Morphological relationships in the chromospheric H-alpha fine structure

A continuous relationship is proposed between the basic elements of the dark fine structure of the quiet and active chromosphere. A progression from chromospheric bushes to fibrils, then to chromospheric threads and active region filaments, and finally to diffuse quiescent filaments, is described. It is shown that the horizontal component of the field on opposite sides of an active region quiescent filament can be in the same direction and closely parallel to the filament axis. Consequently, it is unnecessary to postulate twisted or otherwise complex field configurations to reconcile the support mechanism of filaments with the observed motion along their axis

Solar Cycle Modulation of Total Irradiance: an Empirical Model from 1874 to 1988

Evidence acquired during the past decade indicates that over time scales of the solar cycle, enhanced emission from bright solar faculae cause significant variations in the sun's total irradiance even though, on shorter time scales, the most pronounced variations are those resulting from the passage of dark sunspots across the solar disc. An empirical model which accounts for the competing effects of dark sunspots and bright faculae has been developed from the available radiometry in cycle 21, and extended back to the beginning of solar cycle 12. According to this model, the largest 11-year modulation of total irradiance during the C20th occurred in the most recent cycle 21

Examining the origins of ocean heat content variability in the eastern North Atlantic subpolar gyre

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 11,275-11,283, doi:10.1029/2018GL079122.We analyze sources of ocean heat content (OHC) variability in the eastern North Atlantic subpolar gyre from both Eulerian and Lagrangian perspectives within two ocean simulations from 1990 to 2015. Heat budgets reveal that while the OHC seasonal cycle is driven by air‐sea fluxes, interannual OHC variability is driven by both air‐sea fluxes and the divergence of ocean heat transport, the latter of which is dominated by the oceanic flux through the southern face of the study area. Lagrangian trajectories initialized along the southern face and run backward in time indicate that interannual variability in the subtropical‐origin volume flux (i.e., the upper limb of the overturning circulation) drives variability in the temperature flux through the southern face. As such, the heat carried by the imported subtropical waters is an important component of the eastern subpolar gyre heat budget on interannual time scales.NSF. Grant Number NSF‐OCE‐12‐59102; NASA Grant Number: NNX13AO21H2019-04-2

A study of acoustic heating and forced convection in the solar corona

The S055 EUV spectra was used to perform emission measure and line intensity ratio analyses of loop plasma conditions in a study on the thermodynamics of magnetic loops in the solar corona. The evidence that loops contain plasma hotter than the background corona, and thus, require enhanced local dissipation of magnetic or mechanical energy is discussed. The S055 EUV raster pictures were used to study physical conditions in cool ultraviolet absorbing clouds in the solar corona, and optical data were used to derive constraints on the dimension, time scales and optical depths in dark opaque clouds not seen in H alpha and CaK as filaments or prominences. Theoretical modelling of propagation of magnetically guided acoustic shocks in the solar chromosphere finds it still unlikely that high frequency acoustic shocks could reach the solar corona. Dynamic modelling of spicules shows that such guided slow mode shocks can explain the acceleration of cool spicular material seen high in the corona

A new approach to analyzing solar coronal spectra and updated collisional ionization equilibrium calculations. II. Additional ionization rate coefficients

We have reanalyzed SUMER observations of a parcel of coronal gas using new collisional ionization equilibrium (CIE) calculations. These improved CIE fractional abundances were calculated using state-of-the-art electron-ion recombination data for K-shell, L-shell, Na-like, and Mg-like ions of all elements from H through Zn and, additionally, Al- through Ar-like ions of Fe. They also incorporate the latest recommended electron impact ionization data for all ions of H through Zn. Improved CIE calculations based on these recombination and ionization data are presented here. We have also developed a new systematic method for determining the average emission measure ($EM$) and electron temperature ($T_e$) of an isothermal plasma. With our new CIE data and our new approach for determining average $EM$ and $T_e$, we have reanalyzed SUMER observations of the solar corona. We have compared our results with those of previous studies and found some significant differences for the derived $EM$ and $T_e$. We have also calculated the enhancement of coronal elemental abundances compared to their photospheric abundances, using the SUMER observations themselves to determine the abundance enhancement factor for each of the emitting elements. Our observationally derived first ionization potential (FIP) factors are in reasonable agreement with the theoretical model of Laming (2008).Comment: 147 pages (102 of which are online only tables and figures). Submitted to ApJ. Version 2 is updated addressing the referee's repor

