Measurements on the Lyman alpha corona

Gabriel, A. H.

doi:10.1007/bf00154290

Cited by 99 publications

(52 citation statements)

References 10 publications

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“…The emission of the coronal H I Lyα line is mainly due to two mechanisms: 1) collisional excitation by free electrons impact and 2) resonant scattering of chromospheric photons by neutral hydrogen atoms, with the first one accounting for only a small fraction of the total intensity of the radiative process (Gabriel 1971;Raymond et al 1997). Resonant scattering of the solar chromospheric light by neutral hydrogen atoms thus becomes the primary tool for studying the corona at 121.6 nm.…”

Section: Coronal H I Lyα Intensitymentioning

confidence: 99%

Visible light and ultraviolet observations of coronal structures: physical properties of an equatorial streamer and modelling of the F corona

Dolei

Spadaro

Ventura

2015

A&A

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The present work studies the characteristics of an equatorial streamer visible above the east limb of the Sun on March 2008, during the most recent minimum of solar activity. We analysed the visible light coronagraphic images of SOHO/LASCO and the ultraviolet observations in the H I Lyα spectral line obtained by SOHO/UVCS, and exploited the Doppler dimming effect of the coronal Lyα line to derive the outflow velocity profile of the scattering neutral hydrogen atoms in the streamer region. Taking advantage of the synergy between visible light and ultraviolet observations, we were able to determine all the properties of the coronal structure. In particular, the actual extent of the streamer along the line of sight has been evaluated for the first time. In so doing, the solar wind outflow velocity turned out to be the only free parameter in the theoretical modelling of the Lyα intensity. We found nearly static conditions below 3.5 R along the streamer axis, whereas the solar wind flows at velocities from 40 km s −1 to 140 km s −1 in the altitude range 2.5-5.0 R along the southern boundary of the streamer. We also derived the intensity distribution of the F coronal component in the LASCO C2 field of view, by combining total and polarized brightness data. Finally, we investigated the dependence of the Lyα resonant scattering process on the kinetic temperature of the coronal neutral hydrogen atoms and found that the value of this temperature mostly affects the scattering process at low heliocentric distances, where the solar wind flows with low velocity.

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Section: Coronal H I Lyα Intensitymentioning

confidence: 99%

Visible light and ultraviolet observations of coronal structures: physical properties of an equatorial streamer and modelling of the F corona

Dolei

Spadaro

Ventura

2015

A&A

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“…Comparing the predictions of (2) to the temperatures observed for explosive events, ≈(0.2-5) × 10 5 K (Dere 1994), consistency is obtained if pickup ions typically represent only a fraction ≈10 −2 -0.4 of the total plasma density. This seems reasonable based on equilibrium ionisation balance calculations for the chromosphere (Gabriel 1971;Fontenla et al 1990Fontenla et al , 1991Fontenla et al , 1993, which yield n w /n e ≈ 10 −2 -1 (Section 3.1). For EUV blinkers (Harrison 1997;Harrison et al 1999), with v flow 25 km s −1 , the predictions (2) and (8) yield T ⊥i ≈ 8 × 10 4 K and T e ≈ (3-10) × 10 4 K. These temperatures are very characteristic of the chromosphere and transition region, although as for explosive events, reweighting via (9) will reduce the predicted heating.…”

Section: Chromospheric and Transition Region Eventsmentioning

confidence: 75%

Lower Hybrid Drive in Solar Magnetic Reconnection Regions: Implications for Electron Acceleration and Solar Heating

Cairns

2001

Publ. – Astron. Soc. Aust

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Lower hybrid (LH) drive involves the resonant acceleration of electrons parallel to the magnetic field by lower hybrid waves, often driven by ions with ring or ring-beam distributions. Charge-exchange between hydrogen atoms and protons with relative motions perpendicular to the magnetic field leads to ring distributions of pickup ions, with concomitant perpedicular ion ‘heating’. This paper considers the combination of LH drive and charge-exchange in the outflow regions of magnetic reconnection sites in the solar chromosphere and lower corona, showing that the combined mechanism naturally predicts major perpendicular ion heating and parallel electron acceleration, and exploring the mechanism’s relevance to specific solar reconnection phenomena, heating of the solar atmosphere, and production of energetic electrons that generate solar radio emission. Although primarily qualitative, analysis shows that the mechanism has numerous attractive aspects, including perpendicular ion heating that increases linearly with ion mass, parallel electron acceleration, predicted ion and electron temperatures that span those of the chromosphere and lower corona, and parallel electron speeds spanning those for type III bursts. Applications to chromospheric explosive events and low-lying active regions, and to heating the chromosphere, appear particularly suitable. Sweeping of plasma frozen-in to chromospheric and coronal magnetic field lines across the neutral atmosphere due to motions of sub-photospheric fields represents an obvious and important generalisation of the mechanism away from reconnection sites. The requirements that the neutrals not be strongly collisionally coupled to the plasma and that sufficient neutrals are available for charge-exchange restricts the LH drive mechanism to above the photosphere but below where the corona is essentially fully ionised. LH drive may thus be important in heating the chromosphere and low corona while other heating mechanisms dominate at higher altitudes. Although attractive thus far, quantitative analyses of LH drive in these contexts are necessary before definitive conclusions are reached.

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“…The recent development of a coronagraph capable of observing intensities and line profiles of ultraviolet resonance lines [9] represents an important step in our ability to interpret certain physical variables of the coronal medium. Indeed, : initial considerations and measurements [10,11,12] indicate that combined measurement of profiles and intensities in the ultraviolet and the intensity of Thorn son-scattered radiation in visible light can provide an important new tool for determining temperatures, densities, and flow velocities in solar coronal structures. The early instruments, however, are inevitably limited in their capabilities because of their small scale; this is the result of the small-scale occulting geometries available on ordinary space platforms.…”

Section: Coronagraphic Observationsmentioning

confidence: 99%

The Pinhole/Occulter Facility

Dabbs

Tandberg-Hanssen

Hudson³

1989

EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy and Atomic Physics

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A large occulting system in space can be used for high-re solution X-ray observations and for large-aperture coronagraphic observations in visible and UV light. The X-ray observations will combine high angular resolution in hard (>10 keV) X-radiation with the high sensitivity of a multiple-pinhole camera, and will permit sensitive observations of bremsstrahlung from nonthermal particles in the corona. The large-aperture coronagraphs have two major advantages: high angular resolution and good photon collection. This will permit observations of small-scale structures in the corona for the first time and will give sufficient counting rates above the coronal background rates for sensitive diagnostic analysis of intensities and line profiles for coronal structures in the solar wind acceleration region.

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Measurements on the Lyman alpha corona

Cited by 99 publications

References 10 publications

Visible light and ultraviolet observations of coronal structures: physical properties of an equatorial streamer and modelling of the F corona

Visible light and ultraviolet observations of coronal structures: physical properties of an equatorial streamer and modelling of the F corona

Lower Hybrid Drive in Solar Magnetic Reconnection Regions: Implications for Electron Acceleration and Solar Heating

The Pinhole/Occulter Facility

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