Planetary Ionospheres Research Articles

The Possible Role of Penning Ionization Processes in Planetary Atmospheres

In this paper we suggest Penning ionization as an important route of formation for ionic species in upper planetary atmospheres. Our goal is to provide relevant tools to researchers working on kinetic models of atmospheric interest, in order to include Penning ionizations in their calculations as fast processes promoting reactions that cannot be neglected. Ions are extremely important for the transmission of radio and satellite signals, and they govern the chemistry of planetary ionospheres. Molecular ions have also been detected in comet tails. In this paper recent experimental results concerning production of simple ionic species of atmospheric interest are presented and discussed. Such results concern the formation of free ions in collisional ionization of H2O, H2S, and NH3 induced by highly excited species (Penning ionization) as metastable noble gas atoms. The effect of Penning ionization still has not been considered in the modeling of terrestrial and extraterrestrial objects so far, even, though metastable helium is formed by radiative recombination of He+ ions with electrons. Because helium is the second most abundant element of the universe, Penning ionization of atomic or molecular species by He*(23S1) is plausibly an active route of ionization in relatively dense environments exposed to cosmic rays.

Probability of occurrence of planetary ionosphere storms associated with the magnetosphere disturbance storm time events

Abstract. The ionospheric W index allows to distinguish state of the ionosphere and plasmasphere from quiet conditions (W = 0 or ±1) to intense storm (W = ±4) ranging the plasma density enhancements (positive phase) or plasma density depletions (negative phase) regarding the quiet ionosphere. The global W index maps are produced for a period 1999–2014 from Global Ionospheric Maps of Total Electron Content, GIM-TEC, designed by Jet Propulson Laboratory, converted from geographic frame (−87.5:2.5:87.5° in latitude, −180:5:180° in longitude) to geomagnetic frame (−85:5:85° in magnetic latitude, −180:5:180° in magnetic longitude). The probability of occurrence of planetary ionosphere storm during the magnetic disturbance storm time, Dst, event is evaluated with the superposed epoch analysis for 77 intense storms (Dst ≤ −100 nT) and 230 moderate storms (−100 < Dst ≤ −50 nT) with start time, t0, defined at Dst storm main phase onset. It is found that the intensity of negative storm, iW-, exceeds the intensity of positive storm, iW+, by 1.5–2 times. An empirical formula of iW+ and iW- in terms of peak Dst is deduced exhibiting an opposite trends of relation of intensity of ionosphere-plasmasphere storm with regard to intensity of Dst storm.

ELECTRODYNAMICS ON EXTRASOLAR GIANT PLANETS

Strong ionization on close-in extrasolar giant planets suggests that their atmospheres may be affected by ion drag and resistive heating arising from wind-driven electrodynamics. Recent models of ion drag on these planets, however, are based on thermal ionization only and do not include the upper atmosphere above the 1 mbar level. These models are also based on simplified equations of resistive MHD that are not always valid in extrasolar planet atmospheres. We show that photoionization dominates over thermal ionization over much of the dayside atmosphere above the 100 mbar level, creating an upper ionosphere dominated by ionization of H and He and a lower ionosphere dominated by ionization of metals such as Na, K, and Mg. The resulting dayside electron densities on close-in exoplanets are higher than those encountered in any planetary ionosphere of the solar system, and the conductivities are comparable to the chromosphere of the Sun. Based on these results and assumed magnetic fields, we constrain the conductivity regimes on close-in EGPs and use a generalized Ohm's law to study the basic effects of electrodynamics in their atmospheres. We find that ion drag is important above the 10 mbar level where it can also significantly alter the energy balance through resistive heating. Due to frequent collisions of the electrons and ions with the neutral atmosphere, however, ion drag is largely negligible in the lower atmosphere below the 10 mbar level for a reasonable range of planetary magnetic moments [abridged].

