SOHO Joint Observing Plan 044: The Large Scale Structure of the Solar Minimum Corona
Authors: Douglas Biesecker and Sarah Gibson
Our goal is to understand the large-scale, stable, coronal structure of
equatorial helmet streamers and polar coronal holes that can persist
for several solar rotations at solar minimum. Although the corona
does change dramatically, even during solar minimum, when a Coronal
Mass Ejection blows a huge quantity of mass outwards, it is to this
stable background structure that the corona returns. This
observed behavior suggests that it may prove useful to consider solar
minimum CME's as perturbations of the background corona. Understanding
the large-scale corona is also fundamental to understanding how and
where the solar wind is accelerated. The magnetic field structure in
particular is fundamental to solar wind studies: knowledge of where
the coronal field is open and where it is closed, as well as an
understanding of how the expansion of the field varies from a purely
radial expansion, are essential to studies of how the coronal field is
related to fast and slow solar wind passing the earth. We also would
like to clarify where the wind becomes important to coronal force
balance, i.e. where the sonic surface lies.
We therefore would like to quantify the density, temperature, and
magnetic field distribution throughout the large-scale, stable, mostly
static solar corona (i.e. 1 Rsun - 3 Rsun). We would approach this
problem by using coronal observations as constraints on magnetostatic
models. In previous work (Gibson, Bagenal, & Low JGR, 101, 4813, 1996;
Gibson & Bagenal, JGR, 100, 198651, 1995; Bagenal & Gibson, JGR,
96,17663, 1991), existing coronal models were used and new models were
developed to investigate the balance between magnetic forces and
gravitational and thermal forces in the presence of both bulk and sheet
currents. These models were constrained by observations of the white
light corona (Mauna Loa MkIII and SMM Coronagraphs) and, to a lesser
extent, of the photospheric magnetic field (Stanford WSO Magnetographs).
Density, temperature, and magnetic field distributions were found that
matched the data to within observational accuracy. However, the
considerable degeneracy of acceptable solutions that were found implied
that in order to specify the coronal plasma properties with confidence,
additional observational constraints were needed.
The observations needed for this modeling effort are included in
those described in the Whole Sun Month Campaign. Specifically,
we will use coronagraph density, temperature, and velocity diagnostics,
as well as photospheric magnetic field observations and lower coronal
temperature and density diagnostics to be used as lower boundary conditions.
Observations of the corona above the region we will be modeling (i.e. >
3 Rsun) as well as solar wind observations, both coronal and in situ, are
extremely useful as upper boundary conditions. Details on these observations
are listed below. We plan to make good use of these large and varied
datasets as constraints on our models. We have developed techniques
utilizing genetic algorithms (Gibson and Charbonneau, BAAS, vol. 28 #2)
that will make efficient use of many observational constraints in order
to reduce model degeneracy and so choose the most physically realistic
description of the solar minimum corona.
Details of Observations
1) Outer Coronal Instruments
The outer coronal instruments provide the most direct information about
the large-scale coronal plasma. Electron density can be determined from
white light coronagraph images (particularly from polarized images).
Electron temperature can also be determined from line ratios of the red
and green iron lines, and electron temperatures and densities as well as
velocity distributions can also be studied using the coronal line profile
of the electron scattered component of HI Ly-alpha. The proton density
distribution can also be deduced from the Ly-alpha distribution. Proton and
ion velocity distributions can be determined from various visible and ultra-
violet spectral line profiles. Outflow velocities can be determined from
Doppler dimming and spectral line shifts. Finally, magnetic field direction
can be studied using the Hanle effect on the green iron line.
Contact: Douglas Biesecker
C1, C2 and C3: Daily, or several times daily: 3 polarized
white light (or poss. continuum) images taken at 120 degree
intervals from which polarized Brightness (pB) can be
determined (SEQPW). If possible, some of these polarized
images should be taken between 17:00 and 22:00 UT to overlap
with Mauna Loa observations (particularly C1 and C2).
