Journalof
GEOPHYSICAL RESEARCH
Vonv•m
65
JUNE 1960
No.
6
,
Recent Progressin Solar Physics
WALTER ORR ROBERTSAND HAROLDZIRIN 1
High Altitude Observatory
University of Colorado
Boulder and Climax, Colorado
Abstract. A numberof significant
recentadvances
in solarphysics
arereviewed.
New kinds
of observations
madeduringthepresent
sunspot
cycle,whichbeganin 1954,haveemphasized
still furtherthe apparentcomplexity
of solarphenomena
and the needof nonequilibrium
theoretical
treatments.
Among
thenewdevelopments
discussed
aresolarcosmic
raysandhighenergyparticles,
flaresandprominences,
coronalcondensations,
solarmagnetic
fields,andsolar
radionoiseemission.
Theoretical
analyses
of the conditions
of the solaratmosphere
as a hot
plasma,
andas a gaseous
ensemble
exhibiting
grossdepartures
fromthermodynamic
equilibrium,are discussed.
New workon atomiccollisional
parameters
is alsoreviewed.The article
concludes
witha synopsis
of certainmajorunsolved
problems
of the solaratmosphere.
INTRODUCTION
ary 10-11, 1958, producedan aurora of extra-
by severe
Solarphysics
is growingat an explosive
rate. ordinaryproportions,accompanied
We try, in this review article, to summarize radio and telegraphicdisturbancesand earth
of unusual
magnitude.
August22, 1958,
advancesof the past few yearsthat seemto us currents
most significant.
produced a flare late in the afternoon in the
The stimulatingimpact of the IGY is immediatelyapparent.It is particularlyfortunate
that the IGY coincided
with an extraordinary
peakof solaractivity.On Christmasdayof 1957
the Wolf sunspotnumberreachedthe highest
valueyet recorded.
It is probablethat no higher
SovietUnion,alsoobserved
early in the day
overthe UnitedStates,that initiatedspectacular solarradionoiseoutbursts
at a widerange
of frequencies.
The radiosourceapparentlyex-
sunspotactivity level has occurredsinceGalileo
first beganobservations
in 1611.
The outstandingindividual events of solar
tendedhighinto the solaratmosphere
andoccupieda volumeof spaceroughlycomparable
with
the sun itself.
The outstandingactivity continuedinto the
InternationalGeophysical
Cooperation
1959.For
activity of recentyearshaveproducedterrestrial example,May 12-13 produceda brilliant aurora
effectswhich, thanks to the IGY, have been and strong proton radiation into the terrestrial
over
examinedwith unprecedented
detail. The Feb- polar cap.At the top of the atmosphere
Minnesota
the
proton
flux
reached
1000
times
ruary 23, 1956, cosmic-rayflare, a precursorof
values.The greatpeak
the IGY activity to follow,broughtthe largest the normalcosmic-ray
increasein cosmic-rayneutronactivity yet ob- of flare activityof July 13-16, 1959,alsoproducedstrong proton emissionand was accomserved at the surface of the earth. The Climax
panied
by the largestForbush-typedecrease
of
neutron monitors,for example,reacheda level
cosmic-ray
intensities
yet
detected.
Solar
acof about40 timesnormal.The night of Febru-
tivity in Augustand Novemberof 1959gen-
x Alfred P. Sloan Foundation Fellow.
erated powerful long-wave solar radio noise
1845
1646
ROBERTS
AND
ZIRIN
....
-•'.
..::
..
.......
ß....
.
./:.:•.:....
..-...;.::..•..-
..-: ....-•...:-.,•.-.
..;.:.. ..;::.::;;.
•.!•
Fig. 1. Enlargementof nuclearemulsionfilm exposedat 90,000feet over Minneapolisby Drs. E. P.
Ney and P.S. Freier, University of Minnesota, May 12, 1959, one day after a very large (3-1-) solar
flare. Normally ten such films show about one track. About 1000 times normal flux. Courtesy of Professor Ney.
emissions of unusual nature for which excellent
improved enormouslyin recent years; the work
with rockets and satellites has contributed imradiospectroscopic
observations
exist.
Intensivelyobservedeventslike these,coupled portantly to this development.Moreover, the
with the numerous rocket and satellite observaolder means of observation,such as coronationsof the ionosphere,the Van Allen belts,and graphs,spectroheliographs,
and magnetographs,
interplanetaryspace,have vasfiy enrichedthe have been substantially improved, and these
source materials for sound theoretical advances
more conventionaltechniqueshave been more
in astrogeophysics.
It is no surprisethat solar widely employed.
IGY solar activity patrols. The hundred-odd
physicsproceedsat an enormouslyaccelerated
pace.
solar stations of the IGY have cooperatedin
providing unprecedentedcoveragein solar pheOBSERVATIONAL I-IIGI•LIG tITS
nomena since the beginning of IGY. Some
Observationaltechniquesin solarphysicshave thirty flare patrol stations spread in all longi-
RECENT PROGRESS IN SOLAR PHYSICS
1647
3000•
29oo•_•
i•oo
I
•oo
I
•2000'•9oo -
1800
I
I0
!
II
!
12
I
13
I
14
I
15
I
16
I
17
I
I•
I
19
I
20
I
21
I
22
I
23
I
24
!
25
I
26
!
•7
!
•8
I
29
I
30
i
U.T. JULY 1959
Fig. 2. Intensity recordedby the University of Chicago neutron pile at High Altitude Observatory,
Climax, Colorado (elevation 11,300feet). Graph, courtesyof ProfessorJohn Simpson.
tudeshave kept the sun in virtually unbroken internationalcommitteeheadedby It. J. Smith,
Ha scrutiny since June 1957. These stations of the United States, and includingJ. RSsch,
have photographedthe sun on standardfilms to M. N. Gnevyshev,and R. Giovanelli,of France,
standard scale at intervals not exceeding3 Russia, and Australia, respectively,is overseeminutes. For the first time in history, a con- ing this task. This intensivecoveragewill enable
tinuous solar motion picture coveringthe 24- us to understandmore clearly the interrelation
hour-per-dayactivity of the sun is beingassem- of the varioussolar phenomena.
There has also been a considerable increase
bled for a 2-week period during the IGY. An
Fig. 3. SacramentoPeak Observatoryl:[a photograph of complex loop prominencesmade with 15inch coronagraph.Courtesy of Dr. R. B. I)unn.
1648
ROBERTS
AND
ZIRIN
in the number of nonpatrol-typeinstruments, other short-livedprominencesrepresentativeof
suchas coronagraphs,
large solarspectrographs, high solar activity show a much 'hotter' specand high-resolutionmagnetographs.
