This is the Sakigake Interplanetary magnetic field (IMF) dataset re-processed in 2016 to recover historical data at ISAS/JAXA.

Sakigake IMF data files

• imf1985.txt Sakigake IMF data in 1985
• imf1986.txt Sakigake IMF data in 1986
• imf1987.txt Sakigake IMF data in 1987
• imf1988.txt Sakigake IMF data in 1988
• imf1991.txt Sakigake IMF data in 1991
• imf1992.txt Sakigake IMF data in 1992
• imf1993.txt Sakigake IMF data in 1993
• imf1994.txt Sakigake IMF data in 1994
• imf1995.txt Sakigake IMF data in 1995
• imf1996.txt Sakigake IMF data in 1996
• imf1997.txt Sakigake IMF data in 1997

Note that the quality of the data in 1996 and 1997 is low with a substantial amount of noise caused by bit errors.

Example of the data file

85 2 19 0 56 53.253 0.55 1.48 3.51 3.90 69.5 65.8
85 2 19 0 57 3.128 -0.97 0.66 3.00 3.20 145.9 68.6
85 2 19 0 57 13.004 -0.93 0.72 3.25 3.50 142.1 70.1
85 2 19 0 57 22.879 -0.95 0.66 3.15 3.40 145.1 69.7

Format

1x,I2,1x,I2,1x,I2,1x,I2,1x,I2,1x,F6.3,1x,F6.2,1x,F6.2,1x,F6.2,1x,F6.2,1x,F6.1,1x,F6.1

YY MM DD hh mm ss, Bx By Bz BF PHI THETA
The notation is as follows:

• YY: Year of observation (2-digit integer)
• MM: Month of observation (2-digit integer)
• DD: Day of observation (2-digit integer)
• hh: Hour of observation (2-digit integer)
• mm: Minute of observation (2-digit integer)
• ss: Second of observation (real number with 3 digits after the decimal point)
• Bx, By, Bz: 3-dimensional components of the magnetic field in [nT], averaged over 10 or 40 seconds, in the satellite-centered ecliptic coordinates (The coordinate system is right-handed with the (positive) X-axis being set from the satellite center toward the sun, the (positive) Y-axis set parallel to the ecliptic plane, perpendicular to X-axis and toward the reverse direction to the satellite orbit, and the Z-axis set perpendicular to the ecliptic plane toward the north).
• BF: Root-square mean of the magnitude of the magnetic field averaged over 10 or 40 seconds (Bx, By, Bz)
• PHI: Direction (azimuthal angle) of the magnetic field vector on the ecliptic plane in [degree] for the range between -180 to +180 degrees, where tan(PHI) = By/Bx and PHI = 0 [degree] toward the sun, 180 [degree] toward the anti-sun direction, and 90 [degree] toward the Y-axis.
• THETA: Elevation (latitude angle) of the magnetic field vector from the ecliptic plane in [degree] for the range between -90 to +90 degrees, where sin(THETA) = Bz/BF.

Time resolution (average over 10 or 40 seconds)

The 10-seconds-averaged data were available when the satellite was operated in the high-bit rate mode (0.5 s/frame), which were usually the case when the satellite was close to the earth and when the data were transmitted and received in real time. The data were fitted with the spin period of 9.876 s of the satellite, and the average was calculated.

The 40-seconds-averaged data were available when the satellite was operated in the low-bit rate mode (16 s/frame), which were usually the case when the Data-Recorder was employed or during the real-time data-transmission period when the satellite was distant from the earth. The data were fitted with 40 seconds, which is the least common multiple between the satellite’s spin period and data interval of 4 seconds, and then the average was calculated.

In the archived files, the sets of the data averaged over 10 and 40 seconds were sorted with time, and any duplication has been removed.

Errors of the magnetic field data

In the actual observations, the data were digitized with 12 bits for a range of ±64 nT. However, some other uncertainties come into play; the primary one is the shift of the direction towards the spin axis (roughly along Z-axis), including the zero-point offset and contamination of the internal magnetic field originated in other sub-systems in the satellite. Consequently, the overall error is estimated to be roughly 0.1 [nT].

Notable epochs

• 1986 March 11: Perigee of Halley’s Comet (7 million km)
  • Saito, et al., Nature, 321, 6067, p303, 1986.
  • Saito, et al., Geophys. Res. Lett., 13, 8, p821, 1986.
  • Yumoto, et al., Geophys. Res. Lett., 13, 8, p825, 1986.
• 1992 January 7–8: Sakigake passed through the Earth’s magnetosphere.
• 1992 September 26–27: Extremely radial interplanetary magnetic field, which is the magnetic field generated by, and runs parallel to, the solar wind (The satellite possibly passed the tail of the Earth’s magnetosphere).