TIMED/GUVI Data for IRIR. DeMajistre, L.J. Paxton, D. Bilitza and H.Kil

IRI 2005

GUVI for the ionosphere

• What is TIMED/GUVI

• Measurement methods for the nighttime ionosphere

• Ionosonde validation

• Application – Calibration transfer

• GUVI and IRI

Motivation for future work and collaboration rather than final results

GUVI - Global UV Imager

• Scanning Spectrographic Imager

• Covers 115-180 nm in 160 nm spectral bins

• 14 cross track spatial pixels act as a linear array

TIMED Spacecraft

• High inclination circular orbit, ~625 km altitude

• Covers all local times in 60 days

• Nightside passes ~25 degrees apart

Nightside measurements

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GUVI Colors

• GUVI ‘colors’ summed on board – 5 bright FUV spectral features

• Only 2 colors are usually detected at night

• Color 3, 135.6 nm, due almost entirely from F region recombination

Disk MeasurementsGUVI

F Layer

• GUVI down-looking pixels yield slant column brightness through the F Layer

• Can be converted to TEC with some assumptions about profile shape

• Very high spatial resolution (~ 25x25 km) reveals bubbles and irregularities

Composite 135.6 nm image for day 82, 2002

Limb Measurements

• First 32 steps of each scan are on the limb (100 – 520 km)

• Constrained linear inversion yields volume emission rate/electron densities

Systematic monitoring of the nighttime F Region

Monitoring the IonosphereCoverage• Altitude – Each scan, 150 – 500 km• Latitude – Each orbit• Longitude – Each day• Local time – 60 days• Over 3 years of data yields seasonal coverage as well

Limits• Low signal levels away from the anomalies• Assumption of spherical symmetry• Recombination rate is uncertain• Assumes one constituent ionosphere (O+)

Global Observations

• Electron densities can be used to estimate NMF2

• Coverage allows global maps

• Orbit precession rate allows inter-annual comparisons

• Can be compared directly with ionosonde data or IRI

Ionosonde Comparisons

• GUVI electron density profile fit with a Chapman layer

• NMF2 and HMF2 taken directly from the fit

• Same process can be used with IRI

Ionosonde DataStation Lat. Long.

Juliusruh, Germany 54.6 13.4

Wakkanai, Japan 45 141

Ebro, Spain 40.8 0.5

Kokubunji, Japan 35 139

Yamagawa, Japan 31 130

Delhi, India 28.6 77.2

Okinawa, Japan 26 128

Waltair, India 17 83

Trivandrum, India 8.3 76.6

Sao Luis, Brazil -2.3 315.8

Fortaleza, Brazil -3.9 321.5

C. Paulista, Brazil -22.7 315

B. Aires, Argentina -34.6 301.7

Concepcion, Chile* -36.8 287

Cachimbo, Brazil -9.5 305.2

• Ionosonde Data Supplied to from the various stations

• Data from other stations was supplied but had no suitable GUVI coincident measurement

GUVI Selections

Criteria for selecting coincidences

• Within 200 km and 20 minutes

• Chapman layer fit successful– Good fit to data– Realistic HMF2 and NMF2

• No ‘qualifying letters’ on ionosonde data

427of 1112 observations met these criteria

• GUVI and IRI have similar spread (~30-40%)

• GUVI compares well between 0.5 and 1.0 (106 cm-3)

• IRI bias similar in shape to GUVI

• Both GUVI and IRI show ‘discontinuity’ at 1.0 (106 cm-3)

IRI/GUVI comparison

• Similar spread as (~30%)

• At higher NMF2, GUVI systematically low

• Slope of bias is constant (suggests calibration of GUVI is reasonable)

Conclusion of comparisons

• At moderate NMF2, GUVI predicts ionosonde measurements with 30% or so

• In the same range, IRI behaves the same way with a fairly large positive bias

• At larger NMF2 both under-predict

Calibration Transfer

• The issue – Ionosonde intercalibration– Various instrument designs– Separate calibrations– Independently operated

• Example solution – Ozone monitors– ‘Standard instrument’ carried from site to site– Makes simultaneous measurements

• Suggested approach – Orbiting UV monitoring– Use GUVI (and/or its successors) to transfer

calibration

Regional Analysis - significant geographic differences

Inter-ionosonde Comparisons

• Significant differences between stations

• Significant regional differences

• GUVI statistics can be used to identify areas that may need attention– In principle, GUVI measurements should

have little geographic dependence– Geographic differences in profile shapes and

horizontal gradients may have some influence

GUVI and IRI - Conclusions

• GUVI can provide systematic observations– Can be used to compare with IRI– Once differences are understood,

observations can be included in IRI

• GUVI can be used to refine ionosonde measurements– Provide better IRI validation– Provide better IRI input