The work trum. For example, atoms and ions with low
with these instruments is crucial to understandionization potential, as Ti II, Fe I, and Fe II,
ing the physicsof the sun.
are weakenedas comparedwith lines of neutral
Researcheson prominences and the chro- helium, which has a high ionization potential.
mospherewith coronagraphs.The Lyot coro- Moreover, the ionized-Heline, 4686 A, is greatly
nagraph, because of its low light-scattering enhanced.The followingtypical line ratios may
properties,is an excellentinstrumentfor investi- be given: in cool prominences(Sr II 4077 A)/
gatingsolar-limbphenomena.
Coronagraphs
have (He I 4026 A) ~ 1; in hot prominences,the
evolvedfrom routine survey instruments,with ratio is about 1/7. In cool prominences,(He I
whichonly the red and greencoronallineswere 4713 A)/(He II 4686 A) ~ 5; in hot promiobserved,to instrumentsthat can covera broad nences,~•. The fact that the spectra divide
rangeof spectrumeither by taking rapid series sharply, with few intermediates,suggeststhat
of spectraat differentloci or by usingachro- there are two distinct temperaturesat which
matic first objectives. With such instruments the material is thermally stable. The existence
the relative intensities of a number of coronal
of discretetemperature'plateaus'was predicted
lines can be studied,as well as interestingfaint by Athay and Thomas [1956] and has been
lines in the spectra of prominences,many of substantiated by Zirker [1960]. The kinetic
which are no brighter than coronal lines and temperaturesof the hotter 'plateaus,'however,
are difficultto determineexperimentallybecause
thus are accessible
only to coronagraphs.
R. B. I)unn at Sacramento Peak has obtained
the Doppler profilesare confusedby considerThey must be
outstandingmonochromaticphotographswith a able motion in theseprominences.
high-resolution
coronagraph
of 15-inchaperture at least 30,000øK and perhaps as much as
in studiesof the fine structure of prominences 100,000øK.
and chromosphere.
Photometricdetailsof spicThe observedsimilarity of the spectrumof
to that
ules and the statisticsof occurrenceof spic- tightly loopedactive-regionprominences
ules have been examined with considerable deof flares is not surprising.Many workers,over
tail. The same instrument has .alSorevealed that
the years,have noted that flaresseenin projecthe internalfilamentation
of' largequiescenttion at the limb often have the form of loops
prominences
seemssharp downto the resolution [Waldmeier,1958; Zirin, 1959]. Of course,some
of the telescope(about 0.5 secondof arc). It is flaresappear as eruptingformsof outgoingmaremarkable,too, that both active and quiescent terial; the picture is not simple.
Athay has made successfulobservationsof
prominences,when viewed under high resolution, seemto showa markedtendencyto divide the chromospheric
spiculespectrumin Ha, H•,
into tiny threadlikefilaments,which are prob- D8 (He I), and the K line (Ca II) with the Cliably the result of confinementof prominence max coronagraph.The extreme breadth of the
gasesalong magneticlines of force. We still do spiculeline profilescontrastssharplywith the
not know how fine the filaments are; the ob- narrow profilesof quiescentprominences.
The
servationsprovide only an upper limit. Any line widthsmust be interpretedas beingdue to
theory of prominences,
to be considered
success- kinetic temperaturesof the order of 50,000øK.
ful, must explainthis fine structure.
A difficult problem for studentsof the solar
Observations(by Zirin and Tandberg-Hans- atmosphereis to explain the occurrenceof the
sen) with the coronagraph-spectrograph
at Cli- He II 4686 A line (excitationpotential48 volts)
max have shownthat prominencesfall into two in places where it ought not to be, namely
distinct spectroscopicclasses.The long-lived quiescent prominencesand the low chromofilaments,even when they erupt (disparitions sphere.To be sure, the line is very weak in
brusques), show the same spectrum as the theseplaces,but it oughtnot to appear at all.
chromosphere
at 1500 km height; they appear Explanationsof its appearanceas being due to
to have kinetic temperaturesof approximately ionization by. coronalradiation all seemto fail
10,000øK.Loop prominences,
flares,surges,and quantitatively.A clue is given by the fact that
RECENT PROGRESS IN SOLAR PHYSICS
1649
the line width of 4686 A in quiescentprominencesappearsto be twice as great as that of
the other spectrumlines. In such regionswe
probably do not have a homogeneous
kinetic
temperaturebut sharptemperaturegradients.
Coronal motion pictures. Great advances
have beenmadein the study of the solarcorona
in recent years. At the SacramentoPeak Observatoryroutinemovieshave beenmade of the
corona in the light of X5303 through a highquality birefringentfilter and coronagraph
system developedby R. B. Dunn. Two years of
observationshave revealedmany unsuspected
Coronalspectrummeasurement. From measurements of the intensities and profiles of
coronallines in specificcoronalregions,Billings
and Cooper [1957] have concluded,
however,
that average values of scale heights do correspondto the temperaturesfound from profile
measurements,
and so it is possibleto consider
of excitation rather than actual matter motion
Lyot discovered
a numberof new coronallines,
though its successwas marred by the tragic
death of Lyot. The 1954 eclipsewas clear for
the structural
features in the monochromatic
corona as perturbations of the hydrostatic
equilibriumcorona.They havemadesomeprogressin determiningthe nature of the perturbing
influence.By assumingthat the coronalregion
is in equilibriumbetweenpressure,gravitational,
features. On several occasions structures in the
and magneticforces,they have been able, with
corona, usually fairly low arches over active someuncertainties,to map the direction of the
regions,have beenseento disrupt violently with magnetic field, obtaining in one case a very
a whiplikemotion,whichhasled to the designa- plausiblemap that resemblesprominencestruction of sucheventsas 'whips.'
turesin the region.In the samecoronalregion,
Other active regionshave been found to de- the distributionof line widthssuggested
a very
velop seriesof 'flashingloops'which appear as definiteoscillatorypattern which might be inthe illumination,at successively
greaterheights, terpreted as a hydromagneticwave [Billings,
of curved loops overlying strongly active cen- 1959].
ters, particularly after strong flare activity.
Eclipseresearch. The numberof large and
These loops are shapedlike the trajectoriesof elaborate
eclipseexpeditions
in 1952,1954,1955,
the pronounced'iccp prominences'often found 1958, and 1959 has beennotable.The Khartoum
over extremely active regions.After a sudden eclipse of 1952 yielded valuable results. The
start of activity, they seemto be illuminated, ItAO-SacramentoPeak-Harvardexpeditionobwith no apparentmotionof material,at succes- tained splendidflash spectrawhich greatly exsively greater heightswith the passageof time. tendedourknowledge
of the solarchromosphere,
The coronal motions suggestchangingregions and the French-Egyptianexpeditionheadedby
of gas. In this respectthe coronalmotionsare
unlike the motionsof prominences.
