Contents
Active Region AR 9393 3/27 to 4/2 2001.
Image from
Astronomy Picture of the Day, 4/11/2001
Contents |
Active Region AR 9393 3/27 to 4/2 2001. |
Goals of this project
Grading Policy
You must complete the analysis of 6 weeks worth of recent YOHKOH images,
up through the last data available at the time of the penultimate lab class.
You should observe the Sun optically on at least 4 days during class.
If you wish to observe outside of class time, you will need to coordinate
with a TA.
An observation should take no more than 15 minutes. Keep an observing log:
if the sky is cloudy you will not be penalized for not making the optical
observations.
Grading is based on the quality of the data analysis, the discussion of the uncertainties, and the answers to the questions at the end of this page. The actual rotation period you measure is not important for your grade.
The Sun and its atmosphere are a testbed for plasma physics, atomic physics, and nuclear physics. Solar nuclear reactions, and the solar neutrino problem, provided the first observational eevidence that there might be phase-mixing among neutrinos, and spurred the development of neutrino observatories. Observations of the Solar atmosphere provide a wealth of information on atomic physics that is difficult to obtain in the laboratory. Helium was discovered in the Solar atmosphere before it was found on Earth. The Solar corona, with its low densities, high temperatures, and magnetic fields, offers a marvelous site to undertake (uncontrolled) experiments in plasma physics. The more we observe the Sun, the more detail we see, and the more complicated our star becomes.
Facts about the Sun:
Low mass (solar-like) stars initially spin with rotation periods of about 8 days (Edwards et al 1993, ApJ, 106, 372) when they become visible as T Tauri stars. This period appears to be a consequence of magnetic torquing of the star by the inner disk. After the accretion disk dissipates, the star is free to spin up as it contracts towards the main sequence. Solar-like stars appear to land on the main sequence with rotation periods between 8 hours and a day or two. After that the stars slowly spin down. The angular momentum loss is thought to be due to magnetic torquing of the stellar wind.
The Solar rotation rate is ill-defined because the Sun is a fluid body which rotates differentially. The rotation rate varies from about 25 days at the equator to 29 days at the poles.
An excellent place to start, if you want to look at the state of solar activity today, is the Solar Data Analysis Center (SDAC). The SOHO synoptic database contains a good sampling of daily images. A good public outreach site is the YOHKOH Public Outreach page.
To see a movie of the X-ray Sun rotating, start IDL
on one of the lab computers and type
.run c:/phy515/sunmovie/sole
A movie of the large sunspot group AR 9393 of March 2001 crossing the Solar
disk
is available at
this website
(this is the same image as displayed at the top of this page).
The goal of this lab is to measure the solar rotation. To do so, you need to observe a feature on the solar surface. As a fluid object, the Sun does not have permanent markings like the planets do, but it does (usually) have long-lived magnetic structures. The magnetic fields manifest themselves as photospheric sunspots, or as magnetic loops in the chromosphere and transition region. A secondary goal is to observe the relation between the coronal active regions and the photospheric spots.
You have two options in this lab. Both options involve archival data analysis. Option A also involves visual observations.
The EIT images in the light of Fe IX (171Å), Fe XII (195Å), Fe XV (284Å), and He II (304Å), using narrowband filters to isolate these bright lines. These lines are formed at different temperatures, and thereby probe the solar atmosphere at specific temperatures, or heights.
You can also view daily Fe IX,X (171Å) images from TRACE, which offers better spatial resolution, at the SDAC. These are not full disk images, but show the sometimes incredible detail of the solar magnetic field.
YOHKOH offers poorer spatial resolution than SOHO, and integrates over a wider bandpass containing lines formed at many temperatures.
