# High-frequency spicule oscillations generated via mode conversion

Spicule oscillations involve high-frequency components with a typical period approximately corresponding to $40-50$ s. The typical time scale of the photospheric oscillation is a few minutes, and thus, the origin of this high-frequency component is not trivial. In this study, a one-dimensional numerical simulation is performed to demonstrate that the observed spicule oscillations originate from longitudinal-to-transverse mode conversion that occurs around the equipartition layer in the chromosphere. Calculations are conducted in a self-consistent manner with the exception of additional heating to maintain coronal temperature. The analyses indicate the following features: (1) mode conversion efficiently excites high-frequency transverse waves; (2) the typical period of the high-frequency waves corresponds to the sound-crossing time of the mode conversion region; and (3) simulated root-mean-square velocity of the high-frequency component is consistent with the observed value. These results indicate that the observation of spicule oscillation provides direct evidence of mode conversion in the chromosphere.

Original Article: http://arxiv.org/abs/1801.01254

# The Instruments and Capabilities of the Miniature X-ray Solar Spectrometer (MinXSS) CubeSats

Christopher S. Moore (1, 2 and 3), Amir Caspi (4), Thomas N. Woods (2), Phillip C. Chamberlin (2 and 5), Brian R. Dennis (5), Andrew R. Jones (2), James P. Mason (2 and 5), Richard A. Schwartz (5 and 6), Anne K. Tolbert (5 and 6) ((1) Department for Astrophysical and Planetary Science, University of Colorado, (2) Laboratory for Atmospheric and Space Physics, University of Colorado, (3) Harvard-Smithsonian Center for Astrophysics, (4) Southwest Research Institute, Boulder, CO, USA, (5) NASA Goddard Space Flight Center, (6) American University)

The Miniature X-ray Solar Spectrometer (MinXSS) CubeSat is the first solar science oriented CubeSat mission flown for the NASA Science Mission Directorate, with the main objective of measuring the solar soft X-ray (SXR) flux and a science goal of determining its influence on Earth’s ionosphere and thermosphere. These observations can also be used to investigate solar quiescent, active region, and flare properties. The MinXSS X-ray instruments consist of a spectrometer, called X123, with a nominal 0.15 keV full-width-half-maximum (FWHM) resolution at 5.9 keV and a broadband X-ray photometer, called XP. Both instruments are designed to obtain measurements from 0.5 – 30 keV at a nominal time cadence of 10 seconds. A description of the MinXSS instruments, performance capabilities, and relation to the Geostationary Operational Environmental Satellite (GOES) 0.1 – 0.8 nm flux are discussed in this article. Early MinXSS results demonstrate the capability to measure variations of the solar spectral SXR flux between 0.8 – 12 keV from at least GOES A5 – M5 (5 $\times$ 10$^{-8}$ – 5 $\times$ 10$^{-5}$ W m$^{-2}$) levels and infer physical properties (temperature and emission measure) from the MinXSS data alone. Moreover, coronal elemental abundances can be inferred, specifically Fe, Ca, Si, Mg, S, Ar, and Ni, when there is sufficiently high count rate at each elemental spectral feature. Additionally, temperature response curves and emission measure loci demonstrate the MinXSS sensitivity to plasma emission at different temperatures. MinXSS observations coupled with those from other solar observatories can help address some of the most compelling questions in solar coronal physics. Finally, simultaneous observations by MinXSS and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) can provide the most spectrally complete soft X-ray solar flare photon flux measurements to date.

Original Article: http://arxiv.org/abs/1801.01261

# Synchronized Observations of Bright Points from the Solar Photosphere to Corona

Ehsan Tavabi

One of the most important features in the solar atmosphere is magnetic network and its rela- tionship with the transition region (TR), and coronal brightness. It is important to understand how energy is transported into the corona and how it travels along the magnetic-field lines be- tween deep photosphere and chromosphere through the TR and corona. An excellent proxy for transportation is the Interface Region Imaging Spectrograph (IRIS) raster scans and imaging observations in near-ultraviolet (NUV) and far-ultraviolet (FUV) emission channels with high time-spatial resolutions. In this study, we focus on the quiet Sun as observed with IRIS. The data with high signal to noise ratio in Si IV, C II and Mg II k lines and with strong emission intensities show a high correlation in TR bright network points. The results of the IRIS intensity maps and dopplergrams are compared with those of Atmo- spheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) instruments onboard the Solar Dynamical Observatory (SDO). The average network intensity profiles show a strong correlation with AIA coronal channels. Furthermore, we applied simultaneous observations of magnetic network from HMI and found a strong relationship between the network bright points in all levels of the solar atmosphere. These features in network elements exhibited high doppler velocity regions and large mag- netic signatures. A dominative fraction of corona bright points emission, accompanied by the magnetic origins in photosphere, suggest that magnetic-field concentrations in the network rosettes could help couple between inner and outer solar atmosphere.

