Solar wind and plasma instrument on the Parker Solar Probe
Diagram of SWEAP, SPC subinstrumentDiagram of SWEAP, SPAN-A subinstrumentSPAN-B
SWEAP (Solar Wind Electrons Alphas and Protons) is an instrument on the unmanned space probe to the Sun, the Parker Solar Probe.[1] The spacecraft with SWEAP on board was launched by a Delta IV Heavy on 12 August 2018 from Cape Canaveral, Florida.[2] SWEAP includes two types of instruments, the Solar Probe Cup (SPC) and Solar Probe Analyzers (SPAN).[3] SWEAP has four sensors overall, and is designed to take measurements of the Solar wind including electrons and ions of hydrogen (protons) and helium (these are the main components of the Solar wind and coronal plasma).[4]
Scientists studying emissions spectra from the Sun's corona, the bright halo surrounding it, taken during total solar eclipses, had thought they pointed to the presence of a new element they called "coronium". In the late 1930s, Walter Grotrian and Bengt Edlén postulated instead that they were from highly ionized known elements, particularly iron. Since the levels of ionization they proposed required temperatures in the millions of Kelvins, they were not initially accepted as those temperatures were far hotter than the actual surface of the Sun, which was thought impossible as the corona, without any apparent heat source of its own, should be cooler than the Sun. But later measurements of the corona's temperature by other means suggested this was correct. Why this apparent disparity, now called the coronal heating problem, exists is still not known for certain.[5][6]
Some theories suggest interactions between the various atomic particles in the plasma of the corona could account for the extra heat. To test those theories it would be useful to collect data from the corona itself. NASA thus commissioned the Solar Probe Plus for a 2018 launch (it was renamed the Parker Solar Probe (PSP), after Eugene Parker, who predicted the existence of the later-proven solar wind in the 1950s, the first time NASA had ever named a space mission after a then-living person[7][a]). Following a gravity assist from Earth and Mars, it uses further gravity assists from Venus to make ever-closer orbits to the Sun.[9]
SWEAP consists of the Solar Probe Cup (SPC), a faraday cup which faces the Sun and is designed to measure electrons and ions in the space environment near the Sun; the Solar Probe Analyzers (SPAN-A and SPAN-B);[10][11] and the SWEAP electronics module (SWEM).
The Solar Probe Cup is a Sun facing instrument directly exposed to the Sun, and had to be designed to handle the high temperature conditions at 9-10 Solar radii from the Sun, that are planned for the mission.[12] In operation it peeks out from the probes Sun shield, and is made entirely of refractory materials to endure these conditions. The hottest portion, the grid in front of the cup, can be heated to 3,000 °F (1,650 °C)[13] and is made of tungsten. The wiring connecting to the electronics is made of niobium with sapphire insulators.
SPAN-A and B are behind the heat shield, oriented forward ("ram side") and backward along the spacecraft's orbit, and take electron and ion measurements overs a wide field of view.[14] SPAN-A measure ions and electrons and SPAN-B measures electrons.[15]
