Use of constancy of helium to medium nuclei ratio in solar cosmic rays to estimate solar helium abundanc

Fichtel, C. E.

Reames, D. V.

English

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NA i T IA X; 6 S-Te3
CONSTANCY OF THE He TO MEDIUM
-NUCLEI, RATIO IN THE
SOLAR COSMIC RAYS
C. E. FICHTEL
D. V. REAMES
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GODDARD SPACE FLIGHT CENTER
GREENBELT, MARYLAND
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IConstancy of the He to Medium Nuclei Ratio
in the Solar Cosmic Rays
(A Reply to A Statistical Study of Solar Protons,
Alphas, and Z t- 3 Nuclei in 1967-68
by Armstrong and Krimigis)
C. E. Fichtel and D. V. Reames
Goddard Space Flight Center
Greenbelt, Maryland 20771
Introduction
The constancy of the relative abundances of the multiply charged
nuclei of the same charge-to-mass ratio above about 20 Mev/nucleon in
solar cosmic ray events has been a concept which has developed from
detailed experimental measurements on charge and energy spectra, and
has been tested several times in a number of events (Fichtel and Cuss,
1961; Biswas et al., 1962; Biswas et al., 1963; Biswas et al., 1966;
Durgaprasad et al., 1968; Bertsch et al., 1969). It has been indicated
previously (e.g. Biswa. et al., 1962; Fichtel and McDonald, 1967;
Durgaprasad et al., 1968) that the energetic solar nuclei coming from
the sun with charges ranging from that of helium through at least 16
r
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fi
1
-2-
appear to be in good agreement with the composition of the solar surface,
where comparisons can be made.
Recently, on the basis of data obtained at low energies with a single-
element solid state detector, Armstrong and Krimigis (1971) have questioned
the constancy of the helium to medium nuclei ratio, and the justification
for using it to estimate the solar helium abundance. Specifically, their
conclusion is based on several measurements of the ratio of helium nuclei
in one channel corresponding to an energy interval from 0.5 to 2.5 MeV/
nucleon to heavier nuclei in another single channel, which they indicate
corresponds to an energy/nucleon interval of 0.56 to 9.5 MeV/nucleon for
carbon, 0.51 to 14 MeV/nucleon for nitrogen,and 0.47 to 19 MeV/nucleon for
oxygen. The interval also corresponds to wider ranges of energy/nucleon for
heavier elements which will be considered in detail in the next section of
this paper. The ratio of these two channels, P4/P3, for different events
is found to be variable and generally lower than the consistent 58 + 5
value for the helium to medium (C,N,O) ratio at higher energies where each
s pecies is measured in the same energy/nucleon interval.
In spite of the different energy/nucleon intervals of the P4 /P3
measurements and the fact that nuclei other than medium nuclei. are included,
Armstrong and }rimigis speak of the differEn ce as an "apparent discrepancy"
and use this together with solar wind data as an. argument for questioning
the concept of combining solar spectroscopic and solar cosmic—ray data for
nuclei of the same charge-to-mass ratio to obtain a hydrogen to helium
ratio for the sun. It is the intent of this paper to show that this is
not a valid criticism, and that, although the limited information provided
by the Armstrong and Krimigis detector makes interpretation difficult,
t
-3-
the observed P4/P3 ratios, in fact, fall in a range that would be
expected if the conditions existing at higher energies remain valid
- I	 at lower energies.
Analysis and Discussion
P4 and P3 of the Armstrong and Krimigis experiment are related to
pulse height thresholds in a single solid state detector. Assuming the
channel sensitive to heavy nuclei is not enhanced by pile-up or splash
events (There is no anticoincidence cup around this detector.), the
higher channel, P3, corrEsnonds to various energy/nucleon intervals for
the differen' heavy nuclei and the second highest, P4, to the energy/
nucleon interval for helium nuclei mentioned in the Introduction, together
with very minor contributions from the heavy nuclei. Since there are no
individual charge measurements in the experiment, and there are no energy
spectral measurements for helium or medium nuclei, a charge and energy/
nucleon spectrum must be assumed before P4/P3 can be interpreted. For
the charge spectrum, we shall assume that the composition is the same as
that measured at higher energies (The most recent summary is in Bertsch
et al., 1971; it is essentially the same as that in Durgaprasad et al.,
1968, and Bertsch et al., 1969.), which agrees with the solar spectroscopic
measurements - the fundamental difference from the Armstrong and Krimigis
paper being that nuclei heavier than oxygen will not be ignored here.
The spectral shape will first be assumed to be of the form
J ( >E) — E-a	 (1)
-4-
t
since this form seems to be as good a representation as any in the
low energy region (e.g., Lanzerotti, 1969), at least late in solar
particle events. In the non-relativistic region teing discussed
here this shape is equivalent to a power law spectrum in rigidity for
	 1
particles of the same charge to mass ratio. For the moment, "a" of
Equation (1) will be left as a variable, although its value will be
discussed later.
Table I lists the relative abundances of the more abundant solar
cosmic rays together with the energy/nucleon interval in which the
nucleus would be detected in th y- Armstrong and Krimigis detector
(Armstrong and Krimigis, 1968, and Krimigis and Armstrong, 1966).
For the heavier element: not considered by these authors, the energy
interval was deduced from the description of their detector and the
	
