Feb 24 2010
During the recent solar activity minimum, the Sun was the dimmest X-ray star within 23 light years of its neighborhood - this is suggested by measurements collected by scientists at the Space Research Centre (SRC) of the Polish Academy of Sciences.
The measurements were made using the Polish-built super-sensitive instrument SphinX located aboard the solar orbiting observatory CORONAS-Photon.
The scientists from the SRC were fortunate enough to make some unique observations of the Sun. Using the Polish-designed spectrophotometer SphinX (Solar PHotometer IN Xrays) measuring the X-ray emission of our day-star they were the only ones in the world able to study the X-ray brightness of the Sun during the recent unusually long and deep sunspot minimum which has only just shown signs of ending. SphinX was constructed at the Solar Physics Division of the Polish Space Research Centre in Wroclaw, Poland, and was launched aboard the Russian solar orbiting observatory CORONAS-Photon early in 2009.
"Our instrument has extremely high sensitivity", explains Prof. Janusz Sylwester from the SRC. During the months of lowest solar activity in 2009, the instrument was still able to measure the solar X-ray emission at levels 20 times less than the detection thresholds of the only other X-ray detectors operating - those aboard the American GOES satellites. From February till November last year, SphinX observed the changing X-ray emission from the Sun, including tiny flares that were well below detectability by GOES.
The soft X-ray emission comes from the mega-hot plasma in the region of the solar atmosphere called the corona. The results obtained by the SphinX spectrophotometer enabled a precise measurement of the coronal plasma - 1.8 million Kelvins (degrees absolute) - for the very first time during a sunspot minimum. By comparing this measurement with data from other instruments on the CORONAS-Photon spacecraft, Prof. Sylwester's team was able to estimate the density of the coronal plasma too - about 1014 particles per cubic meter. "This result is especially interesting, since during previous sunspot minima, the coronal density was about hundred times higher", comments Prof. Sylwester.
The data also show that during the last year minimum, the Sun as seen in X-rays was the dimmest star within some 23 light years. As featured in the recent Hollywood movie Avatar, the nearby star Alpha Centauri A, widely considered to be the Sun's twin, was easily a hundred times brighter than the Sun in X-rays.
The SphinX instrument is a multi-channel spectrophotometer making fast (up to 100 Hz) precise measurements of solar X-ray emission including X-ray spectra. A new technological concept was incorporated in the design phase allowing SphinX to see the emission in a number of narrow energy bands, utilising naturally occurring absorption and fluorescence edges of different chemical elements. Each day during the operation of the CORONAS-Photon, nearly a gigabyte of data was transmitted to the ground.
The measurements made by SphinX are helping scientists to understand solar flares better - flares are key events responsible for influencing space weather. Thanks to the quality of the data collected, it will be possible to understand better the physical mechanisms involved in the heating of the hot corona and to determine more precisely its chemical composition.
Source: http://www2.cbk.waw.pl/index.php?id=80