NASA Spacecraft Equipped with Faraday Cup to Collect Sun’s Atmospheric Particles

A critical instrument on a historic space mission that will fly into the sun’s atmosphere is being tested after development by a partnership between The University of Alabama in Huntsville, NASA’s Marshall Space Flight Center (MSFC), and the Smithsonian Astrophysical Observatory (SAO).

Designed to explore the mechanisms that heat the solar atmosphere and create solar weather, the mission’s data will be analyzed with help from UAHuntsville’s computing prowess.

Scientists will use the mission’s data to become better at forecasting and now-casting solar storms, in order to better protect electronics in spacecraft and on Earth.

“We are trying to better understand the solar weather so we can be able to predict it,” said Dr. Gary Zank, director of UAHuntsville’s Center for Space Plasma and Aeronomic Research, “so that we are better able to insure against catastrophic equipment failures.”

The flight’s experiments are also of keen interest to Dr. Zank because they could answer a question about which he has a theory: How can the sun be hot at its core yet stay relatively cool at its surface, while at the same time super-heating its coronal atmosphere? Unlike the Earth’s atmosphere, which cools with increasing height from the surface, the sun’s atmosphere gets hotter as it becomes more distant from the solar surface.

Daisy petals

If all goes according to plan, following a 2018 launch NASA’s Solar Probe Plus spacecraft will begin to orbit the sun in a pattern similar to an artist drawing petals on a daisy.

Flung toward the sun by gravitational assists from Venus, the 1,350-pound craft will have an 8-foot diameter, 4.5-inch thick carbon-carbon/carbon foam heat shield to protect it. It will have close encounters with a sun 20 times larger and 500 times brighter than it is at Earth.

“This will be the closest we have been to the sun,” Dr. Zank said. The craft will at times be about six times closer to the sun than the planet Mercury. “What is going to happen with this mission is that the spacecraft is going to go in and out of the solar atmosphere.”

The flight pattern will keep the Solar Probe Plus from burning up. The craft will experience nine traverses of the solar atmosphere and explore hot plasma arches as well as solar coronal holes and coronal bright patches discovered in the early ‘70s by a Skylab x-ray telescope. Solar Probe Plus will measure the range of energetic particles that compose the features.

“Depending on how far those extend, we will be able to see all those regions,” said Dr. Zank, adding that an orbital mission – instead of a single plunge into the sun – is critical to gaining this type of knowledge. “This is why we want an extended period for the Solar Probe Plus mission, so that it will cover a lot of different areas and depths of the solar atmosphere.”

Onboard are instruments to perform five scientific investigations.

• The Fields Experiment will measure electric and magnetic fields, radio emissions and shock waves in the sun’s atmospheric plasma.
• The Integrated Science Investigation of the sun uses two instruments to monitor electrons, protons and ions in the sun’s atmosphere.
• The Wide-field Imager is a telescope that will make images of the sun’s corona to see the solar wind, clouds and shock waves as they pass by the spacecraft.
• Heliospheric Origins with Solar Probe Plus will provide an independent scientific assessment of scientific performance and be a community advocate with principal investigator Marco Velli of NASA's Jet Propulsion Laboratory in Pasadena, Calif., as the mission's observatory scientist, responsible for serving as a senior scientist on the science working group.
• The Solar Wind Electrons Alphas and Protons (SWEAP) investigation will scoop up samples of the atmosphere of the sun and measure the detailed properties of electrons, protons and helium ions, the main components of the corona and solar wind.

Almost all instruments on the Solar Probe Plus will be protected by the craft’s heat shield. Only two will ride alongside the shield in the full force of supersonic solar particles and radiation – the Fields Experiment antennas and a Faraday cup developed by the UAHuntsville-MSFC-SAO partnership.

Measuring cup

The principal investigator for SWEAP is Justin C. Kasper of SAO in Cambridge, Mass. and it is a multi-institutional project that includes the University of California, Berkeley Space Sciences Laboratory, the Massachusetts Institute of Technology, Los Alamos National Laboratory and NASA Goddard Space Flight Center, as well as UAHuntsville and MSFC

The Faraday cup, held to the Solar Probe Plus by a fixed arm as it collects the sun’s atmospheric particles, is critical to SWEAP determining what happens in the sun’s energy cycle.

“To develop the Faraday cup, UAHuntsville and MSFC are directly involved with the fabrication, testing and building on that equipment,” Dr. Zank said.

Named after Michael Faraday, who first theorized ions around 1830, the device is a metal conductive cup that catches charged particles that create a current that can be used to find out the number hitting the cup and the energy they possess. It’s flown in space before. What’s new is designing one to survive in such a rigorous environment and to provide greater measurement capability than the previous flight versions.

That presents its challenges. CSPAR Research Scientist Dr. Ken Wright is working closely with MSFC to make sure the cup survives. “Mechanically and electrically, you have to put it together differently,” said Dr. Wright. For starters, no solder or plastic parts, and insulation functions must be achieved with high temperature ceramic materials.

The cup and arm assembly will have some thermal protection made from multi-layer niobium, an exotic transition element. In the cup is a grid that is struck by solar particles as the craft flies in the sun’s atmosphere. “Think of a window screen,” said Dr. Wright. “The grid carries voltage, and the particles with sufficient energy pass through, but those that do not have enough energy are reflected away.”

“It’s really like a kind of force field,” said Dr. Jonathon Cirtain, a heliophysicist at MSFC who with his science team in 2010 secured an $8.2 million NASA award to do this work. Depending on how it has been configured, higher-energy particles or particles of interest for study pass through the screen, hit the collector at the end of the cup and are recorded.

