
Helmet streamers, also known as coronal streamers, are elongated cusp-like structures in the
Sun's
corona which are often visible in white-light
coronagraphs and during
solar eclipses. They are closed
magnetic
Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particles ...
loops which lie above divisions between regions of opposite magnetic polarity on the
Sun's surface
The photosphere is a star's outer shell from which light is radiated.
The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
. 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 ...
elongates these loops to pointed tips which can extend a
solar radius
Solar radius is a unit of distance used to express the size of stars in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3:
:1\,R_ = 6.957 ...
or more into the
corona.
During
solar minimum, helmet streamers are found closer to the heliographic equator, whereas during
solar maximum
Solar maximum is the regular period of greatest solar activity during the Sun's 11-year solar cycle. During solar maximum, large numbers of sunspots appear, and the solar irradiance output grows by about 0.07%. On average, the solar cycle t ...
they are found more symmetrically distributed around the Sun.
Structure

Helmet streamers have cusp-like bases that taper radially outward away from the Sun forming long stalks. The base typically extends up to 1.5
solar radii above the surface, whereas the stalk—stretched outward by 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 ...
—can extend over many solar radii.
Helmet streamers are structured by closed magnetic fields and lie above boundaries separating opposite
magnetic polarity in the Sun's
photosphere
The photosphere is a star's outer shell from which light is radiated.
The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
. Their thin stalks consist of oppositely directed magnetic fields which form current sheets.
Surrounding these stalks are open, oppositely directed magnetic fields which are anchored to
coronal holes lower in the corona.
The white-light emissions of helmet streamers is due the high electron density of the confined plasma relative to the surrounding corona. Light from the
photosphere
The photosphere is a star's outer shell from which light is radiated.
The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
is
Thompson scattered off of these electrons with the intensity of scattered light depending on the number of electrons along the observer's line of sight.
Small blobs of plasma, or "plasmoids" are sometimes released from the tips of helmet streamers, and this is one source of the slow component of the solar wind.
Solar cycle
Around
solar minimum, the point of minimum solar activity during the 11-year
solar cycle, helmet streamers are generally located around the heliographic equator in what is referred to as the ''streamer belt''. At the same time, large coronal holes are present at the poles. As solar activity increases near the
solar maximum
Solar maximum is the regular period of greatest solar activity during the Sun's 11-year solar cycle. During solar maximum, large numbers of sunspots appear, and the solar irradiance output grows by about 0.07%. On average, the solar cycle t ...
, helmet streamers appear more symmetrically around the Sun.
Role in coronal mass ejections
Upon the eruption of a coronal mass ejection (CME), the overlying helmet streamer deforms becoming the CME's leading edge. Similarly, the helmet streamer's cavity becomes the CME's cavity and the helmet streamer's prominence becomes the CME's core.
Pseudostreamers
Structures in the corona, similar to a helmet streamer, but connecting holes of the same magnetic polarity, are called pseudostreamers.
They were first observed using space-borne coronagraphs and called a "plasma sheets" by Hundhausen (1972). They were later renamed to "unipolar streamers" by Zhao & Webb (2003) and then termed "pseudostreamers" by Wang et al. (2007).
Material was copied from this source, which is available under
Creative Commons Attribution 4.0
/ref> Pseudostreamers' structure was observed in 2012 by the Solar Dynamics Observatory.[
Magnetic topology of pseudostreamers was described as "contain ngtwin filaments at its base. Such twin filaments are topologically connected, sharing a neutral point and a separatrix dome. This was a case in which two polarity reversal boundaries contain between them fields with a polarity opposite to that of the global unipolar configuration
surrounding them (tripolar pseudostreamer)."][
]
See also
* List of solar cycles
* Coronal loop
* Solar radio emission
References
{{The Sun
Sun
Solar phenomena