1975-15
VOL. 80, NO. 25
JOURNAL
OF GEOPHYSICAL
RESEARCH
SEPTEMBER 1, 1975
SplashAlbedo Protons Between4 and 315 MeV at High and Low
Geomagnetic Latitudes
K.-P.
WENZEL
SpaceScience
Department
(ESLAB),European
SpaceResearch
and Technology
Centre,Noordwijk,
Netherlands
E. C. STONE AND R. E. VOGT
California Institute of Technology,Pasadena,California 91109
The differential energy spectrumof splashalbedo protons has been measuredat high geomagnetic
latitude near Fort Churchill, Manitoba, at three periodsof the solar cycle in 1966, 1967,and 1969and at
low latitude near Palestine,Texas, in 1967 by usinga balloon-bornesolid statedetectortelescope.We
observedsplash albedo proton fluxes between4 and 315 MeV of 81 + 11, 70 + 11, and 48 + 8
protons/(m2s sr) at high latitudein 1966,1967,and 1969and of 37 + 9 protons/(m2s sr) at low latitudein
1967. The decreasesfrom 1966 to 1969 are due to solar modulation of the cosmicray parent nuclei. The
albedospectrumshowsa similarshapefor both latitudes.The differencein intensitycan be explainedby
different local geomagneticcutoffs;i.e., a significantcontributionto the splashalbedo flux arisesfrom
primary particleswith rigidity below4.5 GV. The splashalbedoflux near Fort Churchillis consistentwith
corresponding
fluxespreviouslyreportednear 53ø-55øN.The flux below 100 MeV near Palestineis
significantlylower than that reported by Verma (1967).
As part of a program to study low-energycosmic ray
protons
and a particleswith a balloon-bornedirectionalAE
Primary cosmic ray nuclei entering the earth's atmosphere
range
detector
telescopewe have measuredthe differential
interact with air nuclei and producesecondaryparticles.Some
energy
spectrum
of splashalbedo protons between4 and 315
of these secondariesmove upward and emerge from the atMeV. We report the results of measurementsnear Fort
mosphereas 'splashalbedo.'The chargedsplashalbedopartiINTRODUCTION
cles with rigidities below the local geomagneticcutoff cannot
Churchill, Manitoba, in 1966, 1967, and 1969 and near
Palestine, Texas, in 1967.
escapefrom the earth. They spiral along magneticfield lines
and reenterthe atmospherein the oppositehemisphere.These
INSTRUMENT
particlesconstitutethe 'reentrantalbedo.'The albedoproton
The observationswere made with a directional energyloss
spectrumis relatedto albedoneutronswhichmay be a source
rangesolid statedetectortelescopewhich is physicallysimilar
of protonstrapped in the inner radiation belt (seethe recent
to one of the California Institute of Technology Ogo 6 detector
review by White [1973]).
No comprehensive
measurements
of the spectrumof splash
albedo protons exist. Most observersquote only flux values
over broad energy intervals rather than detailed energy
spectra.McDonaldand Webber[1959] determinedthe sum of
splashplus reentrant proton albedo in the 100- to 350-MeV
energy region at geomagneticlatitudes of 4ø, 53ø, and 55øN
without being able to distinguishbetweenthe two. Estimates
of the reentrant proton albedo [Ormes and Webber, 1964;
Bingham et al., 1968; Teegarden,1967] and of the splash
proton albedo[McDonald, 1958;Hasegawaet al., 1965; Webber, 1967] are discussedfor latitudes mostly about 53ø-55øN
and energies above 70 MeV. Ormes and Webber [1968]
observedreentrant albedo protons between90 and 600 MeV
with a Cherenkov scintillation counter telescopenear Ely,
Minnesota, and Fayetteville, Arkansas. Verma [1967]
observed the vertical splash and reentrant proton albedo
spectrabetween37 and334 MeV nearPalestine,
Texas,with a
systems.The balloon-borneand Ogo 6 instrumentshave
previouslybeendescribed[Althouseet al., 1968;Wenzel,1968,
1970;Garrard, 1972].A crosssectionof the AE rangetelescope
is shownin Figure 1. The energylossAE is measuredin each
of the solid state detectors D1, D2, and D3 by using 256-
channelpulseheightanalyzers.The rangeof an incidentparticle is determinedby the numberof detectorsD4-D7 whichare
penetrated by the particle. The absorbers A2-A6 are
sandwiched between detectors D2-D7.
