Onset Of Equatorial Spread F Research Articles

Equatorial spread-F forecasting model with local factors using the long short-term memory network

The predictability of the nighttime equatorial spread-F (ESF) occurrences is essential to the ionospheric disturbance warning system. In this work, we propose ESF forecasting models using two deep learning techniques: artificial neural network (ANN) and long short-term memory (LSTM). The ANN and LSTM models are trained with the ionogram data from equinoctial months in 2008 to 2018 at Chumphon station (CPN), Thailand near the magnetic equator, where the ESF onset typically occurs, and they are tested with the ionogram data from 2019. These models are trained especially with new local input parameters such as vertical drift velocity of the F-layer height (Vd) and atmospheric gravity waves (AGW) collected at CPN station together with global parameters of solar and geomagnetic activity. We analyze the ESF forecasting models in terms of monthly probability, daily probability and occurrence, and diurnal predictions. The proposed LSTM model can achieve the 85.4% accuracy when the local parameters: Vd and AGW are utilized. The LSTM model outperforms the ANN, particularly in February, March, April, and October. The results show that the AGW parameter plays a significant role in improvements of the LSTM model during post-midnight. When compared to the IRI-2016 model, the proposed LSTM model can provide lower discrepancies from observational data.Graphical

The role of the F-region vertical drift on the onset time of the equatorial spread F over Ilorin, Nigeria

The equatorial ionosphere often shows the occurrence of plasma-density irregularities and velocity fluctuations with a broad range of scale sizes and amplitudes. These irregularities in the F-region are commonly referred to as the equatorial spread F (ESF) and they are predominantly a night-time phenomenon. The vertical drift of ionisation to higher altitudes post-sunset hours has been suggested to be responsible for the occurrence of these irregularities. We have therefore used all the available ionograms at Ilorin, Nigeria ( $$\hbox {latitude} = 8.48^{\circ }\hbox {N}$$ , $$\hbox {longitude}= 4.67^{\circ }\hbox {W}$$ and $$\hbox {dip}= 4.1^{\circ }\hbox {S}$$ ) to study the occurrence of ESF and the effect of the F-region drift on the occurrence of ESF. The available data at Ilorin, though not continuous, cover a period of 6 yrs (i.e., 2002, 2005, 2006, 2007, 2008 and 2010). The F-region vertical drift velocity was estimated starting from around local sunset until the onset of ESF, and the maximum value for a night ( $$V_{Z,\mathrm{max}})$$ is used to represent the strength of the $$E\times B$$ drift for that night. Results obtained showed that the percentage occurrence of ESF increases with the increase in solar activities. A seasonal trend is also observed in the occurrence frequency; occurrence seems to be more frequent during and around the equinox periods. The onset time or the time of commencement (TOC) of ESF was observed to vary from day to day. Also, $$V_{Z,\mathrm{max}} $$ seems to increase with the increase in solar activities, which in turn influences the TOC of ESF. It was found that the higher the $$V_{Z,\mathrm{max}} $$ , the closer the TOC was found to be to the local time $$V_{Z,\mathrm{max}}$$ . In a few cases, there was no noticeable movement of ionisation prior to the commencement of the ESF, and this might be pointing to the fact that the drifting of ionisation to a higher altitude is not the only condition for the occurrence of ESF. Other conditions that have been identified to play a role in ESF occurrence are thermospheric neutral dynamics, equatorial thermodynamic meridional winds and the post-sunset base height of the F-layer ( $$h^{\prime }F)$$ .

Nighttime thermospheric meridional winds as inferred from ionosonde parameters over Indian region and their plausible effects on plasma irregularities

