statistics of solar studies


understood how different the underlying processes are in stars with signifi cantly different stellar parameters and X-ray activity levels’[2]


. Once again, examination of


conclusions reached by inference depends upon statistical testing against all available data and examination of discrepancies, confl icts and contradictions thus uncovered. X-ray emissions are an important area


of study for a wide range of reasons, from solar physics to the effects of solar wind. Imagery and spectrometers on, for example, the British/Japanese/US satellite observatory Yokoh scan the solar disk itself, while more sensitive instruments such as those on the Chandra platform look outwards for lessons to be learnt from subtle manifestations. Recurrent acute phenomena such


as sunspots, coronal loops and mass bulk expulsions can provide particular opportunities for deriving new understanding of solar processes, with computerised statistical methods providing the key to homogenisation of disparate data into coherent models. Reale and Ciaravella[3] describe subtracting the Yohkoh image of a region immediately after disappearance of a coronal loop from the images of the loop itself, thus cleaning the data, but concede that ‘this method cannot take time-variations of the background emission into account and we cannot exclude that crossing structures vary during the observation’. Their solution ‘estimated the average time fl uctuations during the observation by computing pixel- by-pixel the standard deviation of the count rate’. Both parts of this process are realistically possible only by computerised methods. The solar winds (see Fresh blew the


wind…) continually blow through the solar system and beyond, augmented by acute events such as fl ares and coronal mass ejections, producing a continuous catalogue of effects. This is always of interest to everyone from climatologists to cosmologists, but there is currently a particular focus of attention on the effect the winds exert on Mars where they are prime suspects in the denudation of its atmosphere and water. Edberg and others, by analysis of combined Advanced Composition Explorer (ACE) and Mars Express (MEX) data, have recently[4] indicated that loss of Martian atmosphere occurs not in a steady constant stream as the wind blows by, but in differential spurts which coincide with corotating interaction regions (CIRs) and CMEs. Nilsson and others, in an Icarus paper[5]


earlier this year that well illustrates the statistical nature of such work, combine MEX ion data, proxy MGS data and scaled, time- shifted Earth position EUV solar fl ux data to


www.scientific-computing.com


A 256 by 256 arcsec coronal loop region analysed through different spectral fi lters Image source: Reale and Ciaravella [3]


build a remarkably full and detailed picture of interaction between solar wind and Martian ionosphere over considerable distances. The solar winds, particularly the slow


wind, have terrestrial effects too, from the polar auroras through disrupted telecommunications to maintenance and disruption of the ozone layer, generating data whose analysis keeps whole scientifi c computing communities in business.


Alphabet soup


ACE: Advanced Composition Explorer. A robot NASA exploration craft launched in 1994, parked at the L1 Lagrange point (about one and a half million kilometres downwind towards the sun from Earth) and expected to remain functional for about 30 years. CIR: Corotating Interaction Region. A large-scale, long lasting plasma structure produced in low and middle latitude regions of the heliosphere by interaction between a stable fast solar wind stream and the surrounding slow solar wind. CME: Coronal Mass Ejection. A mass magnetized plasma expelled from the sun, typically on the order of a billion tonnes and travelling at 700 km/s or so. ESA: European Space Agency. EUV: Extreme Ultra Violet. The short wavelength end of the ultraviolet electromagnetic band, down to the soft X-ray region.


More fun is the development of a solar


sail. This idea has been around for the whole of my lifetime but could now[6]


, according


to its Finnish-led European development group, be built to a modest operational specifi cation (one Newton thrust and 100 kilogramme mass) in the immediate future. In principle, a solar sail craft would open up all sorts of possibilities. Humans are not likely to travel this way in the near future, and the


JSA: Japanese Space Agency. MEX: Mars Express (the orbital mission, launched in 2003, which carried the ill-fated Beagle 2 lander). MDI: Michelson Doppler Imager. MGS: Mars Global Surveyor. NASA orbital survey mission which ran from 1997 to 2006. NASA: (US) National Aeronautics and Space Agency. SDO: NASA’s Solar Dynamics Observatory, launched in 2010 SDS: Solar Disk Sextant, carried by observatory balloons (see box ‘Up, up and away’). SOHO: Solar and Heliospheric Observatory. A joint cooperative project between NASA and ESA, launched in 1996 and currently approved for operation until 2012. SSM: Standard Solar Model (see separate box). SUMER: Solar Ultraviolet Measurements of Emitted Radiation.


FEBRUARY/MARCH 2011 15


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