Cluster photoemission Aug 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 Recent low photoemission on SC2

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

EFW bias voltage sweeps Usually runs every four hours on every probe  ~10 5 sweeps available, year 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

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) 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 e ph 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 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: –I e ~ n : collected plasma e-, scales with density n –I ph (V sc ) : photoemission Saturation for V sc < 0 Decays for V sc > 0 –I i : negligible ion current – Current balance I e + I ph = 0  V sc = f(n) relation V sc 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

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 I ph (V sc ) depends on UV flux  relation n = g ( V sc ) 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(t)  n(t) /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 SC2  SC2 has recently had much lower perigee than others  Reached 200 km in early June

Severe photoemission drop on SC2

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 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 = March Red = 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 = March Red = 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