PHEBUS Probing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance International Mercury Watch GroupObservatoire de Paris 5-6 April

PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance

International Mercury Watch Group Observatoire de Paris 5-6 April 2008 2

PHEBUS PLACEMENT AND DESIGN

PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy• Signal-to-noise ratio of 3 is achieved in the

radiometric modelling for most important species (depends on integration time)

• EUV = 55 - 155 nm• FUV = 145 - 315 nm • NUV = 404.7 nm (K) and 422.8 nm (Ca)

• EUV: FWHM ~0.48 FW1% ~0.80 • FUV: FWHM ~0.94 FW1% ~1.48

• Ingenious rotating mechanism with attached baffle - impact parameter from 0 to 1500 km

• With current configuration and 3600 s integration time sensitivity to lines as weak as 0.1 Rayleigh is achieved in the radiometric model



PHEBUS DESIGN

• S



PHEBUS DESIGN

Baffle

Mirror

Slith

EUV

FUV

NUV

Grating



PHEBUS MISSION OBJECTIVES

1. To measure the exosphere

(constituents, species densities, vertical structure, seasonal-, local- and temporal variations)

2. To measure the coupling of the surface-exosphere-magnetosphere system

3. Measuring the escape rates of species from the surface and the composition of the eroding regolith

4. Search for surface ice in the polar regions

5. Low priority: Stellar- and interplanetary physics

Magneto- sphereSurface

Exosphere



1. Twilight Mode

2. Vertical Scan Mode

3. Fixed LOS Mode

4. Feature Tracking Mode

5. Nadir Mode

6. Star Mode

7. ???

PHEBUS OBSERVATIONS



PHEBUS MODES

Twilight Mode

Allows search for trace species near surface and studies of the exosphere’s source and dynamics.

Observations when in the shadow of Mercury. The line of sight is as low as possible with no bright emission from the surface is in the field of view

Vertical scan Mode

Give information on the vertical distribution and its variation with solar zenith angle

Fixed LOS Mode

A fixed line of sight respective to the spacecraft. A slow vertical scan is obtained from the altitude variation of the spacecraft.

Gives information about local density of observed species along the orbit



PHEBUS MODES

Line of sight

Line of sight

Feature tracking Mode

The LOS moves slightly as in Twilight Mode to follow a feature (or altitude range) along the orbit

Nadir Mode

The LOS is pointed towards the Nadir on the nightside of the orbit and at high latitude which records the reflectivity of the surface by interplanetry glow (water, sulphur…)

Star Mode

The aim of the star mode is to follow the ageing of the instrument.Observation of some hot stars within the spectral range 100-300 nm. The number of available stars within the Phebus field of view is TBD (by me!).



PHEBUS MODES

?

Tail mode (under construction)



PHEBUS MODELING: Radiometric Model

Complete theoretical model

Calibration model(in progress)

Instrument Response model

Emissions from Mercury(theoretical values)

Collecting: Detectors

Gathering: Entrance Pupil and Baffle

Reflecting: Entrance mirror Gratings

Different reactions due to different photocathodes

Constraints of field of view and stray light

Depends on the separation of the two gratings and the reflectance of coating

Not perfectly reflective

InstrumentResponse



RADIOMETRIC MODEL: Instrument Response and spectra generation

1. Raw list of species with their assumed intensities (preliminary since Mercury’s exosphere is virtually unexplored)

2. List of species subjected to the instrument response

3. ‘Spectrum’ of the species list after addition of noise – 1 s integration time

1.

2. 3.

blue arrow is oxygen at 130,4 nm – ‘easily’ detectablered arrow is clorine at 133,6 nm – ‘difficult’ to detect



RADIOMETRIC MODEL: Instrument Response

10 seconds

60 seconds

600 seconds

3600 seconds

With increasing time the dark current will overtake some lines and change line-ratios



RADIOMETRIC MODEL: Signal-to-Noise Ratio

Species detectable:< 60 s (to make thorough investigations)

EUV: He I, CO, O I, H I, C I, N IFUV: Mg I, Si I, Na I, C I, Fe I, S I, Al I,

CO, Ni I, Mg II, OH, Ca I, Fe II, H2>> 3600 s (to confirm that the species exist)

EUV: Kr I, Ar I, Cl I and Si IIFUV: Al II and Si II

Numbers in the tables are integration time in seconds



RADIOMETRIC MODEL: Detection limit

The lowest (integration time of 3600 s) mean detection limit can thus be estimated to:

EUV range: ~ 0.1 Rayleigh

FUV range: ~ 0.2 Rayleigh

red dotted line marks the limit of 0.1 Rred area marks most common utilised integration time

1-10 s int. time gives lower limit of 10 R

1-10 s int. time gives lowerlimit of 5 R

1-10 s int. time gives lowerlimit of 20 R



PHEBUS DESIGN ACHIEVEMENTS

• Signal-to-noise ratio of 3

• Spectral range of 55 - 315 nm and the NUV of 404 nm and 422 nm

• Spectral resolution: FWHM < 1 nm for EUV, < 1.5 nm for FUV. FW1% < 2 nm for EUV and < 3 nm for FUV

• A vertical scanning range equivalent to an impact parameter from 0 to 1500 km

• Vertical resolution of ~ 1/2 scale height on most species(~ 20 km). From apoherm this gives spatial resolution ≥ 0.37°

• Sensitivity to lines as weak as 0.1 Rayleigh

Is achieved in the radiometric modelling for most important species (depends on integration time)

EUV = 55 - 155 nm, FUV = 145 - 315 nm, NUV = 404.7 nm and 422.8 nm

EUV: FWHM ~0.48 FW1% ~0.80 FUV: FWHM ~0.94 FW1% ~1.48

Ingenious rotating mechanism with attached baffle

With margin the spatial resolution is ~0.4° in the current evaluation of the geometric performance

With current configuration and 3600 s integration time this is achieved in the radiometric model for EUV ~0.1 R (FUV ~0.2 R)

Documents

PHEBUS Probing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance International Mercury Watch GroupObservatoire de Paris 5-6 April