- ID:
- ivo://CDS.VizieR/J/ApJ/738/119
- Title:
- Conversion from magnetoacoustic to Alfven waves
- Short Name:
- J/ApJ/738/119
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Alfven waves may be generated via mode conversion from fast magnetoacoustic waves near their reflection level in the solar atmosphere, with implications both for coronal oscillations and for active region helioseismology. In active regions this reflection typically occurs high enough that the Alfven speed a greatly exceeds the sound speed c, well above the a=c level where the fast and slow modes interact. In order to focus on the fundamental characteristics of fast/Alfven conversion, stripped of unnecessary detail, it is therefore useful to freeze out the slow mode by adopting the gravitationally stratified cold magnetohydrodynamic model c->0. This provides a benchmark for fast-to-Alfven mode conversion in more complex atmospheres. Assuming a uniform inclined magnetic field and an exponential Alfven speed profile with density scale height h, the Alfven conversion coefficient depends on three variables only: the dimensionless transverse-to-the-stratification wavenumber {kappa}=kh, the magnetic field inclination from the stratification direction {theta}, and the polarization angle {phi} of the wavevector relative to the plane containing the stratification and magnetic field directions. We present an extensive exploration of mode conversion in this parameter space and conclude that near-total conversion to outward-propagating Alfven waves typically occurs for small {theta} and large {phi} (80-90{deg}), though it is absent entirely when {theta} is exactly zero (vertical field). For wavenumbers of helioseismic interest, the conversion region is broad enough to encompass the whole chromosphere.
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- ID:
- ivo://CDS.VizieR/J/other/SoPh/294.144
- Title:
- Coronal hole parameters
- Short Name:
- J/other/SoPh/294
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Coronal holes are usually defined as dark structures seen in the extreme ultraviolet and X-ray spectrum which are generally associated with open magnetic fields. Deriving reliably the coronal hole boundary is of high interest, as its area, underlying magnetic field, and other properties give important hints as regards high speed solar wind acceleration processes and compression regions arriving at Earth. In this study we present a new threshold-based extraction method, which incorporates the intensity gradient along the coronal hole boundary, which is implemented as a user-friendly SSW-IDL GUI. The Collection of Analysis Tools for Coronal Holes (CATCH) enables the user to download data, perform guided coronal hole extraction and analyze the underlying photospheric magnetic field. We use CATCH to analyze non-polar coronal holes during the SDO-era, based on 193{AA} filtergrams taken by the Atmospheric Imaging Assembly (AIA) and magnetograms taken by the Heliospheric and Magnetic Imager (HMI), both on board the Solar Dynamics Observatory (SDO). Between 2010 and 2019 we investigate 707 coronal holes that are located close to the central meridian. We find coronal holes distributed across latitudes of about +/-60{deg}, for which we derive sizes between 1.6x10^9^ and 1.8x10^11^km^2^. The absolute value of the mean signed magnetic field strength tends towards an average of 2.9+/-1.9G. As far as the abundance and size of coronal holes is concerned, we find no distinct trend towards the northern or southern hemisphere. We find that variations in local and global conditions may significantly change the threshold needed for reliable coronal hole extraction and thus, we can highlight the importance of individually assessing and extracting coronal holes.
- ID:
- ivo://CDS.VizieR/J/ApJ/710/L58
- Title:
- Coronal type II radio bursts in 2002
- Short Name:
- J/ApJ/710/L58
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Major space weather events such as solar flares and coronal mass ejections are usually accompanied by solar radio bursts, which can potentially be used for real-time space weather forecasts. Type II radio bursts are produced near the local plasma frequency and its harmonic by fast electrons accelerated by a shock wave moving through the corona and solar wind with a typical speed of ~1000km/s. The coronal bursts have dynamic spectra with frequency gradually falling with time and durations of several minutes. This Letter presents a new method developed to detect type II coronal radio bursts automatically and describes its implementation in an extended Automated Radio Burst Identification System (ARBIS 2). Preliminary tests of the method with spectra obtained in 2002 show that the performance of the current implementation is quite high, ~80%, while the probability of false positives is reasonably low, with one false positive per 100-200hr for high solar activity and less than one false event per 10000hr for low solar activity periods. The first automatically detected coronal type II radio burst is also presented.