Regular coronapatrols are now being carried
out in all regionsof pronouncedactivity. From
regionto region,great structuraldifferences
appear. Sometimestightly closedloops are seen,
resemblingprominencetrajectoriesfound above
such regions; sometimes'open' structuresare
observeddivergingoutward above active centers, alsoa characteristicof prominencetrajectories of certain active regions.
Since these loops and open structurespresumablymap out the magneticfield above the
spot groups, it may be that the corpuscular
emissionfrom the two typesdifferssignificantly,
and the chancesfor magneticdisturbancesfrom
one or the other may also differ. One thing is
certain: the old concept of the corona as a
homogeneous
mediumin hydrostaticequilibrium
is a grossoversimplification.
many teams, and goodobservationswere made.
It was particularly interesting becauseit occurredat the minimumof the solar cycle.The
1955 eclipse,which was favorablebecauseof a
very long period of totality, was cloudedout,
but 1958 saw a number of successful
expeditions to the South Pacific. Elaborate Sacramento
Peak-tiAO expeditionswere cloudedout in 1958
and 1959. Nevertheless,rocket investigations
madeby the Naval ResearchLaboratorygroup
at the 1958expedition
werehighlysuccessful,
as
were accurate coronal polarization studiesmade
from Africa by Ney and colleagues
at the October 1959eclipse.
Solar magnetic field measurement. Marked
progresshas been made in the study of solar
magneticfields.The Babcocks,at Mount WilsonObservatory,havedeveloped
automaticscan-
1650
ROBERTS AND
ning instruments(magnetographs)designedto
make longitudinal-component
magnetic-field-intensity measurementsat low field intensities
øVer the entire sun and to provide representations on a convenient scale. The technique
utilizes measurementof the wings of certain
absorptionlines in the solar spectrumthat are
sensitiveto the Zeeman effect. Maps are producedshowingfieldsas low as I gauss.One striking resulthasbeenthe occasional
appearanceof
low-latitudemagneticfieldsof only onepolarity,
covetinglarge regions.The same techniquecan
be applied to high-resolutionstudiesof strong
fields in active regionsby using lines of lower
magneticsensitivityand smallerscanningaper-
ZIRIN
flares, have found no important changesattributable to the flares.
The latest developmentin this field is the
work of Leighton [1959], at Mount Wilson Observatory, who takes spectroheliograms
in the
wing of a Zeeman-sensitive
line, splits the two
states of polarization,and superposes
positive
and negative pictures so that magnetic fields
are shownby deviation from a uniform gray
image. This method allows a picture of the
whole regionto be taken at onetime. Leighton
findsthat the resultantpictureslook very much
like K-line (Ca II) spectroheliograms--regions
of enhancedfield occurover the sameregionsas
calciumplages.This result is of great theoretitures.
cal importance.
The Babcocks'work has given valuableinforResearchwith high-resolutionspectrographs.
mation about the existenceof a weak general High-resolutionspectrographs
coupledwith large
solar field, but the situation is still somewhat magnificationsolar telescopes
have produceda
confused.A changeof the polarity of the gen- wealth of new information about the solar
eral field in recent years has been revealedin a photosphereand chromosphere.McMath and
remarkablestudyby Babcock[1959]. The south colleagues,
for example,have combinedthe Mcpolar field reversedin mid-1957, and the north Gregor tower telescopeof the McMath-Hulbert
polar field in November 1958. The sun'sgen- Observatorywith a new vacuumspectrograph
eral field is now parallelto that of the earth, the to obtain spectraof the solar disk that reveal
oppositeof the situation in 1953-1957.For over the presenceof a fine network of Doppler shifts
a year, during the highest activity, opposite in the Fraunhoferlinesoriginatingin the photorotation poles of the sun thus had the same sphere [McMath, Mohler, and Pierce, 1955].
magneticpolarity. The existenceof a large gen- Study of the line profiles,velocitybehavior,and
eral field (>5 gaussat the poles) hasbeen ruled magneticfield structure of this fine-scalephotoout, at least for the half-dozenyears thus far sphericstructure is under way at the McMathcarefully studied.
•
Itulbert Observatory, aided by a new highThe techniqueof the Babcockshas been ex- resolutionisophotometer[Mohler and Pierce,
tended by A. Severnyand his colleaguesin the 1957]. These disk studies,like the prominence
Soviet Union, who have usedhigh spatial reso- and chromospherework of Dunn, open up a
lution and have studied a number of active renew area of small-scalephenomena.
The large spectrographshave brought regions at the tower telescopeof the Crimean
AstrophysicalObservatory.They concludethat newed interest in the 'bombs' seen in I-Ia and
there is a tendency for flares to occur in steep first describedmany yearsagoby ]•11erman;the
magnetic field gradientswhere the field passes small chromosphericdetails, very similar to
through 'neutral points.' Similar active region Ellerman's bombs, which Severny [1957] has
field mapping has been carried out by J. W. named 'moustaches,'have been accordedgreat
Evans, of the SacramentoPeak Observatory, attention observationally.Thesemoustachesare
using different techniquesand obtainingresults very small bright points which produce exthat are somewhat different, though they are tremely wide (~10 A) emission in the I-Ia
still provisional.Work on the fine structure of line. The mostpuzzlingfeaturefoundby Severny
photosphericfieldshas alsobeendoneby YI. von is that the moustaches show the much narrower
Kliiber at Cambridge.Howard, Cragg, and Bab- YIa absorption line superimposedupon them,
cock [1959] at Mount Wilson Observatory,in so that they must originateat a level below that
somepreliminary studiesof photosphericmag- correspondingto the center of I-Ia. Flares, on
netic fields in active regionsbefore and after the other hand, show no such I-Ia absorption.
RECENT
PROGRESS IN SOLAR PHYSICS
1651
Observatory, suggestthat a 2.5 per cent in-
Scvernybelievesthat the flaresare a massof
many small moustaches
exceptthat perhapsin
flares the overlying layers have been blown
this time in the violet end of the spectrum
away.
where the observations were made. If it was at-
Flare spectrumobservations
have also been
made over an extraordinarilylarge wavelength
range,from 3900 to 7200A, by Je/]eries,Smith,
and Smith [1959]. They utilizedan echellespectrograph and made single exposuressimultaneously,revealinga large numberof emission
linesof a large disk flare of September18, 1957.
