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Radio Sounding in the Polar Regions

Radio Sounding in the Polar Regions. Robert F. Benson Geospace Physics Laboratory (Code 673) Heliophysics Science Division NASA/GSFC Nathan Kurtz University of Maryland Baltimore County/GSFC Kris Atkins NASA Academy/University of Missouri-Rolla/GSFC Thorsten Markus

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Radio Sounding in the Polar Regions

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  1. Radio Sounding in the Polar Regions Robert F. Benson Geospace Physics Laboratory (Code 673) Heliophysics Science Division NASA/GSFC Nathan Kurtz University of Maryland Baltimore County/GSFC Kris Atkins NASA Academy/University of Missouri-Rolla/GSFC Thorsten Markus Hydrospheric and Biospheric Sciences Laboratory (Code 614.6) Earth Sciences Division NASA/GSFC GSFC: Goddard Space Flight Center, Greenbelt, Maryland

  2. OUTLINE • Radio sounding of the polar ionosphere & magnetosphere • Alouette & ISIS topside-sounding satellites • Auroral Kilometric Radiation (AKR) • OEDIPUS dual-payload auroral rocket-sounders • magnetic Field-Aligned electron-density (Ne) Irregularities (FAI) • Radio Plasma Imager (RPI) on the IMAGE satellite • polar magnetospheric Ne response to a magnetic storm • Radio sounding of polar surfaces • Ground-based ionosondes in polar regions as ice-penetrating radars • ionospheric interference between direct and "surface"-reflected rays • Alouette-1 surface reflections over Greenland • rarely observed - attributed to signal absorption by ice • ISIS-2 surface reflections over Antarctica • frequency of occurrence & intensity decreases with increasing ice thickness • Implications for planetary missions • Plasma investigations • Surface investigations

  3. Radio sounding of the polar ionosphere & magnetosphere • Polar-orbiting International Satellites for Ionospheric Studies (ISIS) • Alouette 1 : 1,000 km 10 yrs. of data (1962-1972) • Alouette 2: 500 - 3,000 km 10 yrs. of data (1965-1975) • ISIS 1: 550 - 3,500 km 21 yrs. of data (1969-1990) • ISIS 2: 1,400 km 19 yrs. of data (1971-1990) • Observations of Electric-field Distributions in the Ionospheric Plasma - A Unique Strategy (OEDIPUS) dual-payload auroral rocket-sounders • OEDIPUS A from Andøya Rocket Range (Jan 1989) to 512 km • new record length (958 m) for a space tether • OEDIPUS C from Poker Flat Research Range (Nov 1995) to 824 km • 1174 m tether cut after apogee ~ two free-flying payloads • Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) • 1,000 km - 8 RE radial distance 5 yrs. of data (2000-2005) • RPI used 3 orthogonal dipoles: 500 m spin-plane, 20 m spin axis • Other satellites with radio sounders in the polar regions • ISS b (1978), EXOS C (1984), EXOS D (1989) Japan • Interkosmos 19 (1979), COSMOS 1809 (1986) USSR

  4. ISIS 1 AKR source-region encounter intense X-mode radiation near the electron cyclotron frequency fH = fce from low Neregions fN = fpe = electron plasma frequency ( √ Ne) fN/fH = fpe/fce <<1 [Benson & Akasofu, Radio Sci., 19, 527, 1984]

  5. Fort Yukon aurora & ISIS-1 AKR detection & Ne orbit-plane contours broad Ne cavity AKR source-region magnetic field lines auroral-arc locations all-sky camera images with ISIS-2 orbital path and satellite location [Benson & Akasofu, Radio Sci., 19, 527, 1984]

  6. OEDIPUS-C trajectory with principal events Time after launch at 0638 UT on 7 Nov 1995 (s) [James and Calvert, 1998] Representative accomplishments First in-situ demonstration of Faraday rotation in space [James and Calvert, Radio Sci. 33, 617, 1998] Determination of the depletions (up to 20%) and width (~ 1 km) of Ne FAI [James, JGR, 111, A09315, 2006]

  7. IMAGE/RPI observations near 8 RE in polar cap before & after 31 March 2001 magnetic storm TS04 fce within 15% of observations on quiet & disturbed days [Tsyganenko & Sitnov, JGR, 110, A03208, 2005] fpe can be 2 x fpe(mod) on quiet day and 8 x fpe(mod) on disturbed day fpe(mod): Persoon et al., JGR, 88, 10123, 1983 fpe/fce fluctuations,with 3-hr delay, highly correlated (cc = 0.87) with solar-wind QI index fluctuations QI = (B2/8π)/(v2/2) Osherovich et al., JGR, 112, A06247, 2007

  8. Ellsworth, Antarctica ionosonde as an ice-penetrating radar interference between direct and sea-water reflected rays produce nulls in the ionospheric reflection traces used to calculate thickness and dielectric constant of ice Bowman [J. Atmos. Terr. Phys., 30, 1115, 1968]

  9. Alouette-1 surface reflection investigation by Muldrew et al.[Planet. Space. Sci. 15, 611, 1967] • surface echoes governed by: • directivity of sounder transmissions • radio noise level at the satellite • reflectivity at the Earth's surface • statistical study of ALOSYN tabulations (1: strong, 2: weak, and 3: no echo) • 10 degree by 10 degree bins • low lat obs. limited by 11.5 MHz sounder upper freq. limit • low % ground-echo occurrence ~ high noise regions • some indication that sea is a better reflector than land, yet strong ground echoes were obtained over desert conditions in central Australia • lack of reflections over Greenland imply that the signals penetrate the ice (approximately 3 km thick) and are absorbed before being reflected • above based on Piggott and Barclay [J. Atmos. Terr. Phys.,20, 298, 1961] conclusions that main radio reflections from the Halley Bay, Antarctica ionosonde were 22 dB stronger at the sea/ice interface 150 m beneath the station than the partial reflections from the air/snow surface