The entire detector-
absorberstack is surrounded,exce12tfor the entranceand exit
apertures,by a plasticscintillatoranticoincidencecup D8. D1
is a totally depleted100-•tm-thicksurfacebarrier detector,and
D2-D7 are lithium-drifted detectorswith depletion depthsof
• 1000 tzm.
The discriminatorson range detectorsD4-D7 have a low
and a high threshold.The discriminationthresholdsare set so
thatthelowdiscriminators
will triggeron anycharged
particle
which penetratesthe detector,while the high discriminators
AE range detector telescope. Pennypackeret al. [1973]
will
triggeronly on protonswhichhave an energyof lessthan
measuredthe splashalbedo proton spectrumbetweenabout
•300 MeV at the top of the detectorstack(or on any particle
110 and 1200 MeV with a magnetic spectrometer near
with Z > 1). This feature aids in distir•guishingelectrons,
Palestine. The only calculation of the intensity of albedo
muons, and interacting protons. A second, higher-energy
protonshas beenpublishedby Ray [1962, 1967].No previous
detectorsystemmeasuringthe energylossof a particlein solid
measurementsof splashalbedo protons near Fort Churchill
state detectorsand its velocity in a Cherenkov radiator was
have been reported.
Copyright¸
1975 by the American GeophysicalUnion
flown in the same instrument, but its results will not be discussedin this paper.
358O
WENZEL ET AL.' SPLASH ALBEDO PROTONS AT HIGH AND LOW LATITUDE
The signaturesof the interactingevents were also determined. In the caseof the nominal R7 events,25% triggeredthe
anticoincidenceD8, and 1, 5, 8, and 9% had range signatures
of R3, R4, RS, or R6, respectively.These interactingevents,
which contributeto the background,were further suppressed
in the flight data by requiringthat the appropriatehigh-level
discriminatorson D3-D7 be triggeredand by requiringthat
the energylossesmeasuredin D2 and D3 be consistentwith
the observedrange. By meansof thesetechniquesthe average
interactionbackgroundsubtractionwas • 15%for the splash
albedo flight data.
In order to obtain more spectralinformationwe subdivided
the R4 interval, which correspondsto the relatively wide
energy interval of 48-157 MeV, into three intervals (R4A,
R4B, and R4C) as indicatedin Table 2. This subdivisionwas
basedon the measuredenergyloss in D2 and D3, which was
•2.6 MeV in each for 48-MeV protons, decreasingto • 1 MeV
for 157-MeV protons.
Details of the data analysisprocedurehave been described
by Wenzel[1968, 1970] and Garrard [1972].
GEOMETRICAL
FACTOR
(CM2'SR)
Ul-2 1.521
D2-3 1.76 I
D2.-4 I.;56 I
D2-5 0.761
D2-6 0.451
,D2-7 0.201
ABSORBER
THICKNESS
(G.CM
-2)
A2 AL
0.07
A3 W
2.9
A4 W 27.5
A5 W 31.0
A6 W 38.7
PM
RCA
3581
4439
RESULTS
Fig. 1. Schematic
crosssectionof the detectorsystem.
BALLOON
FLIGHTS
We report results from four series of balloon flights
launched from Fort Churchill, Manitoba, in July 1966, June
1967, and June 1969 and from Palestine,Texas, in April 1967.
In all of theseflightsthe sameinstrumentwas used,and the
detectorsystemwas pointed toward the antizenith.A summary of the pertinentflight data is presentedin Table 1. The
Mount Washingtonneutron monitor rates (J. A. Lockwood,
privatecommunication,
1969)arethe averaged
hourlyvalues
for the time periods when the balloons were at floating
altitudes.
DATA
ANALYSIS
A valid particle event is describedby two pulse heights
(eitherD 1 and D2 or D2 and D3), a rangeparameter,and discriminator parameters.Thesedata are convertedinto energy
spectraby usinga telescoperesponsematrix basedon proton
calibrations obtained at the Space Radiation Effects
Laboratory as describedin detail by Garrard [1972]. The
calibrations,whichwereperformedat sevenenergiesfrom 114
to 569 MeV and at sevenanglesof incidence,provided accurate estimatesof the effectsof nuclear interaction processes
on the telescopedetection efficiency. For example, the
observedrange was correctfor 97 + 1%, 86 + 1%, 65 + 1%,
and 52 + 1% of the protonswhichshouldhave had nominal
R4, R5, R6, or R7 signatures,respectively.
