R. Gladstone

Far Ultraviolet Imaging from the Image Spacecraft: 1. System Design

Direct imaging of the magnetosphere by the IMAGE spacecraft will be supplemented by observation of the global aurora, the footprint of magnetospheric regions. To assure the simultaneity of these observations and the measurement of the magnetospheric background neutral gas density, the IMAGE satellite instrument complement includes three Far Ultraviolet (FUV) instruments. In the wavelength region 120-190 nm, a downward-viewing auroral imager is only minimally contaminated by sunlight, scattered from clouds and ground, and radiance of the aurora observed in a nadir viewing geometry can be observed in the presence of the high-latitude dayglow. The Wideband Imaging Camera (WIC) will provide broad band ultraviolet images of the aurora for maximum spatial and temporal resolution by imaging the LBH N2 bands of the aurora. The Spectrographic Imager (SI), a monochromatic imager, will image different types of aurora, filtered by wavelength. By measuring the Doppler-shifted Ly-α, the proton-induced component of the aurora will be imaged separately. Finally, the GEO instrument will observe the distribution of the geocoronal emission, which is a measure of the neutral background density source for charge exchange in the magnetosphere. The FUV instrument complement looks radially outward from the rotating IMAGE satellite and, therefore, it spends only a short time observing the aurora and the Earth during each spin. Detailed descriptions of the WIC, SI, GEO, and their individual performance validations are discussed in companion papers. This paper summarizes the system requirements and system design approach taken to satisfy the science requirements. One primary requirement is to maximize photon collection efficiency and use efficiently the short time available for exposures. The FUV auroral imagers WIC and SI both have wide fields of view and take data continuously as the auroral region proceeds through the field of view. To minimize data volume, multiple images are taken and electronically co-added by suitably shifting each image to compensate for the spacecraft rotation. In order to minimize resolution loss, the images have to be distortion-corrected in real time for both WIC and SI prior to co-adding. The distortion correction is accomplished using high speed look up tables that are pre-generated by least square fitting to polynomial functions by the on-orbit processor. The instruments were calibrated individually while on stationery platforms, mostly in vacuum chambers as described in the companion papers. Extensive ground-based testing was performed with visible and near UV simulators mounted on a rotating platform to estimate their on-orbit performance. The predicted instrument system performance is summarized and some of the preliminary data formats are shown.

The electron and proton aurora as seen by IMAGE-FUV and FAST

The Far Ultraviolet Instrument (FUV) on the IMAGE spacecraft observes the aurora in three different channels. One of them (SI12) is sensitive to the signal from precipitating protons, while the other two (WIC and SI13) observe auroral emissions which are not only excited by precipitating electrons, but also by protons. We examine a period when in-situ particle measurements by the FAST spacecraft were available simultaneously with global imaging with FUV. The measured electron and proton energy spectra are used to calculate the auroral brightness along the FAST orbit. The comparison with the FUV/IMAGE observations shows good quantitative agreement and demonstrates that under certain circumstances high proton fluxes may produce significant amounts of auroral FUV emission.

Global observations of proton and electron auroras in a substorm

This is the first report of a substorm observed by the IMAGE FUV instruments permitting global observations of electron and proton produced auroras. On the 28th of June 2000 at 1956 UT in the pre-substorm phase at early evening local time the proton aurora was equatorward of the electron precipitation and near midnight they were collocated. There was bright electron and proton aurora in the post midday afternoon side. The sudden brightening of the aurora at substorm onset near midnight is seen in the electrons only although there are protons present at this location. During the expansive phase both the electrons and protons expand poleward. The electron aurora forms a bright surge at the poleward boundary while the protons just show diffuse spreading. The peak intensity of the protons did not change substantially during the entire event. The proton aurora is brighter on the dusk while the electron aurora on the dawn side. As the electron surge expands poleward it leaves the protons behind. The electrons form a discrete auroral feature near the aurora-polar cap boundary, which is devoid of substantial energetic (>1 keV) proton precipitation. The presence of precipitating protons at the point where the initial brightening is seen shows that substorms are initiated on closed field lines.

