The overall flare process involves phenomena characterized as the impulsive and gradual phases, following the X-ray signature first recognized by Kane (1969). In addition, evidence exists for a pre-flare phase in some flares, and recent Solar Maximum Mission data have shown that a post-flare phase, in which extensive and energetically important coronal activity occurs, may also exist. The data to describe the pre-flare and post-flare phases are insufficient to place them properly into an overall picture of the energetics, aside from noting that these phases may indeed be significant from the energetics point of view. What is presently known is reviewed and comments are made about the possible interactions among the flare structures involved

Crannell, C. J.

Hudson, H. S.

English

NASA Technical Reports Server

plasmacoolsislessclearthanit wasbefore.Inreality,our
knowledgeof themicrophysicsof plasmasi sopoorthat
wemustconsideralargenumberofpossiblemodelsbefore
makinganypositiveconclusions.
5.3.6 Summary of Conclusions
Let us address the five conclusions of Moore et al. (1980)
and suggest how future theoretical efforts could clarify them
further.
(1) We find that, under the assumption of filling factors
equal to one, the densities obtained in these five flares are
consistent with those of Moore et al. However, the SMM
results suggest that the particles are contained within fila-
mentary structures occupying no more than 1% of the ob-
served volume (de Jager et al., 1983; Wolfson et al., 1983).
Thus the densities quoted from the Skylab results are too
small by at least an order of magnitude. The large uncer-
tainties in the density and possibility of fine structure dic-
tate our response to the remaining four questions.
(2) Depending upon the flare parameters, either conduc-
tion or radiation can dominate the cooling process. Mass mo-
tions may also play a role in energy transport, a point not
considered by Moore et al. We therefore disagree with their
conclusion that conduction and radiation are generally equally
important. For small filling factors, radiative cooling will
dominate.
(3) In the May 21 flare, continued heating is needed, con-
firming the conclusions of Moore et al. The understanding
of how this heating can occur has improved on the basis of
the work by Forbes and Priest (1982, 1983a, b) and Cargill
and Priest (1982, 1983), which is discussed in Chapter 1 of
this report.
(4) The question of whether compact flares need long-
term heating is now open again as a result of the Idling fac-
tor problem. We do not know what type of filamentary struc-
ture exists in such flares. One possible way of heating such
flares is via turbulence (Bornmann, i985c).
(5) The chromospheric evaporation scenario proposed by
Moore et al. seems to be confirmed by our work on these
five flares.
Thus, two of the original conclusions of Moore et al. have
been unambiguously confirmed by our work. Unfortunately,
the SMM data have confused the other three; but they still
could be true. A crucial issue for theorists and observers is
to determine the nature of the fine structure in flares. This
would appear to hold the key to our understanding of the
decay phase. NS÷-19387
5.4. RELATIONSHIPS AMONG THE
PHASES
C.J. Crannell and H.S. Hudson
5.4.1 Introduction
The overall flare process involves phenomena we have
characterized as the "impulsive" and "gradual" phases, fol-
lowing the X-ray signature first recognized by Kane (1969).
In addition, evidence exists for a pre-flare phase in some
flares, and recent SMM data have shown that a post-flare
phase, in which extensive and energetically important coronal
activity occurs, may also exist. The data to describe the pre-
flare and post-flare phases are insufficient to place them
properly into an overall picture of the energetics, aside from
noting that these phases may indeed be significant from the
energetics point of view. In this section, therefore, we review
what is presently known and comment about the possible in-
teractions among the flare structures involved.
5.4.2 Relationship Between Impulsive and
Gradual Phases
The distinction between the impulsive and gradual phases
of a flare was originally made by Kane (1969). The ener-
getics relationship between these phases has been contro-
versial from the beginning, when Kane and Donnelly (1971)
showed that the large energy in 10-1030 lk bursts correlated
well with the energy inferred for 10 to 100 keV electrons,
assuming non-thermal bremsstrahlung as an explanation of
the hard X-ray bursts. This was the first real evidence that
particle acceleration during the impulsive phase could have
energetically significant consequences in the chromosphere,
the source of the EUV flashes. We now have far better data
with which to examine this question quantitatively, and this
subsection deals with the investigation of the energetic rela-
tionship between the impulsive and gradual phases.
