minute later, quite definite and observable (typical velocities are of order
Mach 1 = 10 © cm s-l in the chromosphere) Doppler shifts in both

directions (red followed by blue) are predicted by simple theoretical models,
and therefore what is required is relatively high time resolution spectral
line scans of suitable chromospheric lines.

In summary, therefore, it appears that properly coordinated
measurements during SMY are definitely capable of resolving some of the
central issues regarding thermalization of energy in flares. It is
emphasized, however, that further theoretical input is also needed to a large
amount of the above material, in order to make best use of the suggested
observations.

10


II. SMM Joint Observing Sequences

Observations with the SMM instruments will be made in Joint Observing Sequences
(JOSs) that are numbered in the same way as the SSOs and COSs. Each JOS has many vari-
ations, A complete list of instrument modes for all JOSs would double the size of this
report and so it is not reproduced; however, the JOSs relevant to SERF are compatible
with the observing sequences in Table 5.II and, in fact, this table is based in part
on the JOSs available in May 1979,

In order to indicate the general nature of the observations of which SMM is capable,
we reproduce below summaries of the SMM instrument characteristics that G. Withbroe, JOS
coordinator for SMM, kindly provided.

, (1) Gamma Ray Experiment (GRE)

© Spectrograms 0.3 - 17 MeV every 16s (7% resolution at 0.6 MeV).

Photometry in 100 - 300 keV band every 64 ms and 300 - 500 keY band every 1s.

Also 10 - 160 MeV gamma rays in 4 channels every 2s, 10 - 160 MeV neutrons
every 2s, and 10 ~ 160 keV X-rays in 4 channels every Is.

Whole-Sun resolution.

Single-point sampling time = 0.064s.

¥ (2) Hard X-Ray Burst Spectrometer (HXRBS)
Spectrograms in 16 energy channels every 100 ms
Photometry (flux integrated over all energies) to 1 ms resolution.
Triggerable memory freezes previous 120-s and succeeding 180-s data
(at 10 ms resolution). Constant count or time bins.
Whole-Sun resolution; FOV = +45°,
20 - 300 keV; 30% resolution at 100 keV.
Single-point sampling time = 0.001s.

v (3) Hard X-ray Imaging Spectrometer (HXIS)

6 spectroheliograms of an area of the Sun in 6 energy channels; various
channels may be added together.

Provides S/C flare alarm (position and intensity) after 2s.

At 8 arc sec resolution FOV = 2.8 x 2.8 arc min and at 32 arc sec resolution
FOV = 6.4 x 6.4 arc min.

3.5 - 30 keV; 18% resolution at 6 keV (mini-proportional counters).

Single-point sampling time = 0.125s.

y (4) X-Ray Polychromator (XRP) - Flat Crystal Spectrometer (FCS)
_ 7 spectroheliograms in fixed wavelengths of a rastered area, e.g., "Home
Position" (0 VIII He-like ions) for Tion.
7 spectroheliograms in flux from "Integral Scans" across short wavelength
bands.
7 short spectrograms/Dopplergrams of a point, e.g., "O VII Components" for
Ne.
Can locate bright points.
10 x 10 arc sec resolution; FOV < 7 x 7 arc min.
1.4 - 22.4 Rs 10° to 10" resolving power.
Single-point sampling time = 0.25s.

(5) X-Ray Polychromator (XRP) - Bent Crystal Spectrometer (BCS)
Spectrograms of HXIS region, especially in Fe XXV, Fe XXVI, Ca XIX, and Fe
Ka lines with 8 detectors

Recirculating queue memory triggerable to store previous 45s of data.
6 x 6 arc min resolution and FOV.

1.769 - 3.231 A; 5 x 10° resolving power,

Single-point sampling time = 0.064s.

31



TABLE 5.11

COLLABORATIVE OBSERVING SEQUENCES RELEVANT TO
THE STUDY OF ENERGY RELEASE IN FLARES

Column 1: Observations of particular importance to each COS. Magnetograms and
other synoptic data (ref. Chapter 5) are highly desirable in each COS. Column 2:
Specific objectives, as outlined at the SERF Workshop and in SMM Investigators*
Working Group meetings. Spatial resolution: Highest available, generally 1 - 10
are sec. Field of view: “a single active region~5 x 5 arc min, or one quadrant of
the solar limb, unless otherwise noted. Time resolution: See Table 5.1.

COS-001 Chromospheric Evaporation (ref. Chapters 4 and 2)

Scheduling criteria: Initiate following flare onset.

Hard X-ray spectra
SHGs
Soft X-ray spectra
SHGs
Doppler shifts
EUV spectra, flux
EUV Dopplergrams
UV Dopplergrams
Chromospheric spectra
O03, H@ filtergrams
Dopplergrams
Coronal photoheliograms
em-A radio spectra, SHGs
m-, radio spectra
SHGs, polarization

COS-002 Thermalization (ref. Chapter 2)

Scheduling criteria: Initiate before

onset.
y-ray spectra, flux

Hard X-ray spectra, flux
SHGs
Soft X-ray spectra, flux

SHGs

X-ray polarization
EUV spectra, flux
UV spectra
Optical spectra

O03, Ha filtergrams

photoheliograms

continuum polarization
em-2 radio SHGs, polarization
m-) radio SHGs, polarization

identify heating agent
locate heating agent

define heated region (e.g. emission measure)

locate ejecta
energy in ejecta

define heated region (e.g. emission measure)

locate ejecta

locate ejecta

study ejecta sources

locate ejecta

locate ejecta

follow material through corona

define heated region (e.g. emission measure)

follow material through corona
follow material through corona

define particle beam and target
properties

define electron energy distribution

locate energy release region

sample electron temperature, get
emission measures

locate energy release region

seek confirmation of electron beam

compare fluctuations with NT emissions

compare fluctuations with NT emissions

seek evidence of electric fields

compare fluctuations with NT emissions

white-light flare search

synchrotron emission test

locate electron beams

locate electron beams

27

or as soon as possible after flare


III. Newletter and Communications

The SMY Newsletter will be the principal means of informaing SERF participants of
Collaborative Observing Sequences and Joint Observing Sequences, of planned observations
and special intervals, and of meetings and recent observational results. The newsletter
will be mailed monthly to all interested parties. In May 1979 there were about 800 sub-
scribers to the newsletter. News of past and future observations relevant to the
scientific objectives of the SMY and requests for newsletters should be sent to

Mr. J.H. Allen

Central Information Exchange Office

World Data Center A for Solar-Terrestrial Physics
NOAA, D64

Boulder, Colorado 80303 U.S.A.

The newsletter is composed with a computerized text editor, and all material is
stored on computer files. By using a system of key words and dates, all references to
observations of a given flare or to a given scientific objective can be recalled and
summarized. A suitable code for the scientific programs, kinds of observations, and
observatory names will be developed from codes already in use at World Data Center A
for Solar-Terrestrial Physics and at the SMM Investigator Working Group meetings.

The purpose of the newsletter is only to supplement such volatile communications
as telephone and Telex, which will be used heavily before, during and after ACTION
intervals. Before an ACTION, participants should Telex or call the SERF coordination
center about observing conditions, predictions of activity, activity in progress, and
recommended ACTION intervals. Before an ACTION is called, the coordinator will consult
with participating observatories, according to the scientific objective of the contem-
plated ACTION, and especially with the following centers:

(1) SibIZMIR (via the Sun-Earth Council)

(2) NOAA and the Solar Observing Optical Network
(3) Solar Maximum Mission Operations

(4) Meudon Observatory

We emphasize, however, that SERF ACTION intervals in the "4 la carte" mode can be
built around requests for assistance from any source, including experimenters on satel-
lites such as ISEE-3 or Interkosmos and theoreticians who desire observational tests of
hypotheses about flare energy release. For practical reasons, SERF will not normally
be able to help with collaborations on problems not concerned with solar flares.

After ACTIONS, participating observatories will be asked to communicate a summary
of observations obtained. This can be done in condensed formats, to be developed, that
will describe the instrument, the observing mode, and the intervals successfully covered
during the ACTION. Codes for this purpose will be included in the catalog of partici-
pants' equipment, interests, and research programs to be issued by the Meudon Observa-
tory for the Flare Buildup Study and for SERF. Most rospective participants have
already responded to questionnaires circulated by SveStka, Zirin, or Rust. Additional
data will be requested in future mailings to make it easy for us to communicate with
you and to understand what you want from the Study of Energy Release in Flares.

IV. Data Exchange and Collaborative Data Analysis

In contrast to the FBS plan and because of the anticipated volume of data, SERF
presently does not plan to be directly involved in data analysis except to indicate to
the participants those data sets that seem applicable to the study of energy release in
flares. Periodically, throughout the SMY, the SERF Organizing Committee and the SMY
Steering Committee will review progress made toward each Specific Scientific Objective.
Periods of successful observations will be identified. Through personal contacts and
via_ the newsletter information on the flares most successfully observed during ACTIONS
will be publicized. We also hope to facilitate data exchanges through meetings spon-
sored by international bodies such as COSPAR and SCOSTEP. We recommend that summaries
of data obtained for outstandingly successful ACTIONS be published by World Data Center
A for Solar-Terrestrial Physics under their series of UAG Reports. Then, workshops will
be organized around the scientific objectives. The participants in the workshops wil
establish the level of collaborative data analysis. It is presumed, however, that all
participants in the SMY will be willing to share data obtained during ACTIONS with
others via collaborative analyses and scientist-to-scientist or institution-to-institu-
tion exchanges.

36



(Model N-T), the characteristic propagation velocity of the disturbance is much
faster (some 2' per second) and would thus be barely observable with the
available time resolution. Further, the location of the hard X-ray source is
extremely useful in discriminating between these models: for model N-T, we should
expect the brightest X-ray images at the feet of the loop; while for model T,
they should be at the top of the structure. This is one of the most vital
observations to make. However, a few words of caution regarding observational
interpretation are appropriate; mamely, the distinction between the motion of a
thermal "front" (as described above) or a macroscopic drift of material. These
both will occur in flares (although perhaps not the latter in the impulsive
flash, due to the long time scales involved - Craig and McClymont 1976) and
simultaneous Doppler measurements (using, for example, the SMM X-ray
polychromator) are desirable in order to discriminate between these possibilities.

N-T

Jreturn < Jcrit

PRECIPITATING
NON-THERMAL,
ELECTRONS

PRIMARY
<_—_. 4 ow RELEASE

TRANSITION
ZONE

FREE-STREAMING
ELECTRON FLUX

REDUCED
PRECIPITATION

OF NON-THERMAL
ELECTRONS

ION-ACOUSTIC
TURBULENT FRONTS
(CONFINE ELECTRONS WITH V$3Ve)

Fig. 2.1: Schematic of the two X-ray burst models N-T and T. In model N-T, the
primary release predominately accelerates non-thermal particles, which
subsequently thermalize and emit X-ray bremsstrahlung in the dense parts of the
loop. In model T, the primary release has predominantly thermal effects; the
current due to free streaming electrons is neutralized by a return current
JRETURN Which has an associated drift velocity higher than the local ion-sound
velocity = jorrt/ne. Thus, turbulent fronts are established which confine the
electrons at the top of the arch, where they emit thermal bremsstrahlung; the
more energetic electrons (v = 3Ve; Ve =(kT/me)!/2) can escape through

these fronts and thermalize as in model N-T

4



Nefedjev, V.P. 1979, Pisma_v. AZh, 5, #2.

Neupert, W.M. 1971, Solar Phys., 18, 474.

Orrall, F.Q. and Zirker, J.B. 1976, Ap. J., 208, 618.

Ox, E.A. 1978, Pisma_v. AZh, 4, 415

Pallavicini, R., Serio, S., and Vaiana, G.S. 1977, Ap. J., 216, 108.

Pneuman, G.W. 1979, Solar Phys., in press.

Ramaty, R., Colgate, S.A., Dulk, G.A., Hoyng, P., Knight, J.W., Lin, R.P.,
Melrose, D.B., Palzis C., Orrall, F.Q., Shapiro, P.R., Smith, D.F., and
Van Hollebeeke, M.A.I. 1979, in (P.A. Sturrock, ed.) "Solar Flares",
Proceedings of the Skylab Solar Flare Workshop.

Robinson, R.D. 1978, Solar Phys., 60, 383.

Roy, J.R. and Tang, F. 1975, Solar Phys., 42, 425.

Rust, D.M. and Hildner, E. 1976, Solar Phys., 48, 381.

Rust, D.M. and Webb, D.F. 1977, Solar Phys., 54, 403.

Rust, D.M., Hildner, E., Dryer, M., Hansen, R.T., McClymont, A.N., McKenna
Lawlor, S.M.P., McLean, D.J., Schmahl, £.9., Steinolfson, R.S.,
Tandberg-Hanssen, E., Tousey, R., Webb, D.F., and Wu, S.T. 1979, in (P.A.
Sturrock, ed.) "Solar Flares", proceedings of the Skylab Solar Flare
Workshop.

Sakurai, T. 1976, Publ. Astr. Soc. Japan, 28, 177.

Santin, P. 1971, Solar Phys., 18, 87.

Shapiro, P.R. and Moore, R.J. 1977, Ap. J., 217, 621.

Shea, M.A., Smart, E.F., and Wu, S.T. (eds.) 1977, "Study of Travelling
Interplanetary Phenomena", Ap. and Space Sci. Library, 71
[Dordrecht :Reidel]

Slottje, C. 1978, Nature, 275, 520.

Smith, D.F. and Lilliequist, C.G. 1979, Ap. J., in press

Somov, B.V. 1975, Solar Phys., 42, 235.

Somov, B.V. 1976, P.N. Lebedev Institute Report 88, 127.

Somov, B.V. and Syrovatskii, S.1. 1976, Soviet Phys. Usp., 19, 813.

Somov, B.V. and Syrovatskii, S.I. 1977, Solar Phys., 55, 393.

Spicer, D.S. 1977, Solar Phys., 53, 305.

Spicer, D.S. 1979, unpublished.

Spitzer, L.J. 1962, "Physics of Fully Ionized Gases" [2d. ed., New
York: Interscience].

Suemoto, Z. and Hiei, E. 1959, Publ. Astr. Soc. Japan, 11, 185.

SveStka, Z. 1973, Solar Phys., 31, 389.

SveStka, A. 1976, "Solar Flares" [Dordrecht:Reidel]

Syrovatskii, S.I. 1976, Pisma v. AZh, 2, 35.

Syrovatskii, S.I. 1977, P AZh, 3, 133.

Syrovatskii, S.I. and Shmeleva, 0.P. 1972, Soviet Astron. - AJ, 16, 273.

Takakura, T. and Kai, K. 1966, Publ. Astr. Soc. Japan, 18, 57.

Tindo, I.P. and Somov, B.V. 1978, in "New Instrumentation for Space Astronomy",
(Oxford: Pergamon] 131.

ma_v

64





of energy by non-thermal electrons enables the close synchronism of emission at
widely separated wavelength ranges (Zirin and Tanaka 1973; Emslie, Brown, and
Donnelly 1978; Emslie and Noyes 1978; Donnelly and Kane 1978) and so, presumably,
depth in the atmosphere, to be realized. Whether this synchronism of emission
over the bulk of the electromagnetic spectrum can be effected by other transport
mechanisms, such as conduction, depends on the temperature gradients in the
atmosphere, coupled with a knowledge of the characteristic velocity associated
with the transport mechanism (cf. Craig and McClymont 1976). Further development
of this line of reasoning requires the inclusion of some a priori concept of the
atmospheric structure in the flare (e.g. plane-parallel geometry - Machado and
Linsky 1975; Brown, Canfield and Robertson 1978) and a determination (Emslie and
Noyes 1978; Machado and Emslie 1979) of the differential emission measure
function &(T) (Craig and Brown 1976), and is still in fact an open question (cf.
Emslie and Noyes 1978). Energy transport by non-thermal electrons beams also
affords a straightforward interpretation of the hard X-ray bursts, i.e. via
non-thermal bremsstrahlung of the beam electrons [although an inverse Compton
mechanism, requiring fewer electrons but of higher individual energy, has also
been proposed (Korchak 1971)], and a great deal of analysis concerning the
intensity, spectrum, and polarization of this bremsstrahlung carried out (e.g.
Brown 1971, 1972; Hoyng, Brown, and van Beek 1976; Lin and Hudson 1976; Bai and
Ramaty 1978).

There are, however, a number of problems with the above electron beam model
scenario. Firstly, it is long recognized that the number of electrons required
to produce typical hard X-ray fluxes by thick-target bremsstrahlung (Hoyng,
Brown, and van Beek 1976) places great constraints on particle acceleration
mechanisms (cf. Heyvaerts 1979) and so on the primary energy release mechanism
[these problems are made even worse if a thin-target (Datlowe and Lin 1973)
source is postulated. or if the importance of return current ohmic losses is
recognized (Emslie 1979b; see below)]. Secondly, recent studies of the
inter-relationship of hard X-ray fluxes with fluxes in other (thermal) wavebands
such as EUV (Emslie, Brown, and Donnelly 1978); Donnelly and Kane 1978) and H
(Brown, Canfield, and Robertson 1978), have strongly suggested that approximately
one to two orders of magnitude less electrons thermalize in the lower atmosphere
than are required to produce the hard X-ray burst by a non-thermal thick-target
bremsstrahlung process. Further, overall estimates of the energy released over
all wavelengths (Canfield et al. 1979b) fall short of the total electron energy
by a similar amount, thus Suggesting that in fact the degree of electron beam
thermalization in the chromosphere is less than previously estimated.

Attempts have been made to resolve some of these discrepancies, such as
invoking detailed source geometries (in the EUV case; Donnelly and Kane 1978),
attempting refinements to existing theoretical models (McClymont and Canfield
1979), or assuming that thermal emission such as Ha in fact originates not in the
chromosphere, but in low-temperature coronal loops (Zirin 1978). However, there
is growing evidence to suggest that the simple collisional degradation model is
not a completely accurate picture. Firstly, it has been frequently noted (Hoyng,
Brown, and van Beek 1976; Knight and Sturrock 1977; Brown and Melrose 1977; Hoyng
and Melrose 1977; Colgate 1978; Hoyng, Knight, and Spicer 1978) that there is a
strong need for a charge-neutralizing return current in beamed flux models, in
order to avoid the establishment of unacceptably large electrostatic fields and
magnetic energy densities associated with an un-neutralized beaml, However,
little work has been done in incorporating the associated return current joule
heating into such beam models, and in fact this joule heating can dominate over
standard collisional losses for sufficiently large events at sufficiently great
heights in the atmosphere (Knight and Sturrock 1977; Emslie 1979b). More
specifically (Emslie 1979b), the effect of this joule heating is to overheat the
corona (thus possibly creating a substantial thermal X-ray component; this
problem is still under development) and underheat the chromosphere (for a given

T This return current is not so important if the electrons have a low degree
of anisotropy (such as in trap models - e.g. Takakura and Kai 1966; Brown
and Hoyng 1975; Melrose and Brown 1976; Bai and Ramaty 1979; Emslie,
McCaig, and Brown 1979), and its presence or absence, as evidenced by the
microwave signature caused by runaway electrons accelerated by the induced
& field, could well be an important flare diagnostic.

44

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The binding energy of the electron is about 7 keV. Thus, this ionization
process is realized by means of electrons accelerated up to energies = 10 keV
coming from outside, i.e., with a directional velocity distribution.

