Detection of negative ions in the deep ionosphere of Titan during the Cassini T70 flyby

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 39,, doi: /2012gl051714, 2012 Detection of negative ions in the deep ionosphere of Titan during the Cassini T70 flyby K. Ågren, 1 N. J. T. Edberg, 1 and J.-E. Wahlund 1 Received 19 March 2012; accepted 9 April 2012; published 16 May [1] We present radio and plasma wave science (RPWS) Langmuir probe (LP) observations that give evidence for a population of heavy, negative ions at altitudes below 900 km in Titan s ionosphere during the Cassini T70 flyby. The negative ion density in this region is comparable to, or higher than, the electron density of 760 cm 3. Both positive and negative ions are moving with a velocity of at least a few hundred m s 1 relative to Titan. We show two limiting cases where we have analysed RPWS/LP ion measurements. The data can be interpreted as either that a population of negative ions with density comparable to the electron density is present, moving at a very high (>2 km s 1 ) velocity, or that the ion population is moving at a few hundred m s 1, but with a density an order of magnitude larger than the electron density in the same region. Citation: Ågren, K., N. J. T. Edberg, and J.-E. Wahlund (2012), Detection of negative ions in the deep ionosphere of Titan during the Cassini T70 flyby, Geophys. Res. Lett., 39,, doi: /2012gl Introduction [2] Titan s atmosphere is principally composed of molecular nitrogen, with methane ( 2%) and molecular hydrogen ( 0.4%) being the most abundant minor species [Waite et al., 2005]. The ionisation of these species and ensuing ionmolecule chemistry create a chemically complex ionosphere with a main peak in the region between km altitude [e.g., Kliore et al., 2008; Edberg et al., 2010]. At these altitudes the main ionisation sources are solar photons and impacting magnetospheric electrons [e.g., Cravens et al., 2005; Wahlund et al., 2005; Ågren et al., 2007, 2009]. Furthermore, transport of ions from dayside to nightside has been indicated to contribute to the ionospheric composition and structure also at chemically-controlled altitudes [Cui et al., 2009]. At lower altitudes, suprathermal ions, protons and oxygen ions, meteoric impacts and cosmic rays contribute to the ionising of Titan s atmosphere [e.g., Molina-Cuberos et al., 2001; Cravens et al., 2008; Galand et al., 2010]. [3] The electron spectrometer (ELS) part of the Cassini plasma spectrometer (CAPS) onboard the Cassini spacecraft has been used to detect heavy, negative ions in the deep (<1400 km) ionosphere of Titan [Coates et al., 2007, 2009]. These ions were detected in the spacecraft ram direction in the same altitude range as where earlier heavy positive ions have been found to dominate the ionosphere [Crary et al., 2009; 1 Swedish Institute of Space Physics, Uppsala, Sweden. Corresponding author: K. Ågren, Swedish Institute of Space Physics, Box 537, SE Uppsala, Sweden. (agren@irfu.se) Copyright 2012 by the American Geophysical Union /12/2012GL Wahlund et al., 2009]. An unexpected feature of the negative ions was their notably high mass (up to amu/q), while the positive ions were detected up to 350 amu - the count limit of the ion beam spectrometer (IBS). [4] Vuitton et al. [2009] have theoretically investigated the formation mechanisms for negative ions at Titan based on laboratory studies. They conclude that dissociative electron attachment to neutral molecules (mainly HCN) leads to the formation of negative ions with the main ions being CN and C 3 N. In a later stage, the negative charge may transfer to other molecules. Further, Vuitton et al. [2009] proposed that negative ions were precursors to the aerosols observed at lower altitudes in Titan s atmosphere. Wahlund et al. [2009] suggested that the aerosol initiation was the result of electron recombination of heavy positive ions, and Sittler et al. [2009] proposed that both complex negative and positive ions act as seed particles for aerosols. A similar scenario was discussed by Waite et al. [2007], where first positive ions, and in a later step, negative ions are both important in the process of tholin formation. [5] The ionisation rate in Titan s ionosphere and consequently the electron and ion densities depend on the solar zenith