The large collecting area and wide energy range of the cosmic ray experiment on Voyager 1 and 2 was used to examine the energy spectra, charge composition, and long term temporal variations of the anomalous components in 1977-1982. Individual energy spectra are obtained for 17 separate quiet time periods during the time interval. The composite spectra of anomalous He, N, O, and Ne are obtained to a new level of precision. This includes the spectral shape and the relative abundance. Essentially, the spectral shape of N, O, and Ne appear to be similar. The ratios of anomalous N and Ne to O are found to be different from both the solar cosmic ray and galactic cosmic ray source composition. Some evidence is found for the enhancement of Ar as well. In the case of elements such as C, Mg, S, and Fe it is difficult to separate a possible lower intensity anomalous component from a quasi-steady interplanetary component that appears to be present at the lowest energies. The long term temporal variations of the anomalous He and O components were studied from 1977-82, a period from minimum to maximum in the modulation cycle. The tracking between these anomalous component intensities and the integral intensity of  75 MeV protons is striking; however, the intensity decrease of the anomalous components is much greater

Cummings, A. C.

Webber, W. R.

English

NASA Technical Reports Server

Voyager Measurements of the Energy Spectrum,
Charge Composition, and Long Term Temporal Variations
of the Anomalous Components in 1977-1982
W. R. Webber
University of New Hampshire, Durham, NH 03824
A. C. Cummings
California Institute of Technology, Pasadena, CA 91125
We have used the large collecting area and wide energy range of the cosmic ray
experiment (CRS) on Voyagers l and 2 to examine the energy spectra, charge composi-
tion, and long term temporal variations of the anomalous components in 1977-1982.
Individual energy spectra are obtained for 17 separate quiet time periods during the time
interval. The composite spectra of anomalous He, N, O, and Ne are obtained to a new
level of precision. This includes the spectral shape and the relative abundance. Essen-
tially, the spectral shape of N, O, and Ne appear to be similar. The ratios of anomalous
N and Ne to O are found to be different from both the solar cosmic ray and galactic
cosmic ray source composition. Some evidence is found for the enhancement of Ar as
well. In the case of elements such as C, Mg, S, and Fe it is difficult to separate a possi-
ble lower intensity anomalous component from a quasi-steady interplanetary component
that appears to be present at the lowest energies. The long term temporal variations of
the anomalous He and O components have also been studied from 1977-82, a period
from minimum to maximum in the modulation cycle. The tracking between these
anomalous component intensities and the integral intensity of > 75 MeV protons is
striking; however, the intensity decrease of the anomalous components is much greater.
Whereas the > 75 MeV intensity decreases by a factor _2 from 1977 to 1982, the 11.5 -
30 MeV/nuc anomalous He decreases by a factor _20 and anomalous O between 4.5 -
14.7 MeV/nuc decreases by a factor --50! Anomalous O is still observed at the lowest
intensity levels, however.
INTRODUCTION
The origin of the anomalous components of low energy cosmic rays, of which He and O are the
most obvious examples, presents many intriguing questions. One would like to know where and how
these particles are accelerated, what their accelerated spectrum looks like, and indeed what their charge
state is. Studies of the effects of solar modulation on these components, their radial gradients, and the
relative composition of the various anomalous species should in principle provide answers to some of
these questions. Previous studies of these components [e.g., Bastian et al., 1979; Klecker et al., 1977;
Webber et al., 1981] have usually been constrained by combinations of limited statistics and/or limited
energy range and therefore cannot examine all of these questions completely. The Voyager 1 and 2 data,
upon which this paper is based, overcomes several of these limitations, utilizing data from eight low en-
ergy telescopes (LET) with a total collecting area --3.5 cm2 sr, and four double ended high energy tele-
scopes (HET) with a total collecting area --4 cm2 sr. The energy range covered, from _4 - 124
MeV/nuc for oxygen nuclei, enables the transition from a quasi-steady interplanetary component at the
lowest energies to the anomalous components and then to the galactic cosmic ray components at the
