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Vibrationally excited H2 in the upper atmosphere of Saturn
0273—1177/90 $0.00 + .50 Copyright © 1989 COSPAR
Adv. Space Res. Vol. 10, No. 1. (l)131—(1)134. 1990. Printed in Great Britain. All rights reserved.
VIBRATIONALLY EXCITED H2 IN THE UPPER ATMOSPHERE OF SATURN Tariq Majeed*.** John C. McConnell*,*** and Roger V. Yelle** **Lunar and Planetary Laboratory, Gould-Simpson Building, University of Arizona, Tuscon, AZ 85721, U.S.A. * **
Observatoire de Besancon, 25044 Besançon Cedex, France
ABSTRACT We have considered the impact of resonance fluorescense of solar EUV radiation by H2 on the distribution of the vibrational levels of H2 in the upper atmosphere of Saturn. This source has not been considered to date. It appears that, for v3, this is the most important source, more important than those due to photoelectron induced fluorescence, recombination of molecular ions such as Ht, and vibrational excitation of H2 by photoelectron impact. Based on the Voyager limb observations of 112 band emission we estimate that some of the higher vibrational levels may have effective temperatures 3500 K. Such high vibrational densities may have an impact on ionospheric densities. INTRODUCTION Recent model calculations of vibrational distribution of H2 molecules in the Jovian upper atmosphere by Cravens /1/ have confirmed earlier speculations /2,3,4/ that it is possible to have enhanced populations of H2(v) large enough to have a significant impact on ionospheric densities via reaction 2 below. Since the publication of this paper Yelle et al. /5/ have suggested that photon-induced fluorescence of Lyman and Werner bands of 112, rather than electron induced fluorescence /6/, is the major contributor to the bright low-latitude H2 band emissions from the outer planets. The former process could thus be a substantial source of vibrationaily excited H2 (H2(v)) throughout the thermospheres of the outer planets. It is prudent therefore to reconsider H2(v) densities in light of this development. SOURCES OF VIBRATIONALLY EXCITED H2 1fl,~states Fluorescence source : The vibrational levels of H2 may be populated via cascading from the B ‘E~and C H 2(B ‘E~or C ‘flu, v’) —. H2(X ‘E~,v) +ht’ which may be excited by absorption of solar EUV radiation h~(<1100A) + H2(X ‘E~, v
=
0)
-.~
H2(B ‘E~or C
‘flu
v’)
We assume that the total excitation rates for the B and C states, as a function of altitude, are given by the volume emission rates inferred from the observed limb profile of H2 band emissions /7/. The relative production rates for each v level for this process, shown in Table 1, have been estimated by calculating the solar EUV flux absorbed by a column of 112 at the ambient temperature, followed by reeniission into the lower vibrational levels of H2, where appropriate allowance is made for multiple scattering of the resonance lines. Thus the excitation rates represent an approximate column average at each height. This should not represent a large source of uncertainty given the approximate nature of the calculations described herein. Direct electron impact excitation: The direct electron impact excitation of molecular hydrogen in its ground electronic state is an important source for vibrational level v = 1 and 2 but becomes less important as the value of v increases; we include this source for v = 1, 2 and 3 only. The excitation rates are calculated from the solar ionization Table 1. Relative Production of H2(v) Lyman and Werner System. V
0
Lyman 0.0599
1
2
3
4
5
6
7
8
0.049
0.036
0.056
0.014
0.053
0.021
0.043
0.028
9
3 Wnor 0.0214 0.0195
0.013
0.024
0.026
0.021
0.015
10
11
12
0.044 0.030 0.036 0.077 0.014 5.6x1O~32.0,dO~4 ~
13 0.096 5x1o6
14 0.020 4x10~7
0.015 9.2x10
rates by assuming that the photoelectrons deposit their energy locally in the upper atmosphere of Saturn. The ratios of the direct production rates for v = 1, 2, 3 can be estimated from the ratios of the respective cross sections at their maximum near 1 - 2 eV /8/ 1:0.1:0.007 respectively. These ratios are almost constant for electron energies greater than a couple of eV. Based on the results of Cravens /1/ we have not included the much smaller photo-electron fluorescence source for these preliminary calculations.
