A contention that Mercury possesses an intrinsic magnetic field sufficient to deflect the solar wind flow was confirmed by the Mariner 10 experiment. Predictions made as to the locations where characteristic bow shock and magnetopause boundaries may be observed were also confirmed

Behannon, K. W.

Lepping, R. P.

Ness, N. F.

Whang, Y. C.

English

NASA Technical Reports Server

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MAGNETIC FIELD OF MERCURY CONFIRMED
by
N. F. Ness, K. W. Behannon
R. P. Lepping and W. C. Whang*
Laboratory for Extraterrestrial Physics
NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771
*The Catholic University of America
Washington, D. C. 20064
April 4, 1975
Submitted to NATURE
MAGNETIC FIELD OF MERCURY CONFIRMED
N. F. Ness, K. W. Behannon
R. Po Lepping and Y. C, Whang*
Laboratory for Extraterrestrial Physics
NASA-Goddard Space Flight Center
Greenbelt, Maryland 20771 USA
*The Catholic University of America
Washington, D. C. 20064 USA
On 16 March 1975, the USA Spacecraft Mariner 10 achieved its third
and final encounter with the planet Mercury. This unique and fortuitous
event occurred because the heliocentric orbital period of Mariner 10 was
176 days, exactly twice that of the orbital period of Mercury, 88 days.
The principal objective of the third flyby was to confirm or reject the
suggestion from the first flyby on 29 March 1974 that Mercury may possess
a modest intrinsic magnetic field sufficient to deflect the solar wind
1-2 3flow . The quantitative analysis of the Mercury I data yielded an
estimate of the planetary magnetic dipole moment equal to 5.1xl022Gauss cm 3
and oriented 70 from the orbit normal. The spacecraft was severely
limited in its ability to function properly since Mercury I due to various
technical failures. However, a dedicated effort by the spacecraft
engineering team at NASA/JPL achieved the desired objective at Mercury III
of a very close darkside pass at the planet. This letter presents
preliminary results from the NASA/GSFC magnetometer instrumentation on
Mariner 10.
The flyby trajectory is shown in planetary centered solar ecliptic
coordinates in Figure 1. This is a preliminary trajectory and will be
updated with post encounter tracking observations when they have been
analyzed. The nominal miss distance at closest approach was approximately
323+8 km. For comparison, also shown in Figure 1 is the trajectory at
Mercury I encounter, for which the closest approach miss distance was
= 704 km. In the right panel, showing the view from the Sun, the much
higher latitude pass of the spacecraft during Mercury III is well
illustrated. This closer and more poleward targeting was chosen so as
to yield magnetic field measurements which would provide unequivocal
evidence regarding the nature of the planetary magnetic field at Mercury.
The Mercury I results were consistent with a reduced scale model of the
solar wind interaction with the terrestrial magnetic field, in which the
planetary field is compressed by the solar wind flow, which itself is
deflected around the planetand a detached bow shock wave forms. As a
result,the field is then confined to a region of space close to the
planet on the Sunward side and extended to form a magnetic tail on the
nightside.
Based upon the observations and analysis at Mercury I and assuming
similar conditions of solar wind momentum flux at Mercury III, predictions
were made of the locations where the characteristic bow shock and magneto-
pause boundaries would be observed. The position of these boundary
surfaces is presented in the left panel of Figure 1. Since the rotation
period of the planet is uniquely coupled to its orbital period, in the ratio
2:3, the "phase" of the planet relative to the planet-Sun line at
successive encounters was identical.
The observed positions of the bow shock and magnetopause
during Mercury III are also illustrated in Figure 1. They show excellent
agreement with the model boundaries, which used a scaling of the magnetic
moment of Mercury equal to 7c10- 4 that of the Earth's dipole moment. In
the case of the solar wind flux impinging normal to a dipole fie d, a
R B _ 1/6
theoretical study has yielded the relationship RMP = 1.07 ----
Rp L4TmnVJ
where R /R = the normalized radius of the stagnation point distance ofmp p
the magnetopause, Bo = equatorial dipole field intensity , and nmV2= solar
wind momentum flux (where n = number density, m = mass of proton and
V = velocity of solar wind). The distance to the magnetopause, as measured
by R is thus weakly dependent upon solar wind momentum flux and henceMp
even if variations of +60% in the solar wind flux occur, they would alter
the position of the characteristic bow shock and magnetopause boundaries
by only +10%.
The magnetic field measurements were accomplished with a
dual magnetometer5 system which sampled the vector magnetic field 25
times each second, with a precision of +.125y for each orthogonal component.
These data are to be considered preliminary in that they are accurate to
+1-2/ or +1%, which ever is larger, because they are derived from quick
look data provided in near real time by JPLo Six second averages of the
observed magnetic field throughout the encounter period are shown in
Figure 2. The occurrence of the well developed bow shock and magnetopause
boundaries are indicated and readily identified by a magnitude and/or
directional change as well as in the Pythagorean mean fluctuation character-
