Release notes for PSP/FIELDS data release 9

Data Release 9 Overview

PSP/FIELDS Data Release 9 includes select data from Encounter 7 (perihelion January 2021), and survey and select data from Encounter 8 (perihelion April 2021).

During PSP Encounter 7, intermittent noise was observed in AC-coupled electric field measurements made by the PSP/FIELDS Radio Frequency Spectrometer (RFS) and Time Domain Sampler (TDS) receivers. The RFS, which is the FIELDS receiver sensitive to the weakest electric fields, was more strongly affected. The appearance of this noise is described in the release notes for FIELDS data release 8.

During the outbound phase of PSP Encounter 8, the noise was observed again in RFS and TDS. At this time, Digital Fields Board (DFB) electric field measurements (both AC- and DC-coupled) were also affected. In the most extreme case, the voltage of the antennas (measured in the dfb_wf_vdc data product) approached the negative rail of the FIELDS power supply at -100 V. The V1 and V2 antennas were affected, with V3 and V4 continuing in nominal operations throughout the interval.

Example data from Encounter 8 showing FIELDS antennas driven to the power supply rails. This figure shows the V1, V2, V3 single ended voltage measurements. Late in May 1, the V1 antenna voltage was driven negative, followed by V2. By the end of May 3, both voltages were driven to the -100 V rail of the FIELDS power supply. V3 (and V4, not shown) remained unaffected. The bottom panel shows the FIELDS quality flag (in magenta) indicating this condition.

After Encounter 8, the “antenna at the rails” condition persisted throughout May and June 2021. On June 30, new FIELDS Antenna Electronics Board (AEB) settings were uploaded during a command pass, which returned the instrument to nominal operations.

The updated AEB settings modified the voltage bias for the shield and stub sections of the FIELDS V1-V4 antenna assemblies. For Encounters 1-8, the shields and stubs for all four antennas were voltage biased to -10 V. For Encounter 9 (and for future Encounters), the shield and stub bias voltages have been set to 0 V.

As of this release, data from Encounter 9 is still being downloaded from the spacecraft. However, FIELDS data already on the ground is sufficient to verify that the antennas at the rails condition did not occur, and noise in the higher frequency receivers is greatly reduced or eliminated.

The exact physical mechanism generating the high-frequency noise has not been determined conclusively, but the DC saturation behavior is consistent with a high-impedance (MOhm) path between the stub and whip sections of the antenna assemblies. When the stub is voltage biased to a non-zero DC voltage, an additional current is driven between the whip and stub. This whip-stub current may generate noise in the sensitive AC-coupled receivers (RFS, TDS) and prevents correct current biasing of the whip for the DC measurements of antenna voltage made by the DFB. If the stub is negative, and the whip-stub current is sufficiently large, the whip can be driven into the negative power rail, also called negative saturation. The attached figure shows the response of the DFB single-ended voltage measurements in early May, when the antenna voltage was gradually driven to the FIELDS power supply rails.

One possibility for creating the high-impedance path between whip and stub is impact-generated spacecraft material, created by a collision between an interplanetary dust particle and the spacecraft. Material released by such a collision, after charging positive due to solar radiation, may have been attracted to the negatively charged stub section of the FIELDS antenna assembly.

After Encounter 9, tests were performed to reproduce the antenna at the rails condition, and investigate whether spacecraft material could be attracted to the antennas by negatively biased stub and shields. Analysis of these tests is ongoing, but initial results indicate that the antenna at the rails condition can be replicated using FIELDS bias settings, and (using WISPR images for verification) that collision-generated spacecraft material can be attracted to negatively biased surfaces.

Two new quality flags have been added to FIELDS data products from Encounters 7 and 8. The first new flag indicates periods of enhanced noise observed in the RFS and TDS receivers. This flag has a value of 2^8 = 256 in the FIELDS bitwise quality flag variable. The second flag indicates the “antennas driven to the rails” condition. This flag has a value of 2^9 = 512. In all cases when the second flag is set to 1, the first flag is also set to 1.

A table containing affected time periods is included below, and all public data products from all affected dates have been reprocessed before this release.

Users of FIELDS data are advised to use caution with all data where either flag is set. In many cases, the data can still be used–for example, with careful analysis the plasma line can still be determined from RFS data while noise is present, and for most of outbound Encounter 8 the antennas at the rails condition only affects the V1 and V2 antennas. Users are strongly encouraged to contact FIELDS team members with any questions about data quality.

Finally, we note that magnetic field measurements from the MAG and SCM are unaffected by this issue.

Updated Quality Flags

The quality flags are contained in a variable included in FIELDS Level 2 CDF files. The metadata for this variable contains a description of the bitwise quality flag variable. This has been updated to the following text, with new additions italicized:

FIELDS quality flags. This is a bitwise variable, meaning that multiple flags can be set for a single time, by adding flag values. Current flagged values are: 

1: FIELDS antenna bias sweep, 
2: PSP thruster firing, 
4: SCM Calibration, 
8: PSP rotations for MAG calibration (MAG rolls), 
16: FIELDS MAG calibration sequence, 
32: SWEAP SPC in electron mode, 
64: PSP Solar limb sensor (SLS) test. 
128: PSP spacecraft is off umbra pointing (heat shield not pointed sunward).
256: High frequency noise affecting RFS and TDS receivers.
512: Antennas driven towards the FIELDS power supply rails.

A value of zero corresponds to no set flags. Not all flags are relevant to all FIELDS data products, refer to notes in the CDF metadata and on the FIELDS SOC website for information on how the various flags impact FIELDS data. For more information on the quality flags describing high frequency noise and antennas driven to the rails, see release notes for PSP/FIELDS releases 8 and 9. Additional flagged items may be added in the future.

Flagged intervals

The intervals specified for both new quality flags are given below:

High frequency noise (flag 256)

Orbit, Start,               Stop,                Flag
07,    2021-01-16/09:33:30, 2021-01-16/11:49:30, 256
07,    2021-01-16/13:16:08, 2021-01-16/13:25:00, 256
07,    2021-01-16/13:55:42, 2021-01-16/14:16:05, 256
07,    2021-01-16/15:21:50, 2021-01-16/17:01:20, 256
07,    2021-01-18/04:39:10, 2021-01-18/05:34:30, 256
07,    2021-01-18/12:06:00, 2021-01-18/12:49:20, 256
07,    2021-01-18/15:16:15, 2021-01-18/15:20:45, 256
07,    2021-01-18/15:42:20, 2021-01-18/16:26:25, 256
07,    2021-01-18/17:52:00, 2021-01-18/18:45:00, 256
07,    2021-01-19/16:23:40, 2021-01-19/17:46:05, 256
07,    2021-01-21/07:46:52, 2021-01-21/08:20:40, 256
08,    2021-04-26/00:41:00, 2021-04-26/01:26:00, 256
08,    2021-04-26/02:45:00, 2021-04-26/02:50:00, 256
08,    2021-04-27/09:06:00, 2021-04-27/14:52:00, 256
08,    2021-04-27/15:32:00, 2021-04-27/15:39:00, 256
08,    2021-04-27/17:07:00, 2021-04-27/18:48:00, 256
08,    2021-04-27/23:43:00, 2021-04-27/23:56:00, 256
08,    2021-04-28/06:31:00, 2021-04-28/06:34:00, 256
08,    2021-04-28/21:15:00, 2021-07-01/00:00:00, 256

Antennas driven to the rails (flag 512)

Orbit, Start,               Stop,                Flag
08,    2021-05-01/18:00:00, 2021-07-01/00:00:00, 512