*
On leave from: York University, North York, Ontario, Canada M3J 1P3 (1)131
T. Majeed et a!.
(1)132
Ionospheric source: Recombination of Ht ions H~(v)+e—~H+H+H —~H;+H where
*
(la) (ib)
represents an electronically or vibrationally excited H
2, is a potential source of H2 (v). Reaction 1 appears be the most important loss process for Ht ions at the altitude of interest. The strength of the source will, of course, depend on the relative efficiency of k,~ versus kib. For the exploratory calculations shown herein we have assumed that k,. = 0. Likewise, since the distribution of energy amount the different vibrational levels of H2(v) is currently not known, we assume that each level is excited with equal probability. to
Although recent investigations /9,10,11,12/ have signalled a substantial uncertainty in the rate of this reaction, this will not greatly impact the column recorubination rate of Ht since recombination is the only loss process for Ht and the major source of H~,ie photoionization of H2 followed by charge transfer of Ht to Ht, does not depend on the details of the ionospheric model. This can be seen in Figure 1, which gives the vertical distribution of the H2(v) source for 2 very different values of kib. The vertical distribution is somewhat different illustrating the different ionospheric structure. However, the column source of H3(v) is the same. Of course, the effect on the ionosphere of the different values of the rate constant is quite dramatic as we show below. MODEL The ionospheric model used in this paper is the same as has been used previously by us /3,13/. However, in this note the calculations of ionospheric and vibrational structure are coupled, so that the ionospheric and vibrational densities are determined in a self-consistent manner. The neutral atmosphere used in this paper was derived from the analysis of the Voyager UVS solar and stellar occulatation data /14/. An important possible loss process for protons is the reaction of 11+ ions with H2. This reaction is endothermic unless the H2 molecules are in the fourth or higher levels H~+H2(v4)—H~+H
(2)
9 cm3 /15/. of this reaction, k2, are available, but we assume that it is close to the gas No measurements for the rate s~ coefficient kinetic value of 2 x10 For our standard ionospheric model, the majority of the Ht ions are in the vibrational level v = 0, and are assumed to recombine with a rate coefficient k?b = 2 x iO~(300/T)°5 cm3 s~ consistent with recent measurements / 10,11/. We have also used the recently reported upper limit for k?b of 1 x 10—il (300/T)°~5cm3 s1 /12/. This we call the slow model. The rate constants used in the vibrational chemistry are those given by Cravens /1/. Figure 2 shows rate constant for most important processes versus vibrational level (v) for a neutral temperature ~ 360 K.
0
IS’
10’ 10’ 10’ Pr,d,,ctl.e Ret. IC.—3
10’
10.
s—u
is’
Figure 1. Production rates due to direct electron impact excitation for the vibrational levels v=1, 2, 3 are
shown as a function of altitude (dashed curves), photon induced fluorescence, v=1, (solid curve), ionospheric source, v=1 standard H~recombination (dotted curve) and slow recombinotion rate (dot-dash
I
2
3
4
5
o 7 e 9 IS V;b~atIoesILs,.I
11
I?
13
14
Figure 2. Quenching (VT) reaction rate constants
both for H 3 and H and vibrational exchange rate constants (VV) for H3 are shown for a neutral ternperature of 350 K. -
curve).
RESULTS Figure 1 shows the vibrational production rates from the sources discussed in section 2. The source for the v=1 level is dominated at all levels by direct electron impact excitation. The production of vibrational quanta for v=2 level is dominated by the electron impact process between 1100 km and 2300 km. At other altitudes the fluorescence source is more important. For v>3 the fluorescence source is more important than both the electron excitation source, the ionospheric source, and tce photo-electron fluorescence source (not shown). We present in Figure 3a and 3b the number densities for all 14 vibrationally excited states of H2 calculated with the standard H~recombination rate coefficient. The peak in H2(v=1) densities for the calculations without fluorescence reflects the structure of the production rate for v=1 and appears in the region of maximum energy deposition for incoming solar EUV radiation. The density of H2(v=1) is much larger than the densities of the other levels as a result of the larger production rate for this level.
