Meyer, et al. have improved their original global crustal model and made a spherical harmonic analysis of the resulting magnetic field to n=50. The Z contours at 400 Km altitude from a field model composed of the first 15 degrees and order of their model and the terms n=16 to 29 from the MAGSAT model M051782 are presented. The main point to consider from such representations is that the lower order terms appear to contribute components comparable in magnitude to those of higher order. Thus, one should allow in making tectonic interpretations of global maps of anomalies such as those published by Langel, that there are likely continental scale (or smaller) features that have been removed along with the core field by the subtraction of the terms n=1 to 13 of the observed field. Planning for the analysis of data to be accrued by GRM should thus address this problem

Cain, J. C.

English

NASA Technical Reports Server

RECENT MAGSAT RESULTS
Joseph C. Cain
U.S. Geological Survey
DFC MS 964, P.O. Box 25046
Denver, CO 80225
Meyer, et al., (1983) have improved their original global crustal
model (Meyer, et al., 1984) and made a spherical harmonic analysis of
the resulting magnetic field to n=50. Figure I shows the Z contours at
400 Km altitude from a field model composed of the first 15 degrees and
order of their model and the terms n=16-29 from the Magsat model M051782
(Cain, et al., 1984). The main point to consider from such
representations is that the lower order terms appear to contribute
components comparable in magnitude to those of higher order. Thus one
should allow in making tectonic interpretations of global maps of
"anomalies" such as those published by Langel, et al., (1982), that
there are likely continental scale (or smaller) features that have been
removed along with the core field by the subtraction of the terms n=1 to
13 of the observed field. The view is also put forward by the paper by
Hahn, et al., (1984). Planning for the analysis of data to be accrued
by GRM should thus address this problem.
In the first determination of secular change from vector
satellite data, Cain, et al., (1983) observed that the small quantity of
observatory data available in 1981 for the Magsat analysis interval
(September 1979 - June 1980) had little effect on the model M061581.
Figure 2 is the secular variation from this model. Figure 3 is the same
projection from the IGRF80 model (Peddle, 1983) which is very similar
except for the north polar area. Cain, et al., (1983) concluded that
the high rate of change in the northern regions was due to the
increasing effect of the polar ionospheric currents as the Magsat orbit
lowered and the current intensity increased as the seasons changed from
northern winter to summer. Thus in order to obtain a model
representative of the secular variation from the Earth's interior it
would be necessary to model and eliminate the effects of these currents.
Pursuant to this elimination our group has collected and reduced,
with the help of the World Data Center for Geomagnetism, the hourly
values from some 60 magnetic observatories as depicted on Figure 4. The
nighttime (22-3h local time), quiet (Kp,<2) values were then corrected
for the Dst variation, the resultant points averaged for each day and a
linear least squares fit made. This correction assumed that Dst
represented the sum of the n=1 internal and external field at the
magnetic equator and that the i/e ratio was 0.28. Likewise, the similar
correction was made to the Magsat data as originally selected for the
M061581 model (and also used for M051782). The corrected Magsat data
were then combined with the new values of X, Y and Z for the 60 stations
and an adjustment made to M051782 to degree and order 10 in both spatial
92
https://ntrs.nasa.gov/search.jsp?R=19860003420 2020-03-20T15:46:49+00:00Z
and first time derivatives. In this instance, unlike in prior
derivations, the observatory SV data were weighted ten times heavier
than previously. These weights were originally taken as the standard
errors of the linear fits (Schmitz and Cain, 1982). An attempt was made
to increase the weights another factor of ten but the result was clearly
