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Associated Research Papers

ApJL_HIs_Lugaz_et_al_2008.pdf - Lugaz et al., The Brightness of Density Structures at Large Solar Elongation Angles: What is Being Observed by STEREO/SECCHI?

JASTP_70_2008_Lugaz_final.pdf - Lugaz et al., Observational Evidence of CMEs Interacting in the Inner Heliosphere Based on MHD Simulations, Journal of Atmosphere and Solar-Terrestrial Physics

ApJ_627_2005_Lugaz.web.pdf - Lugaz et al., The evolution of Coronal Mass Ejection Density Structures

 

    Research

    Modeling of the Sun in Various Filters

    Going about our daily life it is very easy to think of the sun as an unchanging object of the heavens. However, the Sun looks very different depending on where in the electromagnetic  spectrum you view it. For example, in the set of images of the sun on the same day below from the SOHO satellite one can see that even though there is no visible structure (at this scale) on the photosphere (bottom left), the ultraviolet images (top row) show a wealth of detail.

    To see NASA's daily update of the sun in various filters, visit this website: http://sohowww.nascom.nasa.gov/data/realtime/realtime-update.html

    In particular, the outermost layer of the solar atmosphere, the corona, is quite different from the layers below. Because of its extreme temperature and relatively low density, visible light scattered by the corona is overwhelmed by light from the photosphere and becomes difficult to see without specialized instruments. This is usually be overcome in two primary ways: a) by blocking out the bright light from the solar disk and observing off the edge of the sun, or b) by observing at shorter wavelengths such as the EUV and soft X-ray where there is little photospheric intensity. Both of these methods are complimentary in the information they provide about the conditions of the corona (see composite image below).

    See more images like that above at the following link: Find more at http://mlso.hao.ucar.edu/cgi-bin/mlso_homepage.cgi

    A coronagraph creates its images by blocking out the direct radiation from the solar disk and looking off the limb of the sun. As a result, these instruments are able to observe the faint signature of photospheric light scattered off of the corona (in a sense recreating a solar eclipse). Because the amount of scattered light is proportional to the density of the scattering material, this method allows for imaging the corona out to great distances, giving the ability to track dynamic structures as they traverse the solar system. Coronagraphs have been a primary tool for studying the sun for many decades and a large variety are used at the cutting edge of Solar Physics, here some existing instruments:

    LASCO (part of SOHO mission http://sohowww.nascom.nasa.gov/ )

    Cor1, Cor2, HI ( part of the STEREO mission http://secchi.nrl.navy.mil/)

    Solar-C – an of axis coronagraph and spectro-polarimeter  http://www.solar.ifa.hawaii.edu/SolarC/index.html

    As for direct imaging, the corona naturally shines via intense emission lines in the EUV and soft X-ray range (1-1000A). Three common wavelength bands used for imaging the corona from space are centered on the Iron lines at 171, 195 and 284Å (Fe IX, Fe XII, and Fe XV respectively). Because of their high excitation energy, only the corona at its million degree temperatures can excite these lines and thus outshines the photosphere in this wavelength regime. Additionally, since the sources of emission lines in the corona come from various elements and ionization stages, each line has a unique temperature and density dependence. For example, the 171, 195 and 284Å lines are indicative of material at approximately 1, 1.4  and 2 million degrees respectively. The combination of observing a few lines over the entire sun (imaging) and many lines in specific regions (spectroscopy) can thus provide a wealth of information about the temperature structure and dynamics of the corona.  A key feature of observing at these wavelengths is that corona over the disk of the sun can be imaged directly, unlike white light coronagraphs, and, because the emission line intensity is proportional to the square of the density, the low corona and regions of increased activity are preferentially weighted.

    EIT, CDS, SUMER (part of SOHO mission http://sohowww.nascom.nasa.gov/ )

    XRT (X-ray telescope part of Hinode Mission  http://solarb.msfc.nasa.gov/ )

    EUVI ( part of the STEREO mission http://secchi.nrl.navy.mil/)

    AIA (exciting future EUV imaging instrument to fly on SDO http://sdo.gsfc.nasa.gov/mission/about.php )

    When scientists attempt to construct models for the sun and solar corona, we must always keep in mind the wealth of information available to us through multi-wavelength observations and the constraints that they provide. With this in mind, at C2H2 we construct various ‘synthetic’ observations with our simulations that can be compared directly to actual measurements. This type of analysis not only helps us further constrain and understand models for the Corona and CME’s, but also helps in interpreting observations when the projected structures are quite complex.

    Noé Lugaz has spent the past few years producing white-light coronagraph images for both the LASCO instrument and the modern coronagraphs on the STEREO A-B satellites.

    Another current focus in modeling at C2H2 is modeling dynamic events in the low corona such as EIT waves. In order to best validate our models we extended the image synthesis to account for the EUV regime, in particular the 171, 195, and 284Å band passes aboard the SOHO spacecraft (image below). The poster presenting our results applying this method to the Aug 24 2002 CME event can be found here: (poster link).

    Part of Cooper Downs’ Ph.D. thesis project with C2H2 will involve lowering the boundary of the coronal model into the transition region to allow for a more realistic description of the thermodynamic state (density, temperature) in this regime. Once an accurate description is attained we aim to investigate the origin and dynamics of globally propagating coronal waves (also know as EUV waves) and their relation to CME events. A great description of an EUV wave observed by the STEREO satellites can be found here: ( http://secchi.nrl.navy.mil/spwx/index.php?p=20070519event  ). The comparison of synthesis observation for models of real events to the actual measurements will ideally shed light on this interesting phenomenon.