Bottomside spread-F Research Articles

Ionospheric F-region observations over American sector during an intense space weather event using multi-instruments

The critical interaction between the magnetosphere and ionosphere during intense geomagnetic storms continues to be important to space weather studies. In this investigation, we present and discuss the ionospheric F-region observations in the equatorial, low- and mid-latitude regions in both hemispheres over American sector during the intense geomagnetic storm on 01–03 June 2013. The geomagnetic storm reached a minimum Dst of −119nT at 0900 UT on 01 June. For this investigation, we present vertical total electron content (VTEC) and phase fluctuations (in TECU/min) from a chain of 10 GPS stations and the ionospheric parameters foF2 and h′F from a chain of 4 digital ionosonde stations, covering from equatorial to mid-latitudes regions over American sector during the entire storm-time period 31 May–03 June 2013. In addition, the plasma density observed from DMSP satellites is presented. The results obtained show that during the sudden impulse/SSC and throughout the main phase of the storm, a large positive phase was observed in mid-latitudes of the northern hemisphere, which could be due to changes in the thermospheric wind circulation. On the other hand, in the mid-latitudes of the southern hemisphere, no deviations are observed in VTEC and foF2 when compared to the quiet period. During the long recovery phase of the storm on 01–02 June, a north-south asymmetry is observed in the F-region. The study confirms the dominant role of the thermospheric winds on north-south asymmetry in the ionospheric F-region. The ionospheric irregularities are found to be confined in the equatorial region, of the bottomside spread-F type, before and during the geomagnetic storm. It shows that the geomagnetic storm did not affect the generation or suppression of ionospheric irregularities at the stations investigated.

Response of nighttime equatorial and low latitude F-region to the geomagnetic storm of August 18, 2003, in the Brazilian sector

This paper presents an investigation of geomagnetic storm effects in the equatorial and low latitude F-region in the Brazilian sector during the intense geomagnetic storm on 18 August, 2003 (SSC 14:21 UT on 17/08; ΣKp = 52+; Ap = 108; ∣Dst∣ max = 168 at 1600 UT on 18/08). Simultaneous ionospheric sounding measurements from two stations, viz., Palmas (10.2°S, 48.2°W; dip latitude 5.7°S) and Sao Jose dos Campos (23.2°S, 45.9°W; dip latitude 17.6°S), Brazil, are presented for the nights of 16–17, 17–18 and 18–19 August, 2003 (quiet, disturbed and recovery phases). Both stations are equipped with the Canadian Advanced Digital Ionosonde (CADI). Quiet and disturbed conditions of the F-region ionosphere are compared using data collected from the two stations. The relationship between magnetospheric disturbance and low-latitude ionospheric dynamics, and generation of ionospheric irregularities are discussed. On the disturbed nights (17–18 and 18–19 August), the low latitude station S. J. Campos showed strong enhancements in the F-region critical frequency (foF2), whereas the near equatorial station Palmas showed strong uplifting of the F-layer about 1 h earlier. Normally during the June solstice months (May–August) in the Brazilian sector, large-scale ionospheric irregularities in form of plasma bubbles are rarely observed. On the night of 17–18 August, ionsospheric sounding observations at Palmas showed the presence of bottomside spread-F, whereas on the night of 18–19 August, the observations at Palmas and S. J. Campos showed the presence of plasma bubbles when the storm recovery phase had just started. The complementary GPS data available from several stations in the “Rede Brasileira de Monitoramento Continuo de GPS (Brazilian Network for Continuous GPS Monitoring)” are used to obtain the vertical total electron content (VTEC) and the rate of change of TEC per minute on UT days 18 and 19 August, 2003 and presented. Also, several global ionospheric TEC maps from the worldwide network of GPS receivers are presented, showing widespread latitudinal and longitudinal TEC changes during the different phases of the storm. All the observations (local ionospheric sounding and GPS network measurements, and global GPS measurements) presented in this investigation related to the geomagnetic storm on 18 August indicate that the equatorial and low latitude region in the Brazilian sector had much stronger effect during the recovery phase compared with the main phase. A comparison of the observed disturbance drifts with the Fejer–Scherliess storm-time model drifts indicate that the modeled drifts are not consistent with the present observations.

