Evidence for Non‐Maxwellian Electron Energy Distributions in the Solar Transition Region: Si<scp>iii</scp>Line Ratios from SUMER

Pinfield, D. J.; Keenan, F. P.; Mathioudakis, M.; Phillips, K. J. H.; Curdt, W.; Wilhelm, K.

doi:10.1086/308106

Cited by 61 publications

(58 citation statements)

References 27 publications

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“…Vocks & Mann (2003) show that the nonthermal tails in the solar wind can originate in the solar corona. Pinfield et al (1999) conclude that nonthermal distributions with high-energy tails may explain observed Si iii line ratios in the transition region spectra observed by SOHO/SUMER (Wilhelm et al 1995). show that the observed Si iii line ratios are indeed explained well with κ-distributions, once the effect of photoexcitation is taken into account.…”

Section: Introductionmentioning

confidence: 52%

“…In solar and possibly stellar physics, departures from Maxwellian distribution can occur at low plasma densities in combination with strong temperature or density gradients (e.g., Scudder & Olbert 1979a,b;Roussel-Dupré 1980;Shoub 1983;Owocki & Scudder 1983;Dufton et al 1984;Ljepojevic & MacNiece 1988;Scudder 1992;Pinfield et al 1999), in the impulsive phase of solar flares (Seely et al 1987;Dzifčáková & Kulinová 2010;Kulinová et al 2011) or in situations, where the the average particle energies are not held fixed, but can change by about an order of magnitude (Collier 2004), e.g., due to dynamic coronal heating.…”

Section: Introductionmentioning

confidence: 99%

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The bound-bound and free-free radiative losses for the nonthermal distributions in solar and stellar coronae

Dudík

Dzifčáková

Karlický

et al. 2011

A&A

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Context. The radiative-loss function is an important ingredient in the physics of the solar corona, transition region, and flares. Aims. We investigate the radiative losses due to the bound-bound transitions and bremsstrahlung for nonthermal κ-and n-distributions. Methods. The bound-bound radiative losses are computed by integrating synthetic spectra. An analytical expression is derived for nonthermal bremsstrahlung. The bremsstrahlung is computed numerically using accurate values of the free-free Gaunt factor. Results. We find that the changes in radiative-loss functions due to nonthermal distributions are several times greater than the errors due to the missing contribution of the free-bound continuum or errors in atomic data. For κ-distributions, the radiative-loss functions are in general weaker than for Maxwellian distribution, with a few exceptions caused by the behavior of Fe. The peaks of the radiativeloss functions are in general flatter. The situation is opposite for n-distributions, for which the radiative-loss functions have higher and narrower peaks. Local minima and maxima of the radiative-loss functions may also be shifted. The contribution from bremsstrahlung only changes by a few percent except in the extreme nonthermal case of κ = 2. Stability analysis reveals that the X-ray loops are stable against the radiatively-driven thermal instability.

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Section: Introductionmentioning

confidence: 52%

Section: Introductionmentioning

confidence: 99%

The bound-bound and free-free radiative losses for the nonthermal distributions in solar and stellar coronae

Dudík

Dzifčáková

Karlický

et al. 2011

A&A

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“…The temperatures derived from Si IV are above the temperatures expected from ionization equilibrium calculations (Method II). Similarly, lines from the Mg-like ion Si III have been used to derive temperatures (e.g., PinÐeld et al 1999), and higher temperatures than predicted from ionization equilibrium were obtained in this case as well. Other temperature diagnostics involving Si III, C III, and O V lines are discussed by Doschek (1997).…”

Section: Introductionmentioning

confidence: 90%

Intensity Ratios between the 2s²¹S₀–2s2p³P₁and 2s²p¹P₁–2p²¹D₂Transitions in Be‐like Ions as Electron Temperature Indicators for Solar Upper Atmosphere Plasmas