highest energies to be observed. Individual energy spectra for the various charges are obtained for 17
separate quiet time periods during 1977-82. These quiet time intervals are restricted so that there is
negligible contamination of the data for Z I> 2 nuclei above --4 MeV/nuc by corotating streams or solar
energetic particles. Overall, these quiet time intervals contain about 65% of the total active time of the
two spacecraft over the entire time period.
435
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OBSERVATIONS -
SPECTRA AND COMPOSITION
Composite spectra for the entire
time period, constructed using both V 1 and
V2 data, are shown in Figures 1, 2, and 3.
In Figure 1 the overall details of the
anomalous oxygen spectrum are shown
clearly, from the initial rise above the galac-
tic component at --50 MeV/nuc to the
maximum intensity at --5 MeV/nuc. The
anomalous nitrogen spectrum is remarkably
similar. The carbon spectrum falls off like
a modulated galactic component down to
--10 MeV/nuc, below which it begins to
increase. It never seems possible to be
completely free of this low energy turn-up
for carbon and it is not clear whether this is
also an anomalous component, but at a
much lower intensity, or a quasi-stationary
interplanetary component, possibly ori-
ginating on the Sun and probably accelerat-
ed separately from the anomalous com-
ponents. The boron spectrum falls off like
a modulated galactic component down to
--4 MeV/nuc with no evidence of a turn-
Up.
In Figure 2 the behavior of the
anomalous neon component is shown clear-
ly for the first time and looks very similar
to that for N and O. The Mg and Si spec-
tra look very similar to the C spectrum, ex-
hibiting a fall off consistent with galactic
modulation down to _ 10 MeV/nuc with a
turn-up at lower energies. The origin of
this turn-up is probably similar to that for
C. The S and Fe spectra in Figure 3 show
a similar behavior although the turn-up
below 10 MeV/nuc is not as well defined.
The Ar spectrum shows evidence of a
turn-up at lower energies and is well fit by
the Ne spectrum divided by --20. We be-
lieve that this is the first suggestion that Ar
is enhanced as might be expected because
of its high ionization potential.
16 3
-2
t_
k,7
o_ 1(35
, 1 i i , ,ll I i I , , ,,,,I r_
Ne \x Fe _÷20
%%
2" +
I t I I 1 I I II I I I I I I I I[ I I I I I I I
I0 I00 I000
Energy (MeV/nuc)
Fig. 3. Composite V I and V2 spectra for S, Ar, and Ne
nuclei.
I
,6'
x
O
O
O
cr 1(52
,63
Solor Golactic W
C _/C
x
Fe N Mg ....... x.
_i,Ne -- Ne ..... - _
-- Ar S VF e
Ar
i i i i i iiii i 1 i i i i i ii i _ , , _ i 1
I0 I00 I000
Energy (MeV/nuc)
Fig. 4. Abundance of various charges relative to O derived
from smooth curves drawn through the data in Figures l, 2,
and 3.
The abundance of the various charges as a function of energy is shown, relative to O, in Figure 4.
The values on this Figure are obtained from smooth curves drawn through the data in Figures 1-3. Fig-
ure 4 illustrates a progression from a galactic composition above _ 50 MeV/nuc, down to an intermedi-
ate energy range where the anomalous components are dominant, and to the energy range below a few
MeV/nuc, where the abundance ratios should approach values more representative of the quiet time in-
terplanetary components which may have a "solar" composition. This new data provides accurate deter-
minations of the various galactic abundance ratios at -- 100 MeV/nuc. These low energy ratios are in
excellent agreement with ratios measured at higher energies [Engelmann et aL, 1981 ] and will be the sub-
437
ject of future study. At the lowest energies the
turn-up in the C, Mg, and Si ratios toward the
solar value is possibly indicative of a quiet
time interplanetary component of solar com-
position as noted earlier.
At intermediate energies the behavior of
the N and Ne ratios is almost identical to that
of O indicating that all 3 anomalous com-
ponents have essentially the same spectrum.
However, the abundance ratios of these
anomalous components change from the
higher energy galactic value to a value that is
also not equal to the solar value. The average
observed values N/O -- 0.16 and Ne/O = 0.07
for these anomalous components could be the
result of a different composition of the source
of these particles from that of the galactic or
solar cosmic rays, or to preferential effects in
acceleration and subsequent modulation,
although this must be tempered by the fact
that the resultant spectra are nearly identical.
The Ar ratio is not well defined at low ener-
gies, but it appears that the anomalous Ar/O
value must be less than the solar value of
