Detection of photoeletrons from the EFW probes A Fazakerley et al

Photoemission from EFW Photoemission from Cluster Effect of varying bias current Effect of varying guard potential

Probe: 8 cm diameter sphere; attached by thin (0.3 mm tbc) radius wire Trying to achieve effective isolation of the probe Preamplifier Puck: 8 cm diameter disc, 3 cm long, biased to Vprobe+ 1V Idea is to “limit the photoelectrons from the puck coming to the probe” Guard: biased to Vprobe- 6V Idea is to “limit the influence of probe photoelectrons on the spacecraft side of the guard” Wire booms at spacecraft potential thin (1.1 mm tbc) radius Probe-probe distance = 88m Photoemission from EFW EFW Probe photoemission: e-folding energy 1.5 eV based on lab tests Details: Cully et al. 2007, Pedersen et al. 2008

Photoemission from EFW Details: Pedersen et al 2008

(situation with Sun at large +ve x; y = 0) Photoemission from EFW Cully et al. JGR 2007 Vsc = 25 V; Vp + 20 V (case of large Debye lengths – lobes, plasmasheet) Many probe electrons will be attracted to the wire booms and the spacecraft body Probe system

Detection of probe photoemission EDI PEA HEEA PEA LEEA ASPOC EFW HEEA LEEA Spacecraft photoelectrons probe photoelectrons or cold ambient plasma electrons? Note LEEA apparently shows more asymmetry in the eV electrons

PEACE Observations A test was carried out in which the bias current to the probes was changed, turning their photoemission on and off. EFW bias current is periodically perturbed in the second part of this interval There is clear control of the high flux “beam” near the potential. (Note, this was not done for C4, bottom panel) C1 C2 C3 C4 Detection of probe photoemission

EFW: bias voltage 10 V EFW: bias current -100 nA There is clear control of the high flux “beam” near the potential. With a strong –ve bias current, the probe emits all its photoelectrons, some of which are then detected by PEACE. Apparently no significant fluxes of cold ambient plasma – would be expected at similar energies before and after the bias current was set to -100 nA Plot of EFW bias current /voltage history here Detection of probe photoemission

Now EDI produces a strong electron beam current on C1 and C3 and drives Vsc ~100s V EFW cannot provide enough bias current for to make the probes –ve w.r.t. the plasma when Vsc > 100 V, and the probes are positive and emitting much lower fluxes EFW: bias voltage 10 V EFW: bias current -100 nA Detection of probe photoemission Are we seeing cold ambient plasma electrons when bias voltage is at 10 V? It would be good to see if the EFW wake technique reveals anything at this time

Vguard +20V Same for Vguard > 2V Vsc ~ V Limited evidence of probe electrons Role of the Guard Potential

Vguard +2V Vsc ~ V Increasing evidence of probe electrons

Role of the Guard Potential Vguard +0V Vsc ~ V Strong evidence of probe electrons; significant spin asymmetry

Role of the Guard Potential Vguard -2V Vsc ~ V Strong evidence of probe electrons; significant spin asymmetry

Role of the Guard Potential Vguard -6V (standard setting) Vsc ~ V Moderate flux of probe electrons; significant spin asymmetry in LEEA but more symmetric in HEEA... (reason tbc)

Role of the Guard Potential Vguard -10V Vsc ~ 50 V Moderate evidence of probe electrons; significant spin asymmetry in LEEA and some in HEEA...

Role of the Guard Potential Vguard -20V Vsc ~ V Moderate evidence of probe electrons; significant spin asymmetry in LEEA and some in HEEA...

Summary Clear demonstration of detection by PEACE of EFW probe electrons reaching the spacecraft body – will contribute to the electric current balance that controls the spacecraft electrostatic potential Demonstration of the limits of EFW control of probe emission in rarefied plasma plus strong EDI current Demonstration of effect of varying the “guard” potential; – No probe electron flux for strong positive V_g – Quite asymmetric probe electron flux to spacecraft for small positive V_g (leads to spin phase dependent potential tbc) – More symmetric probe electron flux for V_g = -6V (value preferred by EFW to assist making good E field measurements; presumably because get spin independent Vsc) – Potential varies as current balance alters – (could be understood in more detail using future spacecraft-plasma interaction simulations)