Landi

Doron

Feldman

et al. 2001

ApJ

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We investigate the relative intensities of the two moderately bright Be-like 2s2 and 2s2p 1S 0 È2s2p 3P 1 lines as a function of electron temperature. We show that the intensity ratios of the lines in 1P 1 È2p2 1D 2 the beryllium isoelectronic sequence from C III to Ni XXV ions can serve as sensitive temperature indicators for a large variety of solar plasmas. While the C IIIÈNe VII lines can be used to diagnose unresolved Ðne structures in relatively cold solar atmosphere plasmas [(1È5) ] 105 K], the Na VIIIÈAr XV ions can be used to diagnose coronal plasmas [(0.8È3) ] 106 K], and Ca XVIIÈNi XXV lines are useful to measure the temperature in Ñaring plasmas [(5È16) ] 107 K]. We investigate the e †ects on the temperature determination caused by varying the number of energy levels that are included in the atomic model for the considered ions. It is found that a model that includes the 2l2l@ and 2l3l@ conÐgurations is sufficient for adequately describing the relevant level populations of the Be-like ions in coronal conditions. We compare theoretical ratios obtained using collisional cross section and transition probability values derived by di †erent theoretical methods. The atomic data are obtained from the CHIANTI database, the Hebrew University Lawrence Livermore Atomic Code (HULLAC) suite of programs, and other available sources in the literature. Finally, we use spectra of an apparently isothermal coronal plasma observed by the Solar Ultraviolet Measurement of Emitted Radiation instrument on the Solar and Heliospheric Observatory to determine the electron temperature of streamer plasma using the HULLAC and CHIANTI atomic data sets. The result is compared with the temperature derived in an earlier study using di †erent methods.

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“…Apart from that, departures from Maxwellian distributions can occur at low plasma densities if there are strong gradients of temperature and density, i.e., in the transition region (e.g., Owocki & Scudder 1983;Ljepojevic & MacNiece 1988;Scudder 1992;Pinfield et al 1999;, in conditions where the mean energy can change (Collier 2004), i.e., during plasma heating. These processes commonly result in the formation of power-law, high-energy tails.…”

Section: Introductionmentioning

confidence: 99%

The non-Maxwellian continuum in the X-ray, UV, and radio range

Dudík

Kašparová

Dzifčáková

et al. 2012

A&A

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Aims. We investigate the X-ray, UV, and also the radio continuum arising from plasmas with a non-Maxwellian distribution of electron energies. The two investigated types of distributions are the κ-and n-distributions. Methods. We derived analytical expressions for the non-Maxwellian bremsstrahlung and free-bound continuum spectra. The spectra were calculated using available cross-sections. Then we compared the bremsstrahlung spectra arising from the different bremsstrahlung cross-sections that are routinely used in solar physics.Results. The behavior of the bremsstrahlung spectra for the non-Maxwellian distributions is highly dependent on the assumed type of the distribution. At flare temperatures and hard X-ray energies, the bremsstrahlung is greatly increased for κ-distributions and exhibits a strong high-energy tail. With decreasing κ, the maximum of the bremsstrahlung spectrum decreases and moves to higher wavelengths. In contrast, the maximum of the spectra for n-distributions increases with increasing n, and the spectrum then falls off very steeply with decreasing wavelength. In the millimeter radio range, the non-Maxwellian bremsstrahlung spectra are almost parallel to the thermal bremsstrahlung. Therefore, the non-Maxwellian distributions cannot be detected by off-limb observations made by the ALMA instrument. The free-bound continua are also highly dependent on the assumed type of the distribution. For n-distributions, the ionization edges disappear and a smooth continuum spectrum is formed for n 5. Opposite behavior occurs for κ-distributions where the ionization edges are in general significantly enhanced, with details depending on κ and T through the ionization equilibrium. We investigated how the non-Maxwellian κ-distributions can be determined from the observations of the continuum and conclude that one can sample the low-energy part of the distribution from the continuum.

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Evidence for Non‐Maxwellian Electron Energy Distributions in the Solar Transition Region: SiiiiLine Ratios from SUMER

Cited by 61 publications

References 27 publications

The bound-bound and free-free radiative losses for the nonthermal distributions in solar and stellar coronae

The bound-bound and free-free radiative losses for the nonthermal distributions in solar and stellar coronae

Intensity Ratios between the 2s²¹S₀–2s2p³P₁and 2s²p¹P₁–2p²¹D₂Transitions in Be‐like Ions as Electron Temperature Indicators for Solar Upper Atmosphere Plasmas

The non-Maxwellian continuum in the X-ray, UV, and radio range

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Evidence for Non‐Maxwellian Electron Energy Distributions in the Solar Transition Region: SiiiiLine Ratios from SUMER

Cited by 61 publications

References 27 publications

The bound-bound and free-free radiative losses for the nonthermal distributions in solar and stellar coronae

The bound-bound and free-free radiative losses for the nonthermal distributions in solar and stellar coronae

Intensity Ratios between the 2s21S0–2s2p3P1and 2s2p1P1–2p21D2Transitions in Be‐like Ions as Electron Temperature Indicators for Solar Upper Atmosphere Plasmas

The non-Maxwellian continuum in the X-ray, UV, and radio range

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Intensity Ratios between the 2s²¹S₀–2s2p³P₁and 2s²p¹P₁–2p²¹D₂Transitions in Be‐like Ions as Electron Temperature Indicators for Solar Upper Atmosphere Plasmas