suprious since many cells began to appear in the S.V. maps.
This new model, M070284, thus retains all M051782 terms from n=11
to 29, but improves somewhat on the secular terms (Fig. 5). The
degradation of fit to the Magsat data was small, e.g., residuals of the
Magsat scalar data increased from 12 to 15 nT. A comparison of this
result is given in Figures 6, 7 and 8 for Bangui, Kakioka and
Novosibirsk observatories comparing the IGRF80 with M070284. These
plots are of the previously noted daily averages from each observatory
(solid lines) along with the linear least-squares fit (dotted lines) and
the model indicated (dashed lines). The adjustment required to plot the
model values through the middle of each curve is given by the numbers to
the right above each curve. This adjustment is mainly needed for the
neglected very high order terms that result from local magnetic
anomalies and, to a lesser extent, errors in the models. Generally,
these adjustments agree with the observatory "biases" determined by
Langel, et al., (1983). These plots illustrate that most of the M070284
computed secular variation matches the apparent SV for an observatory
better than does IGRF80. However, it would appear that the high
negative F remains for a part of the north polar area. Either there are
not enough stations, the data are too noisy, or the observatories also
reflect this high rate.
In lower latitudes the secular variation on Figure 5 generally
agrees with that of the IGRF80 except where the latter does not follow
the observatory trends. The increase of the secular variation degree
and order to n=10 allowed more detail in the Asian region.
It is not obvious from the analysis to date whether the
variations applied from the Dst corrected observatory data contain
trends due to the external field. On a statistical basis one could
determine the type of behavior that could be expected from quiet data
from annual distortions of the magnetospheric topology (Campbell,
1984). Such average results may or may not apply to this specific
year. Also, the Dst index used was determined only from four low
latitude stations (Sugiura, private communication, 1984) and the levels
subtracted in Figures 6-8 may contain some annual time. Figure 9 is a
plot of the Dst values averaged for a whole UT data selecting only hours
where Kp <2. It is surprising to find such large values even during
such periods. The month-to-month changes in the H levels could be the
result of the Dst correction applied. It is clear from such
uncertainties that a definitive SV for the Magsat interval yet awaits a
complete analysis of the external sources including their induced
components.
93
In summary, it would appear that the use of a mix of observatory
and GRM data even over less than a year intervals would allow an
accurate determination of global secular variation. However, a more
comprehensive analysis of the external currents would be required than
is presented herein.
References
Cain, Joseph C., Jeffory Frayser, Lorant Muth, and Dave Schmitz, The use
of Magsat data to determine secular variation, J. Geophys. Res.,
88, 5903-5910, 1983.
Cain, Joseph C., Dave Ray Schmitz, and Lorant Muth, Small-scale features
in the Earth's magnetic field observed by Magsat, J. Geophys.
Res., 89, 1070-1076, 1984.
Campbell, Wallace H., An external current representation of the quiet
nightside geomagnetic field level changes, J. Geoma$. Geoelectr.,
36, 449-457, 1984.
Hahn, A., H. Ahrendt, J. Meyer, and J.-H. Hufen, A model of magnetic
sources within the Earth's crust compatible with the field
measured by the satellite Magsat, Geol. Jb., (Hannover) A75, 125-
156, 1984.
Langel, R.A., J.D. Phillips and R.J. Horner, Initial scalar magnetic
anomaly map from Magsat, Geoph. Res. Lett., _, 269-272 (1982).
Langel, R.A., and R.H. Estes, The near-Earth magnetic field at 1980
determined from Magsat data, NASA Tech. Memo. 86133, 1984.
Meyer, J., J.-H. Hufen, M. Siebert, and A. Hahn, Investlgations of the
internal Geomagnetic field by means of a global model of the
Earth's crust, J. Geophys., 52, 71-84, 1984.
Meyer, J., J.-H. Hufen, M. Siebert and A. Hahn, On the identification of
Magsat anomaly charts as (the) crustal part of the internal field,
