Document title: Readme.txt for NDADS DE MAGA_AVG datatype
Project: DE
NDADS Datatype: MAGA_AVG
Super-EID: DOCUMENT
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NDADS filename: MAGA_DE_6S_AVG_README.TXT
TRF entry B46599.txt in NSSDC's controlled digital document library. Feb. 1998.
Revised March 19, 1999 to correct a few places in text, (re coordinates) and
delete the duplicated text at the end.
Document text follows:
----------------------
DYNAMICS EXPLORER 1 6-SECOND AVERAGE MAGNETIC FIELD DATA SET
DESCRIPTION OF SPACECRAFT
Dynamics Explorer 1 spacecraft was one of two satellites in the Dynamics
Explorer program. The DE-1 and DE-2 satellites were launched by the same
vehicle so that their orbits would be coplanar, allowing two-point measurements
along magnetic field lines, for the purpose of studying coupling between the
magnetosphere, ionosphere, and upper atmosphere. The DE-1 orbit was highly
elliptical with an apogee of 4.35 Re and a perigee of 500 km whereas the DE-2
spacecraft was placed in a much lower 300 x 1000 km altitude orbit. DE-1 was
spin stabilized with its spin axis normal to the plane of the orbit. DE-2 was
three axis stabilized with one face being nadir oriented.
INVESTIGATION OBJECTIVES
The study of field-aligned currents and MHD waves were the primary objectives
of the DE-1/2 magnetometer investigation. Comparison of the magnetometer data
with measurements of precipitating charged particles yielded new information on
the field-aligned current carriers. In combination with the electric field
measurements, it was possible to determine the vertical Poynting Flux of
electromagnetic energy flowing between the magnetosphere and ionosphere and to
separate small-scale field-aligned currents from MHD waves through the
evaluation of the local ratio of the electric to magnetic field amplitudes in
these perturbations. The field-aligned current measurements and neutral
atmosphere observations also provided an opportunity for investigating
atmosphere-magnetosphere coupling and assessing the total rate of energy
transfer into the upper atmosphere. Finally, the DE-1/2 magnetometer
investigation provided a vital service in so far as a knowledge of magnetic
field direction and intensity is essential to any number of space plasma
science investigations utilizing the various DE-1/2 particles and fields data
sets.
DESCRIPTION OF THE DATA
The DE-1 magnetic field (MAG-A) 6-second average resolution data set consists
of averages of the high resolution triaxial fluxgate measurements taken every
62.5 msec (i.e., 16 vectors/second). The MAG-A data set consists of the three
components of the model magnetic field and difference field, B-Radial (Br),
B-Theta (Bth), and B-Phi (Bph), in *old* Geomagnetic Spherical (GMS) Coordinates,
and the difference field in local *new* Geographic Spherical (GGS) and
Geomagnetic Spherical (GMS) Coordinates, respectively, and the difference field
in local magnetic coordinates (b-para, b-parp1, b-parp2).
The R, Theta and Phi axes are positive in the directions of
increasing radial distance from the center of the Earth (i.e., outward),
increasing magnetic colatitude (i.e., southward) and increasing azimuth angle
(i.e., magnetic east).
The reference for the MAGSAT magnetic field model is Langel et al., Geophys.
Res. Lett., 7, 793, 1980. The following Orbit Attitude (OA) parameters are also
included in the data set: altitude, geographic latitude and longitude, magnetic
local time, and invariant latitude. The data are provided in daily files in
ASCII format.
[updated by Robert.M.Candey@nasa.gov, 2006 Jan 17, per email dated
Date: Thu, 18 Feb 99 17:08:59 JST
From: iyemori@swdcgw.kugi.kyoto-u.ac.jp (Toshihiko_Iyemori)]
As described in Farthing et al. (1981), the DE-1 magnetometer had a digital
resolution of +1.5 nT in its low altitude, least sensitive mode. Two higher
sensitivity modes were used at higher altitudes with digital resolutions of
+0.25 nT and +0.02 nT, respectively.
The data set consists of daily files from 81258 to 91049 in ASCII format.
Each file contains all of the data available for a given day.
