RESEARCH REPORT NO. AAMU-NAG-88-1 

MODEL OF JOVIAN F REGION IONOSPHERE 

by 

A. TAN 

ANNUAL STATUS REPORT OF NASA GRANT NO. NAGW-1067 

(hASA-CR- 182517) MODEL OP JUPIAB P BEG101 N88-2 1105 

IfPsOSEEEfEhE &nnual Status Befcrt (Alabama A 

& t Univ,) 1E CSCL 038 

Dnclas 

63/91 0129558 

ALABAMA AGRICULTURAL AND MECHANICAL UNIVERSITY 

NORMAL, ALABAMA 35762 

MARCH 1988 
Introduction 

To date, seven electron density profiles of the Jovian 

ionosphere have been furnished by the radio occultation 

experiments aboard the Pioneer and Voyager space probes 

(Strobe1 and Atreya, 1983). The data correspond to various 

localities (latitudes and longitudes) and times (dawn and 

dusk) and phases of sunspot cycle (high and low). This 

renders comparative studies difficult. Nevertheless, the 

possibility of existence of diurnal variation (Atreya, et 

al., 1979), equatorial anomaly (Mahajan, 1981) and auroral 

particle precipitation (Waite, et al., 1983) in the Jovian 

ionosphere have been put forth. The grand magnitude and 

depth of the equatorial anomaly, in particular, is a matter 

of great interest and speculation. Correct interpretations 

of the data and the physical processes in the complex Jovian 

atmospheric environment will remain a major task for the 

Aeronomer for decades to come. 

Model studies of a Jovian ionosphere created by solar 

EUV radiation and subjected to model ExB drifts (Tan, 1986) 

showed that equatorial anomaly similar to that in the 

terrestrial ionosphere can indeed be produced in the Jovian 

ionosphere. However, owing to the differences in size and 

rotation period of the two planets and the ionic compositions, 

much larger drift velocities are required to produce a 

comparable anomaly in the Jovian ionosphere. 
, 

2 

Objectives 

The plan of this research project is to further 

develop the model of Tan (1986) to include larger ExB drift 

velocities and invoke other mechanisms which may contribute 

to the magnitude of the Jovian equatorial anomaly. Specific 

objectives include the following. 

(1) Effect of increased drift velocity on the width of the 

anomaly. Run the existing program to accommodate larger 

drift velocities and observe the resulting ionization 

pattern, including the width of the anomaly. 

(2) Incorporation of charged particle precipitation. To 

incorporate auroral and non-auroral charged particle 

precipitation from the study of Waite et al. (1983). The 

high electron densities at high latitudes are almost 

certainly due to the ionization effects of auroral charged 

particle precipitation. 

(3) Magnetic field model. The Jovian magnetic field contains 

substantial quadrupole and octupole moments (Smith, et al., 

1976; Acuna and Ness, 1976). How the departure from a purely 

dipole field will alter the morphology of the ionosphere is 

to be investigated. The effect of the tilt of the dipole 

axis (Tan and Wu, 1981) will also be studied. 

(4) F Region Chemistry. The Jovian ionosphere chemistry may 

be complicated (cf. Shemansky, 1985). The effect of chemical 

reactions on the equatorial anomaly, if any, needs to be 

investigated if possible. 
3 

Effect of Increased Drift Velocity Amplitude 

The formation of the equatorial anomaly in a solar EUV 

radiation induced Jovian ionosphere under the action of ExB 

drifts has been described by Tan (1986). Drift velocity 

amplitudes of up to 100 m/s were considered. We have since 

then increased the drift velocity amplitude to 200 m/s. 

Figures 1 through 10 exhibit the results of the model 

computations with sinusoidal drift velocity models having 

amplitudes of 200 m/s. Figures 1 and 2 display the "planetwide" contour plots of peak electron density in Models 1 and 

2 of Tan (1986). The general features of these plots are 

similar to those for smaller drift velocity amplitudes. The 

width of the equatorial anomaly increased as expected with 

peak electron densities concentrated around 13.5 degrees 

latitude in either hemisphere. 

Figures 3 through 10 are contour plots in meridional 

planes at selected hours in the two drift models. There 

appears to be the formation of a "bubble" of low electron 

density over the equatorial region in the early hours of the 

day in Model 2. The bubble seems to persist until midday 

before disappearing. Whether there is any physical basis 

for this needs to be looked into. 
4 

Estimation of Drift Velocity Amplitude from the "Width" of 

the Equatorial Anomaly 

Model computations show that the amplitude of the 

sinusoidal drift velocity determines the latitude in either 

hemisphere where the peak electron density would be 

concentrated and hence the "width" of the equatorial anomaly. 

