Every morning for the past 16 years, solar physicist Sam Krucker sat down at his desk to check the latest data from NASA's RHESSI. Had the solar observatory seen a flare overnight? If there was a new flare, Krucker, RHESSI principal investigator at University of California, Berkeley, since 2013, would pore over the data, each recorded X-ray telling him something about the giant explosion on the Sun. Now, many years after launching on Feb. 5, 2002, the RHESSI - short for Reuven Ramaty High Energy Solar Spectroscopic Imager - mission has ended; Krucker, and many other scientists, will no longer check the spacecraft's data returns each day. In anticipation of losing touch with the spacecraft's aging receiver, mission operators sent the spacecraft commands to decommission on Aug. 16, 2018. "It does impact everyday life that way," Krucker said. Though it's appropriate timing for RHESSI to stop operations now, he said, while the Sun nears solar minimum, the lull in its activity over an approximately 11-year cycle. "The next two to three years would have been quite boring." RHESSI's job was to watch the Sun for solar flares, some of the most dramatic events on the Sun that can sometimes fling solar energy toward Earth. During a flare, gas in the Sun's atmosphere rapidly soars over 20 million degrees Fahrenheit, sending particles flying at near-light speeds. In turn, the particles emit high-energy emissions like X-rays and even higher gamma rays, which we can detect from far away.
These rays can't, however, penetrate Earth's atmosphere and be measured from the ground, so RHESSI observed them from space. In doing so, RHESSI aimed to understand how flares work, the physics underlying how the Sun generates such powerful bursts of energy.
Ask any scientist who worked on RHESSI what their favorite flare is, and they'll easily rattle off a date, as if it's a birthday or holiday they'll always remember. Krucker said his favorite is either Jan. 20, 2005, or July 19, 2012.
Mission scientist Brian Dennis at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said April 21, 2002, RHESSI's very first X-class flare - the strongest type. For them, these are special dates because RHESSI taught them something new that day.
"When I first looked at the Jan. 20 flare and saw where the high energy was coming from, that brought my understanding to the next level," Krucker said. "One guy even has a most-hated flare. He had many arguments with colleagues over it, trying to explain it, which was not always pleasurable."
Throughout the course of its mission, RHESSI saw more than 75,000 solar flares. These observations have helped scientists craft and refine a model of how solar eruptions form. Launching in early 2002, just after the Sun reached solar maximum in 2001, the spacecraft was poised to see many flares.
"High energies are always interesting in astronomy," University of Minnesota solar scientist Lindsay Glesener said. "They're the biggest explosions, the hottest plasma. But flares are going on at our Sun, which is right next to us and affects us in lots of ways. When I was a grad student, it took just one conversation with Robert Lin, the principal investigator from 2002 to 2012, to convince me flares are the most fascinating thing in the universe."
To understand how solar flares erupt from the Sun, you have to know where their energy comes from. For this, RHESSI carried just one instrument, called an imaging spectrometer, capable of recording both X-rays and gamma rays. For scientists, these high-energy emissions are like fingerprints, showing them how each eruption unfolds: X-rays are associated with electron activity, while gamma rays come from protons and ions.
The instrument combined images of the Sun with its spectroscopy to show all the different energy levels in the flare. This allows scientists to map out where energy comes from during an explosion, and what's producing that energy.
"Previous mission designs had two different instruments, but RHESSI was able to study this large energy range in X-rays and gamma rays, all with the same instrument," said Albert Shih, RHESSI deputy mission scientist at Goddard. "It was an innovative design, and a lot of good science came of it."
RHESSI's observation of so many flares over the years, from one solar cycle to the next, broke new ground in solar physics and enabled a much deeper understanding of flares - where they accelerate particles, and just how much they can vary in scale, from tiny nanoflares to massive superflares, tens of thousands of times bigger and more explosive.
"RHESSI had several firsts," Dennis said. "No one had ever imaged X-rays at this high an energy level before, or imaged gamma rays at all. RHESSI made great strides by making energy measurements with higher resolution than had previously been possible."
Solar Flares Aren't Simple
With these unprecedented capabilities, RHESSI gave scientists entirely novel information, revealing just how complex flares can be.