In this paper, we present nighttime thermospheric meridional winds at Indian low latitude station namely Hyderabad during March–May 2013 derived using (a) h’F method and (b) hpF2 method based on ionosondes. The estimation of meridional winds using h’F method employs the basic principle that the vertical drift at magnetic equator is purely due to ExB drift, while for a station slightly away from magnetic equator, meridional wind also contributes for the vertical drift in addition to diffusion along the field lines. Assuming the EXB drifts are the same at both the locations, the vertical drift contribution is subtracted from the equatorial vertical drifts and then meridional winds are derived, whereas in hpF2 method, winds are derived using the SERVO theory. The winds so derived are compared with HWM07 wind model. The comparison suggests that magnitude of winds derived using h’F method are in better agreement with HWM07 model than hpF2 derived winds where their polarities are better comparable to HWM07 model than hpF2 method. The poor agreement of hpF2 method could be due to uncertainties involved in estimating various ionospheric parameters. Also the winds derived using h’F method showed signatures of the well known phenomenon of Midnight Temperature Maximum (MTM). Based on these results, we suggest that h’F method is better suited for meridional wind estimates over low latitudes than hpF2 method. Accordingly, we further investigated the role of thermospheric meridional winds in equatorial spread F (ESF) occurrence. Our results suggest that equatorward winds along with post sunset height increase could enhance the Rayleigh Taylor (RT) instability growth rate and thereby leading to the generation of ESF/scintillations. The Superposed Epoch Analysis (SEA) technique has been applied to understand the general trend of h’F and meridional winds around the ESF onset time. The study showed reduction in poleward winds, half an hour prior to the onset of ESF. Hence indicating the role of meridional winds in the generation of plasma irregularities/scintillations in addition to post sunset height rise.

Investigation onFlayer height rise and equatorial spreadFonset time: Signature of standing large-scale wave

Equatorial spread F observations have been categorized into three categories based on ionograms recorded over Sriharikota. First category comprised cases where the onset of equatorial spread F (ESF) was concurrent with the peak h′F time. Second and third categories comprised cases where the onset of ESF happened with a delay of 30 min and more than 30 min, respectively, with reference to the peak h′F time. Average peak h′F in the first category was more than 35 km higher than that in the second and third categories. Also, the peak vertical (upward) plasma drift was higher in the first category. Assuming the genesis of F region irregularity to have happened at or before the time of F layer attaining the peak height, late onset of ESF indicates the genesis of irregularities to have happened westward of Sriharikota. The fact that the peak h′F values were remarkably different in the three categories indicates a zonal variation of eastward electric field and postsunset height rise of F layer. The relative magnitude of the F layer height rise in the three different categories over Sriharikota has also been found to be significantly different than that over Thumba, an equatorial (magnetic) station located ~360 km westward of Sriharikota longitude. This scenario points toward the existence of a large-scale zonal standing wave in the F layer and its important role in F region instability process. Results presented in the manuscript have been discussed in the light of current understanding on the large-scale wave structure.

A statistical study of satellite traces and evolution of equatorial spread F

The ionosonde observations made at 5-min intervals at the Indian dip equatorial station Tirunelveli (8.7°N, 77.8°E geographic; 1.1°N dip latitude) from March 2008 to February 2009 during the extended solar minimum period are used to study the interlink between equatorial spread F (ESF) and satellite traces (STs) which are assumed to represent tilts in the bottomside iso-electron density surfaces probably caused by large-scale wave-like structures (LSWS). The data show different patterns of ESF onset in the bottomside F region, which are illustrated through examples. In addition, the statistics of occurrence of ST and its relation to the formation of ESF are studied. The results indicate that (1) the zonally drifting ESF irregularities can be differentiated from those forming over the observing station. (2) Nearly half of the ESF events were preceded by ST. (3) In about 30% of the cases of occurrence of ST, ESF was not formed afterwards implying that LSWS may not always lead to ESF. (4) The percentage of ESF following ST was high in summer and increased with the time of the night. (5) Following the first occurrence of ST, the ESF onset was delayed by about 30 min on the average suggesting that ST may be used as a precursor of ESF. (6) Pre-reversal enhancement (PRE) of upward plasma drift was found insignificant during the period of study. The trapping of high-frequency radio waves between the E and F regions during intense sporadic E is also illustrated.

Impact of meridional winds on equatorial spread F: Revisited

The impact of meridional winds on the onset and evolution of equatorial spread F (ESF) is studied using the NRL ionosphere model SAMI3/ESF. The prevailing belief is that meridional winds have a stabilizing effect on the onset of ESF. In this letter, it is shown that this is not necessarily the case and that meridional winds can be destabilizing. We find that a wind profile with a positive gradient as a function of latitude (∂V m ∕ ∂θ ≥ 0) is a stabilizing influence on the generalized Rayleigh‐Taylor instability; a wind profile with a negative gradient (∂V m ∕ ∂θ < 0) is a destabilizing influence. Here, a northward wind is positive, and θ increases in the northward direction. It is suggested that meridional wind profiles may account for, in part, the longitudinal and day‐to‐day variability of ESF.

