Research at Boston University

BU Joins AAU

Reshaping the
Solar System

By John Rennie

Assistant Professor Merav Opher of the Department of Astronomy is fast becoming one of the most celebrated young scientists in the field of space physics. Her iconoclastic discoveries have literally changed the shape of the solar system in astronomers’ models.

Born in Israel and raised in Brazil, Opher joined the Boston University faculty in 2011 after holding positions at the Jet Propulsion Laboratory (JPL) and George Mason University. As a space physicist, Opher is concerned with the behavior of the ionized gases called plasmas that make up stars and are the most abundant form of visible matter in the universe. Her primary focus is on the plasmas that radiate out from the sun as a solar wind and create a “heliosphere” bubble reaching far beyond the orbit of Pluto. Yet her interests and the implications of her work extend from the very surface of the sun to potentially habitable planets around other stars, as she explains in this interview.

You’ve attracted a lot of attention recently for some surprising discoveries involving the heliosphere at the edge of the solar system. Could you review what some of them were?

OPHER: One surprise at the top of the list is that the solar system is not symmetrical, that the southern part is pushed in by the magnetic field from the rest of the galaxy. Twelve years ago when I was starting out, nobody would have said there was any asymmetry. Very senior researchers were convinced that any variations in the heliosphere came from fluctuations in solar activity over time. Every time I suggested that magnetic fields might be important, people would wave their hands, “No, no.” I practically got tomatoes thrown at me when I’d give talks on the idea.

But in 2006 some colleagues and I published a prediction in the Astrophysical Journal that the heliosphere would be tilted up at an angle, and pushed in on the southern side and bulging out on the north. Other people had published previously that there could be asymmetries but nobody had paid much attention, and we were the first to quantitatively predict how big it would be. And then in 2007 the Voyager 2 spacecraft crossed the termination shock [where the solar wind piles up and slows down as it collides with the interstellar medium] much sooner than expected in the south, in keeping with our prediction. That made people say, “Whoa!”

The Voyager people did simulations that showed fluctuations in solar activity could only account for much smaller asymmetries. Then the space physics community acknowledged that magnetic fields might be strong enough to have an influence after all. But it took a few years.

Voyager also showed that the heliosheath—at the leading edge of the heliosphere where it moves through the interstellar medium—is surprisingly cold. The expectation was that it would be hot, with a temperature of about a million degrees Celsius, because the energy from the shock transition would be dumped back into the kinetic energy of the plasma ions. But instead, that energy seems to be transferred to other particles. That’s also a huge surprise.

Last year you also showed that there was a lot of structure in the heliosheath.

OPHER: Yes. The old idea was that the heliosheath would be smooth, with the solar wind gradually slowing down and deflecting out of the solar system. But instead, Jim Drake [at the University of Maryland] and I found evidence in the Voyager data that showed the plasma in the heliosheath is organized into a thick layer of “bubbles”
100 million miles wide.

How does that happen?

OPHER: In the outer heliosphere, the magnetic field from the sun becomes very tightly folded in the moving plasma, like the pleats in a ballerina’s skirt. We think that where the pleats become very thin and touch one another, the magnetic field can reconnect into closed loops, which can reorganize the plasma into these huge bubble shapes.

“Every time I suggested that magnetic fields might be important, people would wave their hands, ‘No, no.’ I practically got tomatoes thrown at me when I’d give talks on the idea.”

The outer heliosphere isn’t your only professional interest. You’ve also started looking more closely at the innermost part of the heliosphere, very close to the sun.

OPHER: Yes, the lower corona, as it’s called, is this layer of plasma that lies between one and 10 solar radii from the sun. For comparison, the Earth is 200 solar radii away, so you can see the lower corona is very close in—much closer than we have ever sent spacecraft. One probe, Helios, went to 20 solar radii, and another will get to 10 solar radii in 2017–2018. So we only have indirect measurements to go by.