H~in the Upper Atmosphere of Saturn
(1)133
Diffusion controls the distribution of Hz(v=1) for the altitudes greater th~an1500 km. Below this height the distribution is in quasi-photochernical steady state and the main loss process for vibrational quanta is quenching of a single vibrational quantum (VT processes) by H. Normally the peak of a distribution would be formed at an altitude where the diffusion time constant (ra) becomes equal to the chemical time constant (re). However, for the v=1 level, due to the particular form of the sources and sinks this is not the case. The H 2 number densities for 2v5 are mainly controlled by processes involving exchange of a single vibrational quanta (VV) between neighbouring vibrational levels. For levels with v > 5 vibrational quenching processes (VT) with 112 play a more important role (see Figure 2). These processes are very rapid for higher values of v, therefore the densities of 112 (v>5) decrease sharply with increasing v. The cascading from higher vibrational levels contributes to the total production rates of these levels. As for the v=1 level the peak in the distribution occurs in the chemical region, and the transition region between chemistry and diffusion occurs at somewhat higher altitudes. For example, for v = 2, 10, 14 the transition region occurs ~— 2100, 2500, and 3100 km respectively. The density distribution of vibrationally excited H2(v) obtained by including the excitation source due to fluorescence is shown in Figure 3b. The v=1 densities are only increased by 10% except at the bottom of the model where the increased densities reflect the stronger fluorescence source (cf. Figure 1). In this case, due to the extended nature of the source, the peak of the levels with v6 occur near the transition region between chemistry and diffusion. The fall off with density downwards away from the peak reflects the fact that the loss process, quenching by H2, has a smaller scale height than the fluorescence source whose scale height is larger than that for 112 (cf. Figure 1). The H2(v4) densities, which as we have noted above, may impact the ionosphere, maximize in the region of 1800 km and are larger by a factor 30 when the fluorescence is included. This, as we see below, has important implications for ionospheric structure. In terms of equivalent vibrational temperatures the upper levels have temperatures 3500 K.
500 10
10’ l0~’ 10’ 10’
IS’ Ii’ Ii’ ID’ D.ns~tyIC.31
10’ 10’ 10’
10’
SOC
~
10’ 10’ 10’ IS’
Ii’ Ii’ 10’ 10’ IS’ 0.,,.ity IC.—31
10’
Ii’
10’
Figure 3. Densities for all 14 levels of vibrationally excited H3 in the upper atmosphere of Saturn including k2 are shown a) without fluorescence source b) with fluorescence source. The impact of vibrationaily excited H2 (v) on the ionospheric densities is shown in Figure 4. Curve A shows a RSS electron density profile /16/ for a latitude of 31°S. The model calculations for a number of scenarios are shown by curves B, C, and D and are obtained using the standard value of k~,. The model electron densities, n,, with the effects of vibrationally enhanced H2 supressed, (i.e k2=0) are shown by curve B. The results are similar to those obtained by others /17/ and exhibit a peak that is more than 1000 km below and more than 10 times the magnitude of the observed peak. Curve C shows the effect of vibrational excitation but with no fluorescence source included. The calculated electron densities are reduced by a factor 2. The impact of the fluorescence source for H2 (v) on the ionospheric densities is shown by curve D. The vibrational densities for levels v4 have been increased sufficiently by the fluorescence source that k2 is able to reduce the peak in the model n, electron densities to a value comparable to that obtained by the RSS V2-exit occultation experiments. However the altitude of the peak n, is still 1000 km lower than the measured one. This could, however, be modified by the inclusion of a vertical wind /3,13,17/. H+ is the major ion at the ionospheric peak but Ht densities dominate in the 1500 — 2500 km altitude range because of rapid reaction of H+ ions with H2(v4) molecules. We have also done some ionospheric calculations by using the slow Ht recombination rate coefficient. The results of the ionospheric calculations including the fluorescence source of vibrational