spread-F Conditions Research Articles

<i>Letter to the Editor</i>: Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum

Abstract. At dip equatorial stations in the Indian zone, spread-F conditions are known to develop preferentially around midnight during the June solstice (northern summer) months of low solar activity, in association with a distinct increase in F layer height. It is currently held that this onset of spread-F far away from the sunset terminator is due to the generalised Rayleigh-Taylor instability mechanism, with the gravitational and cross-field instability factors (and hence F layer height) playing important roles. We have studied the quarter-hourly ionograms of Kodaikanal (10.2°N; 77.5°E; dip 4°N) for the northern summer months (May-August) of 1994 and 1995 to ascertain the ambient ionospheric conditions against which the post-midnight onset of spread-F takes place. A data sample of 38 nights with midnight onset of spread-F and 34 nights without spread-F is used for the purpose. It is found that a conspicious increase in F layer height beginning around 2100 LT occurs on nights with spread-F as well as without spread-F. This feature is seen in the nocturnal pattern of F layer height on many individual nights as well as of average F layer height for the two categories of nights. The result strongly suggests that the F layer height does not play a pivotal role in the midnight onset of spread-F during the June solstice of solar minimum. The implications of this finding are discussed.Key words. Ionosphere (equatorial ionosphere; ionospheric irregularities)

Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum

At dip equatorial stations in the Indian zone. spread-F conditions are known to develop preferentially around midnight during the June solstice (northern summer) months of low solar activity, in association with a distinct increase in F layer height. It is currently held that this onset of spread-F far away from the sunset terminator is due to the generalised Rayleigh-Taylor instability mechanism, with the gravitational and cross-field instability factors (and hence F layer height) playing important roles. We have studied the quarter-hourly ionograms of Kodaikanal (10.2°N: 77.5°E: dip 4°N) for the northern summer months (May-August) of 1994 and 1995 to ascertain the ambient ionospheric conditions against which the post-midnight onset of spread-F takes place. A data sample of 38 nights with midnight onset of spread-F and 34 nights without spread-F is used for the purpose. It is found that a conspicious increase in F layer height beginning around 2100 LT occurs on nights with spread-F as well as without spread-F. This feature is seen in the nocturnal pattern of F layer height on many individual nights as well as of average F layer height for the two categories of nights. The result strongly suggests that the F layer height does not play a pivotal role in the midnight onset of spread-F during the June solstice of solar minimum The implications of this finding are discussed.

Equatorial spread-F campaign over SHAR

A RH-560 rocket instrumented with a Langmuir Probe and two pairs of double probes was launched from SHAR (dip 14 ° N) at 2130 hr LT (1ST) on 4 October 1988 during spread-F conditions. As a part of this campaign VHF scintillations at 136.1 MHz were also recorded, using the beacon onboard the geostationary satellite ETS-II (130 ° E). A new digital ionosonde (KEL) installed at SHAR during the first week of September 1988 provided the basic ionospheric information. Ionosonde and scintillation data during this campaign (15 September–4October), along with the Langmuir probe data, are described. The occurrence of scintillations and spread-F during the campaign period was very high, with maximum hourly percentage occurrences of about 80 and 70%, respectively. Generally the onset of spread-F occurred slightly before 1900 hr LT when scintillations in the satellite signal also start appearing. This is followed soon by the range spreading on the ionosonde trace. On days with spread-F the minimum virtual height of the F-layer, h ′ F , rose to about 400 km in the post-sunset period while on days without spread-F it rose to only about 300–320 km. The electric field reversal, as inferred from the h ′ F variations, occurred around 1930 hr LT on spread-F days and around 1900 hr LT on days without spread-F. The ionograms, taken at 5 min interval on several nights, were used to determine the vertical drift velocities and these exceeded 50 m/s on spread-F days in the post-sunset period. At the time of the rocket launch, a strong blanketing type of sporadic-E with several multiple reflections appeared around 105 km. The electron density profile obtained from the Langmuir probe data also showed an extremely sharp layer at 105 km and another prominent layer at 130 km, both during the ascent and the descent. The electron density irregularities associated with spread-F were seen in different altitude regions, the most prominent being from 210 to 250 km and from 300 to 320 km. The power spectra of the electron density fluctuations have been obtained both from the in situ probe data and the scintillation data.

Gravity wave activity in the lower ionosphere and in the F2 region — Similarities and differences

The gravity wave activity (T ∼ 10 min – 3 hours) is inferred from the continuous digital nighttime LF radio wave absorption measurements in the lower ionosphere and from ionosonde measurements (from rapid sequence measurements) in the F-region. All measurements have been performed at the Pru̇honice Observatory (∼ 50°N, 15°E). During solar proton events we observe a simultaneous and similar change of the gravity wave activity at both ionospheric levels (an increase of activity at shorter periods), which very probably indicates the important role of high latitudinal ionospheric/thermospheric source of gravity waves. Also spread-F conditions are associated with simultaneous changes of gravity wave activity in both regions.

