The magnetic disturbance fields caused by global external current systems are considered with particular emphasis on improving the understanding of the physical processes which control high latitude current systems. Following processing the MAGSAT data were routinely plotted in the Universal Time (UT) format as well as in a polar plot format. The H'D'U' coordinate system, was adopted as the standard for representing the MAGSAT residual magnetic field vectors. A data file was generated and the TPOLAR computer code was developed to determine from the orbital elements, the time, latitude, and MLT of the extremum latitude of each transpolar segment of orbit. The precision of the vector data set from MAGSAT prompted an extended exploratory phase for data analysis procedures, modeling techniques and phenomenology

Burrows, J. R.

Hughes, T. J.

Wallis, D. D.

Wilson, M. D.

English

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Combined Progress Reports #2 and #3
Studies of High Latitude Current
Systems Using Magsat Vector Data
Investigation 01-40
R E C E1 V J J. Ronald Burrows PI
T.J. Hughes	 COI
DEC ?^ MY	 D. D. Wallis
	
Col
$IS 1902.6	
Margaret D. Wi l son CoI
I"-DYc Herzberg Institute of Astrophysics
7YPf ^t National Research Council of Canada
Ottawa, Canada K1A OR6
Period Reported: November 16, 1980 - August 31, 1981
I1. Introduction
The objective of our investigations is to analyse the magnetic dis-
turbance fields caused by global external current systems with particular
emphasis on improving the understanding of the physical processes which control
high latitude current systems. These objectives and approach have been outlined
in detail in Triennial Progress Report #E 1 (December 1, 1980).
Work towards these objectives has progressed satisfactorily, as
outlined below.
2. Techniques
2.1 Data Display
Following processing of the data as described in Report #1, the
data is routinely plotted in the Universal Time (UT) format (Figure 1) as well
as in a polar plot format (Figure 2). These plots have now been standardized
to consist of the data in the H'D'U' coordinate system (see below). The polar
plot of the H'D' components also has a panel showing the U' component. Both
standard plots are labelled with M Kp, Dst and the X, Y and Z components of
the interplanetary magnetic field (IMF) corresponding to the time of the highest
latitude of that transpolar segment of the orbit.
2.2 Coordinate System
The H'D'U' coordinate system has been adopted as the standard for
representing the Magsat residual magnetic field vectors (i.e., the measured field
- GSFC MGST 06/80 model field). This system is defined as follows:
Let E = unit vector in the direction of the eccentric dipole axis derived from
the value of the coefficients g°, gi and hi in the MGST 06/80 model.
Let U' = unit vector tangent to the MGST 06/80 model field at the point of obser-
vation. U' is antiparallel to the field in the southern hemisphere,
parallel to the field in the northern hemisphere.
Let Z' = unit vector from the point of observation to the eccentric (i.e., offset)
dipole origin derived from the coefficients g2, g 2 and h2i n the MGST
06/80 model.
Then H' = U' x (Z' x E); note that H' lies in the meridional plane of the eccen-
tric dipole and is perpendicular to the model field vector. H' always
has a northward component, but is not usually horizontal.
D'
	
	
x H'; D' is always eastward directed but is not necessarily horizontal,
because of the declination of U'.
!
	