Data Accuracy
The dominant source of error in the DE-1 magnetic field measurements is the
uncertainty in the attitude of the spacecraft. The DE-1 spacecraft was designed
to an attitude uncertainty specification of about 0.3 degree which appears
to have been met much of the time. As a "rule of thumb" each 0.1 degree in
attitude uncertainty near perigee corresponds to an error of approximately 100
nT in each component of the field when the magnetic field measured at the
sensors is transferred to an inertial frame of reference or a model field is
transferred into the spacecraft frame and subtracted from the measured field.
For this reason it is common for the residual, or "delta-B" field obtained by
subtracting the model field at low altitudes (i.e., high fields) to show a
gradual shift of several 100 nT from the start of a passage across the polar
cap to the other side. (These slow shifts in the "baselines" of the vector
field components do not affect most scientific analyses, e.g., field-aligned
current measurements, but they can be effectively dealt with through modeling
if need be.) At higher altitudes the ambient field intensity is less and the
uncertainty due to attitude errors is correspondingly smaller.
The absolute accuracy of the DE-1 total magnetic field measurement has also
been evaluated through comparison with the precision vector/scalar magnetic
field observatories located on the ground which are used to monitor the
geomagnetic field. On the basis of such cross-comparisons utilizing DE-1
perigee data over the life of the mission, R. Langel (private communication,
1994) found excellent agreement between the MAG-A and ground-based observatory
scalar data sets at the 20 to 40 nT level.
In using any unfamiliar data set, caution is advised and tests to screen out
instrumental artifacts should be devised before reaching important conclusions.
De-spinning high sensitivity, boom mounted vector magnetometer data in high
fields (i.e., >1000 nT) frequently results in a readily observable residual
signal at the spin period and its harmonics. In the case of the DE-1
magnetometer measurements, the dominant causes of residual spin tone were found
to be small (0.1 to 0.01%) changes in the instrument scale factors and boom
bending of up to several tenths of a degree in response to varying thermal
inputs due to orbit/attitude driven changes in solar illumination (e.g.,
seasonal variations, eclipses, etc.). These effects were minimized through an
orbit by orbit calibration procedure which analyzed the residual spin tone
around apogee and perigee and adjusted the scale factors and sensor attitude
accordingly. Even after these in-flight calibration activities, residual spin
tone signals in the MAG-A data with amplitudes of tens of nanotesla are common
in high fields around perigee. The most probable cause of these residuals is
the transverse field dependence of fluxgate magnetometers in high fields which
was not well-appreciated at the time that DE-1/2 magnetometers were designed
and calibrated in the late 1970's. As discussed by Luhr et al. (1995) in
regards to the magnetometer on the low altitude, spin stabilized Freja
spacecraft, this non-linear effect can easily produce the residual spin
frequency signals present in the MAG-A data set.
================================================================
Note 1: The MLT and ILAT algorithms were supplied by M. Sugiura (PI for the
Magnetometer Investigation) prior to launch and used in the generation of the
Orbit-Attitude database.
REFERENCES
Farthing, W.H., M. Sugiura, B.G. Ledley, and L.J. Cahill, Jr., Magnetic field
observations on DE-A and DE-B, Space Science Instr., 5, 551, 1981.
Langel, R.A., R.H. Estes, G.D. Mead, E.B. Fabiano, and E.R. Lancaster, Initial
geomagnetic field model from MAGSAT vector data, Geophys. Res. Lett., 7, 793,
1980.
Luhr, H., F. Primdahl, and T. Risbo, The transverse field dependence of
fluxgate magnetometers and its implications for low altitude satellites
(abstract), Chapman conference on Measurement Techniques for Space Plasmas,
Sante Fe, New Mexico 1995.
J. Slavin
Updated 25 August 1995
****************************************************
* *
* The 6-second Average DE-1 Magnetic Field Data *
* *
****************************************************
The magnetic field data obtained by the Dynamics Explorer 1 fluxgate
magnetometer (MAGA(*)) were processed at Kyoto University to make 6 second
average data files. Followings are an explanation of the data screening
procedure and the data record format. For the research and publication by
the use of these data, please consult to;
J.A. Slavin
Code 696, NASA/GSFC
Greenbelt, MD 20771
U.S.A.