Investigation of OI 557.7 nm and OI 630.0 nm nightglow intensity ratios during the occurrence of equatorial F-region plasma bubbles

It is well-known that, at equatorial and low-latitudes, the OI 557.7 nm emission emanates both from the mesosphere and F-region. Earlier investigations using, in general, ground-based simultaneous observations indicate that the contribution to the OI 557.7 nm emission from the F-region is about 20% of the F-region OI 630.0 nm emission. Both of these emissions are excited by the dissociative recombination process (O 2 + + e → O ∗( 1S, 1D) + O) in the F-region. The nocturnal intensity variations of these emissions sometimes show large short-lived depletions, associated with the passage of equatorial spread-F plasma bubbles through the field of view of a photometer. Regular observations of these emissions have been carried out at Cachoeira Paulista (22.7°S, 45.0°W), Brazil, since 1973. In this paper, we present and discuss simultaneous observations of these emissions, during periods of strong equatorial spread-F conditions, when both emissions show large intensity drop-outs. Twenty-five nights of measurements from Cachoeira Paulista have been analysed for this study, which indicate that the average ratio of the intensity depletions ( ΔI557.7 nm ΔI630 nm ) is about 0.26±0.07 for F-region altitudes, and this component has a dependence on the height of the electron density profile.

Short-Period(5-33 min) Variations in Vertical Drift of F-Region Plasma near the Magnetic Equator.

Measurements of F region vertical drift, Vz made at Kodaikanal (dip 4°N) on 133 days during March 1991-February 1992 under non-equatorial spread-F(ESF) conditions, are analysed for the characteristics of short-period (5-33 min) variations in Vz in the evening hours (1630-0130 IST). The spectral content of the fluctuations in Vz is found to depend on local time. The amplitude of the fluctuations, quantified in terms of the variance (σ2), increases during the period of postsunset enhancement of upward vertical drift and decreases thereafter reaching a minimum during 2200-0000 IST. In consonance with this, while functuations in Vz in the entire band 5-33 min manifest in the postsunset hours, the longer-period (13-33 min) components dominate thereafter (2200-0000 IST). The amplitude of the fluctuations and its day-to-day variability are higher in local summer than in local winter. On a day-to-day basis, the amplitude of Vz functuations increases with peak height in the interval 1756-2004 IST corresponding to the epoch of the evening enhancement in Vz. The results support the view that the short-period variations in F region vertical drift near the dip equator during the evening hours could be due to electric fields associated with the commonly occurring atmospheric gravity waves.

Some aspects of mid-latitude daytime ionospheric disturbances

This paper is concerned with the ionospheric irregularities produced during daylight hours in mid-latitude regions by medium-scale travelling ionospheric disturbances (MS-TIDs) as these disturbances pass overhead at a recording station. These irregularities are detected as ionogram trace distortions as well as by spread-F conditions particularly related to the second-hop reflections of HF radio waves. The existence of a significant amount of spread on second-hop ionogram traces during the passage of two particular MS-TIDs is illustrated and discussed. Experimental evidence is presented to suggest that, similar to the results for night-time spread-F, the occurrence of these daytime events is inversely related to variations in the neutral-particle density of the upper atmosphere, for the annual and sunspot-cycle variations. However, the results do not show a similar inverse relationship for the diurnal variation.

Duct‐model explanation of the broad component of plasma‐line spectra observed at Arecibo

The plasma‐line spectrum observed during on‐off cycling of the HF heater at Arecibo contains a broad component extending about 50 kHz from the first decay line. Shortly after HF turn‐on this spectrum can have a maximum intensity 10–20 kHz from the heater frequency (broad bump) or the intensity can decrease monotonically with frequency separation from the first decay line. One possibility is that Langmuir waves (L waves) generated by parametric‐decay of the heater wave are trapped in small‐scale field‐aligned ducts and are responsible for the first decay line. The heater electromagnetic wave can also become ducted near reflection in large‐scale field‐aligned ducts. If the heater wave is not ducted near reflection, its electric field is very nearly parallel to the earth's magnetic field B. The L waves then have wavenormals restricted to a range of angles about B. When these L waves decay parametrically into other L waves that can be detected by the radar, a broad‐bump spectrum results. If the heater wave becomes ducted near reflection (which is probable during spread‐F conditions), its electric field can make a considerable angle to B near reflection and the unstable L waves can propagate at all angles to B (including perpendicular). When these L waves decay parametrically into other L waves that can be detected by the radar, a monotonic spectrum results.