	 It is felt that a field-aligned system is more natural than a verti-
cally aligned system for spacecraft data because of the sensitivity of high
altitude magnetometers to field -aligned current (FAC).
4
22.3 Data Selection
It has become necessary to select subsets of the Magsat data set for
particular studies. For example, to date, only hourly averages of the I14F from
the ISEE-3 satellite have been available. It has been found in preliminary
correlative studies of the polar cap magnetic field and the IMF By that most
significant results are obtained during periods of high IMF stability. The
"stability" is represented by a five-digit number derived from parameters from
the ISEE-3 data tape, as follows:
First (highest order) digit = 1 if the sign of Bx, By and Bz of the IMF (in GSE
coordinates) during the UT hour of interest is the same as during
the previous hour.
- 0 otherwise.
Second digit = IP = [10f]
where [] _ "largest integer in"
f = fraction of milliseconds in the UT hour for which IMF
data exists
and IP is set to 0 if IP < 0
IP is set to 9 if IP > 9.
Remaining digits = [10d]: where d = {(<Bi >)2/<Bi2 >}i
for digits 3, 4, 5 and i = x, y, z (in GSE coordinates):
where <> indicates hourly averages.
3. Accomplishments and Work in Progress
3.1 Data Management
a) A data file containing the following parameters has been generated
i) IMF B BB (GSM coordinates); hourly averages from ISEE-3 only
and IAA stability
ii) Dst, and daily average Ost. The values are those provided on
Sugiura's tape, augmented by NASA-Goddard values when the Sugiura
data are not available
iii) Ap
iv) CppV) f 0.1
vi) 3-hour Kp
vii) daily EKp
b) A computer code (TPOLAR) has been developed to generate, from the orbital
elements, the time, latitude and MLT of the extremum latitude of each
transpolar segment of orbit. The results of TPOLAR are stored on disk.
3c) A computer code (SEARCH) has been developed to identify passes (trans-
polar orbit segments) for which any of the indices described in a) fall
into any desired range. It is possible for example to list all passes
in the southern hemisphere with By < 0, B z > 0 and flO.J > 200.
d) A computer code is being developed to determine equatorward and poleward
boundaries of the regions of transverse perturbation vectors which are
thought to characterize the auroral oval. The code examines the direc-
tion and magnitude of the high latitude perturbation vectors. The
criteria for boundary specification are being refined at this time.
3.2 Modelling
The ex iosting mAgn.t : field modelling code, using a refinement of
Kisabeth's grid cell system (see Report #1), is being used to examine theoretical
models of auroral current systems and to predict Magsat observations. The com-
puter code in use at NRCC permits specification of the current strength with a
resolution of 0.50 in latitude and 4 0 in longitude, with a similar resolution in
the magnetic field calculations. For modelling satellite observations, this
provides a distinct advantage over previous systems which had resolution of 20-30
in latitude and 15 0 in longitude.
At present, because of numerical quadrature restrictions, the grid
cell system does not extend beyond 850N (centered dipole). An additional system
for modelling the perturbation magnetic field due to currents above 85 0N is
under development. This system replaces "cells" by transpolar current "strips".
In a manner similar to the cell system, AB due to current in each strip will be
computed and stored (kernels). Superposition of a number of these strips, at
different orientations, and weighted appropriately, will generate resultant polar
cap current systems of sufficient complexity to satisfy most modelling needs
(Figure 3).
As in the case of the cell system, all quadrature associated with
the solution of the Bio-Savart law is carried out only once for a basic strip
system. All other systems can be generated from this set of kernels.
3.3 Data Analysis
a) A study of the orientation of the polar cap residual vectors in the H'D'
plane has been undertaken. Polar transits satisfying high IMF stability
criteria, and having "well-ordered" vectors have been examined. To date,
"well-ordered" remains subjective, and implies perturbation vectors which
are reasonably uniform in magnitude and direction, directed sunward in
the northern hemisphere and antisunward in the southern hemisphere. Only
data from transpolar passes whose polar cap vectors extend at least 5 0 on
either side of the noon-midnight meridian, and with a total extent of at
least 150
 poleward of the auroral oval are considered. Figure 4 shows
examples of acceptable transits for both hemispheres. Most of these
passes occur at times when the Z-compoAan t of the IMF is negative, and
their frequency increases with Kp.
If the average orientation of polar cap vectors in this data subset is
defined as the average angle of those vectors falling in the central 60
seconds of data (i.e., ;30 s from the noon-midnight meridian), then a
4correlation coefficient of -,0.7 is obtained between this average vector
direction and the Y-component of the IMF. This is true in bath hemi-
spheres.
A similar attempt to examine the average vector magnitude has been some-
what inconclusive, indicating, to date, only that this quantity is larger
in the sunlit (southern) polar cap than in the dark polar cap.
b) A statistical analysis of the fluctuation parameter (FP) (a measure of
the high frequency content of the magnetic perturbation vectors; see
Report #1) has been performed using two months of Magsat data (November
1979, January 1980). Data have been sorted into bins t o of latitude
by 1 hour of magnetic local time, and examined for the presence of
significant FP as a function of Kp (3 bins: 0 to 1+; 2- to 3+, >3+)
and IMF B (B < 0; By > 0). The selection criteria were such that 26
days are Lcl&ed in the low K case, and only 7 in the high KD case.
