A cylindrical stack of G-5 nuclear emulsions housed in the payload section of a four-stage research rocket was flown into the northern edge of the inner Van Allen belt on September 19, 1960. The experimental design permitted, for the first time, measurements of the particle fluxes and energy spectra as functions of position along the rocket trajectory. Eight points along the trajectory have been selected for analysis. Results are presented herein for three of these points, and they are discussed in the light of various theories on the trapped radiation

Kniffen, Donald A.

Naugle,  John E.

English

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THE FLUX AND ENERGY SPECTRA OF THE
PROTONS IN THE INNER VAN ALLEN BELT
John E. Naugle
and
Donald A. Kniffen
Goddard Space Flight Center
Greenbelt, Maryland
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON
https://ntrs.nasa.gov/search.jsp?R=19980227851 2020-06-15T22:47:37+00:00Z

!THE FLUX AND ENERGY SPECTRA OF THE
PROTONS IN THE INNER VAN ALLEN BELT
by
John E. Naugle and Donald A. Kniffen
Goddard Space [:light Center
SUMMARY
A cylindrical stack of G-5 nuclear emulsions housed in the
payload section of a four-stage research rocket was flown into the
northern edge of the inner Van Allen belt on September 19, 1960. The
experimental design permitted, for the first time, measurements
of the particle fluxes and energy spectra as functions of position
along the rocket trajectory. Eight points along the trajectory
have been selected for analysis. Results are presented herein
for three of these points, and they are discussed in the light of
various theories on the trapped radiation.

CONTENTS
Summary ............................... i
INTRODUCTION ........................... 1
METHOD OF ANALYSIS ..................... 2
DISCUSSION OF RESULTS ................... 3
ACKNOWLEDGMENTS ...................... 6
References .............................. 6
iii

