TRACERS uses speedy electrons to trace solar energy’s path to Earth

Physicists led by the University of Iowa have documented in the finest detail to date how energy from the sun interacts with Earth's magnetic field, which could yield greater insights into solar effects on Earth that drive space weather.

In a new study, the researchers measured the velocities and concentrations of electrons in low-Earth orbit at locations called cusps, which act like conduits for charged particles from the sun to enter Earth's ionosphere, the upper reaches of our planet's atmosphere. Through those detailed measurements, the researchers were able to more precisely map the travel pattern of solar energy from magnetic reconnection - solar energy's first encounter with Earth's magnetic field tens of thousands of miles from Earth's surface - to its interactions at cusps a few hundred miles above our planet.

"With magnetic reconnection, we don't really know how it varies at a fine scale. We have a hunch that it's either varying in time or varying spatially," says Jasper Halekas, professor in the Department of Physics and Astronomy at Iowa and the study's corresponding author. "Our electron edge measurements reveal for the first time how these processes vary on small time and spatial scales at the edge of the cusp, helping us to better understand the efficiency of the sun-Earth coupling."

The results come from TRACERS, the approximately $170 million mission funded by NASA and the largest external research award in University of Iowa history. Launched in July 2025, twin satellites swoop through low-Earth orbit, sampling electrons, ions, plasma, and other elements part of the interactions between the sun and the Earth.

"This is important because magnetic reconnection is how the energy from the sun gets into Earth's system," Halekas says. "It's important to know the duty cycle of that reconnection - is it happening continuously, or is it sort of turning on and off?"

Electrons are key to better understanding magnetic reconnection events and how they reverberate closer to Earth. Because of their nearly nonexistent mass and high energies, think of them as ultra-speedy messengers, delivering the first news about magnetic reconnection some 30,000 miles away at the edges of Earth's magnetic bubble and portending the ripple effects at cusps farther downstream in Earth's ionosphere.

"The electrons are saying, magnetic reconnection is taking place way out here, and we're letting you know that there's going to be this wave of mass and energy coming to us," Halekas explains.

The researchers cataloged 149 cusp encounters by one of the TRACERS spacecraft; 57 of those encounters showed characteristic electron dispersion signatures at the equatorward edge. The observations came from data collected by the Analyzer for Cusp Electrons instrument (ACE), designed and built at Iowa.

"The equatorward edge is the leading edge of the cusp, where the solar wind energy and plasma can first reach the ionosphere," says Halekas, principal investigator for the ACE instrument. "The electron and ion signatures we see there are the proof we're seeing the effects of magnetic reconnection."

The study, "Electron dispersion at the electron edge of the Earth's magnetospheric cusp," was published online May 19 in the journal Geophysical Research Letters.

Contributing authors from Iowa are Sarah Henderson, Scott Bounds, Aidan Moore, Ivar Christopher, David Miles, Connor Feltman, George Hospodarsky, Allison Jaynes, Brendan Powers, and Shirsh Soni.

Other authors are Suranga Ruhunusiri and Karlheinz Trattner, from the University of Colorado-Boulder; John Bonnell and Marit Øieroset, from the University of California-Berkeley; Brandon Burkholder, from the University of Maryland-Baltimore County and NASA Goddard Space Flight Center; Iver Cairns, from the University of Sydney in Australia; Li-Jen Chen, Hyunju Connor, and John Dorelli, from NASA Goddard Space Flight Center; Ian DesJardin and Dibyendu Sur, from Catholic University of America and NASA Goddard Space Flight Center; Stephen Fuselier, from Southwest Research Institute and the University of Texas-San Antonio; Katherine Goodrich, from West Virginia University; James Labelle, from Dartmouth College; Steven Petrinec, from Lockheed Martin Advanced Technology Center, in Palo Alto, California; and Robert Strangeway, from the University of California-Los Angeles.