In the Trenches of the Solar-stellar Connection. V. High-resolution Ultraviolet and X-Ray Observations of Sun-like Stars: The Curious Case of Procyon

A joint X-ray (0.2–2 keV) and ultraviolet (1150–3000 Å) time-domain study has been carried out on three nearby bright late-type stars, bracketing the Sun in properties. Alpha Cen A (HD 128620: G2 V) is a near twin to the Sun, although slightly more massive and luminous, slightly metal-rich, but older. Alpha Cen B (HD 128621: K1 V) is cooler than the Sun, somewhat less massive and lower in luminosity. Procyon (HD 61421: F5 IV–V) is hotter, more massive and more luminous than the Sun, half the age, but more evolved. Stellar observations were from Chandra X-ray Observatory and Hubble Space Telescope (HST). The Sun provided a benchmark through high-energy spectral scans from solar irradiance satellites and novel high-dispersion full-disk profiles of key UV species—Mg ii, C ii, and Si iv—from the Interface Region Imaging Spectrograph. Procyon’s flux history was strikingly constant at all wavelengths, in contrast to the other three cycling-dynamo stars. Procyon also displays a strong subcoronal (T ∼ 1 × 105 K) emission excess, relative to chromospheric Mg ii (T ≲ 104 K), although its X-rays (T ∼ 2 MK) appear to be more normal. At the same time, the odd sub-Gaussian shapes, and redshifts, of the subgiant’s “hot lines” (such as Si iv and C iv) are remarkably similar to the solar counterparts (and α Cen AB). This suggests a Sun-like origin, namely a supergranulation network supplied by magnetic flux from a noncycling “local dynamo.”

Simultaneous ALMA–Hinode–IRIS Observations on Footpoint Signatures of a Soft X-Ray Loop-like Microflare

Microflares have been considered to be among the major energy input sources to form active solar corona. To investigate the response of the low atmosphere to events, we conducted an Atacama Large Millimeter/submillimeter Array (ALMA) observation at 3 mm, coordinated with Interface Region Imaging Spectrograph (IRIS) and Hinode observations, on 2017 March 19. During the observations, a soft X-ray loop-type microflare (active region transient brightening) was captured using the Hinode X-ray telescope in high temporal cadence. A brightening loop footpoint is located within narrow fields of view of ALMA, IRIS slit-jaw imager, and Hinode spectropolarimeter. Counterparts of the microflare at the footpoint were detected in Si iv and ALMA images, while the counterparts were less apparent in C ii and Mg ii k images. Their impulsive time profiles exhibit the Neupert effect pertaining to soft X-ray intensity evolution. The magnitude of thermal energy measured using ALMA was approximately 100 times smaller than that measured in the corona. These results suggest that impulsive counterparts can be detected in the transition region and upper chromosphere, where the plasma is thermally heated via impinging nonthermal particles. Our energy evaluation indicates a deficit of accelerated particles that impinge the footpoints for a small class of soft X-ray microflares. The footpoint counterparts consist of several brightening kernels, all of which are located in weak (void) magnetic areas formed in patchy distribution of strong magnetic flux at the photospheric level. The kernels provide a conceptual image in which the transient energy release occurs at multiple locations on the sheaths of magnetic flux bundles in the corona.

Properties of the C ii 1334 Å Line in Coronal Hole and Quiet Sun as Observed by IRIS

Coronal holes (CHs) have subdued intensity and net blueshifts when compared to the quiet Sun (QS) at coronal temperatures. At transition region temperatures, such differences are obtained for regions with identical absolute photospheric magnetic flux density (∣B∣). In this work, we use spectroscopic measurements of the C ii 1334 Å line from the Interface Region Imaging Spectrograph, formed at chromospheric temperatures, to investigate the intensity, Doppler shift, line width, skew, and excess kurtosis variations with ∣B∣. We find the intensity, Doppler shift, and linewidths to increase with ∣B∣ for CHs and QS. The CHs show deficit in intensity and excess total widths over QS for regions with identical ∣B∣. For pixels with only upflows, CHs show excess upflows over QS, while for pixels with only downflows, CHs show excess downflows over QS that cease to exist at ∣B∣ ≤ 40. Finally, the spectral profiles are found to be more skewed and flatter than a Gaussian, with no difference between CHs and QS. These results are important in understanding the heating of the atmosphere in CH and QS, including solar wind formation, and provide further constraints on the modeling of the solar atmosphere.

