THE NATURE AND FREQUENCY OF THE GAS OUTBURSTS IN COMET 67P/CHURYUMOV–GERASIMENKO OBSERVED BY THE ALICE FAR-ULTRAVIOLET SPECTROGRAPH ON ROSETTA

Alice is a lightweight, low-power, imaging spectrograph designed for in situ far-ultraviolet imaging spectroscopy of comet 67P in the spectral range 700–2050 Å. The slit is in the shape of a dog bone, 55 long, with a width of 005 in the central 20, while the ends are 010 wide, giving a spectral resolution between 8 and 12 Å for extended sources that fill its field of view. Each spatial pixel or row along the slit is 030 long. Details of the instrument have been given by Stern et al. (2007).

The strongest of the outbursts, from the period 2015 May–July, are listed in Table 1. The table includes the heliocentric distance of the comet, r_h , the distance of Rosetta from the center of the comet, d, the sub-spacecraft longitude and latitude, and the solar phase angle at the time of observation. With the exception of the June 18 and 23 outbursts, the sub-spacecraft positions of the tabulated outburst are uncorrelated, attesting to their random nature. As an example, we will consider in detail the outburst of May 23 since the pointing was constant over two rotations of the comet enabling us to obtain nearly continuous light curves except for gaps in the data due to a lack of observations during spacecraft maintenance activities. Light curves for the strongest coma emissions are shown in Figure 1. Note that prior to the outburst the relative intensities of H i Lyβ, O i λ1304, and O i λ1356 are consistent with the earlier observations that attributed the emissions primarily to electron dissociative excitation of H₂O (Feldman et al. 2015). One rotation period (∼12 hr) later both the absolute and relative brightnesses have returned to their pre-outburst values, in strong contrast to the persistent diurnal variability seen in mass spectrometer data (Hässig et al. 2015; Fougere et al. 2016; Mall et al. 2016).

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Table 1. Major Gaseous Outbursts Observed by Alice in 2015 May–July

Date	Peak	rh	d	Sub-spacecraft		Phase	Bmax(1356)
	Time (UT) a	(au)	(km)	Longitude (°)	Latitude (°)	Angle (°)	(rayleighs) b
2015 May 23	12:42	1.58	143	151.2	–17.1	61.1	223
2015 Jun 18	03:43	1.42	202	214.8	51.3	89.9	173
2015 Jun 20	15:26	1.40	181	269.3	19.9	89.9	60
2015 Jun 23	20:39	1.39	196	215.0	53.3	89.7	113
2015 Jul 04	08:56	1.34	179	111.6	53.8	89.9	83
2015 Jul 13	01:16	1.30	155	149.3	15.6	88.8	78

Notes.

^a Start time of histogram integration. ^b Maximum O i λ1356 brightness above the sunward limb in a 03 spatial pixel.

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The orientation of the Alice slit projected onto the comet is shown in Figure 2 (left) in an image from the navigation camera (NAVCAM) taken ∼45 minutes after the peak brightness. The Sun is toward the top of the image and the comet's rotation axis is roughly perpendicular to the slit. We attribute the secondary peaks seen in the light curve (top panel of Figure 1) to geometric effects of the rotation, as visualized in the three-dimensional coma models of Fougere et al. (2016). Similar effects are seen in the light curves of the other outbursts listed in Table 1.

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The spectral image in Figure 2 (right) shows the clearly separated coma emissions together with the reflected solar spectrum from the nucleus in rows 11–17 of the slit. Again, the Sun is toward the top of the image. Note that the O i emissions are seen against the solar reflected radiation from the nucleus and into the anti-sunward coma.

To study the evolution of the gas content during the outburst, we present four successive spectra of the sunward coma taken from 10 minute histograms beginning 2015 May 23 UT12:10:07 in the left panel of Figure 3. These correspond to rows 18–21 of the spectral image in Figure 2. The difference between the spectra at the peak of the outburst and the prior spectra is then indicative of the erupting gas. The difference spectrum, shown in the right panel, is characterized by an enhancement in the atomic emissions, particularly atomic oxygen, without a corresponding enhancement in long-wavelength solar reflected light characteristic of dust production. The large increase in O i λ1356/O i λ1304 (intensity ratio ≥1:1) suggests O₂, now known to be present in the cometary ice of 67P (Bieler et al. 2015), as the primary source of the additional gas.