0.005 as well.
OBSERVATIONS - TIME VARIATIONS
We next turn our attention to the long
term temporal variations observed during the
time period 1977-82. The striking and large
short term variations are discussed in an ac-
companying paper. For the temporal varia-
tions we shall use the most abundant com-
ponents - anomalous He and O as well as C as
a reference. The variations of these com-
ponents in 3 energy ranges are shown for the
17 quiet time intervals in Figure 5. It is seen
that the amount of solar modulation is large
and progressively increases as one goes to
lower energies. C + O nuclei between 60 -
104 MeV/nuc represent the modulation of the
galactic component - both C and O show
identical variations in this energy range. The
total intensity decrease of these components is
a factor --8. Helium in the energy range 11.5
30 MeV/nuc represents the peak of the
anomalous He component. The overall de-
crease here is a factor of nearly 20. Oxygen in
the energy range 4.5 14.7 MeV/nuc
represents the peak of the anomalous O com-
ponent. The overall decrease here is a factor
50! The variations of all 3 components ap-
438
0
c
>
i
¢)
ql)
u)
I
N
E
¢J
a_
] r i i r ' i ,
He 11.5-50 MeV/nuc
-'_
0 4.5-14.7 MeV/nu
"l+__ i
C+O 60-104 MeV/nuc
-I
I0
-3
10
_ t_.l-_.J i
I Tr Trr
I t l
1312
1977 i 1978 1 i9_9 1 1980 1 1981 1982
Fig. 5. Temporal variations of He, C, and O nuclei
during the 1977-82 time period.
tJ
c
Q)
i
ffl
I
i0 -I
i
(%1
E
no
, , "T
Helium II,5- 30 MeWnuc
tlt
/
-2.6 -2 -2 2 -2.0
In >75 MeV rate
Fig. 6. Regression curve of 11.5 - 30 MeV/nuc He in-
tensity and integral rate >75 MeV/nuc during the
1977-82 time period.
pearto track well despitea factorof 10 i
difference in energy and possibly different =-
C
origins. The changes of these components \
>
also track closely the changes in the higher
energy integral rate corresponding mainly g
to protons > 75 MeV as indicated in Fig-
i
ure 6. [See also Hovestadt et al., 1979, and _ 16'
Webber et al., 1979, for earlier studies.] ?
%
It is to be noted that the intensity "
drops in several steps and for each step the -_o
intensity remains roughly constant for
several months to a year or longer. For a - _62
detailed analysis of spectral changes we
have, therefore, grouped the data into 4
separate time intervals as indicated in Fig-
ure 5. The He and O spectra observed for
these 4 intervals are shown in Figures 7
and 8. Only spectral points are shown in
these figures that are uncontaminated by _o"
lower energy particles of solar or inter-
planetary origin. It is clear that there are
large and dramatic changes in the spectra
of the anomalous components, although a
clear signature of anomalous O remains \
>
present even at the lowest intensities. _ ,rz F
t)We have tried to analyze the intensi-
ty variations of all the components in a sys- _,
tematic manner by constructing plots of fl
In (J 1/J2) vs p (rigidity). (Jl and J2 are the
intensities at two different times, /3 is the _ _63
velocity.) The quantity B In (J l/J2) is relat- 7_
ed to the modulation parameter of the
o_
steady state modulation theories. Even
though there is considerable uncertainty a-
bout the applicability of these theories,
plotting the relative modulation of the vari- _o4
ous components in this way should help to
illustrate possible fundamental differences
in modulation of the various charges. This
data is shown in Figure 9. Consider first
the C and O modulation. Carbon is as-
i [ i i illl i i i i , ,lli i i i 11111
IO IOO IOOO
Energy (MeV/nuc)
Fig. 7. Helium spectra measured during four time periods.
, , ,,,,i ....... i
m _ Oxygen
L L L i i llll i J ,till , _ .....
IO IOO IOOO
Energy (MeV/nuc)
Fig. 8. Oxygen spectra measured during four time periods.
sumed to be a normal A/Z = 2 component undergoing modulation. This also applies to oxygen above
50 MeV/nuc where the spectra of C and O are the same. The rigidity equivalent to this energy is
shown by a dashed line in Figure 9. Above this energy the modulation of these two charges should be
identical and this is essentially what the data shows. Below this energy the spectra of C and O are dif-
ferent and the relative modulation of O clearly becomes progressively larger than for C. This might be
accounted for purely by the difference in spectra of the two components coupled with the effects of ener-
gy loss, or by this fact plus the additional assumption that anomalous O has a different A/Q than C (e.g.,
O is singly charged as in the model ofFisk, Kovlovsky, and Ramaty [1974]).
Comparing now the modulation of two anomalous components He and O we see that, at the
