A number of theories and hypotheses are currently being developed to explain the often complex behavior of corona and prominence plasmas. In order to test the theories and hypotheses certain crucial observations are necessary. Some of these observations are examined and a few conclusions are drawn. Corona mass balance, corona and prominence classifications, prominence formation and stability, and coronal mass ejection are dicussed

Tandberg-Hanssen, E. A.

English

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WSOME CRUCIAL CORONA ARDPROMINENCE OBSERVATIONS
E. A. TANDBERG-HANSSEN
SPACE SCIENCE LABORATORY
MARSHALL SPACE FLIGHT CENTER, AL 35812
A number of theories and hypotheses are currently being developed to
explain the often complex behavior of corona and prominence plasmas, and later
today Parker will discuss some of the theoretical implications relevant to the
topic of this workshop. In order to test the theories and hypotheses certain
crucial observations are necessary, and I shall in this talk examine some of
these observations and draw a few conclusions.
To set the stage let me remind you that already two thousand years ago
Plutarch commented on an observation of crucial importance for coronal
research when he wrote about solar eclipses: "There always appears around the
circumference of the moon some light that does not permit total darkness." It
took a long time before the proper theoretical explanation of that light was
given. In a more lighthearted way Hirayama (1985) refers to crucial observa-
tions when he comments in his excellent review of Prominence Observations
"Prominences are fascinating objects, abundant in variety, beautiful, and
above all mysterious."
Prominence - corona mass balance
Since it is difficult to quantify "fascinating," "beautiful," and
"mysterious," I shall start by looking at the material involved in the corona
and prominence plasmas. The mass of the corona is, to an order of magnitude,
given by
M _ n m A H , (I)
cor p p
where A is the area of the solar surface (photosphere), H = 1010cm the
coronal scale height, and n_ and m are the number densit_ and mass respec-
tively of hydrogen atoms. __. (1) Pives, with np= 3 x I0_ om-3, the mass of
the corona 17
M _4 x 10 g.
cor
A large _iescent2_ro_inence has a volume of roughly V = 5 x 108 x _ x 109om
x 4 x 10 cm = 10 cm , and therefore a mass, taking n _ 3 x 10_cm - ,-
P
M _ n m V _ 5 x I016g o (2)
prom p p
Consequently we find that half a dozen or so large prominences are as massive
as the whole corona. From this one concludes that either a cycling of mass
must continually be going on or the material must - more likely - come from
lower, denser regions of the atmosphere. In either case we arrive at our
first crucial observational consequence: the d[namic nature of the corona and
prominence plasmas. Static models will no longer do. To understand the
formation of prominences and their interplay with the corona a holistic
approach is necessary. Figure I shows a sketch of the corona observed at the
Nov. 12, 1966, eclipse (Saito and Tandberg-Hanssen, 1973), and two large
quiescent prominences, seen as dark filaments on the disk, are situated under
the helmet streamer at positions dictated by the coronal structure. To
explore this situation further, we must look at solar magnetic fields.
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e_
Figure I
Corona and Prominence Classifications
We are so used to looking at _e changing shape of the corona from solar
maximum to solar minimum conditions that is is easy to forget the important
message this change carries. The classification of coronal shapes, the
observation of helmet streamers and the more recent information on coronal
holes and the solar wind point all to the crucial importance of the magnetic
field configuration. It is the magnetic field that completely determines the
shape and _havior of the coronal plasma; we are in a low-8 plasma,
(8 _ 8_nkT/B ), and a theory that does not include this aspect cannot be
complete.
On the other hand classification of prominences reveals a practical
scheme as given in Table I.
Table I
Prominence Classification
Quiescent occur in queiscent regions
subset: eruptive
Associated with Corona
Ac ti ve Occur in active regions
subset: sprays
Associated with
Sunspots
Loops Occur in active regions Associated with corona
and/or flares
Surges Occur in active regions Often associated with
subflares