--^	 range energy curves given by Northcliffe and Schilling (1970). The
i
	
-	 energy/nucleon intervals determined in this manner for He, C, N. and
0 nuclei agree with those quoted by Armstrong and Krimigis. For the
energy spectral shape mentioned in the last paragraph, P4/P3 becomes
( -a -	 -a )
P4 _ n rf El C	 E2 a	 (2)
P3	 26 n (E -a- E a11r	 z	 lz	 2zlZ=3 /
It is worth noting at this point that the P4/P3 ratio is sensitive to
uncertainties in threshold setting, especially for steep spectra.
Assuming, however, that the experimenters do know the threshold
precisely, P4/P3 can be calculated as a function of "a". The resultF-
of this calculation are shown in Fig. 1. In Fig. 2, the results of a
similar calculation wherein the spectral shape
-5-
J(> E) — exp (-VEC,)
was assumed are given. In both cases, P4/P3 values range from about
10 to 40 for values of "a" and "E " that might be expected. Notice
o
that P4/P3 is smaller for steep spectra (large "a" and small "E of. and
for fat spectra than for intermediate spectra. The former result
is due to the large contribution of nuclei heavier than oxygen at low
energies, and the latter is due primarily to the contribution of C,
N, and 0 nuclei in the higher part of the energy/nucleon inter.,al
available to them. This range of values is in good agreement with
the P4/P3 data presented by Armstrong and Krimigis (1971). Thus there
seems at present to be no discrepancy with the higher energy data.
With regard to the values of "a" and "E 0 ", very little is known
about the spectral shape of helium nuclei at these low energies and
essentially nothing about the medium nuclei energy spectral shape.
Armstrong and Krimigis have tried to use their proton measurement as
an indication of what the helium spectra might be. In addition to the
difficulty of the proton spectra being known to be different from the
IL	 heavier nuclei, the Armstrong and Krimigis measurements discussed in
their paper are ambiguous because they are based on only two pulse
height level, and thereby involve nested energy intervals. These data
	
