Space Environment Center
GOES
Solar X-ray Imager
|
||||
| GOES
Solar X-ray Imager |
||||
Most data product and software development for SXI has focused on the real-time
needs of space weather forecasters. Because of SXI's great potential
to aid scientific investigations of solar processes, SEC has attempted
to ensure both data products and IDL software are compatible with SolarSoft.
This page describes the SXI solar image data products and software tools
that have research utility in addition to operational utility.
The SXI system produces image data products in two levels of processing, each of which are available in a FITS and in a browse formats. All data products follow the same file naming convention. The Level-0 data consists of raw images assembled from telemetry packets. The Level-1 data has had mimimal standard processing applied to the Level-0 data in order to generate exposure normalized images mostly free of instrument effects and artifacts. Because of the operational requirements of SEC's Forecast Center, the Level-1 data are prepared in real-time and should serve as the basic source data for research users. The annotated browse images use Portable Network Graphics (PNG) format.
Level-0 data are assembled into FITS files from telemetry packets on the SXI ingest and preprocessing system. The files consist of a primary header, a primary data array, a secondary header, and a secondary data array. The FITS file contents are summarized below and a detailed description of the Level-0 FITS format is available.
Image Data: The primary header contains virtually all information from the instrument that can be tied directly to an image. It contains 128 FITS keywords. These keywords include the common SSW keywords necessary for time and pointing. The primary data array is a 512x512 16 bit integer array containing 10 bit pixels.
HASS Data: The secondary header array contains detailed summary information taken from the High Accuracy Sun Sensor (HASS) during the image integration period. This includes max/min pointing values, average values, and variances. The secondary data array represents the unfiltered pointing information converted to degrees.
Level-0 FITS Reader: Special care should be taken when using data from the Post-Launch Test (PLT) period of September 7, 2001 through December 20, 2001. Due to updates to the Level-0 processing software, several key FITS keywords had their meanings or computations changed during this period. So inconsistencies, particularly in the pointing keywords apply. A Level-0 SXI FITS reader that corrects for these inconsistencies is available here or via the SXI SolarSoft branch. There is no plan to reprocess the Level-0 data from this period to restore consistency.
Level-1 data are created from Level-0 FITS files and a number of reference files by IDL software. The software is designed to be SolarSoft compatible for the most part and will be available as a branch download. The FITS files consist of a primary data array and header and five extension data arrays and headers. The goal was to include, in addition to the image itself, as complete information as possible on the processing history of the image. This helps operational forecasters and the software they use and should aid researchers. The FITS file contents are summarized below and a detailed description of the Level-1 FITS format is available.
Image Data: The primary header contains 69 keywords representing a subset of the Level-0 keywords and a set of new image summary keywords. The data array is a 512x512 32 bit IEEE floating point array containing exposure normalized signal (DN/s).
Saturated Pixel Data: This FITS extension contains pixel coordinate pairs indicating saturated pixels (DN=1023) or pixels immediately adjacent to saturated pixels. This should be particularly useful as compositing software is created.
Zero Pixel Data: This FITS extension contains pixel coordinate pairs indicating pixels with zero value (DN=0) in the Level-0 data array. These pixels are due to oversubtraction of a background signal in the instrument.
Missing Pixel Data: This FITS extension contains pixel coordinate pairs indicating pixels missing in Level-0 data array. Missing pixels are usually due to occasional telemetry errors and are represented in the Level-0 data as -1.
Fixed Pixel Data: This FITS extension contains pixel coordinate pairs indicating missing pixels in Level-0 data array that have been interpolated over in the Level-1 primary data array.
Median Filtered Pixel Data: This FITS extension contains pixel coordinate pairs indicating pixels that have been replaced by the median of their nearest neighbors because they exceeded a certain relative threshold. This feature is deactivated in the current version of Level-1 processing (indicated by MED_THRS=-1 in the primary header).
Level-1 FITS Reader: Special care should be taken when using data from the Post-Launch Test (PLT) period of September 7, 2001 through December 20, 2001. Due to updates to the Level-0 and Level-1 processing software, several key FITS keywords had their meanings or computations changed during this period. Though the Level-1 data released has been reprocessed, any future updates to processing will be included in updates to the Level-1 FITS reader. In addition, the Level-1 FITS reader provides for easy access to the FITS extensions.
The browse format data for both the Level-0 and Level-1 data are 576x512 pixel PNG files. The files contain substantial annotation and the increase in horizontal size allows for the addition of a scaled color bar. They are the primary 'live-broadcast' product used by space weather forecasters at different institutions across the U.S. The PNG files are also used for whole disk movie generation and, of course, browsing solar activity levels. The images are 8 bits deep and produced in false color. The files are scaled logarithmically from minimum to maximum.
Color Tables: Three standard IDL color tables are used in for the image generation. Red (IDL color table 3 - Red Temperature) is used for the SXI Open filter position. This position provides both the broadest and coolest spectral response of the SXI. Green (IDL color table 8 - Green/White Linear) is used for the polyimide filter set. This filter set is transmissive to the middle range of SXI temperatures. Blue (IDL color table 1 - Blue/White) is used for the beryllium filter set. This set cuts out most of the cooler response of the SXI.
Annotation: Extensive annotation is overlayed on the PNG
images. Many of the elements are derived from specific FITS keywords.
This enables aids users in correctly interpreting browse imagery.
A Level-1 PNG example is shown below in Figure 1. The details are
explained in Table 1 below.
