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### Thirty years of ${\mathrm{H}}_{3}^{+}$ astronomy

#### Abstract

This review covers the work of the three decades since the first spectroscopic identification of the ${\mathrm{H}}_{3}^{+}$ molecular ion outside of the laboratory in 1988, in the auroral atmosphere of the giant planet Jupiter. These decades have seen the astronomy related to this simple molecular ion expand to such an extent that a summary and evaluation of some 450 refereed articles is provided in the review. This enormous body of work has revealed surprises and illuminated the extensive role played by ${\mathrm{H}}_{3}^{+}$ in astrophysical environments in our Solar System and beyond. At the same time the physical chemistry and chemical physics of the molecule that has been revealed and studied during this time has proved to be fascinating and enabled high-resolution spectroscopy to benchmark its achievements against equally high-precision calculations. This review includes a brief look at some of the key foundational articles from before the original 1988 Jupiter detection (including the original 1911 ion discharge tube detection by J. J. Thomson and the key laboratory spectroscopy and quantum mechanics calculations on ${\mathrm{H}}_{3}^{+}$ structure and spectrum). The review explains the original detection and its serendipitous nature and looks at the astronomy that followed, all the way up to the latest results from NASA’s Juno mission. Also covered are the major advances in our understanding of the interstellar medium (known as ISM) that have resulted from the detection of ${\mathrm{H}}_{3}^{+}$ absorption lines there in 1996. The review closes by examining claims for the ion’s presence in other astrophysical environments and its potential role in the atmospheres of exoplanets and brown dwarfs.

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DOI:http://doi.org/10.1103/RevModPhys.92.035003

Gravitation, Cosmology & Astrophysics

#### Authors & Affiliations

• Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom

Thomas R. Geballe

• Gemini Observatory, 670 North A’ohoku Place, Hilo, Hawaii 96720, USA

Tom Stallard

• School of Physics and Astronomy, University of Leicester, Leicester LE2 7RH, United Kingdom

• *Corresponding author. s.miller@ucl.ac.uk

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##### Issue

Vol. 92, Iss. 3 — July - September 2020

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• ###### Figure 2

A small section of the ${\mathrm{H}}_{3}^{+}$ photodissociation spectrum.. From [209].

• ###### Figure 3

Detected unidentified emission lines within Jupiter’s auroral region among the quadrupole-allowed ${\mathrm{H}}_{2}$ emissions. (Top panel) From Trafton et al., these emission lines are well detected. (Bottom panel) A simulated spectrum using a temperature of 1200 K, the spectral resolution of the McDonald Observatory IR grating spectrometer ($\lambda /\mathrm{\Delta }\lambda =1200$) and the [329] line list are shown for comparison, with the line assignments of the brightest five lines. From [454].

• ###### Figure 4

Four images of Jupiter’s ${\mathrm{H}}_{3}^{+}$ aurora, showing the changes in resolution over the past 30 years. (a) One of the first detailed images of Jupiter’s northern aurora, taken by ProtoCam on IRTF. From [28]. (b) Jupiter’s northern aurora taken by NSFCam on IRTF, using a specialist ${\mathrm{H}}_{3}^{+}$ filter, as a part of the large catalog observed by Jack Connerney and Takehiko Satoh [archived at [425] and published in 374]. (c) Two frames of Jupiter’s northern aurora from the adaptive optics images taken by IRCS on Subaru, used to investigate short-term variability in [472]. (d) The first detailed image of Jupiter’s southern aurora taken by the Juno JIRAM imager, as reported by [327].

• ###### Figure 5

High-resolution images of Io’s auroral footprint in Jupiter’s atmosphere. These polar orthographic projections of the radiance of the northern aurora are plotted with parallels and meridians. (Individual frames were supplied by A. Mura, deputy principal investigator of the JIRAM instrument on Juno.) These showed that rather than a sequence of auroral “spots,” the footprint was instead formed from a swirling pattern, which [328] compared with a von Kármán vortex street. The individual black and white pixels are the effect of penetrating radiation from Jupiter’s magnetosphere affecting the detector.