The SOHO data archives are accessible on the web. Go to the SOHO EIT Catalog Interface. Enter the following catalog selection criteria:
The catalog search results will be returned. Each image is labeled by Date, Exposure Time, Filter, Wavelength, and image size. Select the images you want to download. Do not download more than the disk can hold!. You should download a uniform set of images, with the same wavelenth and filter and similar exposure times. You will not need more than one image per day. Enter the requested information at the bottom of the page, and submit your request. It will be fulfilled within a few minutes. The data will be written into a gzipped tar file. Go to the indicated ftp site and download the data. Unpack the data using gunzip and wtar (note that the data will be written into a subdirectory). These are FITS format files, even though the extensions are not '.fits' (the extensions are the universal time at the start of the exposure).
Once the file sohodata.fits is safely on disk,
you can read it using IDL. The command is
d=readfits('sohodata.fits',h)
The header information will be placed in variable h,
the data array is in variable d. A sample SOHO
header follows:
SIMPLE = T / Written by IDL: 25-Aug-2003 15:12:11.00 BITPIX = 16 / Short integer (2 bytes/word) NAXIS = 2 / NAXIS1 = 1024 / Number of columns NAXIS2 = 1024 / Number of rows DATE = '2003-08-01' / Date of file creation TIME-OBS= '00:00:10' / DATE-OBS= '2003-08-01T00:00:10.638' / UTC at spacecraft ORIGIN = 'Rocket Science' / Rocket Science = NASA GSFC DATASRC = 'LZ file ' / TELESCOP= 'SOHO' / INSTRUME= 'EIT' / OBJECT = 'full FOV' / BSCALE = 1. / BZERO = 0. / BUNIT = 'counts / pixel ' / WAVELNTH= 195 / 171 = Fe IX/X, 195 = Fe XII, FILTER = 'Al +1 ' / DATE_OBS= '2003-08-01T00:00:10.638Z' / SCI_OBJ = 'CME WATCH 195 ' / OBS_PROG= '195_10S_AL_1.000 ' / CMP_NO = 1 / Unique campaign instance (1 = synoptic) EXPTIME = 12.697 / s (total commanded + shutter close) EXPMODE = 'backside ' / FILENAME= 'efz20030801.000010' / CFTEMP = -67.57 / CCD cold finger temperature (C) CCDTEMP = 7.37 / CCD temperature (DN/100) CTYPE1 = 'Solar-X ' / CTYPE2 = 'Solar-Y ' / CRPIX1 = 507.19 / Sun center x, EIT pixels CRPIX2 = 517.80 / Sun center y, EIT pixels CRVAL1 = 0.00 / CRVAL2 = 0.00 / CDELT1 = 2.63 / Pixel scale x (arc sec, fixed) CDELT2 = 2.63 / Pixel scale y (arc sec, fixed) SOLAR_R = 362.76 / Solar photospheric radius, EIT pixels SOLAR_B0= 5.75 / Degrees SC_X0 = 0.00 / s/c yaw (arc sec) SC_Y0 = 0.00 / s/c pitch (arc sec; -109.14 after 1996/04/16) SC_ROLL = 180.00 / s/c roll (deg., Solar north + CCW from nominal HEC_X = 92913560.00 / s/c heliocentric ecliptic x (km) HEC_Y = -118435048.00 / s/c heliocentric ecliptic y (km) HEC_Z = 59165.79 / s/c heliocentric ecliptic z (km) CAR_ROT = 2005.00 / Carrington rotation at earth COMMENT CORRECTED DATE_OBS = '2003-07-31T23:59:59.539Z' COMMENT COMMANDED EXPOSURE TIME = 10.000 s COMMENT SHUTTER CLOSE TIME = 2.697 s COMMENT LINE_SYNC = 'no' COMMENT CAMERA_ERR = 'no' COMMENT IMAGE_OF_SEQ = 0 COMMENT READOUT PORT = 'B' COMMENT NUM_LEB_PROC = 3 COMMENT LEB_PROC = 26 (no ROI) COMMENT LEB_PROC = 27 (no occ mask) COMMENT LEB_PROC = 12 (Rice) COMMENT BLOCKS_HORZ = 32 COMMENT BLOCKS_VERT = 32 COMMENT P1_X = 1 COMMENT P2_X = 1024 COMMENT P1_Y = 20 COMMENT P2_Y = 1043 COMMENT N_MISSING_BLOCKS = 0 HISTORY Version 4.0, 2001 December 10 ENDThis is a 195 Angstrom image of the Sun obtained on August 1 2003. The exposure time is 12.7 seconds. The image was obtained through the Al+1 filter. The image is 10242 pixels square, and the sun is nearly centered. The angular scale is 2.63 arcsec per pixel, and the solar radius is 363 pixels. Most of the information is self-explanatory.
