MAGNETIC STORM OF MARCH 22-23, 1920 59

Horizontal Intensity.--The initial motion was an abrupt in- crease followed within two or three minutes by a sharp reversal. At Sitka, the value fell from 15648• at 10 h 08- to 15088• at 10 h 34- and came back to 15675-r at 11 h 22-. This feature had no counter-

part at the other observatories. The first period of great activity was marked by a sweeping decrease in/-/at all the observatories, from which there was a partial recovery during the lull, followed by a still further decrease during the second period of great activity. At Cheltenham, this decrease was so great as to throw the magnet of the Eschenhagen variometer out of equilibrium shortly before 3 • on the 23d and the instrument ceased to record. Rapid oscilla- tions of wide range marked this period, the value at Sitka dropping from 16122-• at 3 • 44- to 14744q, at 4 h 20 m.

Vertical Intensity.•At Porto Rico there was a slight decrease at the beginning followed by an increase one minute later; at Cheltenham there was an increase followed by a decrease, but at the other observatories this initial "kick" could not be distinguished. At Cheltenham during the first period of great activity Z (vertical intensity) was below normal for about 3 hours and then above for 2 hours. During the second period it was high at first and then low. At Sitka Z was alternately low and high during the first period, above normal during the lull and low for the greater part of the second period. At Tucson Z was low during the first period and high during the second period.

Tables.•In the following tables are given the times of occurrence of salient points and the corresponding values for the five observa- tories. There is so little accordance after the beginning that only the extreme values are given for each of the two active periods. A plus sign signifies east declination, and a minus sign the reverse.

LETTERS TO EDITOR

THE AURORA BOREALIS OF MARCH 22-23, 1920, AS SEEN AT THE CHELTENHAM MAGNETIC OBSERVATORY.

Another magnetic storm of extraordinary magnitude was re- corded on March 22-23, 1920, at the magnetic observatory of the United States Coast and Geodetic Survey, Cheltenham, Maryland, latitude 38 ø 44'.0 N., longitude 76 ø 50'.5 W. Its chief features are described in Mr. Hazard's article, pages 57-59. There was no recurrence of the storm 27 days later, April 19-20.

The Aurora was visible soon after sunset on March 22, 1920,

60 LETTERS TO EDITOR [VOL. XXV, No. 2]

but was first noticed by the writer at 8 a 05 m 75th meridian time. At 8 h 10m a display of rays developed, proceeding from a bright band low in the northern horizon some 45 ø in extent. The rays were visible from a point in the western horizon a few degrees (5ø-10 ø) N. of the new moon (azimuth from S- 88 ø ) to a few degrees in the eastern horizon east of Arcturus (azimuth •- -- 76ø.0), and apparently converged to a few degrees north of the planet Jupiter (azimuth 26.5 and altitude 68 ø .5). There was a con- spicuously rosy colored ray in the east. The rays lasted for about 5 minutes. The boundary of the illuminated area was parabolic in form. The ray display was followed by a general diffused glow.

At 9 h 55 m the diffused glow was bounded by a lighter arc extending from near the planet Mars in the east to some 15 ø south of Jupiter. toward the west. At 10 h 00m the rays were developed on a grand scale, and seemed to emanate from an altitude of 20 ø in the north and 30 ø in the south. The rays converged to a point half way between the star Regulus and the planet Saturn (Alt. = 62ø.5; az. = -- 11ø.0). This ray display lasted 15 minutes, and then flickering movements became conspicuous. There was a rosy glow in the west. At 10 h 40•, nearly, the entire sky was illuminated. At 10 h 45 • rays were visible in the north. Rays were again strikingly conspicuous from 11 h 04• to 1! h !5m; they con- verged from all directions to a point near Saturn (alt. = 62ø; az. = -12ø.5), and were followed by flickering movements.

The observations ceased at midnight. GEo. HARTNELL,

Observer-in-Charge. Ckeltenham, Maryland, April 27, 1920.

MAGNETIC STORM OF MARCH 22, 1920, AS RECORDED AT DEL EBRO OBSERVATORY.

At 9a 10m on March 22, 1920, there began a magnetic storm of considerable intensity, accompanied by the corresponding per- turbations in the earth currents and other electrical elements' registered at the Observatory.

The extreme values which give the maximum amplitude of the oscillation are for-

Declination (D): 11 ø 66'.5 at 16• 40 • on March 22; 11 ø 11'.0 at 1 • 39m on March 23; these values correspond to an oscillation of 23'.5W and 32'.0E, with respect to the mean value of the curve.

Horizontal Intensity (H): q-50• at 12a 44• on March 22; --2807 at 18a 15• on March 22.

Vertical Intensity (Z): --49•, at 13 h 50m on March 22; at 17• !7• on March 22.

The center of gravity of an extensive area of solar perturbation crossed the central meridian about 17• of the preceding day, March 21. The spot which precedes the group, crossed the merid- ian at approximately 19 a on March 20.

L•Jxs Ropes, S.J.