CATE 2017: Science Goals

The time sequence of white-light coronal images from the CATE experiment will provide a unique tool for scientific investigations on several fronts.  The broad types of science which the data will address will be static and dynamic studies of the corona.

Static Studies:
Polar Plumes - Using data from the Spartan 201-01 mission in 1993, Fisher and Guhathakurta (1995) measured white light "polar plumes" above the northern and southern solar coronal hole.  These plumes extended from the lower limit of the occulting disk at R=1.25 Rsun up to over 5 Rsun.  Simultaneous ground-based measurements from Mauna Loa suggested that the plumes extended down to R=1.16Rsun. The directions of the plumes, while appearing roughly radial, did not intersect the center of the solar disk, but rather seemed to originate at higher latitudes.  Later work by DeForest and Gurman (1998) traced these structures down to magnetic features at the solar poles using SOHO EIT 171A data, and measured a size of between 3-5 arcsec.  The CATE data will measure these structures in white light with 2 arcsec pixels to very low heights of R=1.05 Rsun, and out to the edge of the field-of-view at R=2 Rsun.  Using simultaneous magnetograms, these structures can be traced using the continuum signal from coronal electron density enhancements back to the Sun with better resolution than these previous studies.

Potential Field Source Surface - The magnetic field in the solar corona has been modeled using the magnetic field measured at the solar photosphere and then extrapolating it to higher regions using different models.  One such model is the Potential Field Source Surface (PFSS) model which uses the assumption that the coronal magnetic field becomes radial at a certain height in the atmosphere (see Riley et al 2006 ApJ 653,1510 for a discussion).  Many authors use a height of R=2.5Rsun for the location of the PFSS in the solar corona, but recent studies suggest the height may vary with time (Arden et al 2014) or it may be lower during solar minimum (Lee et al 2011).  Work from Lee et al (2011) actually suggest the height may lie between 1.5 and 1.9 Rsun, much lower than in other work.  The CATE data will allow us to measure the direction of the coronal magnetic field by tracing small-scale coronal threads.  In each coronal image, we can map the direction to determine where the field becomes radial around the entire limb, and during the 90 minutes of time which elapses, we will be able to use the solar rotation to disentangle some of the line-of-sight effects.  We can use this location at the measured height of the PFSS, and then by using photospheric magnetograms, run a PFSS model to compute the coronal magnetic field.

Dynamic Studies:
The real strength of the CATE data is that for the first time it will reveal a time sequence of coronal continuum intensity at relatively high spatial and temporal resolution for 90 minutes duration.  Time variable phenomenon studied in the corona using narrow-band images which probe particular temperature regimes will now be visible using white light images which probe all temperatures of the corona.  Because total solar eclipses last only a few minutes at each location, these types of studies are impossible unless data from several eclipse locations are combined.

Polar Plume Dynamics - The polar plumes which are dramatically visible in the corona seen at solar minimum show very dynamic changes.  DeForest and Gurman (1998) found upwardly moving density enhancements of 5-10%, traveling at 75-150 km/s velocity and displaying periodicity at 10-15 minute periods using the SOHO EIT 171A observations.  Cranmer (2004) estimated 3-15% variations in the electron density in these events.  Using UVCS observations at two alternating heights in the corona (R=1.9 and 2.1 Rsun), Ofman et al (2000) found quasi-periodic variations of 5-10% in polarized brightness traveling radially at 210 km/s with periods between 6.5 -10.5 minutes.  Morgan et al (2004) used Lyman alpha data to find oscillations with 7-8 minute periods persisting out to 2.2 Rsun.  Gupta et al (2012) used Sumer observations of Ne VIII emission on the solar disk to find 5-10% intensity oscillations traveling at 60 km/s with a period of 14.5 minutes.  While changes in the corona above the southern solar pole are visible in the two images taken 19 minutes apart during a solar eclipse by Pasachoff (2009), it is difficult to find systematic radial motion using the two images.  The CATE data will profoundly impact the study of these events.  With 540 images taken at 10 second cadence across 90 minutes, the motions of these density enhancements will be measured.  Periodicities at the 15 minute time scale will be fully sampled, and the velocities and accelerations of these events will be measured from R=1.05 Rsun out to at least 2Rsun.  The CATE data will be sensitive to transverse velocities of roughly 0.8 to 145 km/sec (3pix/90 min to 1 pix/10 sec) and will easily measure these events.

Prominence-Coronal RT Instabilities - Using data from the Hinode SOT instrument, Berger et al (2007) show upwardly moving hot gas parcels, presumed to be Rayleigh-Taylor instabilities,  rising in prominences.  Typical sizes were 2250 km, with upward speeds of roughly 20 km/s.  Lower resolution white-light eclipse images from Druckmuller et al (2014) point to a new type of static structure observed near a prominence called a "smoke ring", and the authors speculate that these structures may be related to the RT instabilities seen in prominences.  The CATE data will show the motions of these new coronal structures during the 90 minutes of the eclipse, with a transverse velocity sensitivity which easily covers the expected 20km/s motion.  For the first time, these observations will reveal how the instabilities seen in prominences interact with the corona and produce density enhancements or depletions as shown by white-light observations.

Active Region Loop Oscillations - While solar activity should be low during the 2017 eclipse, it is possible that an active region will be close to the solar limb at the time of the eclipse.  Active region coronal loops are known to have several types of oscillations.  First, as seen by the COMP instrument, 5 minute period oscillations are seen to move outwardly through active region coronal loops (Tomczyk and McIntosh, 2009, ApJ 697 1384).  While the transverse motions and density enhancements expected from these oscillations are small or non-existent, the CATE data set should allow smaller lower limits to be set than ever before, especially on the density variations.  Besides these regular oscillations, sporatic transverse loop oscillations have been seen in active region coronal loops.  Aschwanden et al (1999) report that typical spatial displacements are 4000km, with 4.5 minute periods and loop sizes of 130,000km.  These oscillations decay in a few cycles, and the triggers are thought to be associated with solar flare activity.  The CATE data will fully resolve any of these transverse loop oscillations which may occur during the eclipse.

Coronal Inflows - The LASCO C2 coronagraph has been used to study downward moving inflow events in the solar corona, and Sheeley and Wang (2014 ApJ 797 10) show that these inflow events correlate with the locations of sector boundaries in the large-scale photospheric magnetic field, rather than with particular events like flares or CMEs.  These events are thought to represent the return of magnetic flux to the solar surface, are seen in white-light coronal observations, and travel at speeds of roughly 50-100 km/s downward.  While the C2 field-of-view limits measurements to R=2-6 Rsun, many events are seen to disappear behind the C2 occulting disk at R=2Rsun with a significant downward velocity.  While the expected rate of these events may be only 1 per day or less in 2017, if there are such inflow events during the 90 minutes of eclipse, or if there are similar currently unobserved events in the solar corona between 1-2 Rsun, the CATE data set will resolve them with high spatial and temporal cadence.

Coronal Mass Ejections - The likelihood of a CME happening during the 2017 eclipse is low, but CMEs did occur in both the 2008 and 2012 total solar eclipses.  Pasachoff et al. (2009) shows images of a CME at a low altitude separated by 19 minutes of time which shows evolution in the CME material and the surrounding coronal plasma; Pasachoff et al (2014) and Hanaoka et al. (2014) show a CME event higher in the solar corona during the 2012 eclipse, which again shows evolution during the 35 minutes between the two exposures.  With an expected 10 second cadence and 2 arcsecond pixel size, the CATE data set will show fine details in a CME event if one occurs during the 2017 eclipse.

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