Transcript Cluster photoemission

Cluster photoemission
[email protected]
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 2003-2006
• 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 DF
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
 ~105 sweeps available, year 2000 - ...
V  VB  RI
Green line: fitted photoelectron saturation current
TIMED/SEE UV measurements
• TIMED: Sunsynchronous at
625 km
• SEE - Solar
Extreme
Ultraviolet
Experiment
• UV spectra 0.5
nm – 194 nm
• 1 nm bins
• ~2 hour intervals
• 2003 - ...
Can we find eph yield in space? ~No
• UV flux F(l,t)
• Photoemission
saturation current
I(t)
• Photoelectron yield
A(l)
Calculated yield
• I (t )   A(l ) F (l , t )dl
•  formally
possible to derive
material property
• Method: non-negative least-squares fit.
A(l): compare
• Result: unphysical spikes -- method is sensitive to
Cluster I(t) to
data errors and noise.
TIMED F(l,t)
• Solar cycle & annual trends OK, solar rotation
variations higher in UV data than in Iph
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 post2003 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 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
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:
s = 0.99 for raw density data
s = 0.87 for F10.7 correction
s = 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 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 =
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?
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