The EPD instrument suite

The EPT and HET sensors are colocated on two nearly identical sensor units: EPT-HET1 and EPT-HET2. The ICU serves as a single power and data interface between the spacecraft and all the EPD units. STEP, EPT and HET were developed by the Christian Albrechts University (CAU) of Kiel. SIS was developed by the Johns Hopkins University Applied Physics Laboratory (JHU/APL) and CAU. The ICU was developed by the ESA PI institution at the University of Alcalá, Spain. The location of the different EPD sensor units on the spacecraft is shown in the figure.

The EPD energy range starts with suprathermal electrons ( E > 2 keV) and extents up to low-energy galactic cosmic-ray (~ a few hundred of MeV/n), as shown in the left figure. This is achieved by a combination of the four EPD sensors which measure electrons, protons and ions using different techniques. The right figure shows the fields of view of the different EPD sensors in the spacecraft reference frame. The color-coded background corresponds to the hourly-averaged magnetic field vector distribution as observed by HELIOS-1 during 1975-1981. HELIOS-1 had an in-ecliptic elliptical orbit with a perihelion at 0.31 au and an aphelion at 0.99 au. Note the clustering of the sunward and anti-sunward apertures around the mean Parker spiral direction.

EPD main properties are summarized in the following table: