The O IV and S IV intercombination lines in the ultraviolet spectra of astrophysical sources

Keenan, F. P.; Ahmed, S. M.; Brage, Tomas; Doyle, J. G.; Espey, B. R.; Exter, Katrina; Hibbert, A; Keenan, M.T.C.; Madjarska, M. S.; Mathioudakis, M.; Pollacco, D.

doi:10.1046/j.1365-8711.2002.05988.x

Cited by 31 publications

(56 citation statements)

References 33 publications

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“…To the best of our knowledge, this is the first time we can directly diagnose such high electron number densities in the TR plasma during a flare. Density diagnostics based on the use of S iv line ratios with previous instruments in the past were complicated by the presence of line blends which could not be properly resolved and problems in the atomic data (Dufton et al 1982a;Cook et al 1995), as also pointed out by Keenan et al (2002b). In this work we have shown that the S iv 1404.85 Å line can be accurately de-blended from the O iv 1404.85 Å line in the high density interval above 10 12 cm −3 .…”

Section: Discussion and Summarymentioning

confidence: 99%

“…4.3. We note that previous studies (e.g., Keenan et al 2002a) mainly focused on comparing density diagnostics using the O iv and S iv ratios individually but did not report any comparison on the relative intensity of spectral lines from the two ions.…”

Section: Density Diagnostics In the Ar Noaa 12356mentioning

confidence: 92%

“…The main ones are removed with improved atomic data (see Keenan et al 2002a); however, some still remain, especially regarding the 1404.8 Å blend (Del Zanna et al 2002), which is about 30% stronger than predicted. This discrepancy A&A proofs: manuscript no.…”

Section: Atomic Data Blends and Wavelengthsmentioning

confidence: 99%

“…Polito et al (2016) noted that the O iv lines at 1399.78 Å and 1401.16 Å were blended with several photospheric lines, some of them unidentified, during the impulsive phase of an X class flare. Keenan et al (2002a) suggested that Fe ii lines are blended with the S iv 1406.06 Å transition. The presence of two Fe ii lines (at 1405.61 Å and 1405.80 Å) on the blue side of the S iv 1406 Å line is indeed confirmed in some of the observed spectra in this study, see Sect.…”

Section: Atomic Data Blends and Wavelengthsmentioning

confidence: 99%

“…They found that the observed ratios from O iv and S iv would imply electron densities which differed significantly with each other (by up to an order of magnitude). Some of the discrepancies were subsequently identified by Keenan et al (2002a) as due to line blends and low accuracy in the atomic data calculations. They obtained more consistent density diagnostics from O iv and S iv ratios by using updated atomic calculations together with observations from SOHO/SUMER.…”

Section: Introductionmentioning

confidence: 99%

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Density diagnostics derived from the O iv and S iv intercombination lines observed by IRIS

Polito

Zanna

Dudík

et al. 2016

A&A

100

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The intensity of the O iv 2s 2 2p 2 P-2s2p 2 4 P and S iv 3 s 2 3p 2 P-3s 3p 2 4 P intercombination lines around 1400 Å observed with the Interface Region Imaging Spectrograph (IRIS) provide a useful tool to diagnose the electron number density (N e ) in the solar transition region plasma. We measure the electron number density in a variety of solar features observed by IRIS, including an active region (AR) loop, plage and brightening, and the ribbon of the 22-June-2015 M 6.5 class flare. By using the emissivity ratios of O iv and S iv lines, we find that our observations are consistent with the emitting plasma being near isothermal (logT [K] ≈ 5) and iso-density (N e ≈ 10 10.6 cm −3 ) in the AR loop. Moreover, high electron number densities (N e ≈ 10 13 cm −3 ) are obtained during the impulsive phase of the flare by using the S iv line ratio. We note that the S iv lines provide a higher range of density sensitivity than the O iv lines. Finally, we investigate the effects of high densities (N e 10 11 cm −3 ) on the ionization balance. In particular, the fractional ion abundances are found to be shifted towards lower temperatures for high densities compared to the low density case. We also explored the effects of a non-Maxwellian electron distribution on our diagnostic method.

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Section: Discussion and Summarymentioning

confidence: 99%

Section: Density Diagnostics In the Ar Noaa 12356mentioning

confidence: 92%

Section: Atomic Data Blends and Wavelengthsmentioning

confidence: 99%

Section: Atomic Data Blends and Wavelengthsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Density diagnostics derived from the O iv and S iv intercombination lines observed by IRIS

Polito

Zanna

Dudík

et al. 2016

A&A

100

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Solar UV and X-ray spectral diagnostics

Zanna

Mason

2018

Living Rev Sol Phys

205

139

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X-ray and ultraviolet (UV) observations of the outer solar atmosphere have been used for many decades to measure the fundamental parameters of the solar plasma. This review focuses on the optically thin emission from the solar atmosphere, mostly found at UV and X-ray (XUV) wavelengths, and discusses some of the diagnostic methods that have been used to measure electron densities, electron temperatures, differential emission measure (DEM), and relative chemical abundances. We mainly focus on methods and results obtained from high-resolution spectroscopy, rather than broad-band imaging. However, we note that the best results are often obtained by combining imaging and spectroscopic observations. We also mainly focus the review on measurements of electron densities and temperatures obtained from single ion diagnostics, to avoid issues related to the ionisation state of the plasma. We start the review with a short historical introduction on the main XUV high-resolution spectrometers, then review the basics of optically thin emission and the main processes that affect the formation of a spectral line. We mainly discuss plasma in equilibrium, but briefly mention non-equilibrium ionisation and non-thermal electron distributions. We also summarise the status of atomic data, which are an essential part of the diagnostic process. We then review the methods used to measure electron densities, electron temperatures, the DEM, and relative chemical abundances, and the results obtained for the lower solar atmosphere (within a fraction of the solar radii), for coronal holes, the quiet Sun, active regions and flares.

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A closer look at a coronal loop rooted in a sunspot umbra

Chitta

Peter

Young

2016

A&A

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Context. Extreme UV (EUV) and X-ray loops in the solar corona connect regions of enhanced magnetic activity, but they are not usually rooted in the dark umbrae of sunspots because the strong magnetic field found there suppresses convection. This means that the Poynting flux of magnetic energy into the upper atmosphere is not significant within the umbra as long as there are no light bridges or umbral dots. Aims. Here we report a rare observation of a coronal loop rooted in the dark umbra of a sunspot without any traces of light bridges or umbral dots. This allows us to investigate the loop without much confusion from background or line-of-sight integration effects. Methods. We used the slit-jaw images and spectroscopic data from the Interface Region Imaging Spectrograph (IRIS) and concentrate on the line profiles of O iv and Si iv that show persistent strong redshifted components in the loop rooted in the umbra. Using the ratios of O iv, we can estimate the density and thus investigate the mass flux. The coronal context and temperature diagnostics of these observations is provided through the EUV channels of the Atmospheric Imaging Assembly (AIA). Results. The coronal loop, embedded within cooler downflows, hosts supersonic downflows. The speed of more than 100 km s −1 is on the same order of magnitude in the transition region lines of O iv and Si iv, and is even seen at comparable speed in the chromospheric Mg ii lines. At a projected distance of within 1 of the footpoint, we see a shock transition to smaller downflow speeds of about 15 km s −1 being consistent with mass conservation across a stationary isothermal shock. Conclusions. We see no direct evidence for energy input into the loop because the loop is rooted in the dark uniform part of the umbra with no light bridges or umbral dots near by. Thus one might conclude that we are seeing a siphon flow driven from the footpoint at the other end of the loop. However, for a final result data of similar quality at the other footpoint are needed, but this is too far away to be covered by the IRIS field of view.

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The O IV and S IV intercombination lines in the ultraviolet spectra of astrophysical sources

Cited by 31 publications

References 33 publications

Density diagnostics derived from the O iv and S iv intercombination lines observed by IRIS

Density diagnostics derived from the O iv and S iv intercombination lines observed by IRIS

Solar UV and X-ray spectral diagnostics

A closer look at a coronal loop rooted in a sunspot umbra

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