excitation and the loss of H+ ions with Hz(v4) are shown in Figure 5. A serious disagreement obtains between the model calculations (curve B) and observations (curve A) regarding both the magnitude and the location of the electron density peak. In this case the Ht becomes the major ion in the entire Saturnian ionosphere since it recombines very slowly with the electrons. H+ is a minor ion due to very rapid reaction with H2(v4). DISCUSSION AND CONCLUSIONS It is clear that the fluorescence source of vibrational excitation is sufficiently strong to enhance the densities of higher H2 (v) levels to an extent that there is extensive modification of the ionospheric structure if the standard recombination rate constant is used. Thus photon-induced fluorescence represents a potentially important source of H2(v) on all of the outer planets. The disagreement with the ionospheric model and RSS data using the slow rate constant suggests that perhaps there may be a discrepancy with the reported upper limit /12/. However, one solution to this problem has recently been suggested by McConnell and Majeed /18/. They adopted a recombination rate coefficient for ground state Ht ions, based on molecular data, that, at elevated electron temperatures, proceeds much faster than the measured upper limit reported by Adams and Smith /12/. In order for this process to proceed the total energy of the H~ion and the kinetic energy of the electron must be 1 eV. If elevated electron temperatures obtain, then one result of this enhanced recombination process, aside from reducing n,, could be the
(1)134
T. Majeed eta!.
(~
10.
0.naltty lc.—31
Figure 4. Model electron densities for 31°Scompared with the RSS V2-exit measurements, Curve A. Curve B is the model fit with no vibrational excitation, k 3 no added. D included is the model = 0.fluorescence Curve C issource the model fit Curve with 1(3 but fit with fluorescence source included. The standard value for k?b was used. H~and H~densities corresponding to curve D are also shown.
0.n.Ity IC.—3)
Figure 5. Model electron densities for 31°Scompared with the RSS V2-exit measurements, Curve A. Curve 1and cm3 ~ H~and H~ B is the model fit with to1i3curve fluorescence source densities B are also shown. included corresponding and ~?b = 1 x10’
efficient production of H Lyman-a resulting in an important contribution to the planetary H Lyman-a budget (see /5,18/). Another possibility of reducing the plasma densities includes an influx of H 20 in the upper atmosphere of Saturn due to photosputtering of the rings /19/. The charge exchange reaction of H+ and Ht ions with H20 molecules proceeds rapidly with the subsequent formation of rapidly recombining molecular ions, such as H20+ and H30+, so that the ionospheric plasma densities are suppressed /17,19/. ACKNOWLEDGEMENTS JCMcC acknowledges continuing support from the Natural Science and Engineering Research Council of Canada and the Centre National de la Recherche Scientifique for support while on sabbatical in Besançon. TM and RVY acknowledge support from JPL contract 957763 under NASA contract NAS7-918 to the University of Arizona. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
REFERENCES T. E. Cravens, J. Geophys. Rca. 92, 11083 (1987). M. B. McElroy, Space Sci. Rev. 14, 460 (1973). J. C. McConnell et al., Planet. Space Sd. 30, 151 (1982). J. H. Jr. Waite, et al., 3. Geophys. Res. 88, 6143 (1983). R. V. Yelle, et al., 3. Geophys. Res. 92, 15100 (1987). D. E. Shemansky, 3. Geophys. Res. 90, 2673 (1985). R. V. Yelle, et aL, 3. Geophys. Rca. 91, 8756 (1986). H. Erhardt, et aL, Phys. Rev. 173, 222 (1968). T. Amano, Astrophys. 3. 329, L121. (1988). D. Smith and N. G. Adams, Astrophys. 3. 284, L13 (1984). H. Hus, F. Youssif, A. Sen, and 3. B. A. Mitchell, Phys.Rev.A, 38, 658, 1988. N. G. Adams and D. Smith, Astrochemistry, ed. M. S. Vardya and S. P. Tarafdar, 1987, p. 1. 3. C. McConnell and T. Majeed, 3. Geophys. Res. 92, 8572 (1987). G. R. Smith, et al., 3. Geophys. Rca. 88, 8667 (1983). W. T. Jr. Huntress, Ad. Atom. Mol. Phys. 10, 295 (1974). G. F. Lindal, D. N. Sweetnarn, and V. B.. Eshleman, Astron. 3. 90, 1136 (1985). T. Majeed and 3. C. McConnell, EOS Trans., 67, 318, (1986). 3. C. McConnell and T. Majeed, Planet. Space Sci., submitted (1988). 3. E. P. Connerney and J. H. Waite, Nature, 312, 136 1984.
