The author is with the Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024.
The International Magnetospheric Study, presently underway, is an international cooperative enterprise whose aim is a quantitative understanding of the dynamic plasma and field environment of the Earth. Satellite programs play a key role in this study  and the ISEE spacecraft in turn are key elements in the satellite program  . The ISEE 1 and 2 spacecraft travel close together in the same highly elliptical orbit out to almost 23 earth radii. They traverse the entire day side magnetosphere in the near equatorial regions and probe the near-tail region on the night side. Figure 1 shows a sketch of the noon-midnight meridian of the magnetosphere and labels its different morphological features. Standing in front of the magnetosphere is a bow shock which, although collisionless in the classical sense of particle-particle collisions, deflects, heats, and slows down the "supersonic" solar wind.
|Fig. 1. Schematic representation of the noon-midnight meridian of the terrestrial magnetosphere .|
Figure 1 does not do justice to the dynamic nature of the magnetosphere. The boundaries sketched are constantly in motion. If it were not for this constant motion, there might be little need for the ISEE 1 and 2 spacecraft. Many satellites have been launched into this region of space with the express purpose of studying the particle and field environment of the earth, albeit with somewhat less sophisticated instrumentation in general. Much has been learned from these missions, or else we would not have been able to draw Figure 1. Nevertheless because of the constant motion of the boundaries, we have not been able to measure one of the basic parameters of the system, the thickness of the boundaries. How thick is the shock, the magnetopause? What are the magnitudes of the gradients in the plasma mantle and the plasma sheet? Until the results of the ISEE mission are analyzed we will not know, and until we know these thicknesses there can be no quantitative comparisons with theory.
The magnetic fields investigation selected for ISEE 1 and 2 had as its principal objective the study of those magnetospheric phenomena which had a magnetic signature and which moved, and hence to deduce their velocity, thickness, and the electrical current density carried by the magnetospheric plasma. In addition to the motion of the boundaries described above, this objective included the study of magnetohydrodynamic waves in and around the magnetosphere.
A secondary objective was to support other investigations of the spacecraft. Such support might include providing field data to particle investigators that they might calculate particle pitch angles, to plasma wave investigators that they might investigate wave-normal angles or compare the observed wave frequencies with gyro-resonances, etc., or to the electric field investigators so that they can remove V X B electric fields from their instrument readings. It was proposed that these data be provided on board the spacecraft to those who might need them. The state of the art of spacecraft data systems was not quite to the point where this could be accomplished efficiently. Instead we used the magnetometer to generate a pulse, analogous to the sun pulse, at the negative-going zero crossing of the magnetometer boom-aligned sensor. Thus, instruments which preferred to, could synchronize their scanning with the magnetic field rather than the sun.
The most important requirement imposed by the dual spacecraft nature of the mission was to build two instruments as nearly identical as possible. It was also our desire to build as simple an instrument as possible. However, the instrument also needed a wide dynamic range. The field near perigee would be close to 50,000 nT but at apogee would be close to 5 nT, a factor of 10 less. Furthermore, it was important to measure this 5 nT field quite accurately. To maintain simplicity we proposed that the instrument forego measurements above 2048 nT , have no range changes, and return 16-bit digitizations of the field. This would have provided a resolution of 1/16 nT in each field measurement. However, a number of other investigators needed data at higher field values for calibration purposes or for pitch angle data. Thus we changed the range to + 8192 nT and added a command able range change to + 256 nT. At 16-bit digitization these ranges gave 1/4 nT and 1/128 nT resolution.
In order to keep the instruments as identical as possible, we decided to use digital aliasing filters which were the same for all three sensors of both instruments and which relative to the sampling frequency did not change characteristics with changes in telemetry rate. Aliasing filters are essential on magnetic field investigations in the magnetosphere and solar wind  because once sampled it is impossible to distinguish a signal whose original frequency was below half the sampling frequency from one above half the sampling frequency (i.e., Nyquist frequency). The chosen filters were overlapping box car averages whose transfer function has a zero at the Nyquist frequency, and harmonics thereof. Figure 2 shows the transfer function of these filters.
|Fig. 2. Gain of the magnetometers as a function of frequency, relative to the Nyquist frequency. The filters in the instrument are digital and the transfer function remains constant relative to the Nyquist frequency as the telemetry rate and precision of the instrument changes. The dashed lines indicate the decay of the side lobes of the transfer function with increasing frequency.|
Since the ISEE spacecraft were to spin another requirement was linearity. We must reconstruct the field in inertial space from what are predominantly sine waves with the signals of interest, say, e.g., a magnetohydrodynamic wave, possibly 10 times smaller sitting on top of the sine wave. Any distortion of this sine wave will add signals not present in the original medium which in turn could mask the real (small) signals of interest. Thus we attempted to build the most linear system we could.