It’s funny, but for me it’s like the two major unexplored areas of the solar system are what’s very far away and what’s very close. One is hard to study because it’s so distant and the other because it is so hot. They’re both great open areas for science, though. I started working on the lower corona when I was at JPL, but the outer heliosphere was always taking me away from it. But when I first became a professor at George Mason University in 2005, I started putting all my students in this area, and we’re still building on that foundation. We run big computer simulations of the conditions, based on models developed at the University of Michigan and elsewhere.

What makes the lower corona so significant?

OPHER: What my group and I have been finding over the years is that the lower corona is where the magnetic fields primarily exert influence on the solar wind and on other phenomena. Farther from the sun than that, the effect is minor. And the interplay with the solar wind also has a big effect on what those magnetic fields will do.

For example, my graduate student Christina Kay has been running simulations that show when the sun ejects magnetic disturbances called coronal mass ejections, or CMEs, the direction in which they move and expand is determined almost entirely within the first couple of solar radii. This makes sense, of course, because that’s where the magnetic fields are strongest and even tiny deflections will be important.

The CMEs also create shock waves in the coronal plasma where the material piles up, but just within the first two or three solar radii. Within those shock waves, particles could possibly be accelerated up to one gigaelectron-volt of energy. Those are the energetic particles that are most dangerous for astronauts, so it’s very useful to understand where they’re coming from.

But a lot also depends on the conditions in the solar wind, too. Rebecca Evans, another graduate student, found that if the sun launches a CME into different solar wind backgrounds, the distribution of temperatures in the CME and in the surrounding plasma will be completely different, too. So I’m trying to develop a much better description of what happens with the CMEs.

It also goes the other way. We’re finding that identical measurements of the solar wind around Earth can match up with completely different profiles of what’s happening in the lower corona. We really need a much better understanding of how the lower corona relates to the solar wind.

How is your group’s lower corona work relevant to stars other than our sun, and planets other than Earth?

OPHER: We’ve started to look at what kinds of wind conditions will arise from other stars, and what they would mean for planets close to those stars. You know, with the sun and Earth it’s so easy, but for stars with a magnetic axis that is more tilted and more magnetically active, the field will oscillate much more. So planets orbiting close to those very active stars will get slapped by these very changeable conditions. For me, this is convincing evidence—even before doing simulations—that to study the habitability of extrasolar planets, you’ve got to know something about the conditions of the stellar wind. We’re putting the pieces together on that and it should be my next paper.

Astrophysics and space physics are fields in which women are still very much in the minority, but that doesn’t seem to have held you back.

OPHER: I’ve talked about this a lot with my sister, and why we both “made it” in science. [Note: Opher’s twin sister is Michal Lipson, a noted nanophotonics engineer at Cornell University and a MacArthur Foundation grant winner.] My dad was a plasma astrophysicist in Israel and Brazil, and he’s the one who helped get me interested in plasmas and magnetic fields in the first place. What he said to us was, “You can do whatever you like.” I think it didn’t even occur to him that there might be a problem for women in science.

As an undergraduate and graduate student at the University of São Paulo, I didn’t have to think about it much because I was in the astronomy department, and those tend to have more women than physics departments do.

The first time I really felt it was in 1999, when I went to the plasma physics department at UCLA and realized I was the only woman. I spent two years there and wasn’t sure what I would do next. But then I met with Paulette Liewer at the Jet Propulsion Laboratory, who turned out to have interests and objectives like mine. She offered me a postdoc job, and it let me shift back into space physics.

Having good role models and colleagues can make a big difference. My group here at BU used to be just four great women, and we called them “the Ladies of the Heliosphere.” But now we have two guys, too!

BU seems to have worked out well for you.

OPHER: One of the big attractions for me was the chance to work alongside Jeff Hughes and the Center for Integrated Space Weather Modeling group. What I’m doing is different from space weather as they’re doing it, but there’s a lot of overlap.

Also, BU has the only department in the country that is half pure astronomy and half pure space physics. I love that interface. It’s a fantastic marriage! [Laughing] And the astronomy students have to take plasma physics—I love it!

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