MAGNETOSPHERIC STRUCTURE AND ATMOSPHERIC JOULE HEATING OF HABITABLE PLANETS ORBITING M-DWARF STARS

We study the magnetospheric structure and the ionospheric Joule Heating of planets orbiting M-dwarf stars in the habitable zone using a set of magnetohydrodynamic (MHD) models. The stellar wind solution is used to drive a model for the planetary magnetosphere, which is coupled with a model for the planetary ionosphere. Our simulations reveal that the space environment around close-in habitable planets is extreme, and the stellar wind plasma conditions change from sub- to super-Alfvenic along the planetary orbit. As a result, the magnetospheric structure changes dramatically with a bow shock forming in the super-Alfvenic sectors, while no bow shock forms in the sub-Alfvenic sectors. The planets reside most of the time in the sub-Alfvenic sectors with poor atmospheric protection. A significant amount of Joule Heating is provided at the top of the atmosphere as a result of the planetary interaction with the stellar wind. For the steady-state solution, the heating is about 0.1-3\% of the total incoming stellar irradiation, and it is enhanced by 50\% for the time-dependent case. The significant Joule Heating obtained here should be considered in models for the atmospheres of habitable planets in terms of the thickness of the atmosphere, the top-side temperature and density, the boundary conditions for the atmospheric pressure, and particle radiation and transport.

Local time variations in Jupiter's magnetosphere‐ionosphere coupling system

AbstractThe ionization of neutral material ejected by Jupiter's volcanically active moon, Io, results in a plasma disc that extends from Io's orbit out through the Jovian magnetosphere. This magnetospheric plasma is coupled to the planetary ionosphere via currents which flow along the magnetic field. Inside of ∼40 RJ, these currents transfer angular momentum from the planet to the magnetospheric plasma, in an attempt to keep the plasma rigidly corotating with the planet. Jupiter's main auroral emission is a signature of this current system. To date, one‐dimensional models of Jupiter's magnetosphere‐ionosphere (M‐I) coupling have either assumed a dipole field or used a field description appropriate to the postmidnight region of the Jovian magnetosphere. Vogt et al. (2011) described the variation of the N‐S component of the magnetic field in the center of the current sheet, BN, with local time and radius. We apply a 1‐D model of Jupiter's M‐I current system every hour in local time using a modified description of the Vogt et al. (2011) magnetic field to investigate how local time variations in the magnetosphere affect the auroral currents and plasma angular velocity. Our model predicts the strongest aurora at dawn, with a minimum in the auroral currents existing from noon through dusk. This is a few hours duskward of the discontinuity predicted by Radioti et al. (2008). While our model predictions are consistent with some of the observations, future MI coupling models must account for the azimuthal bendback in the magnetic field.

The multiple spots of the Ganymede auroral footprint

The interaction between the moons and the magnetosphere of giant planets sometimes gives rise to auroral signatures in the planetary ionosphere, called the satellite footprints. So far, footprints have been detected for Io, Europa, Ganymede, and Enceladus. These footprints are usually seen as single spots. However, the Io footprint, the brightest one, displays a much more complex morphology made of at least three different spots and an extended tail. Here we present Hubble Space Telescope FUV images showing evidence for a second spot in the Ganymede footprint. The spots separation distance changes as Ganymede moves latitudinally in the plasma sheet, as is seen for the Io footprint. This indicates that the processes identified at Io are universal. Moreover, for similar Ganymede System III longitudes, the distance may also vary significantly with time, indicating changes in the plasma sheet density. We identified a rapid evolution of this distance ∼8 days after the detection of a volcanic outburst at Io, suggesting that such auroral observations could be used to estimate the plasma density variations at Ganymede.

An Analytical Model of ELF Radiowave Propagation in Ground-Ionosphere Waveguides With a Multilayered Ground

A planetary crust with a low electric conductivity has a strong influence on extremely low frequency (ELF) radiowave propagation in the ground-ionosphere waveguide due to the penetration of the ground by the wave's electromagnetic field. The amount of influence it has depends on the actual structure of the ground. Accurate models of radiowave propagation in the ELF range are essential in the study of atmospheric discharges and remote sensing applications. Analytical models are still of primary interest when trying to find inverse solutions and when dealing with large objects such as spherical waveguides that consists of a planetary ground and ionosphere. In this paper, we analyze the influence of a multilayered finite conductivity ground on the propagation of ELF electromagnetic waves in the ground-ionosphere waveguide. We have developed equations that enable us to include different ground models in propagation equations using the concept of complex altitudes, and applied them to the study of ELF electromagnetic field pulses on Mars. This study shows that the structure of the ground and its electrical conductivity has a strong influence on the observed waveforms. Therefore, ELF pulses can serve as a tool for probing planetary grounds. The presented model can also be used to study Schumann resonances.

Electrodynamic coupling of Jupiter’s thermosphere and stratosphere: A new source of thermospheric heating?

