Helmet streamers, also known as coronal streamers, are elongated cusp-like structures in the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

corona Corona (from the Latin for 'crown') most commonly refers to: * Stellar corona, the outer atmosphere of the Sun or another star * Corona (beer), a Mexican beer * Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...

which are often visible in white-light

coronagraph A coronagraph is a telescopic attachment designed to block out the direct light from a star so that nearby objects – which otherwise would be hidden in the star's bright glare – can be resolved. Most coronagraphs are intended to view t ...

s and during

solar eclipses A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of the Earth, totally or partially. Such an alignment occurs during an eclipse season, approximately every six mont ...

. 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 particle ...

loops which lie above divisions between regions of opposite magnetic polarity on the Sun's surface. 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 ...

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\tim ...

or more into the

. During

solar minimum Solar minimum is the regular period of least solar activity in the Sun's 11-year solar cycle. During solar minimum, sunspot and solar flare activity diminishes, and often does not occur for days at a time. On average, the solar cycle takes abou ...

, 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 takes ...

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 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\tim ...

above the surface, whereas the stalk—stretched outward by the

—can extend over many solar radii. Helmet streamers are structured by closed magnetic fields and lie above boundaries separating opposite

magnetic polarity A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nicke ...

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 A coronal hole is a temporary region of relatively cool, less dense plasma (physics), plasma in the solar corona where the Sun's magnetic field extends into interplanetary space as an open field.Freedman, Roger A., and William J. Kaufmann III. "Ou ...

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

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

, the point of minimum solar activity during the 11-year

solar cycle The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surfa ...

, 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

, 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

s 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). CC BY icon

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/ref> Pseudostreamers' structure was observed in 2012 by the

Solar Dynamics Observatory The Solar Dynamics Observatory (SDO) is a NASA mission which has been observing the Sun since 2010. Launched on 11 February 2010, the observatory is part of the Living With a Star (LWS) program. The goal of the LWS program is to develop the ...

. 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)."

References

{{The Sun Sun Solar phenomena

Structure

Solar cycle

Role in coronal mass ejections

Pseudostreamers

See also

References