C1: Daily: Iron X and XIV (red and green lines) done
with as long an exposure as possible to increase signal
* It is not yet known what the daily synoptic program will consist
of. It is likely that most of the images needed will be a part
of the synoptic program. If necessary, extra observations may
be taken during time devoted to special observations. *
Contact: John Kohl
The UVCS observations would consist of its standard synoptic sequences
plus special observations providing additional plasma parameters and
spatial coverage. The emphasis would be on large scale structures, but
fine structures such a polar plumes would also be observed.
The UVCS synoptic program is run everyday (with rare interruptions)
beginning at 01:00 UT and ending at about 14:30 UT. The observations
consist of eight sequences centered at position angles of 270,
315,0,45,90,135,180 and 225 degrees respectively. There are three different
sequences which are designed, respectively, for polar coronal holes,
equatorial streamers, and mid-latitude regions. In all cases, the instrument
is configured for spectral line profiles of HI Ly-alpha and line
intensities of OVI 1032 A and OVI 1037 A. In the case of equatorial
streamers, additional line intensities for heliographic heights below 2.0
solar radii are also obtained. For line intensity measurements, the spatial
resolution is limited by statistics and by the slit widths (84" for
The observed heights depend on the coronal target (i.e., position angle).
Equatorial streamers are observed up to 3.0 solar radii while mid-latitude
structures and polar regions are observed up to 2.5 solar radii. The
synoptic program also includes several visible polarized radiance
measurements at the center of the nominal UVCS instantaneous field-of-view.
The synoptic observations are suitable for line-of-sight HI velocity
distribution measurements and HI Doppler dimming analyses of radial bulk
outflow velocity. The ion intensity measurements are suitable for ion
density determinations, elemental abundance calculations, and Doppler
dimming and brightening analyses of radial bulk outflow velocity.
Electron temperatures can be inferred from charge state intensities and
UVCS special observations are performed about 10 hours per day.
Normally, the observations for those time periods are chosen by the UVCS
Lead Observation Scientist of the Week, but with special provisions for
JOP's, Intercalibrations, and other instrument/spacecraft requirements.
The UVCS Lead Observers for the Global Sun Month would design an
appropriate set of observations which may differ somewhat from week to
week depending on the particular interests of the UVCS Lead Observation
Scientist of the Week.
In general, UVCS would obtain line profile observations for OVI 1032 A
and 1037 A in both polar coronal holes, and profiles for O VI and possibly
other ions in streamers. A long duration observation in streamers might
be used to determine the intensities of week spectral lines. A long
duration observation might also be used to resolve polar plumes. HI
Ly-alpha and Ly-beta observations would be used to investigate density
uniformity along the line-of-sight. In addition, UVCS might attempt to
measure electron temperature from coronal electron scattering of
chromospheric HI Ly-alpha. UVCS observations might also include heights
above those covered in the synoptic program. The details of the special
UVCS observations are yet to be determined.
2) Transition region and lower coronal instruments: CDS, SUMER, EIT.
By taking full Sun images daily, a 3-dimensional picture of the lower
corona would be gained from these instruments. The observations would
then be used to determine temperature, density and emission measure in the
smaller scale structures that underlie the large scale structures observed
by the coronagraphs. Moreover, the three-dimensional images would trace
the coronal hole boundary structure on small and large scales at low and
high altitudes, which could then be compared to large-scale coronal field
predictions. Similarly, observations of polar plumes may trace the
non-radial divergence of the large scale-field at the poles. Finally,
off-limb observations can be used to try to directly compare density
predictions obtained with the upper coronal instruments to those found
with the lower coronal instruments.
The lower coronal observations should be coordinated so that they coincide
temporally and spatially as much as possible. These SOHO observations
will also be coordinated with Yohkoh observations (David Alexander will be
the liason with Yohkoh.)
Contact: Andrzej Fludra
The acronyms listed below are examples of specific planned CDS science studies.
They may be appropriate as they stand for the Whole Sun Campaign, or may
need to be slightly modified.