Relationships between pre-sunset electrojet strength, pre-reversal enhancement and equatorial spread-F onset

Abstract. The virtual height of the bottom side F-region (h'F) and equatorial spread-F (ESF) onsets at Chumphon (10.7° N, 99.4° E; 3.3° N magnetic latitude) were compared with the behaviour of equatorial electrojet (EEJ) ground strength at Phuket (8.1° N, 98.3° E; 0.1° N magnetic latitude) during the period from November 2007 to October 2008. Increase in the F-layer height and ESF onsets during the evening hours were well connected with the EEJ ground strength before sunset, namely, both the height increase and ESF onsets were suppressed when the integrated EEJ ground strength for the period from 1 to 2 h prior to sunset was negative. The finding suggests observationally that the pre-sunset E-region dynamo current and/or electric field are related to the F-region dynamics and ESF onsets around sunset.

F layer postsunset height rise due to electric field prereversal enhancement: 2. Traveling planetary wave ionospheric disturbances and their role on the generation of equatorial spread F

This paper investigates the role played by traveling planetary wave ionospheric disturbances (TPWIDs) on the modulation of the electric field prereversal enhancement (PRE) amplitude, near‐sunset hours, in the equatorial ionosphere, and their influence on the day‐to‐day equatorial spread F (ESF) variability. We show that “fresh” (ESF) day‐to‐day variability is closely related with the postsunset height rise due to the electric field PRE, and there is some indication that the altitude of 300 km may be a threshold height for the generation of fresh ESF. Observations of F layer virtual heights during evening hours (1800 LT, 1900 LT, and 2000 LT) present a strong day‐to‐day variability, even during geomagnetically quiet periods. In fact, this well‐organized variability is induced by TPWIDs‐type oscillations that push the F layer up or down according to the TPWID phase. The preliminary analysis presented here show that, when the planetary TPWID oscillations or geomagnetic activity raises the F layer around sunset hours, the occurrence of fresh ESF is increased, but when the TPWID‐type oscillations or geomagnetic activity pushes the F layer downward, the fresh ESF occurrence is reduced. Also, the TPWID phase modulated the onset time of the fresh ESF during high fresh ESF season. The modulation acts in such way that, when the TPWID phase maximizes, the fresh ESF onset time occurs earlier than when the TPWID phase minimizes and the fresh ESF onset time occurs later.

A study of precursors to equatorial spread F using the Giant Meterwave Radio Telescope

This paper reports the results of an experiment to identify a possible precursor to equatorial spread F (ESF) with the Giant Meterwave Radio Telescope (GMRT) (latitude: 19.10°N, longitude: 74.05°E geographic; dip: 23°N magnetic) near Pune by simultaneously recording the amplitude and phase of the signal from the radio source 3C218 (RA:09h15m, Declination:−11°) at 235, 327, 610, and 1420 MHz in the postsunset‐premidnight period 18.5–22.5 LT on 26, 29, and 31 March 2004. Patches of scintillations both in amplitude and phase were observed on 26 and 29 March 2004 at 235, 327, and 610 MHz frequencies starting around 20 LT (LT = UT + 05:00) and extending until 22 LT while the records for 31 March 2004 do not show any scintillations. Significant amplitude scintillations were not observed at 1420 MHz. The most remarkable feature was the presence of large‐scale periodic structures in phase prior to onset of amplitude scintillations. The Total Electron Content (TEC) as observed on a GPS link also looking through the same ionospheric volume showed periodic variation of carrier phase prior to onset of scintillations and bite‐outs in TEC, which implies that before the onset of ESF, the large‐scale wave structures propagate to the height of maximum ionization and beyond.

