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Register a new userSynthetic IRIS spectra of the solar transition region: Effect of high-energy tails
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The solar transition region satisfies the conditions for presence of non-Maxwellian electron energy distributions with high-energy tails at energies corresponding to the ionization potentials of many ions emitting in the EUV and UV portions of the spectrum. We calculate the synthetic Si IV, O IV, and S IV spectra in the far ultra-violet (FUV) channel of the Interface Region Imaging Spectrograph (IRIS). Ionization, recombination, and excitation rates are obtained by integration of the cross-sections or their approximations over the model electron distributions considering particle propagation from the hotter corona. The ionization rates are significantly affected by the presence of high-energy tails. This leads to the peaks of the relative abundance of individual ions to be broadened with pronounced low-temperature shoulders. As a result, the contribution functions of individual lines observable by IRIS also exhibit low-temperature shoulders, or their peaks are shifted to temperatures an order of magnitude lower than for the Maxwellian distribution. The integrated emergent spectra can show enhancements of Si IV compared toO IV by more than a factor of two. The high-energy particles can have significant impact on the emergent spectra and their presence needs to be considered even in situations without strong local acceleration.
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The heating of the outer solar atmospheric layers, i.e., the transition region and corona, to high temperatures is a long standing problem in solar (and stellar) physics. Solutions have been hampered by an incomplete understanding of the magnetically controlled structure of these regions. The high spatial and temporal resolution observations with the Interface Region Imaging Spectrograph (IRIS) at the solar limb reveal a plethora of short, low lying loops or loop segments at transition-region temperatures that vary rapidly, on the timescales of minutes. We argue that the existence of these loops solves a long standing observational mystery. At the same time, based on comparison with numerical models, this detection sheds light on a critical piece of the coronal heating puzzle.
NASAs Interface Region Imaging Spectrograph (IRIS) provides high resolution observations of the solar atmosphere through UV spectroscopy and imaging. Since the launch of IRIS in June 2013, we have conducted systematic observation campaigns in coordination with the Swedish 1-m Solar Telescope (SST) on La Palma. The SST provides complementary high-resolution observations of the photosphere and chromosphere. The SST observations include spectro-polarimetric imaging in photospheric Fe I lines and spectrally-resolved imaging in the chromospheric Ca II 8542 A, H-alpha, and Ca II K lines. We present a database of co-aligned IRIS and SST datasets that is open for analysis to the scientific community. The database covers a variety of targets including active regions, sunspots, plage, quiet Sun, and coronal holes.
We compute for the first time magnetic helicity and energy spectra of the solar active region NOAA 11158 during 11-15 February 2011 at 20^o southern heliographic latitude using observational photospheric vector magnetograms. We adopt the isotropic representation of the Fourier-transformed two-point correlation tensor of the magnetic field. The sign of magnetic helicity turns out to be predominantly positive at all wavenumbers. This sign is consistent with what is theoretically expected for the southern hemisphere. The magnetic helicity normalized to its theoretical maximum value, here referred to as relative helicity, is around 4% and strongest at intermediate wavenumbers of k ~ 0.4 Mm^{-1}, corresponding to a scale of 2pi/k ~ 16 Mm. The same sign and a similar value are also found for the relative current helicity evaluated in real space based on the vertical components of magnetic field and current density. The modulus of the magnetic helicity spectrum shows a k^{-11/3} power law at large wavenumbers, which implies a k^{-5/3} spectrum for the modulus of the current helicity. A k^{-5/3} spectrum is also obtained for the magnetic energy. The energy spectra evaluated separately from the horizontal and vertical fields agree for wavenumbers below 3 Mm^{-1}, corresponding to scales above 2 Mm. This gives some justification to our assumption of isotropy and places limits resulting from possible instrumental artefacts at small scales.
The Interface Region Imaging Spectrograph (IRIS) has been obtaining near- and far-ultraviolet images and spectra of the solar atmosphere since July 2013. The unique combination of near and far-ultraviolet spectra and images at subarcsecond resolution and high cadence allows the tracing of mass and energy through the critical interface between the solar surface and the corona or solar wind. IRIS has enabled research into the fundamental physical processes thought to play a role in the low solar atmosphere such as ion-neutral interactions, magnetic reconnection, the generation, propagation, and dissipation of various types of waves, the acceleration of non-thermal particles, and various small-scale instabilities. These new findings have helped provide novel insights into a wide range of phenomena including the discovery of non-thermal particles in coronal nanoflares, the formation and impact of spicules and other jets, resonant absorption and dissipation of Alfvenic waves, energy release associated with braiding of magnetic field lines, the thermal instability in the chromosphere-corona mass and energy cycle, the contribution of waves, turbulence, and non-thermal particles in the energy deposition during flares and smaller-scale events such as UV bursts, and the role of flux ropes and other mechanisms in triggering CMEs. IRIS observations have also advanced studies of the connections between solar and stellar physics. Advances in numerical modeling, inversion codes, and machine learning techniques have played a key role in driving these new insights. With the advent of exciting new instrumentation both on the ground (e.g., DKIST, ALMA) and space-based (e.g., Parker Solar Probe, Solar Orbiter), we aim to review new insights based on IRIS observations or related modeling, and highlight some of the outstanding challenges that have been brought to the fore.
Solar spectra of ultraviolet bursts and flare ribbons from the Interface Region Imaging Spectrograph (IRIS) have suggested high electron densities of $>10^{12}$ cm$^{-3}$ at transition region temperatures of 0.1 MK, based on large intensity ratios of Si IV $lambda$1402.77 to O IV $lambda$1401.16. In this work a rare observation of the weak O IV $lambda$1343.51 line is reported from an X-class flare that peaked at 21:41 UT on 2014 October 24. This line is used to develop a theoretical prediction of the Si IV $lambda$1402.77 to O IV $lambda$1401.16 ratio as a function of density that is recommended to be used in the high density regime. The method makes use of new pressure-dependent ionization fractions that take account of the suppression of dielectronic recombination at high densities. It is applied to two sequences of flare kernel observations from the October 24 flare. The first shows densities that vary between $3times 10^{12}$ to $3 times 10^{13}$ cm$^{-3}$ over a seven minute period, while the second location shows stable density values of around $2times 10^{12}$ cm$^{-3}$ over a three minute period.
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