The top panel shows the SKIN current channels from ERPA1 and ERPA2 averaged over one sweep period. There is considerable variation with payload orientation and there are differences between the two detectors.
This sketch illustrates the situation for measuring flow along the field line.
The SKIN channel provides the most straightforward comparison. This channel measures the total current collected by the front disk entrance face (shown in red).
The exact pitch angle response of the SKIN channel is difficult to model but by symmetry arguments the difference between the two ERPAs would be largest when the detectors are aligned with the flow direction, zero when the detectors are 90 degrees to the flow and would change sign when the angle goes past 90 degrees.
The top panel shows the SKIN current channels from ERPA1 and ERPA2 averaged over one sweep period. There is considerable variation with payload orientation and there are differences between the two detectors.
The second panel shows the difference in SKIN current compared to the cos() of the payload pitch angle (calculated from magnetometer data). The collecting disk area is about 25 sq-cm, so 0.2uA*cos(pitch) would be 8nA/sq-cm net current flowing down the field line. The effective area of the disk is probably larger than it's physical size because of the +4V bias so the current density would be something less than 8nA/sq-cm
This interval of data seems to fit fairly well with flow down the field line however this match up does not hold for other times during the flight.
This plot shows the ERPA2-ERPA1 skin current difference over the entire flight. Note that the variation is often out of phase with the assumed pitch angle variation for a field aligned flow. A phase difference can't be accounted for by a difference in flow amplitude or an offset in pitch angle but it might fit with a net flow in some direction other than along the field.
These plots show comparisons between ERPA1 raw spectra and reconstructed ERPA2 spectra at a few times during the flight. The plots show data from 8 consecutive spectra. The spectra change during the plot time because of payload spin and cone motion. The triangle sweep shape means that two consecutive spectra are sampled closely in time. This is sometimes noticeable as pair grouping of the traces.
For times late in the flight, ERPA2 shows a consistent and fairly large (about .25 eV) shift in the sweep voltage where electrons start to be collected. This would normally be interpreted as a difference in payload potential but the two detectors are directly connected each other and to payload ground. This discrepancy is so far unexplained.