Their analysis reveals the complexity of the
flare phenomenon
and the variationsof excitation conditionswith depth in the solar atmosphere,and yet this complexphenomenon
occurs
often sad usually with exactlythe samestruc-
tributed to a changeof the temperature of the
solar surface, the correspondingincreasein the
'solar constant'wouldbe about 2 per cent. The
resultsmust still be viewed with caution, however, since they are in conflict with the rede-
ture.
Balloon solar astronomy. Another new way
to study the photosphere
hasbeenpioneeredby
Schwarzschild,who has flown a 12-inch telescopeto 80,000 feet altitude, in a balloon,with
controlsto point it accuratelyand focusit properly to obtain photographsof granulation.The
picturesso obtainedare significantlybetter than
those that have been obtained on the ground.
They reveal the remarkable structural details
of the granulations,which range from 300 to
1800 km in diameter,often appear to be polygonal, sad resemblethe structures found for
'nonstationary' convection in the laboratory
[Schwarzschild,1959]. It is clear from these
experiments that Schwarzschild'smethod is
limited principallyby telescopicresolution,and
results should improve as better apparatus is
flown. Figure 4 showsgranulesand a sunspot
group photographedby Schwarzschild
and col
leaguesin August 1959.
The solar constant. A great deal of interest
has been centeredrecentlyin the constancyof
the solar constant.A new 'planetary method'
has given improved results.Its advantagesare
that the sun'sradiationoutput is measuredby
comparisonof the reflectionfrom the surfaceof
a planet with the backgroundof starsof known
brightnessin the sagular vicinity of the planet
in the sky. Becausethe method is differential
and is comparedwith the stars, the absorption
and scatter in the terrestrial atmospheredrop
out of the calculations.
Observations taken between 1954 and 1958
by Johnsonand Iriarte [1959], of the Lowell
crease in the solar radiation
termination
has occurred over
of the 'old-method'
values of the
solar constant from Smithsonianmeasuresby
Sterne and Dieter [1958]. These calculations
suggestthat no variations as large as 0.3 per
cent have occurredover the period 1923to 1952,
sincethere is no fluctuationof so large a value
common to the various Smithsonian stations involved in the determination.
Solar radio astronomy. Radio astronomyhas
provided sa enormousvolume of new observational data. Particularly outstandinghas been
the superpositionof interferometric and swept
frequency methods by Wild, Sheridan, and
Trent [1959], of Sydney.These workers have
shown that velociti es hitherto inferred from the
frequency change of the radiation are actual
motionsof the radiatingsource.The application
of radio-frequencypolarimetershas also made
great strides, giving testimony to the role of
solar atmosphericmagnetic fields. Swept frequencysolar radio noise patrols, such as that
operatedby A. Maxwell at the RadioAstronomy
Station of Harvard College Observatory in
Texas, have made important contributionsto
the discoveryand systematicclassificationof
solar radio phenomena.
The classificationof radio noisebursts originally made by Wild has beenextendedas shown
in Table 1, adaptedfrom Denisse.
The mechanismsof the productionof solar
radio
noise are still
not
established.
Plasma
oscillationsseema likely sourceof type II and
type Ill bursts.A traveling disturbsaceis supposedto excite the local plasma frequencyof
the corona as it travels outward. Synchrotron
radiationappearsto be the only possiblesource
of the continuumradiations,but no adequate
theory has beenproposed.
On the other hand, much of the-slowlyvarying solar radiation has been adequately explainedby thermalradiation,eitherby the quiet
sun or by active regionsin the corona.The so-
1652
ROBERTS
AND
ZIRIN
•.' ....
f:::?
•:•c-::'.:':i::i::;'
...:•
Fig. 4. Photographof sunspotand solar granulation taken from Project Stratoscopeballoon at 80,000
feet, August 17, 1959. Courtesy of ProfessorM. Schwarzschild.
called microcondensations
at higher frequencies
may be due to hot loop prominencesforming
above the sunspots.
Radio astronomy has also enabled us to investigatethe outer coronaby studyingoccultations of the Crab nebula by the outer corona
of the sun at meter wavelength. Observations
have been made by many workers, including
Vitkevich, Itewish, Slee,Boischot,and Denisse.
The resultsare somewhatpuzzlingin detail, but
there is no doubt that the corona extends out to
at least 20 solar radii and sho•vsirregularities
capable of scattering and even focusingthe
Crab nebula radio radiation passingthrough it.
Flare-geomagneticstorm relationships.Much
interesthas centeredlately on the radio properties of flares producinggeomagneticeffects.
It. Dodson Prince and colleaguesat the McMath-Itulbert Observatoryand the sun-earth
relationships
group of the National Bureau of
Standardshave devoted particular attention to
this field [see, for example,Dodson and Hedeman, 1958.] If a solarflare hasmajor burst ac-
tivity at its earliestphases,
wellbeforethe radio
noiseactivity that generallyaccompanies
the
peakof flare,thereappearsto be a considerably
heightenedprobability of a sudden-commencement geomagnetic
disturbanceI to 3 dayslater.
Important work on these relationshas also
beendoneby Mustelandcolleagues
in the Soviet
RECENT
TABLE
Time
Duration
Burst
Type
Size
?
Tenths of
seconds
PROGRESS
1.
IN
Classification
of Radio
Polarization
Circular
SOLAR
PHYSICS
1653
Noise Bursts
Associated
Phenomena
Wavelengths
Bursts are at
meter
wave-
lengths, but
associated
centers often
emit weak
Associated with
'R centers' which are in
coronaand roughly overlie
active regions.
continua.
Minutes
II
?
Slow drift
bursts or
outbursts
Usually not
strongly
polarized
Irregular
III
Fast drifts
bursts
Seconds
IV
Continuum
Minutes
to hours
Up to
---105
km
Often
circularly
polarized
Minutes
•-•50,000
Variable
outbursts
or
storm-phase
bursts
Centimeters
to tens of
meters: sweep
in frequency
at •-•Ma/sa 2
Exhibit
harmonics
are
associatedwith flares; are
moving sourcesof ---500
km/secin corona;perhaps
associatedwith magnetic
storm producing solar
corpuscles.
Meters to tens
Associatedwith early
of meters sweep stagesof flares; are probably
in frequency at sourcesmoving at sub•100 Mc/s 2
stantial fraction of speed
of light; perhaps associated
with high-speedproton
Continuum
showers at earth.
Associated with
certain
from centimeters very strong flares, starts
to tens of
•15 min after flash phase;
meters
perhaps associatedwith
synchrontronemissionand
cosmic-raygeneration;
sourcessometimesdisplace
in atmosphereat speedsof
100-1000 km/sec; often
associatedwith type II
bursts.
