Space Weather and Sun Science: A Conversation with Christina Cohen

Space physicist Christina Cohen first joined the Space Radiation Laboratory (SRL) as a postdoctoral scholar in 1996. At the time, the group was helmed by the late Edward Stone, a giant in the fields of space physics and planetary astronomy, who spent six decades at Caltech establishing the SRL and leading numerous space missions, including NASA's twin Voyager spacecraft.

"They were just the nicest people doing really cool stuff, and Ed treated everybody the same regardless of whether you were a grad student or postdoc," Cohen remembers. "You felt like you were an integral part of the team."

As a staff scientist on the SRL team for nearly 30 years now, Cohen has helped design and calibrate numerous instruments to measure solar energetic particles (SEPs), a type of cosmic ray released by the Sun during solar flares and coronal mass ejections. The instruments have played crucial roles on many NASA missions, including the Advance Composition Explorer (ACE), the Solar TErrestrial RElations Observatory (STEREO), Parker Solar Probe (PSP), and the Interstellar Mapping and Acceleration Probe (IMAP) mission, which launched in fall 2025.

Currently a principal or co-investigator on six missions, Cohen studies SEP and energetic storm particle events, which are important contributors to space weather. Understanding the acceleration and transport of these particles is key to developing the ability to accurately predict space-weather hazards.

We spoke with Cohen to learn more about space weather, her favorite mission, and more.

What is space weather, and why is it important to us on Earth?

Space weather generates from large activity that's happening on the Sun—usually a big eruption. You get a solar flare, and then you get this expulsion of material called a coronal mass ejection that moves really fast and goes through the inner heliosphere, a bubble around our solar system created by solar wind. If it's moving in the right direction, that mass can hit Earth and cause lots of problems. It can compress the magnetic field of the Earth, which sometimes does interesting things like induce currents in electrical wires. It can fry transformers, which has happened, and cause blackouts. Our electricity grid is way more interconnected than it used to be, so it is more vulnerable than ever.

Space weather can also affect anything that's using GPS for automatic navigation—it can scramble the memory or put the systems into weird modes. Apparently, farmers use a lot of automated GPS tractors to work in their fields, and, one time, there was a big geomagnetic storm, and, basically, all the tractors suddenly got confused and just came back to the barn.

In space, energetic particles are a radiation hazard for astronauts. If they happen to be doing a spacewalk at the time of an SEP event, that's really bad. Even if they're in a spacecraft that's, say, going to Mars, they'd have to get into a protected area.

In 2020, Congress passed the PROSWIFT Act, which basically declared that space weather is a national priority. We need to track and understand it, and we need to be able to predict it so that we can protect all these things. A lot of what we do with our spacecraft has to do with the SEP events. We're trying to help people figure out how to predict them—which turns out to be really hard—and how to mitigate them to protect astronauts and instrumentation. It's been pretty important.

What are the latest missions you are working on?

The most recent mission we launched is IMAP. It is an interesting spacecraft in that it has two halves in terms of research. I'm most familiar with the half that does in situ measurements: It measures energetic particles, the solar wind, the magnetic fields—all the stuff that you would normally measure for space weather. IMAP is out at the first Lagrangian point, or L1, which is 1 percent of the way between the Earth and the Sun. There's a lot of spacecraft that end up sitting there because it's an ideal location for early detection of solar storms. We don't really call it forecasting because it only really gives you about 30 minutes' notice. We call it now-casting.

The other half of IMAP is energetic neutral-atom-imaging instruments, which I know virtually nothing about, so I'm kind of learning as I go. Those imagers are looking outwards, away from the Sun, at energetic neutral atoms that are basically flowing inward from the outermost reaches of our solar system. The signal from those neutrals can be affected by activity at the Sun that is going outwards, but it takes several years to get out there, have an effect, and come back. The whole point of IMAP is to try and put these two pieces together to learn more about the heliosphere.

There's also a mission called SunRISE, which has not yet launched. We're still hoping that it'll go up this year. It consists of six CubeSats, which, basically, will make a radio antenna in space. We will look at radio bursts coming from the Sun and then connect that to what we're seeing with particle instruments on other spacecraft to understand the solar activity.

You've been a part of many missions. Do you have a favorite?

The ACE mission is the closest to my heart because it's the first one I worked on and got to see launch. I had my 3-month-old daughter with me when I came to Caltech, and she was just over a year old when it was launched, so they're kind of the same age in a way. ACE was doing amazing stuff, and everything we looked at was brand new.

I remember the first events we looked at, and going to Ed [Stone] and some other people in the group thinking I must be doing something wrong because the data didn't make sense to me. And they were like, "No, that's correct. You're getting a really weird signal here." It ended up having a big impact on the community and was really cool. And it's something that we still haven't 100 percent solved; there are still some aspects of it that we don't quite understand.

It was also a nice group of people to work with and interact with, especially as a young scientist. The older people were great mentors, and it's a very weird feeling to think that now I'm the PI on our instruments and it's still running. I'm going to be very sad when it runs out of fuel, which is likely in the next few years. But it was supposed to be a three-year mission, so the fact that it's been almost 30 years is just insane.

Speaking of mentors, you are involved in mentorship and outreach through the American Geophysical Union and other organizations. Why is that important to you?

In my head, it's a no-brainer. If you want science to continue, you have to help the generations behind you. I remember being a young scientist who was just absolutely petrified to ask a question or raise their hand and say, "I don't understand the talk you just gave." I want to be the person that people feel they can approach. And I have to say, sometimes young people ask me questions, and I'm like, "Oh, wow! I didn't even think about that. That's a really good point. We should actually look at that." It makes me feel good to be one of those people that anyone can just come up to.

When did you first become interested in space?

I feel like there is a paradigm in academia of people wanting to do science since they were 2 and having lifelong motivation, but that was not my story. I don't want people to feel like if they weren't driven to do science their entire life, they are either not worthy of being here or a fake. My mother was a high-energy physicist, and my dad was a mathematician. I didn't want to do either. But I visited the University of New Hampshire as prospective student, and they had a big space-physics program. The chair of the department built instruments, put them on rockets, and shot them up into the aurora to make measurements and figure out what was going on. He was showing us videos of all the cool things he did, talking about how he got to go to Alaska, and I was like, "Wow, that looks amazing! That's what I want to do." I didn't end up in aurora physics, but that was how I ended up leaning toward space physics. It wasn't my passion, but I have no idea what I would say if someone would ask, "What would you do if you couldn't do this?"

You first came to Caltech as a postdoctoral scholar three decades ago. What makes you stay?

I came out to California, and the first thing I saw was hummingbirds. It was like paradise. And the SRL group was amazing. I had a young family, and everybody was super supportive. You can only be a postdoc for so long and then you get to the point where you either have to leave or find a new position. Ed basically said, "I would love to keep you, but if you stay, you have to be a staff member." As much as I had it ingrained in me that I should want a faculty position, I really liked what I was doing, and I didn't want to move my kids around. So, I stayed and then suddenly it was 30 years later and I'm still here.