- ID:
- ivo://CDS.VizieR/J/A+A/640/A17
- Title:
- CR Ground Level Enhancements spectra (IGLED)
- Short Name:
- J/A+A/640/A17
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Continuous measurements of ground-based neutron monitors (NMs) form the main data source to study high-energy/high-intensity solar energetic particle (SEP) events, called ground-level enhancements (GLEs). All available data are collected in the International GLE Database (IGLED, http://gle.oulu.fi), which provides formal NM count-rate increases above the constant pre-increase level due to galactic cosmic rays (GCR). This dataset is used to reconstruct energy spectra of GLE events. However, the assumption of the constant GCR background level throughout GLE events is often invalid. Here we made a major revision of the IGLED dataset and provided a dataset of detrended NM count-rate increases accounting for the variable GCR background. The formal GLE count-rate increases were corrected for the variable GCR background which may vary significantly during GLE events. The corresponding integral omnidirectional fluences of SEPs were reconstructed for all GLEs with the sufficient strength from the detrended data using the "effective rigidity" method. The database of detrended NM count-rate has been reset for all GLE events since 1956. The integral omnidirectional fluences have been estimated for all the GLE evens and parameterized for sufficiently strong ones by the modified Ellison-Ramaty spectral shape. The IGLED has been revisited to account for the variable GCR background. Integral omnidirectional fluences reconstructed for most of GLE events, have benn added to IGLED. This forms a basis for more precise studies of parameters of SEP events and, thus, for Solar and space physics.
- ID:
- ivo://CDS.VizieR/J/A+A/590/A118
- Title:
- Differences of atlases of solar spectrum
- Short Name:
- J/A+A/590/A118
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The high-fidelity solar spectral atlas prepared by Delbouille et. al (1973, Liege atlas, Atlas photometrique du spectre solaire de {lambda} 3000 a {lambda} 10000, 1973apds.book.....D) and the atlas by Neckel (1999, Hamburg atlas, 1999SoPh..184..421N) are widely recognised as the most important collection of reference spectra of the Sun at disc centre in the visible wavelength range. The two datasets serve as fundamental resources for many researchers, in particular for chemical abundance analyses. But despite their similar published specifications (spectral resolution and noise level), the shapes of the spectral lines in the two atlases differ significantly and systematically. Knowledge of any instrumental degradations is imperative to fully exploit the information content of spectroscopic data. We seek to investigate the magnitude of these differences and explain the possible sources. We provide the wavelength-dependent correction parameters that need to be taken into account when the spectra are to be compared with synthetic data, for instance.
- ID:
- ivo://CDS.VizieR/J/ApJ/897/L10
- Title:
- Disturbance storm time index; 1903 Sun outburst
- Short Name:
- J/ApJ/897/L10
- Date:
- 15 Nov 2021 09:20:23
- Publisher:
- CDS
- Description:
- While the Sun is generally more eruptive during its maximum and declining phases, observational evidence shows certain cases of powerful solar eruptions during the quiet phase of solar activity. Occurring in the weak Solar Cycle 14 just after its minimum, the extreme space weather event in 1903 October-November is one of these cases. Here, we reconstruct the time series of geomagnetic activity based on contemporary observational records. With the mid-latitude magnetograms, the 1903 magnetic storm is thought to be caused by a fast coronal mass ejection (~1500km/s) and is regarded as a superstorm with an estimated minimum of the equivalent disturbance storm time index (Dst') of ~-531nT. The reconstructed time series has been compared with the equatorward extension of auroral oval (~44.1 in invariant latitude) and the time series of telegraphic disturbances. This case study shows that potential threats posed by extreme space weather events exist even during weak solar cycles or near their minima.