V
Centrimetric
km
bursts
Broad-band
continua but
enhanced at
centimeter
Ascending sourcesof
speeds---3000 km/sec; often
follow type III bursts.
wavelengths
Thereare alsoothertypesof solarradioemissions:
radionoisestorms,slowlyvaryingemissions,
U bursts,
inverted bursts, etc.
Union and by Sinno and associatesin Japan
[Sinno and Hakura, 1958]. We must admit,
however,that it is still not possibleto predict
geomagneticand ionosphericstorm occurrences
with a large measureof reliability from solar
observations
alone,exceptfor certainstormsof
very large, radio-noisyflares.
Rocket solar research. Finally, we come to
the importantobservational
advancesproduced
by rocketresearch.The mostfundamentalones
are the extensionsof our knowledgeof the ultraviolet solarspectrumdownto the 50-A region.
Detailed and well-analyzedspectra have been
obtained down to about 100 A. Prominent are
the resonance lines of MG II and Si II and the
strong absorptiondoublet of A1 I leading to
autoionization.The observedspectrum to the
short-wavesideof 1500A is dominatedby Lyman a, which, accordingto the excellentobservationsof the Naval ResearchLaboratorygroup
under Tousey,appearsto be quite broad (~0.7
A) and to have a broad self-reversal within
which appearsa very sharp absorptionfeature
of still greater depth [Purcell and Tousey,
1654
ROBERTS AND ZIRIN
1960]. The resonancelines of He I and II (576
and 304 A) are also strong, and it has been
suggestedby Rense that the Lyman a-like helium line at 304 A may deliverto the terrestrial
atmosphereenergiesof the order of several
ergsper squarecentimeterper second,equaling
the energy of Lyman a.
In addition, the resonancelines of higher
stagesof ionization of abundant light elements
are seen,suchas 0 VI, Si III, and C III. These
lines presumablyoriginate in the upper solar
atmosphere.Becausethere still is atmospheric
absorptionpresent and, further, very few •
valuesare knownhere,little quantitativeanalysisis possible,but rapid advanceswill be m•e
alsoobtainedinterestingresultson X-ray emissionfrom flares.Despitethe difficultyof bring-
in the next two years.
coronalcondensationto be about 10' lower than
Violeti and Rense [1959], at the University
of Colorado,have obtainedrocket spectraof
the fluxesof 10-• erg/cm2/sec
observed.
Further,the relationof the flareto the corona
hasnot yet beenclearlydelineated.
At leastone
more than 100 solar lines between 1216 and
ing a rocketto altitudeat the precise
time of
the flare,they have maderocketobservations
duringseveralflares,the data showingstrong
enhancement
of X raysin the 2- to 10-A region
[Friedman,1959]Unfortunately,
no conclusive
data on Lymana wereobtained.The originof
theseX raysis probablynonthermal;
the higheststageof ionization
observed
in the corona
is
Ca XV, with 814ev, whichproduces
the yellow
line at 5694A; at 4 X 106degrees
thisproduces
recombination
X rays up to perhaps2000ev, or
downto 6 A in wavelength.However,calcula-
tion showsthe probableflux from a typical
83.9A. Thesespectraare still underanalysisfor
identificationof the many lines present.This
regionand the one borderingit are of great interest becausethey includeresonancelines of
atomsknown to be presentin the corona.In
principlewe shouldsoonbe able to observethe
coronadirectly againstthe disk.
'Of great interestare the monochromatic
photographsof the sunin Lyman a. A crudepicture was obtainedby Rense'sgroup in 1957,
and a very fine one by Tousey'sgroupin 1959
[Tousey,1959]. The picturelooksvery much
like a Ca II spectroheliogram,
exceptthat more
bright area is seen.It is probablyvery similar
alsoto a map of Fe XIV coronalline emission
and to a map of the regionsof the solarphotosphereof magneticfield greaterthan 5 gauss.
Thesefacts,if fully confirmedin future work,
largeflarewasaccompanied
by thecollapse
and
dissipation
of a previously
existing
coronal
condensation[Zirin, 1959]; it couldwell be that
the collapse
increases
the coronalradiation.In
are of obviousrelevanceto the theory of active
region energy balance.
versityof Iowahavemadea series
of remark-
the soft X-ray flux. Rocket measurementsat
production
is rare. Clearly,largenonthermal
effects
mustbe goingonin flares,whichaccelerate particlesthat ultimatelyimpingeon the
high terrestrialatmosphere.
Of course,it is
anotherwell-observedcase [Waldmeier,1958],
however,
little yellowline wasobserved
neara
huge flare.
The effectof the X-ray emission
in producing
SID's is not obvious.On several occasionsthe
Naval Research Laboratory group observed
largeX-ray fluxeswhennoD-layerabsorption
wasapparent,
or whenthe absorption
wasfar
pastitspeak.Theroleof Lymana in producing
the fadeout has not been fully investigated.
Satelliteobservations
are badly needed.
Observation
o/ solarparticleemission.The
Universityof Minnesota
groupheadedby Ney
and Winckler and Andersonof the State Uni-
able observations
of fast protons (100 Mev)
after somelargeflares.The producImportant rocket observationshave been occurring
particlesappearsto
madeby Friedman,Chubb,Kupperian,and co- tion of thesehigh-energy
workers at the Naval ResearchLaboratory on be relativelycommon,
whereas
largecosmic-ray
the October1958 eclipseshowedthat the flux
of 50-A X rays continuedduringtotality but
the Lyman-aflux disappeared.
This findingdi-
rectlyestablishes
a fact predicted
by mostcoro- much easier for some hydromagneticprocesses
particlesto the Mev rangethanto
nal theories,namely that the 50-A radiation to accelerate
range,but our knowledge
of
comesfrom the multimillion-degree-temperature the bey cosmic-ray
regions
of the coronawellabovethe solarlimb. the physicsof flaresdoesnot explainthe deThe Naval Research Laboratory group has
tailed mechanism for either.
RECENT PROGRESS IN SOLAR PHYSICS
1655
ANALYTICAL AND THEORETICALRESULTS
The main contributionto the integral over $,
will
comefrom differentregions,and $, will be
There is a remarkablearray of new observadifferent
in these.Investigations
of theseprobtional solar data. Important progresshas been
made toward its interpretation,particularly as lems have been very difficult,but they have
regardstheoriesof the nonequilibriumsolar at- already explaineda number of phenomena.A
mosphere,in hydromagneticinterpretationsof major stumblingblock is lack of information on
solar corpuscularemission,and in theoriesof transition probabilitiesand atomic crosssections
for the atomsof the atmosphere.
solar radio noisegeneration.