Early RHESSI observations of a strong flare on April 15, 2002, showed X-rays coming from two different places at once: one high in the Sun's atmosphere and another lower down. Researchers interpreted this to mean a huge punch of energy occurred between the two spots.
After tracking the energy in the two-sided explosion, they realized part of the explosion's energy shot high into the Sun's atmosphere and burst into a cloud of hot plasma to become a coronal mass ejection, while the other part bore down towards the surface, exploding into a flare.
These explosions - the most powerful in the solar system - came to be known as solar eruptive events, in which the Sun's strongest flares and coronal mass ejections are linked, occurring at the same time.
RHESSI introduced solar researchers to entirely new questions on the nature of flares. During an intense October 2003 flare, scientists noticed X-rays and gamma rays coming from two different places.
Although electrons and ions have different masses, scientists expected them to originate from the same spot in the flare. Since each kind of emission reflects the presence of different kinds of particles, the unexpected disparity hinted at different mechanisms guiding the movements of each type of particle. Scientists still don't fully understand this process.
"Thanks to RHESSI, we know much more about where particles are accelerated in flares," Glesener said. "But how they get accelerated remains a big mystery. RHESSI told us where we need to look in order to finish answering these questions."
The Shape of Our Star
RHESSI's long-term data sets also led to findings that have nothing to do with flares at all. One major result concerned the shape of the Sun itself. Since it constantly spins, scientists expect the Sun, like Earth, to be a slightly flattened sphere with a bulging waistline. But pre-RHESSI measurements of the Sun determined it to be much flatter than theory dictated, raising questions of whether scientists had overlooked some crucial piece of information in their calculations.
RHESSI put their confusion to rest through the routine information it gathered to assess where its instrument was pointing. To keep perfectly oriented, the spacecraft precisely recorded the position of the Sun's horizon 16 times per second. With such an extensive collection of data, scientists were able to determine the best ever measurement of the Sun's shape - and a close match to theoretical predictions.
Understanding Earth's Lightning Storms
RHESSI's watch of gamma rays throughout the sky made it a prime tool to measure what are called terrestrial gamma-ray flashes, bursts of gamma rays emitted from high in Earth's atmosphere over lightning storms.
The first of these had been spotted before, but RHESSI invigorated an entire field of study when it showed they are more common and luminous than previously thought. Current numbers suggest there may be as many as 400 bursts daily from thunderstorms at different locations around the world. The finding spurred new research, CubeSat observations and computer modeling.
More Science to Come
Throughout RHESSI's lifetime, scientists made the most of the mission by integrating its observations with those from other missions such as NASA's Solar Dynamics Observatory and STEREO, short for the Solar and Terrestrial Relations Observatory. Different instruments focus on different aspects of the Sun, or different levels of energy.
By combining various vantage points across NASA's fleet of heliophysics spacecraft, researchers could piece together a holistic picture of the Sun's complex behavior. Scientists expect RHESSI will continue to shed light on flares for years to come, as they continue to parse its trove of data.
With RHESSI no longer in operation, solar researchers do not currently have a way to monitor high-energy flare emissions. But new missions under development carry the torch of RHESSI's legacy.
"RHESSI established many young scientists, generating an entire class of high-energy solar physicists," Krucker said. "If you look at who's leading the next generation of X-ray telescopes now, they come from RHESSI science."
Glesener and Shih both began their doctoral studies in solar physics with RHESSI data. Now, each is deeply involved with developing and testing advanced high-energy solar instruments for new missions - including the FOXSI sounding rocket and GRIPS high-altitude balloon.
"Throughout its 16 years, it was RHESSI answering questions, but asking new ones along the way, that has motivated future missions," said Shih, who is also the GRIPS project scientist. "RHESSI set the groundwork and framed our expectations for missions that will look at the Sun in even more detail as we investigate entirely new mysteries."
RHESSI first launched as HESSI in February 2002, but was renamed just months after launch, in April that year, in memory of Reuven Ramaty, a deceased NASA scientist who had long championed the mission. The nominal two-year mission was first extended in 2004 for two years, and then extended several times more until its end.
Currently, RHESSI is in a stable low-Earth orbit. But since it has no propulsion, atmospheric drag will continue to tug at its orbit until the satellite reenters Earth's atmosphere, which is expected to occur as early as 2022. NASA will monitor the satellite's deorbiting and reentry.