Adv. Space Res. Vol. 10, No. 1. (l)131—(1)134. 1990. Printed in Great Britain. All rights reserved.
VIBRATIONALLY EXCITED H2 IN THE UPPER ATMOSPHERE OF SATURN Tariq Majeed*.** John C. McConnell*,*** and Roger V. Yelle** **Lunar and Planetary Laboratory, Gould-Simpson Building, University of Arizona, Tuscon, AZ 85721, U.S.A. * **
Observatoire de Besancon, 25044 Besançon Cedex, France
ABSTRACT We have considered the impact of resonance fluorescense of solar EUV radiation by H2 on the distribution of the vibrational levels of H2 in the upper atmosphere of Saturn. This source has not been considered to date. It appears that, for v3, this is the most important source, more important than those due to photoelectron induced fluorescence, recombination of molecular ions such as Ht, and vibrational excitation of H2 by photoelectron impact. Based on the Voyager limb observations of 112 band emission we estimate that some of the higher vibrational levels may have effective temperatures 3500 K. Such high vibrational densities may have an impact on ionospheric densities. INTRODUCTION Recent model calculations of vibrational distribution of H2 molecules in the Jovian upper atmosphere by Cravens /1/ have confirmed earlier speculations /2,3,4/ that it is possible to have enhanced populations of H2(v) large enough to have a significant impact on ionospheric densities via reaction 2 below. Since the publication of this paper Yelle et al. /5/ have suggested that photon-induced fluorescence of Lyman and Werner bands of 112, rather than electron induced fluorescence /6/, is the major contributor to the bright low-latitude H2 band emissions from the outer planets. The former process could thus be a substantial source of vibrationaily excited H2 (H2(v)) throughout the thermospheres of the outer planets. It is prudent therefore to reconsider H2(v) densities in light of this development. SOURCES OF VIBRATIONALLY EXCITED H2 1fl,~states Fluorescence source : The vibrational levels of H2 may be populated via cascading from the B ‘E~and C H 2(B ‘E~or C ‘flu, v’) —. H2(X ‘E~,v) +ht’ which may be excited by absorption of solar EUV radiation h~(<1100A) + H2(X ‘E~, v
=
0)
-.~
H2(B ‘E~or C
‘flu
v’)
We assume that the total excitation rates for the B and C states, as a function of altitude, are given by the volume emission rates inferred from the observed limb profile of H2 band emissions /7/. The relative production rates for each v level for this process, shown in Table 1, have been estimated by calculating the solar EUV flux absorbed by a column of 112 at the ambient temperature, followed by reeniission into the lower vibrational levels of H2, where appropriate allowance is made for multiple scattering of the resonance lines. Thus the excitation rates represent an approximate column average at each height. This should not represent a large source of uncertainty given the approximate nature of the calculations described herein. Direct electron impact excitation: The direct electron impact excitation of molecular hydrogen in its ground electronic state is an important source for vibrational level v = 1 and 2 but becomes less important as the value of v increases; we include this source for v = 1, 2 and 3 only. The excitation rates are calculated from the solar ionization Table 1. Relative Production of H2(v) Lyman and Werner System. V
0
Lyman 0.0599
1
2
3
4
5
6
7
8
0.049
0.036
0.056
0.014
0.053
0.021
0.043
0.028
9
3 Wnor 0.0214 0.0195
0.013
0.024
0.026
0.021
0.015
10
11
12
0.044 0.030 0.036 0.077 0.014 5.6x1O~32.0,dO~4 ~
13 0.096 5x1o6
14 0.020 4x10~7
0.015 9.2x10
rates by assuming that the photoelectrons deposit their energy locally in the upper atmosphere of Saturn. The ratios of the direct production rates for v = 1, 2, 3 can be estimated from the ratios of the respective cross sections at their maximum near 1 - 2 eV /8/ 1:0.1:0.007 respectively. These ratios are almost constant for electron energies greater than a couple of eV. Based on the results of Cravens /1/ we have not included the much smaller photo-electron fluorescence source for these preliminary calculations.