^. t
in general allow two solutions for any assumed one parameter spectral
shape function, one steep and one flat, and cannot distinguish between
spectral shape functions. The analysis here would suggest that a rather
wide range of spectral values is being seen for the helium and medium
nuclei data as would be expected both from the low energy protor data
-6-
(Bryant et al., 1962; Fichtel and McDonald, 1967; Lanzerotti et al.,
1969; Simnett, 1971) and the higher energy helium and medium nuclei
data referenced earlier. If the spectra should be steep as Armstrong
and Krimigis suggest, Figs. 1 and 2 show clearly that it is still
necessary to know how steep and what the spectral shape is before
P4/P3 can be predicted for a specific case.
Even though the results of the Armstrong and Krimigis experiment,
when interpreted more fully, are seen to be completely
consistent with the higher energy solar cosmic ray particle measurements
a. word of caution should be added about using very low energy data for
composition considerations. As has been emphasized from the earliest
papers on solar cosmic ray composition (e.g. Biswas et al., 1962) and
subsequent ones, the solar cosmic ray composition can be expected to
reflect that of the source only for those nuclei whose charge-to-mass
ratios are the same, since both velocity and rigidity effects enter
into the propagation phase of the solar particles, and probably at
least the latter stages of the acceleration process. Particles with
different cl.arge-to-mass ratios are known to have different energy/
nucleon (or velocity) spectra and varying ratios during an event, as
shown by measurements on pro tons and helium nuclei (e.g. Biswas and
Fichtel, 1965).
Ions of any given velocity will attain an equill.brium average
charge state after passage through very small amounts of material
( <5 x 10 -5 g/cm 2 for C and 0). At —1 NeV/nucleon the dominant charge
state for oxygen will be 0+6 giving a mass-to-charge ratio of 2.7
(Heckman et al., 1963); helium at the same energy/nucleon will be
I
r
1-7-
fully stripped of electrons. There is also the possibility that these
lowest energy nuclei may not be fully ionized because they are a part
of a hot thermal distribution at the source. Thus, before consideration
is given to comparing solar cosmic ray composition below a few MeV/
nucleon to solar abundances, it is essential, at a minimum, to know
that the energy/nucleon spectra of helium and medium nuclei are the
same, as one possible indication that the medium nuclei are, or have
remained, fully ionized.
The variation in the helium to medium ratio in the solar wind was
also mentioned by Armstrong and Krimigis. The solar wind is a hot
plasma being convected outward from the sun and is Li effect an extension
of the corona. Theoretically, substantial variations are expected in the
ratio of heavy elements to hydrogen or helium in the solar wind because
a heavy element excess tends to build up in the corona to varying degrees
depending on solar conditions at any given time. As a compensating
effect, heavy elements do not escape from the corona as readily.. Over
a very long time average, the solar wind composition may reflect that
of the photosphere under the concept that the solar wind ultimately is
a net flow of material from the photosphere; however, even if this is
true, and it may not be, substantial variations in the solar wind helium
to medium ratio on a short time scale are to be expected and, indeed,
have been observed. Data from the Vela 3 satellite shows that not only
are there large variations in the helium to hydrogen ratio over periods
of the order of months, but also a general increase in the long time
average value over the period of two ,years (Robbins et al., 1970). At
present, data to provide a long term average over a continuous period
N
I-8-
for heavier elements do not exist; so the solar wind abundance cannot
be considered relevant to the present problem of photospheric composition
at this time (Hundhausen, 1970; Brandt, 1966).
Summary
Ir the previous section, it was sho.wn that the P4 /P3 values measured
by Armstrong and Krimigis are not inconsistent with the solar cosmic ray
composition measured at higher energies. In fact, the P4 /P3 mE:.surements
are in the expected range when it is assumed that the composition of
the solar cosmic rays multicharged nuclei are the same as at higher
energies and all the energy/nucleon spectra of the particles with the
same charge-to-mass ratio are the same. These conclusions were seen to
result basically from the fact that the P3 channel records all heavy
nuclei with energy bands which are increasingly wide as the nuclear
charge increases and that the energy intervals for heavy nuclei are
much wider than that of helium nuclei. The lower energy/nucleon
threshold for heavier nuclei is important for steep spectra and the
differing widths of the energy bands are important for flat spectra.
A word of caution was also added about using very low energy MeV/nucleon
measurements for composition purposes, even when the bare nuclei
