Jupiter boasts some of the most powerful auroras in the solar system. Compared to the Earth's aurora, Jupiter's is a few hundred times more powerful and brighter across the entire spectrum. What causes Jupiter's powerful aurora? Several hypotheses have been proposed, but it has remained a mystery due to a lack of observational evidence.

Jupiter's X-ray aurora, which is observed in the X-ray spectrum region, is thought to sparkle when oxygen and sulfur ion particles moving at nearly the velocity of light strike Jupiter's atmosphere. How can these ions be accelerated to such high speed? There are two leading hypotheses. The first one assumes that the solar wind speeds up the ions, similar to the case of the Earth's aurora. The other proposes that the ions are being accelerated by the rapid spin of Jupiter, Jupiter's own magnetic field, and plasma provided by Jupiter's satellite Io.

Monitor observations of Jupiter's X-ray aurora are essential to compare several parameters of the X-ray aurora, such as brightness at each location, with the changes in the solar wind. For example, a correlation between the solar wind and the X-ray aurora supports the first hypothesis (the ions are accelerated by the solar wind) observationally.

Using the Spectroscopic Planet Observatory for Recognition of Interaction of Atmosphere "HISAKI" (SPRINT-A), the Chandra X-ray Observatory, and X-ray Multi-Mirror Mission (XMM-Newton), scientists monitored Jupiter's X-ray aurora for two weeks in April 2014.

"We estimated the variation of the solar wind at Jupiter by HISAKI's data. The data of the solar wind at the Earth was available. However, when this observation was conducted, Jupiter was in particular far from the Earth. The estimated pressure and velocity of the solar wind at Jupiter is much less accurate if we extrapolated these values observed at the Earth," Kimura, the research team lead, explains. The results show the strong correlation between the velocity of the solar wind and the strength of the X-ray aurora.

Although previous studies indicated the correlation between the pressure of the solar wind and the strength of the X-ray aurora, this is the first time to show the velocity of the solar wind affects the strength of the X-ray aurora. Since Jupiter's aurora at other spectral ranges is thought to get kicked off by the planet-moon interaction, not by solar activity, this study suggests the X-ray aurora sparks by different mechanism from Jupiter's aurora emitting light in other wavelength ranges.

Kimura continues, "We observed the X-ray aurora for 10 hours once daily, 6 times in total. Thanks to the high resolving power of the Chandra X-ray Observatory, we could obtain the detailed spatial structure and its time variations of the X-ray aurora. The spectral data taken by the XMM-Newton told us the volcanic gases from the satellite Io and oxygen atoms can exist in the solar wind emit X-ray."

The long-time observation by Chandra also elucidated the fine spatial structure of the X-ray aurora and precise measurement of time variations. Kimura says, "Using the observational data and a numerical model of Jupiter's magnetic field, we studied the spatial distribution of the magnetic flux line of the X-ray aurora at Jupiter's magnetosphere.

"Our estimation shows the lines of magnetic force piercing the X-ray aurora connect with the boundary surface between Jupiter's magnetosphere and the solar wind. Together with this and the correlation between the velocity of the solar wind and the strength of the X-ray aurora, we believe the solar wind causes the X-ray aurora."

Another study out today, led by William Dunn from UCL and co-authored by the JAXA research team also analyzed the X-ray data taken in 2011. Despite the complete difference in condition of the solar wind observed in 2014, the team found the similar trends, i.e., the correlation between the solar wind and the strength of the X-ray aurora, and the lines of magnetic force connected with the outer region of Jupiter's magnetosphere. These are consistent with the results based on the observations in 2014.

Observation by HISAKI and the Hubble Space Telescope in January 2014 focused on precipitations of Jupiter's aurora in UV. This research concluded Jupiter's high spin resulted in the precipitations of the UV aurora. The results of the X-ray and UV aurora suggest that the both mechanisms, Jupiter's rapid spin and the solar wind, cause Jupiter's powerful aurora phenomenon.

Yamazaki, the HISAKI project manager says, "Auroras in atmospheres of other planets can be formed by the mechanisms similar to Jupiter. For example, there is a high possibility of similar phenomena in the atmosphere of Saturn, because plasma particles originated from water of Saturn's satellite Enceladus are captured by Saturn's magnetic field and turn rapidly around Saturn. Of course, auroras can form in the atmosphere of an exoplanet. Some day, our research may apply to study aurora phenomena of exoplanets."

Kimura tells about his future research as follows: "We plan further observations of Jupiter's auroras by the X-ray astronomical satellite "HITOMI" (ASTRO-H) and JUNO probe which will perform a polar orbit insertion. By using HISAKI's data complementary, we would like to understand the acceleration mechanism of particles inducing Jupiter's aurora phenomena. "