Concurrent study of bottomside spread F and plasma bubble events in the equatorial ionosphere during solar maximum using digisonde and ROCSAT-1

Abstract. Data from the Jicamarca digisonde and the ROCSAT-1 satellite are employed to study the equatorial ionosphere on the west side of South America during April 1999-March 2000 for the concurrent bottomside spread F (BSSF) and plasma bubble events. This study, using digisonde and ROCSAT-1 concurrently, is the first attempt to investigate the equatorial spread F. Results show that BSSF and plasma bubble observations appear frequently respectively in the summer (January, February, November, and December) and in the equinoctial (March, April, September, and October) months, respectively, but are both rarely observed in the winter (May-August) months. The upward drift velocity during the concurrent BSSF and bubble observations has been determined to study the driving mechanism. This analysis shows that large vertical drift velocities favor BSSF and bubble formations in the equinoctial and summer months. Conversely, the smaller upward velocities during the winter months cause fewer BSSF and bubble occurrences. For the geomagnetic effect, the BSSF/bubble occurrence decreases with an increasing Kp value in the equinoctial months, but no such correlation is found for the summer and winter months. Moreover, the anti-correlations between Kp and dh'F/dt are apparent in the equinoctial months, but not in the summer and winter months. These results indicate that in the equinoctial months the BSSF/bubble generations and the pre-reversal drift velocity can be suppressed by geomagnetic activity, because the disturbance dynamo effects could have decreased the eastward electric field near sunset. However, BSSF and bubble occurrences may not be suppressed by the geomagnetic activity in the summer and winter months.

Dependence of the equatorial anomaly and of equatorial spread F on the maximum prereversal E × B drift velocity measured at solar maximum

The relation of equatorial bubbles to the equatorial anomaly is important because scintillation that is most disruptive to transionospheric RF propagation occurs when it passes through the intersection of the two. However, measurement of the relation between the two and of the electric field from which both arise is difficult because of large separations in space and time. This first attempt to perform these measurements employs a latitudinal array of ionospheric sounders spanning 0° to 40° dip latitude (DLAT) in the Western American sector. Measured on each day of a solar maximum year are the following: (1) the maximum electron density of the postsunset equatorial anomaly, Ne, at 16° and at 20.3° DLAT at 2100 LT, the time when the anomaly crest is at its maximum latitude; (2) equatorial spread F (ESF), detected by the occurrence of macroscopic bubbles and of bottomside spread F (BSSF), the latter recorded at levels of none, weak and strong; (3) Kp averaged over the 6 hours before sunset. Ne and ESF are considered functions of the maximum prereversal F layer drift E × B drift velocity measured by the Jicamarca incoherent scatter radar also during solar maximum and at the same longitude. Parameters are averaged over two levels of Kp for the three seasons, the E months (March, April, September, and October), D months (November–February), and J months (May–August) to yield the following results: (1) Ne measured at 16°, at 20.3° DLAT or at the anomaly crest are linearly dependent on maximum E × B drift velocity. (2) Occurrence of each level of ESF increases with Ne approximately linearly during the E and J months but not during the D months. (3) ESF occurrence is dependent on and increases approximately linearly with maximum E × B drift velocity during the E and J months. During the D months this dependence is absent. Except for the D months, these results indicate that scintillation increases with maximum prereversal E × B drift velocity: at L‐band at the bubble‐anomaly intersection because bubble occurrence increases, Ne increases, and the latitudinal extent of the anomaly increases; and at VHF/UHF near the equator because the occurrence of strong BSSF increases.

Dependence of equatorial bubbles and bottomside spread F on season, magnetic activity, and E × B drift velocity during solar maximum