themselves have the same charge-to-mass ratios, until the energy spectra
of at least two multiply-charged species are measured, since these
nuclei may not be fully ionized.
1-9-
Table I
Relative Abundances of Solar Cosmic Ray Nuclei in the Same
Energy/Nucleon Interval Aoove 20 MeV/Nucleon,
And Energy/Nucleon Intervals of the Armstrong and :rimigis Detector
Detector	 Nucleus	
n 
	 El	 E2
p4	 He	 103
	 .54	 2.5
p 3	 C	 .56	 •58	 9.5
N	 .19	 .51	 14.
0	 1.0	 .46	 19
Ne	 .16	 .40	 45
Mg	 .056	 .35	 70
Si	 .028	 .31	 105
S	 .008	 .28	 155
Fe	 .011	 .175
Bertsch, et al. (1971); Durgaprasad, et al. (1968); and Bertsch, et al.
(1969).
-10-
References
Armstrong, T. P. and Krimigis, S. M., "Observations of Protons in the
Magnetosphere and Magnetotail with Explorer 33," J. Geophys. Res.,
22, 143, 1968.
Armstrong, T. P. and Krimigis, S. M., "A Statistical Study of Solar
Protons, Alphas, and Z z 3 Nuclei in 1967-68," to be published
In J. Geophys. Res., 1971.
Bertsch, D. L., Fichtel, C. E., and Reames, D. V., "Relative Abundance
of Iron-Group Nuclei in Solar Cosmic Rays," Ap. J. Letters, LU,
L53, 19,j9.
Bertsch, D. L., Fichtel, C. E., and Reames, D. V., "Nuclear Composition
and Energy Spectra in tie April 12, 1969 Solar Particle Event,"
NASA Goddard prepr i nt X-662-?1-210, submitted for publication
in Ap. J.
Biswas, S., and Fichtel, C. E., "Composition of Solar Cosmic Rays,"
Space Science Rev., 4, 709, 1965-
Biswas, S., Fichtel, C. E., and Cuss, D. E., "Study of -uia Hydrogen,
Helium and Heavy Nuclei in the November 12, 1960 Solar Cosmic-
Ray Event;'Phys. Rev., 128, 2756, 1962.
Biswas, S., Fichtel, C. E., and Cuss, D. E., "Solar Cosmic-Ray Multiply
Charged Nuclei and the July 18, 1961 Solar Event," J. Geophys. Res.,
71, 4071, 1966.
Biswas, S., Fichtel, C. E., Cuss, D. E., and Waddington, C. J., "Hydrogen,
Helium, and Heavy Nuclei from the Solar Event on November 15, 1960,"
J. Geophys. Res., 68 ; 3109, 1963.
-11-
Brandt, J. C., "Chemical Composition of the Photosphere and Corona as
Influenced by the Solar Wind," Ap. J., 1^4, 265, 1966.
Bryant, D. A., Cline, T. L., Desai, U. D., and McDonald, F. B.,
"Explorer 12 Observations of Solar Cosmic Rays and Energetic
Storm Particles after the Solar Flare of September 28, 1961,"
J. Geophys. Res., L, 4983, 1962,
Durgaprasad, N., Fichte., C. E., Guss, D. E., and Reames, D. V.,
"Nuclear-Charge Spe-2tra and Energy Spectra in the September 2,
1966 Solar Particle Event," Ap. J., 1_54, 307, 1968.
Fichtel, C. E., and Guss, D. E., "Heavy Nuclai in Solar Cosmic Rays,"
Phys. Rev. Letters, 6, 495, 1961.
Fichtel, C. E., and McDonald, F. B., "Energetic Particles from the
Sun," Ant.ual Rev. of Astronomy and Astrophysics, 1, 351, 1967.
Heckman, H. H., Hubbard, E. L., and Simon, W.-;,1;"Electronic Charge
Distribution for Heavy Ions at High Velocities," Phys. Rev.,
129, 1240, 1963.
Hundhausen, A. J., "Composition and Dynamics of the Solar Wind Plasma,"
Reviews of Geophysics and Space Physics, 8, 729. 1970•
Krimigis, S. M., and Armstrong, T,P., "Observations of Protons in the
Magnetusphere with Mariner 4," J. Geophys. Res., Z, , 4641, 1966.
Lanzerotti, L. J., "Low-Energy Solar Protons and Alphas as Probes of
the Interplanetary Medium: The May 28, 1967, Solar Event,"
J. Geophys. Res., 74, 2851, 1969.
Northcliffe, L. C., and Schilling, R. F., "Range and Stopping ?ower
Tables for Heavy Ions,"Nuclear Data, Vol. L-Z, 233, Academic
Press, N. Y., 1970.
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Robbins, D. E., Hundhausen, A. J., and Bane, S. J., "Helium in the
Solar Wind," J. Geophys. Res., 21, 1178, 1970.
Simnett, G. M., "Relativistic Electrons from the Sun Observed by IMP-4,"
University of California, Riverside, preprint, 1971 (to be published).
-13-
Figure Captions
Fig. 1.	 Variation of the P4/P3 measure of solar particle composi-
tion on Explorer 35 (see text) as a function of spectral
index "a" assuming spectra of the form J(> E) —E -a for all
multiply-charged nuclei and the relative abundances shown
in Table I. The P4/P3 ratio becomes strongly shape dependent
for steep (large a) and flat (small a) spectra owing to the
effects of the different energy/nucleon regions sampled for
different nuclear species. The predicted ratios are in
reasonably good agreement with those measured by Armstrong
and Krimigis (1971) for typical solar particle spectra.
Fig. 2	 Variations of the P4/P3 measure of solar particle composition
on Explorer 35 (see text) as a function of the spectral parameter
Eo assuming spectra of the form J(> E) — exp (-E/E o ) for all
multiply-charged nuclei and the relative abundances shown in
Table I. The P4/P3 ratio becomes strongly shape dependent
for steep (small E o ) and flat (large E o ) spectra owing to the
effects of the different energy/nucleon regions sampled for
different nuclear species. These predicted ratios are in
reasonably good agreement with those measured by Armstrong
and Krimigis (1971) for typical solar particle spectra.
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Constancy of the He to medium nuclei ratio in the solar cosmic rays

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