Helmet streamers, also known ascoronal streamers, are elongated cusp-like structures in theSun'scorona which are often visible in white-lightcoronagraphs and duringsolar eclipses. They are closedmagnetic loops which lie above divisions between regions of opposite magnetic polarity on theSun's surface. Thesolar wind elongates these loops to pointed tips which can extend asolar radius or more into thecorona.^[1]

Duringsolar minimum, helmet streamers are found closer to the heliographic equator, whereas duringsolar maximum they are found more symmetrically distributed around the Sun.

Helmet streamers have cusp-like bases that taper radially outward away from the Sun forming long stalks. The base typically extends up to 1.5solar radii above the surface, whereas the stalk—stretched outward by thesolar wind—can extend over many solar radii.^[2]

Helmet streamers are structured by closed magnetic fields and lie above boundaries separating oppositemagnetic polarity in the Sun'sphotosphere. Their thin stalks consist of oppositely directed magnetic fields which form current sheets.^[3] Surrounding these stalks are open, oppositely directed magnetic fields which are anchored tocoronal holes lower in the corona.^[4]

The white-light emissions of helmet streamers is due to the high electron density of the confined plasma relative to the surrounding corona. Light from thephotosphere isThomson scattered off of these electrons with the intensity of scattered light depending on the number of electrons along the observer's line of sight.^[5]

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.^{[1][better source needed]}

Aroundsolar minimum, the point of minimum solar activity during the 11-yearsolar cycle, helmet streamers are generally located around the heliographic equator in what is referred to as thestreamer belt. At the same time, large coronal holes are present at the poles. As solar activity increases near thesolar maximum, helmet streamers appear more symmetrically around the Sun.^[3]

Solar minimum

Solar maximum

Coronagraphs taken near the solar minimum betweensolar cycles 22 and23 and near the solar maximum of solar cycle 23 demonstrate the different distributions of helmet streamers around the solar disk at different points in a solar cycle.

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.^[5]

Structures in the corona, similar to a helmet streamer, but connecting holes of the same magnetic polarity, are calledpseudostreamers.^[6][7][8][9] They were first observed using space-bornecoronagraphs 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).^[6] Pseudostreamers' structure was observed in 2012 by theSolar Dynamics Observatory.^[8]

Themagnetic topology of pseudostreamers was described as "contain[ing] twin 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 configurationsurrounding them (tripolar pseudostreamer)."^[9]

Single hybrid magnetic structure that consists of double-streamer/pseudostreamer was observed in the solar corona on May 5-10, 2013 by the SWAP instrument of thePROBA2 satellite. Its structure was described by researchers:^[10]

It consists of a pair of filament channels near the south pole of the Sun. On the western edge of the structure, the magnetic morphology above the filaments is that of a side-by-side double streamer, with open field between the two channels. On the eastern edge, the magnetic morphology is that of a coronal pseudostreamer without the central open field.

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