Nighttime Mid-Latitude Travelling Ionospheric Disturbances Associated with Mild Spread-F Conditions.

Several diverging duplicate traces and one converging duplicate trace for mild mid-latitude spread-F events (on one particular night) have been used to investigate medium-scale traveling ionospheric disturbances (TIDs) during night hours. Swept-frequency and fixed-frequency ionograms (which have phase information added) show spread-F characteristics similar to those recorded for more intense spread-F events. These include modulated height rises in the isoionic contours and electron-density depletions.

VHF radio wave scattering due to range and frequency types of equatorial spread- F

Comparing vertical incidence ionograms during spread-F conditions at the equatorial station Huancayo and modified range time interrsity records of 50 MHz scatter echoes at Jicamarca, it has been shown that the range type of spread- F is very efficient for the back-scattering of VHF radio waves. On the otherhand, the frequency type of spread-F does not seem to produce strong echoes. It is suggested that the range type of spread-F ionogram is due to the reflection ( ω = ω p ) of radio waves from large scale irregularities with structure as small as 3 m below or near the base of the F-region The frequency type spread-F ionogram is suggested due to scattering from large scale irregularities with no 3 m counterpart situated around the region of peak ionisation density.

Storm-time characteristics of low-latitude whistlers

Using the results from special observations, the storm-time effects on whistler characteristics at low latitudes were examined and found to agree with previous statistical studies. A short discussion is made on the link between spread-F irregularities and magnetospheric whistler ducts. The enhanced whistler activity is explained as a consequence of the stable whistler duct region during spread-F conditions.

The height of scintillation-producing ionospheric irregularities in temperate latitudes

The heights of ionospheric irregularities responsible for radio satellite scintillation were directly measured by using spaced receivers on 130 passages of Transit 4A. Observed irregularity heights vary from 90 to 800 km. Most of the irregularities were in the F region between 300- and 400-km height. A significant amount of scintillation was caused by irregularities in the E region near 100-km height. E-region scintillation is most probable at midday, F-region scintillation at midnight. Strong positive correlation was found between both E- and F-region scintillation and sporadic-E and spread-F conditions on ionograms. F-region irregularity heights increased, on the average, toward the north. Most individual regions of irregularities were found to have little change of height with latitude. About ⅓ of the passes showed gradual or abrupt height changes, increasing to the north or south with equal probability, in some part of the pass. Height gradients were seldom uniform for distances of more than 50–100 km.

Characteristics of spreadFat high geomagnetic latitude

A detailed analysis of the fading observed during spread F at Ithaca, New York (74° dip), shows that steady components are present at all ranges within the echo proper. Accordingly, it seems unlikely that theories relying on random scattering processes can properly explain the pronounced spread in range observed. To investigate the mechanism responsible for spread F, a lobe-swept interferometer was designed to estimate the width of the angular spectrum of radio waves reflected from the ionosphere under spread-F conditions. This instrument was operated at spacings from 5½ to 12½ wavelengths. Data obtained with it indicate that the type of spread F-observed at high geomagnetic latitudes has a narrow angular spectrum. These observations are in substantial agreement with recent theories that attribute long delay times to the guiding action of field-aligned irregularities within the F layer. The correlation of spread F (as observed on an ionospheric sounder at Ithaca) with strong scintillation of the radio source Cygnus A at upper culmination is found to exceed 0.8. The height of the irregularities responsible for radio-star scintillation is not known accurately; there are indications, however, that the irregularities lie above the F-layer maximum, and it is tempting to imagine them extending right through the F region on nights of spread-F activity.

Short Wave C.W. Transmissions as Affected by Spread-F Conditions

Regular short wave C.W. transmissions from three stations, namely Bombay, Hyderabad and Madras, have been received at Waltair and their fading characteristics have been studied in relation to the overhead spead-F conditions. It was found that the fading rates were higher under spread-F conditions, compared to those under normal conditions. Allowing for the time shifts in the occurrence of similar peaks in the fading rate and spread-F index temporal variations, a systematic linear increase of the fading rate with spread-F index has been found. It was also observed that the C.W. transmissions in the E-W direction are more affected by spread-F conditions than the transmissions in the N-S direction. From amplitude probability distribution of the fading patterns, it was found that the steady to random component ratio was around 1·8 under spread-F conditions. These distributions under spread conditions were compared with those under normal conditions which are nearer to the Rayleigh type.