On the possible noble gas deficiency of Pluto’s atmosphere

Abstract We use a statistical–thermodynamic model to investigate the formation and composition of noble-gas-rich clathrates on Pluto’s surface. By considering an atmospheric composition close to that of today’s Pluto and a broad range of surface pressures, we find that Ar, Kr and Xe can be efficiently trapped in clathrates if they formed at the surface, in a way similar to what has been proposed for Titan. The formation on Pluto of clathrates rich in noble gases could then induce a strong decrease in their atmospheric abundances relative to their initial values. A clathrate thickness of order of a few centimeters globally averaged on the planet is enough to trap all Ar, Kr and Xe if these noble gases were in protosolar proportions in Pluto’s early atmosphere. Because atmospheric escape over an extended period of time (millions of years) should lead to a noble gas abundance that either remains constant or increases with time, we find that a potential depletion of Ar, Kr and Xe in the atmosphere would best be explained by their trapping in clathrates. A key observational test is the measurement of Ar since the Alice UV spectrometer aboard the New Horizons spacecraft will be sensitive enough to detect its abundance ∼10 times smaller than in the case considered here.

High-latitude ion transport and energetic explorer (HI-LITE): a mission to investigate ion outflow from the high-latitude ionosphere

The proposed HI-LITE Explorer will investigate the global ion outflow from the high-latitude ionosphere, its relationship to auroral features, and the consequences of this outflow on magnetospheric processes. The unique nature of the HI-LITE Explorer images will allow temporal and spatial features of the global ion outflow to be determined. The missions scientific motivation comes from the fundamental role high-latitude ionospheric ions play in the dynamics of the solar wind driven magnetospheric-ionospheric system. These outflows are a major source of plasma for the magnetosphere and it is believed they play an important role in the triggering of substorms. In addition this paper describes the HI-LITE spacecraft and instruments.

Development of a near‐infrared balloon‐borne camera for dayside and sunlit auroral observations

Imaging aurora in daylight is a difficult and challenging task. The brightness of the sunlit atmosphere overwhelms the auroral emissions at visible wavelengths. Modeling of atmospheric brightness suggests that the contrast between auroral brightness and sky brightness makes it possible to image the aurora at near infrared (NIR) wavelengths from sufficient altitudes. Preliminary experiments confirmed that the auroral N2+ Meinel emissions at about 1100 nm are bright enough to be extracted from atmospheric background brightness during daylight at about 40 km of a balloon altitude, which lead to the development of a high-performance NIR InGaAs camera that can be flown on a high-altitude and long-duration balloon. Auroral observations from such a platform are highly accommodated to current space missions (such as THEMIS/ARTEMIS, MMS, Cluster, Geotail, DMSP) and many ground-based measurements and will enhance the science return significantly.

An Early Report on Iue Observations of the Impact of Comet Shoemaker-Levy With Jupiter

W. M. Harris, G. E. Ballester, J. Barker, J. Clarke, M. Combi, M. Vincent (U. Michigan, USA), R. Gladstone (SRI, USA), J. Kozyra (U. Michigan, USA), R. Prangi (IAS, France), L. Ben Jaffel (IAP, France), J.-P. Bibring, C. Emerich (IAS, France), W. Ip (Max Planck I., Germany), S. Miller (U.C.L., UK), D. Rego (IAS, France), D. Southwood, and M. Dougherty (I.C.L., UK), T. A. Livengood (NASA/GSFC, USA), S.A. Budzien (NRL, USA), F. Espenak (NASA/GSFC, USA), G.F.Fireman (CSC, USA), T. Kostiuk (NASA/GSFC, USA), M. A. McGrath (STScI, USA), P. D. Feldman, D.T. Hall, D.F. Strobel, H.W. Moos (Johns Hopkins U, USA), L.M. Woodney (U. Maryland, USA)