SSL-led mission caps over 45 years of discovery in solar physics and space weather forecasting
NASA’s Solar Polarization and Directivity X-Ray Experiment (PADRE) launched from Vandenberg Space Force Base on Monday, June 23. PADRE will measure polarization and directivity of hard X-rays emitted during solar flares with the aim of better understanding how magnetic energy drives those high energy particle emissions. A CubeSat that combines detectors from previous missions with newly developed instruments, PADRE is poised to unlock the secrets of solar flares at a fraction of the cost of previous missions.
PADRE is led by UC Berkeley’s Space Sciences Laboratory (SSL) where one of the instruments was designed and built. Mission partners include NASA’s Goddard Space Flight Center, Southwest Research Institute, University of Applied Sciences and Arts Northwestern Switzerland, CEA-Saclay, andEnduroSat. The small SSL team that leads the mission watched the launch just outside the base in Lompoc, CA.
“I love missions like PADRE,” said Christopher Smith, senior aerospace engineer at SSL and project manager of PADRE. “Small spacecraft offer a very creative environment. We can get new instruments up quickly, troubleshoot them and build on what works.”
PADRE cubesat awaiting thermal testing in SSL clean room. (Credit: Alan Toth) 
Senior aerospace engineer Christopher Smith and aerospace engineer Bradley Pafchek performing thermal testing of PADRE at SSL. (Credit: Alan Toth) 
Members of the PADRE team watching launch outside of Vandenberg Space Force Base. From left to right: Anoushka Chitnis, Christopher Smith, Bradley Pafchek, Milo Buitrago-Casas, Chris Moeckel, Robert Abiad and Juan Carlos Martínez Oliveros.(Credit: Bradley Pafchek) 
Members of the PADRE team watching launch outside of Vandenberg Space Force Base. (Credit: Bradley Pafchek)
PADRE is only the latest in a decades-long series of SSL missions that have revealed the dynamic plasma environment surrounding the Sun. From discovering that solar flares release high-energy charged particles, to building the instrument for the Parker Solar Probe that measured electric and magnetic fields within the Sun’s atmosphere, SSL’s designs, leadership and research have had an outsized impact in our understanding of space weather and its effects on human infrastructure.
Early discoveries
SSL was an early leader in the field of X-ray astronomy. Kinsey Anderson, a professor of physics and director of SSL from 1970-1979, detected X-rays produced by solar flares in 1961 and, along with Robert P. Lin, a senior space fellow and director of the lab from 1998 to 2008, led high-altitude balloon experiments in 1980 that confirmed the existence of nanoflares, or small heating events in the Sun’s atmosphere.
Anderson and Lin knew that a tremendous amount of energy was required to accelerate electrons, which would produce tell-tale signs in the form of X-rays and gamma rays. Lin served as principal investigator of an experiment that mapped those emissions. NASA’s Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) mission produced spectroscopic images of the X-rays and gamma rays produced by solar flares, allowing for a better understanding the basic physics that drive them.
SSL engineers later built a suite of instruments for an experiment consortium known as IMPACT that flew on the Solar TErrestrial RElations Observatory (STEREO) mission in the early 2000’s. IMPACT was led by Janet G. Luhmann, a research geophysicist and senior fellow at SSL. Though STEREO/IMPACT provided previously unattainable measurements of the particles that stream from the Sun, its abilities were limited by its location in Earth’s orbital path. Each pixel of the images it captured represented an area approximately 10,000 miles in diameter—nearly twice the diameter of the Earth. The processes that drive plasma explosions in the Sun’s atmosphere could occur at the subatomic level, so SSL physicists knew they needed a much closer look.
Touching the sun
SSL was heavily involved in the development of the Parker Solar Probe (PSP), which was launched in 2018 with the aim of taking the closest ever measurements of the Sun. SSL built most of the instruments for the Solar Wind Electrons Alphas & Protons (SWEAP) investigation and the entire FIELDS instrument suite.