You will undertake visual observations: look at the Sun though the telescope eyepiece, and sketch what you see in your lab book (click here to download a template for your drawing). Find and mark North and East. In a few observations over a 2-3 week span, you should be able to see the Sunspots moving.
If it is very cloudy, you can download white light images from the Mees Solar Observatory at Haleakela or from the Big Bear Solar Observatory at Big Bear City, CA. These images are in JPEG format.
The YOHKOH SXT data are stored in FITS format, and are accessible via this ftp site. The most recent data are in the main directory; earlier data are in subdirectories labelled by year. File names are of the form sfdyymmddhhmm.fits, where yymmddhhmm are the year, month, day, hour, and minute at the start of the image. (Some of the earlier data are named sf_fitsyymmddhhmm.fits, and may be in a smaller 256x256 format.)
Right-click on the file name to download.
Once the file yohkohdata.fits is safely on disk,
you can read it using IDL. The command is
d=readfits('yohkohdata.fits',h)
The header information will be placed in variable h,
the data array is in variable d. A sample YOHKOH
header follows:
SIMPLE = T / BITPIX = 8 / NAXIS = 2 / NAXIS1 = 512 / NAXIS2 = 512 / DATE-OBS= '02/03/98' / DD/MM/YY [UT] TIME-OBS= '06:59:51' / hh:mm:ss [UT] CRPIX1 = 272.185 / sun E/W center (pixels 1-NAXIS1) CRPIX2 = 293.355 / sun N/S center (pixels 1-NAXIS2) CROTA = 0.929276 / Degrees CCW: PREDICTED (Error +/- .3 Deg) RADIUS = 197.20098 / Radius of sun (pixels) FILTER-A= 'Open ' / 1=Open , 2=NaBan, 3=Quart HISTORY 4=Diffu, 5=WdBan, 6=NuDen FILTER-B= 'AlMg ' / 1=Open , 2=Al.1 , 3=AlMg HISTORY 4=Be119, 5=Al12 , 6=Mg3 RESOLUT = 'Half ' / Full=2.46 arcsec pixel, half=4.92, qrtr=9.84 EXP-TYPE= 'Norm ' / Normal, dark, calibration EXP-DUR = 5369.04 / Exposure duration in milliseconds EXP-LEV = 25 / DPE (commanded exposure level 0 to 32) DP-MODE = 'Quiet ' / S/C Data Processor (DP) Science Mode DP-RATE = 'High ' / S/C Data Processor (DP) Telemetry Rate DATEOBS = ' 2-MAR-98 06:59:51' / Start CCD Integration [UT] ENDThis is a 5.4 second integration in the ALMg filter taken on 2 March 1998. The image is 512X512 pixels in size. Most of the information is self-explanatory.
There is a YOHKOH Analysis Guide available. The Instrument Guide is particularly useful. While it is instructive to read the rest of the manual, we have not installed the software described in these guides on the PCs.
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Note that the features you want to measure in the X-ray images are fuzzy.
This is because the corona is an optically thin three-dimensional
object, whereas the sunspots are features on a two-dimensional
surface. The X-ray-emitting loops are anchored to the solar photosphere
at the sunspots, but diverge with height. Usually, the tops of the loops
are hotter (and brighter in X-rays) than are the footpoints of the loops.
The images to the left show the relation between the two
levels of the atmosphere. As part of this lab, you will need to determine
a justifiable method of determining the location of the active regions
in the coronal X-ray images.
Left: Three images of the Sun taken on March 29 2001. The prominent active region near the center is AR 9393 (the same active region featured in the movie at the top of the page).
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