The converse problem of estimating the drift velocity 

amplitude from the width of the anomaly is examined from 

the geometry of drifting dipolar "tubes" of ionization. The 

range of the vertical drift at the equator can be calculated 

as a function of the drift velocity amplitude and the 

rotational period of the planet. The magnitude of this 

vertical range translates into a definite arc length at 

HmF2 heights from which the drift in latitude can be 

calculated. Twice this latitudinal drift gives the minimum 

width of the anomaly produced. The inverse relation would 

provide an estimate of the drift velocity amplitude required 

to produce an anomaly of a given width. The values of the 

drift velocity amplitudes thus obtained agree closely with 

those obtained by numerical computations for narrow anomalies 

of terrestrial and Jovian ionospheres, but the agreement 

breaks down for wider anomalies (Appendix I). 

--_ 
5 

Jovian Maanetic Field Models 

A Jovian F region model similar to the terrestrial F 

region model of Tan and Wu (1981) has been constructed to 

study the effect of the tilt of the Jovian dipole axis from 

the rotational. Preliminary runs indicate that the resulting 

ionization is symmetrically situated around the dipole 

equator thus reaffirming a strong magnetic control of the 

ionospheric plasma as in the terrestrial ionosphere. 

Whereas the ionization is strongly controlled by the 

orientation of the dipole field lines, the field strengths 

are found to have negligible effect on the ionization 

pattern. For this, we used a simple dipole model but 

calculated the magnetic field strengths at each field point 

from the O4 Model of Connerney (Courtesy of J. H. Waite, Jr.) 

and failed to detect any significant deviation in the 

electron densities. 

The offset-tilted dipole model (Acuna and Ness, 1976) 

provides a better approximation of the Jovian magnetic field 

than the centered-tilted dipole model. But we have 

encountered difficulties in incorporating this in our timedependent scheme. 

Further, since the Jovian magnetic field contains 

substantial quadrupole and octupole moments, a more accurate 

description calls for the incorporation of quadrupole field 

components, particularly when the longitudinal variation 
6 

in the Jovian F region is to be studied. Here also, we have 

encountered numerical difficulties. Attempts are being made 

to resolve these difficulties. 

References 

M. H. Acuna and N. F. Ness in Jupiter, T. Gehrels ed., 

University of Arizona Press, Tucson, 1976. 

S. K. Atreya, T. M. Donahue and J. H. Waite, Jr., Nature, 

280, 795, 1979. 

K. K. Mahajan, Geophys. Res. Lett., - 8, 66, 1981. 

D. E. Shemansky, J. geophys. Res., 90, 2673, 1985. 

E. J. Smith, L. Davis, Jr. and D. E. Jones in Jupiter, T. 

Gehrels ed., University of Arizona Press, Tucson, 1976. 

D. F. Strobe1 and S. K. Atreya in Physics of the Jovian 

Magnetosphere, A. J. Dessler ed., Cambridge University 

Press, 1983. 

A. Tan and S. T. Wu, Geofis. Intl., 20, 11, 1981. _. 

A. Tan, Planet. Space Sci., - 34, 117, 1986. 

J. H. Waite, T. E. Cravens, J. Kozyra, A. F. Nagy, S. K. 

Atreya and R. H. Chen, J. geophys. Res., _. 88, 6143, 

1983. 
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APPENDIX I. Abstract submitted to American Geophysical Union Spring 1988 

Meet i ng . 

1. 1988 Spring Meeting 

Estimation of Electrodynamic Drift Velocity Amplitude -. 2. 000607177 

from the Width of the Equatorial Anomaly of a 

Planetary Ionosphere 

3. (a) A Tan 

A Tan (Department of Physics, Alabama A&M University, Box 447 

Normal, AL 35762) Normal, AL 35762 

The equatorial anomaly in the terrestrial ionosphere 

is known to be caused by a diurnal east-west electric 

field through the "fountain effect". Detailed numerical 

modelling shows that the amplitude of the drift 4. SA 

velocity determines the latitude in either hemisphere 

concentrated and therefore the "width" of the anomaly. 

The converse problem of estimating the drift velocity 

amplitude from the width of the anomaly is examined. 

A semi-empirical expression of the drift velocity 

amplitude is obtained as functions of the latitude of 6. T 

the crest of ionization, the rotational period of the 

altitude of the peak electron density. The results 

obtained from this expression agree closely with those 

obtained by numerical modelling for narrow anomalies 

of terrestrial and Jovian ionospheres, but for wider 

anomalies, the introduction of an adjustable parameter 

is called for. Results for the Saturnian ionosphere 

show that peak ionizations around 5, 10 and 15 degrees 

latitudes would require drift velocity amplitudes of 

41m/s, 134m/s and 246m/s+respectively. 

(b) 205-859-7470 

where the maximum electron density would be 5. (a) 

(b) 2415 Equatorial 

Ionosphere 

planet, the equatorial radius of the planet and the 7. 0% 

8. $35 check enclosed 

9. c 

10.