Post-sunset Rise Research Articles

Effect of the seed perturbation amplitude on the equatorial spread F initiation during solar minimum

The contribution of gravity wave (GW) to the initiation/development of spread F during a solar minimum year was investigated through the comparison of the observed precursory parameters and characteristics of the corresponding equatorial spread F (ESF) events. The ionospheric parameters were recorded at the magnetic equatorial station Sao Luis (2.3°S, 44°W, dip latitude 2°S) during March and October 2010. These data were used to estimate the influence of the relative gravity wave amplitude and the ambient ionospheric condition on the diurnal variation of the spread F initiation. The vertical velocity drift indicated a clear control and defines the threshold for the seasonal variability of the ESF occurrence. However, it was insufficient to solely determine or predict the day to day variation of ESF occurrence. Thus, few days with contrasting ambient ionospheric condition and magnitude of GW amplitude were analysed in order to investigate the role of the different precursory factors in the observed diurnal variation of the plasma irregularity development. The density scale length and gravity wave amplitude were shown to immensely contribute to the linear instability growth rate, especially during the days with a low post-sunset rise. Thus, the experimental observations have demonstrated the strong inter-dependence between the precursory factors and they have also highlighted the probable control of the ESF morphology.

Significant Ionospheric Hole and Equatorial Plasma Bubbles After the 2022 Tonga Volcano Eruption

AbstractThis paper investigates the local and global ionospheric responses to the 2022 Tonga volcano eruption, using ground‐based Global Navigation Satellite System total electron content (TEC), Swarm in situ plasma density measurements, the Ionospheric Connection Explorer (ICON) Ion Velocity Meter (IVM) data, and ionosonde measurements. The main results are as follows: (a) A significant local ionospheric hole of more than 10 TECU depletion was observed near the epicenter ∼45 min after the eruption, comprising of several cascading TEC decreases and quasi‐periodic oscillations. Such a deep local plasma hole was also observed by space‐borne in situ measurements, with an estimated horizontal radius of 10–15° and persisted for more than 10 hr in ICON‐IVM ion density profiles until local sunrise. (b) Pronounced post‐volcanic evening equatorial plasma bubbles (EPBs) were continuously observed across the wide Asia‐Oceania area after the arrival of volcano‐induced waves; these caused a Ne decrease of 2–3 orders of magnitude at Swarm/ICON altitude between 450 and 575 km, covered wide longitudinal ranges of more than 140°, and lasted around 12 hr. (c) Various acoustic‐gravity wave modes due to volcano eruption were observed by accurate Beidou geostationary orbit (GEO) TEC, and the huge ionospheric hole was mainly caused by intense shock‐acoustic impulses. TEC rate of change index revealed globally propagating ionospheric disturbances at a prevailing Lamb‐wave mode of ∼315 m/s; the large‐scale EPBs could be seeded by acoustic‐gravity resonance and coupling to less‐damped Lamb waves, under a favorable condition of volcano‐induced enhancement of dusktime plasma upward E×B drift and postsunset rise of the equatorial ionospheric F‐layer.

Equatorial Plasma Bubble Occurrence Under Propagation of MSTID and MLT Gravity Waves

AbstractWe observed a signature of equatorial plasma bubbles (EPBs) followed by a medium‐scale traveling ionospheric disturbance (MSTID) crossing the solar terminator over the geomagnetic equator during the period of post‐sunset rise (PSSR) of the F layer. Simultaneous observations of the EPBs and the MSTIDs by ground‐based Global Navigation Satellite System receivers and OI 630‐nm imagers, ionospheric parameters by ionosondes, and mesosphere–lower thermosphere (MLT) gravity waves (MLT‐GWs) by OH airglow imagers have been carried out in the equatorial and low‐latitude region of Brazil. On 16–17 September 2015, MSTIDs with very fast horizontal phase speeds, propagating toward northeast (NE), were observed over the latitude region of 0–30°S in the afternoon to evening time zone. It was geomagnetically quiet time (Kp: 2–3). An EPB development was observed after one of the MSTIDs crossed the solar terminator at 22:00 UT. Large‐scale MLT‐GWs with slower phase speeds were also observed both at São João do Cariri (7.5°S, 35.0°W) propagating from southwest (SW) to NE and Cachoeira Paulista (22.7°S, 45.0°W) propagating northward. This is to report on what we consider as a signature of direct seeding of EPBs by an MSTID that was likely induced by a secondary GW from tropospheric deep cloud convections to the south. This study highlights a possible dynamical process from the troposphere to ionosphere via this vertical coupling process over the 2,500 km of horizontal distance.

