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The Forefront of Space Science

Planetary Plasma Environment and Atmospheric Outflow to be Elucidated by Extreme Ultraviolet Spectroscopy
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Science of planetary plasma environment and atmospheric outflow

There are many aspects when discussing the planets in the solar system. Our mission will focus on the strength of its intrinsic magnetic field.

The intrinsic magnetic field exists on earth. The earthís magnetosphere is formed by a fine balance between its magnetic pressure and the dynamic pressure of the plasma flow (i.e., solar wind) in outer space involving the electromagnetic field. When the balance is broken and the solar wind becomes strong, drastic changes occur including explosive aurora, luminous phenomena, and magnetosphere or ionosphere storms.

It is believed that, since strong intrinsic magnetic fields exist on Jupiter and Saturn and their rotation speeds are high, they are not influenced by solar winds in the area. Near them are regions where plasmas in the magnetosphere rotate around the planets at almost the same angular speed as their rotation. These regions are called the inner magnetosphere. It is thought that movement of matter and energy between the inner and outer magnetosphere plasma takes several tens of days. In particular, a distinct feature of Jupiterís inner magnetosphere is the presence of the satellite Io at a distance of about six times the radius of Jupiter from its center. The active volcanoes on Io release volcanic gases including sulfur and sodium in the inner atmosphere. It was revealed by optical observation that plasmas produced by volcanic gases co-rotate with Jupiter and are distributed in a torus-shape along Ioís orbit. This is called the Io Plasma Torus.

Lately, however, observation of luminous phenomena in the plasma torus provided counter-evidence to the traditional interpretation that solar winds do not influence the inner atmosphere. The torus emissions reflect the state of the inner atmosphere while the aurora emissions in the Jovian polar region are an index of activity in the outer atmosphere. If the premise that solar wind does not influence the inner atmosphere is correct, the timing of the respective brightness increases should not be related. Nonetheless, it was observed that brightness increases almost simultaneously in a very short time lag (Fig. 1). Since emission is evidence that energy flows into the regions, this fact suggests an energy-transport process not yet understood. By continuous, simultaneous, high-temporal resolution observation of the Io plasma torus and Jupiterís polar aurora, it is expected that the energy-transport process between the inner and outer atmospheres will be elucidated.

Figure 1
Figure 1. Observation example of Io Plasma Torus (black) and Jupiterís polar aurora (red) measured by an Ultraviolet Spectroscopic Imager onboard the satellite Cassini
There are cases where Io Plasma Torus and Jupiterís polar aurora increase brightness almost simultaneously (Source: Steffl et al., 2004, Pryor et al., 2005)

Meanwhile, because Venus and Mars have weak intrinsic magnetic fields, the solar wind rushes into the environs, and atmospheres directly activated by the wind flow out into outer space. The scale of this outflow becomes a factor to decide whether planets have an atmosphere or not. The earthís outflow is small because of the effect of the existence of the intrinsic magnetic field, so it has evolved into a planet where life flourishes. Venus and Mars are categorized as terrestrial planets, but one is a burning hell of hot temperature and high pressure because of the runaway greenhouse effect while the other is dry terrain of low temperature and low pressure. Their atmosphere environment has become too severe for life to prosper.

The level of solar activity represented by solar-wind dynamic pressure or ultraviolet-radiation quantity is thought to be very high immediately after the birth of the solar system. Therefore, to understand the history of atmospheric-outflow changes from the early stage to the present, we need to trace the atmospheric outflow quantitatively over the range of solar-activity conditions. Optical observation is suitable for all quantity measurements because it can see overall region. The current maximum period of solar activity offers an unrepeatable opportunity for atmospheric outflow measurement although it is far inferior to the ancient Sunís activity level.

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