In the fieldof the chromosphere,
application
Theory o• a solar atmospherenot in thermoof nonthermodynamic
equilibriummethods(ofdynamicequilibrium. Someof the most importen referredto as 'non-LTE' methods)has led
tant advances have centered in methods that
to variousmodelsby differentworkers,all of
treat the solaratmosphereas a gasoutsidetherwhich
try to fit the eclipsedata. At this point
modynamicequilibrium. Since the solar atmosit is difficultto say whichfits the data best.It is
phere is transparent,it is not a hohlraum,and
certainthat the chromosphere
below1500km is
the radiationfield is specifiedby a temperature
cool(• 10,000øK),and the chromosphere
above
much different from the kinetic temperatureof
5000 km hot (•>30,000øK). As was mentioned
the medium. Thus the population of atomic
above,there is someevidencefor the existence
states cannot be calculated by means of the
of temperatureplateausat specifictemperature
Saha equation, but the equilibrium rates of
valuesin the solaratmosphere
(for example,in
many atomic processes
must be laboriouslycalthe work of Athay and Thomas[1956]) with
culated.This procedurewas introducedin 1937
clearly different temperatureregimesprevailby Menzel and others [1945] in a seriesof
ing at different positionsand heights in the
paperson physicalprocesses
in gaseous
nebulas.
atmosphere.But there is no clear agreement
This approachwas not applied to the sun until
aboutthe validity of the differenttwo-tempera-
somewhatlater by a number of workers,including Thomas, Athay, Giovanelli, and Jefferies.
Althoughit seemsclear that this is the appropriate way to attack the problem,extensiveresearchhas revealed the magnitude of the difficultiesinvolved. For example,early treatments
of the formation of absorption and emission
lines consideredthe lines as coming essentially
from one height in the solar atmosphereand
assumedthat their structure couldbe specified
by one line-broadeningformula. Now attention
is directedto the solutionof the equation
ture or even three-temperaturemodelsthat have
been proposed.
Studieso• coronalphysics. In the coronathe
situationis somewhat
simpler,as all linesexcept
the resonancelines, which meansall lines that
fall in the now-observable
regionof the spectrum, are optically thin, making interpretation
easier.Atomic crosssections,which are not well
known, representa major difficulty.
Recent work has resolvedthe discrepancies
betweentemperaturesdeducedfrom Doppler
broadeningand temperaturesdeducedfrom
ionization theory based on observationof the
line intensitiescorresponding
to differentstages
of ionizationof iron. The former methodgave
where I is the emergentintensity at frequency 2,000,000øK;the latter, 700,000øK.Further, the
•, ß is the 'opticaldepth,' and S the so-called ionization theory predicted that only a few
'sourcefunction,' which gives the amount of stagesof ionizationshouldbe seenat once;norenergycontributedby each elementd•. If we mally all the stagesfrom Fe X to Fe XV can be
The ionizationcross
had thermodynamic
equilibrium,S• wouldequal detectedsimultaneously.
sections
that
led
to
discrepant
ionization-theory
B•, the Planck function.Sincewe generallydo
not, S, may have any value less than B•. In temperatureswere classicalonesgiven by J. J.
particular,it may vary considerably
with wave- Thomsonin 1909.A recentquantum-mechanical
lengthacrossa singlespectralline. In the center calculation by Schwartz and Zirin [1959]
of an absorptionline we do not 'see'nearly so showed that the cross sections are less than onedeeplyinto the sun'satmosphere
asin the wings. tenth thosegiven by the Thomsonformula. The
ROBERTS AND
1656
ZIRIN
reasonis that the incident electronis highly
celerated by the large charge of Fe XIV a•d
spends little time near the nucleus, and so
matrix elements are very small indeed. The
temperatureobtainedfrom the ionizationtheory
is now 1,500,000
ø, in agreementwith Doppler
measurements.And a broad range of ionization
is now possible,so that the different lines
pearing simultaneouslycan be explained.More
metric, no-magnetic-fieldmodel of the corona
heated by conduction.He obtaineda coronaof
very great extent, with temperature of around
2,000,000ø K at the earth. He arguedthat the
existenceof such a 'supercorona'would account
for thermal gradientsin the F layer of the ionosphere.The most difficultproblemin this model
is the heat loss through radiation by heavy
atoms in forbiddenlines and through collisions
refined work on these coronal cross sections is
with interplanetarydust. It is rather probable,
needed,however.
however,when all ionizingmechanisms
are conA milestone in solar research was the definite
sidered, that the interplanetary medium is a
identificationof the yellow coronalline, X5694, regionin which hydrogenis completelyionized,
as the 8p• ._> 8Po trsasition in Ca XV. This
and at a temperature of at least 10,000øK,
identificationhad originally been suggestedby though it is not certain that conductionin the
Edl•n in 1942, but doubt had been cast on it
coronaplays a major role in this heat supply.
for severalreasons:(1) its ionization potential
Convection in the solar atmosphere. The
of 814 ev was extremely high, whereas some problem of convectionin the solar atmosphere,
argumentssuggestedthat the regionsemitting long the object of intensiveresearch,has not
it should be cooler than the adjacent regions; comevery much nearer solutionin recentyears
(2) Waldmeierhad discovered
anotherline near s,nd remainsone of the key problems.No theory
by at 5446 A which had similar characteristics of the heat transport in the unstable'hydrogen
but was fainter by a factor of 6 than the yellow convectivezone'just belowthe surfacehasbeen
line, rather than the 50 to 100per centintensity advancedexcept one basedon the Pra•dtl mixpredictedfor the •P• --• •P• transitionif it was ing length. That theory is very sensitiveto the
Ca XV; and (3) the theoreticia•s could not mixing length, which happensto be unknown,
predict wavelengthsthat could be considered and it also faces other objections.Without a
consistent with the observed ones. These diffibetter theory it is impossibleto make a satisfacculties were resolvedby several developments. tory model of the solar interior or reasonable
Pecker, Billings, and Roberts [1954] showed theories for heating of the chromosphereand
that
the 5446-A
line coincided with
a Fraun-
hofer line in the sky background;whenthis was
allowed for, the intensitiesof the yellow lines
cameout appropriatelyclosetogether.Further,
they showedthat comparisonof the profile of
the yellow line in an active regionwith that of
the red and green lines of Fe indicated an element of atomic weight 40. Finally, Rohrlich
[1956] and Layzer [1954] recalculatedthe predictedwavelength,includingconfigurationinteraction,and got wavelengthsfor the two Ca XV
lines satisfactorilycloserto the observedvalues.