• ###### Figure 6

Three maps of Jupiter’s northern auroral ${\mathrm{H}}_{3}^{+}$ temperature. (Top panel) The scanned spectral map of the aurora taken by the Keck NIRSPEC instrument, with the slit aligned north and south and the planet rotating through the slit, with high spectral resolution and high sensitivity ($\lambda /\mathrm{\Delta }\lambda \sim 25 000$ using a 10 m telescope), with the white dots marking the location of the main auroral oval from the VIP4 model ([74]). Adapted from [314]. (Middle panel) Map produced by Juno JIRAM spectral scans of the aurora from the PJ-1 orbit, with low spectral resolution ($\lambda /\mathrm{\Delta }\lambda \sim 200$) and high spatial resolution, in which the dashed line represents the location of the auroral oval from the VIP4 model ([74]) and the solid line is the statistical location of the aurora calculated by [36]. Adapted from [92]. (Bottom panel) Scanned map of the aurora taken by the VLT CRIRES instrument, with a high spectral resolution and high sensitivity ($\lambda /\mathrm{\Delta }\lambda \sim 100 000$ and an 8 m telescope), with the solid line demarking the peak ${\mathrm{H}}_{3}^{+}$ auroral intensity along the main auroral emission, as measured by [201], and the dashed line showing the location of the magnetic footprint of Io according to the [155] model. From [199].

• ###### Figure 7

Three maps of Jupiter’s mid-to-low-latitude ${\mathrm{H}}_{3}^{+}$ emission. (Top panel) Map of the ${\mathrm{H}}_{3}^{+}$ total emission parameter derived from fitted temperature and column densities, )with units of $\mu \mathrm{W}\text{\hspace{0.17em}}{\mathrm{m}}^{-2}\text{\hspace{0.17em}}{\mathrm{sr}}^{-1}$. From [235]. (Middle panel) A relative brightness map, produced by combining tens of thousands of ${\mathrm{H}}_{3}^{+}$ images taken of Jupiter between 1995 and 2000 that were then corrected to remove reflected sunlight and scaled to between 6% and 10% of the peak auroral brightness. From [425]. (Bottom panel) Map of ${\mathrm{H}}_{3}^{+}$ radiance fluctuation that combines multiple ${\mathrm{H}}_{3}^{+}$ emission lines over two nights, using moderate resolution spectra ($\lambda /\mathrm{\Delta }\lambda \sim 30 000$) taken by VLT ISSAC; note that this map has an offset zero in the longitude axis. From [99].

• ###### Figure 8

Jupiter’s and Saturn’s auroral ${\mathrm{H}}_{3}^{+}$ ion wind line-of-sight velocities. (Top panel) Jupiter’s ion winds in the planetary frame of reference observed by VLT CRIRES showing significant structures across the entire auroral region, both equatorward and poleward of the main auroral region (marked by the solid line). The solid line marks the peak ${\mathrm{H}}_{3}^{+}$ auroral intensity along the main auroral emission as measured by [201], the dashed line shows the location of the magnetic footprint of Io according to the [155] model, and the dot-dashed line bounds the fixed dark polar region as defined by [432]. From [201]. (Bottom panel) Saturn’s ion winds in the planetary frame of reference observed by Keck NIRSPEC, shown here as a scanned image, with strong subrotation across the auroral region and narrow arcs of flow across the pole, with lines of latitude and local-time longitude shown by dashed lines separated by 10° and 20°, respectively. From [434].