TABLE 1.
The proton splash albedo intensity shows no significant
variations with altitude above •50 g/cm •'. As an example the
4- to 315-MeV proton ascentdata of two flightsat Palestine,
Texas, which were combined in order to obtain better
statisticalaccuracy,are shown in Figure 2, broken into discrete altitude intervals. The absence of intensity variations
above50 g/cm•'is aswasexpected,sincethe splashalbedoflux
consistsof particles produced in a large thickness of atmospherebelow the instrument. We shall thereforeassume
for all flights that the energy spectrumof the splashalbedo
observedat float altitudes also representsits spectrumat the
top of the atmosphere.
In Table 2 we summarize the results of the splashalbedo
flightsfrom Fort Churchill in 1966, 1967,and 1969and from
Palestine in 1967. Since the Mount Washington neutron
monitor rates(which reflectthe parent nucleiintensity)during
the two Fort Churchill flightsin 1966were nearly identical,we
averagedthe resultsof both flights.Similarly,the two Palestine
flightsweresummed.The total numberof protonsobservedat
floatingaltitude in eachrangebin is listed.We alsopresentthe
observednumber of protons that penetrated the complete
detector-absorberstack (E > 315 MeV). Most of these are
downwardmovingprimary protons.In the lowerpart of Table
2 we list the integral proton and alpha flux above 315 MeV/
nucleon derived from flights during the same observation
periods with the detector system pointing upward. These
primary fluxesare a measureof the level of solar modulation
of the primary cosmicray spectrumresponsiblefor the albedo
production.
Balloon Flights
Mount Washington
Neutron
Monitor Counting
Launch
Site
Fort Churchill
Fort Churchill
Palestine
Palestine
Fort Churchill
Fort Churchill
Average
Floating
Floating
Altitude,
Time,
hours
Date
Rate
g/cm 2
July 3,1966
July 26,1966
April8,1967
Aprill7,1967
2365
2364
2262
2307
2292
2120
2.2
7.5
2.1
10.5
Launch
June25,1967
June25,1969
4.5
4.1
1.7
3.7
3.0
9.7
13.8
14.2
3582
WENZEL ET AL.: SPLASH ALBEDO PROTONS AT HIGH AND LOW LATITUDE
TABLE 2.
SplashAlbedo Proton Flux as a Function of Energy for Three Flight Periodsin Fort Churchill in 1966, 1967, and 1969 and in
Palestine in 1967
Fort Churchi]l
1966
Energyat
Detector,
No.
of
MeV
Range
R2
R3
R4A
R4B
R4C
SplashAlbedo
ProtonFlux,
protons/m2s sr
Protons
3.8-18.5
18.5-47.5
47.5-80
80-113
113-156.5
R5
156.5-235
R6
R7
235-315
>315
92
195
98
90
76
1967
MeV
No.
of
SplashAlbedo
ProtonFlux,
protons/m2 s sr
Protons
MeV
No.
of
0.46 4- 0.05
0.42 4- 0.06
0.27 + 0.04
0.45 4- 0.05
0.36 4- 0.06
0.16 4. 0.04
40
103
54
61
28
19
0.08 4. 0.02
15
0.08.4- 0.02
10
4
1029
0.04 4- 0.02
4
781
0.05 + 0.02
25624-62
200 4- 15
0.69 4- 0.08
0.56 4- 0.05
I
598
2046+ 84
202 4- 17
Palestine, 1967
SplashAlbedo
Proton Flux,
protons/m• s sr
Protons
68
128
77
61
39
Primaryprotons
>315
Primaryalpha >(315 MeV)/n
0.76 4- 0.08
0.69 4- 0.05
1969
MeV
No.
of
SplashAlbedo
Proton Flux,
protons/m•s sr
Protons
MeV
0.39 4- 0.06
0.43 4- 0.04
32
57
0.34 + 0.06
0.26 + 0.04
0.29 + 0.04
0.36 + 0.05
0.09 ñ 0.03
34
27
28
0.21 4- 0.04
0.16 4. 0.04
0.14 4. 0.03
0.05 + 0.02
8
0.04 + 0.02
0.01 + 0.01
3
321
0.04 4- 0.02
1406+ 48
129+ 11
642 4-48
105+ 21
particles
The table includesfor comparisonthe integralfluxesof primaryprotonsand alphaparticles(in unitsof particles/m
• s sr) above315
MeV/nucleon for the same flight periods.