Observation and simulation of the development of equatorial plasma bubbles: post-sunset rise or upwelling growth?

Observation and simulation of the development of equatorial plasma bubbles: post-sunset rise or upwelling growth?

Global sounding of F region irregularities by COSMIC during a geomagnetic storm

Abstract. We analyse reprocessed electron density profiles and total electron content (TEC) profiles of the ionosphere in September 2008 (around solar minimum) and September 2013 (around solar maximum) obtained by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC/FORMOSAT-3). The TEC profiles describe the total electron content along the ray path from the GPS satellite to the low Earth orbit as function of the tangent point of the ray. Some of the profiles in the magnetic polar regions show small-scale fluctuations on spatial scales <50 km. Possibly the trajectory of the tangent point intersects spatial electron density irregularities in the magnetic polar region. For derivation of the morphology of the electron density and TEC fluctuations, a 50 km high-pass filter is applied in the s domain, where s is the distance between a reference point (bottom tangent point) and the tangent point. For each profile, the mean of the fluctuations is calculated for tangent point altitudes between 400 and 500 km. At first glance, the global maps of ΔNe and ΔTEC are quite similar. However, ΔTEC might be more reliable since it is based on fewer retrieval assumptions. We find a significant difference if the arithmetic mean or the median is applied to the global map of September 2013. In agreement with literature, ΔTEC is enhanced during the post-sunset rise of the equatorial ionosphere in September 2013, which is associated with spread F and equatorial plasma bubbles. The global map of ΔTEC at solar maximum (September 2013) has stronger fluctuations than those at solar minimum (September 2008). We obtained new results when we compare the global maps of the quiet phase and the storm phase of the geomagnetic storm of 15 July 2012. It is evident that the TEC fluctuations are increased and extended over the southern magnetic polar region at the day of the geomagnetic storm. The north–south asymmetry of the storm response is more pronounced in the upper ionosphere (ray tangent points h = 400–500 km) than in the lower ionosphere (ray tangent points h = 200–300 km).

Statistical Study on the Occurrences of Postsunset Ionospheric E, Valley, and F Region Irregularities and Their Correlations Over Low‐Latitude Sanya

AbstractThe coupling of equatorial and low‐latitude ionosphere has been an active area of research for many years. At low latitude, the occurrences of E and valley region irregularities during postsunset hours are obviously different from those around the magnetic equator. In this work, we statistically investigated the occurrences of postsunset ionospheric E, valley, and F region irregularities, the postsunset rise of F layer, and their correlations by using the Sanya (18.4°N, 109.6°E, dip latitude 12.8°N) very high frequency radar and ionosonde observations during equinoctial months of 2012–2016. A statistically significant correlation between the presence of valley region irregularities and that of equatorial spread F (ESF) was found over Sanya. For the days without ESF, valley region irregularity was rarely observed. The Doppler velocities of valley region irregularities were mainly upward which resemble those of simultaneous ESF irregularities. On the other hand, an occurrence peak of E region irregularities was detected around sunset. After sunset, E region irregularity echo was frequently disrupted. The disruption can appear on both ESF and non‐ESF days, when the amplitudes of F layer postsunset rise were significantly different. The effects of polarization electric field associated with ESF and of equatorial ionospheric background condition on the E and valley region irregularities over Sanya are discussed.

Upwelling: a unit of disturbance in equatorial spread F

Plasma structure in the nighttime equatorial F layer, often referred to as equatorial spread F (ESF), is not uniformly distributed, either in time or in space. Observations indicate that ESF in the bottomside F layer takes the form of patches; plasma structure within the F layer takes the form of localized plasma depletions, called equatorial plasma bubbles (EPBs), which tend to occur in clusters. Another observed feature is an upwelling, which has been described as a localized, upward modulation of isodensity contours in the bottomside F layer. Interestingly, zonal widths of ESF patches, EPB clusters, and upwellings are similar. Moreover, all display an east-west asymmetry. The objective of this paper is to show, for the first time, that an ESF patch is the bottomside counterpart of an EPB cluster, and that both are products of the electrodynamical process that takes place within an upwelling. The process can be described as having three phases: (1) amplification of upwelling amplitude during the post-sunset rise of the F layer, (2) launching of the first EPB of the evening, from crest of the upwelling, and (3) structuring of plasma within the upwelling. Hence, an upwelling, whose presence is responsible for the formation of ESF patches and EPB clusters, can be envisioned as a unit of disturbance that occurs in the nighttime equatorial ionosphere.