From this point of view it is interesting to compare spectra with
polarization of the X-ray radiation of the flare (Tindo et al. 1972b). This
comparison is presented in Figs. D9 and D10. It can be seen that at the initial
and final stages of flares where the relative intensity of lines Kg and Kg of
lower ions is comparatively high, the polarization value is also high, indicating
directly the presence of directed electron beams. Note that appreciable
polarization was observed in 9 flares out of 14 studied (Tindo and Somov 1978).

Thus, the results of spectral interpretation and Polarization measurements
indicate that at the initial stage of the flare some directed accelerated
electrons evidently appear, which penetrate rather deep into the solar atmosphere
and give raise to hard bremsstrahlung hard X-ray emission and K, and K, lines of
ions in rather low ionization stages as was suggested earlier. “Then nBar the
maximum stage of the flare the flux of high energy electrons increases so that a
certain volume of plasma is heated and the electrons are thermalized both in
velocity value and direction. We cannot say whether the fluxes of directed
electrons are still preserved at this stage since the lines Ky , Kg are lost in
the strong continuous background of thermal radiation. At the final stage of
flares, the accelerated electrons again play an important role, although in large
flares the accelerated mechanism is likely to be different from that at the
initial stage of the flare (de Jager 1976).

Table DIII gives experimental values of radiation fluxes for different lines
in flare spectra taken on 5 November 1970 and evaluations of physical parameters
of the flare region obtained on the basis of these data. As is seen from the
Table, the whole picture is described well within the supposition of a
comparatively small flux of accelerated nonthermal electrons. Our attention is
attracted by the fact that temperature values in the flare region determined by
the ratio of intensities of the resonance and satellite lines of Fe XXV, and also
by the distribution of satellite line intensities, are in good agreement. The
value of the temperature determined by the intensity ratio of resonance lines of
Fe XXVI and Fe XXV turns out to be much higher. This fact also points to the
important additional ionization and excitation of the La line of Fe XXVI by
non-thermal electrons.2

The role of accelerated electrons as a dominant agent which is responsible
for most of a flare's manifestations is confirmed by a number of experimental
facts (SveStka 1976): the power law character of bremsstrahlung spectra z lo
keV, the fine structure of this radiation in the form of "elementary bursts" with
the duration of seconds or less (van Beek, de Feiter, and de Jager 1974), the
presence of accelerated electrons in the interplanetry space, and other
phenomena. On the other hand, this hypothesis has lately met serious theoretical
difficulties (Hoyng, Brown, and van Beek 1976; Emslie 1979b). This problem should
be studied further both experimentally and theoretically.

We think that important information can be obtained if generation heights in the
solar atmosphere of X-ray flare radiation of different energy can be observed
with sufficiently high spatial and temporal resolution. Qur measurements of the
height of the soft X-ray radiation region E<1 keV and also Skylab photos, show
that this radiation appears at the height 2@ - 25000 km in the tops of "arches"
or "loops" (Zhitnik 1974; Pallavicini, Serio, and Vaiana 1977; Vorpahl et al.
1975). It is possible that just in this very region of the corona accelerated
electrons appear, either as a result of the rupture of current sheets (Somov and
Syrovatskii 1976), or due to another mechanism. These particles are indicated by
polarization values although up to now we have these data only for three flares
(Tindo et al. 1972b).

z Possibly a contribution to this ionization is made by hard X-ray emission
from the outside flare regions (Basko 1977).

3 An attempt fo interpret this by a two-component plasma model failed because

the intensity ratio of Fe XXVI satellite and La lines also gives a
temperature of about 20 x 106 kK (Grineva et al. 1975).

54


2. The Problem of the Determination of the Flare Triggers.

It is not yet clear where the disturbance which initiates a flare comes from: from the lower layers
of the solar atmosphere or from above. Is any external disturbance necessary for the onset of a flare at
all or is it a self-exciting system? Finally, one should know the triggering mechanism, the threshold
of instability leading up to a flare. Formulation of the following observational programs is advisable.

2,1 Assessment of the degree of deviation of the character of the magnetic field of the active
region in the upper layers of the solar atmosphere from a potential field and interpretation of these de-
viations. It should be determined whether these deviations of the magnetic field from the potential field
are associated with the existence of an internal current system in the magnetic structure or due to an
external current system from neighboring structures (Molodenskii and Syrovatskii, 1977). The recording
of magnetic fields prior to a flare, of the full vector H at the photospheric level and Hy in the chromo-
sphere in the H-line and acquisition of solar images in UV and X-rays are indispensable here to construct
a three~dimensional magnetic configuration of the active region. In addition, data on soft X-rays will
permit determination of the location of the feet of loop structures of the magnetic field.

2,2 In bipolar arch-like structures, the links between the triggering mechanism and the manner
of the movement of arch feet should be established (Spicer, 1977) and the reason of loops twisting found
out. For this purpose, it is advisable to acquire data in UV and X-rays. By using them, one can reveal
the plasma temperature distribution in the loop configuration, determine the type of plasma MHD-insta-
bility in the arch-like structure and the typical size of this instability at different stages of development
of this structure.

2.3 Recording of the metastable current sheet being in the pre~flare state. This is necessary to
determine the phases of the active region where the current sheet can exist and to determine the param~
eters of the sheet. With this purpose, in addition to the radio observations mentioned in 2.1, spectral
lines at 300-1500 R should be investigated. In particular, the Hel, Hell, CII, Cll, Nell, SII lines
(Syrovatskii, 1977) and possibly lines at optical wavelengths whose emission may be invoked by sec-
ondary effects of the current sheet. Syrovatskii (1976) showed that prior to the thermal instability
switch-on in the sheet and consequently prior to the flare process development, the current sheet can
represent a thin and comparatively cold plasma formation with dimensions of 10!9 cm x 2 109 cm x 20 cm
with a particle density n = 2 x 10! onTM? and a temperature T < 105 K, located in the solar corona.
Such a sheet can persist for a relatively long time ina stationary state until the temperature in it ex-
ceeds a critical value (Tur and Priest, 1978).

2.4 To clear up the nature of the agents which cause sympathetic flares, radio pictures of the
Sun obtained with the radiospectrograph and heliograph (Wild, 1969) at cm, dm and m wavelengths and
acquisition of photoheliograms of the full disk of the Sun in the Ha-line, are needed.

2.5 During the flare process development, the program of observations with the panoramic mag~
netograph with indispensable control of the magnetosensitive line profile variation is required. This
will allow one to establish a link between magnetic field variations and flare location.

ACKNOWLEDGMENTS
In conclusion, we express our gratitude to the members of the "Solar Plasma Physics" Workshop

of SibIZMIR who took part in the discussion of the problems stated above as well as to Dr. B.V. Somov,
Professor V.V. Zheleznyakov and Dr. E.Ya. Zlotnik for useful information.

42



(3) Determination of the elemental composition of the acceleration region
can be carried out using combined measurements of the y-ray "hump" at 1 - 2 Mev
and the electron spectrum in the same energy range [cf. sections I(1) and I(10)].

(4) In order to discriminate between models of 3He rich flares, some of
which favor preferential acceleration (Colgate 1978), and some of which favor
plasma enrichment in the ambient corona (Fisk 1978), spectroscopic determinations
of the abundances of He and heavy nuclei are desirable. in particular, the
detection of y-ray lines from such 3He rich flares would greatly clarify the
issue.

(5) In addition, such 3He rich events seem to be correlated with
low-speed solar wind regions (Zwickl et al. 1978). Since variation in the solar
wind velocity affects interplanetary Propagation of particles (Morfill, Richter,
and Scholer 1978), observations of these events should be coordinated with
measurements of interplanetary structure (section V). In addition, Axford's |
(1977) model predicts that low solar wind velocities are related to transient
open and to closed field regions. Such correlations between field geometry,
particle stream speed, and isotopic composition should be investigated.

III. Location and Temporal Behavior of the Primary Energy Release

(1) High resolution hard X-ray images should be capable of determining
whether subsequent elementary flare bursts (EFB's - de Jager and de Jonge 1978)
are energizations of the same area or of different areas

(2) High time resolution soft X-ray pictures showing the spread of the
energization throughout the flaring volume will also help resolve this question,
as well as possibly discriminating between energy release models (cf. models N-T
and T of Chapter 2).

(3) Radio observations (Slottje 1978) show that solar microwave fluxes
exhibit fluctuations on timescales of the order of milliseconds. The hard X-ray
spectrometer on SMM should be able to test for fluctuations in the X-ray flux on
similar timescales, and so determine whether the EFB is indeed the fundamental
"building block" of the flare energy release.

(4) High spatial resolution vector magnetograms (using the SMM ultraviolet
spectrometer and ground-based Stokes polarimeters) could provide important
information regarding the magnetic field structure in the preflare configuration,
to be compared with the high spatial resolution X-ray pictures mentioned above.

Iv. The Accelerating Agent

(1) High time resolution (about 10 ms) measurements of the total flux at
centimeter wavelengths are urged, the objective being the detection of source
brightness temperatures in excess of 1014 kK, indicative of a plasma wave
acceleration process in centimeter-wave burst sources.

(2) Spectroscopic signatures of strong electric fields are line broadening
and shifts due to the linear and quadratic Stark effects respectively (cf.
Chapter 2); evidence for a turbulent acceleration mechanism would be the
appearance of turbulence-induced forbidden line transitions (Davis 1977).

(3) Simultaneous optical and decametric radio observations of the spatial
evolution of the flare disturbance are capable of detecting collisionless
bow-shock waves (types II and IV radio bursts). A good estimate of the Alfvenic
Mach number of these shocks with time is important for testing theoretical models
of particle acceleration by expanding shock waves.

(4) Particle-accelerating shocks should evidence themselves in the Doppler
broadening of spectral lines, the typical velocities involved being of the order
of 100 km s-l,

14


(6) Ultraviolet Spectrometer/Polarimeter (UVSP)
4 spectroheliograms of a rastered area in any of several sets of lines.
2 Dopplergram rasters produced by slit pairs on 2 different lines.
4 spectrograms (line profiles) across short spectral bands.
2 magne tograms (or polargrams) of a rastered area using waveplates on a pair
oO ines.
Coronal spectroheliograms in Lya to R/Rg = 10,
Can locate bright points.
S/C pointing = #2 arc min
1 x 1 are seg resolution; FOV < 4 x 4 arc min.
1100 - 3000 A; 0.01 A resolution.
Magnetic field noise level ~ 50 G.
Velocity noise level ». 60 km/s.
Single-point sampling time = 0.016s (one detector operating).

(7) Coronograph/Polarimeter (C/P)
Filtergrams of the corona; in white light, coronal emission line FeXIV and Ha
polarization techniques for plane of the sky, magnetic field direction, and
resonance scattering information.
Dopplergrams from tuneable Fe XIV filter
6.4 arc sec; FOV s 6 x 6 Req (at 8 limb positions).
4448 - 6585 R (7 passbands).
Single-point sampling time = 60s.

(8) Active Cavity Radiometer Irradiance Monitor (ACRIM)
Total solar irradiance measurements from three dual conical cavity radi-
ometers.
FOV = 5°,
UV to IR spectral coverage.
Absolute accuracy +0.1% (S.I.
Single-point sampling time =

); precision +0,02%,
2s.

32


Hiei, E. and Widing, K.G. 1979, Solar Phys., in press.

Hildner, E. 1977, in (M.A. Shea et al., eds.) "Study of Travelling Interplanetary
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Hirayama, T. 1963, Publ. Astr. Soc. Japan, 15, 122.

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Nakagawa, Y. 1979, preprint.

63



PREFACE

On February 26, 1979, a workshop was canvened in Cambridge, Massachusetts,
USA, to devise the scientific program and objectives for the Study of Energy
Release in Flares (SERF), a program of the Solar Maximum Year (SMY). The
workshop was sponsored by the National Science Foundation, and organized by
American Science and Engineering, Inc. Most of the workshop was devoted to
parallel meetings of three small teams, where theories of thermalization
particle acceleration, and mass motion in flares were discussed. The teams were
led by D. Spicer, R. Lin, and E. Tandberg-Hanssen, respectively. ach team
outlined a specific scientific program to test current theories. Because of the
nature of the flare problem, most tests require simultaneous observations by
ground-based and space observatories, and it is the participants' hope that the
present volume will help provide the rationale for world-wide collaboration
during the SMY.

Thirty-two scientists, including eighteen from the US, four from the USSR,
three from France, two from the UK, two from the Netherlands, two from Japan, and
one from West Germany, braved the cruel New England winter to attend the
workshop. The names of the participants, the home institutions and the workshop
team each served on appear on the the next page. The enthusiam and plain hard
work of the participants is evident from the cogent scientific program they
created.

An overview of the major scientific issues identified for the Study of
Energy Release in Flares is given in Chapter 1. More specific discussions of the
three program areas appear in Chapters 2, 3, and 4. Chapter 5 explains the
concepts of Joint Observing Sequences and Collaborative Observing Sequences
(COSs) developed by NASA's Solar Maximum Mission investigators. Lists of the
specific observations needed for each COS of the SERF appear in Chapter 5, also.

At the workshop, A. G. Emslie, J. Heyvaerts, R. MacQueen, S. Mandel'stam,
DO. Rust, P. Simon, V. Stepanov and E. Tandberg-Hanssen gave keynote talks. Those
for which texts are available are reprinted in the appendices. The remainder of
the volume was compiled by the editors from discussion notes and from written
material provided by the participants. We are grateful to all the participants
for their guidance and comments on early versions of these Proceedings. Material
on the Joint Observing Sequences and characteristics of the experiments on the
Solar Maximum Mission (due for launch on 19 October 1979) were provided by G.
withbroe and W. Wagner. F. Orrall provided information on the COSs developed at
the SMM Investigators' Working Group meetings

Mrs. Elaine O'Neill and Mrs. Maureen Gaffney, of American Science and
Engineering, Inc., helped greatly to make the workshop a success, from the
initial mailings through the daily arrangements to the production of these
proceedings. The staff of the Massachusetts Institute of Technology Faculty Club
provided warm hospitality and help throughout the meeting.

This work was sponsored by NSF under grant No. ATM78-23322. Publication
costs were assumed by the World Data Center A for Solar-Terrestrial Physics in
Boulder, Colorado, USA. The WDC-A is sponsored by the National Oceanic and
Atmospheric Administration. Any opinions, findings, and conclusions or
recommendations expressed in this publication are those of the authors and do not
necessarily reflect the views of the National Science Foundation or the National
Oceanic and Atmospheric Administration.

David M. Rust

A. Gordon Emslie
Cambridge, Mass.
May 1979

iti

|
j





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Berlin.

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Vlahos, L. and Emslie, A.G. 1979, in preparation.

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press.

Zhitnik, I.A., Korneev, V.V., Krutov, V.V., Mandel'stam, S.L., Tindo, I.P., and
Urnov, A.M. 1979b, Solar Phys., to be submitted.

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Ap. J., 225, 281.

65



‘UAG-48A

UAG-49

UAG-50

UAG-51

UAG-52

UAG-53

UAG-54

UAG-55

UAG-56

YAG~57

UAG-58

UAG-59

UAG-60

UAG-61

UAG-62

UAG-63

UAG-64

UAG-65

UAG-66

UAG-67

UAG-68

UAG-69

UAG-70

‘UAG-71.

“Synoptic Observations of the Solar Corona during Carrington Rotations 1580-1596 (11 October 1971 - 15 January 1973)",
[Reissue with quality images} by R. A, Howard, M. J. Koomen, 0. J. Michels, R. Tousey, C, R. Detwiler. D. E.
Roberts, R. T. Seal and J. D. Whitney, E. 0. Hulbert Center for Space Research, NRL, Washington, D.C. 20375
and R. T. and S. F. Hansen, C. J. Garcia and E. Yasukawa, High Altitude Observatory. NCAR, Boulder, Colorado
80303, February 1976, 200 pages, price $4.27.

“Catalog of Standard Geomagnetic Variation Data", prepared by Environmental Data Service, NOAA, Boulder, Colorado,
August 1975, 125 pages, price $1.85.

“High-Latitude Supplement to the URSI Handbook on Ionogram Interpretation and Reduction", by W. R. Piggott, British
Antarctic Survey, c/o SRC, Appleton Laboratory, Ditton Park, Slough, England, October 1975, 292 Pages, price $4.00.

“Synoptic Maps of Solar Coronal Hole Boundaries Derived from He I1 304A Spectroheliograms from the Manned Skylab
Missions", by J. D. Bohlin and D. M. Rubenstein, E. 0. Hulbert Center for Space Research, Naval Research Lab=
oratory, Washington, D. C. 20375 U.S.A., November 1975, 30 pages, price 54 cents.

“Experimental Comprehensive Solar Flare Indices for Certain Flares, 1970-1974", compiled by Helen W. Dodson and
F; Ruth Hedeman, McMath-Hulbert Observatory, The University of Michigan, 895 Lake Angelus Road North, Pontiac,
Michigan 48055 U.S.A., November 1975, 27 pages, price 60 cents.

“Description and Catalog of Tonospheric F-Region Data, Jicamarca Radar Observatory (November 1966 - April 1969)", by
Het. Clark and T. £. Van Zandt, Aeronomy Laboratory, NOAA, Boulder, Colorado 80302 and J. P. McClure, University
of Texas at Dallas, Dallas, Texas 75230, April 1976, 10 pages, price 33 cents.

“Catalog of Ionosphere Vertical Soundings Data", prepared by Environmental Data Service, NOAA, Boulder, Colorado
80302, April 1976, 130 pages, price $2.10,

“Equivalent Ionospheric Current Representations by a New Method, Illustrated for 8-9 November 1969 Magnetic Disturb-
ances", by Y. Kamide, Cooperative Institute for Research in Environmental Sciences, University of Colorado,
Boulder, Colorado 80302 and Geophysical Institute, University of Alaska, Fairbanks, Alaska 99/01, HM. Kroehl ,
fata Studies Division, NOAA/EDS/NGSDC, Boulder, Colorado 80302, M. Kanamitsu, Advanced Study Program, National
Center for Atmospheric Research, Boulder, Colorado 80303, J. H. Allen, Data Studies Division, NOAA/EDS/NGSOC ,
Boulder, Colorado 80302, and S.-I. Akasofu, Geophysical institute, University of Alaska, Fairbanks, Alaske
99701, April 1976, 91 pages, price $1.60.

“Iso-intensity Contours of Ground Magnetic H Perturbations for the Decenber 16-18, 1971 Geomagnetic Storm”, by
{. Kamide, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder.
Colorado 80372 and Geophysical Institute, University of Alaska, Fairbanks, Alaska 99701 (currently Guest: worker
at Data Studies Division, NOAA/EDS/NGSDC, Boulder, Colorado 80302), April 1976, 37 pages. price $1.39.

“Manual on Ionospheric Absorpticn Measurements", edited by K. Rawer, Institut fiir Physikalische Wel traumforschung,
Freiburg, G.F.R., June 1976, 202 pages, price $4.27.

“ATS6 Radio Beacon Electron Content Measurements at Boulder, July 1974 - May 1975", by R. B. Fritz, Space Environment
Laboratory (currently with Wave Propagation Laboratory), NOAA, Boulder, Colorado 80302 USA, September 1976,
61 pages, price $1.04.

