Investigations From Sun to Earth: An Interview With Bruce Tsurutani

Q: Your research spans the entire Sun-Earth connection. How do you take discrete research topics and piece together a whole picture? A: It is a fascinating task. For example, early on I was studying electromagnetic waves inside the magnetosphere called “chorus waves,” which produce chirps and whistles associated with geomagnetic storms and aurorae. We found that when particles are injected into the magnetosphere from the solar wind, they generate chorus through plasma instabilities, suggesting that we could track the location of chorus waves. They were detected where the electrons went, just as one would have expected. I've also worked on data from spacecraft missions—for example, data from Pioneer 10 and 11, launched in 1972 and 1973, respectively. These were the first to go deep into interplanetary space. We wrote a number of the first papers on the acceleration of energetic particles at interplanetary shocks: When ionized gas is ejected from the Sun at high speeds, such as in coronal mass ejections, collisionless shocks are formed upstream of the plasma, very much like collisional shocks that form upstream of airplanes flying at supersonic speeds. These papers were written with Ed Smith, John Simpson, and James Van Allen—real luminaries in the field. I also helped with the Ulysses mission, which launched in 1990 and orbited over the Sun's poles. Though the mission itself didn't have a direct application to space weather, one of the things we saw as the spacecraft went over the polar regions was that high-speed streamers from the Sun were filled with nonlinear Alfvén waves, a type of low-frequency plasma wave. And then we found later through other spacecraft studies that these waves cause geomagnetic activity at the Earth and eventually lead to the acceleration of relativistic electrons in the Earth's magnetosphere. All these pieces assemble a whole—you start with the Sun, where the Alfvén waves are created; they travel through interplanetary space along with solar wind particles; they affect the Earth's magnetosphere, and somehow chorus is generated and electrons are accelerated to relativistic speeds. It is a long link of microphysical processes going all the way from the Sun to the Earth. Q: How have research collaborations helped you piece together this picture? A: In almost all cases I try to work with people who have different expertise than I do. For example, one of my most notable collaborations was with Frances Tang from Caltech [Pasadena, Calif.] and Walter Gonzalez from the Brazilian National Space Institute [São José dos Campos, Brazil] in our research of magnetic storms when Walter was visiting JPL. Frances was the solar person, I was the interplanetary person, and Walter was the magnetospheric person. And we had that give and take-we all were learning the others’ area of expertise and relying on the others to give good information. This kind of approach is absolutely essential. You have to be willing to teach and to sit down like a new student so that research can take a small step beyond what was known before. I've worked with very bright and curious people who don't have any preset ideas as to what the physics will reveal. We're just taking a look, and when we see something we try to explain it the best we can. Q: Your research on the 1859 Carrington event, culminating in your 2003 paper published in AGU'sJournal of Geophysical Research-Space Physics, generated a lot of attention. How did you find data from an event that occurred so long ago? A: Happenstance, actually. Gurbax Lakhina, an Indian researcher, spent a year visiting us at JPL, and we worked on plasma instabilities together. He was called back to be the director of the Indian Institute of Geomagnetism, based in Mumbai. A couple years later, I was in India for a meeting, and I visited him. We at first discussed plasma waves and somehow segued into magnetic storms. He mentioned that his institute had all this old storm data, way back to 1859 and even earlier. Apparently, as part of the British Empire, scientists in India had been recording magnetic field variations for quite some time at his institute. Now, this was something I'd spent the past decade looking for! Essentially, there was a magnetometer stationed at the institute. It occupied a huge room and had these magnetic needles hung by threads—technicians would use telescopes to measure the torsion and distortion of these needles and from that calculate the Earth's magnetic field. The instrument itself doesn't still exist, but the logs! They would take measurements once every hour, 24 hours a day, 7 days a week, and when things got active, once every 15 minutes. The Sun was very active in 1859 and created many geomagnetic disturbances at Earth, and a record of this is all written down. Q: What was done with these data? A: We measured the magnetic field at different time intervals during this event and then calculated the Earth's magnetic field. From this, our research team worked backward to determine what would have happened inside the Earth's magnetosphere to generate this very large magnetic disturbance, then what the interplanetary electric fields would need to be, what the magnetospheric electric fields would need to be, and finally what the coronal mass ejection that triggered this event must have looked like. A 2008 U.S. National Research Council report, Severe Space Weather Events—Understanding Societal and Economic Impacts, used our research and more fully outlined threats to our way of life from an event of similar magnitude to the 1859 event. The report made it clear that if something like that happened again, it would bring down power grids and would be an extreme problem for humanity. There was a geomagnetic event in 1972 that was very similar to 1859, but the direction of magnetic fields of the coronal mass ejection were the same as the Earth's magnetic field—if they had been directed southward instead of northward, we'd have probably had something like 1859 or even stronger. Quite simply, the Carrington event is much bigger than anything we're used to in our lifetimes. I'd say between the start of the space age in the late 1950s to now, the biggest storm we've had was one that knocked out the power grids in Quebec in 1989. But the 1859 event was 3 times more intense! Q: What new directions do you see for space weather as a field? A: One of the referees on the 1859 storm paper asked us to add information on the probability of such a storm happening again. We know that events have happened in 1972 and 1989, but we found that statistically there just aren't enough data to know if the 1859 storm is a far outlier. Also, the answer lies not with statistics but with physical limitations. This raises several questions about thresholds. What is the biggest magnetic storm that could possibly happen? What is the biggest solar flare that the Sun could have? The problem is that most people aren't thinking like this—they're instead just trying to explain the details of a particular solar flare or magnetic storm. Astrophysicists tell us if the Sun had flares as big as flare stars (1037 to 1039 ergs), there would have been a mass extinction at Earth. So what is the maximum energy of a solar flare on the Sun? Why? I would be very happy if someone could come up with those kinds of numbers. And I think there will be a lot of research in these areas in the future. Mohi Kumar is a staff writer for the American Geophysical Union.

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