Equatorial bubbles occur sporadically, so that the resulting scintillation that disrupts transionospheric RF propagation is unpredictable. Furthermore, this randomness prevents the systematic observation of the formation of bubbles necessary for making progress in their prediction. However, recent results indicate that a necessary condition for bubble formation is bottomside spread F (BSSF); that BSSF is a function of the F region dynamo electric field; and that this field is dependent on magnetic activity and on season. This work measures these conditions using an array of ionospheric sounders in conjunction with other spread F observations, all of which are ground‐based, in the western American sector, during solar maximum. Three spread F levels are observed: total BSSF and strong BSSF, both measured near the dip equator, and macroscopic bubbles measured at the equatorial anomaly. The three independently measured thresholds form a hierarchy in which macroscopic bubbles are a subset of strong BSSF, which is a subset of total BSSF. Each level decreases as a linear function of Kp averaged in the 6 hours preceding observation during March, April, September, and October, and during November–February; however, during May–August the levels are small and are independent of Kp. As a result, the probability of occurrence of each level, most importantly of macroscopic bubbles, is a quantitative function of Kp and season. The seasonal variation of Kp dependence indicates that the suppression of irregularities results from the decrease in the maximum prereversal vertical drift velocity of the equatorial F layer and therefore from the decrease in the eastward electric field.

The equatorial anomaly: Its quantitative relation to equatorial bubbles, bottomside spread F, and E×B drift velocity during a month at solar maximum

The scintillation that is most disruptive to trans‐ionospheric RF propagation occurs when an equatorial bubble intersects the maximum electron density of the equatorial anomaly. This paper reports the first systematic observational study of the important interrelation between bubble and the anomaly together with the maximum pre‐reversal E × B drift velocity, and strong bottomside spread F (BSSF), which is a necessary condition for bubbles. An array of ionospheric sounders located near 75° W longitude measures latitudinal profiles of NmF2 at hourly intervals through 0°–40° dip latitude (DLAT) and 1800–0300 LT during a continuous period of 30 days at equinox and solar maximum. The anomaly is highly variable from day to day, but at 2100 LT, the time of its highest latitude, crest latitude and magnitude increase and decrease together, a relation that is linear above a threshold with coordinates of 38 × 105 el/cm3 and 15.4° DLAT. This threshold is important also because it corresponds to the maximum drift velocity of 50 m/s, and because above it nearly all bubbles are observed, directly as macroscopic bubbles and indirectly as strong BSSF. The importance to C/NOFS and other satellites is that maximum E × B drift, crest NmF2 and crest DLAT are in apparent one‐to‐one correspondence above this threshold, so that measurement of any one of the three implies measurement of the other two. Furthermore, any such measurement that exceeds the threshold indicates that bubbles can occur, whereas one that falls below the threshold indicates that they cannot.

An equatorial bubble: Its evolution observed in relation to bottomside spread F and to the Appleton anomaly

This is a case study that undertakes the first comprehensive description of the concurrent evolutions of an isolated equatorial bubble, bottomside spread F (BSSF), and the enhanced Appleton anomaly. A principal focus is the dynamics of the intersection of the bubble with the anomaly crest which is the source of maximum scintillation on transionospheric RF propagation. Conditions are equinox at solar maximum. Measurements were by an array of eight ionospheric sounders in the Western Hemisphere with a collective field of view extending along the dip equator for about 1600 km and to 40° north dip latitude that resolved rapid changes in the phenomena yet encompassed their extent and duration. Viewed locally, the bubble was triggered by an upwelling of the bottomside F layer during E × B upward drift that accelerated upward for 15–20 min before the onset of irregularities seen as range spread F (RSF). Viewed macroscopically, this onset was part of continuous front of RSF onset that coincided with E × B reversal and moved westward at the velocity of the terminator so as to remain at 1920 LT, to the east of the bubble as BSSF; within the bubble during the upwelling of the bottomside crest; at the west wall of the bubble at the onset of its eastward drift; and west of the bubble as BSSF. The bubble drifted eastward at 185 m/s and rose upward at ≤190 m/s intersecting the maximum of the anomaly crest at 2100 LT, 95 min after bubble onset. During this entire period the bubble was in continuous contact with the anomaly profile which was simultaneously increasing but as a function of LT. Thus the intersection was a function of the upward and eastward velocities of the bubble and the westward motion of the anomaly at the velocity of the terminator. The time and place of the intersection is thus dependent on the entire sequence of events and on the differing dependences on UT and LT, and since each can vary, the time and place of maximum scintillation is highly variable. In light of this variability, observations at least as comprehensive as those reported here appear to be necessary to describe the region adequately and therefore to study it further.