Effect of binary collisions on electron acceleration in magnetic reconnection

Context. The presence of energetic X-ray sources in the solar corona indicates there are additional transport effects in the acceleration region. A prime method of investigation is to add collisions into models of particle behaviour at the reconnection region.<p></p> Aims. We investigate electron test particle acceleration in a simple model of an X-type reconnection region. In particular, we explore the possibility that collisions will cause electrons to re-enter the acceleration more frequently, in turn causing particles to be accelerated to high energies.<p></p> Methods. The deterministic (Lorentz) description of particle gyration and acceleration has been coupled to a model for the effects of collisions. The resulting equations are solved numerically using Honeycutt’s extension of the RK4 method to stochastic differential equations. This approach ensures a correct description of collisional energy loss and pitch-angle scattering combined with a sufficiently precise description of gyro-motion and acceleration.<p></p> Results. Even with initially mono-energetic electrons, the competition between collisions and acceleration results in a distribution of electron energies. When realistic model parameters are used, electrons achieve X-ray energies. A possible model for coronal hard X-ray sources is indicated. Conclusions. Even in competition with energy losses, pitch-angle scattering results in a small proportion of electrons reaching higher energies than they would in a collisionless situation.<p></p&gt

Evidence for solar cycles in a late Holocene speleothem record from Dongge Cave, China

The association between solar activity and Asian monsoon (AM) remains unclear. Here we evaluate the possible connection between them based on a precisely-dated, high-resolution speleothem oxygen isotope record from Dongge Cave, southwest China during the past 4.2 thousand years (ka). Without being adjusted chronologically to the solar signal, our record shows a distinct peak-to-peak correlation with cosmogenic nuclide 14C, total solar irradiance (TSI) and sunspot number (SN) at multi-decadal to centennial timescales. Further cross-wavelet analyses between our calcite δ18O and atmospheric 14C show statistically strong coherence at three typical periodicities of ~80, 200 and 340 years, suggesting important roles of solar activities in modulating AM changes at those timescales. Our result has further indicated a better correlation between our calcite δ18O record and atmospheric 14C than between our record and TSI. This better correlation may imply that the Sun–monsoon connection is dominated most likely by cosmic rays and oceanic circulation (both associated to atmospheric 14C), instead of the direct solar heating (TSI)

Search for solar Kaluza-Klein axions in theories of low-scale quantum gravity

We explore the physics potential of a terrestrial detector for observing axionic Kaluza-Klein excitations coming from the Sun within the context of higher-dimensional theories of low-scale quantum gravity. In these theories, the heavier Kaluza-Klein axions are relatively short-lived and may be detected by a coincidental triggering of their two-photon decay mode. Because of the expected high multiplicity of the solar axionic excitations, we find experimental sensitivity to a fundamental Peccei-Quinn axion mass up to $10^{-2}$ eV (corresponding to an effective axion-photon coupling $g_{a\gamma \gamma} \approx 2.\times 10^{-12}$ GeV$^{-1}$) in theories with 2 extra dimensions and a fundamental quantum-gravity scale $M_{\rm F}$ of order 100 TeV, and up to $3.\times 10^{-3}$ eV (corresponding to $g_{a\gamma \gamma} \approx 6.\times 10^{-13}$ GeV$^{-1}$) in theories with 3 extra dimensions and $M_{\rm F}=1$ TeV. For comparison, based on recent data obtained from lowest level underground experiments, we derive the experimental limits: $g_{a \gamma \gamma} \stackrel{<}{{}_\sim} 2.5\times 10^{-11}$ GeV$^{-1}$ and $g_{a \gamma \gamma} \stackrel{<}{{}_\sim} 1.2\times 10^{-11}$ GeV$^{-1}$ in the aforementioned theories with 2 and 3 large compact dimensions, respectively.Comment: 19 pages, extended version, as to appear in Physical Review