!THE FLUX AND ENERGY SPECTRA OF THE
PROTONS IN THE INNER VAN ALLEN BELT
by
John E. Naugle and Donald A. Kniffen
Goddard Space Flight Center
INTRODUCTION
Prior to the experiment described herein, the proton energy spectrum in the inner
Van Allen radiation belt had been measured (References 1 and 2) only with nuclear
emulsions which integrated the results over the entire flight paths of missiles launched
from Cape Canaveral, Florida. On September 19, 1960, a research rocket especially
equipped for the exposure and recovery of nuclear emulsions was flown along a trajectory
closely corresponding to a magnetic meridian, obtaining the first measurements of the
flux and energy spectra of these trapped protons as functions of position in the belt.
The rocket payload was designed to permit a four-inch cylinder to be extended through
the front end of the nose cone, thereby reducing to a minimum the amount of material
surrounding the emulsion container. Time resolution of the spectrum was obtained by
exposing sections of a cylindrical emulsion stack to the ambient radiation through a port
in the tungsten emulsion container (Figure 1). A point on the periphery of the stack was
behind the port for approximately 80 seconds. During the rest of the flight the point
was shielded by 30.6 gm/cm 2 of tungsten. Therefore, the track population at such a
point consists of particles which came through the port during the 80-second exposure
plus a background of particles which passed through the tungsten during the remaining
900 seconds of flight. A portion of the stack which did not pass behind the port provided
a measure of the background. Protons with energies E >_ 8 Mev were able to penetrate
the aluminum port cover and be detected, whereas only protons with E -> 145 Mev were
recorded in the emulsion after passing through the tungsten shield.
METHOD OF ANALYSIS
The payload trajectory is shown in Figure 2. Numbered points along the trajectory
indicate the locations where the corresponding points on the stack were behind the port.
Table 1 gives the pertinent data regarding three of the eight points selected for analysis.
ALUMINUMPORT COVER
.04 gm/cm. 2
TUNGSTENSHIELD
30.6 gm/cm. 2
ALUMINUM
ORT TUBE
Figure 1 - 25 circular G-5 and G-0 nuclear emulsions
were contained in a tungsten cassette; the numbered
points around the periphery of the st;_ck refer to the
sections of emulsion analyzed.
I
GEOMAGNETICEQUATOR
IOO MAGNETIC FIELD LINES
PROTONS
'.,000 crn!/sec (E >-31 MEV)
6 \
0 0 ()
0 0 0
0 0 0
0.I r'o
0,000
\\
0
0
0
ALTITUDE
KILOMETERS
\
\
Figure 2 - Meridian section of the earth showing tae trajectory of the rocket
payload relative to the magnetic field and the radiation belt
Table 1 -- Summary of Results
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POINT 3
1600 km
28.4 ° N. Lat.
237.7 ° W. Long.
B = 0.231 Gauss
I = 1.67 earth radii
J(E >_ 31) = 70 + 15 protons/(cm 2 sr sec)
4_J(E >- 31) = 900 + 200 protons/(cm _ sec)
(*) Jo(E >- 31) = 220 protons/(cm 2 sec)
(3.2 +.6) x 106
J(E) =
E 4 s ± .5
protons/(cm2 sec sr Mev); 10 -< E < 50 Mev
1884 km
25.2 ° N. Lat.
236.5 ° W. Long.
B = 0.198 Gauss
I = 1.34 earth radii
(6.8 + 0.3) x 10 6
J(E) =
E 4s +0.2
POINT 4
J(E -> 31) = 120 + 20 protons/(cm 2 sr sec)
4_J(E -> 31) = 1500 + 250 protons/(cm 2 sec)
(,)Jo(E >-31) = 1000 protons/(cm2sec)
protons/(cm 2 sec sr Mev); 10 -< E -< 50 Mev
1600 km
21.5 ° N. Lat.
235.4 ° W. Long.
B = 0.209 Gauss
I = 0.946 earth radii
(1 + .5) x 10 3
J(E) -
E 1.7 f .3
POINT 5
J(E -> 31) = 190 + 40 protons/(cm2 sr sec)
47rJ(E >- 31 Mev) = 2400 _ 500 protons/(cm2sec)
(*) Jo(E -> 31 Mev) = 1290 protons/(cm2sec)
protons/(cm 2 sec sr Mev); 40 -< E < 100 Mev
Integral flux behind 30.6 gm/cm 2 of tungsten, averaged over trajectory:
J(E >_ 145 Mev) = 16 + 3 protons/(cm2 sr sec)
",qntegral omnidirectional proton flux at point based on satellite data (Reference 7).
A "line scan" was made around the peripheries of the emulsions, approximately 500
microns from the edge; blackening prevented scanning closer to the edge. All particles
whose tracks, at the scan line, were within +25 degrees of a radius vector and within +45
degrees of the plane of the emulsion were recorded. The portion of the emulsion not
exposed at the port was also scanned, with the same criteria, to obtain an integral flux
averaged over the entire flight and also a background correction for the other sections of
the stack.
4To obtain the differential energy spectrum, all particles which had a possible residual
range in the emulsion at the scan line of at least 2.5 mm were followed until they ended,
interacted, or left the stack. The energy and mass of each of these particles was then de-
termined from range, ionization, and scattering measurements.
DISCUSSION OF RESULTS
I
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In the analysis to date, no evidence has been found for particles other than protons
crossing the scan line. However, the mass measuremenr:s do not rule out a small compo-
nent of singly-charged particles heavier than protons. The thickness of the blackened
edge of the emulsion varies around the periphery of the ._;tack and is consistent with the
stopping in the emulsion of electrons with energies up to about 600 key. There is a high
