Tomographic Imaging of the Velocity and Magnetic Fields in the Sun's Atmosphere

• Main Author: Jefferies, Stuart M.
• Format: Dataset
• Language: unknown
• Published: IEDA: US Antarctic Program Data Center 2016
• Subjects: Antarctic Astrophysics and Geospace Sciences; Sun; Satellite Remote Sensing; Antarctica; Cosmos; US Antarctic Program Data Center (USAP-DC); Antarctic; Austral; Antarc*; antartic*
• Online Access: http://get.iedadata.org/metadata/iso/600152

Abstract: The ultimate goal of this project is to determine the structure and dynamics of the Sun's atmosphere, assess the role of MHD waves in heating the chromosphere/corona and driving the solar wind, and better understand how the Sun's atmosphere couples to the solar interior. As the solar atmosphere is 'home' to many of the solar phenomena that can have a direct impact on the biosphere, including flares, coronal mass ejections, and the solar wind, the broader impact of such studies is that they will lead to an improved understanding of the Sun-Earth connection. Under the current award we have developed a suite of instruments that can simultaneously image the line-of-sight Doppler velocity and longitudinal magnetic field at four heights in the solar atmosphere at high temporal cadence. The instruments use magneto-optical filters (see Cacciani, Moretti and Rodgers, Solar Physics 174, p.115, 2004) tuned to the solar absorption lines at 422 nm (Ca I), 589 nm (Na D2), 770 nm (K) and 1083 nm (He). These lines sample the solar atmosphere from the mid-photosphere to the high-chromosphere. A proof-of-concept run was made in the Austral summer of 2007/2008 using the Na and K versions of the instruments. Here we recorded over 40 hours of full-disk, intensity images of the Sun in the red and blue wings of the Na and K Fraunhofer lines, in both right- and left-circularly polarized light. The images were obtained at a rate of one every five seconds with a nominal spatial resolution of 4 arc-seconds. The run started at 09:44 UT on February 2, 2008 and ended at 03:30 UT on February 4, 2008. Data Quality Assessment: The temperature controls of the instrument housings were unable to fully compensate for the harse Antartic winds encountered during the observing run. This led to large (~15 C) temperature swings which adversely affected the instruments (and thus data quality) in two ways: 1) Crystals of Na and K were deposited on the magneto-optical filter windows leading to "hot spots" in the images. These "hot spots" come and go with time as the temperature changes. 2) The changing temperature caused the optical rails to contract and expand causing the final images to go in- and out-of-focus, thus reducing the resolution to greater than 4 arc-seconds. Both these effect are worse in the K data. Despite these problems, the intensity images can be combined to provide magnetic images that show a very high sensitivity (< 5 Gauss in a 5 second integration). Data Description: The raw data are stored as a series of 1024x1024x4 FITS images. The format is: blue image (left circulary polarized light), blue image (right circularly polarized light), red image (left circulary polarized light), red image (right circularly polarized light). The naming convention for the images is: Type_Instrument_Day_hour_minutes_seconds where Type is I (intensity), F (flatfield), D (dark) Instrument is 0 (Na), 1 (K) Day is the day number from the beginning of the year where January 1 is day 0 For example, I_0_32_12_34_40.fits is an intensity image taken with the Na instrument at 12:34.40 UT on February 2, 2008. Notes: 1) The flatfield images were acquired by moving a diffuser in front of the Sun during the integration. The resulting images therefore have to be corrected for residual low-spatial frequencies due to the non-flat nature of the light source. 2) Each FITS file header contains a variety of information on the observation, e.g., F_CNTO : number of summed frames in each 5 second integration (*) FPS : Camera frame rate (Frames Per Second) FLIP : Rate at which the half-wave rotator (magnetic switch) was switched INT_PER : Integration time (in seconds) MOF : Temperature of magneto-optical filter cell WS : Temperature of wing selector cell TEMP_0 : Temperature of camera 0 TEMP_1 : Temperature of camera 1 TEMP_2 : Temperature inside instrument (location 1) TEMP_3 : Temperature of narrowband filter TEMP_5 : Temperature of magnets surrounding MOF cell TEMP_6 : Temperature inside instrument (location 2) TEMP_7 : Temperature of housing for magnetic switch (*) This is the frame count for the camera. The number of frames in each image for the two different polarization states, is half this number. The measured temperatures are only coarse measurements. 3) Due to reflection in the final polarizing beam splitter (which separates the "red" and "blue" signals into the two cameras), the camera 1 data need to "reversed" along the x-axis (i.e. listed as [1024:1] instead of [1:1024]) 4) Line-of-sight velocity and magnetic field images are generated from the observed intensity images. Doppler images as (red-blue)/(red+blue), magnetic images as the difference between the Doppler images for right- and left-circularly polarized light.