We investigate the vertical penetration of non-uniform electric fields in a planetary ionosphere. We develop a simple theoretical description of the vertical variation of electric fields with altitude in an ionosphere permeated with a vertical magnetic field. This framework is applied to the suggestion that winds in Jupiter’s stratosphere could drive currents in the thermosphere. In particular, we propose that wind structures in the upper region of Jupiter’s stratosphere (200–350km above the 1bar level) couple with the ionospheric plasma in this region to generate electric currents, some of which close by flowing vertically along magnetic field lines and then horizontally in the Pedersen conducting region of the thermosphere. These currents extract kinetic energy from the stratospheric winds and dissipate it, via Joule heating, as thermal energy in the thermosphere, thus providing a possible contribution to the observed high temperatures in this region. While the existence of significant wind structures in the upper stratosphere is speculative, the wind speeds that are required to generate significant heating (∼100ms−1) are not unreasonable in comparison to the observed tropospheric, lower stratospheric and thermospheric wind speeds. The scale size of the wind structures is critical to the degree of penetration of the induced electric fields. Wind structures with scale sizes less than ∼10km do not generate electric fields that penetrate significantly into the thermosphere, while those with scale sizes of greater than ∼100km (which includes very large, planetary-scale wind structures) generate electric fields that penetrate almost unmodified across the whole of the thermosphere. Sharp ionospheric layers or holes can prevent penetration of the electric fields to the thermosphere, doing so more effectively if they are of greater magnitude, of greater vertical width, or located at lower altitude. The timescale for damping of the stratospheric winds by ion drag is found to be strongly altitude-dependent, ranging from ∼1 planetary rotation at 350km altitude to >100 planetary rotations at 200km altitude. The timescales also vary strongly with the nature and scale size of the wind structures. If the timescales of the processes driving stratospheric winds are longer than these timescales, then our mechanism will damp the winds almost to zero, and supply negligible energy to the thermosphere. We also discuss the possible relevance of our results to magnetosphere–ionosphere coupling in the auroral regions of Jupiter and other planets.

Dissociative Recombination of CH4+

CH4(+) is an important molecular ion in the astrochemistry of diffuse clouds, dense clouds, cometary comae, and planetary ionospheres. However, the rate of one of the common destruction mechanisms for molecular ions in these regions, dissociative recombination (DR), is somewhat uncertain. Here, we present absolute measurements for the DR of CH4(+) made using the heavy ion storage ring CRYRING in Stockholm, Sweden. From our collision-energy dependent cross-sections, we infer a thermal rate constant of k(Te) = 1.71(±0.02) × 10(–6)(Te/300)(−0.66(±0.02)) cm3 s(–1) over the region of electron temperatures 10 ≤ Te ≤ 1000 K. At low collision energies, we have measured the branching fractions of the DR products to be CH4 (0.00 ± 0.00); CH3 + H (0.18 ± 0.03); CH2 + 2H (0.51 ± 0.03); CH2 + H2 (0.06 ± 0.01); CH + H2 + H (0.23 ± 0.01); and CH + 2H2 (0.02 ± 0.01), indicating that two or more C–H bonds are broken in 80% of all collisions.

DISSOCIATIVE RECOMBINATION OF VIBRATIONALLY COLD CH+3AND INTERSTELLAR IMPLICATIONS

CH3+ is an important molecular ion in the astrochemistry of diffuse clouds, dense clouds, cometary comae, and planetary ionospheres. However, the rate of one of the major destruction mechanisms of ...

Analytical method for determining the location of ionospheric and atmospheric layers from radio occultation data

We generalize the fundamental principle of the radio-occultation method for studying the atmospheres and ionospheres of planets and the Earth. The criterion containing the necessary and sufficient condition under which the tangential point, at which the refractive-index gradient is normal to the ray trajectory coincides with the radio-ray perigee, is obtained. The method for determining the location and parameters of ionospheric and ionospheric layers, which is based on the relationship between the amplitudes and phases of the analytic functions determined from variations in the phase path (eikonal) and intensity of the radio-occultation signal, is proposed. This method yields qualitative and quantitative estimations of the value of the spatial displacement of the ionospheric or ionospheric layer with respect to the radio-ray perigee and allows one to determine the altitude and inclination of the ionospheric layer. The developed method is, in particular, required for determining the location and inclination of the wind-shear region and the direction of propagation of internal waves in the ionosphere and the atmosphere. This method is simpler and more accurate than the back-propagation, radio-holographic method which was previously used for determining the location of the ionospheric irregularities.