Effects of tidal forcing, conductivity gradient, and active seeding on the climatology of equatorial spread F over Kwajalein

AbstractPlasma structure in nighttime equatorial F layer, referred to as equatorial spread F (ESF), displays climatology whose seasonal variation depends on longitude. At longitudes where ESF favors equinoxes, times when maxima occur can be predicted in terms of the day of year, when E region sunset is simultaneous in conjugate hemispheres (i.e., “sunset nodes”). Aside from occurrences around equinoxes, there are only three longitudes where ESF also occurs during a solstice; one is the central Pacific region. Here ESF activity is strong during the June solstice, when solar activity is high. To understand this puzzling behavior, ESF climatology over the Kwajalein Atoll was compared with properties of the postsunset rise (PSSR) of the F layer and seeding activity in the troposphere. The key findings are as follows: (1) Maxima in PSSR velocity (VPSSR) are better aligned with equinoxes than with sunset nodes; hence, seasonal pattern of VPSSR, not only sunset nodes, should be included in interpretation of ESF climatology. (2) The source of VPSSR during solstice appears to differ from that during equinoxes. (3) Equinoctial maxima in VPSSR could be related to a semiannual variation in equatorial electrojet strength and its contribution to polarization of the F region dynamo current. (4) Enhanced VPSSR during the June solstice is interpreted in terms of tidal forcing with a wave number of 2. (5) Displacements of maxima in ESF climatology from maxima in VPSSR are shown to be consistent with deep convective activity.

Equatorial vertical plasma drift velocities and electron densities inferred from ground-based ionosonde measurements during low solar activity

Average values of ionosonde hmF2 data acquired from an African equatorial station have been used to determine vertical plasma drift (Vz) measurements during period of low solar activity. Pre-noon peak was around 1000h LT for all seasons. The peak daytime F2 drift is higher during the equinoctial months with an average of 18.1m/s than the solsticial months (14.7m/s). At nighttime, Vz is characterized first by upward enhancement around 1900h LT with a range of 0.3–8.0m/s, then by a downward reversal. The highest enhancement was recorded in December solstice and start earliest during the March equinox. The peak reversal values are 13.3, 10.7, 9.0 and 4.2m/s for December Solstice, September Equinox, March Equinox and June Solstice respectively. The observed simultaneous post-sunset rise in hmF2 and in vertical E×B drift together with a sharp drop in NmF2 at all season infer that electrons moving away from the equator are at a region of low recombination loss rate. The abrupt faster drift of the plasma away from the equator as indicated by the pre-reversal enhancement (PRE) in upward plasma drift is responsible for the sharp drop in NmF2 immediately after sunset. Some past results were also confirmed in this work.

Precursor signatures and evolution of post‐sunset equatorial spread‐F observed over Sanya

Recent case studies on the precursor signatures of equatorial spread‐F (ESF) have shown a one‐to‐one correspondence between the large‐scale wave structures (LSWS) and ESF development at equatorial latitude. In this study, the LSWS and the onset and development of the ESF are investigated over Sanya (18°N, 109°E), a station located at 13° north of the magnetic equator, during both geomagnetic quiet and disturbed conditions in September–October 2011. High‐time‐resolution ionograms from Digisonde Portable Sounder (DPS‐4D) provided the satellite trace measurements that were used to indicate the occurrence of the LSWS. The development of local ESF activity was identified using GPS scintillation and VHF coherent radar echo measurements from the same site, together with the range type spread‐F (RSF) in ionograms. Additionally, the Sanya VHF radar five‐beam scanning measurements in east–west direction were used to characterize the longitudinal difference in establishing the initial conditions for ESF development. Correlative studies between the LSWS and ESF activities during the observational period offer consolidated evidence that the LSWS is a necessary precursor for the ESF development. It is shown that the LSWS and ESF have nearly a one‐to‐one relationship when the F layer undergoes an abrupt post‐sunset rise (PSSR), revealing that the magnitude of the pre‐reversal enhancement in zonal electric field (PRE) that elevates the F layer to a high enough altitude is an important parameter controlling the generation of post‐sunset ESF. However, in the absence of the PSSR, the ESF and GPS scintillation did not always occur following the appearance of LSWS. Sometimes the LSWS events preceded the generation of bottom type spread‐F (BSF) that did not develop vertically into ESF and radar plumes. This result may indicate that under inexpressive, weak, or even moderate PRE conditions, the appearance of the LSWS alone may not be sufficient to produce the post‐sunset F region irregularities responsible for ionospheric scintillations. More factors, other than the LSWS, could play crucial roles favoring the growth of ESF instabilities responsible for ionospheric scintillations.

