---

A&A 506, 965-970 (2009)

Ionization processes in the atmosphere of Titan

II. Electron precipitation along magnetic field lines

G. Gronoff¹ - J. Lilensten¹ - R. Modolo²

1 - Laboratoire de Planétologie de Grenoble, Université Joseph Fourier - CNRS, France
2 - LATMOS/IPSL/UVSQ,10-12 avenue de l'Europe, 78140 Vélizy, France

Received 20 March 2009 / Accepted 13 July 2009

Abstract
Context. The Cassini probe regularly passes the vicinity of Titan, providing new insights into particle precipitation by use of its electron and ion spectrometers. A discrepancy between precipitation models and observations of electron fluxes has been found. This discrepancy was suspected to be caused by the geometry of the magnetic field.
Aims. In this article, we compute the electron impact ionization in the nightside ionosphere of Titan, assuming non-trivial geometry for the magnetic field lines.
Methods. We use the TransTitan model, modified to take into account the magnetic field line geometry in the nightside, and we compare these results with the electron flux measurements during the T5 fly-by of Cassini. We use several magnetic field line geometries, including one produced by hybrid simulations.
Results. The geometry of the lines implies a longer path of the electron inside the atmosphere of Titan. The electron fluxes are therefore modified considerably compared to the vertical precipitation hypothesis. At an altitude of , the electron flux can be divided up to ten times with a field line resulting from hybrid simulation. Thanks to the use of more accurate field lines, the model reproduces the experiment well without any further adjustment of the precipitated measured electron flux.
Conclusions. Several hypothesis had been suggested to explain the discrepancies between the different models and the observation of the electron flux during the T5 fly-by of Cassini. Our approach shows that the most probable explanation is the magnetic field line geometry. This work shows that the computation of ion production by electron impact in the atmosphere of Titan needs the consideration of both magnetic field and the input electron fluxes. Based on these considerations, our model can compute the conditions for future fly-by, and could be used to compare models with experiments.

Key words: planets and satellites: individual: Titan - atmospheric effects - space vehicles: instruments

1 Introduction

The magnetic environment of Titan in the magnetosphere of Saturn is very complex, and its characterization and comprehension is one of the main objectives of the Cassini's probe mission (Blanc et al. 2005). Because of the absence of an intrinsic magnetic field, Titan's atmosphere and ionosphere interact directly with Saturn's magnetospheric plasma (Backes et al. 2005). The upper atmosphere is thus partly ionized by mainly solar photons and electron impact. The strength and location of each ionization source is sensitive to the orbital location of Titan, leading to a very complex ionized environment and being highly dynamic. Cassini observations have suggested that photoionisation is the most important ionization process for Titan's dayside (Agren 2009, submitted to PSS). On the other hand, Cassini observations have also shown that, on the nightside, electron impact plays an important role and is the main source of ionization (when the nightside corresponds to the ram-side). This situation happened on April 16th, 2005 on the outbound pass of the T5 flyby by the Cassini spacecraft. The electron flux data acquired during this flyby was compared with several models (Cravens et al. 2008; Agren et al. 2007). These comparisons showed that the ionization and the fluxes detected by the Cassini probe cannot be modeled as a vertical precipitation. This can be interpreted as a consequence of the interaction between the magnetic field lines and the atmosphere of Titan (Cravens et al. 2009; see also, Gronoff et al. 2009, Paper I - Ionization in the whole atmosphere, this issue). In this paper, we use the TransTitan model to compute the electrons fluxes at different altitude by considering different geometry of magnetic lines.

