Ionized Band Encircles the Earth
June 1960 Radio-Electronics

June 1960 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

If I told you I just learned that there exists an ionized region of the upper atmosphere which affects electromagnetic waves, and is modulated in intensity and size by activity on the sun, you would understandably respond with something like, "Where have you been. Tell me something we haven't known for half a century." Sure, but in 1960 when this "Ionized Band Encircles the Earth" article was printed in Radio-Electronics magazine, it was news to most people. The presence of an ionosphere had been theorized and shown to be existent based on ground-based experiments beginning a few decades earlier, but it was not until the International Geophysical Year (IGY) space shots running from 1957 through 1958 that direct measurements confirmed suspicions. Not mentioned by Mr. Warsaw was the big part amateur radio operators (aka Hams) played in both the discovery and early characterization of atmospheric reflection. As I write this, Solar Cycle 25 is just starting to wind down, having provided a couple years of phenomenally good long distance (DX) communications.

Ionized Band Encircles the Earth

By David Warshaw

Electronic studies point toward the discovery of a 100-mile-wide ionized band that circles the earth.

Recordings of atmospherics at 27 kc indicate the presence of an ionized band about 100 miles wide encircling the earth at an altitude of 50 miles. Members of the AAVSO¹ Solar Division, using transistorized receivers² at various locations in the United States, are recording Sudden Enhancements of Atmospherics (SEA's) at 27 kc for the National Bureau of Standards Indirect Flare-Detection Patrol. SEA's caused by solar flares, are received and recorded only during daylight hours, although the equipment operates continuously day and night. As a result, a very strange pattern in the recording traces of the observers was discovered. The pattern showed a small dip and hump, or a fall and rise in level, about a half-hour before sunrise.

What Does It Mean?

The reason for the strange dip pattern on the recordings 36 minutes before sunrise could be an ionized band, about 100 miles wide, 50 miles above the earth. About 36 minutes before the earth's sunrise, reception of the atmospheric pulses is weakened for about 8 minutes, during the period when the ionized band is located directly between the receiving station and the high E- and F-layers. The ionized band temporarily blocks the atmospheric pulses normally reflected via these layers to the receiving station (Fig. 1, path b).

The reason for the hump pattern - which follows the dip, appearing on the recordings about 20 minutes before sunrise - could be the existence of a space, or weakly ionized separation, in the D-layer region. The hump indicates an improvement in the reception of the atmospheric pulses and a consequent rise in the level of the recorded trace (Fig. 1, path c).

When the earth has rotated the receiving station to a location 20 minutes before sunrise, the atmospheric pulses become stronger for about 8 minutes. They are then reflected from the high E- and F-layers, down through this separation to the receiving station. The separation between the ionized band and the beginning of the D-layer is about 100 miles. (At New York's latitude, 1° or 4 minutes of the earth's rotation is equal to about 50 miles. The duration of the dip and the hump on the record is about 8 minutes for each, or 100 miles.)

Not only does the before-sunrise pattern of the dip and hump indicate the presence of a narrow ionized band encircling the earth, but there is also an after-sunset pattern of a hump and dip to offer additional proof of its existence. A recorded trace from an observing station on the West Coast of the United States clearly shows both the sunrise and sunset patterns for Feb. 28 and March 1, 1959 (Fig. 2). The West Coast member of the AAVSO observing group in China Lake, California, is Justin Ruhge, a physicist at a missile base, who operates his own homemade receiver in his spare time. His equipment runs unattended for several days.

When atmospheric pulses are received, they are stored for about 60 seconds, averaged into a direct-current output, then graphically recorded. The normal recorded trace is rather high during the night and low during the day. The D-layer, created only by the sun's rays, does not exist at night, so atmospheric pulses received during the night from the thunderstorm centers are reflected from the higher E- and F-layers. When the reflecting ceiling is higher, it reflects from greater distances and the receiving station is able to receive a greater number of atmospheric pulses from more thunderstorm sources on this very low frequency of 27 kc.

