ERPA (Electron Retarding Potential Analyzer)

This instrument measures ambient thermal electrons in the energy range of about 0 to 3eV with steps of 0.06 eV.

The analyzer uses a selection screen with a swept retarding potential. Electrons with energies less than the retarding potential are rejected and those with higher energies pass through the screen and are collected on an anode below. The current collected by the anode is measured by a low noise electrometer circuit.


This photo shows the ERPA mounted in the ROPA sub-payload. It is mounted in the center of the forward end with several inches of clearance between the front face and any other object on the payload. The payload spin axis is aligned with the magnetic field so the ERPA looks straight up the field line.

The entrance face of the analyzer is held at a fixed +4V relative to payload ground to accelerate electrons into the entrance in the presence a negative payload potential. Gyro radius problems are minimized by keeping the analyzer dimensions small and mounting it in a field aligned direction.

This figure shows a theoretical model of the potential distribution near the ERPA with -1.5V payload potential and +4V bias on the entrance face. The light green area is at the neutral plasma potential. The dark blue is the negative payload potential and the red its the positively biased ERPA. The trace shows a path of a .01eV electron as it moves from its tight spiral in the neutral plasma and accelerates along the magnetic field toward the ERPA.

The +4V bias accelerates the electrons from the external plasma into the ERPA but once they cross the entrance screen they decelerate as they approach the selection screen. The net energy gained relative to the plasma goes to zero when the selection voltage exactly cancels the payload potential. If an electron passes though the selection screen it is then accelerated toward the anode.

This plot shows data from basic operation. The selection screen voltage sweeps from +2V to -2V in 64 1msec steps. When the selection voltage is more positive than the payload potential, the full energy range of electrons is collected. As the sweep becomes more negative, the lower energy electrons are rejected. Since the thermal energy spectrum only extends up a few tenths of an eV, eventually the sweep becomes negative enough to reject all the electrons and the current drops to zero. To see how many electrons were collected at each energy, the change in current at each step is computed to give a differential energy spectrum.

The full sweep is a triangle wave that ramps up and down measuring two energy spectra in 128 msec.



The primary data product of the ERPA is electron temperature measurements. The temperature of the energy distribution is obtained by fitting an exponential to the high energy tail. The algorithm selects points from above the peak extending out until the signal level drops into the noise. It does a linear regression fit to the log of the current. The differential spectrum is used because that removes any flat background level due to high energy auroral electrons. The temperature measurement depends only on the slope parameter so temperature can be derived without knowing density or payload potential.

These plots show a few example fits from ROPA data.

Payload Potential

The sweep voltage corresponding to the location of the peak in the differential energy spectrum is a rough approximation of the payload potential. The payload potential would actually be slightly lower than the peak and dependent on temperature but peak location is the parameter that is easy to extract from the data.


The ERPA does not provide an accurate density measurement. The maximum current collected during a sweep gives an indication of density but the absolute geometry factor of this instrument is difficult to model since it depends on sheath effects.

The total current collected by the +4V bias is measured as the SKIN channel. This channel gives a good indication of relative density but has similar problems in terms of an absolute density calibration. The front disk is basically like a fixed volatge Langmuir probe 2.2 inches in diameter. The current collected gives a rough estimate of number density.

The SKIN channel provides higher time resolution data on fluctuations since it does not require a full sweep.

The SKIN channel sees some variation due the the selection sweep. Most of the current is collected by the outer ring of the disk but the entrance is about 20% of the total area. When electrons are rejected by the screen some of them will return back to the plasma and are not collected. When the sweep is accepting electrons, many that pass the screen are off angle will be collected on the interior of the collimator. This adds to the SKIN current and causes about a 15% overall variation with sweep. The maximum anode current is about 4% of the SKIN current.

The SKIN channel also includes a small square wave current (+/- 6nA) due to capacitive coupling from the sweep. This effect is only noticeable at low signal levels. It is a constant amplitude and can be subtracted out.

High Energy Electrons

High energy electrons will not be affected by the selection screen and will be collected by the anode. The flux from auroral electrons is a small percentage of the thermal flux but it is measurable above the noise level. Current collected at the most negative sweep steps is from electrons above 3eV. The ERPA collimator is about 12 degrees wide so the reading at the top steps is from total flux of all field aligned electrons above 3eV.

This small background has no effect on the differential spectra but it can be used give an indication of when auroral electrons are present.