Constraining the secular variation of the gravitational constant using white-dwarf stars spectra

Context Astrophysical observations play a critical role in the possibility of variations in fundamental physical constants. One of the ways of probing these variations would be based on the evolution of the white-dwarf stars. Aims We use the spectrum of white-dwarf star G191-B2B to find an upper limit on the possible deviation of the gravitational constant with strong gravitational fields. Methods We analyze archive observation of the Hubble Space Telescope Imaging Spectrograph (HSTIS) to determine the possible cosmological deviation of the gravitational constant from the observed gravitational redshift. Results Our analysis provided a strong estimate on an upper bound on the possible space-time variation of the gravitational constant ̇⁄ = (0.238 ± 2.959) × 10 yr comparing with previous results. Conclusions The obtained result in this study offers the possibility of testing parameters of modern unification theories.

A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS)

The Interface Region Imaging Spectrograph (IRIS) has been obtaining near- and far-ultraviolet images and spectra of the solar atmosphere since July 2013. IRIS is the highest resolution observatory to provide seamless coverage of spectra and images from the photosphere into the low corona. The unique combination of near- and far-ultraviolet spectra and images at sub-arcsecond resolution and high cadence allows the tracing of mass and energy through the critical interface between the 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 waves, the acceleration of non-thermal particles, and various small-scale instabilities. IRIS has provided insights into a wide range of phenomena including the discovery of non-thermal particles in coronal nano-flares, the formation and impact of spicules and other jets, resonant absorption and dissipation of Alfvénic waves, energy release and jet-like dynamics associated with braiding of magnetic-field lines, the role of turbulence and the tearing-mode instability in reconnection, 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 various other mechanisms in triggering and driving CMEs. IRIS observations have also been used to elucidate the physical mechanisms driving the solar irradiance that impacts Earth’s upper atmosphere, and the connections between solar and stellar physics. Advances in numerical modeling, inversion codes, and machine-learning techniques have played a key role. With the advent of exciting new instrumentation both on the ground, e.g. the Daniel K. Inouye Solar Telescope (DKIST) and the Atacama Large Millimeter/submillimeter Array (ALMA), and space-based, e.g. the Parker Solar Probe and the Solar Orbiter, we aim to review new insights based on IRIS observations or related modeling, and highlight some of the outstanding challenges.

All that is known about Mars discrete aurorae so far

<p>The discrete aurorae on Mars were discovered with the SPICAM spectrograph on board Mars Express. Now, they have been analyzed in detail using the much more sensitive MAVEN/IUVS imaging spectrograph.</p><p>This presentation gives a summary of the very latest results obtained by Schneider et al. and Soret et al. on this topic.</p><p>The main conclusions are the following:</p><ul><li>the number of auroral event detections has considerably increased since the Mars Express observations;</li> <li>many detections have been made outside of the Southern crustal magnetic field structures;</li> <li>the MUV spectrum shows the same emissions as those observed in the dayglow, with similar intensity ratios;</li> <li>the Vegard-Kaplan bands of N<sub>2</sub> have been observed for the first time in the Martian aurora;</li> <li>the CO Cameron and the CO<sub>2</sub><sup>+</sup> UVD emissions occur at the same altitude;</li> <li>the OI emission at 297.2 nm has been analyzed;</li> <li>the CO Cameron/CO<sub>2</sub><sup>+</sup> UVD ratio is quasi-constant;</li> <li>intensities are higher in B-field regions;</li> <li>auroral emissions are more frequent in the pre-midnight sector;</li> <li>the altitude of the emission layer is independent of local time and presence or absence of a crustal magnetic field;</li> <li>the altitude of the emission layer varies moderately with season (atmospheric effect);</li> <li>the events are spatially correlated with an increase in the flux of energetic electrons simultaneously measured by the MAVEN/SWEA (Solar Wind Electron Analyzer) detectors;</li> <li>the peak altitude of the emission is in good agreement with that expected from the average electron energy.</li> </ul>