Last Update: February 12, 1998

Number: JOP071

Submitted: October 23, 1997

Philip Judge, HAO, NCAR.
Richard Harrison, RAL.
Joe Gurman, NASA-GSFC.
Jeff Kuhn, Michigan State University and NSO.
Haosheng Lin, NSO.  
John Brown and Scott McIntosh, Glasgow University.   
Steve Tomzcyk, Bruce Lites, HAO, NCAR.
Bob Macqueen, Rhodes College.
Ingrid Mann, Max Plank Institut, Lindau.
Rock Bush, Stanford.
Don Hassler, SWRI, Boulder CO.

IMPORTANT NOTE: An earlier goal to determine components of the vector coronal magnetic field using Stokes polarization measurements of infrared emission lines of [Fe XIII] at 10747 and 10798 Angstroms, has been postponed until 11 August 1999, due to technical difficulties with the camera system.
OUTLINE On February 26 1998, a total solar eclipse will be observed by joint NSO/HAO/Michigan State/Rhodes College/Lindau expeditions stationed in Curacao and on an NCAR C-130 aircraft, flying at 18,000 feet near Panama. Prime goals of the aircraft experiments include (1) detection of several infrared lines using an 8x8 solar radii field of view imaging filter experiment (128x128 detector pixels), including Si IX at 3.93 microns, predicted by Judge (1998) as especially promising for coronal magnetometry (NSO/MSU/HAO), (2) accompanying measurements of line spectra between 1 and 2 microns over the brightest region of the corona (NSO/MSU/HAO), using a grating experiment with an E-W aligned slit, (3) measurements of the F corona (Rhodes/NSO/MSU/Lindau). Prime goals of the three ground experiments are to acquire a 2048x2048 image to continue the series of HAO white light eclipse images, to look for coronal fine structure in white light (SWRI/HAO), and to look for wave motions in polar plumes (HAO). The purpose of this JOP is to acquire CDS, MDI, EIT, UVCS and LASCO data that can provide essential support for these experiments. Synoptic EIT data are sufficient to fulfil the needs of this work, but coronal line data from CDS and Fe XII in UVCS, and Fe XIV in LASCO will be extremely useful as a basis for understanding the infrared emission line data. This work should be viewed as essential components for the development of an infrared ``coronal magnetograph''. Such an instrument would routinely measure magnetic fields at the solar limb. The 1998 eclipse presents an important opportunity to assess the viability and needs of such an instrument. SCIENTIFIC JUSTIFICATION The solar corona is dominated by magnetic structures. Even in regions of intense EUV emission, the plasma contributes little to the energy density of the corona. The magnetic field is the mediator of mechanical energy generated by photospheric motions, energy used to heat the corona and drive its dynamics. The magnetic field is therefore the central physical quantity that must be determined when trying to understand basic properties of the corona. Issues such as how the plasma is heated, how CME's are launched, how flares are started, how the Sun's global magnetic field evolves with time over the solar cycle, in principle all could be addressed if the magnetic field could be measured from photosphere through to corona. There are very serious limitations to observational techniques that may be used to determine coronal magnetic fields. In fact, no coronal magnetic field has been successfully measured outside of very active regions or prominences. This is because of the estimated weakness of the magnetic fields- a few Gauss is expected on the basis of (inherently uncertain) extrapolations of photospheric fields, or fields measured close to 1AU. However, the advent of high quality infrared detectors brings previously intractable problems into the tractable domain (Kuhn 1995). The HAO/NSO/MSU Zeeman experiment was designed to detect net field strengths of order 10G, i.e. of a strength comparable to those predicted by field extrapolation upwards from the photosphere, and downwards from in-situ measurements near 1AU. Last minute problems with the detector system meant that this effort had to be postponed until the August 11 1999 eclipse. The focus of this JOP has therefore changed from the original plan. Nevertheless, there remains strong scientific motivation to acquire and understand infrared emission lines that are potentially sensitive indicators of the coronal magnetic field, and SOHO data are needed to complement the infrared data. Furthermore, there is motivation from the recent addition of the coronal fine structure experiment to the expeditions. Some of the originally proposed SOHO observations are largely unchanged, but are now augmented by the needs of the fine structure and plume experiments. We wish to:
  1. Acquire EUV (CDS and EIT) data of plasma loops as they rotate onto the west limb (or from the east limb) of the Sun prior to, during and after eclipse, in support of the aircraft infrared experiments.
  2. Acquire EUV data (CDS/UVCS [Fe XII]) of the brightest region of the limb for comparisons with IR spectral data and the coronal fine structure (SWRI/HAO) experiment.
  3. Acquire EUV data (CDS) for as much of the corona at and above the limb as possible, in support of the infrared filter experiment, in a variety of coronal ions.
  4. Acquire EUV data (EIT) for the southern coronal hole plumes, these are time critical.
