402 MONTHLY WEATHER REVIEW.
In measuring visibility durin the 24-hour day the fol-
First. Angle of light, rays illimiinating the object or
gcond. Condition of sky as to r.loudiness, t.wilight
At ni httime visibilit may be measured by observing
having varyin de ees of traiisparency; or a sing e light could be usef, t8e observer peering through smoked olmsea at the point of observation. Es erhnents are now
gein made to develop a standard visihity measurer in
ing opaqueness.
B
lowing factois should be applie8, owing to the variations several E ghts hi a row, t i e lights to be screened hy lasses
whit % a screened light is viewed through glasses of differ-
in seeing mentioned in the preceding paragraph:
tar et.
starlight, and moonlight.
THE AURORA OF MARCH 7-8, 1918.
By HEHBEHJ* LYMAN and CHARIXS F. BROOKS,
I Untctk Weather Bureau, Washington, July 3, 1819.1
SYNoPsIs.-After discussing auroral phenomeua in general. in- cluding types. latitude variataon, periodicity. height, and cause. tMe article describes the principal features of the remarkably biilliant aurora of March 7-8. 1918, and prewenta a large number or detailed accounta by observers in the rnited Ltates. 4
A chart of the United States shows places from whirh reports of the aurora were received. To facilitate mtercomparison the dwcriptioiis have been grouped and discussed by latitude belts of 24 esrh, lor as would be expected, the display waR pretty much the same at the sdnw latitudes. The aurora became risible at dusk. March 7 and attained ita greatmt brilliance, generally, at 9% p. m. (90th Meri$ian Time). Since DO
display ia homogeneous, however, there are vanations in the times ot
greatest brilliance and in the appeaiance of details of !he display. although there is general unanlmity roncerning the times. colorp. Iirilliance, and aspect, among wtidely wattered observers. The desc~ip- tions of the positions of arc-hea, particularly of B prominFnt red nne.
make it obvious that the actual location of the nurrm 1s the factor
which determines ita aspect, and the distance to which it ran he seen. while the lack of streamers in the dbplay at most southern point* show that the cleamesa of th? air limits its visibility. I t ~eems that the m y t i c disttubance accompmying this a i i r m
wm a r y t i t i o n , after t ree 27-day intervals, of the large magnetic. storm of ecember le-l?, 1917. There was a considerable dbturbanw on January 12, and a m o r one a t the end of the next 37-day interra1 in February. Aurorae on April 4, 5, and 6, maiked the end of. this strung seriea, which waa probably caused by 5 surcepslve preseutlr- tions of an active area on the Bun.
INTRODUCTION.
Aurora,s have always ascited witlesprtwi interest not alone because of their wonderful formation md lwnutiful coloring but also because of their very ni -teriousness.
nessed siniultaneously over practically the entire North American continent and in Europe. In the IJnitetl States auroras are not often seen s0ut.h of latitude 40,
yet in this instance the henomenon was observed as far
 a brilliant illumination of the northern sky. Aside
from the unusual geographical extent of this auroral (lis- play, tlie most noteworthy features were its brilliance, coloring, nnd extent over the sky. Severd ol>servers said the light from the aurora equalled the light from the full moon. The coloring was magnificient. Various shades of red were generally observed (ts well as areens and the usual yellows and white. Reports and notes on the aurora of bfarch 7-8, 1918, received from a lar e number of regular and B few co- operative Weather %ureau stations, where the sky was not obscured (see fi . l), also a few re orts from observers
tabulated in roup by latitudes. It is hopefthat these descriptions may be usable for my studies of the geo rnphic synchronism of the t,imes of great brilliance, anf for comparisons of the t .h e s of occurrence and appearance of pmticular features as seen
A sii erb exam le of the aurora was that w Y iich ocmrretl
i)n life night o P March 7-8, 1915. This tiurora was wit-
south as Miami, Fla., w P iere the observer described it as
in other parts of t 5l e world, have % een assembled and
tit- different places. Thtm is such a ma& of detail, ho\wver, that i t may be best to precede tlie loc.ti1 descrip- tive matter wit.h (1) n gmeral tliscussion of the aurorti
polaris as t.0 its essential features and robable CILUSB, (2)
the inferred make-up of great displays, and (3)
a snnunsry of the principal features of the aurora of
?ularr.h 7-8: 1!)1~.
AU itoai POLARIS.
K>. \\-. J . III.YIHHKYSi.