Diurnal and seasonal variation of spreadFin the Arctic

The diurnal and seasonal variation in the occurrence of spread F is investigated for North American stations by means of hourly ionosonde data for the IGY period. Around the magnetic pole only very small diurnal and seasonal variations are found and explained by the existence of a permanent maximum over the Canadian archipelago (Foxe Basin). To the north of this area small diurnal variations exist during spring and fall months only, and a seasonal variation is noticed. The general level of spread-F occurrence, however, remains so high, that spread-F conditions exist regularly at local noon. For the rest of the Arctic, pronounced diurnal and seasonal variations exist; they are explained by a traveling maximum that moves along the auroral zone.

Some Effects of Nuclear Explosions on the Ionosphere

Examination of spread-F conditions at Brisbane and other stations adjacent to the Pacific area, together with an investigation of h'F2 and foF2 changes at the time, suggest that three disturbances propagated from the site of the nuclear explosion at Johnston I. on August 1, 1958. Somewhat similar variations of ionospheric parameters occurred after the explosion on the same site on August 12, 1958. The average speeds of these disturbances were 1666, 647, and 333 m/s.

EFFECTS OF EQUATORIAL SPREAD-F IRREGULARITIES ON C. W. TRANSMISSIONS

A study is made into the effects of spread-F conditions on continuous wave (c. w.) transmissions between Colombo and Waltair over a distance of about 1300 kilometers. It is shown that under suitable conditions increased fading rate of the received c. w. signals is a sufficient indication of spread F and gives a genuine index of its intensity. Pulse and c. w. methods are used simultaneously to study the time variations of spread-F intensity at separated points. The results indicate that the horizontal extent of spread-F occurrence in the N.–S. direction is greater than 650 km.The probability distributions of signal amplitudes in the c. w. fading records were closer to Rayleigh type under normal conditions than during spread-F conditions. It is suggested that the c. w. flutter fading is caused by the presence of spread-F irregularities in the appropriate zone of reflection in the F-region of the ionosphere.

On the nature of equatorial spreadF

Ionospheric propagation via scattering from the F region was sought at 50 Mc/s over a transequatorial path (with midpoint near Huancayo, Peru) employing a 2580-km transmitter-receiver separation. Propagation via F scatter was present over this path about 10 per cent of the time, though only at night. A condition closely related to the occurrence of F scatter was the presence of equatorial spread-F configurations on the Huancayo ionograms. The height of the propagation medium supporting this F scatter was usually identifiable with the lowest height of the associated equatorial spread F on the ionograms. On the basis of an interpretation of experimental results, equatorial spread F is shown to arise from scattering by relatively thin sheets of irregularities in electron density which occur at the bottom of the F layer or as much as 100 km lower. The thickness of these patches is estimated at the order of 50 km. The scattering irregularities comprising these patches are found to be elongated along the earth's magnetic lines of force. Electron irregularities of scale 10 meters or smaller measured in at least one dimension transverse to the magnetic field lines, and 1000 meters or longer measured along the magnetic field lines, are shown to exist as high as 450 km in the nocturnal equatorial F region during equatorial spread-F conditions. Equatorial spread-F echoes observed at Huancayo are demonstrated to be arriving at all elevation angles in the magnetic east-west plane. The geographical extension of a given scattering sheet in the magnetic east-west direction was at times as great as 1000 km. An observational procedure is suggested for distinguishing two fundamental varieties of spread-F echoes appearing on the equatorial ionograms. A necessary condition, that the contours of mean electron density be parallel to the magnetic lines of force, is proposed as a controlling factor for the occurrence of spread F in the equatorial ionosphere.

Radio Scintillations of Satellite 1958α

THERE has been much speculation in recent years concerning the height of the irregularities in the Earth's ionosphere which are responsible for the scintillations in the signals received from discrete sources of cosmic radio emission. Hewish1 concluded that the irregularities are at a height of about 400 km., whereas Booker2 has recently suggested that they are substantially lower—between the E and F regions; other authors have found correlation between scintillation activity and the occurrence of spread-F and sporadic-E conditions.

Some aspects of the rapid directional fluctuations of short radio waves reflected at the ionosphere

Measurements have been made of the rapid directional fluctuations observed on single-component first-order ionospheric echoes, at vertical and oblique incidence (range 700km), and on frequencies of 2·5?10·5 Mc/s. Under quiet ionospheric conditions the r.m.s. angular deviations are of the order of 1° or less, but they are appreciably greater under ionospheric-storm or spread-F conditions. The fluctuations are uncorrelated at an interval of five seconds. It is shown that a layer of irregular ionization-density within region E would account for the quiet-condition F-layer results, and this would require random variations of the phase path through the region of the order of only 3 m at a frequency of 5 Mc/s. The E-layer results appear to be affected by partial reflections at different levels within the region.