Numerical simulations of the ionosphere of Mars during a solar flare

Electron densities in planetary ionospheres increase substantially during solar flares in response to the increased solar irradiance at soft X‐ray and extreme ultraviolet wavelengths. Here we modify an existing model of the ionosphere of Mars to incorporate time‐dependent solar irradiances and use it to simulate ionospheric conditions during the X14.4 and M7.8 solar flares of 15 and 26 April 2001, respectively. Simulations were validated by comparison to Mars Global Surveyor radio occultation measurements of vertical profiles of ionospheric electron density. Adjustments to the model's representation of the neutral atmosphere were required to adequately reproduce the observations before and during these solar flares. An accurate representation of electron‐impact ionization, an important process in the lower ionosphere of Mars, is required in order to adequately simulate the doubling of electron densities that can occur in the lower ionosphere of Mars during a solar flare. We used the W‐value representation of electron‐impact ionization, in which the number of ion‐electron pairs created per photon absorbed equals the ratio of the difference between photon energy and the ionization potential of carbon dioxide to the W‐value. A range of possible W‐values for Mars have been suggested in the literature, and a value of 28 eV led to the best reproduction of flare‐affected observations. Simulated enhancements in the electron density are largest and persist the longest in the M1 region. We predict that the peak electron density in the M1 region can exceed that of the M2 region for short periods during intense solar flares.

Energetics of Titan's ionosphere: Model comparisons with Cassini data

[1] Observed electron and ion temperatures in planetary ionospheres are higher than the neutral temperature. Instruments on board the Cassini spacecraft have shown this is also true for Titan. The Radio and Plasma Wave Science Langmuir Probe (RPWS-LP) (Wahlund et al., 2005) has measured electron temperatures above 1000 K. Ionospheric ion temperatures were deduced from a combined analysis of data from the Cassini Plasma Spectrometer and Ion and Neutral Mass Spectrometer (INMS) (Crary et al., 2009). Elevated electron temperatures attributed to heating by suprathermal electrons were predicted by pre-Cassini models (e.g., Gan et al., 1992; Roboz and Nagy, 1994) and observed by the Cassini electron spectrometer. Models of the energetic electrons and ions are presented that include Cassini inputs (i.e., measured neutral densities from INMS). The results are compared between 800 and 1800 km with suprathermal electron fluxes and plasma temperatures measured by Cassini instruments emphasizing the thermal electron temperature. Using only solar inputs, the dayside model agrees well with electron temperatures measured by RPWS-LP (Ågren et al., 2009) between 1000 and 1400 km. At higher altitudes energy input from magnetospheric electrons is needed to reproduce the measured temperature. Incorporating typical magnetospheric electron fluxes into the dayside does not noticeably increase ion production near the ionospheric peak; however, effects can be seen near 1350 km. Joule heating effects are shown to be capable of contributing significantly to the ion temperature. Magnetospheric suprathermal electrons are shown to provide sufficient heating for the thermal electron population in the middle to upper ionosphere on the nightside.

Doubly-charged ions in the planetary ionospheres: a review

This paper presents a review of the current knowledge on the doubly-charged atomic and molecular positive ions in the planetary atmospheres of the Solar System. It is focused on the terrestrial planets which have a dense atmosphere of N(2) or CO(2), i.e. Venus, the Earth and Mars, but also includes Titan, the largest satellite of Saturn, which has a dense atmosphere composed mainly of N(2) and a few percent of methane. Given the composition of these neutral atmospheres, the following species are considered: C(++), N(++), O(++), CH(4)(++), CO(++), N(2)(++), NO(++), O(2)(++), Ar(++) and CO(2)(++). We first discuss the status of their detection in the atmospheres of planets. Then, we provide a comprehensive review of their complex and original photochemistry, production and loss processes. Synthesis tables are provided for those ions, while a discussion on individual species is also provided. Methods for detecting doubly-charged ions in planetary atmospheres are presented, namely with mass-spectrometry, remote sensing and fine plasma density measurements. A section covers some original applications, like the possible effect of the presence of doubly-charged ions on the escape of an atmosphere, which is a key topic of ongoing planetary exploration, related to the evolution of a planet. The results of models, displayed in a comparative way for Venus, Earth, Mars and Titan, are discussed, as they can predict the presence of doubly-charged ions and will certainly trigger new investigations. Finally we give our view concerning next steps, challenges and needs for future studies, hoping that new scientific results will be achieved in the coming years and feed the necessary interdisciplinary exchanges amongst different scientific communities.

