Cluster photoemission

Anders.Eriksson@irfu.seAug 24, 2011

Photoemission

• Essential for EFW as the coupling of probes to plasma is mainly through photoemission

• The bias current applied to the probes is carried to the plasma mainly by photoemission

• Photoemission determines the density-s/c potential relation useful for estimating n in tenuous plasmas

This presentation

• Study of photoemission 2003-2006• Recent low photoemission on SC2

Cluster EFW instruments

EFW: Electric Fields and Waves instrumentFour probes on 44 m wire booms on all four Cluster s/c

Double-probe instrument, measures E from

Bonus data products:- Spacecraft potential, Vsc (continuous, 5 Hz)- Photoemission current from bias voltage sweeps (semi-hourly)

EFW bias voltage sweeps

BV V RI Usually runs every four hours on every probe ~105 sweeps available, year 2000 - ...

Green line: fitted photoelectron saturation current

TIMED/SEE UV measurements

• TIMED: Sun-synchronous at 625 km

• SEE - Solar Extreme Ultraviolet Experiment

• UV spectra 0.5 nm – 194 nm

• 1 nm bins• ~2 hour intervals• 2003 - ...

• UV flux F(,t)• Photoemission

saturation current I(t)

• Photoelectron yield A()

• • formally possible

to derive material property A(): compare Cluster I(t) to TIMED F(,t)

dtFAtI ),()()(

• Method: non-negative least-squares fit.• Result: unphysical spikes -- method is sensitive to

data errors and noise.• Solar cycle & annual trends OK, solar rotation

variations higher in UV data than in Iph

Calculated yield

Can we find eph yield in space? ~No

Do lab-determined yields fit? ~Yes

• EFW: Al probes coated with DAG 213

• Lab yield curve for DAG gives too low current

• Lab yield curve for Al gives good fit post-2003 if increased by 10%

• Suggests DAG weared off in space

• Too high predicted currents in 2003 cannot be explained by wear (as DAG has lower yield than Al)

• DAG: Feuerbacher & Fitton, J. Appl. Phys., 1972• Al: Samson & Cairns, Rev. Sci. Instr., 1965

n(Vsc) relation I: Fundamentals

• Currents to spacecraft:– Ie ~ n: collected plasma e-,

scales with density n– Iph(Vsc): photoemission

• Saturation for Vsc < 0

• Decays for Vsc > 0

– Ii: negligible ion current

– Current balance Ie + Iph = 0 Vsc = f(n) relation

• Vsc a proxy for the densityEmpirical relation: EFW Vsc vs. plasma density from Cluster CIS ion spectrometer: 1.1 million data points (spins) from Feb-March 2003,

2004 & 2005

n(Vsc) relation II: Depends on UV

n-Vsc curve clearly varies with solar cycle

[Pedersen et al, JGR 2008]

n(Vsc) relation III: Correct for UV

• Photoemission current Iph(Vsc) depends on UV flux

• relation n = g(Vsc) depends on UV flux

• Should be improved if corrected for UV variations

• Possibilities:– Photoemission current from

sweeps– UV flux from TIMED– F10.7 UV proxy Same data as before, but density now normalized

to the photoemission current derived from adjacent sweeps, thus removing UV variations.

Spread appears less - true?

n(Vsc) relation IV: Improved? Yes!

Does UV variation compensation really improve the use of Vsc as a density proxy?

Compare empirical relations of Vsc(t) to (<> is time average):

Raw density n(t) from CIS

n(t) <F10.7>/F10.7(t)

n(t) <Iph>/Iph(t)

Quality quantified by the root mean square deviation (standard deviation) from a line least-squares fitted to the log-log plots

Resulting RMS deviations:

= 0.99 for raw density data

= 0.87 for F10.7 correction

= 0.81 for sweep photoemission correction

Recent photoemission drop on SC2SC2 has recently had much lower perigee than others

Reached 200 km in early June

Severe photoemission drop on SC2

Photocurrent determined from Vb sweepsSuggest zero (or even wrong sign!) of photoemission

This contrasts to the fact that we do see saturation of the E-field signal only part of the time with -20 nA bias current

Suggests real photosaturation current is something like 20-30 nA

Why do the Vb sweeps not agree with Ib operation?

Difference is ~50 nA

Corresponds to a 50 nA * 5 Mohm = 250 mV offset (to negative) somewhere in the Vb mode

Vb sweeps underestimating Iph0 (compared to Ib) consistent with previous observations

Anyway, we seem to have a drop by approx a factor 10

ISEE-1 saw a drop by a factor 3-4 when going down to 500 km

Very consistent with our data

No recovery signature yet

Data up to early August

Colour codes time:Blue = MarchRed = August

ISEE-1 saw recovery

If our probes were pure Al before, are they Al oxide now?

Langmuir mode data suggest 90 nA

Data up to early August

Colour codes time:Blue = MarchRed = August

ISEE-1 saw recovery

If our probes were pure Al before, are they Al oxide now?

The photoemission current determined from Cluster EFW probe bias sweeps correlate well but not perfectly with UV flux measurements from TIMED SEE

An attempt to derive the photoelectron yield curve of the EFW probes by non-negative least squares fitting failed

Laboratory yield for the original probe coating (DAG) only gives 50% of the photoemission, while pure Al fits within 10% -- has the coating all worn off?

The use of spacecraft potential as a proxy for plasma density is improved by correcting for UV flux variations, preferably from sweeps

SC2 photoemission dropped by a factor 10 when perigee reached 200 km

Consistent with ISEE-1 drop of factor 3-4 at 500 km

Photosaturation currents from Ib and Vb operations not consistent

Conclusions