Figure 5 shows the frequency op occurrence plots for thg Kp analysis.
Now that Magsat data up to May 1980 have been distributed, this analysis
will be performed on t!,e expanded data set. In the meantime, no defini-
tive interpretation of the results has been made.
3.4 Cooperative Activities
a) Discussions with other Magsat investigators brought to light a problem
with the method of computing Magsat magnetic field residuals with accept-
able accuracy. At NRCC (as at a number of institutions), the model
field was initially computed at one-minute intervals along the orbit,
and then interpolated from these values to 1-second values, using a
cubic spline interpolation scheme. Because of rapid changes in the field
in the neighbourhood of the poles, this method introduces oscillations
there up to 40 nT in the a and # components. Use of a 5-point Lagrange
interpolation code leads to even higher am plitude oscillations. However,
altering the scheme so that the model field is calculated every 15 s,
and then interpolated to 1 s values, removes the oscillations. Figure 6
is a comparison of the two methods of interpolation,
b) A cooperative study with Rothwell et al. (University of Southampton) has
been undertaken. Auroral video data from Longyearbyen, Spitzbergen for
January 15, 1980 at 0715 UT indicates an arc near noon M-T which is
coincident with a Magsat orbit. The analysis is incomplete at this time.
(See Daytime auroral motions imaged with an all-sky low light level TV
camera in Spitzbergen, P.A. Rothwell, B. Lanchester, R.W. Thomas, IAGA,
Edinburgh, 1981, paper 2C.08).
3.5 Data Processing Status
We have at present processed 38 days of Magsat data from the interval
launch through January 1980. We are ready to process the remainder of the Magsat
data.
s5
4. Significant Results
The unprecedented precision of the vector data set from Magsat has
made it necessary to undertake an extended exploratory pha3e for data analysis
procedures, modelling techniques and phenomenology. Although a number of signi-
ficant preliminary results have been obtained, as outlined above, the studies
are still in progress and have not yet been submitted for publication.
5. Publications
a) Comparison of quiet day B-field variations in the summer and winter auroral
zones.
M.D. Wilson, T.J. Hughes, J.R. Burrows
Spring AGU 1981, paper GP45.
b) The rc a of crustal magnetic anomalies in modelling ionospheric current
systems.
T.J. Hughes, D.D. Wallis
Spring AGU 1981, paper GP44.
Papers are in preparation for the Magsat special issue of Geophysical
Research Letters.
6. Problems
We reiterate our concern expressed in Report #1 regarding the high
frequency noise that has been introduced into CHRONFIN data by the fine attitude
calculation. We had hoped to study currents at the maximum spatial resolution
permitted by the Magsat data rate. Introduction of this "solution noise" has
reduced this resolution considerably.
In an attempt to understand the source of residual errors in the fine
attitude solutions and perhaps develop a 'work-around' solution to them, we have
made a lim'ted study of the attitude flags and corresponding attitude solutions.
There appears, on occasion, to be an unexpected inconsistency in them. An
example is described in Appendix A. This situation has inhibited our attempts
to develop an algorithm for correcting motion model segments of the fine atti-
tude solution. The study is temporarily suspended.
7. Recommendations
No further recommendations at this time.
8. Conclusions
Although there have been no definitive results to date, preliminary
investigations are encouraging. Analysis should proceed reasonably quickly now
that data from most of Magsat's lifetime has been distributed.
6Figure Captions
Fig. 1. An example of the NRCC standard UT plot of magnetic perturbations in
the H'D'U' coordinate system.
Fig. 2. An example of the NRCC standard polar plot of magnetic perturbation
vectors.
Fig. 3. Schematic of the strip current system for polar cap currents. In prac-
tice, up to 45 strips and 45 rotations may be used.
Fig. 4. Examples of "well-ordered" polar cap perturbation vectors.
Fig. 5. Polar plot of frequency of occurrence of significant fluctuation Para-
meter (FP > 8 nT) for combined north and south hemispheres, for November
1979 and January 1980. Contours are drawn at 20%, 40%, 60% and 800%,
with the 60% contour heavier. No coveraqe occurs where the plot is
blank.
a) OAK <1+
b) 1+<
	 3+
c) Kp > W.
Fig. 6(a). Magnetic perturbations relative to the MGST 06/80 model field calcu-
lated every minute along the orbit and interpolated to values 1 second
apart. Note the H' and D' components between minutes 46 and 48.
(b). The perturbations relative to the model field computed every 15 seconds
and then interpolated to 1 s values. The character of all 3 compo-
nents has been changed significantly from that in Figure 6(a).
Fig. 7. Plot of magnetic perturbations for day 316 1979 near 0440 UT. See
Appendix A for discussion.
Appendix A
There appears, on occasion, to be a problem with the attitude solutions
and their flags. For example, day 316 (1979), near 04:40 UT (Figure 7): there is
a distinct negetive-going step of 95 nT in the X-component, and 11 s later, a
return (positive-going) step of 149 nT. This latter step is flagged as a shift
out of the motion model (flags 7608 to 1022), whereas the first shift occurs
in the middle of a period of motion model. This is not expected from our under-
standing of the attitude solutions and the flags. That is, we do not expect
to see such shifts in the data except when there has been a change in type of
attitude sol , .tion. It is not known at this time if other such unflagged shifts
aia in the data.
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Studies of high latitude current systems using MAGSAT vector data

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