic plane. In this
case, particle drifts yield a 22-year modulation cycle, and thus the
component would not be expected to re-appear during the upcoming solar
minimum. In order to probe these questions we present measurements of
~i0 MeV/nucleon oxygen over the period 1977"1984, thus updating our
previous report for the years 1974-79 (7).
2. Instrumentation. The measurements presented here were carried out
in interplanetary space with the Ultra Low Energy Wide Angle Telescope
(ULEWAT), which is part of the MPI/UMD experiments on the ISEE-I and
ISEE-3 spacecrafts. Figure I shows a cross sectional view of the
ULEWAT, which identifies particle type and energy by the dE/dx versus
residual energy method. For the particles discussed in this paper, the
flow-through proportional counters PI and P2 serve as dE/dx elements,
and the silicon detector DI is the residual energy detector. The
https://ntrs.nasa.gov/search.jsp?R=19850026728 2020-03-20T16:52:21+00:00Z
SH4.6-3
169
hodoscope detectors shown in
ULEWAT the Figure permit correction of
the AE signals for particles
SIC entering the telescope at dif-S_nAxis
Entra_e • ferent angles, thus allowing a
\ Window _ wide opening angle while pre-
it__._Ni.445_icm2 _HODOSCOPE I' serving instrument resolu-
_L / Cou_e_ _ tlon. For the present study,
_-m\qL , I/_I II particles with incident angles
/_ [_ ]']7II Solid State up to ~25 ° were accepted, re-
...... suIting in a geometrical factorI i\X  "l'" lll..c..! i ,,o,.,, -1.3 cm 2 sr. A complete
! Iii7_ I.-H_-D2 20_. SiILi) instrument description has been
i ]_ V'_ ii li|_---AW2030_ Sl(Li) published elsewhere (6).
\ : i!._i! I_y i,__!I In order to remove thepossibility of contamination
F , by, e.g., solar flare parti-
_} cles, a strict criterion wasAE-Propo_nal adopted wherein observation
_-RF-Screen Countesers times were used only if the
counting rate channel for Z > 6
,_Anticoi_idence
Countem AP particles above 300 keV/nucleon
_!! counted less than 30 particles
. /_ " in 24 hours of observingtime. Assuming E-3 spectra,
this criterion extrapolates
roughly to a level of 10-4
_HODOSCOPE--Counters particles/sec-m2-sr-MeV/nuc at
, , i0 MeV/nucleon, a factor of
_cm ~500 below anomalous 0 flux
levels during solar minimum.
Table 1 lists the observing
Figure I time periods and number of
hours observing time for each point. High levels of solar activity
decreased the number of hours of quiet time in some intervals, and,
after ISEE-3 was targeted for comet encounter, spacecraft tracking
coverage was an important factor.
3. Observations. Figure 2 shows the fluxes and upper limits of 6.6-12
MeV/nucleon quiet time O during the survey period. Note that after the
end of 1978, only upper limits are shown. In the figure, upper limit
data points plotted with positive standard deviation bars indicate
finite O measurements where the spectral information was not sufficient
to conclusively identify the particles as part of the anomalous compo-
nent. Points that are shown as upper limits only are 1-count upper
- limits. For comparison with the solar modulation at high energies, the
lower panel shows 3-month averages of the Deep River neutron monitor as
compiled from Solar-Geophysical Data. While the neutron monitor has
been increasing since its 1982 minimum, by late 1984 it has not sur-
passed 1979 levels, and thus it is not surprising that even in 1984
anomalous 0 remained below our detection threshold.
170
SH4.6-3
Table i
OBSERVATION PERIODS AND COLLECTION TIMES
Spacecraft Period Hours Spacecraft Period Hours
ISEE-I 10/26/77 - 12/30/77 343 ISEE-3 7/01/81 - 12/31/81 635
ISEE-I 01/01/78 - 08/08/78 368 ISEE-3 1/01/82 - 06/30/82 734
ISEE-3 08/14/78 - 12/30/78 784 ISEE-3 7/01/82 - 12/31/82 758
ISEE-3 01/01/79 - 06/30/79 709 ISEE-3 1/01/83 - 07/01/83 1299
ISEE-3 07/01/79 - 12/31/79 684 ISEE-3 7/01/83 - 12/31/83 2058
ISEE-3 01/01/80 - 06/30/80 1148 ISEE-3/ICE 1/01/84 - 06/30/84 418
ISEE-3 07/01/80 - 12/31/80 686 ISEE-3/ICE 7/01/84 - 09/28/84 172
ISEE-3 01/01/81 - 07/01/81 586
-I
,u , , , , , , , ,
t-
O
-_ 6.6-12 MeV/nucleon Oxygen
t-"
, I0 -
- +.04 IEI0
_o
_o 5
___ I0- -
o_ • ISEE-I
× • ISEE-5
Lu
10.4 i t i i i l l i
I I I I I I I I
7000 "-Lr]m_ . Deep River Neutron Monitor0
(D U _L
L6600 -O9
o 6200 -0 _
5800 I L i i I
77 78 79 80 81 82 85 84 85
Year
Figure 2
SH4.6-3
171
4. Discussion. From Figure 2 it is clear that at 1 AU anomalous 0
decreased by at least a factor of 50 between 1977 and solar maximum
levels in 1981-82. This is much larger than the factor of ~I0 observed
for cosmic ray protons near i00 MeV (13), but is comparable to the
factor of ~30 decrease in 10-20 MeV/nucleon anomalous He between 1977
and 1981 (9). The fact that the anomalous components show greater
sensitivity to the solar cycle than galactic cosmic rays may be due to a
combination of source (acceleration) and modulation effects; however,
the unique spectral shapes of the anomalous components may also play an
important role (II).
During the 1977-81 time period the NI0 MeV/nucleon 0 flux
observed on the Voyager spacecrafts decreased also by a factor of ~50
(12), almost the same as the upper limits shown in Figure 2. However,
since the Voyagers were at ~i0 AU in 1981, the measurements cannot be
directly compared with the results presented here. Indeed, since the
anomalous He component has typically shown rather large positive radial
gradients, the Voyager results could imply an anomalous 0 flux level at
i AU a factor of 2 or more below our upper limits--ylelding an overall
modulation factor of >I00 between 1977 and 1981.
Since the neutron monitors have not returned to levels at which
our instrumental sensitivity would allow us to see anomalous 0 based on
an 11-year cycle, we are not yet able to address the predictions of a
22-year cycle as in the model in reference (i0). Recently, the anoma-
lous 0 fluxes have been increasing at large distances from the sun (2),
so it may be soon possible to address this question with measurements at
I AU.
5. Acknowledgements. We are grateful to the MPI and UMD individuals
responsible for the success of the ISEE instruments. This work was
supported by NASA under contract NAS5-28704, grants NGR 21-002-224/316
and NAGW-101, by the NSF under grant ATM-84-07546, and by the
Bundesministerium fur Forschung und Technologie, contract RV 14-B8/74.
References
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5. Hovestadt, D. et al. 1973, Phys. Rev. Letters, 31, 650.
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435.
13. Webber, W.R. et al. 1983, 18th ICRC, Bangalore, 3, 35.


Temporal variations of the anomalous oxygen component, 1977 - 1984

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