In the spring of 1944, Tokyo residents experienced numerous aerial attacks from Allied bombers. Air raid sirens warned citizens to get indoors and preceded strategic blackouts across the city. But 28-year old Hisako Koyama saw these blackouts as opportunities. Dragging a futon over her head for protection, Koyama would gaze at the night sky, tracking all sorts of astronomical phenomena. However, her latest endeavor required the light of day. By angling her telescope towards the sun, Koyama could project the star's light onto a sheet of paper, allowing her to sketch the sun’s shifting surface. She spent weeks recreating this set up, tracking every change she saw. But while Koyama didn't know it, these drawings were the start of one of the most important records of solar activity in human history.
To understand exactly what Koyama saw on the sun’s surface, we first need to understand what’s happening inside the star. Every second, trillions of hydrogen atoms fuse into helium atoms in a process called nuclear fusion. This ongoing explosion maintains the sun’s internal temperature of roughly 15 million degrees Celsius, which is more than enough energy to transform gas into churning pools of plasma. Plasma consists of charged particles that produce powerful magnetic fields. But unlike the stable charged particles that maintain magnetic activity on Earth, this plasma is constantly in flux, alternately disrupting and amplifying the sun's magnetic field.
This ongoing movement can produce temporary concentrations of magnetic activity which inhibit the movement of molecules and in turn reduce heat in that area. And since regions with less heat generate less light, places with the strongest magnetic fields appear as dark spots scattered across the sun’s surface. These so-called sunspots are always moving, both as a result of plasma swirling within the sphere, and the sun’s rotation. And because they’re often clustered together, accurately counting sunspots and tracking their movement can be a challenge, depending greatly on the perception and judgment of the viewer.
This is precisely where Koyama’s contributions would be so valuable. Despite having no formal training in astronomy, her observations and sketches were remarkably accurate. After sending her work to the Oriental Astronomical Association, she received a letter of commendation for her dedicated and detailed observations. With their support, she began to visit the Tokyo Museum of Science, where she could use a far superior telescope to continue her work. Koyama soon joined the museum's staff as a professional observer, and over the next 40 years, she worked on a daily basis, producing over 10,000 drawings of the sun’s surface.
Researchers already knew magnetic currents in the sun followed an 11 year cycle that moved sunspots in a butterfly shaped path over the star’s surface. But using Koyama’s record, they could precisely follow specific sunspots and clusters through that journey. This kind of detail offered a real-time indication of the sun’s magnetic activity, allowing scientists to track all kinds of solar phenomena, including volatile solar flares. These flares typically emanate from the vicinity of sunspots, and can travel all the way to Earth’s atmosphere. Here, they can create geomagnetic storms capable of disrupting long range communication and causing blackouts. Solar flares also pose a major risk to satellites and manned space stations, making them essential to predict and plan for.
During an interview in 1964, Koyama lamented that her 17 years of observation had barely been enough to produce a single butterfly record of the solar cycle. But by the end of her career, she’d drawn three and a half cycles— one of the longest records ever made. Better still, the quality of her drawings was so consistent, researchers used them as a baseline to reconstruct the past 400 years of sunspot activity from various historical sources. This project extends Koyama’s legacy far beyond her own lifetime, and proves that science is not built solely on astounding discoveries, but also on careful observation of the world around us.