  10. Anatomy of an ISIS-2 digital topside ionogram Ionosphere Penetration Frequency Surface Reflection Ionospheric Reflections Ground-based transmitter interference

  11. ISIS-2 pass illustrating variations in surface reflection intensity

  12. ISIS-2 pass illustrating variations in surface reflection intensity

  13. ISIS-2 pass illustrating variations in surface reflection intensity

  14. ISIS-2 pass illustrating variations in surface reflection intensity

  15. ISIS-2 pass illustrating variations in surface reflection intensity

  16. Determination of ISIS-2 surface reflection power • satellite rotation (20 s period) affects power values - used maximum reflected signal ~ assumed to represent optimum antenna orientaion • problems with Automatic Gain Control (AGC) voltage prevented power-value calculations in some cases • calculated values compared with expectations from a mirror reflection • independent classification scheme also used which included cases where a reliable power calculation was not possible • 1: no echo • 2: questionable echo • 3: weak echo • 4: medium strength echo • 5: strong echo

  17. Surface reflection power for two ISIS-2 passes over Antarctica Glacial Ice Salt Water Ocean

  18. Surface reflection power for two ISIS-2 passes over the Ross Ice Shelf Ice Shelf Salt Water Ocean  ionograms shown on next two sldes

  19. ISIS-2 Ionogram recorded over Ross Ice Shelf with ionospheric but no surface reflections (-83.4° geographic latitude; 1307:48 UT on 28 Nov 1974) ionospheric reflections

  20. ISIS-2 Ionogram recorded over ocean with ionospheric and surface reflections (-70.7° geographic latitude; 1311:55 UT on 28 Nov 1974) Surface reflections

  21. Many ISIS-2 passes investigated in four distinct areas of Antarctica • Ross Ice Shelf area • area with no known underground lakes • area with a heavy concentration of underground lakes beneath 3,000 to 4,000 m of ice • Lake Vostok (beneath 3741-4150 m of ice) and area which includes a region not explored for underground lakes • Lake information from airborne surveys [Siegert et al., Antarctic Sci., 17, 453, 2005]

  22. ISIS-2surface echo power relative to mirror reflection 1 Ross Ice Shelf 2 no lakes 3 lake region 4 Lake Vostok area 4 1 3 2

  23. Surface-echo strengths from all ISIS-2 ionograms inspected . lake or Ross Ice Shelf boundary no echo + questionable echo + weak echo + medium-strength echo + strong echo +

  24. Summary of observations related to surface reflections • stronger over water than land • salt water ocean reflections had the highest reflectivity; typically +15 dB relative to mirror reflections • average land reflections typically -10 dB relative to mirror reflections • thick glacial ice reflections ranged from -17 dB relative to mirror reflections to no detectable echoes • many strong echoes observed over the Antarctica region with many underground lakes • only two reflections, out of five possible, observed over Lake Vostok - possibly because lake surface approximately 4 km beneath the ice; some near-by reflections possibly due to non-vertical propagation • many reflections, though weak, observed over the area with no known underground lakes • several relatively strong reflections observed in area not explored for underground lakes

  25. Summary of observations related to ionosphere & magnetosphere • extremely limited selection of topics from subjects covered in over 1,000 publications based on space-borne radio sounders • intense AKR is generated in the X-mode very near the electron cyclotron frequency in regions of low electron density (fpe/fce << 1) • key observations for the cyclotron maser mechanism of Wu & Lee [Astrophys. J., 230, 621, 1979] • Ne FAI ~ depletions from a few to 20% with cross-field scales of few km • common high-latitude features that scatter and duct radio waves • may support force-free em cylindrical oscillations as a new wave mode [Osherovich et al., IEEE Trans. Plasma Sci., 33, 599, 2005] • accurate determinations of magnetospheric fpe/fce during magnetic storms • fpe and fce both increase - but fpe more than fce high in the polar cap • fpe/fce as a new magnetospheric index • fpe/fce and solar-wind index fluctuations highly correlated [Osherovich et al., J Geophys. Res., 112, A06247, 2007]

  26. Implications for radio sounders on planetary missions • Planetary ionospheres and magnetospheres (minute sample of results based on space-borne radio sounders in geospace plasma) • terrestrial AKR analog is common in cosmos • [Wu, Space Sci. Rev., 41, 215, 1985; Ergun et al., Apj., 538, 456, 2000] • FAI enable long-range diagnostics & support local oscillations • [Huang et al., Adv. Space Sci., 33, 829, 2004; Osherovich et al., JGR, 98, 18751, 1993] • electron plasma-to-cyclotron ratio fpe/fce comparable in different plasmas • [Osherovich, JGR, 94, 5530, 1989; Benson & Osherovich, Radio Sci., 39, RS1S28, 2004] • Planetary surfaces investigated by ionospheric sounders • Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express Orbiter, currently in operation • [Farrell et al., GRL, 32, L11204, 2005; JGR, in press, 2008; Gurnet et al., Science, 310, 1929, 2005] • advanced planetary radio sounders proposed to explore Galilean icy moons • [Markus et al., Tenth Int. Conf. on Ground Penetrating Radar, 21-24 June, Delft, The Netherlands] • present results over known terrain on Earth are relevant to such missions

  27. Underground lakes from airborne surveys (insert) [Siegert et al., Antarctic Sci., 17, 453, 2005]

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