That the impulsive and gradual emissions are related can
be seen in Figures 5.4.1 and 5.4.2, in which the peak count-
ing rates of hard X-ray bursts (Figure 5.4.1) and the total
hard X-ray counts (Figure 5.4.2) are plotted versus the peak
Ca XIX soft X-ray counting rate. These scatter plots include
all events observed with HXRBS and BCS which have both
a peak hard X-ray counting rate greater than 100 counts s -_
and a peak soft X-ray counting rate greater than 40 counts
s-L The total hard X-ray counts (TOTAL) are better cor-
related with the peak Ca XIX counting rate (BCS) than is
the peak hard X-ray counting rate. This result supports the
conclusion of Neupert (1968) that the gradual soft X-ray
emission resembles an integral of the impulsive hard X-ray
emission.
The significance of these results must be evaluated in view
of the Big Flare Syndrome (BFS) identified by Kahler (1982).
He found, quite simply, that bigger flares are bigger at all
wavelengths. Quantitatively, the BFS is manifest as corre-
lation coefficients of approximately 0.48, with a range of
0.3 to 0.65, between widely diverse parameters. The cor-
relations shown in Figures 5.4.1 and 5.4.2 clearly indicate
a closer relationship than would be expected from the BFS.
5-39
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-r 101
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10 o
NUMBER OF POINTS=101
ORRELATION COEFFICIENT=0.73
101 10 2 10 3 104
BCS PEAK COUNTS PER SECOND
Figure 5.4.1 Scatter plot of the peak HXRBS count-
ing rate versus the peak BCS counting rate in the Ca
XIX channel for all flares recorded jointly by these two
instruments in 1980 with >100 counts s -1 in
HXRBS and >40 counts s-1 in BCS. The three lines
were obtained from least-squares fits to the points
minimizing the vertical, horizontal, or perpendicular
distances of the points from the line.
In an earlier investigation of the relationships between
the energetics of the gradual and impulsive emissions, Cran-
nell et al. (1982) found a good correlation between the energy
in the energetic electrons, estimated from hard X-ray and
microwave observations analyzed with a thermal model, and
the energy in the soft X-ray emitting plasma estimated from
SOLRAD observations. The value of the correlation coeffi-
cient determined in that work lies in the range 0.8 to 0.9,
comparable to that found here for TOTAL versus BCS. In
Section 5.2 a similar comparison based on a non-thermal,
thick-target loop model of the impulsive emissions is pre-
sented. The correlation found there also is comparable to that
for TOTAL versus BCS.
From these results, we conclude that the energetics of
the gradual and impulsive emissions are more closely related
than are parameters which characterize the BFS. On the other
hand, we note with disappointment that the data we have con-
sidered are not sufficient to distinguish between the thermal
and the non-thermal model of the impulsive emissions and,
I--
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UMBER OF POINTS=101
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, A =f,,,,,t , , = ,,,,,I , , , i,,HI , 1 , J ,,,
101 10 2 10 3 10 4
BCS PEAK COUNTS PER SECOND
Figure 5.4.2 Same as Figure 5.4.1 but with the
HXRBS total number of counts integrated over the
duration of the flare (with a 40 counts s -_ back-
ground subtracted) substituted for the HXRBS peak
rate.
in fact, do not enable us to distinguish between the models
and a non-model, i.e. TOTAL versus BCS.
5.4.3 The Pre-Flare Phase
The pre-flare conditions are the subject of an entire chap-
ter of this workshop. As far as this chapter is concerned,
it is important for a given flare theory that the energy source
in the pre-flare state is adequate to supply the flare energy
release. Most modern flare theories derive the energy input
from the energy stored in stressed magnetic fields. The mag-
nitude of the stresses can be estimated from the photospheric
magnetic field distribution, with a model of the coronal field
that permits field-aligned currents to maintain the stress
(Tanaka and Nakagawa 1973). The amount of stored energy
then follows from the model, which can derive the energy
build-up rate from the photospheric _dlandary conditions.
Such models of magnetic storage and free energy genera-
tion in general contain adequate energy for flares. This con-
sistency of observations with models is mainly a result of
inadequate data upon which to determine exact model
parameters.
5 -40
5.4.4 The Post-Flare Phase
A new phase of flare activity, namely a late, high-altitude,
coronal phase, was first identified in SMM data for May
21/22 by Svestka et al. (1982a). Other similar events from
SMM were reported by Lantos et al. (1982) and Svestka et
al. (1982b and c), and in retrospect some of the large-scale
structures observed by Webb and Kundu (1978) probably
fall in this category.
Based upon HXIS observations, Svestka (private com-
