. 

Center for 

Astrophysical 

Sciences 

April 27, 1988 

Ms. Genevieve Wiseman 

Grants Officer, Code 200.3 

National Aeronautics and 

Space Administration 

Goddard Space Flight Center 

Greenbelt, MD 20771 

Dear Ms. Wiseman: 

Enclosed please find the Final Technical Report for the IUE program SUGSD - ''Variability 

Time Scale of H Ly-a from Uranus," by myself, J. T. Clarke, H. W. Moss, and S. Boyer. I am 

sorry for the long delay in getting this report to you. If there is anything else needed for me to 

close out our NASA grant NAG 5-435, please let me know and I will do my best to comply. 

Sincerly , 

Samu CDurrance 

(KlSA-Cii-)827&6) fUE €fiOCFAI SC'GSI;: H88-2 1107 

VBfiAEILIIP TIHE SCALE CF Ei Zj-BLLHA PBGM 

C&ABt,C kiual Sectnical Fiepert (Jchns 

tcFkits UElV,) 14 p CSCL 03B Unclas 

G3/91 01352E3 

The Johns Hopkins University 

Rowland Hall 

Baltimore, Maryland 21218 

(301) 338-7000 
Final Technical Report 

IUE Program SUGSD: Variability Time Scale 

of H Ly-CY From Uranus 

P.I.: Samuel T. Durrance, The Johns Hopkins University 

The scientific motivation for this program was to determine the time scale for 

the variability of the H Ly-a emission from Uranus. It has previously been shown 

to be variable on time scales as short as one day which, coupled with its unusually 

high brightness, indicates that it must be excited by magnetospheric activity. 

The purpose of this series of observations is to determine its variability on shorter 

time scales. This will provide information on the timescale for changes in the 

level of particle precipitation in the Uranian magnetosphere. If this “auroral” 

emission appears to be modulated by the rotation of the planet, as has been 

observed on Jupiter and possibly Saturn, this would provide fundamental information on the nature of the interaction between the Uranian magnetosphere and 

the solar wind. It may also indicate the tilt of the magnetic pole with respect to 

the spin axis and provide information on the rotation of the planetary core. 

A series of observations was carried out in coordination with ESA to cover as 

completely as possible one 24 hour period of the Uranian H Ly-CY emission. The 

observations were obtained on April the 23rd and 24th. Two additional observations on April the 25th and 26th were obtained to search for longer term trends. 

A small modulation in the brightness was observed and the results presented in 

two publications; copies of which are attached to this report. The first publication (Durrance, S. T. and J. T. Clarke, “Lyman a Aurora, in Uranus and Neptune,” edited by J. T. Bergstralh, NASA CP-2330, p. 559, 1984) presents these 

data for the first time and’ shows a variation on a time scale of 18 hours, the rotation period of the planet. The second publication (Clarke, J. T., S. T. Durrance 

et. al., “Continued Observations of the H Ly-a Emission from Uranus,” J. Geophys. Res., 91, p. 8771, 1986) presents a comprehensive compilation of all the IUE 

observations of Uranus to date with the results from this grant included. 
LYMAN-ALPHA AURORA 

Samuel T. Durrance 

Department of Physics, Johns Hopkins University 

Baltimore, Maryland 21218 

and 

John T. Clarke 

Space Sciences Laboratory 

University of California, Berkeley 

Abstract 

The existence of intense and variable H Ly a emission from 

Uranus is demonstrated utilizing the monochromatic imaging 

capabilities of the International Ultraviolet Explorer satellite. 

A series of 14 observations, using the IUE short wavelength 

spectrograph in low dispersion and covering the period from 3 

March 1982 through 2 September 1983, shows the disk averaged LY a 

brightness of Uranus to vary between 690 and 2230 RayleighsModel calculations indicates that < 400 R of this emission can be 

attributed to resonant scattering of solar Ly a radiation. An 

upper limit of 100 R is obtained for the Raman scattering of 

solar LY a by H2 (1280 A). This implies that < 300 R is 

contributed to the planetary Ly a emission by Rayleigh 

scattering. In addition to being unexpectedly strong, the Uranian 

Ly a emission has been observed to vary by a factor of two in one 

24 hr period and by about 50% in one 5 hr period. These data thus 

559 

I 

-. 
offer the first strong evidence for the presence of aurora and 

therefore a magnetic field on Uranus. 

