Spread F and ionospheric F‐region irregularities

Abstract

The results of ionospheric observations by means of a variety of techniques are summarized to show that magnetic field-aligned irregularities exist in the ionosphere. The irregularities appear in patches having horizontal dimensions up to 1000 km in the east-west direction and several hundred kilometers in the north-south direction. Size of the irregularities is typically about I km transverse to the magnetic field and elongated along the field line. Irregularities associated with spread F occur in the altitude range from 250 km up to the height of F .... Correlations between ionogram spread F and other ionospheric observations indicate that spread echoes are closely related to the field-aligned irregularities. A critique of theories explaining why the irregularities cause spread echoes is given. Statistical studies of the geographic and temporal variations in spread-F occurrence are summarized. The fundamental question of how irregularities causing spread F are produced in the ionosphere is discussed. Now at Lowell Technological Institute Research Foundation, Lowell, Massachusetts. 255 256 JOHN R. HERMAN rather well known. These variations and their correlations are summarized in section 4.Two types of irregularities are differentiated in the literature: large patches (hereaoe•er re•erred to as large-scale irregularities), which may have a horizontal exten• up to several hundred kilometers, and individual field-aligned irregularities (small scale), whose sizes transverse •o the magnetic field line are on the order ooe I km. It appears that the large patches are in actuality a collection of small-scale irregularities, and spread F will be seen as long as the patch is over an observing station. Observational evidence connecting the two types of irregularities is discussed in section 5.One ooe the important problems in a complete theory ooe spread F is the explanation of how the field-aligned irregularities are produc,ed. The explanation could, ooe course, be applicable to the phenomena ooe radio star and satellite scintillations as well. An acceptable irregularity production mechanism must be compatible with the observed •emporal and geographical variations in spread-F occurrence frequency and known correlations with o•her geophysical phenomena, and it must permit irregularity sizes that allow spreading in range, fr, equency of the ionosonde echo, or both. The observational evidence suggests that the mechanisms for irregularity production may be different in equatorial regions than in polar regions, so that a single model would not necessarily have. to explain spread F in both regions. In section 6 prevalent theories of irregularity formation are discussed.After a fairly short section of summary and conclusions (section 7), a rather complete bibliography is given. Some pertinent papers which might have been missed in this paper may be found in the references for previous reviews, including those by Little et al. [1956], Dagg [1957a], Glover [1960], Calvert [1962], Singleton [1962/], and Clemesha and Wright [1964]....

Direct measurements of the size and amplitude of irregularities in the topside ionosphere

“…Figure 9 shows that when the plasma frequency is less than 2 MHz, irregularities less than 0.5 km thick and with amplitudes less than 5% could cause spread F but not resolvable fine structure. This value for the thickness of the irregularities agrees with the results of most other techniques which indicate thickness of about 1 km or less (see Herman, 1966a). An exception concerns Bowman's observations of bottomside spread F which he interpreted as being caused by wave like irregularities with horizontal wavelengths of tens of kilometers (Bowman, 1960).…”

“…It would be extremely valuable to employ a modified experimental arrangement whereby irregularities with amplitudes of 1% or less and with dimensions of as much as tens of kilometers could be measured. Such an experiment (to be flown on a future sounder satellite) could readily resolve the question of whether in the region of the F2 peak, spread F is caused by irregularities of about 1 km thick (Herman, 1966a) or by wave-like irregularities with wavelengths of the order of 10 km (Bowman, 1960(Bowman, , 1968. Theoretically it would be possible to detect ionospheric irregularities by determining whe'' er or not the I-V curves deviate from the shape obtained when the medium is homogeneous.…”

“…The size of these irregularities cannot be determined unambiguously from ionograms. However, sizes in the transverse field direction of between 10 meters and tens of kilometers have been inferred from observations made using different radio techniques (Herman, 1966a, Bowman, 1960. Variations in electron concentration of between 1% and 70% have been deduced for these irregularities (Herman, 1966a, Singleton, 1962.…”

“…Many theories on the production of these irregularities have been suggested and were recently reviewed by Herman (1966a) but none of these can be considered entirely satisfactory at present. Hence there is a definite need for direct measurements of irregularity amplitudes and sizes to check the values inferred from indirect measurements and to determine the extent to which irregularities of different amplitudes and sizes exist simultaneously in the ionosphere.…”

Variations of Spread-F Occurrence Rates at Near-equatorial Stations in the Australasian Zone

“…The geographical distribution of spread-F occurrence is characterized by two areas of maxima (Herman 1966). Firstly, there is a belt of high occurrence in the equatorial regions bounded by geomagnetic latitudes of about ± 20°.…”