removal of background light from smei white- light all-sky maps, and an all-sky imager design...
TRANSCRIPT
Removal of background light from SMEI white-light all-sky maps,
and an all-sky imager design suitable for future deep-space missions
Andrew Buffington, Bernard V. Jackson, Mario M. Bisi, John M. Clover and P. Paul Hick
Center for Astrophysics and Space Sciences, UCSD, USA
SMEI – I.
• White-light sky maps from SMEI have demonstrated this instrument’s capability for detecting coronal mass ejections (CMEs) and tracking them from nearby the Sun to beyond Earth.
• Three-dimensional reconstruction using the time series of these maps enables an unfolding of heliospheric density along the line of sight. For example, see Jackson et al., 2005, 2006.
• This in turn permits separation of individual CME structures and determination of their masses and velocities.
• Comparison of results with in-situ measurements certifies this reconstruction process.
• More details in Eyles et al., 2003 and Jackson et al., 2004.
SMEI – II.• For successful 3D reconstruction, the white-light maps must
have non-Thomson-scattering background light, typically 100× larger, removed. This includes– Starlight and nebulae (the sidereal sky), plus– Sunlight scattered from the zodiacal dust cloud.
• Especially for 2D (line-of-sight) interpretations, this can be done approximately by viewing difference maps in which an earlier sky map is subtracted from a present one. This enables tracking of structures as they move away from the Sun.
• However, this method tends to erase any heliospheric structures that move or evolve more slowly than the time difference between the maps.
• The present method preserves even large-scale heliospheric structures, but removes most background which repeats from year to year.
• Its success depends critically upon SMEI’s nearly full-sky coverage.
(Left) The Coriolis spacecraft with the Solar Mass Ejection Imager (SMEI) instrument (Jackson et al., 2004) and the Windsat antenna prior to launch from Vandenberg AFB. The three SMEI camera baffles (circled) are seen on the lower portion of the spacecraft. (Right) SMEI in its polar orbit at 840 km with an orbital inclination of 98. SMEI looks away from the Earth at 30 from the local horizontal to avoid sunlight reflected from the Earth and from the Windsat antenna. The combined fields of view of the three cameras (shown as shaded cones) cover nearly all the sky.
Sidereal Sky
• Use 29 high-resolution calibration periods spanning 5½ years.• Typically 15 to 30 orbital maps/period.• Subtract individual stars brighter than 6th magnitude (~6000).
– Hick et al. 2007
• Remove particle hits & space debris.• Average half the maps below the median (to reduce aurora).• All maps presented here are “as the sky appears”. (Atlas format)
• Intensity scale is SMEI analog-to-digital units (ADUs):– One S10(V) is 0.46 ± 0.02 ADUs (Buffington et al., 2007).
• SMEI optics feeds a “wide open” CCD with only 2 mirrors.– Other uses of our maps might need correction for bandpass differences.
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Zodiacal Light – “Parameter Hell”
• A “work in progress” (formulae + > 30 parameters)
– Gegenschein results accepted for publication in ICARUS
• Empirical formulae fit to all SMEI data, then subtract.• Average over 6 years, fold by day of year (DOY).• Some inevitable ambiguity exists with Sidereal sky.• Current work in progress (~ last 10%):
– DOY-dependent corrections.
– Reducing camera 3 artifacts {planets (especially Venus), residual auroral light & data near shutter closure}.
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Zodiacal-light map for January 1, 2003 from our empirical model. The intensity scale here has a minimum of 20 ADUs to more clearly show the Gegenschein enhancement near R.A.= 6h 40m and Declination +23º. 1-year MOVIE
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Current Status – Zodiacal Light Removal• Main zodiacal light is subtracted per present formulae.• Additional stage of processing refines removal (~last 10%)
– Subtract a small additional modeled residue, and overall residue map.– Includes adjustment based on {SMEI orbital axis-to-Sun} angle.– Includes empirical baseline shift of camera 3 relative to other cameras,
when the in-flight mask is used to stamp out “hot pixels”, which alters the dark-current median-to-mean relationship.
– Finally, a weekly residue map is subtracted.• Final end-product result expected to yield the original SMEI
specification of 0.1% differential photometry over most (> 70%) sky for cameras 1 & 2, and perhaps a factor of two worse for camera 3. 6-year antisolar-centered and solar-centered MOVIES
• Six years of data permits a reasonable separation of annually repeating zodiacal signal, and removal of various artifacts (mostly auroral) .
• Final product is mostly Thomson-scattered sunlight.
REFERENCES (SMEI):
Buffington, A., Jackson, B.V., Hick, P., Price, S.D., 2006. An Empirical Description of Zodiacal Light as Measured by SMEI. EOS Trans. AGU 87(52), Fall Meeting Suppl., Abstract, SH32A-06.Buffington, A., Morrill, J.S., Hick, P.P., Howard, R.A., Jackson, B.V., Webb, D.F., 2007. Analyses of the comparative responses of SMEI and LASCO. Proc. of SPIE 6689, 66890B 1−6, doi: 10.1117/12.734658.Buffington, A., Bisi, M.M., Clover, J.M., Hick, P.P., Jackson, B.V., Kuchar, T.A., and Price, S.D., 2009. Measurements of the Gegenschein brightness from the Solar Mass Ejection Imager (SMEI). Paper accepted for publication in Icarus.Cox, A.N., 2000. Allen’s Astrophysical Quantities, fourth edition. Springer-Verlag, New York, p. 330.Eyles, C.J., Simnett, G.M., Cooke, M.P., Jackson, B.V., Buffington, A., Hick, P.P., Waltham, N.R., King, J.M., Anderson, P.A., Holladay, P.E., 2003. The Solar Mass Ejection Imager (SMEI). Solar Phys. 217, 319-347.Hick, P.P., Buffington, A., Jackson, B.V., 2007. A Procedure for Fitting Point Sources in SMEI White-Light Full-Sky Maps. Proc. of SPIE 6689 66890C 1−8, doi: 10.1117/12.734808.Jackson, B.V., and 23 co-authors, 2004. The Solar Mass Ejection Imager (SMEI) mission. Solar Phys. 225, 177-207.Jackson, B.V., Buffington, A., Hick, P.P. and Wang, X., 2005. Low Resolution Three Dimensional Reconstruction of CMEs Using Solar Mass Ejection Imager (SMEI) Data, Proc. SPIE 5901, 590101, 1-12. Jackson, B.V., Buffington, A., Hick, P.P., Wang, X. and Webb, D., 2006. Preliminary three-dimensional analysis of the heliospheric response to the 28 October 2003 CME using SMEI white-light observations. J. Geophys. Res. 111, A4, A04S91.