J. Geoph. Res., _ (paper 4B1171), in press, 1984.
Peddle, Norman W., International geomagnetic reference field - its
evolution and the difference in total field intensity between new
and old models for 1965-1980, Geophys., 48, 1691-1696, 1983.
Schmitz, Dave R. and Joseph C. Cain, Geomagnetic spherical harmonic
analyses I. Techniques, J. Geophys. Res., 88, 1222-1228, 1983.
94
90° N
60°N
30°N
30°S
60 oS-
Figure I. Vertical field derived from composite crustal model by
Meyer, et al., (1984) for n=I-15, plus terms n=16-29 from
model by Cain, et al., (1983). Contours in nanoTesla at
400 Km altitude.
95
90'N
', .',, .... ._,_. ., :_,:
.... H-.,/" ,"_' ,._ "-_ k_'_"-J _
! ',, . : ,'
,_, # : ..... ,o.,
go's ...... "'" "'- -......_ .......
18Q'w 12o'w 6o'w a' 6o'c 12o'{ IBO'{
LONGITUDE
Figure 2. 1980 secular variation in total field from the model
M051782 (Cain, et al., 1983).
....... - ._ .,'-.....'/,_" ii i,, ..- . .,
_'s .,. _....... "".......," ,: k _ "'-_.......... "'. " ......."" .,_
IDO'w 120'w 60°w O' 60"K 120°E IDo'E
¢,_ LONGITUDE
Figure 3. 1980 secular variation in total field from the 1980 IGRF.
96
90 oN
o Yg v o uR _
SIT omEA E [_NK OSVD ONV5 [R_
Vl ONEW B_H 5TJ Ha ° CURo OOE o NKOKGO O
O BOU O B E°_[]TIF Ta52 emB
o HON _%_X35JG LNP
i_) 80 GU8
(j2'
_° _6E o Pnr
o CZT
60 ° 5
90 oS
80o W 120° i4 60 ° N 0° 60 ° E 120oE 180oE
LONGITUDE
Figure 4. Magnetic observatories providing hourly values during the
Magsat analysis interval, September 1979 - June 1980.
_'. ,. -,, ,. ... -;............... _ ........ .-... -.
N '" _ " "' .... " i_:'_ "' " "
., _'. _, )
30,5 ..... ,, ._ ." .;.-
..'--... .__-_-.1".-.... ...:.,'_'.,,',,.'"
. " i{',,:,..'"" ._......_ &
\_' ",:-::i';::::.......,-
_o._ _'." ', ", k ".............. -,...... ,+"
I_01_ 120I_ 60'w O; 60'E 120'£ 180'£
_¢_ LONGITUDE
Figure 5. 1980 secular variation in total field from the model
M070284.
97
AD- -0
-3.100"
,y,: ",
T_
_--f
2 -3.175. " v-
= / ._.... _'-_'_"_" _',_-'_-_'--_'Y""_"_"....
-3.250
AH- -197
32160 •
c 3213s-/ - -' .... ;:,_.L,_ ,L
32110
AZ- 259
-9050 """" •
C -9070- ,.T,A:. : .. '
_ )._/"--_ ,-,,
-9090 """""""" ""-
SEP OCT NOV DEC JaN FEB MFIR RPR MFIy JUN
_-_s 1979-1980 ( IGRFSO )
AO- -0
-3.100 '
_,_,-,4j-¢-"j
-3.250
AH- -126
32160-
k/'
= " "F _ " _ .... r .........
32110
AZ- 212
-9050
?'ZZ._ . •
'_%,- -V - -V_- _." k3, f'V',,
C- 2--_ .... -\__ _ j \
-9090
5EP OCT NOV DEC JflN FEB NRR flPR NflY JUN
1979-1980 [ M070284)
Figures 6a-6b. Bangui quiet, nighttime, daily means (solid lines) best
fit (dotted lines), and model value (dashed lines) adjusted by
values over each plot. A = 5° .
98
AO- 0
-6.50
34640
5EP OCT NOV DEC JRN FEB MRR RPR MRY JUN
1979-1980 [ |GRF80 ]
AD- 0
-6.50
o_
o /:.- r ....... _--_-c_.___ _.... t_____,¢____c___,_.¢:__,_ .... I-&J'I--JL_--'Z-'-I _-_!
-6.60
&H- i
30210
t '
/ ^ , / ..........._k__.__.___,__V_:{:-v....: :- _t-"................
30170
AZ- -i01
34660
__,0_o::-_--_'-J-!";_!:-_-_---__--Vl-_%--_ii-"_'
N
34640
SEP OCT flOV OfiC JRN FEB MF_R RPR MRY JUN
1979-1980 {M07028'_ ]
Figures 7a-7b. Kakioka quiet, nighttime, daily means (solid lines),
best fit (dotted line) and model value (dashed lines)
adjusted by values over each plot. A = 26 ° •
99
60- LO
8.55
_o I I f
....... \ __L__}____%__<._=._L_ 4_V_,_y.
o- °.45...._j< -l:-,;6-b- _,____>;___i=......
8.35
&H- 148
17310
,_ ---l!-_,---,--T>_,-:i ..............
17260
AZ- -33
56480 -
56450
SEP OCT NOV DEC JRN FEB NRR RPR tIRY JUN
1979-]980 (IGRF80 }
8.55- &D- -0
__°.,5 - . 3........... -_\--,-_--_,:,,Jv_--i;-::;__rm
8.35
]73]0 &H- 189
h, ............<:
s ,,_o5...... :--i-/-?_-:i-__i-/--i_______<___,.,_A:_,,_' l _G__,,/
]7260
564eo &Z- -16
l_- / _< !66466-:._ :-:_--f_L_,_/f_,___,_ '-'-t- ----4_-_-.._
56450
5[P OCT NOV DEC JRN FEB MRR RPR tIRy JUN
1979-1980 {N070284}
Figures 8a-8b. Novosibirsk quiet, nighttime, daily means (solid
lines), best fit (dotted lines), and model value (dashed lines)
adjusted by values over each plot. A = 44° .
100
30
I
-I00 , i , , , i , , ,
SEP OCT NOV DEC JRN FEB IIRR RPR P1RYJUN
t 979- 1980 (5Ut I URAl
Figure 9. Average value of Dst for U.T. intervals for which
K <2 using the data from M. Sugiura.
P
101


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