spread-F Occurrence Research Articles

Post midnight spread-F occurrence over Waltair (17.7° N, 83.3° E) during low and ascending phases of solar activity

Abstract. A study carried out on the occurrence of post midnight spread-F events at a low-latitude station, Waltair (17.7° N, 83.3° E), India revealed that its occurrence is maximum in the summer solstice months of the low solar activity period and decreases with an increase in the sunspot activity. The F-region virtual height variations show that 80% of these spread-F cases are associated with an increase in the F-region altitude. It is suggested with the support of the night airglow 6300 A zenith intensity data obtained with co-located ground-based night airglow photometer and electron temperature data from the Indian SROSS C2 satellite that the seasonal variation of the occurrence and probable onset times of the post midnight spread-F depend on the characteristics of the highly variable semipermanent equatorial Midnight Temperature Maximum (MTM).Key words. Ionosphere (ionospheric irregularities; ionosphere atmosphere interactions) Atmospheric composition and structure (airglow and Aurora)

Spread-F at anomaly crest regions in the Indian and American longitudes

Occurrence features of spread-F over Ahmedabad, situated near the anomaly crest region in Indian zone are studied from the quarter hourly ionograms for the years 1983–1995. The occurrences of range and frequency types of spread-F over Ahmedabad are seasonal and solar cycle dependent. Frequency type of spread-F is most frequent during June-solstices of low sunspot years. The range type of spread-F is frequent during equinoxes and December-solstices (with lower occurrence) of high sunspot years. The occurrence features of range spread at Ahmedabad are compared with the occurrence features of range spread over Cachoeira Paulista, a station near the anomaly crest region in Brazil in the American sector. The occurrence of range spread-F over Cachoeira Paulista shows strong seasonal dependence with a distinct maximum during local summer (December solstices) for both low and high sunspot years. The occurrence of range spread over Cachoeira Paulista is much higher than over Ahmedabad.

On the region of mid-latitude spread-F

Mid latitude spread-F has extensively studied recently as compared to the earlier periods. In the course of these investigations two concepts have appeared in the literature concerning the origin of the mid-latitude spread-F. The first concept based on radar measurements attributes the development of spread-F to plasma instabilities (Fukao et al. 1988), while according to the other concept based on the vertical incidence sounding of the ionosphere the occurrence of mid-latitude spread-F is related to travelling ionospheric disturbances (Bowman 1981). Therefore, it seemed worthy to study mid-latitude spread-F from the point of view of its origin. This study has been carried out by analysing the connection between the occurrence of spread-F and the occurrence of sporadic E layers, keeping in mind that the development of sporadic E layers at mid-latitude is due to atmospheric gravity waves, which initiate plasma instabilities. On the other hand, the relation of the occurrence of spread-F to enhancements of the auroral electrojet index (AE) has been studied indicating the occurrence of large scale travelling ionospheric disturbances.

Some results referring to the long-term change of ionospheric parameters

Mid latitude spread-F has extensively studied recently as compared to the earlier periods. In the course of these investigations two concepts have appeared in the literature concerning the origin of the mid-latitude spread-F. The first concept based on radar measurements attributes the development of spread-F to plasma instabilities (Fukao et al.\ 1988), while according to the other concept based on the vertical incidence sounding of the ionosphere the occurrence of mid-latitude spread-F is related to travelling ionospheric disturbances (Bowman 1981). Therefore, it seemed worthy to study mid-latitude spread-F from the point of view of its origin. This study has been carried out by analysing the connection between the occurrence of spread-F and the occurrence of sporadic E layers, keeping in mind that the development of sporadic E layers at mid-latitude is due to atmospheric gravity waves, which initiate plasma instabilities. On the other hand, the relation of the occurrence of spread-F to enhancements of the auroral electrojet index (AE) has been studied indicating the occurrence of large scale travelling ionospheric disturbances.