The aurora 1)olaria is a \vell-kiicwn hui iruperIectly uiidelutond lumi- nous phenomenon of the upper at.mosphcre. * * Typm.-bVhile no two aiiroras we exaptly alike, si<> era1 types haw lieair rerognixed. such rn arcs. bands, rays, curMua or draperiea. corniias.
luniinoris patches. and diffuse g h v ~ The WCH noriiid to the magnetic meridiau, ohen. but not always. reach the boiizoii. Their under edge
ip rather sharply defined. NO that lq7 contrast the adjacent portinii of t h
s k r appais exce tionally dark. The r a p . sometimes extending up- wvdrcl from an mr\i at other times isolated. are parallel to the linw of
uiiiguetic forre. Many auroiaa are quiescent. others exceedingly diaiigmble?. Hitting frnm side to side like wandering Perurhlights, aiitl
i i i snnw ram ewn waving like giant t n n g u ~ of flame. f,ditiid<~ iwiu/ioii. -The aurora of the Northern Hcllliaphere wcum
nirmt. frcqusutly. rthJUt I I J O per year, at. the latitudes MIo (ovex the Sortti Atlantic. and :Yalrth Imerica) to TO (off the c w t of Bibern,.
l i s f r w p m -y ap WSIH I O be l m within this boundary, while with iic.crewe nf lat.ii iidv it. fall^ off sn rapidly that even in southern Europ
it icr il rare phtwmiptioii. .it t,he Rame 1atit.ude it is distinrtly more frequent i n Bnrth .\mciica thau in either Eurnpe or Asia. The distrilJiriiciii c i f auroras in t.he Pout.heri1 Hemisphere is not. sti
\vd1 known. 1iiIt i i appears to h? siinilar. in general. to that of the nurt,hern. fwiodir.ily.---lt ia aell established that on the averagv ~urom are
illore iiuruerous during yeam of Nunspot ruaxinia than duriiig yeam nf
spit minima. They 111~o appear to be more niinierous before midnight. than aft.er. RehtionR of frequency t.o phme of the nirion, scaron. et. c..
huvr also been dieruMed. but wit.h no coiic*lu&ive remilti.
Red, yellon-, +iid greet1 are a~Ro rnninion auroral colors. sonit! streaks and kiaIldR are reddidi through t.heir lower (northerni portion, then yellowish. aiitl finally greenish t.hrough t.he higher portions. Much of t.he light, i n due 1.0
nitrngeu bands, but the mource of t,he most rominctnt line of the auroral spect.rum. A .557p ( reen1, is not known. ft has ohen been at.tributad to krypton. Init otier couspicunw krypt,on lilies are absent: beside* krypt.oii is too heavy to exist at auroral heights in siiiiiricnt. ahundanc*e to produce a spectrum of such brilliance. There is good evidence that this green light.. the light t.hnt prw1ilr.w t.he auroral line. is always present in t.he sky, though whrt.her wholly of auroral origin, or due in part t.0 bombardment by meteoric dust.: o r
t.o some other cause. is not known. Mri!~ht.-The problem of t.he height of aurorw has often been inveati- gat.ed. but only recently solved. By t&multaneously )hotographiug t.he same aurora from two stations against a comninn &diground ol stam. and measuring the parallax obtained, St3rnier2 and Vegard and
IGoguew have seruwd manv excellent height measurements. The opper linuta of t.he auroral iight vary from about 100 kilomet.ers t i l
over 300 kilometers; and the lower limits from perhap 85 kilometers tu
170 kilometers, d t ,h two well-defined maxima. one a t I W kilometem, rhe other at 106 kilometers.
Ihk~.--Many aurora are wartically whit.e.
I Quoled from Pbydcs of the Air Journ. Franklin Imt.. vrt., 1918, pp.. 191-48;.
2 Terr. Maagnef. and Amos. Elm., i916.21: p. 16:.
J Tern. Magnet. and A tmm. Elec.. 1916,21: p. 16%
,JUNE, 1919. MONTHLY WEATHER REVIEW. 403
f'nrcse.--The fact that brilliant. shifting auium we accompanied by magnetic storms renders it practically certain that they, and presum- ably therefore all auroras, are due to electric discharges: aiid thc furthvr fact that they vary in frequency with the sunspot period indicates that this ciineiit either comes from or is induced by the sun. For some time it waa thought probable that aurom are caused by negative particlw shot off from the BUII and entra ped by the magnetic field of the earth. (Jn the other hand. VegardI ku given strong arguments in favor of the CI particle which is prwcitively eharged, and Wrmer' hup
found at least one case that required the p i t i v e charge to awount for the observed magnetir dkturbance. The evidence, then, while not conclusive, indicates that aumma
:ue due to strearnfl of o particlee in the upper atmosphere ahot off by radioactive substances in the sun.*
the sun. W e may assume that at such times great jets or streams of electrified mutter (electrons perhaps), akin to cathode rays in a vacuum, are projected with high velocities outward. and that occaaioiiolly some of these jets cross the earth's orbit or p m near to it, Leiiig when leaving thesuii inu general radial direction, bent backward. for obviousremom That electrified miitter in a vacuum does move in jets or streams for indefinite distances is a fundamental fact. Afoving charges of the same irame in paths. dtraight or curved. act like parallel currents and attract one another. the more as their velocity is greater, iuiitil the static repulsion of wrh like charges sets a limit to further approach. It is possible that the space uroiiud the siin may be crossed by many such jetn or stream of electrified matter moving at very high velocities into the vdcuous space. The coroiial atreamerR may be the visible composite effccta of the projected jet#. Such electrified jets may act