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Register a new userHelium line emissivities in the solar corona
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We present new collisional-radiative models (CRMs) for helium in the quiescent solar corona, and predict the emissivities of the He and He$^+$ lines to be observed by DKIST, Solar Orbiter, and Proba-3. We discuss in detail the rates we selected for these models, highlighting several shortcomings we have found in previous work. As no previous complete and self-consistent coronal CRM for helium existed, we have benchmarked our largest model at a density of 10$^{6}$ cm$^{-3}$ and temperature of 20,000 K against recent CRMs developed for photoionised nebulae. We then present results for the outer solar corona, using new dielectronic recombination rates we have calculated, which increase the abundance of neutral helium by about a factor of two. We also find that all the optical triplet He I lines, and in particular the well known He I 10830 and 5876 A lines are strongly affected by both photo-excitation and photo-ionisation from the disk radiation, and that extensive CRM models are required to obtain correct estimates. Close to the Sun, at an electron density of 10$^{8}$ cm$^{-3}$ and temperature of 1 MK, we predict the emissivity of the He I 10830 A to be comparable to that of the strong Fe XIII coronal line at 10798 A. However, we expect the He I emissivity to sharply fall in the outer corona, with respect to Fe XIII. We confirm that the He$^+$ Lyman $alpha$ at 304 A is also significantly affected by photo-excitation and is expected to be detectable as a strong coronal line up to several solar radii.
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We calculate neutrino emissivities from self-annihilating dark matter ($chi$) in the dense and hot stellar interior of a (proto)neutron star. Using a model where dark matter interacts with nucleons in the stellar core through a pseudoscalar boson ($a$) we find that the neutrino production rates from the dominant reaction channels $chi chi rightarrow u bar{ u}$ or $chi chi rightarrow a a$, with subsequent decay of the mediator $ a rightarrow u bar{ u}$, could locally match and even surpass those of the standard neutrinos from the modified nuclear URCA processes at early ages. We find that the emitting region can be localized in a tiny fraction of the star (less than a few percent of the core volume) and the process can last its entire lifetime for some cases under study. We discuss the possible consequences of our results for stellar cooling in light of existing dark matter constraints.
The structure of the excited $2^{3}$S and $2^{3}$P triplet states of $^{3}$He and $^{4}$He in an applied magnetic field B is studied using different approximations of the atomic Hamiltonian. All optical transitions (line positions and intensities) of the 1083 nm $2^{3}$S-$2^{3}$P transition are computed as a function of B. The effect of metastability exchange collisions between atoms in the ground state and in the $2^{3}$S metastable state is studied, and rate equations are derived, for the populations these states in the general case of an isotopic mixture in an arbitrary field B. It is shown that the usual spin-temperature description remains valid. A simple optical pumping model based on these rate equations is used to study the B-dependence of the population couplings which result from the exchange collisions. Simple spectroscopy measurements are performed using a single-frequency laser diode on the 1083 nm transition. The accuracy of frequency scans and of measurements of transition intensities is studied. Systematic experimental verifications are made for B=0 to 1.5 T. Optical pumping effects resulting from hyperfine decoupling in high field are observed to be in good agreement with the predictions of the simple model. Based on adequately chosen absorption measurements at 1083 nm, a general optical method to measure the nuclear polarisation of the atoms in the ground state in an arbitrary field is described. It is demonstrated at $Bsim$0.1 T, a field for which the usual optical methods could not operate.
We study the relationship between solar wind helium to hydrogen abundance ratio ($A_mathrm{He}$), solar wind speed ($v_mathrm{SW}$), and sunspot number (SSN) over solar cycles 23 and 24. This is the first full 22-year Hale cycle measured with the Wind spacecraft covering a full cycle of the solar dynamo with two polarity reversals. While previous studies have established a strong correlation between $A_mathrm{He}$ and SSN, we show that the phase delay between $A_mathrm{He}$ and SSN is a monotonic increasing function of $v_mathrm{SW}$. Correcting for this lag, $A_mathrm{He}$ returns to the same value at a given SSN over all rising and falling phases and across solar wind speeds. We infer that this speed-dependent lag is a consequence of the mechanism that depletes slow wind $A_mathrm{He}$ from its fast wind value during solar wind formation.
An observing campaign (SOHO JOP 139), coordinated between ground based and SOHO instruments, has been planned to obtain simultaneous spectroheliograms of the same active region in several spectral lines. The chromospheric lines CaII K, Halpha and Na D as well as HeI 10830, 5876, 584 and HeII 304 AA lines have been observed.These simultaneous observations allow us to build semi-empirical models of the chromosphere and low transition region of an active region, taking into account the estimated total number of photoionizing photons impinging on the target active region and their spectral distribution. We obtained a model that matches very well all the observed line profiles, using a standard value for the He abundance ([He]=0.1) and a modified distribution of microturbulence. For this model we study the influence of the coronal radiation on the computed helium lines. We find that, even in an active region, the incident coronal radiation has a limited effect on the UV He lines, while it results of fundamental importance for the 5876 and 10830 lines. Finally we build two more models assuming values of He abundance [He]= 0.07 and 1.5, only in the region where temperatures are larger than 1.* 10^4 K. This region, between the chromosphere and transition region, has been indicated as a good candidate for processes that might be responsible for strong variations of [He]. The set of our observables can still be well reproduced in both cases changing the atmospheric structure mainly in the low transition region. This implies that,to choose between different values of [He], it is necessary to constrain the transition region with different observables, independent on the He lines.
In this paper we seek to understand the timescale on which the photospheric motions on the Sun braid coronal magnetic field lines. This is a crucial ingredient for determining the viability of the braiding mechanism for explaining the high temperatures observed in the corona. We study the topological complexity induced in the coronal magnetic field, primarily using plasma motions extracted from magneto-convection simulations. This topological complexity is quantified using the field line winding, finite time topological entropy and passive scalar mixing. With these measures we contrast mixing efficiencies of the magneto-convection simulation, a benchmark flow known as a ``blinking vortex, and finally photospheric flows inferred from sequences of observed magnetograms using local correlation tracking. While the highly resolved magneto-convection simulations induce a strong degree of field line winding and finite time topological entropy, the values obtained from the observations from the plage region are around an order of magnitude smaller. This behavior is carried over to the finite time topological entropy. Nevertheless, the results suggest that the photospheric motions induce complex tangling of the coronal field on a timescale of hours.
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