文章基本信息

标题：Non-equilibrium ionization by a periodic electron beam
其他标题：II. Synthetic Si IV and O IV transition region spectra★
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作者：Elena Dzifčáková ; Jaroslav Dudík
期刊名称：Astronomy & Astrophysics
印刷版ISSN：0004-6361
电子版ISSN：1432-0746
出版年度：2018
卷号：610
DOI：10.1051/0004-6361/201731744
语种：English
出版社：EDP Sciences
摘要：Context.Transition region (TR) spectra typically show the SiIV1402.8 Å line to be enhanced by a factor of 5 or more compared to the neighboring OIV1401.2 Å, contrary to predictions of ionization equilibrium models and the Maxwellian distribution of particle energies. Non-equilibrium effects in TR spectra are therefore expected.Aims.To investigate the combination of non-equilibrium ionization and high-energy particles, we apply the model of the periodic electron beam, represented by aκ-distribution that recurs at periods of several seconds, to plasma at chromospheric temperatures of 104K. This simple model can approximate a burst of energy release involving accelerated particles.Methods.Instantaneous time-dependent charge states of silicon and oxygen were calculated and used to synthesize the instantaneous and period-averaged spectra of SiIVand OIV.Results.The electron beam drives the plasma out of equilibrium. At electron densities ofNe= 1010cm−3, the plasma is out of ionization equilibrium at all times in all cases we considered, while for a higher density ofNe= 1011cm−3, ionization equilibrium can be reached toward the end of each period, depending on the conditions. In turn, the character of the period-averaged synthetic spectra also depends on the properties of the beam. While the case ofκ= 2 results in spectra with strong or even dominant OIV, higher values ofκcan approximate a range of observed TR spectra. Spectra similar to typically observed spectra, with the SiIV1402.8 Å line about a factor 5 higher than OIV1401.2 Å, are obtained forκ= 3. An even higher value ofκ= 5 results in spectra that are exclusively dominated by SiIV, with negligible OIVemission. This is a possible interpretation of the TR spectra of UV (Ellerman) bursts, although an interpretation that requires a density that is 1–3 orders of magnitude lower than for equilibrium estimates.
关键词：Key wordsenSun: UV radiationSun: transition regionradiation mechanisms: non-thermal