“Auroral Electrojet Magnetic Activity Indices AE(11) for 1974", by Joe Haskell Allen, Carl C. Abston and Lestie D.
Norris, National Geophysical and Solar-Terrestrial Data Center, Environmental Data Service, Decenber 1976,
144 pages, price $2.16

“Geomagnetic Data for January 1976 (AE(7) Indices and Stacked Magnetograms)" by J. H. Allen, C. C. Abston and
L. D. Morris, NGSOC/EDS/NOAA, July 1977, 57 pages, price $1.07.

“Collected Data Reports for STIP Interval II 20 March - 5 May 1976", edited by Helen E. Coffey and John A.
Nekinnon, National Geophysical and Solar-Terrestrial Data Center, Environmental Data Service, August 1977,
313 pages, price $2.95.

“Geomagnetic Data For February 1976 (AE(7) Indices and Stacked Magnetograms)" by J. H. Allen, C. C. Abston and
L. 0. Morris, NGSOC/EDS/NOAA, September 1977, 55 pages, price $1.11.

“Geomagnetic Data for March 1976 (AE(7) Indices and Stacked Magnetograms)" by J. H. Allen, C. C. Abston and
L. D. Morris, NGSDC/EDS/NOAA, September 1977, 57 pages, price 111.

“Geomagnetic Data for April 1976 (AE(8) Indices and Stacked Magnetograms)" by J. H. Allen, C. C. Abston and
L. D. Morris, NGSDC/EDS/NOAA, February 1978, 55 pages, price $1.00.

"The Information Explosion and Its Consequences for Data Acquisition, Documentation, and Processing" by G. K. Hartmann,
Max-Planck-Institut fiir Aeronomie, 0-341 Katlenburg-Lindau 3, GFR, May 1978, 36 pages. Price 75 cents.

“synoptic Radio Maps of the Sun at 3.3nm 1970-1973" by Earle B. Mayfield, Space Science Lab., and Fred I. Shimabukuro
Electronics Res. Lab., The Ivan A. Getting Laboratories, The Aerospace Corp. , £1 Segundo, California 90245,
May 1978, 30 pages, price 75 cents.

“Tonospheric D-Region Profile Data Base, A Collection of Computer-Accessible Experimental Profiles of the D and Lower

E jRegions’, by L. F, McNamara, Ionospheric Prediction Service, Sydney, Australia, August 1978, 30 pages, price
cents.

“A Comparative Study of Methods of Electron Density Profile Analysis", by L. F. McNamara, Tonospheric Prediction
Service, Sydney, Australia, September 1978, 56 pages, price $1.41.

“Selected Disturbed D-Region Electron Density Profiles. Their relation to the undisturbed D region", by L. F.
NcWiamara, onospheric Prediction Service, Sydney, Australia, October 1978, 50 pages, price $1.29.

“Annotated Atlas of Ha Synoptic Charts for Solar Cycle 20 (1964-1974) Carrington Solar Rotations 1487-1616", by
Patrick S. McIntosh, Space Environment Laboratory, ERL/NOAA, February 1979, 327 pages, price $3.50,
“Magnetic Potential Plots Over the Northern Hemisphere for 26-28 March 1976", by A.D. Richmond, SEL/ERL/NOAA, H.W.

Sroehl» NGSOC/EDIS/NOAA, M.A. Henning, Lockheed Missiles and Space Co.,’Aurora, CO. ang? Kamide, Kyoto
Sangyo Univ., Kyoto, Japan, April 1979, 115 pages, price $1.50.



Chapter 5
RECOMMENDED OBSERVATIONS

I. Collaborative Observing Sequences

In each of the preceding chapters a group of closely related scientific problems
was discussed and appropriate kinds of observations were mentioned somewhat inciden-
tally. In this chapter readers inspired by the scientific discussions will find spe-
cific needed observations grouped together as Collaborative Observing Sequences (COSs).
During the SMY, whenever it appears that one of the scientific objectives of the SERF
subprogram can be pursued profitably, an SMY Coordinator (see Appendix E) will suggest
that observers institute a specific COS. (Details of the operations are summarized in
Appendix A.)

Following the lead of the Solar Maximum Mission investigators, we recommend that
COSs be numbered and identified with Specific Scientific Objectives (SS0s) as follows:

® 001 Chromospheric Evaporation
e002 Thermalization
© 003 Nature of Electron Acceleration
004 Coalignment
005 Ejecta
006 Flare Energy Budget
007 Bright Points
e 008 Impulsive Phase
e 009 Location of Energy Release Sites
010 Evolution of Active Regions
011 (None)
012 Active Region Structure
013. Limb and Disk Surveys
014 Flare Buildup (FBS objective)
e 015 Flare Precursor (FBS objective)
016 Coronal Heating Mechanisms
017 Coronal Evolution
018 Flare Decay (FBS objective)
@ 019 Flare Morphology
e 020 Ion Acceleration
021 Hydrodynamics
022 Spotless Flares (FBS objective)

Topics without bullets (e) are not SERF objectives, although some like 004 Coalignment
are of obvious importance to SERF operations. Very detailed COSs for SSO 014 have been
developed by Z. Svestka for the SMY Flare Buildup Study and we will not repeat them.

Table 5.1 summarizes the varieties of flare-specific observations that are called
for in the SERF COSs. The interval At between successive observations is, in most
cases, the limit set by available instrumentation, Where instruments are capable of
recording data much more rapidly than required by SERF COSs, the shortest required sam-
pling rates are given. In Table 5,1 "IP" stands for interplanetary, "“SHGs" stands for
spectroheliograms, and the At intervals given are the approximate times required to
complete an SHG of a region of roughly 5 x 5 arc min. The At intervals are based on
the assumption that the highest possible resolution will be used

25


Chapter 3
MECHANISMS OF PARTICLE ACCELERATION

Particle acceleration in solar flares can be considered to occur in two distinct
classes: the so-called first-stage, common to all flares, in which 10-100 keV
electrons are accelerated; and the second-stage acceleration phase, responsible
for the production of relativistic electrons and heavy ions, which occurs only in
a small number of (usually large) events. In addition to these two main
acceleration processes, there is also acceleration of electrons to produce type
III radio bursts (which may in fact be due to an entirely different mechanism
than that responsible for the first-stage acceleration), and the steady
production, for hours or even days, of sub-MeV ions. For a review of these
acceleration processes, see Ramaty et al. (1979), while a schematic
representation of these processes and their relevance to the flare as a whole is

illustrated in Figure 3.1.
p e PB @ 2nd PHASE
ION & ELECTRON
ACCELERATION ?
—y, x SHOCK WAVE

TM. TYPE IE BURST
~

“N
~

MASS EJECTION
MOVING TYPE IZ

IMPULSIVE PHASE —=4 BURST

ACCELERATION ?

~10' % SOFT
X-RAY PLASMA

A

7raoiativery UNSTABLE
7 EUV
y EH
| WHITE LIGHT
f | | CONTINUUM
YEH Solar Molar HEY === WAVE BURST
\ }
SEA, BE

Fig. 3.1: Schematic illustration of the particle acceleration phenomena and their
relation to the overall flare process. The existence of a distinct second-stage
acceleration process is not clear; nor is the implied cospatiality of the type II
shock front and the second-stage acceleration region.

11



Chapter 4

MASS MOTIONS

Mass motions play an important - perhaps even dominant - role in the flare
phenomenon. They can occur on several scales and at various times thoughout the
flare, and may be broadly classified into four somewhat overlapping categories:

(1) "Intrinsic" motions; i.e., internal hydrodynamic motions of the
heated flare plasma -- these are considered in Chapter 2 [cf. Sections I(1)
and I(7) therein], where tests are proposed to distinguish between these
motions and thermal conduction fronts.

(2) "Thermally driven" motions; i.e., motions caused by external heating
of material. These are also clearly connected with energy thermalization
through the process of "evaporation", i.e., upward convection of material
(cf. Kostyuk and Pikel'ner, 1975; Antiochos and Sturrock, 1978; Hiei and
Widing 1979). Intrinsic motions may result in significant mass and energy
transport, but the motions are all constrained by local magnetic flux tubes.

(3) "Infall;" i.e., relaxation of previously evaporated material back into
the chromosphere during the decay phase of the flare. Impact of this
material may cause further flare brightening (Hyder 1967a, b). These
“infall-impact" brightenings will not be considered further in this report,
but we note that the pattern of such brightenings can help illuminate the
magnetic field configuration of flaring regions.

(4) "Ejections"; i.e., macroscopic motions due to extensive rearrangement
of the magnetic field or to large-scale thermodynamic disturbances. These
exhibit themselves in the form of (e.g.) spray prominences, eruptive
prominences and coronal transients. It is possible also to summarize many
optical soft-X-ray, radio and white-light observations of mass ejection
events in a common time-height diagram (Figure 4.1).

Mass motions may account for a large fraction of the total flare energy
(Canfield et al. 1979b; Rust et al. 1979). In order to fully understand the
energy budget in flares, therefore, it is especially necessary to know the nature
of large-scale and thermally driven ejections. In what follows, we discuss what
seem to us to be the most important questions to be answered about these
phenomena. Briefly, an observational program should:

e Determine the total mass and energy losses in mass motions;

e Determine the influence of thermal mechanisms in driving flare
ejecta;

e Determine the three-dimensional shape and volume of the various
kinds of mass ejecta;

e Determine the spatial relationship of ejecta and particle
acceleration regions;

° Differentiate between self-driven expanding magnetic loop and
gas-dynamic models of coronal transients;

6 Determine whether important motions are present in the corona
before flares: e.g., are 'forerunners' initiated prior to flare
onset?

o Determine the interplanetary signature of solar mass ejections.

I. Mass and Energy

Ia. Energy of filament eruptions and coronal transients

(1) Filament eruptions accompany the onset of many flares, both in young
active regions and in old regions which have lost their sunspots and have weaker
magnetic fields. These eruptions are obviously involved in some way in the
conversion of magnetic energy to thermal and kinetic energy. To understand this
energy release, we need to know the magnetic field configuration. The form and

17



Ib. Inference From Radiation Signatures

(6) Since particles emitted from flares occurring in the W20 - W90 solar
longitude range are most likely to be detected by near-Earth spacecraft, it is
strongly urged that SMM and other satellite observations be preferentially made
in this region of the solar disk, for comparison with measurements made under
program Ia. The location of HELIOS and other interplanetary spacecraft should
also be taken into account in deciding the optimum area to be observed.

(7) In principle, the X-ray spectrum observed at the Earth can be inverted
to yield the energetic electron spectrum responsible for its production. This
process is, however, well known to be mathematically ill-posed, and very accurate
X-ray spectral measurements are necessary in order to obtain substantially
quantitative results for the electron spectrum. Since the precise form of this
electron spectrum has important bearing on the flare problem (e.g., it is
substantially different for the two hard X-ray models N-T and T of Chapter 2), we
urge that such accurate spectral observations be made, such as with balloon-borne
cooled semi-conductor detectors, which have a typical energy resolution of about
1 kev.

(8) The polarization of X-ray bursts has also important implications for
the accelerated particle population (cf. discussion in Chapter 2 and Appendices C
and D), and cooperation between Russian experimenters and those involved with the
solar X-ray polarimeter experiment sheduled for the forthcoming Space Shuttle
Orbital Flight Test is encouraged.

(9) It may be possible to determine the relationship between impulsive
phase electron acceleration and type III burst electron acceleration by comparing
radio and hard X-ray source locations. This involves obtaining two-dimensional
high time resolution images of total intensity and circular polarization during
impulsive bursts to be compared with information obtained from the SMM hard X-ray
imaging spectrometer. Even better would be to directly measure the hard X-ray
burst component produced by the type III burst electrons; such measurements are
conceivably possible using the cosmic X-ray telescope currently planned for
Spacelab 2, suitably modified for a later Spacelab flight.

(10) For diagnostics of the second-stage acceleration, which is
characterized by a flux of heavy particles (notably protons) and relativistic
electrons, y-ray measurements are of course of great use. Such observations can
be used to infer the energy spectra of the particles (nuclei from line ratios,
and electrons from the continuum), the total energy of such particles, the
directivity of the accelerated ions (from Doppler shifts and asymmetries in
spectral line profiles - cf. Orrall and Zirker 1976; Canfield and Cook 1978), and
possibly the enrichment of heavy nuclei in the energetic particle stream [cf. (1)
above]. In addition to the y-ray detector on SMM, cooled semi-conductor
detectors, which have a high (a few keV) spectral resolution, are also of use;
such detectors are on the Air Force P78-1 spacecraft and scheduled for the HEAO-3
satellite.

II. The Acceleration Region

(1) Spectroscopic, measurements made with the SMM X-ray polychromator
instrument are capable of determining both the ion (via line widths) and electron
(via line intensities and ratios) temperatures in the high temperature regions of
the flare. Comparison of these two temperatures permits inference of the likely
acceleration agent (cf. section III) and of conditions in the acceleration
region, to be compared with models (cf. Chapter 2). in addition to the SMM X-ray
polychromator, Russian spacecraft will also be helpful in obtaining this
information (cf. Appendix D).

(2) High spatial (=1") and temporal (=10s) resolution radio mapping of the
flare at centimeter wavelengths is necessary to fully study the site of primary
energy release and particle acceleration, for it permits inference of the
magnetic field structure in such regions; this can again be tested against models
of flare energy release.

13



TABLE 5.1
DATA FOR COLLABORATIVE OBSERVING SEQUENCES

Column 1: The kind of flare-specific observation SERF calls for in the Collaborative
Observing Sequences. "“IP" stands for interplanetary and "SHGs" for spectroheliograms.
Column 2: Time interval in seconds (At) between successive observations. In almost

a cases At represents the limit set by available instrumentation. The table gives
the shortest sampling rate for those experiments capable of recording data much more
rapidly than required by SERF cooperative observations.

y-ray spectra, flux 10,1

X-ray (E>10 keV) spectra 1073
SHGs 1
X-ray (E<10 keV) spectra 10
SHGs 102

Doppler shifts 10?
X-ray polarization 10
EUV (100 - 3000 R) spectra 10
flux 1
Dopplergrams 10

Uv (1000 - 3000 &) spectra 10?
SHGs 10?

magnetic fields 10°

Dopplergrams 103

Optical (3000 - 11000 A) spectra 10?
HGs 10
filtergrams (disk, corona) 10

magnetic fields 10?

Dopplergrams 103

photoheliograms (disk, corona) 10,10
_ continuum polarization (disk, corona) — _ 20,107

cm-A radio spectra 10-7
SHGs, polarization 10

m-A radio spectra 10-7
SHGs, polarization 1

~~“ Solar wind velocity, density, magnetic field ~~ ~~ 7 7 777 1020

IP electron spectra, flux 10?
IP ion spectra, flux 102
abundances, charge states 103

IP scintillations 10*

26


(8) The Stark broadening, both linear and quadratic, associated
with the presence of an external (beam induced?) electric field permits us to
test for such external fields. Such studies naturally subdivide into a number
of categories.

(i) Examination of the increase in line half-widths with increasing

principal quantum number, or of the continuum merging of the
Balmer series or, even better, a series of suitable coronal
lines, and use of the Inglis-Teller law to give the local
electric field. This has been done for prominences by Hirayama
(1963), and flares by Suemoto and Hiei (1959), but primarily as
a density diagnostic. In this study we propose to use an
Inglis-Teller relationship of the form

In € = C - 5 In amaxy

where E is the electric field strength, max the principal
quantum number of the lowest wavelength line discretely

visible, and C is a known constant (Inglis and Teller 1959). A
certain part_of E will be made up of the local interionic field
E* = 3.7 en2/3, where n is the number density; thus if an

independent estimate of n (e.g., by spectroscopic analysis of
line intensity ratios) can be made, a value for the external

electric field Eg = E - E* can be found, to be compared with

theoretical models.

(ii) Since Model N-T predicts an anisotropy (towards the vertical)
of the electron velocity distribution function, the Stark
broadening of lines (in itself a useful diagnostic of E fields,
if E* can be eliminated) will be directional in strength
Thus, observations of Stark-broadened lines in flares at
different central meridian distances provides us with a measure
of this anisotropy and so of the likely X-ray emitting
mechanism,

(iii) The excitation of forbidden and/or satellite lines by the
action of time-independent and time-dependent Stark effects
respectively provides another diagnostic of this effect. At
this stage suitable candidates for investigation are, however
unclear.

(iv) Finally, it is conceivably possible to measure the quadratic
Stark shift in a line, if the instrumentation is accurate
enough. Typical values of the quadratic Stark Shift are o
order 0.03 R (observed in the He I lines at 3705 and 4026 A).

At this point, it is worth remarking on the use of Stark broadening
measurements to determine & fields in the corona. These fields may be capable
of creating a a_x_B drift of material across the magnetic field lines at the
top of a coronal loop; this production of material is needed in models which
predict a downward convection of coronal material in flares (cf.Craig and
McClymont 1976).

(9) Since the SMM hard X-ray imager is capable of unprecedented
temporal resolution ( * 0.2 seconds), it is capable of perhaps observing real
time delays between bursts at different frequencies hitherto considered
"synchronous to within the timing accuracy of the data" (cf. Emslie, Brown,
and Donnelly 1978, Emslie and Noyes 1978, Donnelly and Kane 1978), even for
the non-thermal burst model (a 30 keV electron travels 0.5 arc minutes in 0.2
seconds). Delay times of the form At = Qt + o_ should be derivable and
provide a potentially useful test of theoretical models of energy transport

(10) in order to assess the importance of thermal effects (such as
the thermally driven motion of the conduction fronts in Model T), accurate
values of source temperature gradients, inferred from simultaneous density and
emission measure determinations, are desirable. At this point, it appears
that there are very few density-sensitive spectroscopic diagnostics in the
very high temperature regime covered by the available instruments’ wavelength
range, and theoretical input in this direction from atomic physicists is urged.

Other questions and potential diagnostics addressed by the group will
now be presented.
8


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TABLE DIT.

Solar Flare Data

+ Maxi 4 Mc Math Temperature Emission Measure
Date Siar “or = BGS) Importance —Region 7, (10° k) Y (cm73)
10/24/70 0447 0582 0631 2N 11002 18 3.0 x 1049
11/05/70 0310 0335 0437 3B 11019 15 7.5 x 1048
11/16/70 2215 2221 2233 iN 11029 20 1.5 x 1048
11/16/70 0045 0052 0139 2B 11029 23 2.5 x 1049
Cht-s* Fe Kul Rwy E1170 28 FLARE cnt-s" fe 51170. 3B FLARE
90) Ky ‘in R
eee OA eh ee a ae)
0 3 nce
a onvo(o}
geval ty 9 an
hanna, :
° a wrabaad Jan all) mm
0 MA.
* “at ‘| 0331 (VT)
ost) or Ll they Uk,
Q
500 ons(u7) 0332/07)
g eis a
is 190 ,
a” oo Ll Ly als
ght “
Fig, D5: Spectra of the Initial ms Tso as 18 X
Stage and Maximum for the Flare
16 November 1970 (Spectra 0039-0048
have somewhat lower spectral
resolution. ) Fig. D6: Spectra of the initial

52

State and Maximum for the Flare
5 November 1970 (The spectra at
0314 and 0331 UT have somewhat
lower resolution.)

i
|





are associated with the compression of the plasma at the "contact surface",
i.e., the leading edge of the plasma from the site of the initial burst.
The ambient coronal plasma moves ahead of this piston; its leading edge is
compressed by the shock wave. The entire ensemble fills a large solid
angle, = 1/2 to 2m steradians, which is a function of the energy, momentum,
and mass input. If 8 < 1.0, the plasma will be constrained: only outward
wave motions will occur, and no mass will cross the magnetic field lines.