2 The TransTitan model

To compute the electron fluxes in different geometric configurations, we need a code capable of computing electron transport in the atmosphere. In particular, it must compute the influence of elastic and inelastic collisions with the different species of the atmosphere. This code exists: TransTitan, which has been studied in details in the Paper I. In the present paper, we only use the kinetic part for the electrons in the nightside. The geometry of the code is modified to take into account the bending of the magnetic field lines around Titan: in the original version of the code, the electron transport was computed along a straight line, which cannot be strongly inclined with respect to the vertical. The vertical line represents the grid where the ionization is computed. In the present version, we compute the ionization along the magnetic field line. For this study, the neutral atmosphere used is described in Cui et al. (2009), and we take into account only the densities of the two major components of the atmosphere, and . To study the influence of the neutral atmosphere, we also used the neutral atmosphere of Waite et al. (2005) and the pre-Cassini model of Müller-Wodarg et al. (2000). The differences between these models can be seen in Fig. 1. The main difference is that the pre-Cassini model has a higher CH₄ density in the upper atmosphere. We assume that the exospheric temperature is constant at 175 K. The altitude range covered by our computation is from 900 km to 1600 km, the lower altitude being determined by the geometry of the line. The corresponding length of the lines can be more than 4000 km.

3 Electron precipitation and magnetic field line geometry

In situ electron-flux spectra are taken from Cravens et al. (2009). The aim of this present work is to compute the ionospheric parameters, by taking into account the presence of the Kronian magnetic field and mimicking the behavior of the electron fluxes, which are affected by the magnetic field draping. So far, vertical magnetic field lines or parabolic lines have been used to compute the electron energy deposition in Titan's atmosphere (Cravens et al. 2005). We assume several magnetic field line configurations, including horizontal magnetic field lines above 1200 km. Titan's ionosphere acts as an obstacle and modifies the magnetospheric plasma flow. The kronian magnetic field lines close to Titan are twisted and wrap the ram-side of the body, as they diffuse slowly in the dense and cold ionosphere. We assume curved magnetic field lines, which enter inside the ionosphere to emerge outside several thousand of kilometer apart. To demonstrate how the magnetospheric electron fluxes are sensitive to the magnetic field line topology, we use simulation results describing the draping around Titan. These field lines are extracted from a three-dimensional multi-species hybrid simulation. In this model, a kinetic description is used to describe ions, while electrons are characterized as an inertialess fluid contributing to electric currents that produce quasineutrality. This model depicts the dynamics of the plasma environment in the vicinity of Titan, and includes the effects of ionization by solar photons, ionization by electron impact, and charge exchange. We note that the description below 1400 km is limited in this model since the complex ionospheric chemistry is not included. Nevertheless, the model describes the photoabsorption and the deceleration of ions in the atmosphere caused by elastic collisions. More details concerning the model are presented in Modolo & Chanteur (2008). The different configurations are summarized in Fig. 3, where each number corresponds to the following cases:

1.

the vertical case;

2.

a straight line with a minimum altitude at ;

3.

a straight line with a minimum altitude at ;

4.

a bent line with a minimum altitude at ;

5.

a bent line with a minimum altitude at ;

6.

the hybrid simulated line. This line is cut at the lowest altitude of 1000 km because the simulation is not valid below. To show the draping at the origin of this line, we put two other simulated lines (label 7), which does not reach the altitude of our simulation.

4 Result

Given the incident precipitation flux shown in Fig. 2, we considered the impact of several magnetic geometries on the production and fluxes at different altitudes.

4.1 First case: vertical precipitation

This is the simplest case, we consider that electrons simply precipitate vertically. The resulting ionization can be seen in Fig. 4. The ionization peak is at , and has an intensity of , this simulation having been completed using the Müller-Wodarg et al. (2000) neutral atmosphere, because electrons reaches the mesosphere. We computed the resulting electron flux (which corresponds to the degradation in energy of the primary flux plus the secondary electron flux) at different altitudes (Fig. 5, with the Cui et al. 2009, atmosphere in that case). We find a significant discrepancy between the Cassini flux measured at and the model predictions. As in Agren et al. (2007) and Cravens et al. (2009), we have to divide the input flux by a factor of about 10 to reach good agreement. As shown in Fig. 5, that hypothesis allows us to explain the flux in the 10-1000 eV energy range, but underestimates the flux at both lower and higher energies.