Observations and modeling of the coupled latitude‐altitude patterns of equatorial plasma depletions

The equatorial ionosphere is host to the most dramatic and enigmatic plasma instability mechanism in the geospace environment. Equatorial spread F (ESF) was discovered in early ionosonde measurements and interpreted theoretically using Rayleigh‐Taylor theory. Subsequent diagnostic and modeling advances have improved substantially our understanding of ESF onset and evolution and its associated effects on the ionosphere throughout the low‐latitude domain. The degree to which ESF mechanisms penetrate into the lower midlatitudes is a topic of current study, a reverse of the familiar concept of high‐to‐low latitude coupling for space weather phenomena. Optical diagnostic systems, first ground based and now space based, reveal the presence of ESF structures via images of airglow depletions that are aligned in the approximately north‐south direction spanning the geomagnetic equator. Ground‐based all‐sky camera systems used to capture the two‐dimensional horizontal patterns of airglow depletions are the main source of observations showing that ESF processes intrude to midlatitudes in the L ∼ 1.5 domain. In this paper we review the process of mapping airglow depletions along geomagnetic field lines to the equatorial plane, hence defining the maximum apex heights achieved. A case study comparison of simultaneous radar backscatter data from Kwajalein with optical data from Wake Island, sites that share common magnetic meridians in the Pacific section, confirms the utility of the approach and its applicability to sites at other longitudes. Modeling studies based on buoyancy arguments using flux tube–integrated mean density values versus L shell apex heights show that instability‐induced plasma depletions starting at F layer bottomside heights easily reach altitudes above 2000 km in the equatorial plane, implying that ESF intrusions to lower midlatitudes should be a relatively frequent occurrence.

Toward a synthesis of equatorial spread F onset and suppression during geomagnetic storms

We present initial steps toward unifying our understanding of storm time equatorial spread F (ESF) by searching for the common elements in past case studies and statistical occurrence patterns. We show that the development (or inhibition) of equatorial irregularities during magnetically active periods can be understood using the AE‐parameterized Fejer‐Scherliess model for disturbance vertical drifts versus storm time and local time. This model takes into account the different sources of perturbation electric fields (magnetospheric and ionospheric dynamos) that ultimately drive the equatorial vertical drifts, showing prompt and delayed effects in the premidnight sector (where both generation and suppression can occur), as well as in the postmidnight period where generation dominates. The postsunset period exhibits the greatest variability for storm time ESF versus longitude, and thus we demonstrate the Fejer‐Scherliess model's applicability in a test case (6 April 2000) that had an AE pattern compatible with their parameterization scheme. The model successfully accounts for the pronounced longitude confinement in the observed postsunset ESF patterns. Finally, we move beyond the empirically derived relationships between geomagnetic indices and the occurrence of ESF (Aarons, 1991) into a framework of true solar‐terrestrial parameters that drive such effects. Additional case studies taken from the published literature are then used to show a consistent linkage between postsunset ESF onset and the interplanetary electric field (IEF) Esw. While AE, Dst, Kp and Dst/dt indices were used in earlier studies to determine the dusk‐longitude sector of disturbance electric fields, here we attribute to the IEF the main role in the determination of this longitude sector.

Suppression of equatorial spread F by sporadic E

We have examined quantitatively the influence a low‐latitude, premidnight sporadic E layer might have on the daily and hourly development of equatorial spread F (ESF). In particular, we calculated changes in the flux tube–integrated Pedersen conductivity as it affects the growth rate of the Rayleigh‐Taylor instability, which governs the initial development of ESF. We find that the growth rate is lowered by an order of magnitude with a density of 1 × 106 cm−3 in a slab from 115 to 120 km. Since sporadic E layers observed after dusk do not regularly reach these values, they are not a likely source of the daily variability in ESF. However, even a mild enhancement in the postsunset E region could lead to a significant suppression of ESF if it also inhibits the upward plasma drift of the prereversal enhancement, a key variable in the growth rate of the equatorial spread F instability. Thus, consistent with the nature of an instability, the second‐order effect (suppressed upward drift) is more important than the first‐order cause (reduced F region to E region conductivity) of inhibited ESF onset.

