<i>A Guide to the Solar Corona</i>

Billings, Donald E.; Aarons, J.

doi:10.1063/1.3034941

Cited by 120 publications

(169 citation statements)

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“…For our purposes, we need to estimate the density in the blobs, by knowing the blob location determined via triangulation, assuming a coronal electron density profile and measuring the brightness due to Thomson scattering of photospheric light by free electrons (Minnaert 1930;Van De Hulst 1950;Billings 1966). The STEREO/COR1 coronagraphs (Thompson & Reginald 2008) observe in a 22.5 nm wide waveband centred at the Hα line at 656.3 nm, which is the result of the electronic 3 → 2 transition by neutral hydrogen atoms.…”

Section: Estimate Of Blob Electron Densitymentioning

confidence: 99%

Measurements with STEREO/COR1 data of drag forces acting on small-scale blobs falling in the intermediate corona

Dolei

Бемпорад

Spadaro

2014

A&A

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In this work we study the kinematics of three small-scale (0.01 R ) blobs of chromospheric plasma falling back to the Sun after the huge eruptive event of June 7, 2011. From a study of 3D trajectories of blobs made with the Solar TErrestrial RElations Observatory (STEREO) data, we demonstrate the existence of a significant drag force acting on the blobs and calculate two drag coefficients, in the radial and tangential directions. The resulting drag coefficients C D are between 0 and 5, comparable in the two directions, making the drag force only a factor of 0.45-0.75 smaller than the gravitational force. To obtain a correct determination of electron densities in the blobs, we also demonstrate how, by combining measurements of total and polarized brightness, the Hα contribution to the white-light emission observed by the COR1 telescopes can be estimated. This component is significant for chromospheric plasma, being between 95 and 98% of the total white-light emission. Moreover, we demonstrate that the COR1 data can be employed even to estimate the Hα polarized component, which turns out to be in the order of a few percent of Hα total emission from the blobs. If the drag forces acting on small-scale blobs reported here are similar to those that play a role during the CME propagation, our results suggest that the magnetic drag should be considered even in the CME initiation modelling.

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Section: Estimate Of Blob Electron Densitymentioning

confidence: 99%

Measurements with STEREO/COR1 data of drag forces acting on small-scale blobs falling in the intermediate corona

Dolei

Бемпорад

Spadaro

2014

A&A

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“…We know P F / B F = 0.002 at 10 R [Mann, 1992]. We calculated P K /B K using well-known expressions [van de Hulst, 1950;Billings, 1966;Hayes et al, 2001]. At 10 R the familiar coronal density models are not valid, so we used two spherically symmetric solar wind density models: a polar coronal hole model [Guhathakurta et al, 1999]; and an equatorial streamer model [Muhleman and Anderson, 1981].…”

Section: Observationsmentioning

confidence: 99%

A comparison of mean density and microscale density fluctuations in a CME at 10 R_⊙

Lynch

Coles

Sheeley

2002

Geophysical Research Letters

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[1] We have observed intensity scintillation (IPS) of the radio source 0854 + 201 at 8 GHz on August 2, 2000 during the passage of a coronal mass ejection (CME) across the line of sight. The source was at a distance of 10 R over the north solar pole. Simultaneous observations with the LASCO C3 instrument allow us to model the mean density N e and the microscale density fluctuations dN e within the CME. We find that N e increased by a factor of 2.18 but dN e increased by only 1.76, so the ratio dN e /N e is 19% smaller than in the pre-CME slow wind. During the passage of the CME a short burst of enhanced turbulence doubled the IPS variance but was not visible in the C3 images. This was likely caused by a thin flux tube crossing the line of sight. Detailed modeling indicates that the diameter of the tube was 41,000 km and its density was 14.5 times the CME density. Observations[2] Intensity scintillation at 8 GHz is ''weak'' at 10 R , so it provides a linear measure of the density fluctuations with spatial scales of the order of 50 km. With the LASCO C3 instrument [Brueckner et al., 1995] it is possible to make a direct comparison between these ''microscale'' fluctuations and the mean density. Both observations involve line of sight integrals which cannot be directly inverted, but a useful comparison can be made by fitting a simple CME model to the two data sets simultaneously. The comparison can be made with respect to the pre-CME slow wind without requiring absolute calibration of the IPS or the coronagraph.[3] The structure of the CME is shown clearly in Figure 1. This is a grey-scale version of the summary images available on the SOHO web site (http://sohowww.nascom.nasa.gov/). A description of the image processing is available on the LASCO web site (http://lasco-www.nrl.navy.mil/idl_pros/ help_synoptic.html).[4] Eight C3 images taken during the passage of the CME are shown with the same processing in Figure 2. A 3-hour average of the pre-event corona is used as the slow wind background reference. These images were used to estimate the CME velocity using the running difference technique described by Sheeley et al. [1997]. Height-time plots at the position angle of the radio source show that the velocity of the leading edge was 546 ± 26 km/s and the velocity in the center of the CME was 444 ± 21 km/s. As the CME is relatively fast, one might expect to see some compression at the leading edge.[5] To obtain an estimate of the integrated mean electron density from the C3 images, we must separate the K (electron) component from the F (dust) component. This is done using polarization measurements because the polarization of the K and F components are very different. We assume that the F component is independent of the CME and calculate it from the pre-CME reference. We know P F / B F = 0.002 at 10 R [Mann, 1992]. We calculated P K /B K using well-known expressions [van de Hulst, 1950;Billings, 1966;Hayes et al., 2001]. At 10 R the familiar coronal density models are not valid, so we used two spherically symmetric s...