560 
Spectroscopy of the outer planets at far ultraviolet 

wavelengths has progressed rapidly over the past few years with the 

advent of the International Ultraviolet Explorer. satellite and the 

Voyager flybys of Jupiter and Saturn. Measurements of H Ly a emissjon 

(1216 A) from these planets has been a particularly useful tool to 

understand the physical conditions in their upper atmospheres and 

magnetospheres. The three principal excitation nechanisms to produce H 

Ly a emission from the outer planets are (a) resonant scattering of 

solar Ly a radiation, (b) Rayleigh scattering of solar Ly a radiation, 

and (c) 

these mecbanisn;s provides conplementary information on the physical 

state of the atmosphere. 

charged particle excitation of atmospheric H and H2. Each of 

Resonant scattering of solar Ly a photons by atmospheric H 

provides a direct measure of the atonic hydrogen column abundance above 

the lower UV absorbing layers. Atomic hydrogen is produced in the upper 

atmosphere by photodissociative processes and mixed downward by eddy 

diffusion. Methane is also produced photochemically in the upper 

atmosphere and mixed downward. The column abundance of H above an 

absorbdng CH4 layer can thus be used, in conjunction with model 

calculations, to determine the eddy diffusion coefficient and the level 

of the homopause in the atmosphere. Resonant scattering of solar Ly a 

photons has been observed from both Jupetir and Saturn. 

I 

Rayleigh scattering of solar Ly a photons by atmospheric H2 

is another plausible mechanisn to produce planetary Ly a emission. At I 

low spectral resolution (>2 A) this emission is indistinguishable from 

I 

561 
-,=I; 

>:"4 * that produced by resonant scattering. Rayleigh scattering will, -..-?:c IS . -2G.G 

-324.2 

. I+ 

..- 1 . I_ 

however, produce a Ranan-shifted line at 1280 A with about 1/3 the 

intensity of the Rayleigh scattered line (Dalgarno and Williams, 1962, 

and Brandt, 1963). This Raman-shifted line can be used to determine the 

amount of Rayleigh scattering present at 1216 A. The Rayleigh 

scattering cross section is low (- 10 CCI 1, so it requires a 

large H2 column density (- cm'*) above the absorbing CH,, 

layer. Such large E2 columns are not present on Jupiter of Saturn and 

. $0. 

- 

. IC ,* -- -2i-*. 

-..-. 4 

-r. 

-r 

.. 

-24 2 ,, 

the Raman-shifted emission at 1280 A has not been detected. 

Charged particle excitation can produce Ly a emission by 

2 direct excitation of H or by dissociative excitation of H . On 

Jupiter and Saturn this type of emission has been observed in both the 

polar aurorae and in the equatorial regions. Its presence is indicated 

by H band ercission and by the nore varriable nature of particle 

precipation. This emission is a usefull indicator the level of 

2 

magnetospheric activity and as a tool to study the dynamics of the 

magnetosphere. 

Recently H Ly a emission from Uranus has been detected by 

three independent groups (Frike and Dauius, 1982, Durrance and Eoos, 

1982 and Clarke, 1982). Frike and Darius interpret their measurements 

as most likely due to resonant scattering of solar Ly a by H. The 

Hopkins and Berkely grogps, however, interpret their data as most 

likely due to auroral excitation. These data will be reviewed and newer 

observations will be presented in a hope to clarify the situation. 

