Data Set Overview  :  The data set consists of calibrated observations. The MAG measures  the vector magnetic field.   There are three principal coordinate systems used to represent the data  in this archive - the payload (pl) coordinate system, the Sun-state (ss)  coordinate system, and the planetocentric (pc) coordinate system.  Cartesian representations are used for all four coordinate systems. The  pc and ss coordinate systems are specified relative to a 'target body'  which may be any solar system object. Primarily the 'target body' is  Jupiter. In what follows we will reference Jupiter as the target body,  but, for example, if observations near a satellite (such as Io) are  desired in Io-centric coordinates, the satellite Io may be specified as  the target body.   The ss coordinate system is defined using the instantaneous Jupiter-Sun  vector as the primary reference vector (x direction). The X-axis lies  along this vector and is taken to be positive toward the Sun. The  Jupiter orbital velocity vector is the second vector used to define  the coordinate system; the y axis lies in the plane determined by the  Jupiter-Sun vector and the velocity vector and is orthogonal to the x  axis (very nearly the negative of the velocity vector). The vector  cross product of x and y yields a vector z parallel to the northward  (upward) normal of the orbit plane of Jupiter. This system is sometimes  called a sun-state (ss) coordinate system since its principal vectors  are the Sun vector and the Jupiter state vector.   The planetocentric (pc) coordinate system is body-fixed and rotates  with the body as it spins on its axis. The body rotation axis is the  primary vector used to define this coordinate system. Z is taken to  lie along the rotation axis and be positive in the direction of  positive angular momentum. The X-axis is defined to lie in the  equatorial plane of the body, perpendicular to Z, and in the direction  of the prime meridian as defined by the IAU. The Y axis completes the  right-handed set.   Data in the vicinity of the moons of Jupiter (Io, Callisto) may be  provided in separate files in moon centered coordinate systems, if it  turns out that the mission plan affords an opportunity  to acquire data in the immediate vicinity of either of those bodies. Data in the vicinity of Ganymede and Europa has been provided in  planetocentric coordinates. The planetocentric and SS data follows the  definitions above with the reference body being the moon or target  specified via option in the command line All of the archived data files  are simple and readable ASCII files with attached documentation in a  header that precedes the columns of data. Files using a coordinate system centered on a target body other than Jupiter are identified via the  target body listed on the command line which appears in the header along  with an audit trail of supplementary engineering (kernel) files.   The output from the processing program is in Standard Time Series (STS)  format. The Object Description Language (odl) header is included in the  STS file. There will also be a detached PDS label file describing the  contents of the STS file.   Each data file contains the observations collected on a given UTC day.   Instrument Overview  :  Please see JNO_FGM_INST.CAT.   Parameters  :  The FGM powers up in operational mode and returns telemetry  immediately every clock tic (2 seconds). The FGM may be operated  in autoranging mode, or manual range commands may be sent to fix  the instrument in any of its dynamic ranges. Likewise any telemetry  mode may be selected, depending on telemetry resource allocation. In  addition, packets of engineering telemetry (in addition to science  telemetry packets) are telemetered at a variable rate, from one per  2 seconds to one per 512 seconds, per commanded state.   Calibration Overview  :  The FGMs were calibrated in the Planetary Magnetospheres Laboratory  and the GSFC Mario H. Acuna (MHA) Magnetic Test Facility (MTF), a  remote facility located near the GSFC campus. These facilities are  sufficient to calibrate the FGMs to 100 parts per million (ppm)  absolute vector accuracy. An independent measurement of the magnetic  field strength in the 0.25, 1, and 4 Gauss ranges was provided by  Overhausen Proton Precession magnetometers placed near the FGM. Scale  factor calibration is extended to 16 Gauss using a specialized high  field coil and measurement techniques (see JUNO Magnetic Field  Investigation instrument paper: (Connerney et al., Space Science  Reviews, 2017, doi: 10.1007/s11214-017-0334-z)). A nuclear magnetic  resonance magnetometer (Virginia Scientific Instruments) provided the  absolute field strength measurements in the 16 Gauss range when it was  working.   