An advantage, however, of spinning spacecraft is that one is not sensitive to spacecraft fields or sensor offsets in the spin plane of the spacecraft. A novel use of the dual spacecraft nature of the mission would have been to have the spacecraft fly with spin axes at right angles to one another, thus allowing three component vector measurements solely from spin plane data of both spacecraft, permitting quick calculation of spacecraft fields along the spin axes. The mission planners did not agree to this concept. However, we were permitted one full week of crossed spin axis operation after launch from which base our initial estimates of spacecraft fields. Additionally we installed flippers on each spacecraft which interchanged a sensor in the spin plane with one along the spin axis.
Finally, it was deemed desirable on occasion to sacrifice amplitude resolution for increased temporal resolution. Thus we proposed to transmit at times only 8 of the 16 bits. We proposed two such 'single precision' modes but in implementation it was found to be just as easy to select any 8 contiguous for this mode.
Three Naval Ordnance Laboratory ring core sensors in anorthogonal triad are enclosed in a flipper mechanism at the end of the magnetometer boom, 3 m from the skin of the spacecraft on ISEE l, and 2 m on ISEE 2. The flipper mechanism is actuated by heating a bimetallic strip which rotates the sensor from one stable spring-held position through 90o to a second position. During a "flip left" operation sensor which is initially anti parallel to the spin axis in the flip position, is rotated into the spin plane to look in the direction opposite spacecraft rotation. Sensor 3 is rotated from the spin plane looking in the direction of spacecraft rotation to a direction anti parallel to the spacecraft spin axis. A flip takes about 4 min at room temperature in vacuum, and requires about 5 W. If a flip does not occur within 8.5 min the power to flip mechanism automatically shuts off. Figure 3 shows a sketch of the sensor configuration. Table I lists instrument weights, power, and dimensions.
The instrument has two commandable ranges, + 8191 nT + 256 nT and is accurate to 0.025 percent or one part in 2 The linearity was mcasured by calibrating with a computer controlled field generated with a 16-bit digital-to-analogue converter. The temperature dependence of the magnetometer zero levels was measured to be less than 10nT per degree(Celsius in its most sensitive mode, and the overall gain depends to be from one part in 10 to 10 per degree Celsius. The basic noise level of the instrument is about 10 nT/Hz at the and is approximately inversely proportional to frequency
|Fig. 3. Sensor configuration (not to scale).|
Weight: Sensor Assembly 0.53 Kg Electronics 1.90 Kg Power: Normal Operations 3.9 watts During Flip Operations 7.8 watts Dimensions: Electornics 21 X 12 X 15 cm Sensors 11 cm X 9 cm (dia)
One complication in data reduction in past programs was the determination of the exact time of instrument status changes. To simplify this procedure in this investigation, commands were stored by the instrument for execution at a major frame pulse. Since the experiment data records, including the instrument status words, are handled in blocks beginning with the major frame pulse, this procedure insured that the measurement status could be readily obtained for every data point. It was also decided to not allow the instrument to change gain automatically . Since only two-range-change commands were required per orbit. and since the times for the range changes could be specified quite accurately from orbit predictions, this proved to be no hardship on spacecraft controllers. Again this decision simplified data reduction operations.
As mentioned above the analog-to-digital converter in the data handling assembly has only 12 bits, yet a 16-bit sample is transmitted to ground. This is accomplished by sampling the basic magnetometer at a rate of 512 samples/s on each axis and averaging those readings in overlapping groups of 32, 64, 128 or 256 to produce instrument output rates of 32, 16,8, and 1 samples/s, respectively. If the instrument output were constant during an averaging interval there would be no advantage to this averaging operation. However, the satellites spin with nominally a 3-s spin so there are always significant changes in the spin plane readings between samples. Furthermore, even the sensor nominally aligned along the spin axis is a few degrees off alignment so that it sees some spin tone and hence some time-varying field even when the ambient field is constant. Since the averaging procedure in general averages over many digital windows, the discontinuities in the analog-to- digital converter are smeared out providing a more accurate sample than an instantaneous reading of one window would provide. Furthermore, digital noise, which has limited many magnetometer investigations in the past  has been reduced to insignificant values. At the lowest sampling rate, when the instrument is returning 16-bit samples every 4 s, the digital noise level is 2.5 X 10 nT /Hz in high gain. At the highest rate, it is 6.4 X 10 nT / Hz. We note that increasing the number of bits in a sample at the expense of sampling frequency is an effective means of reducing digital noise. For example, in the single precision mode at the highest data rate, if only the most significant 8 bits are transmitted, the digital noise level in high gain is 2.1 X 10 nT / Hz On the other hand, if one knows a priori that the lowest order 8 bits will be sufficient to transmit the signal, one could use the single precision mode to decrease the digital noise level by a factor of 2 over the double precision level to 3.2 X 10 nT / Hz. Such an increase would be moot in earth orbit where the natural noise levels far exceed this value over the bandwidth of the magnetometer. The main advantage of the single precision mode is to double the temporal resolution of the instrument. Since in most applications the lowest order 8 bits are not sufficient to return a full measure of the ambient field, use of the single precision mode increases the digital noise of the instrument, and this impact has to be assessed for each individual situation in which increased temporal resolution might otherwise seem desirable.