Responses of the low-latitude ionosphere to very intense geomagnetic storms

In this work, we investigate the ionospheric responses to exceptionally high-intensity and long-duration magnetic storms over Brazil. Disturbed ionospheric F-region vertical drifts and peak electron density changes observed at the equatorial station Fortaleza — Fz (3°55′S; 38°25′W; dip−3.5°) and the low-latitude station Cachoeira Paulista — CP (22°41′S; 45°00′W; dip 24° S ), for three magnetospheric storm events that occurred in December 1980, April 1981 and September 1982, are analyzed. These storms had minimum Dst indexes −240,−311 and −289 nT, respectively. The interplanetary magnetic field (Bz) data from the ISEE-3 satellite, the auroral activity index AE, and the ring current index Dst are used as indicators of the magnetospheric conditions. The ionospheric response features are analyzed using the F-layer critical parameters h'F, hpF2 and foF2, from ionograms obtained at Fz and CP. The Bz and the AE index variations were much higher than those in many previous studies. Therefore, many of the observations reported here either have not been observed or are not readily explained by current models for predicting the penetration/dynamo disturbance electric fields. The altitude of the nocturnal ionospheric F-layer at low latitudes may undergo significant variations during storm-time, caused by magnitude variations on the local zonal component of the F-region electric field intensity. During the period studied here, clear association of the F-layer rise (vertical velocity and altitude) and spread-F occurrence is observed. It is shown that the storm-time layer rise has a dominant role on the equatorial spread-F. An attempt is made to identify the origin of electric fields responsible for the disturbed F-layer alterations. The main conclusions of this study are that (a) some effects on the F-layer height and peak electron concentrations are consistent with model predictions. Some others are in discrepancy or have not been either predicted by model studies or experimentally detected, (b) the F-layer rise over Fz played a major role in the generation of spread-F, (c) the maximum disturbance electric field intensity observed was about 1.09 mV m s −1 , (d) in some cases, foF2 increases (decreases) over CP were seen to be related to increases (decreases), in the fountain effect mechanism, (e) storm-time-induced h'F post-sunset height rise inhibitions over Fz may extend for at least 4 days, as observed, (f) daytime foF2 depressions of amplitude up to −9 MHz are observed over Fz, (g) in particular, a rather unexpected disturbance rise in h'F over Fortaleza, on September 8 at 08 LT, does not seem directly associated with either disturbance winds or penetration electric fields with origin at high latitude, where the convection remained low during the preceding 10 h.

Quantitative estimates of relationships between geomagnetic activity and equatorial spread-F as determined by TID occurrence levels

Using a world-wide set of stations for 15 years, quantitative estimates of changes to equatorial spread-F (ESF) occurrence rates obtained from ionogram scalings, have been determined for a range of geomagnetic activity (GA) levels, as well as for four different levels of solar activity. Average occurrence rates were used as a reference. The percentage changes vary significantly depending on these subdivisions. For example for very high GA the inverse association is recorded by a change of −33% for Rz ≥ 150, and −10% for Rz < 50. Using data for 9 years for the equatorial station, Huancayo, these measurements of ESF, which indicate the presence of TIDs, have also been investigated by somewhat similar analyses. Additional parameters were used which involved the local times of GA, with the ESF being examined separately for occurrence pre-midnight (PM) and after-midnight (AM). Again the negative changes were most pronounced for high GA in Rz-max years (−21%). This result is for PM ESF for GA at a local time of 1700. There were increased ESF levels (+31%) for AM ESF in Rz-min years for high GA around 2300 LT. This additional knowledge of the influence of GA on ESF occurrence involving not only percentage changes, but these values for a range of parameter levels, may be useful if ever short-term forecasts are needed. There is some discussion on comparisons which can be made between ESF results obtained by coherent scatter from incoherent-scatter equipment and those obtained by ionosondes.

Equatorial spread-F occurrence statistics in the American longitudes: Diurnal, seasonal and solar cycle variations

Incorporation of equatorial spread-F (ESF) occurrence statistics into the IRI requires first a detailed characterization, as a function of longitude, of the spatial and temporal distribution of the different types of ESF irregularity activity indices based on data from the different available techniques, ionosondes/digisondes, scintillation and satellite in-situ measurements. In this paper we present and discuss the distribution statistics of ESF events, mainly from ionograms, using the long term data sets available from equatorial and low latitude stations in Brazil and Argentina. Significant longitudinal dependence in the ESF occurrence is observed. The results are discussed on the basis of the longitude dependent magnetic declination angle that characterizes this region and on relevant thermospheric parameters.

Effect of equatorial ionization anomaly on the occurrence of spread-F

Abstract. The unique geometry of the geomagnetic field lines over the equatorial ionosphere coupled with the E-W electric field causes the equatorial ionization anomaly (EIA) and equatorial spread-F (ESF). Ionosonde data obtained at a chain of four stations covering equator to anomaly crest region (0.3 to 33 °N dip) in the Indian sector are used to study the role of EIA and the associated processes on the occurrence of ESF. The study period pertains to the equinoctial months (March, April, September and October) of 1991. The ratios of critical frequency of F-layer (ƒ0F2) and electron densities at an altitude of 270 km between Ahmedabad (33 °N dip) and Waltair (20 °N dip) are found to shoot up in the afternoon hours on spread-F days showing strengthening of the EIA in the afternoon hours. The study confirms the earlier conclusions made by Raghava Rao et al. and Alex et al. that a well-developed EIA is one of the conditions conducive for the generation of ESF. This study also shows that the location of the crest is also important in addition to the strength of the anomaly.