Daily (or every other day): get full disk images (FSUN.)
Use strongest lines to minimise duration of observation.
Weekly: run a slower, more comprehensive version of FSUN.
(Use weaker lines if necessary for accuracy.)
Every other day: run observations of coronal hole
boundaries (e.g. run BOUND - on-disk, and CHSTR - off-limb.)
Other possible observations:
Off-limb observations, out to 1.1 Rsun, using the same ions
as LASCO (LABP2W, or OLICS).
Contact: Don Hassler
Daily (or every other day - coordinated with CDS):
observations of full disk at lower resolution, either
just intensity in one line, or moment observations (i.e.,
emission measure, temperature, and non-thermal broadening
analysed in flight.)
Weekly (coordinated with CDS): high resolution full disk
image in 3 lines.
Every other day (coordinated with CDS): synoptic
observations of swaths across both poles, possibly also
at equator, extending off limb.
Contact: Joe Gurman
Daily synoptic maps of Iron lines Fe IX-X, Fe XII, and Fe XV.
* No special observations required. *
3) Photospheric boundary conditions
Full disk images over a full rotation can be analyzed for a spherical
harmonic decomposition of the large-scale photospheric field, and be used
as a lower boundary condition on the coronal field. Also, high-resolution
observations of dynamic changes of the magnetic field when combined with the
high resolution observations of the transition region and coronal instruments
will show if changes in photospheric field affect coronal structures.
Contact: Todd Hoeksema
Normal daily full-disk magnetograms for the entire rotation
(~15 per day at 4" resolution)
Weekly high (1.2 ") resolution campaign to be coordinated
with SUMER/CDS/YOHKOH (several observations can be taken
during the time needed to run the high-res SUMER/CDS images)
of a smaller region (approx. 1/3 the solar disk, starting
just north of the equator).
* must schedule high/res during VC2/VC3 high rate data hours *
4) Solar Wind Instruments:
Given a prediction of the large-scale coronal density, magnetic field, and
temperature, solar wind models can be evolved along the field lines, and
compared to in situ observations. In situ observations of fast and slow
wind can be mapped back to the solar surface to see if they come from open
or closed regions. In situ magnetic field and mass flux can be compared
to extrapolated solar wind predictions. Finally the in situ measurements
can give a coronal electron temperature prediction based on where in the
wind different elements freeze in.
In addition to SOHO/CELIAS, other non-SOHO solar wind instruments such as
Ulysses, Wind, and IMP, may wish to contribute data from the month of
September. Toni Galvin will help coordinate these external involvements,
under the auspices of IACG Campaign IV, Solar Structures of Heliospheric
Structure Observed out of the Ecliptic.
Contact: Toni Galvin
Standard daily observations
* No special observations required. *
* The actual data set will be partly determined by the solar wind
conditions. (Some instrumental responses are a function of the solar wind
speed or intensity.) *
CELIAS MTOF Proton Monitor can give solar wind proton speeds (within 5%
uncertainty) and densities (within 20% uncertainty). This can be used to
help identify the type of solar wind, and for mapping back to the
Carrington longitude to find the "source". The MTOF mass spectrometer
will provide what it can on elemental composition solar wind speed, so we
will have to look at the data but should come up with at least some
selected time periods.) Elemental abundances are expected to vary
depending on the source of the solar wind (slow vs. coronal hole). Fred
Ipavich, the MTOF Lead CoI, has approved MTOF participation.
CELIAS CTOF can give solar wind minor ion (e.g., oxygen, silicon, iron)
velocities, abundances, thermal velocities, and charge state
distributions. The charge state distributions are used by modelers to
estimate coronal electron temperatures and perhaps temperature profiles.
Heiner Gruenwaldt, the CTOF Lead CoI, has approved CTOF participation.
CELIAS STOF - if the sun is not as quiet as hoped, there may be SEP
events. Martin Hilchenbach, the STOF Lead CoI has approved STOF