- ID:
- ivo://CDS.VizieR/J/A+A/623/A126
- Title:
- Effects of a revised ^7^Be e^-^-capture rate
- Short Name:
- J/A+A/623/A126
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The electron-capture rate on ^7^Be is the main production channel for ^7^Li in several astrophysical environments. Theoretical evaluations have to account for not only the nuclear interaction, but also the processes in the plasma where ^7^Be ions and electrons interact. In the past decades several estimates were presented, pointing out that the theoretical uncertainty in the rate is in general of few percents. In the framework of fundamental solar physics, we consider here a recent evaluation for the ^7^Be+e^-^ rate, not used up to now in the estimate of neutrino fluxes. We analysed the effects of the new assumptions on Standard Solar Models (SSMs) and compared the results obtained by adopting the revised ^7^Be+e^-^ rate to those obtained by the one reported in a widely used compilation of reaction rates (ADE11). We found that new SSMs yield a maximum difference in the efficiency of the ^7^Be channel of about -4% with respect to what is obtained with the previously adopted rate. This fact affects the production of neutrinos from ^8^B, increasing the relative flux up to a maximum of 2.7%. Negligible variations are found for the physical and chemical properties of the computed solar models. The agreement with the SNO measurements of the neutral current component of the ^8^B neutrino flux is improved
- ID:
- ivo://CDS.VizieR/J/MNRAS/436/1452
- Title:
- Electron impact for OV and OVI levels
- Short Name:
- J/MNRAS/436/1452
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Radiative atomic and electron impact excitation data for OV and OVI ions have been calculated. The radiative atomic data have been calculated with the AUTOSTRUCTURE code. Besides the one-body and the two-body fine structure interactions, the two-body non-fine structure operators of the Breit-Pauli Hamiltonian, namely contact spin-spin, two-body Darwin and orbit-orbit are incorporated in AUTOSTRUCTURE. The scattering problem has been treated in the Breit-Pauli distorted wave approximation using the same code AUTOSTRUCTURE. The OV atomic calculations have been extended from 46 to 92 levels. We have calculated excitation cross sections for the OV 2s^2^-2s2p and the OVI 2s-2p transitions at energies near the corresponding excitation threshold regions. We have also calculated collision strengths for transitions from the most important levels for collisional excitation at electron energies up to 120Ry for OV and up to 140Ry for OVI. Our results have been compared with the available theoretical/experimental ones and a satisfactory agreement have been found between them.
- ID:
- ivo://CDS.VizieR/J/ApJ/759/141
- Title:
- Emission measures in solar active regions
- Short Name:
- J/ApJ/759/141
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The recent analysis of observations taken with the EUV Imaging Spectrometer and X-Ray Telescope instruments on Hinode suggests that well-constrained measurements of the temperature distribution in solar active regions can finally be made. Such measurements are critical for constraining theories of coronal heating. Past analysis, however, has suffered from limited sample sizes and large uncertainties at temperatures between 5 and 10MK. Here we present a systematic study of the differential emission measure distribution in 15 active region cores. We focus on measurements in the "inter-moss" region, that is, the region between the loop footpoints, where the observations are easier to interpret. To reduce the uncertainties at the highest temperatures we present a new method for isolating the Fe XVIII emission in the AIA/SDO 94{AA} channel. The resulting differential emission measure distributions confirm our previous analysis showing that the temperature distribution in an active region core is often strongly peaked near 4MK. We characterize the properties of the emission distribution as a function of the total unsigned magnetic flux. We find that the amount of high-temperature emission in the active region core is correlated with the total unsigned magnetic flux, while the emission at lower temperatures, in contrast, is inversely related. These results provide compelling evidence that high-temperature active region emission is often close to equilibrium, although weaker active regions may be dominated by evolving million degree loops in the core.
- ID:
- ivo://CDS.VizieR/J/A+A/624/A36
- Title:
- EUV irradiances of the quiet Sun
- Short Name:
- J/A+A/624/A36
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We benchmark new atomic data against a selection of irradiances obtained from medium-resolution quiet-Sun spectra in the extreme ultraviolet (EUV), from 60 to 1040{AA}. We used as a baseline the irradiances measured during solar minimum on 2008 April 14 by the prototype (PEVE) of the Solar Dynamics Observatory Extreme ultraviolet Variability Experiment (EVE). We took into account some inconsistencies in the PEVE data, using flight EVE data and irradiances we obtained from Solar and Heliospheric Observatory (SoHO) Coronal Diagnostics Spectrometer (CDS) data. We performed a differential emission measure and find overall excellent agreement (to within the accuracy of the observations, about 20%) between predicted and measured irradiances in most cases, although we point out several problems with the currently available ion charge-state distributions. We used the photospheric chemical abundances of Asplund et al. (2009ARA&A..47..481A). The new atomic data are nearly complete in this spectral range for medium-resolution irradiance spectra. Finally, we used observations of the active Sun in 1969 to show that the composition of the solar corona up to 1MK is nearly photospheric in this case as well. Variations of a factor of 2 are present for higher-temperature plasma, which is emitted within active regions. These results are in excellent agreement with our previous findings.