*
On leave from: York University, North York, Ontario, Canada M3J 1P3 (1)131
T. Majeed et a!.
(1)132
Ionospheric source: Recombination of Ht ions H~(v)+e—~H+H+H —~H;+H where
*
(la) (ib)
represents an electronically or vibrationally excited H
2, is a potential source of H2 (v). Reaction 1 appears be the most important loss process for Ht ions at the altitude of interest. The strength of the source will, of course, depend on the relative efficiency of k,~ versus kib. For the exploratory calculations shown herein we have assumed that k,. = 0. Likewise, since the distribution of energy amount the different vibrational levels of H2(v) is currently not known, we assume that each level is excited with equal probability. to
Although recent investigations /9,10,11,12/ have signalled a substantial uncertainty in the rate of this reaction, this will not greatly impact the column recorubination rate of Ht since recombination is the only loss process for Ht and the major source of H~,ie photoionization of H2 followed by charge transfer of Ht to Ht, does not depend on the details of the ionospheric model. This can be seen in Figure 1, which gives the vertical distribution of the H2(v) source for 2 very different values of kib. The vertical distribution is somewhat different illustrating the different ionospheric structure. However, the column source of H3(v) is the same. Of course, the effect on the ionosphere of the different values of the rate constant is quite dramatic as we show below. MODEL The ionospheric model used in this paper is the same as has been used previously by us /3,13/. However, in this note the calculations of ionospheric and vibrational structure are coupled, so that the ionospheric and vibrational densities are determined in a self-consistent manner. The neutral atmosphere used in this paper was derived from the analysis of the Voyager UVS solar and stellar occulatation data /14/. An important possible loss process for protons is the reaction of 11+ ions with H2. This reaction is endothermic unless the H2 molecules are in the fourth or higher levels H~+H2(v4)—H~+H
(2)
9 cm3 /15/. of this reaction, k2, are available, but we assume that it is close to the gas No measurements for the rate s~ coefficient kinetic value of 2 x10 For our standard ionospheric model, the majority of the Ht ions are in the vibrational level v = 0, and are assumed to recombine with a rate coefficient k?b = 2 x iO~(300/T)°5 cm3 s~ consistent with recent measurements / 10,11/. We have also used the recently reported upper limit for k?b of 1 x 10—il (300/T)°~5cm3 s1 /12/. This we call the slow model. The rate constants used in the vibrational chemistry are those given by Cravens /1/. Figure 2 shows rate constant for most important processes versus vibrational level (v) for a neutral temperature ~ 360 K.
0
IS’
10’ 10’ 10’ Pr,d,,ctl.e Ret. IC.—3
10’
10.
s—u
is’
Figure 1. Production rates due to direct electron impact excitation for the vibrational levels v=1, 2, 3 are
shown as a function of altitude (dashed curves), photon induced fluorescence, v=1, (solid curve), ionospheric source, v=1 standard H~recombination (dotted curve) and slow recombinotion rate (dot-dash
I
2
3
4
5
o 7 e 9 IS V;b~atIoesILs,.I
11
I?
