The Solar Wind Electrons Alphas and Protons (SWEAP) is an instrument on the Parker Solar Probe, designed for an unmanned mission to the Sun's outer corona. [1] The Parker Solar Probe 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 solar wind including electrons, ions of hydrogen (protons), and helium (the main components of solar wind and the coronal plasma). [4]
Scientists studying emission spectra of the Sun's corona during total solar eclipses thought they had found the presence of a new element they called "coronium." In the late 1930s, Walter Grotrian and Bengt Edlén postulated instead that these spectra were from highly ionized known elements, particularly iron. Since the levels of ionization they proposed required temperatures in the millions of Kelvins, this idea was initially not accepted as those temperatures were far higher than the actual surface of the Sun. This disparity was thought impossible as the corona should be cooler than the Sun, given that it lacks any apparent heat source of its own. Later measurements of the corona's temperature by other means suggested this idea was correct. Why this apparent disparity exists, now called the coronal heating problem, is still not known for certain. [5] [6]
Some theories suggest that interactions between the various atomic particles in the plasma of the corona could account for the extra heat. To test those theories, NASA decided that it would be useful to collect data from the corona itself. They then commissioned the Solar Probe Plus for a 2018 launch (which was later 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 used 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 that faces the Sun and is designed to measure mostly protons and alphas with the occasional measurement of electrons 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 it 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 probe’s Sun shield, and it 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 over a wide field of view. [14] SPAN-A measure ions and electrons and SPAN-B measures electrons. [15]
Summary: [16]
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By September 2018, SWEAP had been turned on, and its first light data was returned. [17]
The data collected from the first and second encounters were released in November 2019 and are publicly available. [18]
A corona is the outermost layer of a star's atmosphere. It is a hot but relatively dim region of plasma populated by intermittent coronal structures known as solar prominences or filaments.
The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV. The composition of the solar wind plasma also includes a mixture of particle species found in the solar plasma: trace amounts of heavy ions and atomic nuclei of elements such as carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron. There are also rarer traces of some other nuclei and isotopes such as phosphorus, titanium, chromium, and nickel's isotopes 58Ni, 60Ni, and 62Ni. Superimposed with the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field. The boundary separating the corona from the solar wind is called the Alfvén surface.
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