munication) has estimated that the total energy content of
the post-flare arch structures in the 21 May event was be-
tween 1.5 × 1029 and 4.7 × 1030 ergs; the total energy in the
analogous structure of the November 6 event was 1.2 × 10 at
ergs. In both cases, the estimated energies are comparable
to the energies of other major flare components. It is neces-
sary in a complete picture of flare energetics to understand
the relationship between these manifestations and those of
the more well-known flare phases. It is premature, based
upon the limited number of events -- not all of which may
be of the same type -- to draw general conclusions yet.
5.4.5 Phenomena in the Distant Corona
Coronal transients, coronal mass ejections, interplanetary
shock waves, and the like have an uncertain but important
place in flare energetics. These phenomena can be observed
by coronagraphs and by meter-wave radio telescopes, as well
as by in situ techniques at larger distances from the Sun. The
Skylab coronagraph observations provided the earliest com-
prehensive views of coronal transients (Rust et al., 1980),
and observations have continued both in space (SMM and
P78-1) and on the ground (notably with the Manna Loa
K-coronameter).
The relationship of these coronal phenomena with clas-
sical Hot "chromospheric flares," or with the high-energy
flare events, remains problematical. There is no doubt that
major flares often produce major coronal transients, but we
have to guard against inferring a causal relationship: the BFS
may confuse the picture (Kahler 1982). Indeed the suggested
existence of "forerunner" coronal transients (Jackson and
Hildner 1978) could imply that the coronal phenomena cause
the flare rather than the other way round, and this is con-
sistent with some theoretical views. The relationships are ob-
scured by two major factors: there are only limited
quantitative observations in the key inner corona, and in the
outer corona there is confusion and uncertainty in the as-
signment of a given event to a given flare because of over-
lapping in time. Finally, it is known that coronal transients,
especially with low speeds, may arise without the occurrence
of a flare (Wagner, 1984). These events tend to fall in the
"eruptive prominence" classification.
The energetics analysis of coronal phenomena has not ad-
vanced appreciably since the Skylab Workshop treatment
(Webb et al., 1980). Among the prime flares studied by the
energetics team in this chapter, only one (June 29) had C/P
observations. However, even this limb flare was not satis-
factory for quantitative energetics analysis because it could
not properly be compared with the disk flares in the re-
mainder of our list.
N87-19338
5.5 CHARACTERIZATION OF TOTAL
FLARE ENERGY
H.S. Hudson
5.5.1 Statement of the Problem
5.5.1.1 Introduction
The total energy released by a solar flare has a certain
distribution in form as well as a certain pattern of flow among
the several forms, as described above. As data have grown
more comprehensive, the definition of this distribution has
improved; classical assessments are found in the works of
Ellison (1963), Bruzek (1967), Smith and Smith (1963), and
Smith and Gottlieb (1975). Most recently the Skylab flare
workshop (Sturrock 1980) addressed this question in two sur-
veys of a single well-observed flare on 1973 September 5.
These surveys dealt with the radiant energy (Canfield et al.,
1980) and the mechanical energy (Webb et al., 1980), and
their results have become the definitive data on flare ener-
getics despite acknowledged gaps in coverage and in theo-
retical understanding.
This section aims at updating our knowledge of this funda-
mental matter. Unfortunately, there are still limitations in
data coverage, as described in detail below. In some areas,
notably the X-ray and gamma-ray ranges, there have been
striking improvements, as reported above. We summarize
the improvements here and take the further step of attempt-
ing to fill in the gaps in coverage to estimate the total radi-
ant energy. One purpose for doing this is to rr__--rmita
comparison of the observed or estimated total with the up-
per limits derived from the precise total-irradiance monitor
(the Active Cavity Radiometer Irradiance Monitor-ACRIM)
on SMM.
5.5.1.2 Availability of Data
What are the key limitations in the data set available to
us? The foregoing discussions have naturally emphasized the
observed forms and have relied on theoretical considerations
to bridge the gaps. Where are the largest gaps? We discuss
these items briefly here and present recommendations for
future observations in Section 5.6.
The most important omissions from the data set fall into
two major areas: the radiant energy in optical and EUV
wavelengths, and coronal observations of all types. The
brightness of the quiet Sun makes the optical wavelengths
5-41


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