Recent work at the High Altitude Observatory
showsthat the positionsand times of appearancesof the yellow lines coincidewith thoseof
the Ca XIII line 4086, which is well identified,
and therefore
the identification
is confirmed in
corona. It is further
difficult to understand
the
connection between photosphericgranulation
and subsurfaceconvection,particularly in view
of the recent observationsof granulation,or to
see how we can explain the coolingof the sunspotsby suppression
of convectionby magnetic
fields, as some authors have suggested,when
regionsof enhancedcoronaand plage activity
coincidewith regionsof strengthenedmagnetic
fields.
Cosmicrays from the sun. Solar magnetic
fields and their relation to observed nonthermal
phenomenaare of great interest,but the efforts
at explanationare still speculative.
A great deal
of work hasbeendoneonpropagation
of auroral
particles and cosmicrays. The February 23,
1956, flare and associatedcosmic-rayincresse
was peculiarin that, althoughthe initial particles came directly from the sun,later on the
estingsuggestion
was recently made by Chap- particle flux was isotropic.Parker has postuman [1957], who calculateda sphericallysym- lated from this that the solar magneticfield is
another way.
The Chapman conductioncorona. An inter-
RECENT
PROGRESS IN SOLAR PHYSICS
stretchedout radially by corpuscularemission
but becomes
irregularbeyondthe earth. The irregularityreflectsthe cosmicrays backand producesan isotropicflux. The problemof where
the magneticfield of a rotating body stopsrotating with it is important. Presumably this
point is the same as that at which the atmosphere also no longer sharesthe rotation.
Thermal processesin prominences. Some
work has been done on the possibility that
prominencesare formed by condensationfrom
the solarcoronaunder the influenceof the magnetic field; also, many workers have given attention to problemsof the heating and cooling
of flare material
and to maintenance
of the
coronaagainst'drainage'by prominencesif the
condensationprocessactually goeson. Coronal
material is unstableagainstcooling,so that, as
was first pointed out by Kiepenheuer,the more
it cools,the more it radiates.But problemsarise
becauseit radiates so slowly that prominences
would take very long to form. Investigations
have been made by L'•t and Zirin [1960] and
by Kleczek [1957] on possiblemeansof increasing the radiation and the rate of prominence
formation.The answerprobablyliesin compression by magneticfields.It is a very interesting
fact that the yellow line often appears high
above the sun near the point where the loops
are forming,and we have severalrather definite
observationsshowingthat the yellow line first
appears as the loop starts to condense.This
phenomenonis also accompaniedby increased
continuumemission(high density). It therefore
looksas thougha compression
startsthe process
going and incidentally lets us seethe yellow line
either becauseof the temperature rise or the
density increaseor both.
PRINCIrAL UNSOLVEt) PROBLEMS
1657
lence, a better knowledge of hydromagnetics
and of plasma physics.
First in importanceis the problemof observing and describingthe atmosphericmagnetic
fieldsof the sun. These fieldsundoubtedlycontrol prominenceand flare formation and many
other solar processes.Certainly they are also
responsiblefor the guidance,acceleration,and
general control of the solar particle emission,
includingcosmicrays, and the particlesthat replenishthe Van Allen belts.It has recentlybeen
discoveredthat after many solar flares strong
proton emissionis detectedin the high terrestrial atmospherewithin the polar cap. These
particles travel with perhapshalf the velocity
of light from sun to earth. The nature of their
ejectionfrom the sun and guidanceby magnetic
fieldsin the solar atmosphereis crucial.It seems
probable that studies of prominence motion
fields, coronal structure, and polarization of
radio noise are the most promising modes of
attack on this problem,althoughthere are other
possibilities.
The secondimportant problemis study of the
distributionof photosphericmagneticfieldsand
the relationship of such fields to solar flares.
Small fractional chargesin the energy contained
in the magneticfield of an active region could
easilyexplain the entire output of a solar flare,
includingits radiational and corpuscularemission.Consequentlyobservationaland theoretical
studiesin this area are of highestimportance.
A great deal of further work is necessaryto
explainin detail the accelerationof cosmicrays,
the generationof solar7 rays, and the processes
involved in the proposedsynchrotronradiation
in the sun. Extraordinary incidents like the
August 22, 1958, and August 22-24, 1959, solar
radio noise outbursts,with their large size and
strong continuousemission,deserve particular
attention. A theory of the mechanism,to be
satisfactory, must explain the enormous increasesof continuousradio noise emission,the
polarization of the radio noise, and the large
observed sizes and heights of the radio-noise
sources.It also seemsclear that 7 rays are
sometimesgeneratedin such events, as in the
August 22, 1958, incident, and that high-speed
protons are frequently ejected.
The unsolvedproblemscould be a long list,
but we shall confineourselvesto those having
the greatestinterest and the best prospectof
solution.For example,a very important problem is the cause of the solar cycle, but most
people think that progress waits on better
knowledgeof the origin of sunspotsthemselves.
Further, there is a host of problemsin allied
fields whosesolutionwill greatly advancesolar
physics,for example,better atomic crosssecThere still remainsthe resolutionof the questions, a soundand applicabletheory of turbu- tion of the relative roles played by coronalX
1658
ROBERTS
AND
ZIRIN
rays and by ultraviolet radiationin producing sorptionlines [Mohler and Dodson,1958] (also
the radio fadeoutsassociatedwith large flares. unpublished communication)deserve further
Straightforward observationsfrom an artificial attention.
satellite seem to be called for. It was heartAlso of high priority is the problemof the
breakingthat the July 16, 1959,satellitefiring flare-corona
relationship.
It is knownthat enwasunsuccessful,
becauseit containedthe neces- hancementsof the corona,especiallyof the
sary experimentsto solvethis questionand was yellow line, are associated
with flares,but it is
followedin a few hoursby an extremelylarge not definitelyknown whether the enhancement
and geoactivesolarflare.Consequently,
an early precedesthe flare, as seemsmore probable,or
repeat, and success,
are highly to be desired.
followsit. To answerthis questionwill necessiAnotherproblemrelatedto solaratmospheric tate long vigils at the coronagraph,waiting for
magneticfieldsis the closerelationof coronal, lucky circumstances.We still know little about
prominence,and plageactivity. The fact that a the fine structure and the vertical structure of
magnetogramresemblesa K-line spectrohelio- flares.All this is researchthat requirespatient
gram, whichin turn resembles
a coronalgreen- work on the ground with large-scalecoronaline map as well asa Lyman-a spectroheliogram, graphs capableof giving us large imageswith
is intriguing.Onewouldthink that the magnetic very low scatteredlight.
field comesfirst, sinceit is hard to picture a
We have only touchedon a few of the many
coronalcondensation
producinga magneticfield. problems of today. Although their number
Presumablythe explanationwill bring with it seemsto be increasingrapidly, we should be
the answer to the whole questionof coronal comforted by the thought that we have made
heating,a highly importantunsolvedproblem great advances in recent years, have solved
of solar physics.
many difficult problems,and can see pathways
The understandingof temperature plateaus of progressfor many other solarenigmaswhich
and the multitemperaturechromospheric
struc- today seemfar lessmysteriousand disconnected
ture shouldbe a key to our solutionof several than they did 10 to 20 years ago. This is an
problemsof the solar atmosphere;it is in- optimisticomen for the comingdecade.
triguing that a cloud of gas suspendedhigh
abovethe atmosphere,
as in a quiescentpromiREFERENCES
nence,showsmuch the same spectrumas the
low chromosphere.