Answers to the following questions should reveal which model is the more accurate
description of mass ejections:

(a) What is the magnetic configuration before, during, and after a
mass-ejecting flare?

(b) What are the various energy contents (i.e. kinetic, potential,
magnetic, thermal and particles) as a function of time? What are the total
outputs in these various forms? How does the radiative flare output vary
during periods of large mass motion?

(c) What is the source region of the particles responsible for "prompt"
energetic solar protons relative to the flare ejecta?

(d) How does the coronal transient evolve into the travelling
interplanetary disturbance? Can we identify one or more interplanetary
signatures with different types of coronal transients?

We recommend the following observational studies for answering the above
questions:

(1) Use K-coronagraph views at 1 x R/Re < 2 to search for transients.

(2) Radio spectra and spatial views are needed to determine shock velocity
(cf. Chapter 3) and type IVm and stationary type IV characteristics in
association with the white-light transients.

(3) Determine, using soft X-ray observations, the density and temperature
of the plasma at the flare site as a function of time. This information is

needed for modeling of input pulse functions. Image the emission-line corona.

(4) Obtain "before" and "after" magnetic field directions in the plane of
the sky.

(5) Obtain inner-coronal Faraday rotation of spacecraft telemetry signals
to sample magnetic field polarity changes during a transient. Use
telemetry phase shifts and dual frequencies to get velocities and
densities during and after transients.

(6) Use IPS observations to get the temporal and spatial developments of
large-scale mass motion changes in the solar wind.

(7) Use multiple spacecraft observations to detect in situ changes. Obtain
unambiguous associations of shocks and pistons with specific solar
transient events. Sample energetic particles which come directly from
flare sites; distinguish them from locally-~accelerated particles.

IV. Precursors and Forerunners

Not all mass motions are consequences of flares. Activated filaments and,
perhaps, forerunners precede flare onset. Filament and coronal disturbances may,
in fact, cause flares. We urge increased effort to establish the physical
reality of the various reported and debated precursors. Specifically, one should
search for preflare changes in coronal loops over activated filaments and
determine whether such activity is associated with forerunners. The magnetic
fields in the photosphere and chromosphere should be monitored for evidence of

23



APPENDIX E

The following resolution was adopted unaminously at the Workshop on Energy
Release in Flares on 1 March 1979.

The SERF (Study of Energy Release in Flares) Workshop collectively recommends the
establishment of the position of an SMY (Solar Maximum Year) coordinator, whose
role is to

(a) provide a central focus for the communication of detailed
observational plans and priorities between observatories (ground
and space-borne), and

(b) assist in the development of the plans to carry out coordinated
observing sequences, and

(c) assist in the coordination of the scientific goals of the
individual ground and space-borne observatories toward the ultimate
goal of maximizing the collective science.

59



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Henoux, J.-C. and Nakagawa, Y. 1978, Astr. Ap., 66, 385.

Henoux, J.-C. and Rust, D.M. 1979, in preparation.

Heyvaerts, J. 1979, these proceedings (cf. Chapter 3).

62



II. Thermally Driven Motions

Ila. Flare-initiated Events

(1) The sudden input of heat in the low corona that is a flare should cause
an explosive increase in the local gas pressure. Some flare sprays and surges
as well as the sudden coronal loop enhancements found in the Skylab soft X-ray
data, may be evidence of mass motions driven by this increased gas pressure.
Very little quantitative data are available on such thermally driven motions. A
few, rapid, flare-associated coronal loop brightenings were found in the Skylab
data (Rust and Webb 1977). (We refer to enhanced loops stretching from the flare
kernel to a point = 0.1 Re away as illustrated in Figure 4.2.) The propagation
velocity of the excitation is in the range 400 to 1000 km s-l, anda
significant fraction of the total flare energy may be carried away in the process
(Craig and MacClymont 1976). We recommend that high time resolution, wide-field
soft X-ray observations be obtained to quantify the energy in the heated (or
shocked) gas.

(2) Ha sprays frequently accompany coronal enhancements, so high time
resolution Ha or He 410830 A observations should be obtained simultaneously with
the above soft X-ray spectroheliograms. Flares with sprays are easiest to study
when within 20° (heliographic) of the limb, since the origin of the spray can
be compared with the disk position of the flare, while the spray trajectory can
be traced easily once it clears the limb. Magnetograms of the active center and
its surroundings are useful for understanding how spray trajectories and
enhanced coronal loops connect to the flare core

(+) |

ACTIVE REGION

(-)
MAGNETIC PLAGES |
(a) t,-100s OUTSIDE ACTIVE
° REGION
(-)

Fig. 4.2, a-c: Schematic model of an active region X-ray enhancement. | (a) X-rays
(shaded area) are emitted from a filament cavity shortly before the filament
explosive phase. The emission seems to come from along the top of or just below
a canopy of loops that extend from positive to negative magnetic footpoints in
the active region. The emitting region appears 1 - 3 x 10% km above the solar
surface, and its edges are very sharply defined. (b) Just after filament
eruption, X-ray emission is brightest in several low active region loops. Other
fieldlines are open and may carry shocks (hatched area) to the outer corona.

Slow-mode shocks (see Kopp 1972) also follow extended flux tubes ‘to the_

chromosphere. (c) About 500 s later, soft X-ray flux is at a maximum with

emission from an arcade of loops over the active region. Flux tubes terminating
outside the active region brighten, bow out and expand under increasing gas

pressure as matter is evaporated from the chromosphere.

20



SERF WORKSHOP PARTICIPANTS

(Note: The letters T, A, and M refer to the teams on Thermalization, Particle
Acceleration and Mass Motions, respectively; the number refers to the position in

the group photograph)

Dr. C. de Jager (A;6)
Space Research Labs.
Beneluxlaan 21

Utrecht

The Netherlands

Dr. Murray Dryer (M;4)
Space Environment Lab
NOAA/ERL

Boulder, CO 80302

Dr. A. Gordon Emslie (7;8)
Center for Astrophysics

60 Garden Street

Cambridge, MA 02138

Mr. Gary Heckman (A316)

Space Environment Services Center
NOAA-325 Broadway

Boulder, Colorado 80302

Or. Jean-Claude Henoux (T;12)
Observatoire de Paris

92190 Meudon

France

Or. Jean Heyvaerts (A;-)
Observatoire de Paris
92190 Meudon

France

Dr. E. Hiei (73.5)

Tokyo Astronomical Observatory
Mitaka

Tokyo 181

Japan

Or. S. Hinata (1;21)
Sacramento Peak Observatory
Sunspot, NM 88349

Or. Gordon Holman (A;-)
Astronomy Program
University of Maryland
College Park, MD 20742

Dr. S. W. Kahler (A310)

American Science & Engineering, Inc.

955 Massachusetts Avenue
Cambridge, MA 02139

Or. M. R. Kundu (A537)
University of Maryland
Astronomy Program

Space Sciences Building
College Park

Maryland 20742

Dr. R. P. Lin (A;-)
Space Science Lab
University of California
Berkeley, CA 94720

Or. R. M. MacQueen (M; 26)
High Altitude Observatory
P. 0. Box 3000

Boulder, Colorado 80303

Dr. S. Mandel'stam (M;24)
Institute for Spectroscopy

USSR Academy of Sciences

Troitsk, Moscow obl., 142092, USSR.

Or. Ron Moore (M31)

California Institute of Technology
Solar Astronomy

1201 East California Blvd.
Pasadena, CA 91125

Dr. Yoshi Nakagawa (7313)

—S-51 Space Sciences Laboratory
NASA/Marshall Space Flight Center
Huntsville, AL 35812

Dr. Dennis Peacock (-318)
Div. of Environmental Sciences
Room 312

National Science Foundation
1800 G. Street, N.W.
Washington, D.C. 20550

|
|



v


Ir. Soft X-Ray Irradiation

The study of chromospheric heating by soft X-ray irradiation is a fairly
recent aspect of theoretical flare modeling. Analysis for simple source
geometries has already been carried out (Somov 1975; Henoux and Nakagawa 1977,
1978; Machado 1978), with very promising results. Recently Henoux and Rust
(1979) have carried out a calculation involving the heating of a chromospheric
flare by X-ray radiation from an arcade of congruent semi-toroidal loops, with
material both uniformly and non-uniformly distributed within them. They find
that the degree of illumination of the chromosphere (defined by optical depth
unity in "typical" EUV lines) has a geometric structure which bears a strong
similarity to the classic two-ribbon Ha structure in flares. Although the
analysis is still incomplete (for example, it remains to be shown whether deep
chromospheric optically thick radiative transfer leads to a brightening or in
fact a darkening in Ha of these brightly illuminated regions), a strong
diagnostic of what such a model predicts lies in the comparison of the soft
X-ray and Ha radiation geometries. Although care must be exercised due to
projection effects when observing loops, this is nevertheless a strong and
easily obtained diagnostic, requiring merely the cooperation of SMM X-ray
polychromator experimenters and ground-based Ha observatories. Other
diagnostics associated with this chromospheric heating model are polarization
and non-LTE effects introduced in chromospheric lines by the incidence of
photo-electrons (this is theoretically treated very similarly to the case of
non-thermal beamed electrons, and shares a number of features in common -
Chambe and Henoux 1979). Although a theoretical prediction of the strength of
polarization in the La wings introduced by this effect gave rather negative
results (due largely to the great optical depth of the la radiation; Chambe
and Henoux 1979) it is hoped that other low-temperature EUV lines, observable
with the SMM ultraviolet spectrometer-polarimeter will provide better results,
and work is at present in progress along these lines.

As regards NLTE effects, the ionization of Helium is strongly sensitive
to an impingent flux of photoelectrons, and careful observations of the He I
and II continua, and lines of both species may provide a potential diagnostic
of this mechanism. Observations of the spectrum of other atomic species may
also be relevant to this study.

Tir. White-Light Flares

In order to test mechansims responsible for producing white-light
flares, simultaneous white-light flare observations and, in particular, Ca II
K-line observations, should be made simultaneously with observations in other
wavelength bands. In particular, the inference of the accelerated proton flux
and the spectrum (E 10 MeV) is of great interest in studies of
temperature-minimum heating during flares (Machado, Emslie, and Brown 1978).
The gamma-ray detector on SMM is sensitive to both of the important 2.2 and
4.4 MeV lines (which permit inference of the proton spectra upwards of 7 MeV)
and, when coupled with white-light observations and heating theories of the
deep chromosphere, such measurements are potentially capable of increasing our
understanding of the deep chromospheric flare. Hard X-ray observations are
also of use in this study, since white-light flares may be explainable by
electron bombardment alone (Machado and Rust 1974; Lin and Hudson 1976).

Iv. Mass Flows

The detection of chromospheric and coronal macroscopic motions and
their associated turbulence by measurement of Doppler shifts and broadening of
suitable spectral lines is important in assessing the contribution of mass
motions as a mechanism of thermalization of energy (Craig and McClymont
1976). Since the initial motion is downward (flare-driven) followed by an
upward "evaporation" of overheated chromospheric material some 30 seconds to 1



PREFACE

CHAPTER 1:

CHAPTER 2:

CHAPTER 3:

CHAPTER 4:

CHAPTER 5:

APPENDIX A:

APPENDIX B:

APPENDIX C:

APPENDIX D:

APPENDIX E:

REFERENCES .

CONTENTS

OVERVIEW OF THE STUDY OF ENERGY RELEASE IN FLARES .

THERMALIZATION OF ENERGY IN FLARES.
MECHANISMS OF PARTICLE ACCELERATION
MASS MOTIONS.

RECOMMENDED OBSERVATIONS.

SERF ORGANIZATION AND OPERATIONS
David M. Rust. .

SCIENTIFIC PROBLEMS OF THE SMY SERF PROGRAM

A.T. Altyntsey, ¥.G. Banin, G.Y. Kuklin, V.P. Nefedjev,

A.V. Stepanov, Y.E. Stepanov, and Y¥.M. Tomozov .

ENERGY TRANSPORT AND THERMALIZATION IN SOLAR FLARES

Gordon Emslie

X-RAY SPECTRA OF SOLAR FLARES
S.L. Mandel'stam .

RESOLUTION
SERF Workshop Participants

vii

Page

iii

ell

17

225

+35

137

+43

47

.59

» 61



(5) Observations of the evolution of coronal mass ejections (cf. Chapter 4)
with the SMM coronograph-polarimeter, coordinated with observations of particle
fluxes, spectra, and compositions, are required in order to investigate the
possible relation between ejections and particle acceleration. HELIOS data are
particularly useful for studying the variation of particle flux with size of the
coronal mass ejection. Off-band Ha measurements should also help to determine
the position and velocity of each part of the ejected feature (cf. Chapter 4), to
be related to positions of the observed manifestations of the second-stage
acceleration process, if any.

Vv. The Relationship of Particle Acceleration to the Overall Flare

(1) The overall flare energy budget should be derived for a number of
events (cf. Canfield et al. 1979b; Rust et al. 1979) and the contribution of
energetic particles assessed. This has important implications for mechanisms of
primary energy release.

(2) The interaction of accelerated particles with the ambient solar
atmosphere needs to be better modeled and understood. We refer the reader to
Chapter 2 and Appendix C for a more detailed discussion of this aspect of the
flare problem.

VI. Coronal and Interplanetary Structure

Observations of the structure of coronal holes and the solar wind are
necessary in order to ease the interpretation of near-Earth observations of
particle streams.

(1) Coronal hole maps that define the probable sources of high-speed
streams will enable the Sun-Earth propagation of ejected particles to be better
modeled. To this end, daily, whole-Sun, soft X-ray spectroheliograms should be
made an integral part of the observing program. Also, daily He (10830 A images
will be of use.

(2) Solar wind plasma and field parameters, obtained from (e.g.) ISEE-3,
HELIOS, and Pioneer-Venus, will help define the spatial extent and shape of
solar-induced interplanetary disturbances.

(3) We also recommend that three-dimensional probes of the interplanetary
medium be carried out using the interplanetary scintillation (IPS) technique, at
regular intervals throughout SMY, with specific emphasis on periods of special
study as identified by FBS, SERF, and STIP.

with the above observations properly coordinated, we feel that we can make
significant progress during SMY toward understanding the causes and effects of
particle acceleration in solar flares.

15



REPORT UAG-72

WORLD DATA CENTER A
for
Solar-Terrestrial Physics

er
coy

ENERGY RELEASE IN SOLAR FLARES

Proceedings of the Workshop on
Energy Release in Flares
26 February - 1 March 1979
Cambridge, Massachusetts U.S.A.

r+

BZ
ZS

July 1979

=



by "runaway" electrons (Kaplan et al., 1974) is problematic since it is difficult to estimate the density
of fast particles. Consequently, only ion-acoustic turbulence is fairly substantiated in the current
sheet.

Ion-acoustic waves have a frequency much less than the electron plasma frequency pes There-
fore for the conversion of s-waves to high-frequency (w > ®pe) Langmuir and electromagnetic waves, for |
example, by scattering on particles (inverse Compton~effect), a sufficient number of superthermal par- t
ticles is needed. |

The processes of particle acceleration, plasma heating and generation of plasma turbulence in the
flare seem to be interconnected. Consequently, using the radio emission characteristics one may in
principle determine not only the levels of the s= and £-turbulence but also get information regarding the
spectrum of particles directly in the acceleration region. For this purpose, in our opinion, observations
of flare radio emission at millimeter, centimeter and decimeter wavelengths are needed. In addition, for
the current sheet diagnostics during the flash-phase of the flare measurements of polarization are im~
portant. In this connection, let us mention the work of Nefedjev (1979), in which the growth of the
linear polarization at ) = 3,2 cm prior to the flash phase of the flare is linked with, the presence in the
corona of a sheet about 102 cm thick and with an electron density of n~ 104* cmTM

Linear polarization of radio emission, as is well known (Zheleznyakov, 1970), arises by radio
wave propagation through a region of the quasi-transversal magnetic field. If the waves then get into a
region where the geometrical optics approach is violated, polarization can remain also during further
propagation. Thus linear polarization of a microwave burst can be accounted for by the presence of a
large density gradient in or just above the burst source. According to simple estimates performed on the
basis of the bremsstrahlung mechanism, linearly polarized emission originating from a dense sheet
should be observed in a narrow wavelength range (AX ~ 0.5 cm) at wavelengths between 2 cm and 6 cm.

Note that a very promising diagnostic of current sheets is the method of the sheet "transillumina-
tion" by radio emission from a local source. The current sheet formation process must be accompanied
by a decrease in emission of the local source (absorption burst) located below the current sheet.

1.2 X-Ray Emission, Measurements of X-ray emission from the flare should be performed simul-
taneously with optical and radio observations. These data are essential for the determination of fast
particle spectra. Particularly useful are observations of the hard X-ray emission polarization of the
flare. These allow one to select among different theoretical models which explain the nature of hard
X-ray bursts: deceleration of electron beams in dense layers of the solar atmosphere (thick and thin
targets), Thompson scattering of X-ray quanta in the solar atmosphere (Beigman, 1974), or the multi-
temperature model (Colgate, 1978).

Somov (FIAN) suggested a program "Hard X-rays and brightening ¢& the lower layers of the Sun's
atmosphere from flares" (and for a flare on the limb - investigation of coronal condensations) with the
aim to compare the X-ray image with the supposed illumination of the lower atmospheric layers by flare
X-rays and to verify the reality of the low chromosphere and high photosphere heating by flare short-
wave radiation at X < 912 A. Three mechanisms of energy transfer from the source above the chromo-
sphere (magnetic reconnection region) into the low-temperature (optical) part of a chromospheric flare
are widely discussed and investigated. These are heat conduction, energetic particles, and generally
speaking, gasdynamic flows. However, as was shown by Somov (1975), the radiation from low-lying
X-ray structures can make an important contribution to the heating of the under-lying He (and other line)
flare structures. X-ray flare radiation may also cause diffuse optical haloes around the flare, as a
whole, or around flare kernels, The X-ray heating efficiency depends essentially on the X-ray source
geometry.

The HXIS experiment will be the first and only one to allow a possible illumination of the low
solar atmosphere by flare X-rays to be tested. The whole necessary calculation has been published by
Somov (1975, 1976) (see also Somov and Syrovatskii, 1977). The HXIS data will offer the possibility
of checking the X-ray heating theory both in principle and quantitatively. Data to be used:

1. The X-ray images to be obtained with the HXIS instrument at different spectral channels from
3.5 to 30 keV with spatial resolution 8" x 8". (Et would be very useful for comparison to
have EUV-images of the same flare with \ > 912 A because this non-ionizing radiation can
also make a contribution to heating the solar atmosphere at large depths near the temperature
minimum (Somov and Syrovatskii, 1977).

38



- Are there several different kinds of flares or are there various
thresholds at which successively more effective energy release mechanisms take
hold? [e.g., is there a clear distinction between "small" and "large" (e.g.,
proton) events?]

(3) The mechanisms which create the radiation signature of the flare remain
ambiguous:

- Are hard X-rays produced by non-thermal bremsstrahlung of a large
number of fast-moving, streaming electrons, or are they predominantly thermal in
nature?

