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== Solar Hard X-ray Albedo ==
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This page describes the steps necessary to create and edit your own article.  This information is generally valid for creating any kind of article on this wiki but may be particularly useful to new Nugget authors.
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In the early days (1972) of solar hard X-ray flare observations,
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== Creating an Article ==
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[http://adsabs.harvard.edu/abs/1972ApJ...171..377T] Fred
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All pages on this wiki are articles including the RHESSI science nuggets. In order to create a new article a title must first be chosen. The title of an article cannot be changed so choose your article title carefully! It is possible to move the content from one article to another with a different title but this is not a recommended practiceThere are two main ways to create an article.
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Tomblin published theoretical arguments
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that the hard X-ray spectrum of solar flares in the 5-40 keV
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range must have an albedo component due to Compton back-scattering
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in the photosphere of those primary bremsstrahlung
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photons that are emitted downward. In a more complete
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analysis, [http://adsabs.harvard.edu/abs/1978ApJ...219..705B]
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Taeil Bai & Reuven Ramaty showed that this albedo component would be polarized and its size must depend on the height of the primary source.
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The "reflected" photons form what is called an albedo patch.
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*Simply search for the title of the article you wish to create using the search bar on the left.  The wiki should confirm that no other articles have this title by not finding it and will give a choice to create this article. Click the link (at "you searched for") and your new article will open in edit mode.
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For sufficiently high primary source altitudes, the albedo would
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be much larger in extent than the primary source, with a size
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scale that increases with source height. (See Fig. 1 below.)  
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Furthermore, the albedo source would be displaced toward
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disk center by a distance h sin θ,
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where θ is the heliocentric angle.  
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*Articles are organized through categories.  An article is added to a category by adding meta-data into the article text.  The next method for creating an article is to navigate to the category page and use the Form to create a new article in that category.  The RHESSI Science Nugget category page can be found [[:Category:Nugget|here]].  The category meta-data will then automatically be included in the article text.
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<h3> Why albedo has not been imaged before </h3>
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== Writing your article ==
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A significant fraction (possibly as high as 40%) of the X-ray flux from solar flares comes from X-rays that propagate back to the solar surface from coronal sources and "reflect" off the photosphere. This component of flares is called the <i>albedo</i>, and it is remarkably difficult to observe because it is very diffuse with an intensity that is one or two
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orders of magnitude smaller than the primary flare sources themselves.
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Its importance for flare physics is that it both distorts the spectral interpretation of X-ray emission and offers a potentially powerful diagnostic of electrons accelerated in solar flares. Our study uses the unique capabilities of the Ramaty High Energy Spectroscopic Imager (RHESSI) to isolate this albedo component, determine its properties such as size, shape and centroid location as a function of energy. We have focused on single-component flares in the 12-30 keV range that appear a within 45&deg; of disk center. Using standard techniques, we have obtained the X-ray visibilities
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[http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/RHESSI_Visibilities] {RHESSI Nugget # 39) of a number of such flares and applied Forward-Fitting methods to determine the parameters of the primary component (position, flux, and size) and the albedo-related parameters (primary source height and albedo flux). 
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Writing a wiki article is very straightforwardEvery article has an "edit" tab above itClicking that will open in the article in an editable windowMany formatting shortcuts can be found above the editing window. Since each article on this wiki can be edited, it is easy to learn by example. For more basic help with editing read the following [[Help:Editing|article]].
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<h3>Sensitivity to source size</h3>
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The modulation of RHESSI's count rates depends exquisitely on the grid pitch and the source size, a fundamental property of Fourier imaging.
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When the source FWHM is less than the subcollimator angular
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resolution, the modulation amplitude falls rapidly. This is illustrated below
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for three subcollimators with angular resolutions of 23'', 69'' and 217''. <br>
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== Previous attempts to infer albedo properties ==
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<ul>
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  <li> '''Statistical center-to-limb variations'''
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  [http://arxiv.org/abs/astro-ph/0701871]
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  Jana Kasparova, Eduard Kontar & John Brown
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  demonstrated a center-to-limb  variation of photon spectral
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  indices in the 15-20 keV energy range and a weaker
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  dependency  in the 20-50 keV range, which is consistent with
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  photospheric albedo as the cause.
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  [http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=74]
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  Nugget #74 illustrates albedo's anisotropy effect on the spectrum.
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  <li> '''Spectroscopy of individual flares'''
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  [http://www.iop.org/EJ/article/1538-4357/653/2/L149/21151.html]
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  Eduard Kontar and John Brown  analyzed the 2002/08/
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  20 and 2005/ 01/17 flares  in terms of double-power-law fits.
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  To fit the HXR spectrum with a low-energy cutoff <math>E<sub>c</sub></math> and ignoring albedo  requires  an unusually high value of <math>E<sub>c</sub></math>< &#126; 30 &#177; 2 keV. 
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  This produces a clear gap in the range E = 15 to 30 keV, which is 
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  likely to be unphysical and suggests that albedo is important.
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  A related [http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=42] nugget #42 shows how the albedo "mirrors" the primary flux.
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  <li> '''Fourier methods'''
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  The above statistical and spectral methods give no information
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  about the spatial characteristics of albedo patches.   The only
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  hope for getting such spatial information is by using  the
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  Fourier amplitudes and phases determined by RHESSI.  In 2002,
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  [http://adsabs.harvard.edu/abs/2002SoPh..210..273S] the authors made a first step towards this by assuming circular symmetry. It is now possible
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  to go beyond this, at least for some flares.
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</ul>
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== Full Exploitation of Fourier methods ==
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We have found 9 flares with reliable enough amplitudes and phases to compare models of simple sources with
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albedo patches. The flares all lie within 45&deg; of sun center, where albedo is expected to be strong. These flares form a small subset of RHESSI events for which MEM maps show only single, compact primary components in the range 12 to 30 keV.
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<h3>Forward Fitting</h3>
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Given the RHESSI amplitudes and phases for a flare, one may compare the observed values with those computed
