A heliospheric imager is a wide-field camera that is designed to image the
solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
in interplanetary space, far from the Sun itself.
Overview
The solar wind is composed of
plasma
Plasma or plasm may refer to:
Science
* Plasma (physics), one of the four fundamental states of matter
* Plasma (mineral), a green translucent silica mineral
* Quark–gluon plasma, a state of matter in quantum chromodynamics
Biology
* Blood pla ...
and contains both
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s and free
electrons
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
. The electrons, in particular, scatter incident sunlight via
Thomson scattering
Thomson scattering is the elastic scattering of electromagnetic radiation by a free charged particle, as described by classical electromagnetism. It is the low-energy limit of Compton scattering: the particle's kinetic energy and photon frequency ...
, and clouds of plasma can therefore be photographed using
visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
. Heliospheric imagers are simple in principle – they are simple visible light cameras placed inside deep optical baffles. However, the solar wind features are extremely faint – as little as 0.1% of the brightness of the background starfield and
zodiacal light
The zodiacal light (also called false dawn when seen before sunrise) is a faint glow of diffuse sunlight scattered by interplanetary dust. Brighter around the Sun, it appears in a particularly dark night sky to extend from the Sun's direction in ...
– and therefore heliospheric imager data require extensive post-processing to remove these backgrounds.
Heliospheric imagers are generally flown in space because the Earth's atmosphere itself interferes with the signal even at night. Terrestrial features such as clouds, aircraft, moonlight, small variations in dust and moisture content of the air column, and even high altitude
airglow
Airglow (also called nightglow) is a faint emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky never to be completely dark, even after the effects of starlight and diff ...
and
aurora
An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...
, can obscure the desired solar wind signal. Instruments flown to date include the Solar Mass Ejection Imager (SMEI) flown on a
USAF
The United States Air Force (USAF) is the air service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part of the United States Army Sign ...
satellite in high polar orbit, and the HI imagers that formed part of the SECCHI instrument suite
on board NASA's twin
STEREO
Stereophonic sound, or more commonly stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible perspective. This is usually achieved by using two independent audio channels through a configuration ...
spacecraft in deep space. The concept was first demonstrated in the 1980s via detailed post-processing of photometer data from the two HELIOS spacecraft flown in the 1970s.
References
Space weather
Space imagers
{{sun-stub