Measurement of the latitudinal distributions of total electron content during equatorial spread F events

We have constructed latitudinal profiles of the total electron content (TEC) using measurements from six GPS receivers conducted during 1998. The TEC profiles have been divided into two groups: One corresponds to days when plumes or equatorial spread F (ESF) develops, and the second group portrays days of no‐ESF condition. The presence/absence of ESF is based on the signature of the coherent echoes measured by the Jicamarca Unattended Long‐Term Investigation (JULIA) radar and records of scintillations from two sites spaced in latitude. One scintillation station is located near the magnetic equator (Ancon) and the other 12° southward (Antofagasta). The TEC profiles display the typical day‐to‐day and seasonal variability seen at low latitudes. During the equinoxes, we observed quite often the crests of the anomaly located between 12° and 20° away from the magnetic equator and a trough in‐between. The monthly distribution of the appearance of the anomaly and the local time of their appearance are in very good agreement with the reported variability of the upward vertical drifts and the current theory of the equatorial fountain effect. During the equinoxes and the December solstice, the TEC anomaly is observed almost every day, sometimes when there is no ESF activity. Nevertheless, fine inspection of the TEC latitudinal profiles suggests the existence of a close relationship between the temporal evolution of the TEC profiles near sunset and the onset of ESF. We have examined the TEC latitudinal distributions in two different ways. First, we calculated time difference profiles using the distributions corresponding to 1800 and 2000 LT. Second, we used a parameterization of the TEC distributions obtained at 2000 LT. The first method indicates quite drastic increases of the crest values and sharp decreases near the trough during ESF days. In contrast, during days of no ESF there exist almost uniform TEC decreases at all latitudes. The second method displays a preferred high crest/trough ratio (>2), small TEC values at the trough, and large latitudinal integrated values during ESF events.

The early phase of spread F development studied in situ with impedance and Langmuir probes

The postsunset equatorial ionosphere is studied with rocket‐borne plasma diagnostic instruments within the Dynamics of the Equatorial Ionosphere Over Shriharikota Range (DEOS) campaign. Data from an impedance probe and a Langmuir probe allow for a critical comparison between the instruments, with ionosonde data, and with the International Reference Ionosphere model. During flight F05, which was launched before the onset of equatorial spread F (ESF), a generally smooth density profile was found. Density depletions (bubbles) are only found when the rocket crosses the lower edge of the F layer during the downleg of the trajectory. In flight F06, launched shortly after onset of ESF, a large‐scale depletion is found on the upleg of the trajectory below 360‐km altitude, where a transition to an unperturbed density profile occurs. Impedance probe and Langmuir probe data show close agreement of the density depletions and their fine structure with a resolution of 100 m. The morphology of the plasma depletion is compared with models and previous investigations. Electron temperature measurements with Langmuir probes show that the temperature in the depleted region is lower than that in the ambient plasma.

Testing the thermospheric neutral wind suppression mechanism for day‐to‐day variability of equatorial spread F

The determination of the physical processes that cause the day‐to‐day variability of equatorial spread F (ESF) has long been one of the outstanding problems in terrestrial space physics. Within the context of the Rayleigh‐Taylor instability model for ESF, mechanisms that either enhance or inhibit the growth of a seed perturbation offer potential sources of variability that can be tested. In this study the hypothesis that enhanced thermospheric meridional winds play a critical role in suppressing ESF is examined during the Multi‐Instrumented Studies of Equatorial Thermospheric Aeronomy (MISETA) campaign of September 1998. New, high‐time‐resolution Fabry‐Perot interferometer (FPI) observations at 6300‐Å nightglow made at Arequipa (Peru) provided the neutral wind measurements during the critical postsunset hours that had been sampled only sparsely in earlier morphology studies. Evidence of local ESF activity was obtained using GPS‐based observations of phase fluctuations (Fp) and 6300‐Å all‐sky optical images from the same site. Additional GPS measurements of Fp and total electron content (TEC) from Bogota (Colombia) and Santiago (Chile) were used to determine the full flux tube development of ESF plumes and to characterize the F region morphology of the interhemispheric Appleton anomaly. Correlative studies between the nightly magnitudes of the meridional winds (Um), ESF activity (Fp), and indices describing the strength (Is) and asymmetry (Ia) of the Appleton anomaly offered no convincing evidence for the wind suppression mechanism. The best available precursor for premidnight ESF appeared to be the strength of the electrodynamically driven Appleton anomaly pattern at sunset. If one assumes that the required seed perturbation for ESF onset is essentially always available, then for all practical purposes, the magnitude of the eastward electric field that causes upward drift is both the necessary and sufficient parameter to forecast ESF with reasonable success. These results reconfirm 60 years of study pointing to the dominance of electrodynamical processes in the onset and growth of plasma instabilities at low latitudes.