istic, R4S. The maximum magnitude of the field is 400y, 4 times larger
than that observed at Mercury I encounter and 20 times larger than the
interplanetary field, which is close to 207. This large value
precludes any reasonable possibility that the magnetic barrier to solar wind
flow is associated with a complex induction process occurring at the
planet. The direction of the magnetic field, as measured by latitude
E and longitude 0, is primarily towards che Sun (and planet) with
gradual changes along the trajectory within the magnetosphere boundaries.
This orientation is opposite to that observed at Mercury I encounter but
is perfectly consistent with the model of a Mercurian magnetosphere in
which the planetary field is represented by a centered dipole normal to
the solar wind flow and which is then highly distorted by magnetopause currents.
A preliminary spherical harmonic analysis of the data within the
magnetosphere has been conducted using the available trajectory. The
analysis assumes a centered planetary dipole field, i.eo,harmonic order
N = 1, as well as external field sources described by harmonic terms up
to order N = 2. The preliminary least squares fit yields a residual of
77 with the deduced parameteis for the planetary dipole listed in Table
I. These are compared with those values derived from Mercury I and are
seen to be in remarkably good agreement, considering the preliminary
nature of these results. The magnitude of the dipole moment is the same
within 10% and 'ts direction only 240 from that derived earlier. The
uncertainty associated with the present analysis precludes attributing
any significance to these small differences.
In summary, the observations at the third encounter with Mercury by
the NASA/GSFC magnetic field experiment have dramatically confirmed the
earlier tentative conclusion that Mercury possesses a modest magnetic
field intrinsic to the planet. The origin of this magnetic field is at
present uncertain. It may be associated with remanent magnetization of
portions of the planet, i.e.,material presently below its Curie point,
or it may be due to an active dynamo in the presumably large iron/nickel
core. It is difficult to construct a plausible sequence of events
leading to the model in which the Mercurian magnetic field is due to
remanent magnetization. Thus, we conclude tentatively that these
observations of an intrinsic magnetic field require the existence of
an active dynamo mechanism in the interior of the planet. Future detailed
and final studies of the Mercury III encounter results and intercomparison
with those from Mercury I encounter, as well as considerations of possible
interior structures,should yield significant insight into the present
physical state of Mercury.
We express sincere appreciation to Dr. James Dunne, JPL Project
Scientist on Mariner 10, and the JPL spacecraft team for their out-
standing effort in achieving the third encounter. We also appreciate
the special efforts of D. H. Howell, F. W. Ottens and R. F. Thompson
of the NASA/GSFC for their special efforts and prompt analysis of
these data
References
1. Ness, N. F., Behannon, K. W., Lepping, R. P., and Whang, Y. C., Science,
185, 151 (1974).
2. Ogilvie, K. W., et al., ibid, pg. 145.
3. Ness, N. F., Behannon, K. W., Lepping, R. P., and Whang, Y. C.,
J. Geophys. Res., 80, in press (1975).
4. Choe, J. T., Beard, D. B., and Sullivan, E. C., Planet. Space Sci.,
21, 485 (1973).
5. Ness, N. F., Behannon, K.-W., Lepping, R. P., and Schatten, K. W.,
J. Geophys. Res., 76, 3564 (1971).
6. Whang, Y. C., and Ness, N. F., J. Geophys. Res., 80, submitted (1975).
Table I
Comparison of Planetary Dipole Moments Obtained by
Spherical Harmonic Analysis of Magnetic Field Data
Mercury I Mercury III
29 March 1975 16 March 1975
Dipole Moment (Gauss cm 3) 5.1x10
2 2  4.8(+.5)x102 2
(350 7 R3) (330+307 Rm m
Tatitude (ME coordinates) -800 -760+5 0
Longitude (ME coordinates) 2850 90+300
Root Mean Square Residual 0.97 7y
Comment (Quiet Data Only) (Preliminary but
entire data set)
List of Figures
Figure 1 Mariner 10 spacecraft trajectory in Mercury centered
solar ecliptic coordinates. The left panel plots the
trajectory as distance, XSE
, 
from the dawn-dusk terminator
of the planet, versus distance from the planet-Sun line,
P Z The right panel presents the view from the
Sun with ZSE positive to north ecliptic pole.
Figure 2 Measurements by the NASA/GSFC Magnetometer on Mariner 10
during Mercury III encounter. Magnetic field direction
latitude , E, is referenced with respect to a plane parallel
to the ecliptic and longitude, 0, with respect to the planet-
Sun line reckoned positive relative towards the east limb
of the Sun.
YME- ZME MARINER 10
ALIGNED FLOW
=o10 MERCURY ENCOUNTERS
Cf - MAo =20+12- MAO I and Il
SCALED ME
SMAGNETOPAUSE
SOLAR \-+6 +6
---- S M
MP MP
r- WIND
XME MERCURY '
(D4 -4 10km -12 -6 +6 2 ME
BOUNDARY CROSSINGS
-6 B SHOCK (S) -6
\ MAGNETOPAUSE VIEW FROM SUN
(MP)
M ------ 16 MARCH 1975
\ I 29 MARCH 1974
ME+ ME
-14- 103 km
NASA- GSFC MAGNETIC FIELD -MARINER 10
BS MP CA MP BS
1000 - - T-- - -T-- --- -- W. 1000
500 I 500
BS-BOW SHOCK I
200 - MP- MAGNETOPAUSE 200
CA- CLOSEST APPROACH i l
100 100
7--I
50- 50
2 i -20
101
40 i-- - -40
RMS 20- 20
8 9I I _Ogoo--t .. 90°
200
'°°o' ------T r . Li ' _ . o
-90 _ 1_ _ _ _ _il
2210 2220 2230 2240 2250 2300 UT
16 MARCH 1975


Magnetic field of Mercury confirmed

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