X-RAY, FUV, AND UV OBSERVATIONS OF α CENTAURI B: DETERMINATION OF LONG-TERM MAGNETIC ACTIVITY CYCLE AND ROTATION PERIOD

We have been carrying out a study of stellar magnetic activity, dynamos, atmospheric physics, and spectral irradiances from a sample of solar-type G0-5 V stars with different ages. One of the major goals of this program is to study the evolution of the Sun's X-ray through NUV spectral irradiances with age. Of particular interest is the determination of the young Sun's elevated levels of high-energy fluxes because of the critical roles that X-ray through FUV emissions play on the photochemical and photoionization evolution of early, young planetary atmospheres and ionospheres. Motivated by the current exoplanetary search missions that are hunting for earth-size planets in the habitable zones of nearby main-sequence G-M stars, we are expanding our program to cooler, less luminous, but much more numerous main-sequence K-type stars, such as alpha Centauri B. The long life (2-3x longer than our Sun) and slow evolution of K stars provide nearly constant energy sources for possible hosted planets. Presented here are X-ray, UV, and recently acquired FUV observations of the K1 V star alpha Cen B. These combined high-energy measures provide a more complete look into the nature of alpha Cen B's magnetic activity and X-UV radiances. We find that alpha Cen B has exhibited significant long-term variability in X-ray through NUV emission fluxes, indicating a solar-like long-term activity cycle of P_cycle = 8.84 years. In addition, analysis of the short-term rotational modulation of mean light due to the effects of magnetically active regions has yielded a well-determined rotation period of P_rotation = 36.2 days. alpha Cen B is the only old main-sequence K star with a reliably determined age and rotation period, and for early K-stars, is an important calibrator for stellar age/rotation/activity relations.

Results of solar wind and planetary ionosphere research using radiophysical methods

The magnetic field created by this antenna was measuredat the distance 1200 km from the transmitter, in the Bare-ntsburg Observatory, by a three-component inductionmagnetometer. The radiation experiments were conductedwithin a few days, mostly at night, when the IAR formationprobability is high.Themainresultoftheseexperimentsisthatinanumberofcases, a nonmonotonic dependence of the received signal onthe frequency was detected for the horizontal magnetic fieldcomponent(Fig.5b,c).WebelievethatthisisduetotheIARinfluenceonthepropagatingsignal,becausetheschemeoftheexperimentallowedinvestigatingthefieldintheregionwithasignificant contribution of the wave propagating in the outerwaveguide (above the F-layer) to the total field.Wealsoestimatedtheeffectivenessofvariousionosphericmodels for interpretation of the results of the conductedmeasurements. We showed that the results of the interna-tional reference ionosphere (IRI) model calculations agreepoorly with the experimental data. The upper atmospheremodel (UAM) calculations better correspond to the experi-ment,buttheyarealsoimperfect.Theresultsofexperimentalionospheric studies (such as satellite tomography) should beused to refine the ionospheric characteristics obtained bymodel calculations.

Physical Processes in Magnetically Driven Flares on the Sun, Stars, and Young Stellar Objects

The first flare on the Sun was observed exactly 150 years ago. During most of the long history, only secondary effects have been noticed, so flares remained a riddle. Now the primary flare products, high-energy electrons and ions, can be spatially resolved in hard X-rays (HXRs) and gamma rays on the Sun. Soft X-rays (SXRs) are observed from most stars, including young stellar objects. Structure and bulk motions of the corona are imaged on the Sun in high temperature lines and are inferred from line shifts in stellar coronae. Magnetic reconnection is the trigger for reorganization of the magnetic field into a lower energy configuration. A large fraction of the energy is converted into nonthermal particles that transport the energy to higher density gas, heating it to SXR-emitting temperatures. Flares on young stars are several orders of magnitude more luminous and more frequent; they significantly ionize protoplanetary disks and planetary ionospheres.

