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 Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
in interplanetary space, far from the Sun itself.
Overview
The solar wind is composed of
plasma 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 convent ...
s and free
electrons
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
. 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, visible light, or visible radiation is electromagnetic radiation that can be perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400–700 nanometres (nm ...
. 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 direct ...
– 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 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 diffuse sky radiation, diffuse ...
and
aurora
An aurora ( aurorae or auroras),
also commonly known as the northern lights (aurora borealis) or southern lights (aurora australis), is a natural light display in Earth's sky, predominantly observed in high-latitude regions (around the Arc ...
, 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 force, air service branch of the United States Department of Defense. It is one of the six United States Armed Forces and one of the eight uniformed services of the United States. Tracing its ori ...
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, commonly shortened to 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 configurat ...
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
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