- In the former case, do the electrons escape out of the bremsstrahlung
emitting layers (thin-target model), do they lose all their energy in a
high-density target (thick-target model), or are they confined (e.g.,
magnetically) in a trap?

- In the latter case, what is the density and temperature distribution
within the X-ray source?

- Is the white-light flare produced by non-LTE effects at relatively
great heights in the atmosphere (cf. Machado and Rust 1974; Lin and Hudson 1976),
or does some powerful, as yet undefined, heating mechanism produce it at
photospheric depths (Machado, Emslie and Brown 1978; Emslie and Machado 1979)?

(4) The exact sequence of events in the flare is unclear:

- Does the white-light flare precede the hard X-ray burst in some events?

- Are the hard X-ray and EUV time profiles really synchronous enough
(Donnelly and Kane 1978) to necessitate a non-thermal excitation mechanism for
the latter?

- Does mass ejection occur as a result of the flare energy release, or
does it in fact cause the release to occur?

These questions are just some of the many that remain unanswered at this
point. However, the spatial, spectral, and temporal resolution of the
instruments available to SMY experimenters is conceivably good enough to resolve
some of these issues. Thus we hope that the following chapters will clarify the
various theoretical points at issue in the study of energy release in flares.
These proceedings are being published in the further hope that the best possible
sequence of observations will be carried out, and the best possible use of the
resulting data made. In the chapters following, the key issues above will be
re-raised, along with others. The observing sequences deduced as being possibly
capable of resolving the problems identified therein will appear in the final
chapter.



Chapter 1

OVERVIEW OF THE STUDY OF ENERGY RELEASE IN FLARES

Early in 1980, the Sun promises to pass through one of the highest solar maxima
in recent history, thus providing an unprecedented opportunity to observe and
study the puzzling phenomena known as solar flares. Realizing the importance of i
this opportunity, the ICSU Scientific Committee on Solar-Terrestrial Physics
(SCOSTEP) accepted a resolution in 1978 to inaugurate the Solar Maximum Year
(SMY) program of study, to commence in August 1979 and extend until February
1981. During this period there will be a concerted, world-wide effort to
coordinate observations of solar flares and their effects at every possible
energy and wavelength, most notably with the Solar Maximum Mission (SMM)
satellite, currently planned for launch in October 1979.

The SMY investigations center on three subjects, each an important aspect
of the solar flare problem - Flare Buildu udy (FBS), the Study of Traveling
Inte ena (STIP), and the Study of Energy Release in Flares
(SERF). As part of the organizational effort to ensure that the best and most
useful data are obtained during the SMY, it was decided that theoreticians
involved in these thréé-aspects—-vuf—the—fLar proprem~should meet to discuss the
types Of ~opservations—thatTMsnourd—be-mace vuring the SMY to develop and test
theoretical models” of flares. This was the Principal aim of the SERF workshop.

Even after more than a century of observation, with a decade and a half of
this spacecraft-based, the solar flare remains basically an object about which
very little is known. Even some of the gross characteristics of the flare
Process elude a definitive explanation. In this brief introductory chapter, we
shall briefly discuss a few of the major unresolved problems of energy release in
flares, in order to "set the stage" for the more detailed discussions in the
chapters to follow.

One of the most basic questions about energy release in flares is of course
the following: how does the Sun release some 102 ergs s~l over some 1000
seconds in an area apparently no larger than 1019 om2? Although it is
undoubtedly the case that preflare energy storage is stored in magnetic fields,
it is by no means clear how a process of magnetic reconnection can in fact cause
such a violent release of energy over such a telatively long period of time.
Other, more specific areas of the problem of energy release in flares also have
their fair share of unresolved issues, as we now highlight.

(1) Basic questions on the site of the energy release remain unanswered:

- Is the site of the initial flare energy release in the corona, the
photosphere, the deep layers of the solar interior, or even a substantial
fraction of a solar radius above the photosphere?

- Is the characteristic size of the energy release region of the order of
arc seconds (so that resistive magnetic tearing can proceed rapidly enough; cf.
also the observed sizes of Ha bright points) or arc minutes (to supply a large
enough energy release rate)?

(2) In addition, the nature of the energy release itself is not clear:

- Does the bulk of the flare energy go into macroscopic ejecta or into
thermalization of the lower atmospheric layers? The results of Canfield et al.
(1979b) and Rust et al. (1979) seem to suggest the former, although no sound
theoretical reason for this has yet been given.

- Is the thermalization of the lower layers accomplished by fast
non-thermal particles (e.g., electrons) or by thermal conduction and shock
fronts? The mechanism actually responsible must be able to explain, among
other issues, the synchronism of the flare radiation components at different
wavelengths and the relative intensity of these components. There appear to be
modeling difficulties with each of the mechanisms mentioned above (see Appendix
C), whereas no convincing observational evidence points as yet to either one.



The upper levels of satellite lines in the hydrogen line spectra and some
of satellite lines (e.g. j,k,a) in the helium- like spectra are populated by
dielectronic recombination:

Fet25 4 e- + Fet24** (21 21') + Fet24* (15 21") + hy
Fet24 4 e- + Fet23** (15 21 21') > Fet24* (152 21") 4 hv

and the ratio of their intensity to the intensity of the resonance line is given
by: _ SE SE

2 2 16/1, = KT. ©*PlkT.) *
where 6— = Z°Ry/h© , the binding energy of the electron, is sufficiently large.

This makes the ratio Ig/Iy a rather sensitive "thermometer" to measure ;
the electron temperature Te of the flare plasma (cf. Chapter 2; Section 1.5,
Figure 2.2). It is important to note here that this ratio is independent of the
ion number density and can therefore be used for plasma which is nonstationary
and spatially inhomgeneous ~ in temperature and in density. Table DII gives
values of Te thus determined by the use of the Fe XXV spectra and emission
measures obtained from absolute values of radiation fluxes in the resonance line
for four flares.

These data refer to the time which corresponds approximately to the flare
maxima. Recently evaluated spectra (Zhitnik et al. 1979b) which refer to initial
and final stages of flares are of obvious interest.

Fig. 05 shows several spectra which refer to the inital stage of the flare |
16 November 1970 (which began at 0045 UT) and to the vicinity of its maximum.
The intensity of resonance and satellite lines Fe XXV in spectra of the initial
phase are 30-50 times less than in spectra close to the flare maximum. However,
in spectra of the initial phase one can clearly see resonance lines of
hydrogen-like ions Fe XXVI and lines Kg and Kg , which correspond to inner-shell
transitions L+K and M+K in "colder" iron ions. These lines are not seen and,
perhaps, are absent altogether in the spectra near the maximum stages of flares.

The Ky lines were observed previously, but with lower dispersion (Doschek
et al. 1971, Neupert 1971). The Kg lines apparently are observed for the first
time. Our preliminary identification suggests for Ky and Kg lines, according to
laboratory data and calculations, ions with ionization stage beginning within the
interval from Fetl® to Fet3,1

In Fig. D6 several spectra of the initial phase and spectra close in time
to the flare maximum for the flare 5 November 1970 are presented. The same
picture is observed: 30-40 times lower intensity of resonance and satellite
lines Fe XXV and relatively high intensity of the line Kg, Kg of lower ions, and
Fe XXVI La.

The same picture is also observed for the flare 24 October 1970 (Figs. D7,
D8): low intensity of resonance and satellite lines Fe XXV at the initial and
final stages as compared to the maximum, and relatively high intensity of lines
Ky» Kg of lower ions and the Fe XXVI La line in relation to the Fe XXV lines

Fetl5 ions and those of lower ionization degrees where the ls electron
is removed have their their maximum concentration within the temperature range
2x 108 - 5 x 10" K, i.e., in the lower layers of the corona and the transition t
region.

1 Lines K, correspond to the transitions: |
Fe XVII: 1s2s22p63s + 1s22s22p53s; 4 = 1.9278
Fe IV: 182822p63s23p63d6 + 1522s22p63p53d6; » = 1.936R |

Lines Kg correspond to the transitions: |
Fe XIV: 182s22p63s23p2+ 1522s22p63s23p; , = 1.7398 |
Fe VII: 1s2s22p63s23p63q3 +1822522p63523p53d3; d = 1.7568 |

$1


IV. Coronal Hole Observations

Reconstruction of energetic particle spectra at the Sun, based on observations
made at distances greater than several solar radii, can be improved by a knowledge of
the structure of the solar wind. Sun-to-Earth propagation and interplanetary acceler-
ation can both be better described with knowledge of the location of high-speed solar
wind streams. Coronal hole maps that define the probable sources of high-speed streams
and in situ measurements of solar wind plasma characteristics are essential for this
reconstruction. Access to the following observations, through established data centers,
should be facilitated during the SMY:

(1) daily whole-Sun, moderate-resolution soft X-ray heliograms, e.g., those from
the U.S, Air Force satellite P78-1;

(2) daily He 10830 full-disk solar images, e.g., those made at Kitt Peak National
Observatory and

(3) solar wind plasma parameters of which the primary sources are ISEE-3, HELIOS,
and Pioneer-Venus.

Vv. In-Situ Plasma _and Interplanetary Magnetic Field Observations

Recognizing the value of solar plasma and interplanetary magnetic field measure-
ments, particularly in the Earth-Sun region, in defining the spatial extent and shape
of solar-induced interplanetary disturbances, we recommend rapid dissemination of
"quick look" plasma and field measurements from satellites such as ISEE-3 and the Venus
orbiter, and the timely reduction and deposit of these data into the World Data Centers.

Moreover, recognizing that the interplanetary scintillation (IPS) technique offers
a three-dimensional probe of the interplanetary medium, we recommend that solar wind
and interplanetary disturbance measurements by the IPS technique be routinely performed
during the SMY and that they include specific emphasis on periods of special studies
as identified by FBS, SERF, and STIP. We further encourage the rapid dissemination of
these data, particularly when these routine measurements indicate a current disturbance
in the interplanetary medium.

VI. A Final Comment

The advanced solar instrumentation that will be used during the SMY is at a point
where resolution is not a strong function of wavelength, Almost all Sun-viewing instru-
ments except meter-wave radio telescopes and gamma ray detectors can resolve 1-10 arc
sec. This is an important point to remember, since it facilitates intercomparison of
observations, but if the fundamental physical processes in flares take place on a scale
of less than 1 arc sec, progress in the SERF subprogram will be limited and we must
await the arrival of the next generation of instruments, Nevertheless, we certainly do
not wish to end on a pessimistic note. The instruments available now are an order of
magnitude better than those available during the last solar maximum. These instruments
will answer most of the questions raised in Chapters 2 - 4, and therefore our under-
standing of flares and flare-associated phenomena will be much greater

34



IIb. Models of Thermally Driven Mass Motion

(3) In order to understand the detailed physics of the observed events,
theoretical modeling is needed. A model should be designed to study the
dynamical response of a flaring loop of very high B. In this case the shape of
the loop is prescribed and will remain unchanged, and the mass motion will be
confined by the loop. The thermal pulse takes place at one footpoint of the loop
and the effects travel over the top of the loop (in the corona) to the
chromosphere at the other footpoint, where reflection and explosive convection
must be taken into account. The effects of thermal conduction and radiative
losses should also be incorporated. Multiple loop models could be investigated
to determine whether the spreading of excitation in the impulsive phase can be
simulated.

(4) An alternative to the above model would place the source of loop
brightenings outside of the loops themselves. That is, shocks from the flare
would strike closed loops broadside and cause brightening at every point almost
simultaneously. This model is not excluded by the available data. In order to
test these models, we need the magnetic field configuration and strength before
and after the event, and the plasma temperature, density and velocity as a
function of space and time for each event. By comparing two-dimensional MHD
models with observations, we can get some degree of understanding of the physical
processes at work during the event; e.g., thermal conduction, shocks, shocks
modified by thermal conduction, radiative cooling, etc.

IIc. Filament Activation and Flare Onset

(5) Most, perhaps all, large flares are preceded by a filament eruption.
Soft X-ray observations show that some filament material is heated before flare
onset, but we do not know whether filament ejection is a result of this heating.
Conceivably, heating could signal the destabilization of a magnetic
configuration, which then acts directly by Lorentz forces to propel the filament
outward. We urge that the source and location of pre-flare filament heating be
investigated and that its link to filament motion be clarified. High time
resolution observations of soft X-ray emission should be combined with Ha
observations to determine the degree of filament activation - twisting or
untwisting - that is associated with preflare heating. Spectra from the SMM
ultraviolet spectrometer-polarimeter, and field measurements in the eruption,
will help specify the thermal release process.

III. Loops versus Bubbles

There are two physical models which have been proposed to describe coronal
mass ejections. At the risk of oversimplification, we categorize the two
approaches as follows:

(a) Discrete ("loop") model - Motivated by the visual appearance of
white-light coronal transients, one hypothesizes a loop with plasma
confined by a helical magnetic field. One assumes, further, the presence
of a low g plasma (8<<1, where g is the ratio of thermal to magnetic
pressure) as well as an uncertain Alfven Mach number motion Ma, where
Ma is the ratio of dynamic pressure, 1/2 nmv2, to magnetic pressure.
The loop is supposed driven outward by magnetic stresses. Some features of
this magnetic loop model can successfully simulate coronal mass ejections
of the kind most frequently seen during the Skylab mission. The magnetic
loop is considered to be pinched off eventually by reconnection; the inner
part returns to the Sun, and the outer plasma bubble moves into the solar
wind.

(b) MHD continuum ("bubble") model - Motivated by the successes of
collective fluid mechanics in describing explosive phenomena (e.g., bombs),
one starts with an atmosphere in stationary equilibrium and introduces an
impulsive burst of energy, mass and/or momentum at or near the base of the
atmosphere. The fast, slow and intermediate (Alfven) waves propagate
anisotropically; the fast waves steepen into an MHD shock. The slow waves

22


2. Optical images of flares in lines and continuum formed at low heights (Z < 500 km in the
model of Vernazza et al, (1973) for X-ray energies E, = 3.5 keV) above the photosphere.
To compare optical and X-ray images there is no need for high spatial resolution. It will be
enough to compare the X-ray images with optical images having the same (8" x 8") resolu-
tion. Another reason that high spatial resolution is not required is that radiative heating
(unlike heat and particle fluxes traveling along magnetic tubes) produces diffuse flare im-
ages. The program can be implemented jointly with the Space Research Laboratory of the
Astronomical Institute, Utrecht, The Netherlands,

1.3  Qptical. To solve the problem of the role of plasma turbulence in the flare mechanism it is
advisable to obtain spectrograms of hydrogen lines of the Balmer series (He, HB, Hy... toH,, and
probably of the Hel 3705 and 4026 A lines) with good time resolution (as much as 5 s) with coverage of
all phases of the flare. Analysis of line profiles with due regard for the Stark~effect will permit deter~
mination of the electron density and the level of plasma turbulence.

Traditional mechanisms determining the behavior of flare hydrogen lines are the Doppler effect
due to unresolved mass motions and the Stark-effect associated with the electric fields of thermal elec-
trons and ions. However, if plasma turbulence ig grcited in the solar flare and the amplitude of electric
field oscillations exceeds the value E, = 2.6 ¢ n°/° (Holtsmark field), then a dominating role will be
played by the Stark effect in turbulent electric fields. This possibility was first indicated’ by Tomozov
(1973), Tsytovich (1973), Dolginov and Yakovlev (1973).

The Stark effect theory in turbulent electric fields stems from a separate consideration of the effect
on the emitting atom by the low-frequency and high-frequency components of the turbulent electric field
(HF-oscillations of the field are produced by Langmuir turbulence, and the LF-fields by the ion-acoustic
one). According to Ox's estimates (1978), HF-noise in the flare plasma has a stochastic character and
appearance of nonadiabatic effects should be expected. These arise when wp = 3 nea F/2h® oe (ag
is the Bohr radius, R is the amplitude of the quasi-static field) and are characterized by the appearance
of gaps on the spectral line profile at frequencies:

&g = Ey =n), ~ mpm) (ny - a )glon, +

Og = [ (n/n) fy -a)y7 mo ng], ;

which correspond to the distances A, and XB from the line center ) |

re

2
Of My ree (e/4 0) oy X= Ape 2ne -
Here @ and f are the initial and final states of the atom, n, and n, are the parabolic quantum numbers.
Using the gap half-width determined from the line profile, one can calculate the amplitude of Langmuir
noise (Zhuzhunashvili and Ox, 1977):

9
7.6x 10° Ad, 1»

E (kV/om) =

[n° -(n, =n

2. 2 1/2
pT mm Uy? ago

2
1 ~ny), 7g (ay -n)g XS

Here ¥= a, B and X% and AX ya me expressed in angstroms, m is the parabolic quantum number.

f2

Banin et al, (1978) have investigated the profiles of the central and middle parts of the Ha and
Hf lines in the spectrum of the 26 September 1963 large flare. Using the locations of typical gaps,
they determined the electron density in the Ha, Hf emitting region to be 3 x 10 cm73, whereas the
corresponding level of Langmuir turbulence was W, {nk T, is too large. Its real value seems to be
much less than that obtained since the electron temperature in the flare region generally exceeds the
yon temperature.

Unlike Langmuir turbulence, ion-acoustic turbulence determines the character of the line profile
as a whole by changing the intensity decrease law in the line wings. Ox's estimates (1978) also yield
a large value for the ion-acoustic turbulence level:

W.~ 0.1 nkT.
s e

39



OPEN FIELDS

Ha@ FLARE BEGINS

IN ACTIVE REGION SHOCK FRONT WILL

‘\<—— PRODUCE TYPE II BURST

SHOCK ALONG
FLUX TUBE

SHOCK WILL
PRODUCE FLARE
WAVE IN

CHROMOSPHERE

(b) ty (FLARE START)+100s

OPEN FIELDS

EXPANSION DUE
TO INCREASED
GAS PRESURE

LOOPS ROOTED IN
Ha FLARE RIBBONS

HEATED

CHROMOSPHERIC

GAS
EXPANSION
ALONG
FLUX

(ce) tyt500s TUBE

Ha

CHROMOSPHERIC

BRIGHTENINGS

21




Or. Ken Phillips (1317)
Astrophysics Research Unit
Culham Laboratory
Abington, Berkshire
United Kingdom

Or. Reuven Ramaty (A319)
Goddard Space Flight Center
Building 2, Room 149, Code 611
Greenbelt, Maryland 20771

Or. David M. Rust (M322)

American Science & Engineering, Inc.
955 Massachusetts Avenue

Cambridge, MA 02139

Ms. M. A. Shea (A315)

Air Force Geophysics Laboratory (PHG)
Hanscom AFB

Bedford, MA 01731

Or. E. J. Smith (A;3)
183-401 Jet Propulsion Lab.
4800 Oak Grove Drive
Pasadena, CA 91104

Or. P. Simon (M; 25)
Observatoire de Paris
Section de Astrophysique
92190 Meudon, France

Or. Dan Spicer (T;-)

Institute for Physical Sci. & Tech.
Space Sciences Bldg.

University of Maryland

College Park, MD 20742

Or. V. E. Stepanov (1327)
P.O. Box 4

Sib IZMIR

Irkutsk 33

U.S.S.R.

vi

Or. N. V. Steshenko (M39)
Crimean Astrophysical Observatory
USSR Academy of Sciences

p/o Nauchny

Crimea, 334413

USSR

Dr. Zdenek Svestka (T;20)
Space Research Laboratory
Beneluxaan 21

Utrecht

The Netherlands

Or. S. I. Syrovatskii (7323)
P.N. Lebedev Physical Institute
Academy of Science of U.S.S.R.
Leninski Prospekt 53

Moscow, U.S.S.R.

