Steven H. Saar

The Interface Region Imaging Spectrograph (IRIS)

The Interface Region Imaging Spectrograph (IRIS) small explorer spacecraft provides simultaneous spectra and images of the photosphere, chromosphere, transition region, and corona with 0.33u2009–u20090.4xa0arcsec spatial resolution, two-second temporal resolution, and 1xa0kmu2009s−1 velocity resolution over a field-of-view of up to 175xa0arcsec × 175xa0arcsec. IRIS was launched into a Sun-synchronous orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a 19-cm UV telescope that feeds a slit-based dual-bandpass imaging spectrograph. IRIS obtains spectra in passbands from 1332u2009–u20091358xa0Å, 1389u2009–u20091407xa0Å, and 2783u2009–u20092834xa0Å, including bright spectral lines formed in the chromosphere (Mgxa0ii h 2803xa0Å and Mgxa0ii k 2796xa0Å) and transition region (Cxa0ii 1334/1335xa0Å and Sixa0iv 1394/1403xa0Å). Slit-jaw images in four different passbands (Cxa0ii 1330, Sixa0iv 1400, Mgxa0ii k 2796, and Mgxa0ii wing 2830xa0Å) can be taken simultaneously with spectral rasters that sample regions up to 130xa0arcsec × 175xa0arcsec at a variety of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to emission from plasma at temperatures between 5000 K and 10 MK and will advance our understanding of the flow of mass and energy through an interface region, formed by the chromosphere and transition region, between the photosphere and corona. This highly structured and dynamic region not only acts as the conduit of all mass and energy feeding into the corona and solar wind, it also requires an order of magnitude more energy to heat than the corona and solar wind combined. The IRIS investigation includes a strong numerical modeling component based on advanced radiative–MHD codes to facilitate interpretation of observations of this complex region. Approximately eight Gbytes of data (after compression) are acquired by IRIS each day and made available for unrestricted use within a few days of the observation.

Recent Measurements of Stellar Magnetic Fields

I review recent measurements of magnetic fields on two interesting cool stars: a nearly pole-on BY Dra variable (BD +26°730) and a (possibly) very young K dwarf (HD 17925). These data are combined with previous measurements to investigate correlations between magnetic, chromospheric and coronal fluxes. Implications for atmospheric heating are briefly discussed.

Calibrating Data from the Hinode /X-Ray Telescope and Associated Uncertainties

The X-Ray Telescope (XRT) onboard the Hinode satellite, launched 23 September 2006 by the Japan Aerospace Exploration Agency (JAXA), is a joint mission of Japan, the United States, and the United Kingdom to study the solar corona. In particular, XRT was designed to study solar plasmas with temperatures between 1 and 10 MK with ≈u20091″ pixels (≈u20092″ resolution). Prior to analysis, the data product from this instrument must be properly calibrated and data values quantified to accurately assess the information contained within. We present here the standard methods of calibration for these data. The calibration was performed on an empirical basis that uses the least complicated correction that accurately describes the data while suppressing spurious features. By analyzing the uncertainties remaining in the data after calibration, we conclude that the procedure is successful, because the remaining uncertainty after calibration is dominated by photon noise. This calibration software is available in the SolarSoft software library.

The Magnetic Fields on Cool Stars and Their Correlation with Chromospheric and Coronal Emission

I discuss the results of recent measurements of magnetic fields on cool stars and how these measurements relate to stellar “activity”. Special emphasis is given to the correlations between the magnetic field strength, surface filling factor, and the nonthermal emission from the hot outer atmospheres of these stars.

Stars in magnetic grand minima: where are they and what are they like?

We explore various ideas of what a star in a Maunder-like magnetic minimum would look like, and ways of finding stars in such a state, and make some estimates of their physical and magnetic activity properties. We discuss new X-ray observations of a small selection of candidates for being in magnetic grand minima. These are then compared with the Sun and other low activity stars.