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from a model. This process is called "Forward Fitting". At the present time, this method is reliable for computing albedo patch parameters only for single elliptical primary  sources because the number of parameters  (8 in this case) must be much smaller than the number of amplitudes. (For a discussion of Forward Fitting of amplitudes and phases, see
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[http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=35] RHESSI Nugget #35)
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<h3>Amplitude model of a primary source both with and without albedo</h3>
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Modeling the albedo patch in addition to the primary source makes it possible to infer the height of the primary source, and the fraction of the total flux emitted by the reflected photons. Here we show an example of a flare
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where the primary source amplitudes (blue crosses) are fit by a 6-parameter model (flux, position, ellipse FWHMs and orientation) with an albedo patch fit by 2 parameters (primary height and albedo fraction). For comparison we show a fit made for a primary source without albedo.
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<h3> Model Albedo Visibility Back-Projection</h3>
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In those cases where the primary source and albedo patch are both well represented by a model, it is possible to display both sources using back projection. We have done this for a number of flares, with one example "bpmap" here (for the flare and band of Fig. 3.)
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We have visualized the albedo patch by back-projecting our Forward-Fit model of the best-fit albedo parameters.  If the back-scattering
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process is isotropic (as we assume in the Forward-fit model), the
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albedo patch is displaced toward sun center from the projected
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location of the primary source, such that it is vertically below the
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primary source.
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== Amplitude Fits ==
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Top row: Amplitudes vs subcollimator (SC) and grid position angle (PA) for flare models including back-scattered emission (solid) and without (dashed). The axes in these figures are the same as for Fig. 3. The black dashed curve is the difference between the models with and without albedo. Much of the errors in fitting are due to inter-detector calibration errors. Although these can be compensated for, they introduce uncertainties in the inferred
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albedo fraction and the source height.
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Bottom row: Back-projection maps for 15-20 and 20-30 keV for the flare above.
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As in Fig. 4, the arrows point from map center to Sun center. In each case, the model albedo patch was back-projected from a Fourier plane with uniformly spaced roll bins.
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== Conclusions ==
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Using Fourier amplitudes and phases for nine simple (single primary,
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slowly varying) flares we have found evidence for X-rays back-
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scattered from the photosphere (the albedo patch).  Note that these
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results are only for the so-called "thermal phase" when a single
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component dominates the emission.
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We have visualized the albedo patch by back-projecting our Forward-Fit model of the best-fit albedo parameters.  If the back-scattering
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process is isotropic (as we assume in the Forward-fit model), the
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albedo patch is displaced toward sun center from the projected
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location of the primary source, such that it is vertically below the
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primary source.
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<h3> We make several inferences from our results for 9 flares:</h3>
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<ul>
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<li>The model fits of the amplitudes and phases to the observations
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    are significantly better when back-scattered (albedo) emission is included than if it is not. (Compare the dashed and solid red curves in Figurs 3 & 5.)
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<li> The albedo fraction increases with energy in the range 12-30 keV,
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    in qualitative agreement with theory (Bai and Ramaty 1978).
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<li> We have inferred primary heights ranging from about 10 to 30
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    Mm, in agreement with the range seen in limb flare observations.
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<li> In any given flare, the heights of the primary source determined
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    by Forward Fitting do not significantly change with energy,
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    consistent with a thermal interpretation, and also consistent with
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    the single-component nature of our flares.
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<li> Relative detector-to-detector responses affect these results, and
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    improved calibration would improve our albedo measurements
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    significantly.
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<li> Extension of these results to some 2-component ("footpoint")
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    flares may be possible, and this would have great significance for
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    spectral work.
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</ul>
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== Acknowledgements ==
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The RHESSI software team has given invaluable help in making visibility software available to the community. Without their continued support, this research would have been impossible.
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Biographical notes: Ed Schmahl is a retired University of Maryland and GSFC scientist, currently employed at NWRA/CoRA and Gordon Hurford is a senior RHESSI team member based at UC Berkeley.
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=== Adding Figures ===
=== Adding Figures ===
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Writing as though for a newspaper, rather than as for a boring archival journal, would be best - there is no need really to have complete literature  
Writing as though for a newspaper, rather than as for a boring archival journal, would be best - there is no need really to have complete literature  
citation, since those ''really'' knowledgeable about the field will certainly know where to go (ADS).
citation, since those ''really'' knowledgeable about the field will certainly know where to go (ADS).
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==Meta data==
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In order to provide better tools to organize and search for nugget topics, we encourage authors to use the following meta data tags in their articles.
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* To note that an article discusses a particular event use the following <pre>[[Has event date:: November 09, 2002 13:12]]</pre>
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* To denote that an article makes use of data from a particular observatory add <pre>[[Has observation by:: EIT| ]]</pre> at the bottom of the article.
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* Subject topics should also be added. The syntax for that is <pre>[[Has article subject:: theory]]</pre>.
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The following subjects are available.
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* theory
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* educational
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* microflares
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* CMEs
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== Source Code ==
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If you'd like to add actual source code in your article please use the following syntax.
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<source lang="csharp">
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// Hello World in Microsoft C# ("C-Sharp").
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using System;
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class HelloWorld
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{
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    public static int Main(String[] args)
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    {
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        Console.WriteLine("Hello, World!");
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        return 0;
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    }
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}
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</source>