background of grains extending another 500 microns inward from the black edge and pos-
sibly due to the presence of soft X-rays.
The number of protons with energies greater than 100 Mev crossing the scan lines
was approximately equal to the number in the background area in the same energy range.
Therefore it is not possible, with the statistics available at this time, to make reliable
measurements of the spectrum above about 100 Mev.
The spectra obtained at the three points analyzed are shown in Figure 3. Three
things are apparent from these results:
1. At the higher latitudes, the slope of the energy spectrum below 40 Mev is very
steep when compared to predictions from conventional neutron albedo theory (References
3, 4 and 5).
2. The shape of the spectrum and the flux both chan!._e with latitude; points 3 and 5 are
at the same altitude and geomagnetic longitude.
3. At a comparable position in the belt (Point 5) the flux and the shape of the spectrum
agree with previous data (Reference 1).
A change in the slope of the spectrum at higher enel gies with a change in latitude has
been predicted by Singer (Reference 6) on the assumptio_l that the maximum energy of the
trapped protons decreases with altitudeowing to the bre:tkdown of the adiabatic invariance
of the magnetic moments of the trapped particles. Itis :tlsopossible that the change in
the spectrum at lower energies is due either (I)to solar protons trapped inthe geomag-
netic fieldor (2) to protons produced by the decay of albedo neutrons arising from solar
proton interactions over the polar cap. In either case there would be a preferential trap-
ping of low energy particles at the higher latitudes;but itisnot clear which of these
mechanisms is responsible for the change in the spectrum. A further study of these
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Figure 3- The differential energy spectra obtained at points 3, 4, and 5 on the trajectory,
together with the three spectra superimposed for purposes of comparison
emulsions will be made to determine the maximum energy at which protons can be trapped
at a given point. Additional flights are scheduled for late 1961 to determine whether there
is a time variation. If the low energy protons are associated with solar activity, their
flux should decrease as the number of solar proton events per year decreases during the
solar cycle. Changes in the spectrum after a large solar flare have been reported by
Armstrong and Heckman (Reference 2).
The unidirectional integral flux, J, for protons with E >_ 31 Mev is given in Table 1.
In order to compare this flux with the satellite measurements, the assumption has been
made that the flux is isotropic; therefore the integral omnidirectional flux can be obtained
by multiplying J by 4m The values obtained by this method are listed in Table 1 with the
best available value of the flux at the three points based on the analysis of the satellite
data (Reference 7). The quoted errors are large because of the effects of the background
on the integral flux; and the agreement is quite good considering these errors. The omni-
directional flux computed by assuming isotropy is consistently higher than the omni-
directional flux at all three points, indicating that the assumption is not valid.
The rocket nose cone was spin-stabilized nearly parallel to the magnetic field lines,
and the unidirectional flux in the emulsion was measured primarily at angles near 90 de-
grees with respect to the field lines. At low altitudes, where there is a high density of
mirror points, the flux should not be isotropic but should be higher at 90 degree angles,
as was indeed observed.
ACKNOWLEDGMENTS
The solution of the problems associated with the exT_osure and recovery of emulsions
at 1880 km required outstanding and unique contributions from a number of people. These
efforts are gratefully acknowledged by the authors. In particular, the flight would not have
been successful without the work, over and above his normal responsibility, of Project
Engineer Charles E. Campbell, of the Goddard Space Flight Center.
REFERENCES
1. Freden, S.C. and White, R.S., "Protons in the Earth's Magnetic Field," Phys. Rev.
Letters 3(1):9-11, July 1, 1959
2. Armstrong, A.H., Harrison, F.B., Heckman, H.H., aid Rosen, L., "Charged Particles
in the Inner Van Allen Radiation Belt," J. Geophys. ]_es. 66(2):351-357, February
1961
3. Freden, S.C., and White, R.S., "Particle Fluxes in the Inner Radiation Belt," J. Geo-
phys. Res. 65(5):1377-1383, May 1960
4. Singer, S.F., "Trapped Albedo Theory of Radiation t_elt," Phys. Rev. Letters 1(5):181-
183, September 1, 1958
5. Hess, W.N., "Van Allen Belt Protons from Cosmic-Ray Neutron Leakage," Phys. Rev.
Letters 3(1):11-13, July 1, 1959
6. Singer, S.F. "On the Nature and Origin of the Earth'+ Radiation Belts," in "Space Re-
search," Proc. 1st Internat. Space Science Sympos., Ed. by H. Kallman Bijl, Amster-
dam:North Holland Publ. Co., 1960, pp. 797-820
7. McIlwain, C.E. and Van Allen, J.A., State University of Iowa, Private Communication
!
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NASA - Langley Field, Va.




The Flux and Energy Spectra of the Protons in the Inner Van Allen Belt

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