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“…The irregularly shaped, luminous white halo originates from the scattering of photospheric radiation by free electrons in the Sun's million degree atmosphere and by dust particles in interplanetary space [Billings, 1966]. The electron or "true" corona is dominated by bright streamers, whose bulbous or cusplike bases taper into long spikes or "stalks" that extend more or less radially outward from the Sun.…”

Section: Introductionmentioning

confidence: 99%

“…We also assume the presence of a plasma layer of total angular width -3 ø centered around the polarity reversal. Thomson scattering of photospheric radiation by electrons in the plasma sheet then gives rise to the observed streamer intensities, which can be calculated as a function of sky plane position using standard scattering formulae [see Billings, 1966]. Since both the incident radiation flux and the density of scatterers fall off rapidly with heliocentric distance, the main contribution to the integrated line-of-sight intensity comes from the streamer material nearest the sky plane (i.e., closest to the Sun); the scattered radiation is strongly polarized, with the electric vector aligned in the direction tangential to the solar limb.…”

Section: Sheetmentioning

confidence: 99%

The dynamical nature of coronal streamers

Wang¹,

Sheeley²,

Socker³

et al. 2000

J. Geophys. Res.

217

204

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Abstract. Recent high-sensitivity imaging of the Sun's white-light corona from space has revealed a variety of unexpected small-scale phenomena, including plasma blobs that are ejected continually from the cusplike bases of streamers, fine raylike structures pervading the outer streamer belt, and inflows that occur mainly during times of high solar activity. These phenomena can be interpreted as different manifestations of magnetic field line reconnection, in which plasma and magnetic flux are exchanged between closed and open field regions of the corona. The observations provide new insights into a number of longstanding questions, including the origin of the streamer material in the outer corona, the sources of the slow solar wind, and the mechanisms that regulate the interplanetary magnetic field strength. The Streamer Belt as a Warped Plasma SheetAs has long been known from eclipse observations, the large-scale structure of the corona changes systematically over the l 1-year sunspot cycle. This effect is illustrated in Figure 1, which shows a pair of polarized-brightness images recorded with the LASCO C2 coronagraph in May 1997 and September 1999, respectively. In the earlier image, representative of the white-light corona near sunspot minimum, the streamers lie close to the heliographic equator, the higher latitudes being dominated by dark polar coronal holes traversed by faint polar plumes. In the later image, representative of the corona near sunspot maximum, the bright streamers are distributed more or less symmetrically about the solar disk, and the large polar holes are no longer present.The visual impression obtained from images like those in Figure 1 is that of a disjunct collection of cylindrical stalks.However, as we now demonstrate, the observed streamer structure beyond r • 2.5 Rs can be described by a model in which the scattering electrons are confined to a narrow, warped sheet located where the magnetic field reverses its polarity.We employ Sun-centered spherical coordinates (r, X, •b), with ;t denoting heliographic latitude and •b denoting Car-25,133

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A Guide to the Solar Corona

Cited by 120 publications

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Measurements with STEREO/COR1 data of drag forces acting on small-scale blobs falling in the intermediate corona

Measurements with STEREO/COR1 data of drag forces acting on small-scale blobs falling in the intermediate corona

A comparison of mean density and microscale density fluctuations in a CME at 10 R_⊙

The dynamical nature of coronal streamers

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A Guide to the Solar Corona

Cited by 120 publications

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Measurements with STEREO/COR1 data of drag forces acting on small-scale blobs falling in the intermediate corona

Measurements with STEREO/COR1 data of drag forces acting on small-scale blobs falling in the intermediate corona

A comparison of mean density and microscale density fluctuations in a CME at 10 R⊙

The dynamical nature of coronal streamers

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A comparison of mean density and microscale density fluctuations in a CME at 10 R_⊙