Examining pictures of some of this objects, like active, sunspot promi-
nences or loops, we again cannot doubt the decisive influence of the magnetic
field structure on these prominences
This structure also plays an important part in quiescent prominences, and
measurementsof prominence magnetic fields (e.g. Leroy et al 1984) are crucial
in distinguishing between models of prominences (Malherbe et al 1983a), taking
the dynamic nature of these objects into account (Malherbe et al 1983b).
Prominence formation and stability
Prominences can, theoretically, form either by ejection of matter from
below or by condensation of matter from the corona. Surges and sprays from
according to the former mechanism; coronal rain and postflare loops seem to
owe their existence to the latter. In the case of quiescent prominences it is
often assumed that they form by condensation of coronal matter, and even
though this process may take place, we have seen above that it is difficult to
account for the material needed for a big quiescent prominence by this mechan-
ism. Rather, an ejection or a siphon- type mechanism probably supplies matter
into a pre-existing magnetic structure capable of supporting the prominence
Pikel'ner (1971). The very stable nature of many quiescent prominences is
also due to the action of the magnetic field, i.e. to its loop-shaped struc-
ture. Observations leave little doubt that it also is the loop-shaped
magnetic field that accounts for the shape and stability of phenomena like
coronal arches, postflare loop prominences, sprays, and transition region
loops. We therefore arrive at another crucial observational consequence: The
ubiquity of the magnetic loop. Table 2 illustrates this phenomenon, and shows
the importance of loop structures on nearly all observable lengthscales on the
Sun.
Table 2
Loops - a basic structure in solar physics
o Coronal arches - interconnect active regions
o Coronal loops, hot (> 106 k), cool (< 106 k)
o Flare loops - relationship to coronal loops?
o Loop prominences = post-flare loops
o Loop structures in quiescent prominences
o Transition-region loops, high and low
o Bright points (X-rays, UV) = small loops?
Eruptive Promlnences-Coronal Mass Ejection
The last crucial observation I want to direct your attention to is the
disappearance of prominences during flares and the correlated coronal
response. It seems that only a holistic approach will suffice to let us
properly explain this complicated flare manifestation. Borrowing from work by
Moore et al (1986), we can ascertain that the prominence eruption and the
accompanying coronal mass ejection both are caused by an underlying change in
the magnetic field - a global instability of the field configuration in the
region where the flare occurs - and are not caused by the energy release in
the flare. Fig. 2 illustrates both the change in hard x-ray intensity, show-
ing the i.npulsive phase of the flare, and the eruption of the He
prominence. We note that the eruption begins before the onset of the impul-
sive phase, probably caused by the same global instability in the magnetic
field that also is responsible for the energy release that causes the flare.
1956 1958 2OOO 2OO2 2OO4 2006
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Figure 2
Other observations than the ones I have discussed may certainly be label-
ed crucial, and we are, for example interested in the diagnostic being dis-
cussed in Vial's and Lang's groups to furnish temperatures, densities, veloci-
ties etc. to properly model the observed prominences and coronal manifesta-
tions. However, the list of observations I have discussed and the preliminary
conclusions drawn from them should form a basis from which we now can proceed
to better explain the "fascinating" and "mysterious" objects that are among
the topic of this workshop.
References
Hirayama, T. 1985, Solar Phys. 100____,415.
Leroy, J.L., Bommier, V., and Sahal-Brechot, S., 1984, Astron. Astrophys. 131,
33.
Malherbe, J.M., Priest, E.R., Forbes, T.G., and Heyvaerts, J. 1983a Astron.
Astrophys. 127____,153
Malherbe J.M., Schmieder, B., Ribes, E., Mein, P., 1983b Astron. Astrophys.
119____,197.
Moore, R.L., Kahler, S.W., Kane, S.R., and Zirln, H. 1986, Bull. AAS 18, No.2.
Pikel'ner, S.B., 1971, Solar Phys. 17__, 44
Saito, K. and Tandberg-Hanssen, E. 1973, Solar Phys. 31, 105.


Some crucial corona and prominence observations

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