The splash albedo proton spectra measured near Fort
Churchill are comparedin Figure 3. The shapesof the spectra
in all 3 years of observationare very similar. The percentage
changein the albedo is comparedin Table 3 with changesin
other cosmic ray fluxes. As is indicated in this table, the
changes in the splash albedo intensity are comparable to
changesin the >320-MeV proton fluxesdue to solar modulation. Primaries between 0.3 and 2 GeV seem to make significant contributions to the albedo production.
The splashalbedo spectra near Fort Churchill, Manitoba,
and Palestine,Texas,are comparedin Figure 4. The spectraat
the two latitudesare very similarin shape.They areessentially
flat at low energies(about 10-40 MeV) and can be described
by a power law E -• with 'V -• 2 above about 100 MeV. There
is, however,a differencein intensityat lower energiesof about
primariesof energiesabove the cutoff at Palestine(•4.5 GV)
and that therefore the albedo intensity would become less
latitude dependenttoward higher energies.The higher intensity at Fort Churchill indicatesthat primary cosmicrays below
the 4.5-GV cutoff at Palestinecontribute significantlyto the
production of splashalbedo protons.A similar conclusionwas
reached by Israel [1969] for splash albedo electrons.
No previous measurementsof splash albedo protons at
latitudesabove •60 ø have beenreported. But severalobservations at geomagneticlatitudes of about 53ø-55øN exist. Since
primariesbelow a few hundred MeV do not contributesignificantly to the albedo production, as can be seenfrom Table 3,
albedo observationsat latitudes below a cutoff rigidity of
about 1 GV (450-MeV protons) should be essentiallylatitude
independent. Thus observations at geomagnetic latitudes
above about 55ø should be comparable to our high-latitude
data (Figure 5a).
Hasegawaet al. [1965] measuredthe integral splashalbedo
proton flux in 1959 near Sioux Falls (Xm = 53.5øN), using
emulsions. Their flux of 0.15 ñ 0.01 proton/(m • s sr MeV)
between66 and 300 MeV is an upper limit, sincetheir data are
a factor of 2 between the two latitudes. Above • 100 MeV the
not correctedfor sidewardparticles producedin the at-
We observedsplashalbedoproton fluxesbetween4 and 315
MeV of 81 ñ 11, 70 ñ 11, and 48 + 8 protons/(m• s sr) at Fort
Churchill for 1966, 1967, and 1969. The flux near Palestine in
1967 was 37 ñ 9 protons/(m• s sr).
DISCUSSION
difference
is lessobvious.
It canbeassumed
thatthe•plash mosphereand packing materialsduring ascent.It
albedo particles of higher energy,are mainly produced by
is in agreement with our average flux for the 70- to 315-MeV interval
ioo
i
z
o
i-
u")
o
z
o
i-
o
x
x
[![t]tll
Ii
i
•00
t
IO
i0
20
30
40
50
HIL
KINETIC
I02
I03
ENERGY (MeV)
ATMOSPHERIC
DEPTH(G/CM
2)
Fig. 2. Flux of splashalbedoprotonsbetween4 and 315 MeV near
Fig. 3. Differentialenergyspectraof splashalbedoprotonsnear Fort
Palestine,Texas, as a function of atmosphericdepth.
Churchill, Manitoba, in 1966, 1967, and 1969.
WENZEL ET AL.' SPLASHALBEDO PROTONSAT HIGH AND LOW LATITUDE
I
,11,
I
]
i
i
[ i 1 , i[
I
i
,
] i •
3583
TABLE 3. Comparison of Intensity Changes in the Albedo and
Primary Fluxes
Intensity Changes,%
Component
o
I-o
Splashalbedo
Primary protons
1967 FT. CHURCHILL
1967 PALESTINE
Energy
4-315 MeV
70-320 MeV*
> 320 MeV*
> 2 GeV•
1966-1967
1966-1969
-14
-48
-20
- 8
-41
-81
-46
- 28
x
IO
KINETIC
Fig. 4.
*Garrard [1972]; see also Garrard et al. [1973].