Or. Katsuo Tanaka (M314)
Tokyo Astronomical Observatory
Mitaka

Tokyo 181

Japan

Or. E. Tandberg-Hanssen (M311)
NASA, MSFC

Code ES-51

MSFC, AL 35812

Or. G. Wibberenz (A;-)
Institut fur Kernphysik
Universitat Kiel

2300 Kiel, FRG

Or. Shi Tsan Wu (M32)

Department of Mechanical Engineering
University of Alabama

P.O. Box 1247

Huntsville, AL 35807



In order to understand the causes and mechanisms of these processes, it is
necessary to obtain observations which can satisfactorily define:

(a) the accelerated particle Population ~- its spectrum, temporal evolution,
composition, etc.;

(b) the characteristics of the acceleration region - its location,
temperature, density, magnetic configuration, etc.;

(c) when and where the particle acceleration takes place;

(d) the responsible accelerating agent - e.g., plasma turbulence, electric
fields, shocks, etc.;

(e) the relationship of the particle acceleration process to the flare as a
whole - e.g., the fraction of the total flare energy carried by first-stage
accelerated particles;

(#) the coronal and interplanetary structure, in order to facilitate the
interpretation of terrestrial and spacecraft measurements of particle streams.

In addition, knowledge of the preflare configurations which lead to such
acceleration processes is of use; this problem is common to both the SERF and FBS
areas of the SMY.

We now discuss each of the broad areas mentioned above in greater detail.

I. The Accelerated Particle Population

This section breaks down naturally into direct detection of particles in
the interplanetary medium (including Earth-orbiting satellites), and inference of
the particle distribution by means of its associated radiation signatures
(X-rays, y-rays, etc.). We deal with each of these in turn:

Ia. Direct Detection

(1) Spacecraft measurements of the electron spectrum in the MeV range will
give information on the likely mechanism for the excess y-ray emission in the 1 -
2 MeV range which is observed in some flares(e.g., the 4 August 1972 event). At
present, this excess emission is interpretable as being due to either the
presence of MeV electrons or to the enrichment of heavy nuclei (Mg, Si, Fe) in
the particle stream, or ambient solar atmosphere, or both. Deviations in the
shape of the electron spectrum from a power-law in the MeV range indicate the
former possibility, while no substantial deviation indicates the latter.

(2) Measurements of the energy spectra of protons and heavy nuclei (C, N,
0, Ne, Mg, Si, Fe) from a few MeV to at least 100 MeV, are urged. The
intercomparison of these spectra will give information on the interaction mode of
the particles at the Sun (e.g., thick- or thin-target), and also on the effects of
the interplanetary medium on the propagation of these particles (see section V).

(3) Measurements of the charge states of energetic nuclei are potentially
capable of determining the electron temperature of the acceleration region (see
section II).

(4) Measurements of the abundance ratio of rare nuclides (e.g., D, Li, Be,
8B) to C and O can place limits on the column density of material encountered by
the particles since their production, and hence on the likely interaction mode
(ef. (2)]. This information can also be coupled with measurements of the total
particle flux (obtained from simultaneous widely-separated multi-spacecraft
measurements) and the total y-ray flux [see (10)] in order to obtain a reasonably
complete picture of the confinement and escape mechanisms of accelerated
particles.

(5) Since very high energy particles can be detected by terrestrial
cosmic-ray neutron monitors, with the low energy cutoff in the instrument
response being a function of geomagnetic latitude, use of the world network of
neutron monitors can provide information on the energetic particle spectrum in
excess of 17 GeV. The SMM y-ray experiment is conceivably capable of detecting
the 19 decay y-rays produced by particles with energies of several hundred MeV,
and this would provide a comparison flux at lower particle energies.

12


3 20 609
g % 8
x 2 S
y 150 § js008
§ sys
& SY
a 400 >
S S
“N , @
50 / p00.
j 8
0 lab plasma 200 8

_solar
flare 100
= 0
1.85 186 187 —= dK

Fig. D3: Comparison of Flare and Laboratory Spectra within the Region of Fe XXV
Lines

counts! Fe X\ ep XXVI

20 G_-sat.

7 Ue

140 180 MA)

Figs 04: Comparison of Flare and Laboratory Spectra within the region of Fe XXVI

bnme
f=)

arb.units

49


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TABLE 5.11 (continued) |

COS-006 Flare Energy Budget

Scheduling criteria: A large flare-prone active region is required.
Weather must be clear at an adequate number of ground-based
observatories. Interplanetary probes must be on alert. |

Very complete observations are required to specify the total energy output
especially in the optical spectrum and in interplanetary space. Special
coordination will be required at least on some occasions when this COS is carried
out, since an energy measurement in every waveband and for each form of ejection is
required. In addition, magnetograms before and after the event are required for
estimates of the available energy. While SMM is admirably suited to obtain the
total energy of the hotter and energetic plasma, it does not have the diagnostic
capability to derive either the total mass or energy flux of the lower temperature
component. An intensive effort at ground-based observatories is required to
specify the total energy.

COS-008 Implusive Phase (ref. Chapters 3 and 2)

For the study of impulsive phase questions the SMM investigators have defined a
series of JOS's for which the collaborative observations are essentially the same
as those for COS-003, Nature of Electron Acceleration.

COS-009 Location of Energy Release Sites (ref. Chapter 2)

Distinctions between this COS and COS-002, Thermalization, have not yet emerged.
Basic observation sequences are the same as for COS-002. |

COS-015 Flare Precursor (ref. Chapter 4)

Scheduling criteria: Initiate on reasonable probability of flare
occurrence, esp. when many ground-based observatories are clear (ref.
Flare Build-Up Study). |

Soft X-ray SHGs
UV SHGs
Dopplergrams °
Optical SHGs (D3, 10830 A)
Ha filtergrams
Coronal emission-line
filtergrams
Magnetograms
Dopplergrams

Photoheliograms (disk)
Photoheliograms (corona)
cem-r% radio SHGs
m-A radio spectra

SHGs

search for preflare heating

search for preflare heating

search for preflare heating

preflare motions, absorption
preflare motions, absorption
preflare coronal motions

emerging flux; other field changes

photospheric shear; filament
activation

sunspot changes

look for forerunners

search for preflare heating

record preflare bursts

locate preflare bursts

29

|
|


TABLE 5.11 (concluded)
COS-018 Flare Decay (FBS objective)

Scheduling criteria: Initiate following flare maximum. Continue
intermittently for several days following large flares.

Soft X-ray spectra define emission-measure changes
SHGs define morphology of cooling regions
Doppler shifts seek hot component of cooling loops
EUV spectra define emission-measure changes

Dopplergrams identify precipitating material
UV spectra examine line wings for evidence of
asymmetries induced by non-thermal
particles
UV SHGs structure of cooling region

search for electric field and
energetic particle effects

morphology vs time

magnetic field relaxation

sunspot changes

streamer reformation; coronal
transients

define emission-measure changes

decay of flare effects on IP medium

Optical spectra

Ha, 5303 R filtergrams
Magnetograms

Disk photoheliograms
Coronal photoheliograms

cm-\ radio spectra

Solar wind velocity,

density, magnetic field

IP electron spectra, flux particle acceleration or trapping
with leakage in decay phase

(same as above)

persistence of flare effects

IP ion spectra, flux
IP scintillations

COS-019 Flare Morphology
Scheduling criteria: Initiate on regions producing frequent flares.

This COS is a sequence of pre-flare, flare onset and flare decay COSs. See
COS-015, Flare Precursor; COS-002, Thermalization; and COS-018, Flare Decay.

COS-020 Ion Acceleration (ref. Chapter 3)

Scheduling criteria: Initiate on reasonable probability of flare

occurrence.

y-ray spectra, flux

Hard X-ray spectra, flux

Soft X-ray spectra

Soft X-ray SHGs

X-ray polarization

UV spectra (Ly@)

UV SHGs

Optical spectra
magnetograms

Photoheliograms (disk)

m-» radio spectra

SHGs, polarization
IP ion spectra, flux

abundances
charge states

Solar wind data

Neutron-monitor data

seek evidence of heavy nuclei enrichment

Provide electron temperature and energy data

yield ion and electron temperatures

provide flare morphology

seek evidence for particle beams

proton beam effects in line profile

provide flare morphology

search for deuterium lines

magnetic gradients

white-light flare search

presence of shocks in corona

shape of coronal shocks; magnetic fields

infer interaction with ambient medium,
total energy

infer interaction mode with ambient medium

infer electron temperature in acceleration
region

separate propagation effects from
acceleration mechanism

define deka-GeV particle population

30


injected flux, as compared to the purely collisional model), and to cause a
reduced non-thermal X-ray flux (since the electrons are stopped quicker in the
target by the additional deceleration of the — field). While not satifactorily
resolving the X-ray/(EUV, Hq) inter-relationship difficulties of Emslie, Brown,
and Donnelly (1978) and Brown, Canfield, and Robertson (1978) respectively, the
inclusion of ohmic heating terms does, however, on a purely non-thermal
interpretation of the X-ray event, worsen the electron number problem to a degree
which might well be considered totally unacceptable.

It thus appears that the present view of the thermalization of electron
beams is very much in need of repair and review. This has led to a renewed
interest in thermal models of the hard X-ray burst (Chubb 1971; Brown 1974; Smith
and Lilliequist 1979; Brown, Melrose and Spicer 1979; Vlahos and Papadopoulos
1979), in which the X-rays are emitted from a very hot (=3 x 108K) source, the
bulk of whose electrons are prevented from free-streaming out of the source (e.g.
Kahler 1971) by the ion-acoustic turbulence created by instabilities in the
aforementioned beam return current. A small, high energy, fraction of the
electrons are not‘effectively confined by the thin turbulent fronts bounding the
source and can escape to, and thermalize in, the chromosphere. In this way, a
number of the problems mentioned above are possibly resolved: the greater
efficiency of thermal (as opposed to non-thermal) bremsstrahlung results in a
reduced number of electrons requiring to be accelerated, and the precipitation
rate into the chromosphere is reduced, easing the X-ray (EUV, Hq)
inter-relationship difficulties mentioned above (for more details, see Vlahos and
Papadopoulos 1979; Vlahos and Emslie 1979). Further, the anisotropy in the
source effected by the heat flux along the arch results in finite polarization
and directivity of the X-rays (Emslie and Brown 1979) which is quite compatible
with current observational requirements (Tindo, Shuryghin, and Steffen 1976;
Datlowe et al. 1977), and the computed X-ray spectrum from such a source (Emslie
and Brown 1979, Smith and Lilliequist 1979) compares very favorably with those
observed (e.g., Hoyng, Brown, and van Beek 1976).

Finally turning to the deeper layers of the atmosphere, it is apparent that
significant temperature enhancements are produced at photospheric depths during
flares (Machado, Emslie and Brown 1978; Cook and Brueckner 1979). Various
attempts at reproducing these enhancements on the basis of standard mechanisms
(electron impact, X-ray and EUV irradiation, joule heating) have met with little
or no success (Machado, Emslie, and Brown 1978; Emslie and Machado 1979) and it
appears that we must either invoke a very high flux of deka-MeV protons or a
separate primary energy release at photospheric levels. Proton fluxes as high as
those necessary should evidence themselves in strong y-ray bursts and therefore
SMM is ideally suited to testing the first of these hypotheses. Further, as
Machado, Emslie, and Brown (1978) point out, more ground-based (Ca II H and K and
Mg II h and k line) data are necessary to strengthen their observational claims;
this is also appropriate to the proposed high level of co-operation of satellite
and terrestrial observations during SMY.

In conclusion, the task facing modelers of energy tranport and
thermalization is to construct, to as high a level of self-consistency and
exactness as possible, theoretical emission measure profiles and other such
signatures of their theories, to such a level that the SMY network of satellite
and ground-based observations is capable of discriminating observationally
between such models.

45


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18


Fig. D2 shows records of 6 high resolution spectra in the region of lines
In Fig. D3 one of these spectra

of the helium-like ion Fe XXV, A= 1.85-1.87

is fitted with the laboratory gne (Zhitnik et gl.
Ss

resonance line ls 2p IP, +1s
intercombination line ls 2p 3P}+1s2 1sg
magneto-dipole line ls 2s ?P

forbidden magneto-quadrupole line 1s 2p 5P) + 1s4

density). Moreover, a large number of satellite lines are observed.

= 1.8596

et 1979 a).
A= 1.8506 A ( w ), the
, (y), the forbidden
1 +1s? lsg, ) = 1.8685 A (z), and the,

One can see the

Isg, a= 1.8557 A (x)
(these two lines cannot be seen in the laboratory spectrum due to large electron

These lines

are due to transitions ls 21 21'+1s¢ 21" in the lithium-like ion Fe XXIV;

the upper levels of these lines correspond to the excitation of two electrons and
Fig. D4 shows a section of the
solar and the laboratory spectra in the region 1.70-1.80 A. One sees the
unresolved resonance line La of the hydrogen-like ion Fe XXVI 2p 2Py/2,3/2 >

is 2S1/2, X= 1.78 R and the adjoining group of unresolved satellite lines

lie above the ionization limit of Fe XXIV ions.

which correspond to transitions 21 21°+1s 21" in helium-like ions.

The

upper levels of these lines also lie above the ionization limit of the ion Fe
XXV. Table Dl contains experimental and theoretically calculated wavelengths for
the observed lines of Fe XxXV and for the satellite lines

counts

4 |

oo”
16.14.7(
p70
of'os*
16.9170

0

ae

96.1170

03°31"
05.1170

05°42"
2410.70

Fig. D2: Spectra in the Region 1.85-1.87 R of 6 solar flares

48


As to the generation regions of more hard radiation (greater than 10 keV),
direct experimental data are lacking. From the viewpoint of the picture
developed here, the excitation of the Kg and Ke lines and further heating of
plasma take place within a volume of about 1027-1029 cm? at a height of
Several thousand km or less with the density in the range 109-10l! cm
(Kane and Lin 1972), though we cannot exclude a somewhat higher density. Thus,
determination of the regions of generation of X-ray radiation of various energies
can be very useful for clarification of flare development mechanism.

We are planning to investigate, during the year of maximum solar activity,
spectra, polarization and localization of generation regions of X-ray radiation
for different energies. These studies will be performed by means of AUOS
stations - a new generation of satellites for solar research.

57





APPENDIX B
SCIENTIFIC PROBLEMS OF THE SMY SERF PROGRAM

A.T, Altyntsev, V.G. Banin, G.V. Kuklin, V.P. Nefedjev
A.V. Stepanov, V.E. Stepanov, and V.M. Tomozov

USSR Academy of Sciences
Siberian Branch
Siberian Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation
Irkutsk, USSR

We propose to concentrate our attention on the two problems which have bearing not only on the
SERF problem but also on the FBS problem.

Problem I, Investigation of particle-heating and acceleration processes in the flare region.
Problem If. Determination of the flare triggers.
1. Let us consider problem I in more detail.

According to current knowledge, the solar flare is associated with the formation, interruption and
turbulization of the current sheet. Plasma heating and particle acceleration are accompanied by the ex-
citation of ion-acoustic(s) and Langmuir () turbulence at a comparatively high intensity: W, < W_~ 0.1
nk T_, where W is the wave energy density. Such a high level of ion-acoustic turbulence résults*also
from laboratory experiments on current sheets.

Fast particles are assumed to contain about 10 percent of the flare energy and to be accelerated by
plasma turbulence. Therefore it is essential to acquire observational data which permit the determina-
tion not only of the plasma heating rate ad of the efficiency of particle acceleration but also of the
plasma turbulence level in the acceleration region. These data can be inferred from radio, X-ray and
optical observations of flares.

1.1 Radio emission, Plasma heating in the flare region and the existence of plasma turbulence
both will cause radio emission. Theoretically the electron density in the current sheet n~ iol4 - 10
cm”, i.e., the radio emission frequency in the transmittance region (» = w__) corresponds to the milli-
meter and centimeter range. Knowing from observations the characteristics &f the current~sheet radio
emission and having available an appropriate theory one can create the flare (current sheet?) diagnostics
from radio emission.

The current-sheet radio emission theory is only in its first stages. Kuznetsov and Syrovatskii
(1977) made an estimate of the level of the current sheet bremsstrahlung emission. Zheleznyakov and
Zlotnik (1979) calculated the thermal cyclotron emission of the Harris sheet (1962) - a classical model
of the current sheet. It resembles a series of relatively narrow spectral lines with a simple ratio of
their frequencies. The formation of individual lines is explained by cyclotron emission only on the
periphery of the current sheet, where the magnetic field has quasi-homogeneous features. Therefore,
the frequencies of the lines correspond to electron cyclotron harmonics in this magnetic field. Detec-
tion of cyclotron harmonics at the background of a slowly varying component is possible for sufficiently
hot current sheets with a temperature of 107-108 x. Using these lines, diagnostics of the magnetic
fields and kinetic temperature in the current sheet is possible.

But the chief problem of the current sheet radio emission theory is the problem of radio emission
of the ion-acoustic(s) turbulence of a comparatively high level: W_~ 0.1 nkT_. The point is that if
together with s-waves also £ -waves of approximately the same level exist, the problem then reduces to
calculation of the plasma electromagnetic wave conversion coefficients under conditions of strong tur-
bulence. However, the statement that in the flare W, ~ (10~° - 1) W_ cannot be regarded as substanti-
ated. The linear mechanism of Langmuir wave generation by the current is not realized. With the current
speed u > y,,, Buneman instability with a real part of the frequency Rew ~(m_/m,) ve arises. Ifu
exceeds the Sound velocity (u> Cs), ion-acoustic waves are then generated? Tne non-isothermicity of
plasma (T, > T,) is provided either from the preliminary electron heating in the Buneman stage of current

instability or by Joule heating. Generation of Langmuir waves of a sufficiently high energy density level

NOTE: Some minor, stylistic changes have been made from the text supplied by the authors.
37



APPENDIX A
SERF ORGANIZATION AND OPERATIONS

David M. Rust
American Science and Engineering, Inc.
Cambridge, Massachusetts U.S.A.

I. Introduction

The SERF is one of three subprograms of the SMY. The other two subprograms are
the Flare Buildup Study (FBS) and the Study of Traveling Interplanetary Phenomena (STIP).
Each of these subprograms is guided by an organizing committee headed by the individuals
listed below.

FBS -- Z. SveStka, Space Research Laboratory, Beneluxlaan 21, 3527 HS Utrecht, The
Netherlands.

SERF -- D. Rust, American Science and Engineering, Inc., 955 Massachusetts Ave,,
Cambridge, MA 02139, U.S.A

STIP -- M. Dryer, Space Environment Laboratory, NOAA/ERL, Boulder, CO 80303, U.S.A.

The actions of the three subprograms are coordinated and integrated by a Steering Com-
mittee appointed by SCOSTEP, COSPAR, IAU, IUPAP and IUGG with C. de Jager (Chairman),

Z. SveStka (deputy-Chairman), P. Simon (Secretary): DASOP, Observatoire, 92190 Meudon,
France, and members M. Dryer, M. Kundu, D. Rust, E.d. Smith, V.E. Stepanov, G. Wibberenz,
and R. MacQueen.