Stuart D. Bale, a professor of physics and director SSL from 2008 to 2018, led development of FIELDS, which was designed to measure electric and magnetic fields in the solar atmosphere. These measurements would help determine whether coronal mass ejections (CME’s)—vast clouds of plasma that explode from the Sun’s atmosphere—and solar flares are produced by magnetic reconnection, which occurs when opposing magnetic field lines intersect and snap into different orientations. To effectively take those measurements, the antenna of FIELDS extends beyond PSP’s main heat shield where it is exposed to the full force of the solar wind and heated to temperatures of approximately 1200° Celsius (C).
“It’s like trying to blow-dry your hair with a flamethrower,” said David Glaser, a mechanical engineer at SSL and member of the FIELDS design team.
That image is as daunting as the task presented to the FIELDS design team in 2008. A key part of the instrument is an antenna composed of niobium—a rare, lightweight metal with a melting temperature of 2350°C. Even with such a high melting temperature, the niobium antenna might not survive direct exposure to the solar wind. To protect the antenna, the design team came up with an auxiliary heat shield, also made of niobium. The shield stood up well in testing, but the design team couldn’t be completely sure that it would survive PSP’s closest encounter with the Sun.
On December 24, 2024, PSP’s elliptical orbit took the spacecraft further into the Sun’s atmosphere than ever before. Moving faster than any previous human-made object, it streaked through the solar sky only 3.8 million miles from the Sun’s surface. FIELDS survived the encounter, and the data it gathered has already allowed Bale to establish that magnetic reconnection drives plasma acceleration and contributes to CME formation. In a paper published in May in Astrophysical Journal Letters, Bale described how PSP/FIELDS passed through a magnetic reconnection event in 2022 that accelerated particles back toward the Sun. These findings may contribute to understanding why the Sun’s atmosphere is hotter than its surface.
Specific X-ray measurements may lead to better solar predictions
Magnetic reconnection events, like those described in Bale’s paper, are also thought to create solar flares, but details of the mechanisms that convert magnetic energy to kinetic energy in the form of particle emissions are not well understood. High energy X-rays, like those detected by PADRE, are the most direct way to measure solar flare electron acceleration. Specifically, PADRE aims to determine whether those electrons have isotropic (uniform) or anisotropic (non-uniform) distribution.
There are two ways of determining the electron distribution based on X-rays emissions: polarization and directivity, and PADRE does both. In coordination with the STIX instrument aboard the Solar Orbiter spacecraft, the MeDDEA instrument measures directivity (how much of the radiation is emitted in a single direction), and the SHARP instrument measures polarization (the orientation of the waveform).
SHARP measures polarization through a process known as Compton scattering whereby X-ray photons penetrate the instrument and collide with electrons in the beryllium detector surface. The collision deflects the electron and creates a secondary X-ray that collides with another electron and so forth. By measuring the interval between collisions, SHARP can determine the orientation of the wave. PADRE spins to allow its 8 identical SHARP detectors to gather measurements of X-ray emissions. The multiple readings provide information about the fluctuating power of those emissions and their angular distribution (differences in particle intensity based on their direction and point of origin).
“PADRE will help us unlock the secrets of one of the most extreme and dynamic process in our solar system: the acceleration of electrons to high energies in seconds,” said Juan Carlos Martínez Oliveros, associate research physicist at SSL and principal investigator of PADRE.
CME’s and solar flares have the potential to cause massive disruptions to satellites and electricity infrastructure, and a recent report by the Johns Hopkins Applied Physics Laboratory, in collaboration with several US government agencies, found a lack of coordination within government in terms of reacting quickly enough to protect infrastructure from these harmful space weather events. The report called for more space weather monitoring and improved forecasting models. Martínez-Oliveros is confident that PADRE will reveal the distribution pattern of the electrons that generate solar flares, and that his research, combined with that of Bale and other solar scientists at SSL, may lead to the predictive models we need to better protect ourselves.