Original Article: http://arxiv.org/abs/1801.01307

# Solar Flares and the Axion Quark Nugget Dark Matter Model

Ariel Zhitnitsky

We advocate the idea that the nanoflares conjectured by Parker long ago to resolve the corona heating problem, may also trigger the larger solar flares. The arguments are based on the model where emission of extreme ultra violet (EUV) radiation and soft x-rays from the Sun are powered externally by incident dark matter particles within the Axion Quark Nugget (AQN) Dark Matter Model. The corresponding annihilation events of the AQNs with the solar material are identified with nanoflares. This model gives a very reasonable intensity of EUV radiation without adjustments of any parameters in the model. When the same nuggets enter the regions with high magnetic field they serve as the triggers igniting the magnetic reconnections which eventually may lead to much larger flares. Technically, the magnetic reconnection is ignited due to the shock wave which inevitably develops when the dark matter nuggets enter the solar atmosphere with velocity $v\sim 10^{-3}c$ which is much higher than the speed of sound $c_s$, such that the Mach number $M=v/c_s\gg 1$. These shock waves generate very strong and very short impulses expressed in terms of pressure $\Delta p/p\sim M^2$ and temperature $\Delta T/T\sim M^2$ in vicinity of (would be) magnetic reconnection area. We find that this mechanism is consistent with x -ray observations as well as with observed jet like morphology of the initial stage of the flares. The mechanism is also consistent with the observed scaling of the flare distribution $dN\sim W^{-\alpha}dW$ as a function of the flare’s energy $W$. We also speculate that the same nuggets may trigger the sunquakes which are known to be correlated with large flares.

Original Article: http://arxiv.org/abs/1801.01509

# Solar polarimetry in the K I $D_2$ line: A novel possibility for a stratospheric balloon

Of the two solar lines, K I $D_1$ and $D_2$, almost all attention so far has been devoted to the $D_1$ line, as $D_2$ is severely affected by an O$_2$ atmospheric band. This, however, makes the latter appealing for balloon and space observations from above (most of) the Earth’s atmosphere. We estimate the residual effect of the O$_2$ band on the K I $D_2$ line at altitudes typical for stratospheric balloons. Our aim is to study the feasibility of observing the 770 nm window. Specifically, this paper serves as a preparation for the third flight of the Sunrise balloon-borne observatory. The results indicate that the absorption by O$_2$ is still present, albeit much weaker, at the expected balloon altitude. We applied the obtained O$_2$ transmittance to K I $D_2$ synthetic polarimetric spectra and found that in the absence of line-of-sight motions, the residual O$_2$ has a negligible effect on the K I $D_2$ line. On the other hand, for Doppler-shifted K I $D_2$ data, the residual O$_2$ might alter the shape of the Stokes profiles. However, the residual O$_2$ absorption is sufficiently weak at stratospheric levels that it can be divided out if appropriate measurements are made, something that is impossible at ground level. Therefore, for the first time with Sunrise III, we will be able to perform polarimetric observations of the K I $D_2$ line and, consequently, we will have improved access to the thermodynamics and magnetic properties of the upper photosphere from observations of the K I lines.

Original Article: http://arxiv.org/abs/1801.01655

# Spatial Inhomogeneity In Solar Faculae

In this paper, we investigate the inhomogeneous spatial distribution of solar faculae. The focus is on the latitudinal and longitudinal distributions of these highly localised features covering ubiquitously the solar surface. The statistical analysis is based on white light observations of the Solar and Heliospheric Observatory (SOHO) and Solar Dynamics Observatory (SDO) between 1996 and 2014. We found that the fine structure of the latitudinal distribution of faculae displays a quasi-biennial oscillatory pattern. Furthermore, the longitudinal distribution of photospheric solar faculae does not show homogeneous behaviour either. In particular, the non-axisymmetric behaviour of these events show similar properties as that of the active longitude (AL) found in the distribution of sunspots. Our results, preliminary though, may provide a valuable observational constrain for developing the next-generation solar dynamo model.

Original Article: http://arxiv.org/abs/1801.01716

# Ghost Images in Helioseismic Holography? Toy Models in a Uniform Medium

Dan Yang

Helioseismic holography is a powerful technique used to probe the solar interior based on estimations of the 3D wavefield. Porter–Bojarski holography, which is a well-established method used in acoustics to recover sources and scatterers in 3D, is also an estimation of the wavefield, and hence it has the potential to be applied to helioseismology. Here we present a proof of concept study, where we compare helioseismic holography and Porter–Bojarski holography under the assumption that the waves propagate in a homogeneous medium. We consider the problem of locating a point source of wave excitation inside a sphere. Under these assumptions, we find that the two imaging methods have the same capability of locating the source, with the exception that helioseismic holography suffers from "ghost images" (i.e., artificial peaks away from the source location). We conclude that Porter–Bojarski holography may improve the current method used in helioseismology.

Original Article: http://arxiv.org/abs/1801.01759
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