angle (SZA) [Ågren et al., 2009]. On the dayside (SZA < 50 degrees) the electron density is typically cm 3, which is on average four times higher than on the nightside (SZA > 100 degrees). In a statistical study of the negative ions, Coates et al. [2009] showed that the maximum negative ion mass is higher at low altitudes and high latitudes. In addition, a weak dependence of the maximum mass of the negative ions on SZA, with a tendency to find highest masses near the terminator, was found. [6] Here, we use LP data to report on findings from the T70 Titan flyby. This is the only detection of negative ions at such low altitudes at Titan, since the spacecraft attitude for this flyby was unfavourable for the CAPS/ELS instrument, which could have performed complementary measurements. 2. Observations [7] The RPWS instrument onboard the Cassini spacecraft consists of sensors for measuring the electric and magnetic fields, low, high, and wide band receivers for processing the data and a spherical LP. In this study we have primarily used data from the LP. Sweeping the probe in bias voltage (from +4 V to 4 V for flybys below 1400 km) causes ions and electrons to be attracted by the probe. Given the bias voltage and the collected current, plasma parameters such as the electron density, n e, the electron temperature, T e, the ion density, n i, the ion temperature, T i, the ion speed, v i, and the average ion mass, m i can be derived. In addition, the LP provides an independent measurement of the spacecraft potential, U sc. Moreover, at low altitudes in Titan s ionosphere the measured current can not be fitted to theoretical 1of5

2 Figure 1. (top) The raw data from the LP with U bias on the y-axis and the magnitude of the current given by the color scale to the right. (middle) The electron density from the LP (blue dots), the electron density derived from the f UH line (black dots, multiple lines) and the ion density (red dots). (bottom) The spacecraft potential, U sc. The odd sweep close to 01:30 UT is due to an instrumental effect and is not real. models without including a population of negative ions (as shown below). Adding a component made up by negative ions (hence dependent on the density, n i, mean ion mass, m i, and velocity, v i, of the negative ions) to the total current results in a first order estimate of the negative ion density. For a more thorough description of how the LP works, see Morooka et al. [2011]. [8] The T70 flyby is to date the flyby of Titan reaching the lowest altitude. The closest approach (CA) occurred at 880 km, about 70 km deeper than any previous flyby. The SZA at CA was 81 degrees and the latitude was 82 degrees north. This means that T70 occurred both in the latitude region and very close to the region of SZAs where maximum negative ion masses have previously been detected by ELS. According to Coates et al. [2009], ion masses of up to 9000 amu/q are expected for this flyby based on ELS measurements at higher altitudes. [9] Figure 1 shows a time series plot of 15 minutes around CA. The appearance of negative ions is detected already in the raw data (Figure 1, top). The minimum current separating the electron and ion populations gives the zero current potential, U 0. In dense plasmas and in the absence of negative ions U sc U 0. Above this potential, electrons are attracted, and below the ion current is collected (together with smaller contributions from photoelectrons and secondary electrons). Thus, the upper half of the plot shows the sampled electron current; a broad region around CA with a large ionospheric population of thermal electrons. Similarly, the lower half of Figure 1 (top) shows the ion current. The two (inbound and outbound) ionospheric peaks can be identified a few minutes before and after CA, at 01:23 and 01:31 UT, but around CA the ion current clearly decreases and partly becomes positive. This can also be seen in Figure 1 (middle), where the ion density is comparable to the electron density, but near CA apparently vanishes, given that a pure positively charged ion component is assumed in the fitting analysis. A detailed investigation of the sweep data shows that negative ions must be present in order to explain the measurements, as shown below. Also note, since the Debye length, l D, is only a few cm, the sampled