The brightness of Io's magnetic footprint, an indicator of electromagnetic interaction at the satellite, appears to be strongly connected to the satellite's distance from the plasma equator. As a result, the brightest footprints were detected when Io is near the interception location between the satellite's or- bital plane and the plasma equator. However, volcanic activities on Io show strong correlation with the equatorward shift of Jupiter's main auroral oval, consequently causing the disappearance of Io's footprint. The same conclusion was suggested via the observation of Jupiter's hectometric radio emission, called HOM, which closely corresponds to Jupiter's auroral activity. The plasma environment near the Jovian satellites was found to vary significantly at different observational epochs. The electron density increased by approximately a factor of three from the Voyager epoch (1979) to the Galileo epoch (1995), while the electron density was found to be significantly higher (~ 5 times) in the Cassini epoch (2001). In this current study, the magnetic footprints were clearly brighter ten years ago (from peak brightness in 1998 2001) than the footprints detected in 2007. For volcanic activities on Io in 2007, there are two clear activities in February and late May. The magnetic footprint appeared to be dimmer in March 2007, expected to be the result of volcano activities in Feb 2007. However, the magnetic footprint brightness in June appeared to be slightly brighter than the footprints observed in May. The reason could be the time delay between the brightening of the sodium nebula on approximately May 31st and, a while later, the enhancement of ux tube content peaking on approximately June 5th. On the other hand, Io's magnetic footprints were observed during June 1st - 10th when they may not yet have been affected by the increase in mass out ow due to the increase of plasma density.

ABSTRACT
1. INTRODUCTION
2. OBSERVATIONS AND DATA REDUCTION
3. DATA ANALYSIS
4. RESULTS
5. DISCUSSION
6. CONCLUSIONS
REFERENCES

• Suwicha Wannawichian(Department of Physics and Materials Science, Faculty of Science, Chiang Mai University/ThEP Center)
• Tatphicha Promfu(Department of Physics and Materials Science, Faculty of Science, Chiang Mai University)