HF radar observations of equatorial spread-F over West Africa

Abstract. New experimental data depicting equatorial spread-F were taken during an HF radar sounding campaign in Korhogo (Ivory Coast, 9°24N, 5°37W, dip 4°S). Range-time-intensity maps of the radar echoes have been analyzed to identify the signatures of density depletions and bottomside spread-F. Density depletions are well known features of equatorial spread-F, and are believed to emerge after the development of a Rayleigh-Taylor instability on the bottomside F-layer. A simple model is developed and used to simulate the flow of density depletions over the radar field of view. The simulation permits an interpretation of the data that yields the zonal flow velocity as a function of local time. Comparisons with previous measurements are undertaken to assess the consistency of the computational results, and qualitative arguments are presented to identify bottomside spread-F. Using the computational results as reference, a morphological study of ionograms showing spread-F is undertaken which reveals the specific signature of bottomside spread-F on ionograms recorded just after sunset.

HF radar observations of equatorial spread-F over West Africa

New experimental data depicting equatorial spread-F were taken during an HF radar sounding campaign in Korhogo (Ivory Coast, 9°24N, 5°37W, dip 4°S). Range-time-intensity maps of the radar echoes have been analyzed to identify the signatures of density depletions and bottomside spread-F. Density depletions are well known features of equatorial spread-F, and are believed to emerge after the development of a Rayleigh-Taylor instability on the bottomside F-layer. A simple model is developed and used to simulate the flow of density depletions over the radar field of view. The simulation permits an interpretation of the data that yields the zonal flow velocity as a function of local time. Comparisons with previous measurements are undertaken to assess the consistency of the computational results, and qualitative arguments are presented to identify bottomside spread-F. Using the computational results as reference, a morphological study of ionograms showing spread-F is undertaken which reveals the specific signature of bottomside spread-F on ionograms recorded just after sunset.

A study of scintillations at low latitudes during a period from sunspot minimum to sunspot maximum

Records taken of the 20 and 40 MHz transmissions from the satellite S 66 during the period October 1964 to July 1968 have been analysed for scintillations both north and south of Hong Kong (latitude 22.2°N, longitude 114.2°E, dip 30.5°N). Scintillation activity has been found to be greatest (1) around midnight, (2) in the summer and (3) at sunspot minimum. Three correlations have been established: negative correlation between scintillation activity and solar flux, positive correlation with magnetic activity for values of ap>30, and, for a region north of Hong Kong, positive correlation with values of foEs. For values of ap from 10 to 30 the correlation with scintillation activity has been found to be negative. Good positive correlation between bottom side spread-F occurrence and scintillation activity for the region south of Hong Kong has been obtained at sunspot minimum. It is considered that irregularities in the topside of the ionosphere contribute significantly to scintillation activity. Estimates of the sizes of the irregularities have been made using three independent methods and these estimates range from 0.8 to 4.7 km for heights of occurrence from 95 to 435 km. From results of the variation of normalized scintillation index with zenith angle of satellite crossing the magnetic meridian plane it seems that the field alignment effect of irregularities was obscured. Some association has been observed between high scintillation activity and low electron content values for a region south of Hong Kong.

The morphology of spread-Foccurrence over half a sunspot cycle

An analysis has been made of bottomside spread-F data obtained at sunspot minimum at stations separated in latitude but grouped about 80° west longitude. At this part of the solar cycle, the temporal variations of the occurrence of the frequency-spreading component of spread F change with geomagnetic latitude in a manner comparable to that at sunspot maximum, the main differences being the following. The low-latitude region of high occurrence expands to higher latitudes as the sunspot number decreases. The high-latitude region of high occurrence is not so strong in sunspot minimum as in sunspot maximum. A third region of high occurrence about 10° latitude wide centered on 50° geomagnetic latitude, which is not so obvious at sunspot maximum, appears at sunspot minimum. This midlatitude region of occurrence on the bottomside is identified with the topside spread F attributed to the ducting process. A comparison of the high- and low-latitude regions of high bottomside frequency-spreading occurrence with their topside counterparts results in the conclusion that, at these latitudes, the occurrence properties of topside ducting are not substantially different from those of topside scattering.

Behavior of topside and bottomside spreadFat equatorial latitudes

Diurnal variation of topside and bottomside spread-F and geomagnetic effects from Alouette I ionograms