current is from the near-local region around the LP. [10] The fact that the LP sweep current for a positive U bias becomes positive puts constraints on the amount of the negative ions. This effect would not occur if the flux of negative ions, q i n i v i, was not comparable to that of positive ions, q i n i v i, where q i = +e and q i = Ze (Z < 0). Given that the positive ions are singly charged and that the velocities are equal (but oppositely directed) for positive and negative ions we may put limits to the negative ion density. [11] To investigate this further, we adapted our analysis to also consider negative ions. However, as this increases the number of free variables, this results in an underdetermined problem with several possible solutions. [12] As I e is very large compared to the other involved current contributions for U bias > 0, the electron density is fixed and constrains the analysis. Nevertheless, multiple 2of5

3 Figure 2. The current-voltage characteristics of a sweep at UT of flyby T70. The x-axis shows applied bias voltage to the probe. The data: (top) linear and (bottom) logarithmic. The LP data are shown as blue dots. Superposed are the total current (red line), the negative ion current (blue line), the ion current (red dot-dashed line) and the photoelectron current (black dashed line). For this sweep the ion velocity, v i, was set to 260 ms 1, which results in a high negative ion density (> cm 3 /Z). Please note that the ion velocities here are given relative to the spacecraft, which moves at a speed of 6 km s 1. good fits to the data can be obtained, depending on the choice of the negative and positive ion density, the mean mass of the ions, as well as the relative speed between them. Note also that the ion mass comes in non-linearly into the equations for probe current contributions, which means that the mass distribution for positive and negative ions do matter. However, as a first approximation the mean ion mass suffice, and the error associated to this problem does not change our conclusions. The measured LP sweeps cannot be explained without the addition of a negative ion component, since it is not possible for a population consisting of only positive ions to produce a positive current (compare with Figures 2 and 3). The electron contribution to the current is exponential for U bias < 0, and is starting to influence the measurements when U bias > 0.5 V in Figures 2 (top) and 3 (top). For these sweeps, the electron part is removed and focus is put on the positive, I i, and negative, I i, ion current contributions. For (U bias + U float ) < 0 these are given by I i ¼ q i n i v i prlp 2 1 q iðu bias þ U float Þ ð1þ I i ¼ q i n i v i prlp 2 qi UbiasþUfloat e ð ð Þ=Wi Þ ð2þ where r LP is the radius of the LP, U float is the floating potential and W i ¼ m iv 2 i 2, and similarly for W i. For the negative ions the large ion mass and high velocity leads to an exponential function e ( q i (U bias +U float )/W i ) 1, which simplifies the expression into W i I i ¼ q i n i v i pr 2 LP : ð3þ Hence, the negative ion current will give a near constant contribution in the following analysis and is not significantly dependent on the mean ion mass of the negative ions. 3. Discussion and Conclusions [13] Figures 2 and 3 show two possible fits to the ion current measured by the LP at 01:27:51 UT. The different solutions are restricted by the zero crossing and the inclination of the derivative. As seen in equation (1), the mean ion mass of the positive ions also affects the fit. For the sweeps presented in this paper we use mean ion masses of 52 and 260 amu to achieve good fits to the data. In Figures 2 and 3 we present two limiting cases, based on assumptions for the relative velocity and negative ion density as described below Case I Figure 2 [14] Crary et al. [2009] used combined CAPS/ion beam spectrometer (IBS) and ion and neutral mass spectrometer (INMS) spectra to show clear evidence of strong ionospheric winds at Titan. They modelled the wind speeds to commonly be in the range of 100 m s 1 with the highest values reaching up to 260 m s 1. As flyby T70 reached 70 km deeper than the flybys used in these results, we may put 260 m s 1 as an upper limit to the ion wind speed. Figure 2 shows a fit to the data given an ion speed of 260 m s 1. This implies a negative ion density of cm 3 /Z and a resulting (assuming charge neutrality) positive ion density of cm 3. This gives us an upper limit to the negative ion density at this altitude. 3of5