E-mail: slavin@lepjas.gsfc.nasa.gov
or
M. Sugiura
Institute of Research and Development
Tokai University
Tokyo 151
Japan
E-mail: KYOTO::SUGIURA
On the data processing at Kyoto University, please consult to;
T. Iyemori
Data Analysis Center for Geomagnetism and Space Magnetism
Faculty of Science
Kyoto University
Kyoto 606
Japan
E-mail: Iyemori@kugi.kyoto-u.ac.jp
(December 25, 1995)
(*)Farthing,W.H., M.Sugiura, B.G. Ledley and L.J. Cahill,Jr., Magnetic field
observations on DE-A and -B, Space Science Instrumentations, 5, 551-560,
1981.
*** The Record Format of the Screened Data ***
A file contains the data for each day. The record format of the files is as
follows:
YYDDD [I5] year and day (ex.,81365)
IXR [I9] universal time (milli second)
Alt [F8.1] geodetic altitude (km)
Lat [F6.2] geographic latitude (degree)
Long [F7.2] geographic longitude(degree)
Mlt [F6.2] magnetic local time (hour)
Ilat [F6.2] invariant latitude (degree)
B [3F8.1] reference magnetic field (B-r,B-th,B-phi) in GEO
(Geographic Spherical coordinate system) (nT)
YR [3F8.2] difference magnetic field (b-para,b-parp1,b-parp2) in LMG
(local magnetic coordinate system) (nT)
YGR [3F8.2] difference magnetic field (b-r,b-th,b-phi) in GMS (nT)
Flg [I2] error code ( =0 : no error in data reduction)
-----------------------------------------------------------------------------
A Screening Procedure
of the 6-second Average DE-1 Magnetic Field Data
and an Explanation of the Record Format
S. Nakabe
Geophysical Institute , Kyoto University
(January 31,1995)
The 6-second average DE-1 magnetic field data processed at Kyoto University
include many strange points. Some of them are artificial noises and others are
generated in the process of data reduction. For example, we find
1. Extraordinary points lasting for a period
( this case often takes place at the beginning of a successive data
period.)
2. Continuous periods of the data having the same value
(the value is often equal to zero.)
Therefore we screened the 6-second average data. The procedure was taken with
making a new database in which both the 6-second average data and the orbit
information are written together.
We checked one point at a time, taking the location of the DE-1 satellite
into consideration. The criteria of screening are as follows,
1. when 3 components in GMS (geomagnetic spherical coordinate system) of a
point are all equal to zero
2. the first 3 points of a continuous period
3. when at least 1 component in GMS is the same with the respective component
of the next point.
4. when the value jumps suddenly from a normal (i.e., not so much different
from the model field) value and until it comes back again to a normal
(reasonable) value.
5. when the 4th point of a continuous period is extraordinary (the first
3 points are deleted) and until it comes back to a reasonable value
The software of screening consists of 3 parts, "the main part", "the first
point part", and "the jumping part". While values are considered to be normal,
i.e., while points don't satisfy any conditions of 1-5 , the software is
running in "the main part". But if a point satisfies any conditions of 1-3,
the point is skipped. If a point satisfies the condition 4 or 5, we go into
the "first point part" or "the jumping part". And we don't come back to
"the main part" until the values fall in a reasonable range.
This procedure may not be perfect. There may remain some bad data points,
or the criteria could be too strong. Keep these possibilities in mind when
you use the data set.
*** The Main Part ***
Usually we are in this part. There are 5 check points.
1. Elimination of zeros
If
YGR(i,1)=YGR(i,2)=YGR(i,3)=0
then we skip the point i.
(YGR(i,k) (k=1,2,3) represents the components r,th,phi in GMS
and i represents a point of data)
2. Skipping the first 3 points of a continuous data period
If
| IXR(i)-IMBEF | > 60 s
or
IMBEF=0
then we do not write the points i, i+1, and i+2.
(IXR(i) represents the time of the point i.
IMBEF is the last time of the point we accessed.)
3. Elimination of the periods having a constant value
If
YGR(i+1,1) - YGR(i,1)=0
or
YGR(i+1,2) - YGR(i,2)=0
or
YGR(i+1,3) - YGR(i,3)=0
then we do not write the point i and i+1.
4. Elimination of sudden jumps
If
|YGR(i+1) - YGR(i,1)| > DB
or
|YGR(i+2) - YGR(i,2)| > DB
or
|YGR(i+3) - YGR(i,3)| > DB
then we do not write the point i, and go into "the jumping part".