II. Operations During the SMY

Based on the COSs defined in Chapter 5, SERF “ACTION” intervals will be declared
as special opportunities for observations arise throughout the SMY. The first SERF
ACTIONS will be scheduled during a special ALERT period shortly after the launch of the
SMM satellite. This period is likely to be 1 - 22 November 1979. All subprograms of
the SMY will be involved. The goal of this first ALERT is to obtain some simultaneous
ground and satellite data as early as possible in the SMM flight. Thus, should any
satellite experiment fail in the first month of operation at least the best advantage
will have been taken of the period of successful operation. To this end, the SERF
program will follow SMM operations rather closely during this initial interval,

_ During the rest of the SMY, it appears that the following two modes of collabo-
ration will be most effective:

(1) "A la carte" - observing efforts built around Specific Scientific Objectives
(SSOs) and particular observing opportunities. This is basically to say that the whole
of the SMY is an ALERT interval for SERF, except for May and June 1980, when the Flare
Buildup Study will be the main subject of the SMY. STIP will assign ALERT intervals to
coincide with SERF and FBS ALERTS when appropriate.

Observing efforts called "ACTIONS", of 1 to 3 days, will be triggered by good pros-
pects for collaborative observations. Although notice of the ACTION (including coordi-
nates of the activity center) will be distributed by Telex and telephone to all inter-
ested participants, it is understood that not everyone will be able to react to every
call_for observations. We hope, in the course of a year's work, however, to obtain
simultaneous and relatively complete coverage of several dozen flares for most, if not
all, of the specific scientific objectives of the SERF program. ACTIONS may be called
at the average rate of 2 or 3 per month. Advance notice for ACTIONS will vary from 1
to 7 days.

(2) "Menu conseil1é" - one or more pre-established intervals for especially in-
tense efforts. This is the same as an ALERT-ACTION interval in the FBS program, and
we hope that all participants will schedule their telescopes, maintenance programs,
vacations, etc., in order to ensure that a set of observations as complete as possible
will be obtained during this period. The candidate time for a special SERF ALERT is
15 September - 15 October 1980. During this month, or during another period to be
chosen at the IAU meeting in Montreal, the ACTION intervals should yield especially
complete coverage.

35





(2) As mentioned above, reliable measurements of the polarization of
the hard X-ray bursts provide a clear test between models N-T and T. Since Model
T is only capable of producing at most some 5% polarization (Emslie and Brown
1979), while model N-T can produce much more (e.g., Bai and Ramaty 1978), any
observation of more than 5% polarization of the emitted X-rays clearly points to
Model N-T. However, past polarization observations (Tindo et al. 1972a, b; 1973;
1976) are somewhat uncertain, with polarizations in excess of 10-20 percent
claimed in some events, and only a few (2-3) percent in others, but the recent
results reported by Mandel'stam (1979) of some 10-20 percent polarization in half
of a data sample of 12 events in 1970 clearly require further confirmation. To
this end, the Soviet AUOS satellite due to be in operation during SMY, is a
potentially very valuable instrument. It will have the ability to deconvolve the
effect of an anisotropic X-ray background, and the results it produces should be
made an integral part of the SMY observing program.

(3) Radio imaging of the flare in two dimensions, with as high a time
resolution as possible will also be a great asset in the discrimination between
models N-T and T, especially if polarization measurements are also available. In
this way, the location of very high energy (several hundred keV) electrons, and
their pitch angle to the magnetic field lines, may be inferred, and compared with
the predictions of the two models. These electrons will form part of the
suprathermal "tail" in Model T and should therefore manifest themselves outside
the X-ray source. They are simply part of the non-thermal (e.g., power-law
distribution in model N-T and should therefore appear cospatially with the X-ray
source.

(4) Still, in the radio waveband, it has been observed (Allissandrakis
and Kundu 1978) that 6-cm radio bursts, measured with 6" spatial resolution, show
an intrinsic polarization during the impulsive phase of the flare which they lose
during the post-maximum phase. Kundu and Vlahos (1979) have interpreted this as
due to asymmetry in the magnetic field geometry in the flaring loop, and point
out that the electrons spiralling down the other (weaker B) leg of the arch should
emit most strongly at wavelengths nearer to 20 cm. Thus, simultaneous
measurements of the radio emission at 6 cm and 20 cm respectively should provide
useful information as to the distribution function for the streaming electrons
and so possibly discriminate between models T and N-T.

(5) The measurement, using the SMM Bent Crystal Spectrometer, of the
intensities of the Fe,XXV resonance line ls? 1s Sg + Is2p +P, (E =

6.701 keV; + = 1,850 R) and its Fe XXIV dielectronic recombination satellites
1s2p2 2D5/2 + 1822p 2P 342 (n =2; line "j", in the notation of Gabriel

1972) and Is 2p(1P) 3p 4205/2 + 1s2 3p 2P3/2 (n=3; line "d13" in the

notation of Bely-Dubau, Gabriel, and Volonte 1979) provide us with samplings of
the electron energy distribution function at E = 4.649keV (line j), 5.815 keV
(line d13) and E 2 6.701 keV (resonance line). The satellite lines are excited
by electrons having the exact energies quoted above (to within the autoionizing
width), due to the absence of a third body to remove excess energy in the
dielectronic recombination process. Two temperature values may be obtained from
the intensities of these lines: Tj, from the intensity ratio of the satellites;
and Tg, from the ratio of either satellite intensity to that of the resonance
line (see schematic in Figure 2.2). These two temperatures, and their
difference, can be compared with the electron energy distribution function of the
two hard X-ray burst models, and the high ( = 0.0002 R) resolution of the BCS
instrument, which will observe the 2h Fe lines continuously, permits measurements
of the required quality to be made.

(6) The intercomparison of hard X-ray and EUV fluxes, for which the
two hard X-ray models predict a different ratio (Emslie, Brown and Donnelly 1978;
Vlahos and Emslie 1979), is also important. It is especially important to know
where the EUV radiation originates, and also its spectral distribution, in order
to strengthen the previous analysis of Emslie, Brown, and Donnelly (1978) and
Donnelly and Kane (1978). This diagnostic clearly requires an interplay between
the SMM hard X-ray imager and the ultraviolet spectrometer-polarimeter, although
ground-based (e.g., SFD) observations would be also useful.


APPENDIX D
X-RAY SPECTRA OF SOLAR FLARES

S. L. Mandel'stam
USSR Academy of Sciences
Institute of Spectroscopy

Troitsk, Moscow

Spectra of solar flares provide vast information on the physical
parameters of the plasma of the flare region. In the present communication the
results of recent evaluation of the spectra of large X-ray flares at the Sun on
24 October, 5 November and 16 November 1970 are presented.

Observations were performed from the satellite "Intercosmos-4" launched on
14 October 1970 (Mandel'stam 1978). The principal layout of this satellite-borne
instrument is shown in Fig. Dl. The instrument contained two spectrometers with
quartz crystals with spacingsd= 1.800 & and d = 4.2546 A, an X-ray heliograph
and an optical sensor. The instrument was mounted on the front panel of the
satellite; the longitudinal axis of it was pointed at the Sun and the solar disk
was scanned automatically three times at each orbit. By scanning the angle of
incidence for the radiation from the flare region onto the erystal @ varies and,
therefore ~ according to Wulf-Bregg law - the radiation wavelength recorded at
the moment is A = 2d sin ®@ . By fitting two of the lines in the solar spectra with
two reference lines in laboratory spectra and knowing the scanning velocity de/dt.
wavelengths of other jines in the spectrum were determined. Scanning velocities were
determined by passing the flare region through three collimator slits of the helio-
graph and from the optical sensor data. The error in the } determination with regard
to the possible error of alignment of spectrometer and heliograph axes ( = 1') and the
dimensions of the flare radiation region ( = 1') did not exceed +0.0904 f; spectral
resolution (depending on scanning velocity) was 3 x 107" - 1 x 1073 (Zhitnik 1974).

( olirection an the Sun
bey radiation | sinning angle 240"

electronics

oa telemetry

Fig. Ol: Layout of X-Ray spectrometer-heliograph: 1 - quartz crystals; 2 -
collimators; 3 - photon counter; 4 - heliograph collimator; 5 - optical sensor.

NOTE: Some minor, stylistic changes have been made to the text si lied b
author (editors). , sppened by he
47



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53


é
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Fig. 2.
of the "shape parameter” m.

Behavior of an idealized flux-vs-time profile o(t), for various values

20 12,
1h
sa}
1
1“ oa}
12
ost
to om
dom e e
ae or 1 es
os
o2t
4h
oF °
°
a2
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at rst L L n n eal A
° YT a Yr
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tn
Fig 2.4 Trajectories in the (g,$) plane for each of the functions of Figure 2.3.

The general shape of this curve provides an indication of the character of the

process responsible for the flux.


WORLD DATA CENTER A
for
Solar-Terrestrial Physics

REPORT UAG-72

ENERGY RELEASE IN SOLAR FLARES

Proceedings of the Workshop on
Energy Release in Flares
26 February - 1 March 1979
Cambridge, Massachusetts U.S.A.

Edited by

David M. Rust
American Science and Engineering, Inc.
Cambridge, Massachusetts U.S.A.

and

A. Gordon Emslie
Harvard-Smithsonian Center for Astrophysics
Cambridge, Massachusetts U.S.A.

=
SS

y,

£

WS

July 1979

Yj

4%

Published by World Data Center A for
Solar-Terrestrial Physics, NOAA, Boulder, Colorado
and printed by

U.S. DEPARTMENT OF COMMERCE
NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION
ENVIRONMENTAL DATA AND INFORMATION SERVICE
Boulder, Colorado, USA 80303

SUBSCRIPTION PRICE: $25.20 a year; $17.30 additional for foreign mailing; single copy price varies.*
‘Checks and money orders should be made payable to the Department of Commerce, NOAA/NGSDC.
Remittance and correspondence regarding subscriptions should be sent to the National Geophysical and
Solar—Terrestrial Data Center, NOAA, Boulder, CO 80303.

*PRICE THIS ISSUE $1.50



TABLE 5.II (continued)

COS-003 Nature of Electron Acceleration (ref. Chapter 3)

Scheduling criteria: Initiate upon reasonable probability of flare

COS-005

occurrence.

y-ray spectra, flux

Hard X-ray spectra, flux
HGs

Soft X-ray spectra
HGs

X-ray polarization

UV polarization

H@ filtergrams

Magnetograms
Coronal photoheliograms
cem-» radio spectra, flux

SHGs polarization
m-, radio spectra

SHGs, polarization
IP electron spectra, flux

Ejecta (ref. Chapter
Scheduling criteria:

Hard X-ray SHGs
Soft X-ray SHGs
Doppler shifts

UV SHGs

Dopplergrams
Optical spectra
5303 8, Ha, Dz filtergrams

(Limb)

Off-band He filtergrams

(full disk)

Dopplergrams (Ha)
Coronal photoheliograms
Continuum polarization
(corona)

Magnetograms
m-A radio spectra

SHGs, polarization
Solar wind velocity,

density, magnetic field

4)

Initiate

IP ion abundances, charge states

IP scintillations

infer beam and target properties

infer beam and target properties

find acceleration region size and location

sample electron temperature

follow spread of energization

search for electron beam effects

search for electron beam effects

locate flare kernels, other
phenomena associated with electron
beam evidence, e.g., Type III bursts

show field structure and changes

define Type III burst trajectories

measure NT source brightness
temperature

locate acceleration region

follow accelerated beam

seek second-stage acceleration region

define particle population

with active prominence at R/Rg>0.7

burst location re. ejecta position

locate heated ejecta

infer kinetic energy of hot ejecta

define mass as a function of temperature

infer kinetic energy and trajectories

infer sources and sinks of kinetic energy

infer kinetic energy and trace material
through low corona

search for Moreton waves

infer sources and sinks of kinetic energy
follow mass ejection through corona
determine shape of coronal transients

infer magnetic field in prominence

follow shock through corona

infer magnetic field and location of shock
infer mass and energy of ejecta

infer conditions in source region

infer size, field and velocity of IP
disturbance

28




WORLD DATA CENTER A
National Academy of Sciences
2101 Constitution Avenue, N.W.
Washington, D.C., U.S.A., 20418

World Data Center A consists of the Coordination Office
and seven Subcenters:

World Data Center A
Coordination Office
National Academy of Sciences
2101 Constitution Avenue, N.W.
Washington, D.C.,-U.S.A., 20418
[Telephone: (202) 389-6478]

Glaciology [Snow and Ice]: Rotation of the Barth:
World Data Center A: Glaciology World Data Center A: Rotation
[Snow and Ice] of the Earth
Inst. of Arctic & Alpine Research U.S. Naval Observatory
University of Colorado Washington, D.C., U.S.A. 20390
Boulder, Colorado, U.S.A. 80309 (Telephone: (202) 254-4023]

[Telephone: (303) 492-5171]
Solar-Terrestrial Physics (Solar and

Metéorology (and Nuclear Radiation): Interplanetary Phenomena, Ionospheric
Phenomena, Flare~Associated Events,
World Data Center A: Meteorology Geomagnetic Variations, Magnetospheric
National Climatic Center and Interplanetary Magnetic Phenomena,
Federal Building Aurora, Cosmic Rays, Airglow):
Asheville, North Carolina, U.S.A. 28801
[Telephone: (704) 258-2850] World Data Center A
for Solar-Terrestrial Physics
Oceanography : Environmental Data and Information
Service, NOAA
World Data Center A: Oceanography Boulder, Colorado, U.S.A. 80303
National Oceanic and Atmospheric [Telephone: (303) 499-1000, Ext. 6467]
Administration
Washington, D.C., U.S.A. 20235 Solid-Earth Geophysics (Seismology,
[Telephone: (202) 634-7249] Tsunamis, Gravimetry, Earth Tides,
Recent Movements of the Earth's
Rockets and Satellites: Crust, Magnetic Measurements,
Paleomagnetism and Archeomagnetism,
World Data Center A: Rockets and Volcanology, Geothermics) :
Satellites
Goddard Space Flight Center World Data Center A
Code 601 for Solid-Earth Geophysics
Greenbelt, Maryland, U.S.A. 20771 Environmental Data and Information
[Telephone: (301) 982-6695] Service, NOAA

Boulder, Colorado, U.S.A. 80303
[Telephone: (303) 499-1000, Ext. 6521]

NOTES:

1. World Data Centers conduct international exchange of geophysical observations
in accordance with the principles set forth by the International Council of Scientific
Unions. WDC-A is established in the United States under the auspices of the National
Academy of Sciences.

2. Communications regarding data interchange matters in general and World Data
Center A as a whole should be addressed to: World Data Center A, Coordination Office
(see address above).

3. Inquiries and communications concerning data in specific disciplines should
be addressed to the appropriate subcenter listed above.



The processing of spectrograms is generally complicated by various photometric errors. Further-
more, when flares on the Sun's disk are observed the determination of the flare emission is complicated
by the effect of spectral lines in the photosphere. To eliminate the effect of the Fraunhofer spectrum of
the "substrate" it is necessary to obtain spectra of flares at the limb of the Sun. The quantity W, de-
termined independently using the Stark effect can be compared with the value Ww, obtained from radio
data.

1.4 Laboratory experiment, Laboratory experiments serve as a check of the correctness of the
theory of particle heating and acceleration in current sheets. A part of the results concerning annihila-
tion of opposite magnetic fields has already been achieved at SibIZMIR and FIAN. The neutral current
sheet that forms in the theta-pinch device has been selected as an objective of investigations. The de-
scription of the device is given by Altyntsev et al, (1973). To study the tearing-instability, a sheet
with a width one order more than its thickness is formed. The main parameters are: Magnetic field
B< 10° G, density n= 1042 ~ 1014 om=3,

With the aim to use the results of the experiment for the interpretation of solar flares, the param-
eters of the process as a function of B and n are investigated, which provides a possibility in many
cases of describing these parameters in dimensionless quantities. Assuming that the sheet under study
is a fraction of the solar current sheet, when modeling one can assume the scaling parameter to be
equal to unity for such results as the maximum rate of the magnetic field energy conversion into plasma
energy, the current dissipation mechanism, fast electron density and rate of electron acceleration.

The first problem of the experiment which has been mainly solved so far is to study the magnetic
field topology in the sheet (Altyntsev and Krasov, 1974). From the experiments it follows that the sheet
thickness is A= 100/a,.-

By direct measurements of the magnetic field vector it has been shown for the first time that the
evolution of the magnetic field structure in the sheet is consistent with the development of the collision-
less tearing-instability. The ratio of perturbation wavelength to sheet thickness is constant and equals
six, The instability stabilizes and does not lead to a catastrophic sheet interruption. The stabilization
level, B Bey = 0.2 - 0.3, where B, is the maximum value of the resulting transversal field component
in the sheet and Bip is the value of magnetic field at the sheet boundary. This phenomenon was ex~
plained theoretically by Galeev and Zelenyi (1975).

The second problem is to study plasma heating and acceleration in the sheet. The experiment
(Altyntsev and Krasov, 1977, 1978) has shown that the electron temperature reaches the values of 0.5 - 1
keV and is due to the attainment in the sheet of the quasi-linear stage of the ion-acoustic instability.
Electron energy distributions are measured in a turbulent current sheet, Spectra characterized by ¥
from 0.5 to 8 have been obtained if the distribution is approximated by a power law n(E) « EY and is
a function of initial conditions and recording site with respect to the sheet, The acceleration rate can
be accounted for by the existence in the sheet of a superthermal Langmuir turbulence.

Applications of these results to the problem of solar flares are given in the work of Altyntsev et al,
(1978). The efficiency of the process investigated is sufficient for the observed energy release rate in
flares with an exclusively small total volume of current sheet fractions.

The technique developed at SibIZMIR permitted the first detection of accelerated electrons in the
experiment of Frank's group (FIAN) at the interruption of the current sheet Produced from the zero line of
the multipole magnetic field (Altyntsev et al., 1978).

The chief trend of further experiments is to study the efficiency of annihilation with due regard for
the effects of plasma compression into a sheet, conductivity, etc. Study of the generation mechanism
of Langmuir oscillations and of their effect on the spectrum of accelerated electrons is being planned.
The lifetime of the current sheet in the experiment was 5 x 107’ s. This time it is intended to increase
it up to 4x 107 s,

1.5 Summary. Above we have mainly dealt with the processes occurring directly in the accel-
eration region (current sheet). To get a more comprehensive picture about the dynamics of particles and
emissions in the flare it is necessary to investigate other possible mechanisms of particle acceleration
and the behavior of energetic particles in solar magnetic traps.

40


emerging flux or shear, which could destabilize the fields supporting filaments.
Ha, 5303 R, and X-ray observations can show precursor activity quite clearly.
More patrols in these wavebands are needed. Type III bursts may give a warning
that preflare motions are imminent, and microwave interferometer observations
will indicate where the preflare heating is occurring. Radioheliograms showing
the path of type III and type II bursts can help indicate the rate of propagation
of disturbances in the corona near 1.5 Re, where coronagraph coverage is poor.

Vv. Interplanetary Effects

We have barely started to identify, from both observational and theoretical
viewpoints, the interplanetary extensions of solar eruptions. During the SMY, we
hope to clarify how solar events affect the interplanetary medium and the normal
solar wind, to categorize a spectrum of forcing functions, as it were, and to
make unambiguous solar-planetary associations with, at least, the most energetic
events.

with these observational recommendations carried out to the fullest extent

possible, we believe that several key issues regarding the role of mass motions
as a mechanism of energy release in flares can be resolved.