4 Figure 3. Same as for Figure 2, except for the negative ion density being set to the electron density, n i = n e, which results in a high ion velocity Case II Figure 3 [15] In a previous study, Coates et al. [2007] showed that the negative ion density at low altitudes could reach 10% of the negatively charged population. As the observations leading up to this conclusion did not include any flyby reaching below 950 km, the negative ion density for flyby T70 is supposedly higher than so. We set n i to be 50% of the negatively charged population, i.e., n i =n e. This results in very high ion velocities of 2.55 km s 1. Such velocities are not probable at these altitudes as the ionosphere is dense and collision dominated. An electric field of 50 mv/m could produce velocities of this order, however, such strong fields have so far not been detected at Titan [Ågren et al., 2011]. Cassini does not carry any instrument for directly measuring the electric field. [16] For the data shown here we have used a mean mass of 3600 amu for the negative ions. As discussed above, Coates et al. [2009] showed that the negative ions reach masses of up to 9000 amu/q in this latitude region. Hence, the mean negative ion masses we use are reasonable. [17] It is beyond the scope of this paper to further investigate which of the above scenarios is most likely at these altitudes. Probably the correct answer lies somewhere in between, since the flux q i n i v i must be constant. We simply conclude that there are substantial amounts of negative ions present, with a density in the range of around 1000 up to more than cm 3 /Z and a velocity of at least a few hundred m s 1, possibly as high as a few km s 1. [18] In summary, we have for the first time used the LP alone to detect a significant amount of negative ions in the deep ionosphere of Titan and confirm the findings of Coates et al. [2007, 2009]. This result is important as it constrains the ionospheric plasma composition around Titan. Furthermore, negative ions are believed to be important in the formation of tholins and these findings provide further clues to this process. [19] Acknowledgments. The Swedish National Space Board (SNSB) supports the RPWS/LP instrument onboard Cassini. [20] The Editor thanks Thomas Cravens and an anonymous reviewer for assisting with the evaluation of this paper. References Ågren, K., et al. (2007), On magnetospheric electron impact ionisation and dynamics in Titan s ram-side and polar ionosphere A Cassini case study, Ann. Geophys., 25, Ågren, K., J. Wahlund, P. Garnier, R. Modolo, J. Cui, M. Galand, and I. Müller-Wodarg (2009), On the ionospheric structure of Titan, Planet. Space. Sci., 57, , doi: /j.pss Ågren, K., et al. (2011), Detection of currents and associated electric fields in Titan s ionosphere from Cassini data, J. Geophys. Res., 116, A04313, doi: /2010ja Coates, A. J., F. J. Crary, G. R. Lewis, D. T. Young, J. H. Waite Jr., and E. C. Sittler Jr. (2007), Discovery of heavy negative ions in Titan s ionosphere, Geophys. Res. Lett., 34, L22103, doi: /2007gl Coates, A. J., A. Wellbrock, G. R. Lewis, G. H. Jones, D. T. Young, F. J. Crary, and J. H. Waite (2009), Heavy negative ions in Titan s ionosphere: Altitude and latitude dependence, Planet. Space. Sci., 57, , doi: /j.pss Crary, F. J., B. A. Magee, K. Mandt, J. H. Waite, J. Westlake, and D. T. Young (2009), Heavy ions, temperatures and winds in Titan s ionosphere: Combined Cassini CAPS and INMS observations, Planet. Space. Sci., 57, , doi: /j.pss Cravens, T. E., et al. (2005), Titan s ionosphere: Model comparisons with Cassini Ta data, Geophys. Res. Lett., 32, L12108, doi: / 2005GL Cravens, T. E., I. P. Robertson, S. A. Ledvina, D. Mitchell, S. M. Krimigis, and J. H. Waite Jr. (2008), Energetic ion precipitation at Titan, Geophys. Res. Lett., 35, L03103, doi: /2007gl Cui, J., M. Galand, R. V. Yelle, V. Vuitton, J.-E. Wahlund, P. P. Lavvas, I.C.F.Müller-Wodarg,T.E.Cravens,W.T.Kasprzak,andJ.H.WaiteJr. (2009), Diurnal variations of Titan s ionosphere, J. Geophys. Res., 114, A06310, doi: /2009ja of5

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