Elsevier

Icarus

Volume 261, 15 November 2015, Pages 31-33
Icarus

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Brightening event seen in observations of Jupiter’s extended sodium nebula

https://doi.org/10.1016/j.icarus.2015.07.037Get rights and content

Abstract

Jupiter’s sodium nebula, which originates from Io’s volcanic gas, shows variations in its brightness due to the volcanic activity on Io. Imaging observation of D-line brightness in the sodium nebula was performed from 2013 through 2015 in a conjunction with the HISAKI mission. The D-line brightness of the sodium nebula had been stably faint and dim until January 2015, but it showed a distinct enhancement from February through March, 2015. The brightness increased by three times during this enhancement. Details in variations of Jupiter’s sodium nebula are shown in this paper.

Introduction

Io, one of the remarkable moons of Jupiter called Galilean moons, is the most volcanically active body in the Solar System. Io’s atmosphere originates from the volcanic gas. The volcanic gas is distributed beyond Io’s and Jupiter’s gravitational-sphere through interactions between Io’s volcanic atmosphere and Jupiter’s magnetic field.

Jupiter’s sodium nebula, which consists of neutral sodium atoms, has its origin in unidentified sodium molecules included in Io’s volcanic gas. The sodium molecules (NaX+) picked up from Io’s ionosphere by Jupiter’s co-rotating magnetic fields and the subsequent destruction of these molecular ions in the Io plasma torus produce a flow of fast sodium atoms called a “stream” (e.g., Schneider et al., 1991, Wilson and Schneider, 1994, Wilson et al., 2002). The most likely molecular ion as NaX+ is NaCl+. Kuppers and Schneider (2000) found Cl+ in their observation of the Io plasma torus, and neutral NaCl gas was detected in Io’s atmosphere by Lellouch et al. (2003). Moses et al. (2002) suggested that NaCl gas is produced from Pele-type volcanic hotspots that have plumes based on their model study.

Although atmospheric sputtering is also a major loss process that leads to Io’s atmospheric escape, its contribution seems to be limited in giving a certain thickness in the north–south direction, but not to the large extent of the nebula in the east–west direction (Wilson et al., 2002).

The sodium nebula can be seen at wavelengths of D lines (589.6 and 589.0 nm), and its angular size is 5°. Therefore, small telescopes with wide FOVs are very adequate for observing the sodium nebula. The D-line brightness of the sodium nebula faithfully reflects the column density of the sodium atoms since the emission is due to the resonant scattering of the solar light caused by the sodium atoms.

In addition, Mendillo et al. (2004) showed that there is a clear correlation between the D-line brightness of the nebula and Io’s infrared brightness at 3.5 μm which is due to thermal emissions from Io’s volcanoes. It can be said the D-line brightness of the sodium nebula is a good index for Io’s volcanism especially with plumes. Brown and Bouchez (1997) observed an enhancement in the sodium D-line emissions followed by that of S+ ions in the torus. Yoneda et al. (2010) found that flux tube contents of S+ ions in the torus had increased during an enhancement in the sodium D-line emissions of Jupiter’s sodium nebula. We can expect D-line emission of Jupiter’s sodium nebula shows changes in Io’s atmosphere and the plasma torus as well as that of volcanism on Io.

A UV telescope onboard the HISAKI spacecraft has been observing Jupiter’s magnetospheric phenomena since 2013 (e.g., Yamazaki et al., 2014, Yoshikawa et al., 2014), and several observations are being made from the ground and space to support the HISAKI mission. In this study, we show our observation results of Jupiter’s sodium nebula in order to contribute to this international campaign.

Section snippets

Observations

The optical instrument used in the observations consists of a 10-cm refractor and two interference filters; one is for Na D-line emissions, and the other one an off-band filter for obtaining scattering light, whose origin is in the solar continuum reflected at the surface of Jupiter. An occulting mask made of a neutral density filter was used for reducing the scattering light as far as possible. Details on the instruments, observation sequences and data analysis are described in Yoneda et al.

Results and discussion

The D-line brightness of the sodium nebula is shown in Fig. 1, and three images take of the nebula are shown in Fig. 2. Poor weather conditions and full Moon phases prevented us from obtaining successive data, but variations in the brightness were clearly seen. The D-line brightness was around 20–25 Rayleighs at 50 RJ from Jupiter from November 2013 through January 2015. This value is smaller than those showed by most of past studies (Flynn et al., 1994, Mendillo et al., 2004, Takahashi et al.,

Summary and conclusions

We made ground-based observations of Jupiter’ sodium nebula from November 2013 through April 2015 in a conjunction with the HISAKI mission. Although the nebula had been faint and dim until January 2015, it showed a distinct enhancement from the end of January through the end of March, 2015. Year-to-year variations in Jupiter’s sodium nebula have been known by the past study. Moreover, this study captured the temporal scales of both increasing and decreasing phases in brightness of the nebula

Acknowledgments

The authors would like to thank Professor Krishna K. Khurana as the editor and two anonymous reviewers for their kind suggestions to improve this paper. The authors would like to thank Prof. Jeff Kuhn and his colleagues for their support at Advanced Technology Research Center, Institute for Astronomy, University of Hawaii. This research was supported by a special fund for education and research provided by the Ministry of Education, Culture, Sports, Science and Technology to the Planetary

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