same rigidity, the modulation of He is always larger than O. This is true even though the He spectrum
is flatter than O; thus it is not easy to account for this difference purely in terms of differences in spectral
shapes which should act to produce a larger O modulation in conventional theories because of the larger
439
Compton-Getting coefficient associated
with the spectrum.
We are continuing to study the im-
plications of this modulation. We are
hopeful that having accurate modulation
data from 3 charges in overlapping energy
ranges and with different spectra should en-
able one to uniquely determine the A/Z of
the charges involved, with a minimum of
necessary assumptions regarding the modu-
lation itself.
Acknowledgments. We greatly ap-
preciate the efforts of R. E. Vogt, both in
his capacity as CRS Principal Investigator
and as a colleague who has provided useful
discussions. We are grateful to the Caltech
and Goddard groups who have supported
the CRS experiment. We appreciate help-
ful discussions with E. C. Stone. This work
was supported in part by NASA under con-
tract NAS 7-918 and grants NAGW-200
and NGR 05-002-160.
I
C
IO
0.1
( Helium •
Modulotion { Corbon
[Oxygen o
(¼)
I l i i l i i l I i i i l , i , i I i I I i i _-L
0.I I I0
Rigidity(GV)
Fig. 9. Modulation parameters observed for He, C, and O
nuclei intensity changes in 1977-82.
REFERENCES
Bastian, T. S., R. B. McKibben, K. R. Pyle, and J. A. Simpson, Variations in the intensity of galactic
cosmic rays and the anomalous helium as a function of solar latitude, Proc. 16th Int. Cosmic Ray
Conf., Kyoto, 12, 318, 1979.
Engelmann, J. J., P. Goret, E. Juliusson, L. Koch-Miramond, P. Masse, N. Petrou, Y. Rio, A. Soutoul,
B. Byrnak, H. Jakobsen, N. Lund, B. Peters, I. L. Rasmussen, M. Rotenberg, and N. Westergaard, The
elemental composition of cosmic rays from Be to Zn as measured by the French Danish Instrument
on HEAO-3, Proc. 17th Int. Cosmic Ray Conf., Paris, 9, 97, 1981.
Fisk, L. A., B. Kovlovsky, and R. Ramaty, An interpretation of the observed oxygen and nitrogen
enhancements in low energy cosmic rays, Ap. J. Lett., 190, L35, 1974.
Hovestadt, D., B. Klecker, G. Gloeckler, F. M. Ipavich, C. Y. Fan, and L. A. Fisk, Temporal variations
of the anomalous oxygen (1974-1979) and disappearance in 1978, Proc. 16th Int. Cosmic Ray Conf.,
Kyoto, 3, 255, 1979.
Klecker, B., D. Hovestadt, G. Gloeckler, and C. Y. Fan, Composition and energy spectra of cosmic rays
between 0.6 and 24 MeV per nucleon during quiet times : transition from a solar to the anomalous
component, Ap. J., 212, 290, 1977.
Webber, W. R., E. C. Stone, and R. E. Vogt, The elemental composition of quiet time low energy cos-
mic rays measured on the Voyager spacecraft, Proc. 16th Int. Cosmic Ray Conf., Kyoto, 5, 357, 1979.
Webber, W. R., F. B. McDonald, T. T. Von Rosenvinge, and R. A. Mewaldt, A study of temporal and
radial dependencies of the anomalous helium and oxygen nuclei, Proc. 17th Int. Cosmic Ray Conf.,
Paris, 10, 92, 1981.
440


Voyager measurements of the energy spectrum, charge composition, and long term temporal variations of the anomalous components in 1977-1982

The large collecting area and wide energy range of the cosmic ray experiment on Voyager 1 and 2 was used to examine the energy spectra, charge composition, and long term temporal variations of the anomalous components in 1977-1982. Individual energy spectra are obtained for 17 separate quiet time periods during the time interval. The composite spectra of anomalous He, N, O, and Ne are obtained to a new level of precision. This includes the spectral shape and the relative abundance. Essentially, the spectral shape of N, O, and Ne appear to be similar. The ratios of anomalous N and Ne to O are found to be different from both the solar cosmic ray and galactic cosmic ray source composition. Some evidence is found for the enhancement of Ar as well. In the case of elements such as C, Mg, S, and Fe it is difficult to separate a possible lower intensity anomalous component from a quasi-steady interplanetary component that appears to be present at the lowest energies. The long term temporal variations of the anomalous He and O components were studied from 1977-82, a period from minimum to maximum in the modulation cycle. The tracking between these anomalous component intensities and the integral intensity of 75 MeV protons is striking; however, the intensity decrease of the anomalous components is much greater

Voyager measurements of the energy spectrum, charge composition, and long term temporal variations of the anomalous components in 1977-1982

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