THE NEED FOR SOHO OBSERVATIONS The infrared line data acquired with the 8x8 solar radius field of view will be the first to determine the dependence of intensity of infrared forbidden lines in the plane of the sky. This is very important to determine to assess the potential performance and limitations of future magnetographs. The line list includes a line at 3.93 microns of Si IX that is predicted by Judge (1998) to be strong. Based upon atmomspheric transmission and Judge's calculations, it is potentially the most promising line as a candidate for Zeeman measurements of magnetic fields in the quiet Sun (Kuhn 1997, unpublished). This is a difficult measurement to make owing to relatively high thermal emission in the instrument. CDS data are needed for EUV Si IX lines for comparison with Si IX. EUV SOHO data are needed in general to aid in assessing the infrared line data, both for the filter and grating experiments on the aircraft. The SWRI/HAO fine structure experiment on Curacao island would benefit support from the UVCS instrument in the Fe XII channel, and the LASCO C2 telescope Fe XIV channel: the fine structures (obtained by spatially differencing the white light data, as done by November and Koutchmy 1996) need to be correlated with more sensitive indicators of the temperature / density structure as can be obtained with the Fe XII line of UVCS. The plume experiment would benefit from near simultaneous observations from EIT. The calibration of the LASCO experiment may benefit from the high quality coronal white light images to be obtained with the 2Kx2K camera. The proposed SOHO data to be acquired would use thus use primarily CDS, UVCS, EIT, LASCO, with support from and MDI. Details are outlined below. References Judge, P. G., 1998. ApJ (in press). Kuhn, J. R. 1995, in J. R. Kuhn, M. J. Penn (eds.), IR Tools for Solar Astrophysics: What's Next?, No. 15 in National Solar Observatory/ Sacramento Peak Summer Workshop, World Scientific, Singapore, p. 89 November, L.J., and S. Koutchmy, 1996. White-Light Coronal Dark Threads and Density Fine Structure, Ap. J., 466, 512. REQUESTED SOHO OBSERVATIONS We propose the following (tentative) plan, to be refined after further discussions with co-Is following final integration of the IR experiments. To maximize the chances of seeing "simple" coronal loops at the limb, we must allow the possibility of observing the Sun at either the East, or West limb. The choice will be simple, and will be made days in advance. If there are obvious active regions on the disk whose overlying coronal loops seen, e.g., in EIT, will intercept the west limb, we will observe that limb. If not, we will look at the possibility that active regions will appear on the East limb, on the basis of earlier EIT data, for example. Examination of the EIT synoptic data shows that it is very likely that at least one active region will appear on the limb on 26 February 1998. Currently (February 11, 1998) we are looking to target a decaying active region which was at SW limb on 29-30 January. Needed Data from EIT EIT synoptic data will serve to reveal the near-limb coronal structure in Fe IX, Fe XII, Fe XV for comparison with the infrared images. EIT data in support of the plume experiment are being discussed- the target is the southern coronal hole. Needed Data from CDS Data for CDS have been decided in conjunction with Harrison and Fludra. Here is an extract from the CDS plan as of February 11, 1998: Eclipse Support Activity (18 February to 5 March) Leader: Phil Judge (see e-mail address above). Target near C.M. to be selected for 18th. Observe once per day, i.e. following it as it rotates towards the limb. (Actual eclipse is on 26th Feb). On each day, use a set of LARGEBP2 (Study ID 10, Variation 47) rasters forming a mosaic covering the AR target. i.e. each raster is 4x4 arcmin, and use several to cover 8x8, 8x12 or whatever is required to cover region. Keep mosaic area large enough to cover region, but not too large (to minimise impact on other Studies). Duration of one raster about 1h 45m hours. Track target region with the mosaic arrangement once each day. Target should be selected in conjunction with Phil Judge. Most likely to be decaying AR which was at SW limb on 29-30 January. Modified LARGEBP2: NIS, 4x240 slit, 4x4 area, Fe XIII 320.80, 348.18, 359.64, Si IX 349.8, 345.0, 341.95, 349.62, He I 594, O V 629, Mg IX 368, Fe XVI 360, Fe XII 364, Fe XIV 354, Fe XIV, 334, - 14 lines. 25 pix. Duration about 2 hours. Other data not mentioned here include two CDS studies requested on the day of the eclipse: (1) a rather complete image of the corona above the entire solar limb, (2) a deeper study of the "target" mentioned above on the day of eclipse. Needed Data from UVCS Maps of Fe XII data in the "target" region would be very useful to support the fine structure coronal experiment. Needed Data from LASCO Synoptic data would be fine, and Fe XIV data from C2 would be extremely useful for comparison with the IR forbidden line data. Details still to be decided in conjunction with LASCO team. Needed data from MDI MDI synoptic data will serve to assess the underlying photospheric field structure for the state of the corona observed on 26 feb 1998. CIRCUMSTANCES OF ECLIPSE AT SABINE WESTPUNDT, CURACAO ----------------------------------------------------- Duration of Totality = 3 m 34 s Time of mid Totality = 18:13:11 UT Second Contact = 18:11:26 UT Third Contact = 18:14:56 UT The orientation of the Sun on 26 February is : P angle = -20.77 deg B angle = -7.18 deg apparent Radius = 16 arc min 9.01 arc sec