How the D-Layer Works

As the earth rotates the receiving station into daylight, the D-layer, formed by the sun's rays, gradually begins to take form about 50 miles overhead, below the higher E- and F-layers. As atmospheric pulses at 27 kc cannot pass through the D-layer, it becomes the reflecting ceiling. Because the D-layer is lower, the number of reflections increases and each reflection absorbs some of the energy of the atmospheric pulses. As a result, the receiving station receives fewer and weaker atmospheric pulses and the recorded trace gradually falls to a lower level. The trace usually remains low during daylight hours, except when a solar flare causes an SEA.

On the daylight side of the earth, the sun's rays create the D-layer region, an umbrella of ionization, 50 miles above the sunlit half of the earth's surface. The amount of sunlight intensity concentrated on a surface increases with the sine of the angle formed by the sun's rays and the surface (Fig. 3).

For this reason the earth's surface is weakly illuminated at sunrise, but not by the sun's direct rays. 50 miles directly above the earth at sunrise, the concave or under surface of the D-layer region is also weakly concentrated for the same reason. At this same 50-mile altitude the sun's rays form an angle of about 10° to a neighboring layer or region which curves into the earth's shadow, and the concentration on this layer is more intense.

The neighboring layer, about 100 miles wide, is the ionized band, which is separated from the actual D-layer. It is illuminated and ionized by the sun's rays penetrating the earth's atmosphere below the 50-mile altitude. Until recently it was not thought probable that the ionizing rays could penetrate this lower altitude enough to induce formation of the ionized band. The existence of radiation that penetrates below the D-region is taken for granted nowadays.

For each day of August, 1958, when there were thunderstorms in the mountains of New England, the recording traces received in New York failed to show the pre-sunrise dip and hump patterns. When reception is from the northeast, instead of the usual southwest, the reflecting ceilings are the E- and F-layers. The propagation path from this source to the receiving station passes through the separation or space between the D-layer and the ionized band (Fig. 1, path d). Because neither the D-layer nor the ionized band blocks the path of propagation, the recording trace shows no dip or hump pattern.

Position of the Band

On December 21, at geographic latitude 77.5° north, the ionized band is located directly overhead for about 2 hours before and after noon, as shown in Fig. 4. However, it is not difficult to plot the ionized band for the other seasons. For example, during the summer it is directly overhead for about 4 hours around midnight at geographic latitude 56.5° north. During the March and September equinoxes, at all latitudes except 80° north, the ionized band is directly overhead a half-hour before local sunrise and a half-hour after local sunset, for only a few minutes. At 80° north latitude, the ionized band is directly overhead from about 10 pm to 2 am. When plotting the position of the ionized band for any day of the year, remember that the ionized band encircles the earth in a plane perpendicular to the sun's rays and situated about 10° farther than the tangent point made by the sun's rays to the earth's surface (sunrise). Some of the communications problems of airlines using the arctic areas should be eased with the knowledge of the exact location of the ionized band.

Recent rocket observations of the upper atmosphere have indicated that shortwave radio fadeout³ (5 to 30 mc) are caused by an extra ionized layer which is formed by an increased intensity of X-rays emitted by the sun during solar flares. This extra layer of ionization may extend downward to about 12 miles below the normal D-layer while the higher layers appear to remain undisturbed during the fadeout. The extra or additional layer, present during solar flares, temporarily makes a better reflecting ceiling for atmospheric pulses on 27 kc, thus causing an SEA at the same time as the solar flare.

AAVSO Solar Division members receive and record SEA's and send the recordings to their chairman, H. L. Bondy in Flushing, N. Y. He analyzes the tabulations for the Central Radio Propagation Laboratory, Radio Warning Services Section of the National Bureau of Standards, in Boulder, Colo. SEA record tabulations are correlated and published monthly by the National Bureau of Standards CRPL in series F, part B, Solar-Geophysical Data, listed under Ionospheric Effects of Solar Flares. The Indirect Flare Detection Patrol gathers the information appearing under SEA's, SCNA (Sudden Cosmic Noise Absorption) at about 18 mc and bursts of solar radio noise, which are also at 18 mc.

1 American Association of Variable Star Observers, Solar Div. 6130 157th St., Flushing 67, N. Y.

2 Solar-Flare Indicator-Transistorized, Radio-Electronics, August, 1956, and Improved Solar-Flare Indicator, Radio-Electronics, January, 1959.

3 Science, Jan. 17, 1958