Coronal physics from eclipse observations

Koutchmy, S.

doi:10.1016/0273-1177(94)90156-2

Cited by 70 publications

(56 citation statements)

References 6 publications

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“…The inter-plume densities are very low, approximately a factor of five lower than in plumes (confirming the forward calculation). They are in excellent agreement with white-light measurements in voids of coronal holes during eclipses (Koutchmy 1994) and with Si viii line-ratio observations in very dark regions of coronal holes (Wilhelm 1999a). The plume and inter-plume density ratios also agree with eclipse observations in 1998 (Lites et al 1999).…”

Section: Discussionsupporting

confidence: 87%

Solar coronal-hole plasma densities and temperatures

Wilhelm¹

2006

A&A

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Polar plumes extending from the Sun into the solar corona have long been seen during eclipses, and can now be studied without this restriction with telescopes and spectrometers on board of spacecraft. Despite the large amount of observational data available on this prominent phenomenon, it is not clear whether plumes contribute substantially to the fast solar-wind streams emanating from coronal holes. An understanding of the processes leading to the formation of bright plumes and the surrounding darker inter-plume regions in coronal holes requires a good knowledge of the physical conditions in plumes and their environment. This investigation aims at measuring the electron densities and temperatures in these regions with the help of radiance ratios of ultraviolet emission lines obtained by SUMER on SOHO. It finds densities of about 7 × 10 7 cm −3 in bright plumes and 1.3 × 10 7 cm −3 in inter-plume lanes at ≈45 Mm above the limb. At this height, the total plume cross-section relative to the size of the coronal hole was found to be less than 8%. The densities drop by a factor of roughly two over the next 80 Mm in height, in lanes a little less than seen in plumes. In this height range, the electron temperatures in plumes are ≈7.5 × 10 5 K and ≈1.13 × 10 6 K in inter-plume regions. The effective ion temperatures, deduced from the line widths, are higher and nearly independent of the altitude in plumes, whereas they increase in inter-plume regions, starting from an even higher level. No systematic dependence of the line-of-sight bulk velocities on the brightness could be found in the coronal-hole plasma.

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Section: Discussionsupporting

confidence: 87%

Solar coronal-hole plasma densities and temperatures

Wilhelm¹

2006

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“…Radio emission near the harmonic of the local plasma frequency at 237 and 164 MHz comes from electron densities of, respectively, 1.7 × 10 8 and 8.3 × 10 7 cm −3 . In a hydrostatically stratified flux tube at coronal temperatures -a model that agrees with electron density measurements at eclipses for heights 1 R above the photosphere (Koutchmy 1994 This pattern can be understood if the flux tubes guiding the radio emitting electron beams are bent toward the observer so that the same, or nearly the same line of sight intercepts the radio sources at several frequencies. The view on the potential field lines from above the northern solar pole shows the geometry expected from this interpretation: field lines bending earthward from the Sun have a pronounced kink and make a small angle with the line of sight at the projected altitudes of the radio sources, at the place and with the geometry required to have similar lines of sight to the sources at 164 and 237 MHz.…”

Section: Type III Burst Mapping and Open Flux Tubes From The Pfss Modelmentioning

confidence: 62%

“…From the bursts occurring near the limb (2002( Oct. 05, 2003 Oct. 04) we infer that this level lies about 0.3 R above the photosphere. The density in the flux tubes guiding the type III emitting electrons is hence about an order of magnitude above electron densities in coronal holes inferred at this altitude from white light eclipse observations (Koutchmy 1994;Aschwanden 2004, Fig. 1.20).…”

Section: Open Magnetic Flux Tubes and Coronal Holesmentioning

confidence: 66%

Open magnetic flux tubes in the corona and the transport of solar energetic particles

Klein¹,

Krucker²,

Lointier³

et al. 2008

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We investigate how magnetic fields guide energetic particles through the corona into interplanetary space and eventually to a spacecraft near the Earth. A set of seven simple particle events is studied, where energetic electrons (30-500 keV; Wind spacecraft) or protons (5-55 MeV; SoHO) were released together with low-energy electron beams producing metric-to-kilometric type III emission. Imaging of the coronal (metre-wave) part of this emission with the Nançay Radioheliograph is used to identify the open flux tubes that guide these electrons -and by inference all particles detected at 1 AU. Open coronal field lines are also computed using potential magnetic field extrapolations, constrained by a source surface and by SoHO/MDI measurements in the photosphere (code by Schrijver and DeRosa). We find that in all events the type III radio sources lie in open flux tubes in the potential magnetic field extrapolations. The open flux tubes are rooted in small parts of the parent active region, covering a heliocentric angle of a few degrees in the photosphere. But they expand rapidly above the neighbouring closed magnetic structures and cover several tens of degrees in longitude on the source surface. Some of these open field lines are found to connect the parent active region to the footpoint of the nominal Parker spiral on the source surface, within the uncertainty of about ±10• inherent to the evaluation of its connection longitude. This is so even when the parent active region is as far as 50• away. In two cases where the coronal flux tubes point to high heliolatitudes, the detection of Langmuir waves at the Wind spacecraft in the ecliptic plane suggests that the interplanetary field lines curve down to the ecliptic before reaching 1 AU. We conclude that non-radial open flux tubes in the corona can transport particles over several tens of degrees in longitude even in simple impulsive particle events. In all events we studied, potential magnetic field models give an adequate description of these structures.

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“…(N 0 = 4.2 × 10 4 cm −3 ; R, distance from the center of the Sun; R , radius of the Sun) is adopted as an appropriate one above active regions (Koutchmy 1994). According to this model, the maximum electron number density at the bottom of the corona (i.e.…”

Section: Plasma Parameters In the Flare Regionmentioning

confidence: 99%

Budget of energetic electrons during solar flares in the framework of magnetic reconnection

Mann

Warmuth

2011

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Context. Among other things, solar flares are accompanied by the production of energetic electrons as seen in the nonthermal radio and X-ray radiation of the Sun. Observations of the RHESSI satellite show that 10 32 −10 36 nonthermal electrons are produced per second during flares. They are related to an energy flux in the range 10 18 −10 22 W. These electrons play an important role, since they carry a substantial part of the energy released during a flare. Aims. In which way so many electrons are accelerated up to high energies during a fraction of a second is still an open question. By means of radio and hard X-ray data, we investigate under which conditions this acceleration happens in the corona. Methods. The flare is considered in the framework of magnetic reconnection. The conditions in the acceleration region in the corona are deduced by using the conservation of the total electron number and energy. Results. In the inflow region of the magnetic reconnection site, there are typical electron number densities of about 2.07×10 9 cm −3 and magnetic fields of about 46 G. These are regions with high Alfvén speeds of about 2200 km s −1 . Then, sufficient energetic electrons (as required by the RHESSI observations) are only generated if the plasma inflow towards the reconnection site has Alfvén-Mach numbers in the range 0.1−1, which can lead to a super-Alfvénic outflow with speeds up to 3100 km s −1 .

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Coronal physics from eclipse observations

Cited by 70 publications

References 6 publications

Solar coronal-hole plasma densities and temperatures

Solar coronal-hole plasma densities and temperatures

Open magnetic flux tubes in the corona and the transport of solar energetic particles

Budget of energetic electrons during solar flares in the framework of magnetic reconnection

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