Figure 1: Annotated Level-1 PNG Image Explained
Table 1: Definitions of Browse Format Annotations and the Relationship to FITS keywords and File Name Substrings
| Element | Value in Example | Explaination | FITS Keyword | File Name SubString Columns* |
| Credit Line | NOAA / SEC Boulder, CO | NOAA / SEC generates the GOES SXI image products. | ||
| Spacecraft Identification | GOES-12 | The satellite platform for SXI. | TELESCOP | |
| Instrument Identification | SXI-0 | The serial number assigned by SEC to the instrument. | INSTRUME | 26-27 |
| Image Type Code | CS | This parameter displays a set of image categories which are based on
the intent of the image. These intent-based categories aid forecasters
in selecting images. The categories are:
|
IMG_CODE | |
| Processing Level | Level-1 | Level of data processing applied:
|
23 | |
| Exposure Index | 952 | Unique numerical index for a given combination of integration time (EXP_TIME), filter (WAVELNTH), and detector gain (MCP_GAIN) | EXP_INDX | |
| Source Image Scaling | LIN | Instrument linear or log amplifier used | LIN_LOG | |
| Circular Mask | Circular mask applied only to Level-1 PNG images. Reduces visual impact of seasonal detector rotation and short term pointing errors. | |||
| Detector Gain | 549V | Gain setting for the MCP detector in volts | MCP_GAIN | |
| Integration Time | 3.000s | Exposure duration setting. The actual exposure time is 0.5 ms longer than the setting due to the rise/fall time on the | EXPTIME | |
| Filter | OPEN | Filter mnemonic for the analysis filter position used for the image. | WAVELNTH | |
| Observation Date | 2001/10/22 23:27:21 | Earth-adjusted UTC time of observation. On the PNG image, the precision displayed is 1 second. In the the FITS header and filename have a precision of milliseconds | DATE_OBS | 4-21 |
| Error Note Area | (Empty) | Used to present error messages to users, e.g., background subtraction errors. Error messages here reflect errors in the FITS Level-1 products. | ||
| Color Bar and Scale | The color bar and and scale are displayed logarithmically, as is the image. | |||
| Detector Edge | The SXI detector undergoes seasonal rotation with the spacecraft, which is Earth-pointing, not Sun pointing. This seasonal rotation, in conjuction with pointing errors, sometimes brings the detector edge into the nominal | |||
| Scale Multiplier and Units | The scale multiplier is the factor by which to multiply the scale to get the true signal. Tthe units are DN for Level-0 and DN/s for Level-1 |
Software tools have been developed for the SXI data for operational use by space weather forecasters. Where possible, these tools have been written in IDL and made compatible with SolarSoft. NOAA/SEC is in the process of examining these tools and determining if modifications need to be made for SolarSoft distribution.
The initial SXI SolarSoft branch is ready for distribution. Since the IDL software was developed for first for operations and second for research, a limited set of the software has been reviewed for release. For the same reason, the branch structure for SXI follows:
Note that all SXI IDL software has been developed on MS Windows platforms. While attempts have been made to keep cross platform compatibility, no testing on other platforms has been done. In addition, to ensure stability of the operational IDL code for SXI, the SSW library routines used were 'frozen' at development time. Thus, the possibility exists that the SXI code will not run except with the frozen version of SSW. Thus, in the IDL branches for SXI, seemingly redundent SSW routines are included. These will eventually be moved to an SXI specific library directory.
The image processing software that converts Level-0 FITS files into Level-1 FITS files is collectively referred to as the 'image algorithms.' This software is driven by the program SXIG12_PREP.PRO. Its function is to apply minimal standard processing to individual raw images to create individual exposure normalized images mostly free of instrument effects and artifacts.
The image algorithm software is designed primarily for production of Level-1 FITS files in real time and must meet the following operational requirements:
Since the Level-1 FITS files are made available in real time, it is anticipated that this software may find only niche applications in the research environment.
The software performs the following functions in sequence to Level-0 FITS files:
Figure 2: Image Algorithms (SXIG12_PREP.PRO) Interfaces
The IDAC is a forecaster tool used to display and analyze FITS images. Since it is distributed to a number of 'remote' client sites, the code will run without a full IDL installation. So there is an executable 'runtime' version avaialble in addition to the IDL source code version. The IDAC performs a number of basic functions such as image scaling, zooming, and displaying the FITS header.
Current Coronal Structure images are accessible with static filenames. SEC customers can use these URLs to show, or link to, the latest SXI images from their web pages. Use of an appropriate image credit is requested; Space Environment Center, NOAA, US Dept. of Commerce
There are 3 files for each image type: a small 128x128 pixel PNG file, about 10kbytes; a full size 576x512 pixel PNG file, 90-140kbytes; and a text file with the date/time of the current image. SXI Corona Structure images usually update every 4 minutes.
| Current SXI 4MK Corona image | Static filenames |
|---|---|
 |
current_sxi_4MKcorona_stamp.png current_sxi_4MKcorona.png current_sxi_4MKcorona.txt |
These images show the outer Solar
atmosphere, or corona, as viewed by the GOES 12 Solar X-ray Imager (SXI). This
region of the Sun has a temperature of one to several million degrees kelvin
(Mega Kelvin MK) and is active on a time-scale of seconds to weeks. Here the
red image represents the ionized gas (plasma) of the corona at about 3.0 MK
and the green image shows the plasma at 4.0 MK. Bright areas, referred to as
active regions, are associated with sunspots seen in white light images and
may produce explosive events known as flares. Dark regions are coronal holes
where the fastest solar wind originates.
Last update June 23, 2006