13
14
Figure 2. Quenching (VT) reaction rate constants
both for H 3 and H and vibrational exchange rate constants (VV) for H3 are shown for a neutral ternperature of 350 K. -
curve).
RESULTS Figure 1 shows the vibrational production rates from the sources discussed in section 2. The source for the v=1 level is dominated at all levels by direct electron impact excitation. The production of vibrational quanta for v=2 level is dominated by the electron impact process between 1100 km and 2300 km. At other altitudes the fluorescence source is more important. For v>3 the fluorescence source is more important than both the electron excitation source, the ionospheric source, and tce photo-electron fluorescence source (not shown). We present in Figure 3a and 3b the number densities for all 14 vibrationally excited states of H2 calculated with the standard H~recombination rate coefficient. The peak in H2(v=1) densities for the calculations without fluorescence reflects the structure of the production rate for v=1 and appears in the region of maximum energy deposition for incoming solar EUV radiation. The density of H2(v=1) is much larger than the densities of the other levels as a result of the larger production rate for this level.
H~in the Upper Atmosphere of Saturn
(1)133
Diffusion controls the distribution of Hz(v=1) for the altitudes greater th~an1500 km. Below this height the distribution is in quasi-photochernical steady state and the main loss process for vibrational quanta is quenching of a single vibrational quantum (VT processes) by H. Normally the peak of a distribution would be formed at an altitude where the diffusion time constant (ra) becomes equal to the chemical time constant (re). However, for the v=1 level, due to the particular form of the sources and sinks this is not the case. The H 2 number densities for 2v5 are mainly controlled by processes involving exchange of a single vibrational quanta (VV) between neighbouring vibrational levels. For levels with v > 5 vibrational quenching processes (VT) with 112 play a more important role (see Figure 2). These processes are very rapid for higher values of v, therefore the densities of 112 (v>5) decrease sharply with increasing v. The cascading from higher vibrational levels contributes to the total production rates of these levels. As for the v=1 level the peak in the distribution occurs in the chemical region, and the transition region between chemistry and diffusion occurs at somewhat higher altitudes. For example, for v = 2, 10, 14 the transition region occurs ~— 2100, 2500, and 3100 km respectively. The density distribution of vibrationally excited H2(v) obtained by including the excitation source due to fluorescence is shown in Figure 3b. The v=1 densities are only increased by 10% except at the bottom of the model where the increased densities reflect the stronger fluorescence source (cf. Figure 1). In this case, due to the extended nature of the source, the peak of the levels with v6 occur near the transition region between chemistry and diffusion. The fall off with density downwards away from the peak reflects the fact that the loss process, quenching by H2, has a smaller scale height than the fluorescence source whose scale height is larger than that for 112 (cf. Figure 1). The H2(v4) densities, which as we have noted above, may impact the ionosphere, maximize in the region of 1800 km and are larger by a factor 30 when the fluorescence is included. This, as we see below, has important implications for ionospheric structure. In terms of equivalent vibrational temperatures the upper levels have temperatures 3500 K.