24


TIT. Synoptic Observations

Most of the observations to be made during the SMY will provide high time and
spatial resolution measurements that will comprise the primary set of data for attack-
ing the problems of energy release in flares. As a complement to these intensive data
sets, selected synoptic observations, made daily over the period of the SMY and cover-
ing the entire Sun, are essential to providing a comprehensive survey of solar flare
activity. The following specific problems require synoptic data:

(1) Establishment of trends of solar activity. Initiation of COSs depends on a
knowledge of the recent trend of development of active regions and the rates of flare
occurrence, Synoptic data, such as full-disk and large-scale Ha and white-light images,
if made available on a rapid time scale will provide a sound basis for selection of SMY
target regions. Magnetograms are also especially important for this purpose

(2) Provision of a continuous record of the development of each active region
studied. By the time of solar maximum, active regions may persist for several months,
as magnetic field structures decay and are replaced by new fields in the same general
location. Synoptic observations will provide the record necessary to follow the evolu-
tion and topology of the magnetic field and emission sources in target regions.

(3) Isolation of sources of particles observed by interplanetary satellites. Major
flares often occur within hours in widely separated active regions. A knowledge of all
possible sources of energetic particles at any given time is necessary before initiat-
ing studies that relate interplanetary particle fluxes and spectra to specific flare
sources.

(4) Study of important events in regions not selected as SMY targets. In particular,
great particle events may occur in active regions other than those being observed by the
high spatial resolution instruments participating in the SMY. Analysis of the gamma ray
and hard x-ray observations will then depend on the availability of lower resolution but
temporally complete synoptic observations.

Primary sources of synoptic data include the following:

(1) The U.S. Air Force Solar Observing Optical Network (SOON). The SOON tele-
scopes produce regional and whole-Sun Ha and continuum images, providing a consistent
and nearly continuous record of almost all flares and sunspot activity. The digital
SOON flare histograms are the most precise way of differentiating between flares that
occur almost simultaneously in different active regions. Whole-Sun radiation measure-
ments of soft x-rays, hard x-rays, gamma rays, and radio emission can be differentiated
and attributed to the correct flare sources by using these regional histograms. Recog-

nizing the unique capabilities of the Solar Observing Optical Network, we strongly recom-

mend the timely dissemination of the SOON data to the scientific community and the per-
manent archival of these data by a World Data Center.

(2) The U.S. Air Force Radio Solar Telescope Network (RSTN). The RSTN provides
the only source of continuous whole~Sun radio flux measurements in microwave and meter
wavelengths. Thus, these measurements are capable of showing probable times of high
energy particle acceleration in all solar flares. We recommend that (a) routine 1-s
data be rapidly disseminated to the scientific community in digital form and archived
by a World Data Center and that (b) for designated periods of detailed interdisciplinary
solar flare and particle event studies, solar radio data acquisition rates be increased
to give significantly higher time resolution.

(3) The NOAA GOES Satellite Whole-Sun X-Ray Monitor. The GOES X-ray measurements
provide the only source of continuous soft X-ray fluxes, and since it is a whole-Sun
instrument, it is the only comprehensive record of X-ray emission for all solar flares.
GOES data should continue to be archived through established procedures.

33


structure of a filament prior to and during its eruption gives some indication of
the magnetic field structure in the immediate vicinity of the flare. We urge
intensive study of high-resolution Hz movies, which are the most effective means
of studying this structure.

(2) Observations of coronal transients as well as in situ measurements of
travelling interplanetary disturbances give estimates of the total mechanical
energy output of a flare. The ratio of the mechanical energy output to the
radiative energy output is a fundamental physical characteristic of the energy
release process. The best past estimates (Canfield et al. 1979b; Rust et al.
1979) indicate that this ratio may be substantially larger than unity, ‘So that
the energy of flare-associated coronal transients and filament eruptions, in
addition to the total radiative output, must be documented in order to understand
the energy release mechanism of flares.

(3) The total energy of moving material is the sum of the kinetic,
potential, magnetic, thermal, and non-thermal energies, and while Ha and
white-light images can give good measurements of the kinetic and potential
energies, observations of the polarization and intensity of radio emission are
necessary in order to infer the magnetic field. Observations of line emission
from the outer and inner corona are required to determine the thermal energy of
transients. Radio spectroheliograms and interplanetary particle observations
(cf. Chapter 3) will give the energy in the non-thermal component.

Ib. Mass of Flare Sprays

(4) Flare sprays are composed of both cool and hot material catapulted into
the corona under the action of magnetic (or thermodynamic) forces. The
prominence (i.e., flare spray) material, seen in Ha emission, is contained within
an expanding or stretching magnetic flux tube and constitutes a substantial mass |
ejection (Rust et al. 1979). It will be important to obtain optical spectra of
this ejected mass to distinguish, by Doppler shift measurements, between genuine
mass motions and wave phenomena, where we observe the locus of a density
disturbance at successive times. We encourage measurements in some lines formed
at higher temperatures than Ho, in addition to Ha measurements, so as to follow
the material as it is heated in the corona, Possible lines for these
observations are HI 1216 8, He II 4686 &, He II 1640 &, and various lines of
Si II, C III, C IV, and Si IV.

(5) Mass ejections reach velocities of several hundred to several thousand
kilometers per second, but the material visible in Ha seems to be confined within
flux tubes. Broad-band ( = 10 A) Ho filtergrams will allow observation of the
material moving at an angle with the plane of the sky, so that an accurate
estimate of the total mass may be obtained.

Ic. Mass of Coronal Transients

(6) The source of the mass in coronal transients should be investigated
with coordinated observations of the low and outer corona. One study (Rust and
Hildner 1976) indicated that the mass seen leaving the Sun through the outer
corona all came from the inner corona, but some coronal transients may possibly
carry off more mass than was present beforehand in the underlying corona. Excess |
mass may be supplied to the corona during the main flare phase: Ho fibrils
connecting the outer borders of flare ribbons to nearby plages have been seen to
open up. There follows immediately a surge-like mass ejection from the feet of
the opened feature. It is important to determine the mass ejected in the surges
and also the surge positions relative to the base of coronal transients, to
determine whether the surges could be at the feet of field lines leading into the
coronal transient. High-resolution filtergrams, Ho and high Balmer line spectra
(to determine the density) as well as coronagraph observations are needed.

19

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Chapter 2

THERMALIZATION OF ENERGY IN FLARES

The problems addressed by the members of this workshop team were the
various (sometimes conflicting) theories of energy transport and thermalization
in flares (see Appendix C) and the measurements Possible during SMY that could
conceivably discriminate between, and enhance our knowledge of, these theories.

I. Hard X-Ray Emission

One of the most important and controversial questions addressed by the
group was the character of the hard X-ray bursts observed during flares (see,
e.g., Hoyng, Brown, and van Beek 1976). Essentially there are two models
currently in vogue, which we shall now briefly summarize.

Model N-T (Non-Thermal)

In this model, electrons are accelerated by the action of some non-thermal
process, such as a direct electric field or plasma turbulence, and are injected
downwards into the solar atmosphere. The beam and its associated return current
are considered to be largely stable to the generation of plasma turbulence, and
as a result the beam thermalizes by Coulomb collisions deep in the atmosphere,
producing the enhanced thermal emission (EUV, Ha) at these levels (see, for
example, Syrovatskii and Shmeleva 1972; Brown 1973; Brown, Canfield, and
Robertson 1978; Emslie, Brown, and Donnelly 1978). The X-rays in this model are
emitted in a "thick-target" source of high density at the foot of the flare
structure (e.g., loop) and have a large amount of polarization and directivity due
to the large anisotropy of the electron phase-space distribution function (see,
e.g., Brown, 1972).

Model T (Thermal)

In this model, the plasma at, say, the top of an arch-like structure is
heated locally to thermal hard X-ray temperatures (=3x108 K). Due to the long
collisional mean free paths associated with such high temperatures (Spitzer
1962), the electrons form a steady stream down the magnetic field lines in the
arch. However, the large return current (required to ensure charge neutrality
and zero net current density - see, e.g., Brown and Melrose 1977; Spicer 1977)
associated with the stream has a drift velocity which exceeds the local ion sound
velocity, thus exciting ion-acoustic turbulence in the plasma. As a result, the
X-ray emitting electrons are effectively confined between two conduction
"fronts", each moving at the local ion-sound velocity down the arch (B8rown,
Melrose, and Spicer 1979), with a few "suprathermal" (v 2 3 Ve, where Ve is
the mean electron thermal velocity) electrons escaping due to the finite
thickness of the conduction front and the long turbulent mean free path of such
high energy particles. The velocity distribution function for the confined
electrons is a slightly (~1%) skewed Maxwellian. Thus, in this model, the X-rays
are emitted predominantly at the top of the flare structure and are not polarized
to such a large degree, although the above anisotropy of the velocity
distribution function (caused by the presence of a thermal heat flux) does in
fact create finite polarization and directivity (Emslie and Brown 1979).

These two models are illustrated schematically in Figure 2.1., and with
these model characteristics in mind, it is a fairly straightforward matter to
prepare a number of distinguishing tests, using both satellite (SMM) and ground-
based observations.

(1) Using the Utrecht hard X-ray imaging spectrometer (HXIS) on SMM at
very high time resolution ( = 0.2 seconds), it should be possible to detect the
motion of the thermal conduction fronts along the loop. More specifically, such
fronts will move at a typical velocity of 2" per second along a structure
potentially as long as 1'. Thus some hundred or so consecutive images of the
source with the 8" spatial resolution of the instrument could be produced,
covering this entire event. If in fact the hard X-ray burst model is non-thermal



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55


(a) Dynamics of energetic particles and emissions in coronal magnetic traps. After a large
flare, there arises a broad-band (cm-dm) radio emission from energetic particles trapped in magnetic
bottles in the corona (type IV, flare continuum, etc.). The trapping of particles by magnetic bottles
may be the cause of the extended duration (up to 10-15 hours) of the particle flux from flares. It is not
excluded that energetic particles may additionally be accelerated by MHD-oscillations of the traps (be-
tatron acceleration, Fermi mechanism, magnetic pumping).

Using X-ray and radio emissions one can determine the energetics of post-flare particles, the
processes of their accumulation and escaping from traps as well get information regarding the power of
fast particle sources. In this respect, of special importance and interest are observations of X-ray and
radio emission pulsations (periods 0.1- 102 s) probably associated with the modulation of high-frequency
radiations by a vibrating magnetic bottle (Brown and Hoyng, 1975; McLean et_al., 1971), Such oscilla~
tions can arise when energetic protons with B_ > 0.1, where B. = 4m_m_V“/B%, are injected into the
magnetic trap (Meerson et al., 1978). Note that type IV radio"pulsatfdn8 correlate with solar proton
satellite-bome observations (McLean et al., 1971).

(b) Proton acceleration in a flare. From observations it follows that energetic protons are ac-
celerated in the solar corona. Gubchenko and Zaitsev (1977) proposed a mechanism of injectionless
regular proton acceleration. This implies a repeated reflection of proton from the front of the perpen-
dicular collisionless shock wave playing a role of a moving "mirror." It is clear that in order to study
the particle acceleration due to shock waves it is necessary to know the shock wave front structure.
Only shock waves propagating in the solar atmosphere across the magnetic field have been studied in
sufficient detail up to now with due regard for the effect of the plasma turbulence on the front structure
(Zaitsev et al., 1979). It is not excluded however that effective mechanisms of proton acceleration
also exist in longitudinal and oblique shock waves. In this connection, observations of type II radio
bursts made together with the recording of protons on satellites and observations at optical and X-ray
wavelengths are useful. In addition, we recommend that observations of type IV bursts linked with
type II radio emission be performed. The point is that shock waves can supply energetic particles into
magnetic traps which are sources of type IV bursts, Using the intensity of type IV bursts or the manner
of pulsations of type IV emission, one can estimate the energy of particles supplied to the trap.

Sometimes, type III bursts display an intensity modulation with a characteristic period of about
1s (Santin, 1971), Zaitsev (1974) suggested that such bursts are generated by energetic protons rather
than electrons. Generation of plasma waves by a proton beam due to induced wave scattering on the
coronal plasma occurs periodically with T~ 1s, The electron beam does not lead to any modulation of
plasma wave energy density. Therefore, when performing investigations of type III bursts together with
optical and X-ray observations and with the recording of particles at the Earth's orbit, one will be able
to get information on which types of magnetic structure in the Sun's atmosphere are associated with
proton acceleration during the flare and which accelerate electrons only. Also, the bandwidth of the re-
ceiver at a frequency of 100 MHz should be not greater than 1 MHz,

1.6 Tosumup, the following list of observational data needed to solve the first problem can be
given:
a. Data on intensity and linear polarization of radio emission at the centimeter wave-
length for diagnostics of current sheets. In particular, at 2, 3 and 5 cm wavelengths.

b. Radio emission (intensity, polarization) at decimeter and meter wavelengths to get
additional information regarding spectra of particles and the plasma turbulence level
in a current sheet to establish the mechanisms of proton acceleration during flares.
Of special interest are observations of type II bursts and pulsations of type IV bursts
(radioheliograph, spectrograph).

c. Data on X-ray emission of flares to investigate spectra of energetic particles and the
possibility of solar atmosphere heating due to X-ray emission of the flare.

d. Spectrograms of H and He lines for an independent determination of the plasma wave
level using the Stark effect.

41


APPENDIX C
ENERGY TRANSPORT AND THERMALIZATION IN SOLAR FLARES

A. Gordon Emslie
. Harvard-Smithsonian Center for Astrophysics
60 Garden Street
Cambridge, Massachusetts 02138

The means by which energy, liberated by magnetic field annihilation in the
primary release site, is transported and subsequently thermalized throughout the
flare atmosphere provides us with a potentially important set of signatures of
the physical processes responsible for, and operating within, a solar flare.

A solar flare is characterized by its thermal emission in optical and EUV
wavelengths and by its non-thermal emission in microwave and X-rays [although the
X-ray burst may in fact be significantly thermal (see below)]. Any model which
contends to explain the solar flare phenomenon must be capable of reproducing the
characteristics (intensity, spectral distribution, and polarization) of this
composite radiation field.

One of the most obvious mechanisms for thermalizing energy in the lower
atmosphere is thermal conduction (SveStka 1973). In fact, it is apparent
(Machado and Emslie 1979) that this is the dominant energy redistribution
mechanism during the post-flash and decay phases of the flare, although the
significantly long timescales associated with conductive transport (Bessey and
Kuperus 1970; Craig and McClymont 1976) seem to effectively rule this out as a
viable heating mechanism during the impulsive phase of the event. However, these
timescales may be somewhat overestimated (Shapiro and Moore 1977; Spicer 1979),
and I shall return to this point later

Similar timescale arguments rule out convection as flash-phase transport
and thermalization mechanism (Craig and McClymont 1976), although study of the
full set of hydrodynamic equations (instead of the frequency assumed static
energy equation) is doubtless necessary for later stages in the flare and is a
very worthwhile pursuit. The original work in this area by Kostyuk and Pikel'ner
(1975) is somewhat lacking in detail (in particular, no density values are
presented; see Canfield et al. 1979a for more details), and further hydrodynamic
modeling, including the very difficult treatment of the behavior of a pressure
shock wave upon encountering the sudden density discontinuity at the transition
zone, is at present is progress (McClymont and Canfield 1979). Such studies will
hopefully enable us to construct reasonably accurate theoretical atmospheric
structures which can then be tested against empirical structures (Machado and
Linsky 1975; Machado, Emslie and Brown 1978; Lites and Cook 1979; Canfield et
al. 1979a), based on observation of spectral features

Another mechanism which undoubtedly plays an important role in the heating
of the lower atmosphere, particularly in the post-impulsive phase, is irradiation
by both soft X-ray and EUV flux. Modeling of soft X-ray heating was first
carried out by Somov (1975) and subsequently developed (a) in steady-state and
(b) including hydrodynamic terms by Henoux and Nakagawa (1977, 1978
respectively). These authors demonstrated that significant chromospheric heating
could be indeed produced by this process. Also Machado (1978) has demostrated
that observed Skylab SO55 Lg intensities over the flare area correspond very well
to those expected on the basis of a point source of X-ray emission with a
temperature of some 107K. However, Machado, Emslie, and Brown (1978) have
demonstrated that plausible soft X-ray sources cannot in fact produce enough
heating at photospheric levels to be compatible with observation (see below).
These authors suggested that, instead, EUV radiation might be responsible for the
heating and cited the very strong C IV 1549 line as a possible source;
however, Emslie and Machado (1979) have now shown that insufficient heating is in
fact produced by this process. Nevertheless, it is clear that a fully consistent
treatment of chromospheric modeling must take both radiation sources into account
in the local energy balance.

Of course, one of the mechanisms of thermalization which has received a
great deal of attention in the literature is the collisional degradation of a
non-thermal beamed electron flux. First proposed by Dubov (1963) and
subsequently developed in some detail by Brown (1972, 1973), Lin and Hudson
(1976), Emslie (1978), and others, this mechanism remains one of the most viable
mechanisms of transport and thermalization of energy in a solar flare. Transport

43


(7) The study of the general shape of the flux-versus- time profile
in a number of different wavelength bands, such as EUV and X-ray, does in fact
permit inference of the type of mechanism responsible for the flux
enhancement. Using theory due to Nakagawa (1979), the path of a disturbance
g(t) in the ($,$) phase-space (a dot denoting differentiation with respect to
time) gives information on the "degree of impulsiveness" (and hence,
presumably, of non-thermal/thermal character) of a burst profile. in order to
clarify the discussion somewhat, we now briefly reproduce some of the
essential theoretical results:

The rate of growth of a quantity @ (e.g., X-ray flux) is governed by
the action of a driving force a(t) and a restoring force b(t)$; viz.,

$= [a(t)-b(t)]¢

-a Further, since 9+ 0 at both t = 0 and t =o » we must have a(t)

t ,q > 1; b(t) ~t®& g 20. If we further relate the magnititudes of a(t)
and b(t) by the quantity m, we can formally integrate the linear ordinary
differential equation above, using the principle of superposition, to obtain:

(co glo n 8th
$= | J [ | Ca,ppm Pe) dat cy gt | exp (Fy) aban

m=0 B=0 =1

Each “elementary Function" o, B,m in the above expression has a unique
maximum turning point further of Figure 2.3, and therefore gives a unique
trajectory in the (6,6) plane of Figure 2.4. Thus comparison of the
trajectories of different features gives us insight into the closeness of
their associated values of a, 6B and m; i.e., into the possible relation of
their physical origin, and it is Proposed to treat SMY data ina variety of
wavelength intervals in this way.

1
\
/|

f(E)

46 58 67

E (keV)

Fig. 2.2: Use of Fe XXV resonance and Fe XXIV satellite lines to determine
features of the electron energy distribution. Satellite line strengths give
values of the electron distribution function F(E) at 4.649 and 5,815 keV, while

combination of these with the resonance line strength gives us | f(E)dE, as shown

6.7 kev
shaded. The temperature Tj, determined by the two satellite line intensities
and the temperature Tz, determined by the ratio of the satellite and resonance
line strengths, can be compared with the predictions of flare models.

6

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