Two independent methods are used to calibrate the magnetometers. The  vector fluxgates are calibrated in the 22' facility using a method  ('MAGSAT method') developed by Mario Acuna and others. This technique  uses precise 90 degree rotations of the sensing element and a sequence  of applied fields to simultaneously determine the magnetometer  instrument model response parameters (the 'A matrix') as well as a  similar set of parameters (the 'B matrix') that describe the facility  coil orthogonality [instrument paper reference]. The second calibration  method (called the 'thin shell' and 'thick shell') uses a large set of  rotations in a known field (magnitude) to obtain the same instrument  parameters, subject to an arbitrary rotation [Merayo 2000 & 2001]. In  the 'thin shell' method, the sensor is articulated through all  orientations in a fixed, or known field magnitude. This can be done in  a facility like the GSFC 22 foot coil system, wherein any fixed field  up to about 1.2 Gauss may be utilized, or it may be done in the Earth's  field using the ambient field in a gradient-free region and a system  to compensate for variations in the ambient field (normally corrected  via a secondary reference magnetometer coupled with a Proton Precession  total field instrument). Application of this method in a coil facility  (with closed loop control for ambient field variations) allows for the  'thin shell' to be performed at many field magnitudes ('thick shell').   The MAGSAT calibration method provides the instrument calibration  parameters referenced to the optical cube mounted on the sensor  (or MOB) which defines the instrument coordinate system. These  parameters include the instrument scale factors, 3 by 3 instrument  response matrix (or 'A' matrix), and zero offsets for each instrument  dynamic range. The 'thin shell' method provides the same parameters,  but since the method conveys no attitude information, only the  symmetric part of the instrument response matrix is determined via  'thin shell'. Nevertheless, it provides a useful independent verification of the MAGSAT calibration.   Inflight calibration activities are designed to monitor instrument  parameters, primarily zero offsets, and to monitor the relative  alignment of the magnetic field sensor platforms (the MOBs) and the  spacecraft attitude reference (Stellar Reference Units, or SRUs).  Spacecraft generated magnetic fields will be monitored using the dual  magnetometer technique and a series of magnetic compatibility tests  designed to identify the source of any magnetic signals (if any)  associated with spacecraft payloads. Since Juno is a spinning  spacecraft, spinning at 1 or 2 rpm nominally, any field fixed in the  frame of reference of the spacecraft (e.g., fixed spacecraft-generated  magnetic fields, sensor offsets, etc.) is easily identified. In practice  we apply an algorithm developed independently by several groups (Acuna,  Reviews of Scientific Instruments, 2002) to estimate bias offsets using  differences in the measured field. This method handily corrects for  biases in the spacecraft x and y axes, but since the spacecraft spins  about the z axis, biases in z must be estimated using different methods.  One technique utilizes the Alfvenic nature of fluctuations in the solar  wind, that is, the magnitude preserving nature of variations in the  field. Of course, not all fluctuations are Alfvenic (preserving  magnitude) so some care is taken in application of this method to select  appropriate events.   The process used to correct for the contribution of eddy currents is  described in Kotsiaros, et al. 2020.   Coordinate Systems  :  The MAG data are represented in the following coordinate systems:   - spacecraft-solar equatorial  - spacecraft payload  - planetocentric  - sun-state   all described above.   Data  :  Data products contain the observations collected on a given  UTC day. Each coordinate system in a separate file.   Through 2017 DOY 174, magnetometer observations in weak field  environments (sensor range 0, +/- 1600 nT nominal dynamic range) are  sourced from the outboard sensor. Subsequent to 2017 DOY 174,  magnetometer observations in weak field environments (sensor range 0,  +/- 1600 nT nominal dynamic range) are sourced from the inboard sensor  to alleviate minor sporatic interference appearing in the z axis of the  outboard sensor. This substitution is noted in the STS header that  identifies the content of each record. The STS header should be  consulted for file content each time a file is read, in the event that  file content changes (this was the design purpose of the STS header).  We anticipate further file format changes (in upper dynamic ranges) as  additional corrections are introduced.

Confidence Level Overview  :  Not applicable.   Review  :  The FGM data set was reviewed internally by the MAG team prior to  release to the PDS. PDS also performed an external review of the MAG  data.    Limitations  :  The Juno magnetic field investigation was designed to measure fields  to 16 Gauss per axis over 6 dynamic ranges of the instrument, the most  sensitive of which is +/- 1600 nT with a quantization step size of  0.05 nT (16 bit A/D). Moreover, the spacecraft magnetic requirement was  not to exceed 2 nT static and 0.5 nT variable spacecraft-generated  magnetic field. In very weak field environments, such as encountered in  outer cruise, accuracy may be expected to be limited by sensor offset  and spacecraft magnetic field variations. The combined (static)  spacecraft-generated magnetic field and sensor offset may be  continuously monitored in flight in the spacecraft x and y axis, since  the spacecraft spins (nominally at 1 or 2 RPM) about an axis closely  aligned with the spacecraft payload z axis. However, offsets in the z  axis need be estimated using the Alfvenic properties in the solar wind  (Connerney et al., Space Science Reviews, 2017,  doi: 10.1007/s11214-017-0334-z). Statistical in nature, estimates of z  axis zeros are not continuously available and are less accurate than the  x and y zeros. Also, variations in spacecraft field over a time span  comparable to a spin period will also lead to larger errors.