It is further interestingthat Athay, 1•. G., and R. •. Thomas, Temperature
gradientsin the chromosphere,
Astrophys.J., 123,
when a huge surgeemergesfrom the chromo299, 1956.
sphereit already has the hot spectrumcorre- Babcock, H. D., The sun's polar magnetic field,
Astrophys. J., 130, 364, 1959.
spondingto >30,000øK. Progressin understanding thesephenomenashouldbe highly reward- Billings, D. E., Velocity fields in a coronal region
ing.
with a possible hydromagnetic interpretation,
Astrophys. J., 130, 215, 1959.
Billings, D. E., and R. H. Cooper, Heigh5 gradient
The equatorialaccelerationof the sun'srotation is a problemthat has thus far defiedsoluof the emissioncorona,Z. Astrophys.,43, 218,
1957.
tion, and it deservesmore specificattention
than it is now getting.As Plaskett [1959] has Chapman, S., Notes on the solar corona and the
terrestrial ionosphere,Smithsonian Contribs. to
suggested,
it may involvea solarthermalgradiAstrophys., • (1), 1, 1957.
ent 'wind,' but further observationaland theo- Dodson, H. W., and E. 1•. Hedeman, Geomagnetic
retical studies are needed for confirmation.
disturbances
associated
with
solar flares with
Beforewe canexpectto answerthe problems major premaximum bursts a5 radio frequencies
200 Mc/s, J. Geophys. Research,63, 77, 1958.
of particle productionin flares we need much
Friedman, H., Rocket observationsof the ionobetter knowledgeof the physicalconditionsof
sphere,Proc. IRE, 47, 272, 1959.
the flaresthemselves.The appearanceof flares Howard, R., T. Cragg, and H. W. Babcock, Magobserved in the center of the Ha line and in the
netic field associatedwith a great solar flare,
Nature, 184, 351, 1959.
wings is still a virtually unexploredfield. Yet
Jefferies,J. T., E. v. P. Smith, and tI. J. Smith,
unnamedsolar featuresthat showup on specThe flare of September18, 1957,Astrophys.J.,
troheliograms
takenin the wingsof strongab-
129, 146, 1959.
RECENT
PROGRESS
Johnson,/5. L., and B. Iriarte, The sun as a variable star, Lowell ObservatoryBull. 96, 1959.
Kleczek, J., Condensationof solar prominences,1,
Equation of isobariccooling,Bull. Astron. Inst.
Czech., 8 (5), 120, 1957.
Layzer, D., Energy levels in Ca XV, Monthly
Notices Roy. Astron. Soc., 114, 692, 1954.
Leighton, R. B., Observationsof solar magnetic
fields in plage regions, Astrophys. J., 130, 366,
1959.
Liist, R., and H. Zirin, Condensationof promi-
IN SOLAR PHYSICS
1659
Schwarzschild,M., Photographsof the solar granulation taken from the stratosphere, Astrophys.
J., 180, 345, 1959.
Severny, A. B., Investigation of the fine structure
of emissionin active regions on the sun," Izvest.
Krimskoi Astrophys. Obs., 17, 129, 1957.
Sinno,K., and Y. Hakura, On the relation of solar
eruptions to geomagnetic and ionospheric disturbances, 1, On the power spectrum of solar
radio outbursts,Rept. Ionosphere Research Japan, 12, 285, 1958.
Sterne, T. E., and N. Dieter, The constancyof the
1960.
solar constant, Smithsonian Contribs. to Astrophys., 3 (3), 9, 1958.
McMath, R. R., O. C. Mohler, and A. K. Pierce,
Doppler shifts in solar granules,Astrophys.J., Tousey, R., Solar photographs in Lyman-a light,
Sky and Telescope,18, 440, 1959. (Highlights of
122, 565, 1955.
papers presented at 102nd Meeting of Am.
Menzel, D. H., and others,Physicalprocesses
in
gaseousnebulae,Astrophys.J., 85 (4), 193, 1945.
Astron. Soc. by R. Tousey, J. D. Purcell, D. M.
M ohler, O. C., and/5. W. Dodson,Observationof
Packer, and W. R./sunter.)
solar 'Points,' Astron. J., 63, 309, 1958.
Violett, T., and W. A. Rense, Solar emissionlines
M ohler,O. C., and A. K. Pierce,A high-resolution in the extreme ultraviolet, Astrophys. J., 180,
isophotometer,Astrophys.J., 125, 285, 1957.
954, 1959.
Pecker, C., D. E. Billings, and W. O. Roberts, Waldmeier, M., Die weisseSonnen-Eruptionvom
Identification of the yellow coronal line, Astro23. M•rz 1958, Z. Astrophys., 46, 92, 1958.
phys. J., 120, 509, 1954.
Wild, J.P., K. V. Sheridan, and G./5. Trent, The
nences from the Corona, Z. Astrophys., 49, 8,
Plaskett,It./5., Motions in the sun at the photosphericLevel, 8, Solar rotation and photospheric
circulation,Monthly NoticesRoy. Astron.Soc.,
119, 197, 1959.
Purcell, J. D., and R. Tousey,Profile of solar
Lyman-a, Astron. J., 1960.
Rohrlich,F., Classification
of emissionlines5694
and 5446 in the solar corona,Astrophys. J., 123,
521,1956.
Schwartz,S. B., and /5. Zirin, Collisionalionization cross-section for Fe XIV
transverse
motions
of the sources of solar radio
bursts, Paris Symposium on Radio Astronomy,
edited by R. Bracewell, Stanford University
Press, paper 32, p. 176, 1959.
Zirin, I-I., Physical conditionsin limb flares and
active prominences,2, A remarkable limb flare,
December 18, 1956, Astrophys. J., 129, 414, 1959.
Zirker, J., Helium equilibriumof the solarchromosphere,Astron. J., 65, 1960,in press.
in the solar corona
and the coronal electron temperature, Astrophys.
J., 130, 384, 1959.
(Manuscript received March 10, 1960;
revised March 25, 1960.)