• ###### Figure 9

Various images of Saturn’s ${\mathrm{H}}_{3}^{+}$ aurora, observed by the Cassini VIMS instrument. These images, taken throughout Cassini’s decade-long orbit of Saturn, reveal many of the instances where VIMS provided a unique insight into Saturn’s aurora. Each image was constructed from light gathered from a range of VIMS wavelength bins bright in ${\mathrm{H}}_{3}^{+}$ between 3.4 and $4.2 \mu \text{m}$, with background light removed using a range of adjacent ${\mathrm{H}}_{3}^{+}$ dark bins, described in more detail by [429]. Latitudes are shown in steps of 5° (dotted lines) and 15° (three-dot-dashed lines), local-time longitudes in steps of 30° (dotted lines) and 90° (three-dot-dashed lines), and the “ray height” (the projected height above the 1 bar planetary surface on the limb) in steps of 1000 km (blue lines). The data and time are shown at the top of each image and the image exposure time (in milliseconds per pixel and minutes for the total image) is at the bottom. (Top left and center panels) Images among those used by [419] to highlight a narrow and bright main auroral emission, similar to that observed in the UV but with polar aurora unlike any previously seen at other wavelengths. (Top right panel) Image resolving Saturn’s auroral curtain, used by [429] to reveal the ${\mathrm{H}}_{3}^{+}$ altitudinal structure. (Bottom left panel) Image is one of those used by [237] to directly compare auroral emission from ${\mathrm{H}}_{3}^{+}$ and UV auroras with simultaneous measurements of low-frequency radio emissions and images of energetic neutrals surrounding Saturn. (Bottom middle panel) Image is one of the highest-spatial-resolution images taken of any planetary aurora from space, which [284] compared with UVIS images to correlate the H, ${\mathrm{H}}_{2}$, and ${\mathrm{H}}_{3}^{+}$ auroras. (Bottom right panel) Image is one of the last ${\mathrm{H}}_{3}^{+}$ auroral images taken by Cassini before it crashed into Saturn; this image reveals complex morphology (partly obscured by sunlight on the right-hand side of the image, the result of the difficult observing conditions high above Saturn’s pole), which will be described in more detail in future publications.

• ###### Figure 10

Variations in Saturn’s ${\mathrm{H}}_{3}^{+}$ density with infalling water ions from the rings. This sketch shows the pathway of infalling water ions from their ionization within the rings, along magnetic field lines and into the planet, where they result in either increases or decreases in the ${\mathrm{H}}_{3}^{+}$ column density $N\left({\mathrm{H}}_{3}^{+}\right)$ as a function of planetocentric latitude and corresponding magnetic field mapping.

• ###### Figure 11

Images of Uranus in the UV and IR. (Top panels, gray scale) One of the HST STIS instrument UV auroral images observed by [238], before (left panel) and after (right panel) background reflected sunlight was removed, showing a small auroral spot on the disk of the planet. (Bottom panels) Three images of ${\mathrm{H}}_{3}^{+}$ emission from Uranus taken with the NFSCam on IRTF. These show the planet rotating by half of a Uranian day, with clear variability over this time. [278] concluded that the degree of spatial variability indicated possible auroral variation, but that no clear structure could be resolved. From [278].

• ###### Figure 12

Discovery spectrum of ${\mathrm{H}}_{3}^{+}$ in a dense cloud toward the young massive protostar RAFGL 2136 by Geballe and Oka in 1996, obtained at UKIRT at a spectral resolving power ($\lambda /\mathrm{\Delta }\lambda$) of 15 000. The shifts in the observed wavelengths of the two lines between the two observation dates are caused by the change in Earth’s orbital velocity.

• ###### Figure 13

Spectrum toward a bright star within the Galactic Center’s Central Molecular Zone, approximately 100 pc from the central supermassive black hole, Sgr A*, obtained at the Frederick C. Gillett Gemini North Telescope at a spectral resolving power of 950. The star is embedded in an opaque shell of warm dust and gas, which emits a continuum steeply rising to longer wavelengths. All of the detected absorption lines in the spectrum are due to ${\mathrm{H}}_{3}^{+}$, whose concentration in the absorbing foreground gas is typically a few parts in ${10}^{8}$. The line labeling system is described in Sec. 3 and the detected transitions are shown schematically in Fig. 14. The detection of the $R\left(2,2{\right)}^{l}$ line is somewhat marginal; however, it is present in higher resolution spectra ([147]). The column density of absorbing ${\mathrm{H}}_{3}^{+}$ toward this star is the largest observed to date.