562 
In the spring of 1982 two research groups, one at Johns 

Hopkins University and one at the University of Callfornia Eerkely, 

began observational programs to search for H Ly a emission from Uranus 

and Meptune. The observational techniques used take advantage of the 

monochromatic imaging capabilities of the IUE short wavelength 

spectrograph. The large entrance aperture of the spectrograph has a 

projection onto the sky of abou,t 10 x 20 arc sec as indicated in the 

upper portion of figure 1. The disk of Uranus, which has a diameter of 

about 3.8 arc sec, was inaged into the center of this aperture as is 

also indicated in figure 1. The background geocoronal and 

interplanetary Ly a emissions are varriable on both short and long time 

scales; however, they are spatially uniform over scale lengths 

comparable to that of the aperture so it should be possible to resolve 

the planetary Ly a emission from that of the background. The file with 

line by line spectra froa the standard NASA/IUF data package (in which 

a data point corresponds to an area 1.1 arc sec in the dispersion 

direction and 2.1 arc sec high) was used to obtain a two-dimensional 

Plot of the focal plane flux. A projection of the resulting data set at 

Ly a is shown in figure 1. The dispersion direction is parallel to the 

axes pointing downward and to the right. Figure la shows an exposure 

taken about 3 arc min south-west of the planet and thus contains 

background Ly a emission only. Figure lb shows an exposure with Uranus 

centered in the aperture, so it contains both planetary and background 

Ly a emissions. The planetary emission is clearly resolved froL the 

background emisuioo. There are curently about 30 Uranus observations of 

563 
t 
this type scattered throughout the period from 3 March 1982 to 2 

September. 

The Uranian Ly a brightness is obtained from these data by the 

following process. The two or three Uranus exposures obtained in a 

given 8 hr IUE shift are combined into one. The exposure with 

background emission only is scaled to match the background contribution 

to this combined exposure and the two data sets are subtracted. The 

resulting array is summed over a region about 9 arc sec diameter, which 

is centered on the planetary emission, to give the total measured 

planetary Ly a signal in IUE flux numbers. This summing region is 

chosen to include all of the flux from the - 4 arc sec planetary disk 

after convolution with the - 5 arc sec point spread function of the 

instrument. This is converted to a disk averaged brightness using ,the 

absolute calibration of the SWP camera given by Bolin et a1 (1980) and 

the size of the Uranian disk at the time of the observations as given 

in the Astronomical Almanac. 

The data from Clarke (19821 ..were analized in a somewhat 

different manner. The photcmetrically corrected image segment from the 

standard NASA/IUE data package was used instead of the line by line 

file. This file should provide somewhat better spatial resolution. 

There is, however, a transformation necessary to convert the 

photometrically corrected irnabe data into the IUE flux numbers in which 

the absolute calibration is given. When the IUE image processing system 

was changed in 1980 to increace the digitation of the low dispersion 

spectra by a factor of 2 this transformation was normalized by a factor 

of 2 (Turnrose, 1980) so that the same calibration could be used. The 

LY a brightnesses of Uranus given by Clarke (1982) should thus be 

increaced by a factor of 2 to account for this. 

The Ly a brightness of Uranus from the above analysis is 

565 
plotted as a function of time in figure 2. The data shown as squares 

are from Durrance and Moos (1982) and several subsequent observations 

analyzed using the same technique. The data shown as diamonds are from 

Clarke (1982) but they have been increaced by a factor of 2 to account 

for the propper normalization. Two additional observations by Clarke 

are also shown as diamonds. The largest uncertianty in these 

measurements is a systematic error introduced by the background 

subtraction and is estimated to be + 20%. However, it is estimated that 

extracting different images by this method introduces a relative error 

of < 10%. The quoted uncertainty in the IUE calibration is 2s = 15% in 

a 25 A band at 1216 A (Bolin, et al. 1980), and the instrument has been 

determined to be stable in time to within this uncertainty by observing 

standard stars. The systematic error in the background subtraction and 

the uncertainty in absolute calibration are not shown in figure 2; only 

the relative error between different observations is shown. A 

variability by 2s much as a factor of 3 on relativly short time scales , 

is clearly indicated by these data. 

566 
, - - . .. 
The data presented here clearly show Uranus to be an intense 

and highly variable source of H Ly a radiation. To understand this 

emmision we will now address the three most plausable excitation 

mechanisms. First consider the resonant scattering of solar H Ly a 

photons by an optically thick layer of atonic hydrogen in the upper 

atmosphere. There is evidence that CH4 is condensed out of the 

atnosphere on Uranus at the temperature ninimun (danielson, 1977). This 

could provide a larger column of A than one would expect based on 

analogy with Jupiter and Saturn. However, since both I! and CH4 are 

produced photochemically in the upper atnosphere and mixed downward and 

since the solar flux at Uranus is relatively weaker, it is difficult to 

I produce the large H column required to explain the high Ly a brightness 

I by resonant scattering. Detailed nodel calculations by Atreya and 

Ponthieu (1983) and by Atreya (this volunn) show that for a range of ’ 