Ionospheric photoelectrons: Comparing Venus, Earth, Mars and Titan

The sunlit portion of planetary ionospheres is sustained by photoionization. This was first confirmed using measurements and modelling at Earth, but recently the Mars Express, Venus Express and Cassini-Huygens missions have revealed the importance of this process at Mars, Venus and Titan, respectively. The primary neutral atmospheric constituents involved (O and CO 2 in the case of Venus and Mars, O and N 2 in the case of Earth and N 2 in the case of Titan) are ionized at each object by EUV solar photons. This process produces photoelectrons with particular spectral characteristics. The electron spectrometers on Venus Express and Mars Express (part of ASPERA-3 and 4, respectively) were designed with excellent energy resolution (Δ E/ E=8%) specifically in order to examine the photoelectron spectrum. In addition, the Cassini CAPS electron spectrometer at Saturn also has adequate resolution (Δ E/ E=16.7%) to study this population at Titan. At Earth, photoelectrons are well established by in situ measurements, and are even seen in the magnetosphere at up to 7 R E. At Mars, photoelectrons are seen in situ in the ionosphere, but also in the tail at distances out to the Mars Express apoapsis (∼3 R M). At both Venus and Titan, photoelectrons are seen in situ in the ionosphere and in the tail (at up to 1.45 R V and 6.8 R T, respectively). Here, we compare photoelectron measurements at Earth, Venus, Mars and Titan, and in particular show examples of their observation at remote locations from their production point in the dayside ionosphere. This process is found to be common between magnetized and unmagnetized objects. We discuss the role of photoelectrons as tracers of the magnetic connection to the dayside ionosphere, and their possible role in enhancing ion escape.

Carbon−Carbon Coupling Reactions of Medium-Sized Nitrogen-Containing Dications

The reactivity of molecular dications C(m)H(n)N(2+), generated by (dissociative) double ionization of different heteroaromatic compounds, with methane, ethane, and nitrogen is investigated using a triple-quadrupole mass spectrometer. About a quarter of the set of dications studied undergo carbon-carbon coupling reactions with methane, whereas only nonspecific reactions leading to the formation of monocations (such as proton-, electron- and hydride transfer as well as Coulomb explosions) occur in the presence of either ethane or nitrogen. Here, we report the reactivity observed for 42 different C(m)H(n)N(2+) dications formed via electron ionization of eleven different heteroaromatic precursors. The potential relevance of these results is discussed in the context of the growth of hydrocarbon species and their nitrogen-containing homologues at extremely low temperatures and pressures, such as in interstellar environments or the ionospheres of planets and moons, Saturn's moon Titan in particular.

Threshold Photoelectron Spectroscopy of the Methyl Radical Isotopomers, CH3, CH2D, CHD2 and CD3: Synergy between VUV Synchrotron Radiation Experiments and Explicitly Correlated Coupled Cluster Calculations

Threshold photoelectron spectra (TPES) of the isotopomers of the methyl radical (CH(3), CH(2)D, CHD(2), and CD(3)) have been recorded in the 9.5-10.5 eV VUV photon energy range using third generation synchrotron radiation to investigate the vibrational spectroscopy of the corresponding cations at a 7-11 meV resolution. A threshold photoelectron-photoion coincidence (TPEPICO) spectrometer based on velocity map imaging and Wiley-McLaren time-of-flight has been used to simultaneously record the TPES of several radical species produced in a Ar-seeded beam by dc flash-pyrolysis of nitromethane (CH(x)D(y)NO(2), x + y = 3). Vibrational bands belonging to the symmetric stretching and out-of-plane bending modes have been observed and P, Q, and R branches have been identified in the analysis of the rotational profiles. Vibrational configuration interaction (VCI), in conjunction with near-equilibrium potential energy surfaces calculated by the explicitly correlated coupled cluster method CCSD(T*)-F12a, is used to calculate vibrational frequencies for the four radical isotopomers and the corresponding cations. Agreement with data from high-resolution IR spectroscopy is very good and a large number of predictions is made. In particular, the calculated wavenumbers for the out-of-plane bending vibrations, nu(2)(CH(3)(+)) = 1404 cm(-1), nu(4)(CH(2)D(+)) = 1308 cm(-1), nu(4)(CHD(2)(+)) = 1205 cm(-1), and nu(2)(CD(3)(+)) = 1090 cm(-1), should be accurate to ca. 2 cm(-1). Additionally, computed Franck-Condon factors are used to estimate the importance of autoionization relative to direct ionization. The chosen models globally account for the observed transitions, but in contrast to PES spectroscopy, evidence for rotational and vibrational autoionization is found. It is shown that state-selected methyl cations can be produced by TPEPICO spectroscopy for ion-molecule reaction studies, which are very important for the understanding of the planetary ionosphere chemistry.