]'Basedon changesin the Mount Washington neutron monitor rate
and the regressioncurvesby Ormes and Webber [1968].
IO2
ENERGY (MeV)
Differential energyspectraof splashalbedoprotonsnear Fort
Churchill
and Palestine
in 1967.
CALCULATION
OF ALBEDO
SPECTRUM
The intensity of secondaryprotons created in nuclear interactionsof primary cosmicray particles(protons and a particles with air nuclei) and subsequentlytraveling through the
atmosphere gyrating about magnetic field lines and losing
energyby ionization in air was calculatedby Ray [1962, 1967],
between 100 and 150 MeV from their data. Webber [1967] including the effects of nuclear interactions.
derived a splashalbedo proton spectrumbetween80 and 300
We have made a number of improvementsin the approxMeV at 5-g/cm2 atmosphericdepth near Minneapolis from imations and functional relationshipsused by Ray and have
measurementsof differentinvestigators.Sinceextrapolationto numerically integrated the resulting equation [cf. Ray, 1967,
the top of the atmospheredoes not produce any significant equation (4)]. The calculated albedo spectra were, however, a
change, this spectrum may be compared directly with our factor of 2 (at E < 100 MeV) to 4 (E > 100 MeV) higher than
spectrumat Fort Churchill.Both spectraagreerelativelywell, the observedspectra.This discrepancyis most likely due to unat least below 150 MeV.
certainties in using the Bristol data [Camerini et al., 1950,
In Figure 5b we compare our splash albedo spectrum 1951; Powell et al., 1959] to describethe angular distribution
measurednear Palestine,Texas, in April 1967 with that of and number of secondaryprotons produced by a nuclear inVerma [1967] measuredin May 1965and that of Pennypacker teraction.
et al. [1973] measuredin 1970-1971. All spectraagree above
We believe that the measurements present the correct
about 100 MeV within the experimental errors. Below 100 spectrumand that improved calculationsof the albedo proton
MeV, however,Verma's spectrumcontinuesto rise, while our spectrumshould be basedon a Monte Carlo techniqueusing
spectrumflattens.The 2-year time differencebetweenthe two detailed nuclear cross-section
data for protons in air.
flightscannotbe responsible
for the discrepancy
in the spectral
Acknowledgments. We thank T. L. Garrard for his assistance,esshape.Differencesin instrumentalresolutionand background
pecially during the data analysis. We are very grateful to W. E.
may be important; the present instrument was specifically Aithouse for the design of the electronicsand his support during
designedfor high-resolution, low-background measurements measurements. J. A. Lockwood kindly supplied the Mount
(Figure 5a). Ormesand Webber[1964]measuredthe spectrum
of splashplus reentrantalbedoand the spectrumof reentrant
albedo in Minneapolis (cutoff 1.3 GV) in 1963.By subtraction
we deduceda splashalbedoflux of 0.17 proton/(m2 s sr MeV)
below
•45
MeV.
Washingtonneutron monitor data. Most of this work was performed
111111
+
(0)
HIGH
LATITUDES:
•
r--I HASEGAWAET AL. [1965],SIOUXFALLS
ORMESANDWEBBER[1964],
MINNEAPOLIS
PALESTINE,
TEXAS:
VERMA [1967]
• PENNYPACKER
ET
AL.
[1973]
•'• WEBBER
[1967],
MINNEAPOLIS
Io
(b)
,.• THIS EXPERIMENT
"• THISEXPERIMENT,
FT.CHURCHILL
10-3
I0
IO
102
KINETIC
ENERGY
IO2
IO3
(MeV)
Fig. 5. Differential energyspectraof splashalbedo protons (a) at high latitudesand (b) near Palestine.
3584
WENZEL
ET AL.: SPLASH ALBEDO PROTONS AT HIGH AND LOW LATITUDE
during a stay of one of the authors(K.P.W.) at the California Institute
of Technology,supportedby a NASA International Fellowship. It
wascompletedaspart of a Ph.D. thesisat the Universityof Heidelberg
in Germany. This research was supported in part by the National
Aeronauticsand SpaceAdministration under grant NGR-05-002-160.
The Editor
thanks
W.
R. Webber
and R. S. White
for their as-
sistancein evaluating this paper.
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(Received October 1, 1974;
revised February 14, 1975;
accepted March 6, 1975.)