Understanding Photoemission in Plasma: Cluster EFW Study 2003-2006

1. Cluster photoemission Anders.Eriksson@irfu.se Aug 24, 2011

2. 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

3. This presentation • Study of photoemission 2003-2006 • Recent low photoemission on SC2

4. Cluster EFW instruments EFW:Electric Fields and Waves instrument Four probes on 44 m wire booms on all four Cluster s/c Double-probe instrument, measures E from DF Bonus data products: - Spacecraft potential, Vsc (continuous, 5 Hz) - Photoemission current from bias voltage sweeps (semi-hourly)

5. EFW bias voltage sweeps Usually runs every four hours on every probe  ~105 sweeps available, year 2000 - ... Green line: fitted photoelectron saturation current

6. 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 - ...

7. Can we find eph yield in space? ~No • UV flux F(l,t) • Photoemission saturation current I(t) • Photoelectron yield A(l) •  formally possible to derive material property A(l): compare Cluster I(t) to TIMED F(l,t) Calculated yield • 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

8. 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

9. 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 density Empirical relation: EFW Vsc vs. plasma density from Cluster CIS ion spectrometer: 1.1 million data points (spins) from Feb-March 2003, 2004 & 2005

10. n(Vsc) relation II: Depends on UV n-Vsc curve clearly varies with solar cycle [Pedersen et al, JGR 2008]

11. 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?

12. 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: • s = 0.99 for raw density data • s = 0.87 for F10.7 correction • s = 0.81 for sweep photoemission correction

13. Recent photoemission drop on SC2 • SC2 has recently had much lower perigee than others • Reached 200 km in early June

14. Severe photoemission drop on SC2

15. Photocurrent determined from Vb sweeps • Suggest 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

16. No recovery signature yet Data up to early August Colour codes time: Blue = March Red = August ISEE-1 saw recovery If our probes were pure Al before, are they Al oxide now?

17. Langmuir mode data suggest 90 nA Data up to early August Colour codes time: Blue = March Red = August ISEE-1 saw recovery If our probes were pure Al before, are they Al oxide now?

18. Conclusions • 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