Where we take DB as,
(1) Altitude > 12500 km ,|Invariant Latitude| > 55 deg
DB = 80 nT
(2) Altitude > 12500 km, |Invariant Latitude | =< 55 deg
DB = 30 nT
(3) 2500 km =< Altitude =< 12500 km, |Invariant Latitude| > 55 deg
DB = 150 nT
(4) 2500 km =< Altitude =< 12500 km, |Invariant Latitude| =< 55 deg
DB =100 nT
(5) Altitude < 2500 km
DB = 300 nT
DB is a criteria of jump scales. Large jumps are allowed in the low altitude and the auroral zone.
5. Checking the forth point of a continuous period
If
|(YGR(i,1)| > ABSB
or
|(YGR(i,2)| > ABSB
or
|(YGR(i,3)| > ABSB
then we skip the forth point i, and go into "the first point part" directly.
Where we take ABSB as,
(1) Altitude > 12500 km
ABSB = 500 nT
(2) Altitude =< 12500 km
ABSB = 1100nT
ABSB is a criteria of the absolute value of the magnetic field.
*** The First Point Part ***
While we are in this part, any points are not written. There are
3 conditions to escape from here.
1. The first point of a continuous data period
If
|IXR(i)-IMBEF| > 60 s
then we go back to the beginning of "the main part" directly.
2. There is a jump and the value of the next point is reasonable.
If
( |YGR(i+1,1) - YGR(i,1)| > DB
or
|YGR(i+1,2) - YGR(i,2)| > DB
or
|YGR(i+1,3) - YGR(i,3)| > DB )
and
|YGR(i+1,1)| < ABSB
and
|YGR(i+1,2)| < ABSB
and
|YGR(i+1,3)| < ABSB
then we go back to "the main part".
3. We have deleted(skipped) the data points for an hour.
If any points don't satisfy the above two conditions, we delete the
data for an hour , and escape from this deleting routine.
*** The Jumping Part ***
While we are in this part, any points are not written. There are 5 conditions
to escape from here.
1.The first point of a continuous data period
If
|IXR(i) - IMBEF| > 60 s
then we go back to the beginning of "the main part".
2. There is a jump in the opposite direction to the jump with which we came
in this "jumping part".
If
the sign of (YGR(i+1,1) - YGR(i,1)
is not equal to the sign of Ajump(1)
and
the sign of (YGR(i+1,2) - YGR(i,2)
is not equal to the sign of Ajump(2)
and
the sign of (YGR(i+1,3) - YGR(i,3)
is not equal to the sign of Ajump(3)
and
0.8 * |Ajump(1)| < |YGR(i+1,1)|
< 1.2 * |Ajump(1)|
and
0.8 * |Ajump(2)| < |YGR(i+1,2)|
< 1.2 * |Ajump(2)|
and
0.8 * |Ajump(3)| < |YGR(i+1,3)|
< 1.2 * |Ajump(3)|
then we go back to "the main part".
Where
Ajump(k) = YGR(n,k) - YGR(n-1,k) k = 1, 2, 3
(n is the i when the data made the first jump)
3. The value comes back to the last value just before the first jump.
If
Alast(1) - DB/2 < YGR(i+1,1) < Alast(1) +DB/2
and
Alast(2) - DB/2 < YGR(i+1,2) < Alast(2) +DB/2
and
Alast(3) - DB/2 < YGR(i+1,3) < Alast(3) +DB/2
then we go back to "the main part".
Where
Alast(k) = YGR(n-1,k) k = 1, 2, 3
(n is the i when the data made the first jump)
4. The value comes back to the value of 5 points ahead of the first jump.
If
Alast5(1) - DB/2 < YGR(i+1,1) < Alast5(1) +DB/2
and
Alast5(2) - DB/2 < YGR(i+1,2) < Alast5(2) +DB/2
and
Alast5(3) - DB/2 < YGR(i+1,3) < Alast5(3) +DB/2
we go back to "the main part".
Where
Alast5(k) = YGR(n-5,k) k = 1, 2, 3
(n is the i when the data made the first jump)
5. We have deleted the data points for 5 minutes by the above criteria.
If any points don't satisfy the above four conditions, we continue to
delete the data, and escape from this deleting routine after 5 minutes.