Voyager 1 9.6-s Averaged Triaxial Fluxgate Magnetometer (MAG) Interplanetary Magnetic Field in CDF Format

This data set includes the Voyager spacecraft number (1 or 2), the date-time in decimal year (90.00000 is day 1 of 1990), the magnetic field strength, F1, computed from high-resolution magnitudes, the elevation and azimuth angles in heliographic (RTN) coordinates, and the magnetic field strength, F2, computed from 1-hr averages of the components. The vector components of B can be computed from F2 and the two angles. The elevation angle is the latitude angle above or below the solar equatorial plane, and the azimuth angle is in the direction orbital motion around the Sun from the projection of the Sun-to-spacecraft axis into the solar equatorial plane. The Voyager MAG experiment and coordinates are further described in the following publication: Behannon, K.W., M.H. Acuna, L.F. Burlaga, R.P. Lepping, N.F. Ness, and F.M. Neubauer, Magnetic-Field Experiment for Voyager-1 and Voyager-2, Space Sci. Rev., 21 (3), 235-257, 1977. At the time of experiment proposal, it was expected that the required accuracy of the measurements would be 0.1 nT, determined by the combined noise of the sensors and the spacecraft field. The spacecraft magnetic field at the outboard magnetic field sensor, referred to as the primary unit, was expected to be 0.2 nT and highly variable, consistent with current estimates. Hence, the dual magnetometer design (Ness et al., 1971, 1973; Behannon et al., 1977). At distances > 40 AU, the heliospheric magnetic fields are generally much weaker than 0.4 nT; the average magnetic field strength near 40 AU and 85 AU is about 0.15 nT and 0.05 nT, respectively. The use of roll calibrations lasting about 6 hours permits determination of the effective zero levels for the two independent magnetic axes that are perpendicular to the roll axis, which is nearly parallel to the radius vector to the Sun, at intervals of about 3 months. There is no roll calibration for the third magnetic axis. Comparison of the two derived magnetic vectors from the two magnetometers permits validation of the primary magnetometer data with an accuracy of 0.02 to 0.05 nT. A discussion of the uncertainties that must be considered when using these data is given in the Appendix of Burlaga et al. (1994) and in Appendix A of Burlaga et al. (2002). References: Behannon, K.W., M.H. Acuna, L.F. Burlaga, R.P. Lepping, N.F. Ness, and F.M. Neubauer, Magnetic-Field Experiment for Voyager-1 and Voyager-2, Space Science Reviews, 21 (3), 235-257, 1977. Burlaga, L.F., Merged interaction regions and large-scale magnetic field fluctuations during 1991 - Voyager-2 observations, J. Geophys. Res., 99 (A10), 19341-19350, 1994. Burlaga, L.F., N.F. Ness, Y.-M. Wang, and N.R. Sheeley, Jr., Heliospheric magnetic field strength and polarity from 1 to 81 AU during the ascending phase of solar cycle 23, J. Geophys. Res., 107 (A11), 1410, 2002. Ness, N., K.W. Behannon, R. Lepping, and K.H. Schatten, J. Geophys. Res., 76, 3564, 1971. Ness et al., 1973. Coordinate Systems: Interplanetary magnetic field studies make use of two important coordinate systems, the Heliographic Inertial (HGI) coordinate system and the Heliographic (HG) coordinate system. The HGI coordinate system is used to define the spacecraft's position. The HGI system is defined with its origin at the Sun. There are three orthogonal axes, X(HGI), Y(HGI), and Z(HGI). The Z(HGI) axis points northward along the Sun's spin axis. The X(HGI)-Y(HGI) plane lays in the solar equatorial plane. The intersection of the solar equatorial plane with the ecliptic plane defines a line, the longitude of the ascending node, which is taken to be the X(HGI) axis. The X(HGI) axis drifts slowly with time, approximately one degree per 72 years. The magnetic field orientation is defined in relation to the spacecraft. Drawing a line from the Sun's center (HGI origin) to the spacecraft defines the X axis of the HG coordinate system. The HG coordinate system is defined with its origin centered at the spacecraft. Three orthogonal axes are defined, X(HG), Y(HG), and Z(HG). The X(HG) axis points radially away from the Sun and the Y(HG) axis is parallel to the solar equatorial plane and therefore parallel to the X(HGI)-Y(HGI) plane as well. The Z(HG) axis is chosen to complete the orthonormal triad. An excellent reference guide with diagrams explaining the HGI and HG systems may be found in L.F. Burlaga, MHD Processes in the Outer Heliosphere, Space Sci. Rev., 39, 255-316, 1984.

本数据集包含旅行者号（Voyager）航天器编号（1或2）、以十进制年份表示的日期时间（格式如90.00000即代表1990年第1天）、由高分辨率幅值计算得到的磁场强度F1、日球坐标系（heliographic RTN）下的仰角与方位角，以及由分量1小时平均值计算得到的磁场强度F2。可通过F2与上述两个角度计算磁场矢量B的各分量。其中仰角为相对于太阳赤道平面的南北纬度角；方位角则是将太阳-航天器轴投影至太阳赤道平面后，沿绕太阳轨道运动方向的角度。

旅行者号磁强计（MAG）实验及相关坐标系的详细说明可参见下述文献：Behannon, K.W.、M.H. Acuna、L.F. Burlaga、R.P. Lepping、N.F. Ness与F.M. Neubauer，《Voyager-1与Voyager-2磁场实验》，《空间科学评论》，21(3), 235-257, 1977。

在实验提案阶段，预计测量所需精度可达0.1纳特（nT），该精度由传感器与航天器自身磁场的综合噪声决定。被称为主单元的外侧磁强计传感器的航天器背景磁场预计为0.2纳特，且具有高度可变性，这与当前的估算结果一致。因此采用了双磁强计设计（Ness等, 1971, 1973; Behannon等, 1977）。

当日球层距离大于40天文单位（AU）时，日球层磁场强度通常远低于0.4纳特；在40 AU与85 AU附近的平均磁场强度分别约为0.15纳特和0.05纳特。

通过持续约6小时的滚转校准，可每隔约3个月对垂直于滚转轴的两个独立磁轴的有效零点进行标定，而滚转轴几乎平行于指向太阳的径向矢量。第三个磁轴未设置滚转校准方案。

通过比对两台磁强计得到的两组磁场矢量，可验证主磁强计数据的精度，验证精度可达0.02~0.05纳特。关于使用该数据集时需考虑的不确定性分析，可参见Burlaga等(1994)的附录以及Burlaga等(2002)的附录A。

参考文献：
1. Behannon, K.W., M.H. Acuna, L.F. Burlaga, R.P. Lepping, N.F. Ness, and F.M. Neubauer. Magnetic-Field Experiment for Voyager-1 and Voyager-2. 空间科学评论, 21(3), 235-257, 1977.
2. Burlaga, L.F. Merged interaction regions and large-scale magnetic field fluctuations during 1991 - Voyager-2 observations. 地球物理学研究杂志, 99(A10), 19341-19350, 1994.
3. Burlaga, L.F., N.F. Ness, Y.-M. Wang, and N.R. Sheeley, Jr. Heliospheric magnetic field strength and polarity from 1 to 81 AU during the ascending phase of solar cycle 23. 地球物理学研究杂志, 107(A11), 1410, 2002.
4. Ness, N., K.W. Behannon, R. Lepping, and K.H. Schatten. 地球物理学研究杂志, 76, 3564, 1971.
5. Ness et al., 1973.