4.2 Second case: straight line, horizontal at 1200 km

In that case, we use a straight horizontal line, whose lower altitude is , e.g., it is perpendicular to the vertical at this altitude. This geometry could not be simulated with the initial TransTitan code. The maximum altitude of this line is the top of our atmosphere at , and corresponds to the line with a entry in the thermosphere and exit of the thermosphere, in Fig. 6. In that figure, we can see that the ionization peak is not at the middle of the line, but at , with an intensity of . The corresponding altitude is , but is about away from the lowest point ( altitude). The corresponding flux comparison can be seen in Fig. 7, the agreement being closer than in the previous case: the length flux is inside the measurement error bars between and , and corresponds to an altitude of . This results shows that the division of the input flux is unnecessary to account for this part of the spectrum. For the highest energy, our modeling departs from the measurements, being one order of magnitude higher in flux.

4.3 Third case: straight line, horizontal at 1000 km

This case is similar to the previous one, except that the lower altitude of the line is and the maximum altitude is always , which is reached at the extremities. The ion production maximum occurs at length, with an intensity of (Fig. 8). The peak corresponds to an altitude of . The corresponding flux comparison can be seen in Fig. 9, where, fluxes at altitudes close to show agreement with measurements when energies are above .

4.4 Fourth case: bent line, minimum at 1200 km

In this simulation, we use a curved line with a minimum altitude at (Fig. 3). The ionization peak is at length, which corresponds to an altitude of . Its intensity is (Fig. 12). The corresponding flux comparison can be seen in Fig. 11. A more accurate description of the reference flux is achieved for an altitude of , where the magnetic field line goes outside of the atmosphere.

4.5 Fifth case: bent line, minimum at 1000 km

This is the same configuration as in the previous case, except that the lower altitude is . The ionization peak is at length, which corresponds to an altitude of . Its intensity is (Fig. 12). The corresponding flux comparison can be seen in Fig. 13. The fluxes simulated below are very close to the reference flux. For the two bent cases, we can conclude that it is possible to find a magnetic field line configuration capable of explaining the entire spectrum without modifying the input energy spectrum. However, to determine the reality of this approach, we need to use global simulation computed lines.

4.6 Sixth case: line from a hybrid simulation of the Ta fly-by of Cassini

In this case, we use a magnetic field line computed from a hybrid simulation performed in the conditions of the Ta fly-by of Cassini (Modolo & Chanteur 2008).

We considered several magnetic field lines in that simulation, and we only show the one that provide closer agreement with measurements. This line is of course an approximation, used as a representative field line when Titan is inside the magnetosphere of Saturn: the measurement were completed for the T5 fly-by and the MHD simulation is incomplete below . The ionization peak is at length, which corresponds to an altitude of . Its intensity is (Fig. 14). The corresponding flux comparison can be seen in Fig. 15. The agreement between model and experiment is good and within the error bars in that case, even if a difference is still seen in high energies.

5 Discussion

Cravens et al. (2009) consider the problem of the discrepancy between the vertical precipitation model and the comparison of the fluxes at . In Fig. 5, we confirm this problem, with the important discrepancy between our model and the experiment at this altitude. The conclusion of this work was that the electrons are embedded in different magnetic tubes, each one having a different electron flux.

In the present work, the use of several magnetic field configurations shows that the geometry of the precipitation has an important effect on the electron flux at altitude. In Figs. 7 and 15, we find good agreement between model and experiment, between approximatively and few hundred eV. The differences at high energies could be explained by the spatial resolution: in Fig. 15, the high energy fluxes vary significantly between altitude and altitude. Another explanation of this discrepancy could be that in the neutral atmosphere model used in our simulations, or in the uncertainties in the flux measurement at these energies. Concerning the influence of the neutral atmosphere, some minor differences in the fluxes are visible when we replace the Cui et al. (2009) model with either the Waite et al. (2005) or the Müller-Wodarg et al. (2000) model. However, the neutral atmosphere cannot notably explain the differences between model and experiment in the vertical case. In Fig. 16, we can see the resulting flux at 1200 km for the hybrid simulated line with the different neutral atmospheres. The neutral atmosphere of Cui et al. (2009) produces higher fluxes (about two times higher between 10 and 1000 eV). This is because of the lower density of that model, although the model predictions are within the error bars of the observations. Further improvements will require either a precise measurement of the neutral atmosphere during the fly-bys or a neutral atmosphere model that considers the variation in the density with e.g., solar zenith angle, solar activity.

