Helmet streamer#Pseudostreamers

File:Helmet streamers at min.jpg

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—can extend over many solar radii.{{cite journal |last1=Koutchmy |first1=Serge |last2=Livshits |first2=Moissei |title=Coronal streamers |journal=Space Science Reviews |date=1992 |volume=61 |issue=3–4 |page=393 |doi=10.1007/BF00222313 |bibcode=1992SSRv...61..393K |s2cid=189775835 |url=https://link.springer.com/article/10.1007/BF00222313 |access-date=30 July 2022|url-access=subscription }}

Helmet streamers are structured by closed magnetic fields and lie above boundaries separating opposite magnetic polarity in the Sun's photosphere. Their thin stalks consist of oppositely directed magnetic fields which form current sheets.{{cite journal |last1=Wang |first1=Y.-M. |last2=Sheeley |first2=N. R. |last3=Socker |first3=D. G. |last4=Howard |first4=R. A. |last5=Rich |first5=N. B. |title=The dynamical nature of coronal streamers |journal=Journal of Geophysical Research: Space Physics |date=1 November 2000 |volume=105 |issue=A11 |pages=25133–25142 |doi=10.1029/2000JA000149 |bibcode=2000JGR...10525133W |doi-access=free }} Surrounding these stalks are open, oppositely directed magnetic fields which are anchored to coronal holes lower in the corona.{{cite journal |last1=Cranmer |first1=Steven R. |title=Coronal Holes |journal=Living Reviews in Solar Physics |date=2009 |volume=6 |issue=1 |page=3 |doi=10.12942/lrsp-2009-3 |doi-access=free |pmid=27194961 |pmc=4841186 |arxiv=0909.2847 |bibcode=2009LRSP....6....3C }}

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 the photosphere is Thomson scattered off of these electrons with the intensity of scattered light depending on the number of electrons along the observer's line of sight.{{cite journal |last1=Gopalswamy |first1=N. |title=Coronal mass ejections: Initiation and detection |journal=Advances in Space Research |date=January 2003 |volume=31 |issue=4 |pages=869–881 |doi=10.1016/S0273-1177(02)00888-8 |bibcode=2003AdSpR..31..869G |url=https://cdaw.gsfc.nasa.gov/publications/gopal/gopal2003AdvSpRes31_869.pdf |access-date=27 August 2021}}

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.{{Better source needed|date=July 2022}}

{{Further|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, helmet streamers appear more symmetrically around the Sun.

{{multiple image

|direction = horizontal

|align= center

|caption_align = center

|image1 = Solar minimum coronal streamers from SOHO LASCO C2.jpg

|image2 = Solar maximum coronal streamers from SOHO LASCO C2.jpg

|caption1 = Solar minimum

|caption2 = Solar maximum

|footer = Coronagraphs taken near the solar minimum between solar cycles 22 and 23 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.

}}

File:Parker Solar Probe Encounters Streamers on the Way to the Sun.webm passed through the Sun's corona in early 2021, the spacecraft flew by coronal streamers.]]

{{Further|Coronal mass ejection}}

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.

Structures in the corona, similar to a helmet streamer, but connecting holes of the same magnetic polarity, are called pseudostreamers.{{cite journal |last1=Wang |first1=Y.-M. |last2=Sheeley, Jr. |first2=N. R. |last3=Rich |first3=N. B. |title=Coronal Pseudostreamers |journal=The Astrophysical Journal |date=April 2007 |volume=658 |issue=2 |pages=1340–1348 |doi=10.1086/511416 |bibcode=2007ApJ...658.1340W |s2cid=250768165 |url=https://iopscience.iop.org/article/10.1086/511416/fulltext/|url-access=subscription }}{{cite journal |last1=Masson |first1=Sophie |last2=McCauley |first2=Patrick |last3=Golub |first3=Leon |last4=Reeves |first4=Katharine K. |last5=DeLuca |first5=Edward E. |title=Dynamics of the Transition Corona |journal=The Astrophysical Journal |date=13 May 2014 |volume=787 |issue=2 |pages=145 |doi=10.1088/0004-637X/787/2/145|arxiv=1301.0740 |bibcode=2014ApJ...787..145M |hdl=2060/20150008396 |s2cid=119182452 }}{{cite journal |last1=Panasenco |first1=Olga |last2=Martin |first2=Sara F. |last3=Velli |first3=Marco |last4=Vourlidas |first4=Angelos |title=Origins of Rolling, Twisting, and Non-radial Propagation of Eruptive Solar Events |journal=Solar Physics |date=2012 |volume=287 |issue=1–2 |pages=391–413 |doi=10.1007/s11207-012-0194-3 |arxiv=1211.1376|s2cid=118374791 }} 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).{{cite journal |last1=Scott |first1=Roger B. |last2=Pontin |first2=David I. |last3=Antiochos |first3=Spiro K. |last4=DeVore |first4=C. Richard |last5=Wyper |first5=Peter F. |title=The Dynamic Formation of Pseudostreamers |journal=The Astrophysical Journal |date=1 May 2021 |volume=913 |issue=1 |pages=64 |doi=10.3847/1538-4357/abec4f|bibcode=2021ApJ...913...64S |s2cid=235281960 |url=https://discovery.dundee.ac.uk/en/publications/fdb4f5d7-e55e-415a-94ec-aedc4eece448 |doi-access=free }}50px Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/3.0/ Creative Commons Attribution 4.0] Pseudostreamers' structure was observed in 2012 by the Solar Dynamics Observatory.

The magnetic 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 configuration

surrounding them (tripolar pseudostreamer)."

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 the PROBA2 satellite. Its structure was described by researchers:{{cite journal |last1=Rachmeler |first1=L. A. |last2=Platten |first2=S. J. |last3=Bethge |first3=C. |last4=Seaton |first4=D. B. |last5=Yeates |first5=A. R. |title=Observations of a Hybrid Double-streamer/Pseudostreamer in the Solar Corona |journal=The Astrophysical Journal |date=1 May 2014 |volume=787 |issue=1 |pages=L3 |doi=10.1088/2041-8205/787/1/L3 |arxiv=1312.3153 |bibcode=2014ApJ...787L...3R |url=https://iopscience.iop.org/article/10.1088/2041-8205/787/1/L3 |access-date=10 March 2023 |issn=0004-637X|hdl=10023/5318 |hdl-access=free }}

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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