Julia L. R. Saba

Coronal dynamics of a quiescent active region

Post-SMM repair Flat Crystal Spectrometer (FCS) observations of coronal X-ray line broadening in a nonflaring active region are discussed. The resulting map of FCS line profiles across the central X-ray bright portion of a large active region shows a clear variation in line width for the Mg XI resonance line at 9.17 A, but no evidnece of line shifts. Line ratios yield T(e) of about 3 {times} 10 to the 6th K within 10 percent for most of the map. The brightest, broadest profiles are consistent with emission from the tops of loops overlying the magnetic neutral line. For the spatially averaged Mg XI resonance line profile, the excess velocity v(ex) corresponding to the excess width above the thermal value equals the thermal velocity for Mg ions, 45 km/s; for some individual bright pixels v(ex) is 60 km/s or more. The observed coronal line broadening is interpreted as a signature of the mechanisms responsible for coronal heating in active regions. 36 refs.

Flare heating and ionization of the low solar chromosphere. II - Observations of five solar flares

Two neutral Mg spectral lines formed in the temperature-minimum region and the low chromosphere, at 4571 and 5173 A, are used to quantify the changes in the atmospheric structure as a function of time during five solar flares. Eight proposed flare heating and ionization mechanisms and predictions of the effects of each on the temperature minimum region are discussed. Two Mg spectral observations made at the National Solar Observatory (Sacramento Peak), along with observations of hard and soft X-rays from the SMM and GOES satellites, are compared to the predictions of the eight proposed mechanisms. The initial effects in all five flares are consistent with backwarming by enhanced Balmer- and Paschen-continuum radiation originating in the upper chromosphere. Extended heating observed in two of the flares is most likely due to UV irradiation. In all cases heating by the dissipation of nonreversed electric currents, collisions with an electron or proton beam, irradiation by soft X-rays, and dissipation of Alfven waves are eliminated. 61 refs.

Understanding Space Weather: The Sun as a Variable Star

The American Meteorological Society has recently adopted space weather as a new core competency. This is the first in a series of papers discussing the multidisciplinary aspects of space weather. This paper concerns the physics behind solar variability, the driver of space weather. We follow the tortuous journey of the energy from its production in the solar core until it escapes into interplanetary space, showing how the internal dynamics and structure of the Sun change its nature. We show how the production and dissipation of magnetic fields are a key clue to untangling the riddle of the sunspot cycle and how that, in turn, affects the amount of radiation that the Earth receives from the Sun—the total solar irradiance.

Understanding Space Weather: Part II: The Violent Sun

AbstractThe Sun is often racked by short-term violent events such as flares and coronal mass ejections (CMEs) but these two phenomena are often confused. Both are caused by the release of energy due to the reconnection of stressed and unstable magnetic fields. Flares bathe the solar system in electromagnetic radiation from gamma rays to radio emissions. CMEs throw billions of tons of solar plasma into interplanetary space at velocities of over 1,000 km s−1. Flares can occur without significant ejecta being spewed out from the Sun into the solar system. CMEs can occur without a significant flare being detected. The most violent and dangerous events occur when a large flare is accompanied by a major eruption. These violent events are much more common near solar maximum but can occur at any time during the solar cycle, so we are rarely completely immune to their effects. Various types of solar activity can lead to problems with electrical grids, navigation systems, and communications, and can present a hazar...

Understanding Space Weather: Part III: The Sun’s Domain

CapsuleThe solar wind is a continuous, varying outflow of high-temperature plasma, travelling at hundreds of km s-1 and stretching to over 100 AU from the Sun.