To identify which of the two possible explanations of the observed electron flux is the most appropriate at , we need to measure the magnetic field line for the T5 conditions. Such work has not yet been published. The detection of a magnetic field oriented to the vertical at along the Cassini's path (Cravens et al. 2009) would be insufficient to retrieve the data by modeling, but would be able to constrain models. The observation of the magnetic draping (Bertucci et al. 2008) shows that an analysis of the magnetic environment of Titan is necessary to compute the ion production caused by electron impact. The ion production in the different cases (Figs. 7, 9, 11, 13, and 15) shows an enhancement in the production at high altitude, and a lack of production at low altitude.

6 Conclusion

The new version of TransTitan is able to compute electron precipitation along magnetic field lines in the atmosphere of Titan. Because of the magnetic field line geometry computed by hybrid simulation, we find agreement between model and observations of electron fluxes inside the atmosphere of Titan, without needing to divide the input flux. The magnetic field line geometry can therefore provide an explanation of the discrepancy between model and observation observed in Cravens et al. (2009), but lack of knowledge about the magnetic configuration still prevents us from being able to decide between our explanation and the hypothesis of magnetic tubes with different electron fluxes. The present work can be considered as a basis for future work on the comparison between MHD models and precipitation observations on Titan and other non-magnetic planets or bodies. It illustrates the importance of considering draped magnetic field lines inside the atmosphere.

Acknowledgements

The authors wish to thank C. Simon (BIRA, Belgium), M. Barthélemy (LPG, France) and V. Vuitton (LPG, France) for useful discussions.

References

• Agren, K., Wahlund, J. E., Modolo, R., et al. 2007, Annales Geophysicae, 25, 2359 [NASA ADS]
• Backes, H., Neubauer, F. M., Dougherty, M. K., et al. 2005, Science, 308, 992 [NASA ADS] [CrossRef]
• Bertucci, C., Achilleos, N., Dougherty, M. K., et al. 2008, Science, 321, 1475 [NASA ADS] [CrossRef]
• Blanc, M., Kallenbach, R., & Erkaev, N. V. 2005, Space Sci. Rev., 116, 227 [NASA ADS] [CrossRef]
• Cravens, T. E., Robertson, I. P., Clark, J., et al. 2005, Geophys. Res. Lett., 32
• Cravens, T. E., Robertson, I. P., Ledvina, S. A., et al. 2008, Geophys. Res. Lett., 35
• Cravens, T. E., Robertson, I. P., Waite, J. H., et al. 2009, Icarus, 199, 174 [NASA ADS] [CrossRef]
• Cui, J., Yelle, R. V., Vuitton, V., et al. 2009, Icarus, 200, 581 [NASA ADS] [CrossRef]
• G. Gronoff, J. Lilensten, L. Desorgher, & E. Flückiger, A&A, 506, 955 (Paper I)
• Modolo, R., & Chanteur, G. M. 2008, J. Geophys. Res., 113
• Müller-Wodarg, I. C. F., Yelle, R. V., Mendillo, M., Young, L. A., & Aylward, A. D. 2000, J. Geophys. Res., 105, 20833 [NASA ADS] [CrossRef]
• Waite, J. H., Niemann, H., Yelle, R. V., et al. 2005, Science, 308, 982 [NASA ADS] [CrossRef]

 

All Figures

---

Copyright ESO 2009