Effect of equatorial ionization anomaly on the occurrence of spread-F

The unique geometry of the geomagnetic field lines over the equatorial ionosphere coupled with the E-W electric field causes the equatorial ionization anomaly (EIA) and equatorial spread-F (ESF). Ionosonde data obtained at a chain of four stations covering equator to anomaly crest region (0.3 to 33 °N dip) in the Indian sector are used to study the role of EIA and the associated processes on the occurrence of ESF. The study period pertains to the equinoctial months (March, April, September and October) of 1991. The ratios of critical frequency of F-layer (ƒ0F2) and electron densities at an altitude of 270 km between Ahmedabad (33 °N dip) and Waltair (20 °N dip) are found to shoot up in the afternoon hours on spread-F days showing strengthening of the EIA in the afternoon hours. The study confirms the earlier conclusions made by Raghava Rao et al. and Alex et al. that a well-developed EIA is one of the conditions conducive for the generation of ESF. This study also shows that the location of the crest is also important in addition to the strength of the anomaly.

Spread-F during quiet and disturbed periods at midlatitudes

Spread-F has been investigated at a European vertical sounding station in Spain (40.6° N, 0.6° E) using modified ARTIST software. Analysis of color computer ionograms for 1991, year of high solar activity, was performed. Occurrence of spread-F was studied during magnetic storms and during quiet periods with Dst > −30 nT (nano Tesla) and Kp < 3. It is shown that spread-F is observed during disturbed periods as well as during very quiet periods lasting 3–8 days. Most frequently spread-F appears in dawn hours. Special analysis of ionospheric parameters and the spread-F index were made for the great magnetic storm of 24–26 March, 1991.

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.

The Influence of the 27-Day Variation of the Upper Atmosphere Neutral-Particle Density on Ionospheric Spread-F Occurrence.

Days of maximum 10.7 cm solar flux associated with solar rotations have been used as controls to investigate, using superposed-epoch methods, spread-F occurrence levels over a range of latitudes from sub-auroral regions to equatorial-anomaly crest regions. For all latitude regions, 27-day periodicities, and other periodicities of lesser significance, were recorded in the spread-F plots, with minima in the oscillations located near the central positions. For these oscillations spread-F occurrence levels changed by 5 percent in sub-auroral regions, about 15 percent in mid-latitude regions and 10 percent in low-latitude regions. It can be seen that, as the sun rotates, solar-activity changes have a significant influence on spread-F occurrence. With some complications, generally the spread-F minima were delayed after the solar-flux maxima by a few days. These and other experimental results suggest an explanation in terms of an hypothesis which considers the so-called 27-day variation of the upper-atmosphere neutral-particle density (neutral density) associated with the solar-rotations. The proposal is that it is the control by the neutral density of the medium-scale travelling ionosphere disturbance wave amplitudes which determines whether or not spread-F is observed on ionograms.

Occurrence statistics for spread-F over Indian subcontinent

An attempt was made to study occurrence statistics of spread-F over Indian subcontinent. Spread-F data from eight Indian stations have been used. Solar and magnetic activity effects on the occurrence of spread-F have been studied.

Seasonal variation in ionospheric electron content and irregularities over Waltair — A comparison with SLIM model

Beacon satellite measurements made during 1984–1985 and 1989–1990 at a low latitude station Waltair (17.7°N, 83.3°E) are used to study the diurnal and seasonal variations in ionospheric electron content (IEC) and amplitude scintillations. In 1989–1990, under high solar activity, the daytime IEC shows winter anomaly in the peak values and local time differences in their occurrence. By night, enhancements in IEC and the occurrence of scintillations and spread-F were frequent, particularly at equinoxes. The seasonal variations of the meridional wind direction and phase computed from the SLIM model/1/ compare well with the daytime peak density anomaly and with nighttime IEC over Waltair. A probable effect of neutral winds on the occurrence of ionospheric irregularities is discussed.

Seasonal variations of equatorial spread-&amp;lt;i&amp;gt;F&amp;lt;/i&amp;gt;

Abstract. The occurrence of spread-F at Trivandrum (8.5°N, 77°E, dip 0.5°N) has been investigated on a seasonal basis in sunspot maximum and minimum years in terms of the growth rate of irregularities by the generalized collisional Rayleigh-Taylor (GRT) instability mechanism which includes the gravitational and cross-field instability terms. The occurrence statistics of spread-F at Trivandrum have been obtained using quarter hourly ionograms. The nocturnal variations of the growth rate of irregularities by the GRT mechanism have been estimated for different seasons in sunspot maximum and minimum years at Trivandrum using h'F values and vertical drift velocities obtained from ionograms. It is found that the seasonal variation of spread-F occurrence at Trivandrum can, in general, be accounted for on the basis of the GRT mechanism.

Short-Term Delays in the Occurrence of Mid-Latitude Ionospheric Disturbances Following Other Geophysical and Solar Events.