500 10
10’ l0~’ 10’ 10’
IS’ Ii’ Ii’ ID’ D.ns~tyIC.31
10’ 10’ 10’
10’
SOC
~
10’ 10’ 10’ IS’
Ii’ Ii’ 10’ 10’ IS’ 0.,,.ity IC.—31
10’
Ii’
10’
Figure 3. Densities for all 14 levels of vibrationally excited H3 in the upper atmosphere of Saturn including k2 are shown a) without fluorescence source b) with fluorescence source. The impact of vibrationaily excited H2 (v) on the ionospheric densities is shown in Figure 4. Curve A shows a RSS electron density profile /16/ for a latitude of 31°S. The model calculations for a number of scenarios are shown by curves B, C, and D and are obtained using the standard value of k~,. The model electron densities, n,, with the effects of vibrationally enhanced H2 supressed, (i.e k2=0) are shown by curve B. The results are similar to those obtained by others /17/ and exhibit a peak that is more than 1000 km below and more than 10 times the magnitude of the observed peak. Curve C shows the effect of vibrational excitation but with no fluorescence source included. The calculated electron densities are reduced by a factor 2. The impact of the fluorescence source for H2 (v) on the ionospheric densities is shown by curve D. The vibrational densities for levels v4 have been increased sufficiently by the fluorescence source that k2 is able to reduce the peak in the model n, electron densities to a value comparable to that obtained by the RSS V2-exit occultation experiments. However the altitude of the peak n, is still 1000 km lower than the measured one. This could, however, be modified by the inclusion of a vertical wind /3,13,17/. H+ is the major ion at the ionospheric peak but Ht densities dominate in the 1500 — 2500 km altitude range because of rapid reaction of H+ ions with H2(v4) molecules. We have also done some ionospheric calculations by using the slow Ht recombination rate coefficient. The results of the ionospheric calculations including the fluorescence source of vibrational excitation and the loss of H+ ions with Hz(v4) are shown in Figure 5. A serious disagreement obtains between the model calculations (curve B) and observations (curve A) regarding both the magnitude and the location of the electron density peak. In this case the Ht becomes the major ion in the entire Saturnian ionosphere since it recombines very slowly with the electrons. H+ is a minor ion due to very rapid reaction with H2(v4). DISCUSSION AND CONCLUSIONS It is clear that the fluorescence source of vibrational excitation is sufficiently strong to enhance the densities of higher H2 (v) levels to an extent that there is extensive modification of the ionospheric structure if the standard recombination rate constant is used. Thus photon-induced fluorescence represents a potentially important source of H2(v) on all of the outer planets. The disagreement with the ionospheric model and RSS data using the slow rate constant suggests that perhaps there may be a discrepancy with the reported upper limit /12/. However, one solution to this problem has recently been suggested by McConnell and Majeed /18/. They adopted a recombination rate coefficient for ground state Ht ions, based on molecular data, that, at elevated electron temperatures, proceeds much faster than the measured upper limit reported by Adams and Smith /12/. In order for this process to proceed the total energy of the H~ion and the kinetic energy of the electron must be 1 eV. If elevated electron temperatures obtain, then one result of this enhanced recombination process, aside from reducing n,, could be the
(1)134
T. Majeed eta!.
(~
10.
0.naltty lc.—31
Figure 4. Model electron densities for 31°Scompared with the RSS V2-exit measurements, Curve A. Curve B is the model fit with no vibrational excitation, k 3 no added. D included is the model = 0.fluorescence Curve C issource the model fit Curve with 1(3 but fit with fluorescence source included. The standard value for k?b was used. H~and H~densities corresponding to curve D are also shown.
0.n.Ity IC.—3)
Figure 5. Model electron densities for 31°Scompared with the RSS V2-exit measurements, Curve A. Curve 1and cm3 ~ H~and H~ B is the model fit with to1i3curve fluorescence source densities B are also shown. included corresponding and ~?b = 1 x10’
efficient production of H Lyman-a resulting in an important contribution to the planetary H Lyman-a budget (see /5,18/). Another possibility of reducing the plasma densities includes an influx of H 20 in the upper atmosphere of Saturn due to photosputtering of the rings /19/. The charge exchange reaction of H+ and Ht ions with H20 molecules proceeds rapidly with the subsequent formation of rapidly recombining molecular ions, such as H20+ and H30+, so that the ionospheric plasma densities are suppressed /17,19/. ACKNOWLEDGEMENTS JCMcC acknowledges continuing support from the Natural Science and Engineering Research Council of Canada and the Centre National de la Recherche Scientifique for support while on sabbatical in Besançon. TM and RVY acknowledge support from JPL contract 957763 under NASA contract NAS7-918 to the University of Arizona. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
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