• ###### Figure 14

Energy level diagram for the lowest rotational levels of the ground vibrational and ${v}_{2}=1$ states of ${\mathrm{H}}_{3}^{+}$. Absorption lines from the $v=0$ $\left(J,K\right)=\left(1,0\right)$ and (1,1) levels are shown by vertical continuous lines. Four other absorption lines originating in the (2,2) and (3,3) levels, important in studies of the Galactic Center, are denoted by vertical dashed lines. The $J=0$ and even-numbered levels of the ground vibrational state do not exist. The values of $G$ at the bottom and top of the figure are for the ground and first excited vibrational states, respectively.

• ###### Figure 15

Number densities $n\left(\mathit{X}\right)$ of ${\mathrm{H}}_{2}$, CO, ${\mathrm{C}}^{+}$, and ${\mathrm{H}}_{3}^{+}$ in dense clouds and in diffuse clouds with visual extinctions $>1$ magnitude as a function of cloud density $n\left(\mathrm{H}\right)$. The dashed line shows the density of hydrogen atoms. Adapted from [345].

• ###### Figure 16

Levels of ${\mathrm{H}}_{3}^{+}$ for the lowest three rotational states of its ground vibrational state. Thick upward arrows denote IR absorption lines to $v=1$ levels. Thin diagonal lines are allowed spontaneous radiative transitions between ortho (red) and para (blue) levels. Energies of the four lowest levels above the (1,1) state and radiative lifetimes of the $J=2$ states are shown. From [129].

• ###### Figure 17

Discovery spectrum of ${\mathrm{H}}_{3}^{+}$ toward Galactic Center source IRS3 ([125]), compared with spectra of the same two lines toward a bright protostar in the dense cloud W33A ([127]) and the star Cygnus OB2 No. 12, located in a diffuse cloud ([119]). Note the much stronger absorption toward the star in the Galactic Center than toward the other stars. All three spectra were obtained at UKIRT. From [129].

• ###### Figure 18

Velocity profiles of three lines of ${\mathrm{H}}_{3}^{+}$ and one line from the first overtone band of CO toward a bright IR star in the Quintuplet Cluster, near the center of the CMZ, obtained using the Gemini South Telescope and UKIRT at resolving powers of 50 000 and 37 000, respectively. The intensities of the spectra are scaled by different factors as indicated. The thick dashed lines delineate wedges of blueshifted absorption, present in the upper two spectra but absent in the lower two. The narrow vertical dashed lines mark the radial velocities of dense gas in foreground spiral arms. The velocity scale is relative to the local standard of rest (LSR). Adapted from [351].

• ###### Figure 19

Spectra of lines of ortho ${\mathrm{H}}_{2}{\mathrm{D}}^{+}$, para ${\mathrm{H}}_{2}{\mathrm{D}}^{+}$, para ${\mathrm{D}}_{2}{\mathrm{H}}^{+}$, and ortho ${\mathrm{D}}_{2}{\mathrm{H}}^{+}$ in the cold dense cloud IRAS 16293-2422. The rotational energy levels (e.g., ${1}_{10}$) are specified by the three quantum numbers $J$, ${K}_{a}$, and ${\mathit{K}}_{c}$ ([310]). The black histograms are observed spectra obtained by [47] and [170]. The solid lines are model spectra. The four transitions are shown schematically in the center of the figure and their frequencies are shown below each spectrum. The velocity scale is relative to the LSR. From [53].

• ###### Figure 20

Three spectra of ${\mathrm{H}}_{3}^{+}$ lines toward the nucleus of the galaxy IRAS 08573+3915, obtained using UKIRT and the Subaru Telescope. The spectra are displayed at a resolving power of 5000. At bottom the mean of the spectra is shown along with the profiles of the same lines toward a star in the Galactic Center, redshifted to the radial velocity of IRAS 08572+3915 and binned to the same resolving power. From [120].

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