Five years of ionograms recorded at a mid-latitude station, Moggill (near Brisbane, Australia) have been examined to determine days of high and days of low activity for 3 ionospheric disturbance parameters. These are nighttime spread-F, daytime first-hop distortions and daytime second-hop spread. Superposed-epoch analyses using these days as controls show that statistically above-average occurrence levels for these parameters can be expected for low geomagnetic activity a few hours before the recording periods for these parameters. Conversely when geomagnetic activity is high a few hours earlier the occurrence levels of these parameters are suppressed. Also, a 3 day delay after enhanced geomagnetic activity at specific local times (reported earlier) is also illustrated and discussed. When mid-latitude spread-F occurrence levels are considered on a global scale it is found that changes in the levels of occurrence are in antiphase with the 10.7 cm solar flux, in particular the 27-day variation due to the solar rotation. It seems likely that these relationships with geomagnetic activity and the 10.7 cm solar flux can be explained in terms of the upper-atmosphere neutral-particle density (UA-NPD) changes which are associated with these geophysical parameters. It is argued that the UA-NPD changes modulate the wave amplitudes of the medium-scale travelling ionospheric disturbances in such a way that large wave amplitudes (needed for significant disturbance conditions) result from lower levels of the UA-NPD. The results suggest that further work on these short term relationships may in the future facilitate the short-term forecasting of mid-latitude disturbance conditions.

A review of some recent work on mid-latitude spread-F occurrence as detected by ionosondes.

A review of some recent work on mid-latitude spread-F occurrence as detected by ionosondes.

HF doppler observations on the occurrence of equatorial spread-F

HF doppler observations of the vertical drift velocity and group height of the 5.5 MHz plasma frequency level of the post-sunset bottomside F-region obtained on a fewESF (equatorial spread-F) and non-ESF days at Trivandrum are presented. The results show that on the non-ESF days, the maximum group height attained is about 400 km and the maximum velocity is less than 30 m/sec. On theESF days, however, the corresponding values are found to be in the range of 400–650 km and 30–50 m/sec. TheESF onset is found to be significantly delayed relative to the velocity peak indicating that it is more closely linked to the layer height than to the drift velocity.

Spread-F ionospheric irregularities and their relationship to stable auroral red arcs at magnetic mid-latitudes

The signature of the stable auroral red arc (SAR arc) as it appears on ionograms is described. The key features are a very significant increase in the amount of spread-F and a reduction in the maximum plasma density compared with regions just equatorward and poleward of the SAR arc Identification of the SAR arc signature is made by using complementary data from the global auroral imaging instrument on board the Dynamics Explorer-1 satellite. At sunspot minimum there is a positive correlation between the occurrence of spread-F on ionograms from Argentine Islands, Antarctica (65°S, 64°W; L = 2.3) and magnetic activity. In contrast, at sunspot maximum there is a weak negative correlation when the K magnetic index is less than 6. but a significant increase in spread-F occurrence at K ⩾ 6. Detailed study of ionograms shows that there are two distinct regions where considerable spread-F is observed. These are the region where SAR arcs occur and the poleward edge of the mid-latitude ionospheric trough. They are separated by a region associated with the trough minimum, where comparatively little spread-F is seen. It is suggested that the movement of these features to lower latitudes with increasing magnetic and solar activity can explain the lack of correspondence between variations of spread-F occurrence as a function of magnetic activity at sunspot maximum compared with that at sunspot minimum at Argentine Islands.

Some initial results on mid-latitude spread- F irregularities using a directional ionosonde

Specific phase delays imposed on returning ionospheric signals received at 8 antennas in one antenna arm (500 m long) are effective in producing an interference pattern (which is displaced for off-vertical angles) on what would otherwise be a standard ionogram. Recordings from 2 antenna arms at right angles allow the calculations of zenith angles and azimuths-of-arrival for returning ionospheric signals. The system is therefore a directional ionosonde. The records show that ionospheric spread- F signals arrive predominantly from directions in the northwest quadrant consistent with earlier experimental results from Brisbane. Also, the records suggest that at these times of spread-F occurrence it is not unusual to find ‘tongues of ionization’ which extend some tens of kilometres below the normal level of the F2-layer. These observations are consistent with earlier reports of the existence of almost-vertical ionospheric contours of equal-ionization density. For one particular event which was analysed in detail an enhanced maximum electron-density of the F2-layer was found at the position of the ‘tongue’. The occurrence of isolated patches of sporadic E with associated spreading of the E s trace (spread- E s ) at the times of occurrence of 18 ‘tongue’ events has been found to be statistically significant. It is suggested that a knowledge of the existence of these tongues at the time of occurrence of mid-latitude spread- F may give a new lead in the quest to explain how and why spread- F irregularities are formed.