ESA Plasma Instrument End of Prime Mission Review. Dr. James McFadden and Dr. Charles Carlson UC Berkeley SSL. Requirements and Specifications. Measurement
End of Prime Mission Review
Dr. James McFadden and Dr. Charles Carlson
UC Berkeley SSL
ESA data demonstrated to be accurate in when ESA energy range contains all significant plasma.
Pressure balance demonstrated across the magnetopause.
Good density agreement in sheath & foreshock where ions are hot.
Solar wind mode allows the THEMIS ESAs, which were not optimized for the intense solar wind ion flux, to perform adequately, without saturation as demonstrated by Ni=Ne.
With corrections for s/c charging, solar wind electrons are accurately measured as demonstrated by agreement between components of Vi and Ve.
Accurate density measurements
Errors in electron velocity measurements due to density fluctuations
ESA’s MCP detectors have been monitored monthly since the commissioning phase to test for any degradation and to maintain proper gain.
The tests toggle the preamplifier threshold between the nominal value of ~30 fC to a value equal to the nominal gain (~160 fC). Variations in counts that are less than a factor of 2 demonstrate good MCP detector gain.
MCPs show little evidence of any average degradation. MCP bias voltages have not required changes for a year.
Concern existed that intense solar wind ions might saturate the MCP detectors and degrade response.
Detector threshold is toggled from nominal values through increasing threshold level to reveal actual MCP gain.
These tests show that MCPs signals decrease in amplitude, but not enough to impact detection at nominal threshold.
Solar wind electrons with nominal MCP full threshold toggle test provide a baseline for comparison.
Energetic electrons measured by the SST detectors produce X-rays that contaminate the ion ESA measurement.
Moment calculations without removal of this background will overestimate ion density (green). Simple background removal algorithms (black) help, but still do not provide good agreement with electron density (red).
We estimate background using both the eSST and iESA data.
Cross fitting these two estimates allows corrections due to changes in response with time.
Using eSST rather than iESA estimate eliminates removal of real counts at low energy.
Density after background subtraction
ESA data is not perfect and the THEMIS web site and instrument papers outline sources of non-ideal response.
For example, even with accurate density measurements,
errors in electron velocity measurements may be present due to density fluctuations on the time scale of the spacecraft spin period.
Another minor source of non-ideal behavior was discovered after the first tail season, when the eESA experienced high count rates at low energy due to photo-electrons from the axial booms.
This change in detector response necessitated modifications to the ground software and added several months to the in-flight calibration effort.
Such changes were necessary in order to get accurate electron moments, especially in the solar wind.
Response before first tail season.
Response after first tail season.