We report an analysis of seven Apollo 14 Apollo Lunar Surface Experiments Package (ALSEP) Suprathermal Ion Detector Experiment (SIDE) "resonance" events from January 1972 through January 1973. The events appear to be associated with traversals of the Moon through the terrestrial bow shock

Collier, M. R.

Hills, H. K.

Stubbs, T. J.

NASA Technical Reports Server

LUNAR SURFACE POTEKTIAL CHANGES POSSIBLY ASSOCIATED WITH TRAVERSALS OF THE 
BOW SHOCK. M. R.. Collieri, H. K. ~ i l l s ~ ,  and T. J. ~tubbs',', 'NASA's Goddard Space Flight Center, Code 673, 
Greenbelt, Maryland 20771 (Michael.R.Collier@nasa.gov), "erot Systems Government Services, 8270 Willow 
Oaks Corporate Drive, Fairfax, Virginia 22031, ~ o d d a r d  Earth Science and Technology Center, University of 
Maryland, Baltimore County, Maryland 2 1228. 
Introduction: We report an analysis of seven 
Apollo 14 Apollo Lunar Surface Experiments Package 
(ALSEP) Suprathermal Ion Detector Experiment 
(SIDE) "resonance" events from January 1972 through 
January 1973. The events appear to be associated with 
traversals of the Moon through the terrestrial bow 
shock. 
ALSEPlSIDE Resonance Events: The Apollo 
ALSEPISIDE instrument made the first measurements 
of the potential of the lunar surface [I]. The SIDE in- 
strument contains a Total Ion Detector (TID) curved 
plate analyzer. To determine lunar surface potentials 
[ 2 ] ,  the instrument is equipped with a ground plane 
grid which makes contact with the lunar surface and 
whose potential with respect to a wire grid at the ion 
entrance aperture is stepped through a series of volt- 
ages ranging from zero to ~k27.6 V. 
Frequently, the SIDEITID data reveal narrow low- 
energy ion flux spectra that show a correlation with the 
ground plane stepper voltage. These events were 
named "resonance" events and allow a determination 
of the lunar surface potential. Fig. 1 shows an example 
of an Apollo 14 resonance event. At about 1000 UT on 
day 004 of year 1972, the 17 eV TID ion channel 
showed a dramatic count rate increase when the 
ground plane was at 27 V while there was no count 
rate enhancement in the 17 eV channel for the 16 V 
and 0 V ground plane settings (first panel). This indi- 
cates a lunar surface potential of about 10 V because 
the ground plane accelerates positive thermal ions by 
minus 27 V while a lunar surface potential of 10 V 
decelerates them so that they are observed only in the 
17 eV energy channel (27 V - 10 V). The second panel 
which shows a 7 eV energy channel resonance indi- 
cates a surface potential of 9 V (16 V - 7 V) similar to 
the surface potential inferred from the 17 eV channel. 
The third and fourth panels show both the IMP6 
and Apollo 12 Solar Wind Spectrometer (SWS) meas- 
urements [3] of the solar wind density and speed, 
respectively. Prior to the time of the event, the solar 
wind density and speed on the lunar surface do not 
correlate very well with the undisturbed solar wind 
observed by 1MP6. After event commencement, al- 
though the solar wind density observed on the lunar 
surface is a bit higher and the velocity a bit lower than 
in the undisturbed solar wind, the IMP6 and Apollo 12 
SWS measurements track each other much better. This 
suggests that the Moon transitioned through the bow 
shock from the magnetosheath into the solar wind at 
the beginning of the resonance event. 
Table 1 summarizes the seven Apollo 14 SIDE/TID 
resonance events analyzed in this study along with the 
geocentric solar ecliptic (GSE) coordinates of the 
Moon at the start of the event and the solar wind den- 
sity, velocity, and ram pressure. 
Coinparison with Bow Shock Position: Fig. 2 
shows the location of the Moon during the Apollo 14 
SIDE events plotted with typical magnetopause and 
bowshock predicted locations. The solar wind condi- 
tions used in the models were solar wind pressure of 
3.0 nPa, a solar wind speed of 400 km/s, a GSE-z 
https://ntrs.nasa.gov/search.jsp?R=20080031646 2019-08-30T04:55:56+00:00Z
/ e\ent 1 jcar 1 da j  I start 1 Lfuon 1 kfoon I \.toon 1 s\\ dct~sity 1 sv. speed / mtn prcss 
Table 1 - Summary of the seven ALSEPiSIDEiTlD resonance events analyzed in this study. 
component of the solar wind magnetic field of 0.0 nT 
and a density of 11.2 em". The magnetopause model 
used here is from Shue et al. [4] and the bow shock 
model is from Howe and Binsack [5] modified by 
Stubbs et al. [6]. As can be seen, the events correlate 
very well with the bow shock, with a spread that is 
easily explained by variations in the solar wind ram 
pressure and the other parameters. This behavior was 
noted by Freeman and Ibrahim [7 - see their Fig. 5 
which also shows that the lunar surface potential tends 
to be positive on the dayside and negative on the night- 
side]. 
Fig. 3 shows that the GSE-y position of the Moon 
at the beginning of the resonance events appears corre- 
lated with the solar wind ram pressure. This is consis- 
tent with an association between these events and tra- 
versals of the how shock as higher ram pressure will 
compress the bow shock leading to encounters at GSE- 
y values closer to zero. 
Possible Explanations: It has been known for 
Fig. 2 - The distribution of the ALSEP/SIDE events 
quite some time that the terrestrial bow shock acceler- 
ates energetic particles [e.g. 81, so it is tempting to try 
to explain the positive drift of the lunar surface poten- 
tial as due to the increased flux of suprathermal posi- 
tive ions resulting from shock acceleration. However, 
the suprathermal ions, although extremely energetic, 
have a very small density relative to the solar wind 
electrons and, as such, contribute very little to the cur- 
rent balance. It may very well be rhe case that secon- 
dary emission is the source of the increased potentials. 
References: [I]  Freeman, J.W., Jr., Fenner, M. A., 
and Hills, H.K. (1973) JGR, 78, 4560-4567. [2] 
Manka, R.H. (1 973) in Photon and Particle Interactions 
with Surfaces in Space, R.J.L. Grard (ed.), 347-361. 
[3] Neugebauer, M. et al. (1972) PSS, 20, 1577-1591. 
[4] Sbue, J.H. et al. (1998) JGR, 103, 17691-17700. [5] 
Howe, H.C. and Binsack, J.H. (1972) JGR, 77, 3334. 
[6] Stubbs, T.J. et al. (2004) JGR, 109, A09210. [7] 
Freeman, J. W. and Ibrahim, M. (1975) The Moon, 14, 
103-114. [8] Ipavich, F. M. et al. (1981) JGR, 86, 
4337-4342. 
1.00 3.50 2.80 2.50 3.W 3.50 4.00 4.50 
solar wind ram pressure during event (nPa) 
Fig. 3 - The solar wind ram pressure versus the GSE-y 
position of the Moon at the start of the resonance 
events. 


Lunar Surface Potential Changes Possible Associated with Traversals of the Bow Shock

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