The ionospheric modification experiments provide an opportunity to better understand the aeronomy of the natural ionosphere and also afford the control of a naturally occurring plasma, which will make possible further progress in plasma physics. The ionospheric modification by powerful radio waves is analogous to studies of laser and microwave heating of laboratory plasmas (20). " Anomalous" reflectivity effects similar to the observed ionospheric attenuation have already been noted in plasmas modulated by microwaves, and anomalous heating may have been observed in plasmas irradiated by lasers. Contacts have now been established between the workers in these diverse areas, which span a wide range of the electromagnetic spectrum. Perhaps ionospheric modification will also be a valuable technique in radio communications.

An experiment resulting in ionospheric modification of the F region through heating by an HF transmitting facility located near Boulder, Colorado, has begun. This facility has a transmitter capable of producing nearly 2 Mw of average power, which, when used in conjunction with a 10‐element ring array antenna, results in a power‐aperture product of the order of 104 Mw m². Salient effects observed with radio‐wave measurements after the heating transmitter had been turned on are: a prompt ionospheric response appearing within 30 sec as a deformation in the traces on ionosonde records; a development and growth of spread F starting within tens of seconds, frequently followed by multiple splitting of the O and X traces; appearance of a new time‐varying broad‐band echo which, at times, occurs after 10 min or more of heating and which changes in range with time; and a decrease, within 10 sec, of about 10 db in the amplitude of the O component alone, measured on an oblique path when heating with the O wave. Photometric measurements of 6300‐A airglow from the heated region indicate about a 30% rise in electron temperature. Infrared radiation at 1.27 µ is enhanced in a region located down the magnetic field lines traversing the higher region initially heated by the radio wave.

Within the past four years, experiments with high power, high frequency radio waves have proved the feasibility of temporarily altering the ionosphere's properties. Many different radio and photometric effects have been observed as a result of the ionospheric modification. This paper provides a survey of some of the effects observed with vertical incidence radio techniques when the US Department of Commerce's Platteville facility, near Boulder, Colorado, is used to illuminate the overhead ionosphere with an equivalent isotropically radiated power of the order of 100 Mw. A variety of different and repeatable phenomena are observed to occur in the F region. A few always occur; for others, apparently, certain ionospheric conditions must exist. E‐region effects observed by these techniques are small, but substantial effects are noted in the D region.

Some early observations of significant ionospheric modification resulting from F‐region heating by a very high power, high frequency transmitter located near Boulder, Colorado, are presented in this paper. A description of the Platteville heating transmitter facility, the locations of ionosonde and photometric diagnostic equipments, and the geometry of the experiment are given in a companion paper by Utlaut [1970]. A single‐plane inverted log periodic antenna, which has a half‐power beam‐width of the order of 90°, was used for transmitting. The ionosonde transmitter, which has a peak power of about 25 kw, was operated by using a 200‐Hz pulse repetition rate with a pulse duration of 64 μsec.

Ionospheric modification by high‐powered HF transmissions has produced many different radio and optical effects, some of which have been reported. Additional effects observed with ionosondes are presented here in more detail so that researchers interested in ionospheric phenomena can examine some of the major and subtle changes and then, perhaps, contribute to new understanding about ionospheric processes. The data shown were obtained with O‐mode excitation and provide examples of (a) major deformation of nighttime ionospheric structure, (b) prompt, strong, wideband attenuation of diagnostic O waves on all frequencies exceeding the modifier frequency, and (c) daytime creation of artificial spread F, extending over a range of about 200‐km virtual height, and showing different responses in the F1 and F2 regions.