Latest revision as of 02:15, 18 September 2018

This page describes the steps necessary to create and edit your own article. This information is generally valid for creating any kind of article on this wiki but may be particularly useful to new Nugget authors.

Contents

Creating an Article

All pages on this wiki are articles including the RHESSI science nuggets. In order to create a new article a title must first be chosen. The title of an article cannot be changed so choose your article title carefully! It is possible to move the content from one article to another with a different title but this is not a recommended practice. There are two main ways to create an article.

Writing your article

Writing a wiki article is very straightforward. Every article has an "edit" tab above it. Clicking that will open in the article in an editable window. Many formatting shortcuts can be found above the editing window. Since each article on this wiki can be edited, it is easy to learn by example. For more basic help with editing read the following article.

Adding Figures

In order to add a figure it is first necessary to upload the image to the wiki. This can be done by using the link on the left titled "Upload File". Please use a descriptive name for the file and add a description of its contents. Once uploaded the file can be added to articles using the following command

[[Image:POF.jpg|200px|thumb|right|'''Figure 1''': This is the figure caption.]]

whose results can be seen to the right. Most of these parameters should be self explanatory. The "thumb" parameters tells the wiki that this is a figure and therefore displays the figure caption. The float parameters (in this case set to "right") tells the wiki where to float the image. Possibilities include "left", "center", or "right".

Figure 1: This is the figure caption. Note that the figure has properlyy appeared on the right, but that you can't really control how the text will wrap around it - that depends on the browser.

If you'd like to experiment with more advanced methods read this article on Wikipedia's website.

Style and content

We try to write the Nuggets so that a technically literate reader won't be baffled. That means that the text should appear in plain English, and that jargon is a no-no. Many scientists don't realize they are writing gibberish so please be careful about this. On the plus side, you can freely use cgs units if you wish. Also please be aware that many of the audience are not native English speakers, so phrases like "X must have been a gutsy operator to have diagonalized those macrospicules" would not do so well. Writing as though for a newspaper, rather than as for a boring archival journal, would be best - there is no need really to have complete literature citation, since those really knowledgeable about the field will certainly know where to go (ADS).

Meta data

In order to provide better tools to organize and search for nugget topics, we encourage authors to use the following meta data tags in their articles.

The following subjects are available.


Source Code

If you'd like to add actual source code in your article please use the following syntax.

// Hello World in Microsoft C# ("C-Sharp").
 
using System;
 
class HelloWorld
{
    public static int Main(String[] args)
    {
        Console.WriteLine("Hello, World!");
        return 0;
    }
}
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