Global Observations of Earth s Ionosphere/Thermosphere. John Sigwarth NASA/GSFC Geoff Crowley SWRI

Transcription

1 Global Observations of Earth s Ionosphere/Thermosphere John Sigwarth NASA/GSFC Geoff Crowley SWRI

2 Overview Remote observation of Nighttime ionospheric density Daytime O/N 2 thermospheric composition Daytime ionospheric density

3 Nighttime Ionospheric Densities Nighttime ionospheric densities can be determined from nightglow emissions + e - The electron density n e2 is related to the nightglow emission by J = k O + e _ 2 n e

4 Dynamics Explorer/SAI

5 Polar/VIS

6 O/N 2 Composition Observations An important indicator for the condition of the upper atmosphere is the relative abundance of O and N 2 in the thermosphere. These are the major constituents of the thermosphere and play dominant roles in the atmospheric chemistry. The relative abundance of the O/N 2 can be determined using FUV wavelengths.

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9 st In this plot of the D st for April, 2002, the geomagnetic storm begins on April 17 and reaches D st minima of -126 nt at 8 UT on April 18 and -151 nt at 7 UT on April 20. The provisional D st are provided by the World Data Center for Geomagnetism, Kyoto.

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11 The decrease in O/N 2 results from heating of the lower thermosphere by direct impact of auroral precipitation and the associated joule heating in the ionosphere. Heated molecular species N 2 and O 2 in the lower thermosphere upwell to higher altitudes. Subsequently, these molecular species are carried to middle latitudes by thermospheric winds where they cool and dissipate. (Strickland et al., 1999 and references therein.)

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28 Hemispheric power: IMAGE/FUV and NOAA There are 6 intervals in the 4 days during which IMAGE provided total coverage of the northern auroral oval during which determination of auroral hemispheric power input with 2 minute resolution is possible. The agreement is generally good, but several differences stand out. Peak hemispheric power from FUV can exceed NOAA power by x2. (Immel, private communication)

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30 The O/N 2 neutral density ratio can be used to infer the daytime ionospheric density. Where solar illumination is the dominant ionization source and charge exchange followed by molecular ion recombination is the dominant ionization loss path, the ionospheric density is proportional to the O/N 2 ratio. e 2

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32 The nighttime ionospheric density can be determined from remote observations of the nightglow. The nightglow intensities vary quickly in space and time. Observations on the time scale of a few minutes will be required for comparisons to the insitu observations of the LWS IT spacecraft. The nighttime ionspheric densities are highly structured with spatial scales ~ 10 km.

33 The decrease in O/N 2 results from heating of the lower thermosphere by direct impact of auroral precipitation and the associated joule heating in the ionosphere. Heated molecular species N 2 and O 2 in the lower thermosphere upwell to higher altitudes. Subsequently, these molecular species are carried to middle latitudes by thermospheric winds where they cool and dissipate. These observations can be used to test and validate thermospheric models such as TIMEGCM.

34 Three results are immediately evident from inspection of the images. The O/N 2 column density ratio reaches a maximum reduction of 45%- 60%. The O/N 2 depletion regions are highly structured. Depleted regions are significantly different between the northern and southern hemispheres.

35 In general for the comparison of O/N 2 derived from Polar/VIS Earth Camera with the modeled output from TIMEGCM, the minimum O/N 2 ratios are reproduced. However, the details of the structures in the north and south as observed with the VIS are not reproduced in the TIMEGCM. Refined auroral inputs may produce better agreement.

36 Observations of the O/N 2 intensity ratios are proportional to the electron densities in dayside regions where photochemistry is a rapid process. These observations can then be used to infer the highly-structured dayside ionosphere electron densities.

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38 Craven, J. D., A. C. Nicholas, L. A. Frank, and D. J. Strickland, Variations in FUV dayglow brightness following intense auroral activity, Geophys. Res. Lett., 21, 2793, Drob, D. P., R. R. Meier, J. M. Picone, D. J. Strickland, R. J. Cox, and A. C. Nicholas, Atomic oxygen in the thermosphere during the July 13, 1982, solar proton event deduced from far ultraviolet images, J. Geophys. Res., 104, 4267, Immel, T. J., J. D. Craven, and L. A. Frank, Influence of IMF By on large- scale decreases of O column density at middle latitudes, J. Atmos. Terr. Phys., 59, 725, Meier, R. R., R. J. Cox, D. J. Strickland, J. D. Craven, and L. A. Frank, Interpretation of Dynamics Explorer far UV images of the quiet time thermosphere, J. Geophys. Res., 100, 5777, Nicholas, A. C., J. D. Craven, and L. A. Frank, A survey of large-scale variations in thermospheric oxygen column density with magnetic activity as inferred from observations of the FUV dayglow, J. Geophys. Res., 102, 4493, Strickland, D. J., R. J. Cox, R. R. Meier, and D. P. Drob, Global O/N 2 derived from DE 1 FUV dayglow data: Technique and examples from two storm periods, J. Geophys. Res., 104, 12,747, Strickland, D. J., J. D. Craven, and R. E. Daniell Jr., Six days of thermospheric-ionospheric weather over the Northern Hemisphere in late September 1981, J. Geophys. Res., 106, 30,291, 2001.