The search-coil magnetometer carried on Pioneer I has yielded evidence of complex geomagnetic behavior at great distances from the earth. This paper is intended to report only some preliminary observations; in particular, what appears to be directional instability in the field. A comprehensive statistical analysis, to be reported later, is still in progress

Judge, D. L.

Kelso, J. M.

Sonett, C. P.

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N7 47 99 
Aoovsr. Jous&i. or GEOPRT6ICAL RazARvH	 \oz.uii 4, No. 8 I
Evidence Concerning Instabilities of the 
Distant Geomagnetic Field:

Pioneer I 
C. P. SONET'r, D. L. JUDGE, AND J. M. KELSO 
Space Technology Laboratories, Inc. 
,) ()	
Los Angeles 45, California 
Abstract—The search-coil magnetometer carried on Pioneer I has yielded evidence of 
complex geomagnetic behavior at great distances from the earth. This paper is intended to 
report only some preliminary observations; in particular, what appears to be directional 
instability in the field. A comprehensive statistical analysis, to be reported later, is still in 
progress. 
Preliminary results of the flight of Pioneer I * 
have indicated that certain instabilities appar-
ently occur in the geomagnetic field at great 
distances from the earth. A comprehensive sta-
tistical analysis of the data is in progress. Since 
the results of this analysis are pending, it was 
considered desirable to present qualitatively 
some of the characteristics of the data at the 
present time. 
The magnetometer which was used to obtain 
the data consisted of a search coil wound about 
a nickel-iron alloy core. The emf generated by 
the search coil was applied to an amplifier 
which had a center frequency of 2 cps and a 
passband 2 cps wide. The dynamic range of the 
amplifier was approximately 3000. The 2-cps 
spin rate of the vehicle was utilized to generate 
the emf, the coil being fixed in the vehicle's 
frame of reference. 
As a consequence of the low frequency, the 
narrow passband, and the large amount of auto-
matic gain control, the transient response for 
the amplitude excursions highly modified the 
data. On the other hand, the phase transient 
response time was considerably shorter than the 
time associated with a number of phase shifts 
which have appeared in preliminary examina-
tion of the data. The geometry for the apparatus 
is indicated in Figure 1 where XYZ is an iner-
tial frame centered in the vehicle and X'Y'Z is 
a rotating frame fixed in the vehicle. 
Since the rocket was spin stabilized, the orien-
tation of the spin axis was fixed inertially. The 
distance from the center of the earth for the 
particular discontinuity which is shown in Fig-
ures 2 and 3 is 84,000 km. The geocentric lati-
tude was 5° and the orientation of the spin axis 
was approximately 85° to the dipole field di-
z
VI 
V 
x
Fic. 1—The magnetometer frame X'Y'Z referred 
* This program was carried out under the direc- to an inertial set XYZ. The spin axis of the vehicle 
tion of the National Aeronautics and Space Ad- is colinear with Z and the search coil is mounted in 
ministration,	 the plane X'Z. 
941
942
	 SONETT, JUDGE, AND KELSO
0.2 
_lio-
-110-4 
i10-5 
0 
Fia. 2—A typical magnetometer recording showing a signal zero associated with large phase change 
(between arrows), signal oscillations, and rapid changes in signal level. The magnetometer trace shows 
a characteristic spin rate. The telemeter rf signal strength is shown below, so as to serve as a phase 
reference for the magnetometer. The scale for the magnetic field is indicated at the right-hand side, 
the units being oersted (emu). This record represents the signal at approximately 86,300 km from the 
earth's surface. 
rection; thus, within the error assignment for 
the experiment, the magnetometer, at this dis-
tance, measured the total field. The velocity of 
the vehicle at this time was 2.2 km/ sec. 
The received data shown in Figures 2 and 3 
consists of the spin-rate sinusoid corresponding 
to H1 . Comparison of the phase of H1 with the 
rf signal-strength spin modulation can be made. 
Referring to Figure 1, we note that amplitude
excursions of H1
 would be explained by changes 
in II, the polar angle 8, or a combination of 
the two. In Figure 2, the arrows isolate a zero 
with which is associated a phase change (rota-
tion of ) of about r radians. Because of the 
amplitude zero, this appears to be a combina-
tion of a rotation of H through the spin axis 
coupled with a change in HI. The asymmetric 
form of the signal to the right of the zero would 
10-s 0 
Fin 3—A time-expanded section of Figure 2 (the region about the indexing arrows) showing clearly

the phase change of about r radians associated with the zero. The scale at the right of the diagram is

the same as for Figure 2.
INSTABILITIES OF THE DISTANT GEOMAGNETIC FIELD 	 943 
imply fa.st (r < ½ sec) changes in either or 
both HI and e. Preliminary examination of the 
data has not disclosed large phase changes oc-
curring in this time. 
Figure 3 is a more expanded version of the 
data between the arrows. This figure distinctly 
indicates the phase changes discussed in the pre-
vious paragraph. The amplitude excursions are 
accentuated by the amplifier gain character-
istics; and thus the time fluctuations, if real, 
are not as extreme as shown. The removal of 
amplifier effects from the data is planned as a 
part of the computer program. 
The generation of hydromagnetic waves at the 
geomagnetic field termination due to a magnetic 
instability associated with a coronal wind has 
been proposed by both Biermann [1957] and 
Hoyle [1956]. Slow variations in amplitude are 
seen in Figure 2. The periods were typical for 
the region of 13 to 15 earth radii. The day of 
firing (October 11, 1958) was extremely quiet 
magnetically. A proton magnetometer at Palo 
Alto disclosed surface fluctuations of about 10-8 
gauss rms (A. J. Dessler, private communica-
tion). Extrapolation of these values to about 13 
earth radii would yield wave amplitudes con-
sistent with those obtained [Dessler, 1958]. It 
has been suggested that 10 earth radii represent 
a suitable geomagnetic-field coronal-wind in-
terface for a quiet day, though the region of 
instability sampled by Pioneer I is much broader 
than that suggested by J. A. Simpson (private 
communication). A further flight will be neces-
sary to determine the boundaries of the ob-
served phenomena, if, in fact, an outer bound-
ary exists. The possibility that some or all of 
the fluctuations represent a deep and random 
infusion of interplanetary gas into the geomag-
netic field is being explored. A detailed statistical 
analysis of the available data is being prepared 
at the present time. 
Some of the amplitude diseontinuities in the 
data appear to occur in times limited by am-
plifier response. These would be attributable to 
compressional waves. Sudden phase changes as-
sociated with the above amplitude discon-
tinuities may enable longitudinal and transverse 
wave motion to be sorted out, since Alfvén 
waves would be expected to show an abrupt 
phase discontinuity. 
The following conclusions can now be drawn: 
1. Rotations of the magnetic intensity vector, 
through both large and small angles, appear for 
the distant field. At least some of those asso-
ciated with large angular excursions have char-
acteristic times of the order of 10 seconds. 
2. Almost periodic oscillations in amplitude 
occur (sometimes accompanied by rotational 
changes) having lifetimes of 2 to 5 cycles and 
periods of the order of 10 seconds. 
REFERENCES 
L. BIERMANN, Observatory, 77, 109, 1957. 
F. HOYLE, Suggestion concerning the nature of 
the cosmic-ray cutoff at sunspot minimum, Phys. 
Rev., 104, 269-270, 1956. 
A. J. DESSLER, Large amplitude hydromagnetic 
waves above the ionosphere, J. Geophys. Re-
search, 63, 507-511, 1958. 
(Manuscript received April 2, 1959.)


Evidence Concerning Instabilities of the Distant Geomagnetic Field: Pioneer I

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