NTB: You have also become one of the world’s foremost authorities on magnetospheres. What are magnetospheres, and what is it about them that captured your interest?
Dr. Green: Some solar system bodies generate their own magnetic field. The extent and shape of that magnetic field into the space around a body is called a magnetosphere. My interest in magnetospheres started when I was at the University of Iowa. I really enjoyed anything to do with the electromagnetic spectrum and, in particular, the radio end of the spectrum. Radio waves are actually generated by magnetospheres. We’ve known that one of the brightest radio objects – and the first planetary object – discovered at radio frequencies was Jupiter. Jupiter generates decametric radio emissions that we are observing from Earth even today, and that’s actually a magnetospheric phenomenon. So, my interest in planetary science actually started with my interest in understanding how emissions in the radio end of the spectrum are generated by magnetospheres.
Now that we have spacecraft that actually make radio frequency measurements in magnetospheres, we see a tremendous host of other in situ radio and extremely low frequency and ultra-low frequency radio waves. It’s fascinating to me to understand the physics of how these waves are generated and what they tell us about the space environments of these magnetospheres.
NTB: While we’re on the subject of magnetospheres, when you were at the Goddard Space Flight Center you were a co-investigator and deputy project scientist on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission. Tell us about that mission and what it accomplished.
Dr. Green: IMAGE was a real delight to be involved with. My actual start in IMAGE really occurred in the late 1980s. I wrote a proposal to study the use of electromagnetic waves to measure various aspects of magnetospheres just like a cop uses a radar wave to understand your speed. Same principle, but I was taking it into the magnetosphere. I did get approved to do that research. I began in the 1990s to pull a team together, and we eventually ended up developing an instrument called the Radio Plasma Imager – RPI – and we were able to get it on the IMAGE mission, which had another group of fabulous remote sensing instruments. RPI was just an unbelievable instrument in the sense that it would not only measure electromagnetic waves that are intrinsic to a magnetosphere, but it would generate waves, bounce them off magnetospheric structures, and it enabled us to really visualize what these plasmas look like and how they moved in time. The instrument was so successful that we generated almost a peer-reviewed publication a month from a team of 8 – 10 of us.
It was just a tremendous opportunity that allowed us to really understand magnetosphere-solar wind dynamics, how things change, and really bring that to a visual perspective through imaging techniques. We could see the plasmasphere, which is the evaporated ionosphere. We could see its structure and how it changed with time. RPI could also observe intense electromagnetic waves that are generated above auroras, called auroral kilometric radiation. With IMAGE we could simultaneously observe the aurora and the auroral kilometric radiation, and understand a lot more about their relationship and dynamics and how the magnetosphere was generating these radio waves. By making these measurements at the Earth, we were then developing an understanding of how these processes could also work in other locations in our solar system, such as Jupiter. Jupiter does the same kind of stuff, but it does it in a bigger way because it is a much more intense magnetic field. And a much higher intensity set of energetic particles.
So, IMAGE was a spacecraft launched in 2000, it lasted for about five years, and it was highly successful and a very exciting time in my career to be associated with it.
NTB: Looking ahead, what would you say is the Planetary Science Division’s top priority over the next five years?
Dr. Green: Well, I would go back to our key goals – understanding the origin and evolution of the solar system and looking for the potential of life elsewhere in the solar system. I believe the Planetary Science Decadal that comes out early next year and will chart our course for the next ten years will keep these same priorities going. These are just fundamental areas for us to continue to explore. We have made some astounding discoveries, but many more are on the way. In the next few years I believe we will make major progress on understanding the potential for life or habitability. I think we’re on track to make some fabulous measurements at Mars that will take us one giant step closer to answering that question, at least for that particular planet.
In addition, we are just now seeing a scientific revolution that is going on in that field and we have much more to learn about it. I’ll give you a perfect example. Is Pluto a planet or not? That is a topic that even kids can resonate with. And the reason why we now think of Pluto as part of a different class of objects shows an evolution in our thinking. If I can take a minute, perhaps I can give you a little understanding of how that goes.
If we go back in time, let’s say 1850, and you went into a library, pulled out the physics book, and looked at the chapter on solar system because you wanted to memorize the names of all the planets, how many planets would you need to know? You would probably be very surprised if I told you 23. The reason why there were 23 is because in the early 1800s telescopes were getting much better at observing fainter objects. Many people in the general public were starting to use them. Amateurs were finding all sorts of objects and it was quite an exciting time to be a ground-based astronomer. New objects were found like Vesta, Ceres, Eros… In other words, you were observing bodies in the asteroid belt.