The X-Ray Emission Spectrum of a Solar Active Region

Abstract

Absolute intensities are presented foi twenty-eight identified and four unidentified solar X-ray emis- sion lines between 11 and 22 A measured during a Skylark rocket flight on May 5, 1966 It is found that the total line emission exceeds the X-ray continuum by at least a factor of 5, and this is shown to be con- sistent with an emission model derived from the observed line spectra. Excellent agreement is found be- tween the total flux and a simultaneous observation obtained with a broad-band (8-20 A) ion chamber of the NRL SOLRAD satellite series. Of the measured emission lines, those of 0 vii are distinct, both in their observed profiles and in consideration of the coronal temperatures at which they are efficiently produced. These lines are interpreted as arising principally from the general coronal disk at a temperature between 106 and 1.5 X 106 ° K. In contrast, all the other observed lines show a strong component clearly associated with a coronal active region, and the absolute line intensities are used here to derive a model of this region Good agreement of the calculated and observed spectra of 0 viii, neon, iron, and nickel is obtained with the bulk of the active region plasma at a temperature of 3 X 106 ° K The corresponding value of N2V = 1 7 X 1048 cm3 is consistent both with the non-observation of a solar continuum below 14 A and with the separate values of Ne2 and V obtained for similar active regions from recent broad- band X-ray photographs Element abundances for neon, iron, and nickel are determined relative to oxygen and are found to be in good general agreement with values obtained from analyses of solar ultra- violet spectra