, :model parameters the expected H column density above an absorbing CH4 

2 layer is - -5 x 1015 cm . A simple model for the resonantly 

scattered emission was presented by Clarke (1982). The two curves of 

growth from that model are presented in figure 3; this model assumes 

monochromatic scattering in a plane-parallel isothermal layer with a 

completely absorbing lower boundry. This is an idealization but should 

b adaquate for our purposes here. For the lower temperature model, 

which appears more reasonable, the above column of H would produce a Ly 

a brightness of a 100 R considerably less than the 2.1 KR of the 

I 

I highest brightness observed. To produce this level of Ly a emission via 

resonant scattering would require an H column density in excess of 

10l8 cm2 which seems unlikely. 

568 
ICL 

w 

> 
If an extended H atmosphere existed, as a result of a hot 

hydrogen corona for example, then our conversion of the measured flux 

to a disk averaged brightness would be an over estimate. Two things 

argue against this. First, the size of Uranus' image in these 

observations is - 5 arc sec which is the same size as the point spread 

function of the telescope. If Uranus were emitting over an area much 

larger than the visible disk then it would show up as a more extended 

inage. Second, calculations using a 60,000 K hydrogen corona (Shemansky 

and Smith, private communication) show that the optically thick H 

column would extend out to < 1.1 RU. This would reduce the derived 

disk averaged brightness by only about 15%. 

If CH4 is condensed out of the atmosphere, Uranus may have a 

deep H2 atmosphere and thus Rayleigh scattering could be an important 

contributer to the Ly a brightness. However, the Rayleigh scattering 

cross section is - cm2 (Dalgarno and Williams, 1962) and the 

model calculations of Atreya (this volumn) give N(H2) - 1015 cm'2 

so this mechanism is probably not important. Also, in a sum of several 

long low dispersion IUE exposures using the small aperture an upper 

limit to the Raman-shifted emission of < 30 R is set (Frike and Darius, 

1982). This sets an upper limit to the Rayleigh scattered Ly a emission 

of < 100 R. 

Charged particle excitation by direct impact of solar wind 

particles on the Uranian disk could also produce H Ly a emission. If 

one assumes an average velocity of 400 Km/sec for solar wind protons 

and an average excitation energy of 12 eV per emitted photon and ir 

addition that the entire disk captures and converts these protons to U\ 

photons with 100: efficiency, then the resulting Ly a bribhtness woulc 

be < 50 R. There does not appear to be enough energy available for thi: 

process to be important. 5 70 
I ‘An auroral emission from a zone between 80 and 90 degrees 

latitude and centered on the Earth facing pole would be able to produce 

the observed Ly a emission if the average surface brightness of this 

aurora were 15 KR. This is comparable in brightness to the aurora 

observed on Jupiter and Saturn. Recent theoretical calculations by 

Hill, Dessler and Rassbach (1983) indicate that, if Uranus has a 

magnetic field, it will have an aurora that is highly variable in 

respofise to variations in the solar wind velocity. The highly variable 

nature of the Uranian emission 1.9 indicative of this type of emission. 

Jf the brightest emission observed is interpreted as intitely due to 

auroral particle precipatation, the required power is estimated to be “ 

3 X 10’’ W using a 5% conversion efficiency (Gerard and Sing, 1981). 

This amount of power could be supplied by a magnetospheric interaction 

with the solar wind ( Hill, Dessler, and Rassbach). 

571 
Intense and highly variable H Ly a emission has. been observed 

fron Uranus. The disk averaged brightness is seen to vary between 717 R 

and 2.1 KR. Resonant scattering of solar Ly a by atmospheric H appears 

insufficient to explain this emission; model calculations limit this 

contribution to the ,Uranian Ly a brightness to < 400 R. Rayleigh 

scattering also is insufficient and an upper limit to this contrabution 

was derived to be < 100 R. Auroral precipatation of charged particles 

is the most likely cause of this emission. This implys that Uranus has 

a magnetic field and thus a magnetosphere. The required input power to 

the aurora is about 3 x 10" W. 

5 72