Initially, the UAH/MSFC partnership helped fabricate a candidate grid material. The Solar Probe Plus, including the cup assembly, has recently completed a NASA Mid-Phase B review – the green light to continuing development. Now, the focus has turned to testing instrument designs.

“You have to be able to test your instrument in conditions in which it flies,” said Dr. Wright. “MSFC is developing a facility that simulates both photon and charged particle environments to verify it can measure currents in those conditions.”

Testing is being done at the High Intensity Solar Environment Test (HISET) site at MSFC created for the purpose. So far, testing has involved aiming a beam of proton particles at the instrument and measuring its response. In order to control the beam location, a system of Helmholtz coils has been arranged to produce a region of nearly zero magnetic field, nullifying the effects of Earth’s magnetism. That allows the test particle beam at any energy level to be precisely aimed and moved around on the cup.

UAHuntsville graduate student Phyllis Whittlesey designed the first stage of the Helmholtz coils and has been working on coil placement and other facility aspects, according to Dr. Wright. Whittlesey comes to UAHuntsville from the University of Texas at Dallas, where the chair of the physics department program was Dr. James L. Horowitz, now deceased, who had spent 25 years at UAHuntsville and encouraged her to attend the university.

Creation of the HISET facility has also resurrected a vacuum test chamber that Dr. Wright used while a student at UAHuntsville to test and calibrate instruments previously flown on the space shuttle.

“In the next phase of mission development, we hope to simulate the power and environment of the sun with high-power Xenon lamps,” said Dr. Wright. They will shine with the equivalent of 520 suns as seen from Earth.

Testing is comprehensive in simulating the sun’s atmosphere because the orbit of the Solar Probe Plus will result in tremendous temperature swings.

“Going into and coming back out of the sun’s atmosphere is actually more technically difficult” than a mission that would remain at a constant temperature, Dr. Cirtain said. In cooperation with SAO, the work in Huntsville is being done to confirm the cup’s performance and calibration of data.

“The bulk of our contribution is in these facilities and in these tests and making sure the cup can function,” said Dr. Cirtain. “We’re now testing out new materials from a set of finalist materials for the grid.”

The entire Solar Probe Plus project remains on track to fly in 2018.

“This mission was first thought of 50 years ago,” Dr. Wright said, “and it’s been 50 years in development. It’s great that we are the ones who happen to be around to see this through.”

Data analysis

UAHuntsville’s CSPAR will also be involved in the Level I and Level II data analysis project that will result from the mission, Dr. Zank said. “We will be developing and archiving all the SWEAP data that comes.” The university’s computational abilities are one reason it received the project award, but the data tracking the density, temperature and velocity of the solar plasma and how it relates to the magnetic field will require even more capacity in future years.

“When the time comes, we will upgrade and update our equipment to facilitate that,” Dr. Zank said. “It’s not only gathering and analyzing the data, but being able to allow access to it to researchers all across the globe.”

Heated question

For thousands of years, the true nature of the corona – the sun's outer atmosphere – was a mystery to astronomers, until it was determined that coronal gases are super-heated to temperatures greater than 1 million degrees C (1.8 million degrees F). At these high temperatures, the two dominant elements of hydrogen and helium are completely stripped of their electrons. Even minor elements like carbon, nitrogen and oxygen are stripped down to bare nuclei. Only the heavier trace elements like iron and calcium are able to retain a few of their electrons in this intense heat.

While the corona is superheated, the visible solar surface has a temperature of only about 6,000 degrees C (10,000 degrees F). Yet at a comparatively small distance of 100 kilometers above the relatively cool surface, the temperature rises to 1-7 million degrees C. (12,600,000 degrees F).

“One really puzzling thing is, what is the mechanism that leads to the heating of the solar corona?” Dr. Zank asks. “There are essentially five theoretical models involved in the heating of the sun, one of which I have helped to develop.”

The theory Dr. Zank helped develop views the sub-surface interior of the sun as akin to a cauldron of boiling water. In the case of the sun, the water is actually heated gas in the convective zone. The boiling-like movement of the hot gas induces motions in the sun’s magnetic fields, converting kinetic (motional) energy into waves that carry magnetic energy into the corona.

The heating of the solar corona results from the turbulent dissipation of energy carried by the magnetized waves. The turbulent dissipation mechanism results from the cascading of energy from large to smaller and smaller scales. At the smallest scales, the energy is converted to the heating of the charged particles in the solar corona.

“The cascading of energy to smaller scales is one that we are all intimately familiar since it occurs whenever we make a cup of coffee,” said Dr. Zank. “We create large-scale motions when we stir a cup of coffee with cream. The large eddies cascade to smaller and smaller scales, entraining the cream and mixing it more and more smoothly into the coffee until coffee and cream can no longer be distinguished separately.”

Pieces of the sun

Solar Probe Plus instruments will be in a position to distinguish between the different coronal heating theories, including the turbulent dissipation mechanism. The craft’s Faraday cup can measure protons, electrons or heavy ions as it scoops up pieces of the sun. “We’ll measure in principle all those species,” said Dr. Zank.

Once the sun’s particles hit the cup’s collector, the number of strikes, their position on the collector and the energy of each strike will be recorded and used to calculate the distribution function.

“You’ll end up with a curve that will tell you the number of particles at a particular energy,” said Dr. Zank. This data will provide information about the amount of heating going to what particles and so provide clues as to how the heating of the solar atmosphere is occurring. From this knowledge, scientists hope then to have new insights into the basic generators of solar weather.

“From the heated particles,” Dr. Zank said, “we will be able to infer the precise mechanism of the heating of the solar corona.”