Finally, as noted above, the averages are overlapped by using six averaging registers and reading into pairs of them simultaneously but computing the average alternately from one and then the other of the pair. This procedure ensures that the instrument transfer function has an exact zero at the Nyquist frequency, and multiples thereof. As referenced to the Nyquist frequency the instrument transfer function remains fixed. However, as referenced to the spin rate the transfer function does change with bit rate and this fact must be taken into account in using data obtained with the instrument because it effectively makes slight alterations in the gain of the instrument in the spin plane. There are no phase changes associated with this digital filter which make it particularly attractive. However, in this regard it should be noted that the center time of each sample of the field transmitted by the magnetometer, is one sample period prior to transmission. If this fact is not taken into account, a linear phase shift with frequency will be introduced.
Both the ISEE l and 2 magnetometers were turned on a few hours after launch and have operated continuously since that time except for brief periods during interference testing. The only operational anomalies have been a couple of status changes of the ISEE 2 instrument which were not commanded from the ground. These both occurred during the first two weeks and have not reoccurred. The flippers have been exercised every five days on both spacecraft for a total of over 50 flips to date with no evidence of aging.
|Fig. 4. Interplanetary shock of October 26, 1977.|
Shortly after launch on October 26, 1977, while we were testing a magnetometer for the IMS mid-latitude chain, a sudden commencement was recorded at our site in the San Gabriel Canyon, later to be confirmed worldwide by the regular network of ground stations. To see what caused this ground disturbance we examined our ISEE records. The ISEE spacecraft were in the solar wind at the time, and ISEE 1 was transmitting at its high bit rate providing us with sixteen 16-bit samples of the 3 vector components of the field every second. ISEE 2 provided us with data at 1/4 of this rate. Figure 4 shows the field magnitude on ISEE 1 during this event. These data were reduced using preliminary versions of the data reduction software and instrument calibration. The shock, which would be expected to be traveling at a velocity of about 400 km/s relative to the spacecraft, crosses the spacecraft in about 1/10 s. ISEE 2, transmitting at one-quarter of this rate, saw the shock essentially at the same instant of time but with poorer resolution. The field jump is less than a factor of 2, and thus is weaker than the Earth's bow shock is typically. Otherwise, the shock is very reminiscent of the structure of the terrestrial bow shock. On October 28 at 0650 UT, the same shock crossed the Voyager spacecraft at 1.28 AU (F.L. Scarf, personal communication), as would be expected if the shock traveled at about 400 km/s radially outwards from the sun.
In summary, we are very pleased with the initial returns from the ISEE spacecraft. The instrumentation is functioning very well and shows every indication of achieving every objective of the ISEE program.
The successful implementation of this investigation is in large measure due to efforts of R. C. Snare, F. R. George, J. D. Means, and R. C. Elphic at UCLA and B. Plitt at Westinghouse Electric Corporation, Systems Development Division, Baltimore, MD, who have provided the scientists on the investigation, E. W. Greenstadt of TRW Systems, P. C. Hedgecock of Imperial College, London, England, M. G. Kivelson and R. L. McPherron of UCLA, and the author himself with such an excellent instrument with which to work. Testing and integration on the spacecraft went smoothly largely due to the efforts of M. Davis and M. Lids ton of GSFC and D. Eaton and R. Gruen of ESTEC. Special thanks go to E. Iufer and R. Murphy of the Ames Research Center for their care in calibrating the magnetometers.
 K. W. Ogilvie, T. Von Rosenvinge, and A. C. Durney, "International sun-earth explorer: A three-spacecraft program," Science, vol. 198, pp. 131-138, 1977.
 C. T. Russell, "The configuration of the magnetosphere," in Critical Problems of Magnerospheric Physics, E. R. Dyer Ed. Washington, DC: National Academy of Sciences, 1972, p. 1.
 , "Power spectra of the interplanetary magnetic field near the earth," in Solar Wind, C. P. Sonett, P. J. Coleman, Jr., and J. M. Wilcox, Eds. NASA SP-308, 1972.
Last Modified: July 29, 1996