Ionization hole campaign—a coordinated rocket and ground-based study at the onset of equatorial spread-F: first results

Abstract A comprehensive multi-technique campaign involving the launch of two high altitude RH-560 rockets was carried out from Sriharikota (SHAR), India, a near-equatorial rocket launching station at the onset of equatorial spread-F, along with a host of ground-based complementary experiments at other locations spread over the country. The main objectives were to obtain the background ionospheric and thermospheric conditions at the onset of equatorial spread-F, and to be able to evaluate the relative importance of the various agencies, the neutral dynamics in particular. Multiple barium cloud releases, in situ plasma diagnostic measurements, along with complementary optical and radio probing experiments were carried out as a part of this campaign. The presence of large scale gradients in the ambient electric fields at ~185 km altitude and also of vertical winds of significant magnitudes at higher altitudes were found. The presence of large scale irregularities in the ion densities at heights above 250 km in a region of negative background density gradient is one of the significant new results. The details of the Ionization hole campaign along with the first results are presented and discussed in the context of the present knowledge of the phenomenon of equatorial spread-F.

HF Doppler and ionosonde observations on the onset conditions of equatorial spread F

The relative importance of height, vertical drift velocity, and electron density gradient of the postsunset bottomside (5.5 MHz) equatorial F region for the onset of spread F is studied using simultaneous HF Doppler radar and ionosonde observations. The study conducted for the periods January‐March of 1984 and 1985 shows that the height of the F layer, determined by the time history of the prereversal enhancement of the drift velocity, is the deciding factor for the onset of equatorial spread F (ESF) with little contribution from the electron density gradient. Maximum growth rate of linear collisional Rayleigh‐Taylor instability occurs at the time of peak height rather than at the time of peak velocity confirming that, for the onset of ESF, the layer should attain a threshold height. The threshold (group) height of the 5.5 MHz layer falls from ∼450 km in 1984 (mean F10.7 equals 120) to ∼350 km in 1985 (mean F10.7 equals 70); the corresponding evening peak upward drift velocities decrease from about 30 m s−1 in 1984 to about 20 m s−1 in 1985. The significant fall of the thresholds with the declining solar activity is due to the decrease in the ion‐neutral collision frequency with declining solar activity; the fall of the thresholds is reflected in large decreases in the intensity and duration of the spread F.

Onset conditions for equatorial spread F

The problem of day‐to‐day variability in the occurrence of equatorial spread F (ESF) is addressed using multidiagnostic observations and semiempirical modeling. The observational results are derived from a two‐night case study of ESF onset conditions observed at Kwajalein Atoll (Marshall Islands) using the ALTAIR incoherent scatter radar and all‐sky optical imaging techniques. The major difference between nights when ESF instabilities did not occur (August 14, 1988) and did occur (August 15, 1988) in the Kwajalein sector was that the northern meridional gradient of 6300‐Å airglow was reduced on the night of limited ESF activity. Modeling results suggest that this unusual airglow pattern is due to equatorward neutral winds. Previous researchers have shown that transequatorial thermospheric winds can exert a control over ESF seasonal and longitudinal occurrence patterns by inhibiting Rayleigh‐Taylor instability growth rates. We present evidence to suggest that this picture can be extended to far shorter time scales, namely, that “surges” in transequatorial winds acting over characteristic times of a few hours to a day can result in a stabilizing influence upon irregularity growth rates. The seemingly capricious nature of ESF onset may thus be controlled, in part, by the inherent variability of low‐latitude thermospheric winds.

Evidence for a large scale electric field gradient at the onset of equatorial spread-F

Barium-strontium release experiments were conducted at Sriharikota Rocket Range (SHAR. 5.5 N dip latitude) at the onset of equatorial spread-F. We report here an unusual phenomenon of the development of two barium ion clouds from a single release around 200 km altitude, moving with different speeds indifferent directions. This is the first experimental evidence for the presence of large scale electric field gradients with a scale size of 15km. By incorporating neutral wind measured during spread-F into a numerical model for equatorial electrojet we interpret these gradients to be the manifestation of effects due to the meridional winds and wind shears. It is possible that the electric field gradients observed may lead to the generation of plasma holes during the onset of equatorial spread-F.