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However, laboratory data on the electron impact dissociative excitation of O₂ (Kanik et al. 2003) suggest that this ratio should be ∼2, as seen in the exospheric spectra of Europa (Hall et al. 1995) and Ganymede (Hall et al. 1998). The observed ratio varies in the other outbursts listed in Table 1 and likely reflects additional sources of O i λ1304 such as photodissociation of O₂ (Beyer & Welge 1969; Lee et al. 1974) or electron impact on O (if present). The dramatic change in relative intensities implies that the outburst cannot be due to a sudden increase in photoelectron flux or change in the electron energy distribution. For May 23 this is borne out by data from RPC/IES (Clark et al. 2015) that shows only a modest uniform increase in electron flux beginning about two hours before the outburst observed by Alice and lasting for 12 hr (K. Mandt 2015, private communication).

A significant amount of H₂O is also released as evidenced by the presence of H i Lyα and Lyβ in the difference spectrum. If we ignore the blending of Lyβ with O i λ1027, also produced by e+O₂ (Ajello & Franklin 1985), and assume that all of the emission at 1026 Å is Lyβ, then we can use the e+H₂O cross section for Lyβ at 100 eV from Makarov et al. (2004) relative to the e+O₂ cross section for O i λ1356 from Kanik et al. (2003) to estimate the relative O₂/H₂O abundance in the outburst. From the relative fluxes in the difference spectrum (Figure 3) we find O₂/H₂O $\geqslant$ 0.5, considerably greater than the mean quiescent value of 0.038 reported by Bieler et al. (2015). The presence of H₂O in the outburst is confirmed by concurrent submillimeter measurements of the H₂O column density by MIRO (Lee et al. 2015) along a line of sight contiguous with the central row of the Alice slit that showed a threefold increase at the same time (P. von Allmen 2015, private communication), consistent with the increase in Lyβ seen in the top panel of Figure 1.

CO began to appear regularly in Alice coma spectra in 2015 June and continues to be present through early 2016. While not detected in the May 23 spectrum the CO Fourth Positive system is seen in several other spectra listed in Table 1. However, it does not appear in any of the difference spectra and thus is unlikely to be the major driver in any of the outbursts presented here. The origin of the C i λ1657 emission in the difference spectrum is not clear. Electron impact on CO₂ would produce emission at 1561 Å with an intensity about half that of C i λ1657 (Mumma et al. 1972) as well as CO Cameron band emission at longer wavelengths, which is not observed. Other carbon bearing molecules are not precluded as a source (Le Roy et al. 2015).

We note that N₂, also discovered for the first time in 67P (Rubin et al. 2015), also has a rich electron excited spectrum of N₂ bands and atomic and ionic N multiplets within the Alice spectral range (Heays et al. 2014). None of these are detected in the outburst spectrum so that N₂, whose mean abundance relative to CO is found by Rubin et al. to be less than 1%, also plays no role in the gas outbursts.

Additional information about the outburst can be obtained from the profiles of the O i emissions along the slit. Figure 4 shows the profiles of O i λ1304 and O i λ1356 for the four spectra shown in Figure 3. Both profiles peak just above the sunward limb and decrease radially outward. The brightness increases nearly uniformly in each successive 10 minute integration. With an outflow velocity of 0.5 km s⁻¹ the escaping O₂ molecules will exit the Alice field of view in ∼10 s, so the ejection is continuous. From the light curve we see that one rotation later (∼12 hr), at the same sub-spacecraft longitude, the emission has returned to its quiescent level. Emission is seen against the nucleus and off the anti-sunward limb, indicating that the outburst is not in the form of a collimated jet but is rather diffuse. As noted above, any dust produced would have been detected by an increase at long wavelengths of reflected solar radiation.

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For the other dates in Table 1, the light curves, difference spectra, and spatial profiles are all similar to those of the May 23 event. A search through the Alice database for earlier events reveals multiple outbursts with similar spectra on 2015 April 15 and 29/30. Prior to April, the geometry for observing outbursts (closer distance to the nucleus) was less favorable. In April the spacecraft was at southern latitudes but the longitudes of the outbursts was also random. The rate of detected gas outburst events decreased after perihelion on 2015 August 13. Post-perihelion observations, through the present, continue to show variable O i λ1356/O i λ1304 intensity ratios although these measurements are not always confined to the short timescales of the "outburst" events described above. These observations provide a means of monitoring the O₂/H₂O abundance in the coma even when Rosetta is at large distances ($\geqslant 100$ km) from the nucleus, complementing ROSINA measurements closer to the comet.