On seeding, large-scale wave structure, equatorial spreadF, and scintillations over Vietnam

[1] Understanding the day-to-day variability in occurrence of equatorial spread F (ESF) remains as a high-priority objective in space weather research. A major difficulty has been an inability to resolve the roles being played by large-scale wave structure (LSWS) and the post-sunset rise (PSSR) of the equatorial F layer, in the production of ESF. In this paper, we show conclusively that total electron content (TEC), measured as a function of latitude and longitude, provides clear, routine descriptions of LSWS. Then, together with ionosonde data, we show, for the first time, that while a seed for LSWS can occur in the late afternoon, its amplification takes place mostly during the PSSR. Implications of these findings are discussed in light of existing theories.

Day to day variability of h ′F and foF2 during some solar cycle epochs

Comparison of the diurnal and seasonal changes of the variability (VR) of the virtual height of reflection (h′F) and those of critical frequency of the F layer (foF2) is carried out at Ibadan (7.4°N, 3.9°E, 6°S dip) in the African sector. The effect of latitude on both characteristics is investigated by combining data from Singapore (1.3°N,103.8°E, 17.6°S dip) in the East Asian sector and Slough (51.5°N, 359.4°E, 66.5°S dip) in the European sector. The variability of foF2 is found greater than that of h′F except during high solar activity when night-time h′F VR is about the same as night-time foF2 VR possibly due to high post-sunset rise in F region heights at this epoch. Both characteristics have pre-midnight and post-midnight peaks with the latter being the greater one at all epochs with the exception of all the seasons of 1958 but June Solstice, very likely because of the same greater F region height during maximum solar activity. Night-time foF2 VR is greater in September Equinox and June Solstice at all epochs while night-time h′F VR is greater in June Solstice during the three epochs, on a general note. No seasonal trend is observed in the daytime variability of both characteristics during the three epochs except for h′F VR of December Solstice during high solar activity, which is greater than those of other seasons. VR of both parameters are found to increase and decrease alternately with Zurich sunspot number (Rz) at Ibadan and Singapore throughout the day. VR of foF2 at Slough is found to increase from midnight to 1300 h after which it alternates. Generally, post-midnight VR of both characteristics are greater than those of pre-midnight at low latitudes during all epochs while daytime VR are the least. At the mid-latitude station of Slough, foF2 VR is about the same throughout the hours of the day during 1968 and 1971, its post-midnight value is however greater than those of pre-midnight and daytime during 1970. Daytime h′F VR is greater than post- and pre-midnight values during 1968, but about the same throughout the hours of the day during 1971.

On large‐scale wave structure and equatorial spread F without a post‐sunset rise of the F layer

Large‐scale wave structure (LSWS) and the plasma structure, referred to as equatorial spread F (ESF), are shown, for the first time, to develop on a night, when the post‐sunset rise (PSSR) of the F layer was absent. Using ionograms from two nearby locations, together with measurements of total electron content, LSWS and ESF are also shown to coexist in regions that are bounded geographically. These results point to the undeniable importance of LSWS (as well as the PSSR) in the day‐to‐day variability in occurrence of ESF, and a need to use of a two‐dimensional description of the electrodynamics.

First observations of large‐scale wave structure and equatorial spread F using CERTO radio beacon on the C/NOFS satellite

First observations of large‐scale wave structure (LSWS) and the subsequent development of equatorial spread F (ESF), using total electron content (TEC) derived from the ground based reception of beacon signals from the CERTO (Coherent Electromagnetic Radio Tomography) radio beacon on board C/NOFS (Communications/Navigation Outage Forecasting System) satellite, are presented. Selected examples of TEC variations, using measurements made during January 2009 from Bac Lieu, Vietnam (9.2°N, 105.6°E geographic, 1.7°N magnetic dip latitude) are presented to illustrate two key findings: (1) LSWS appears to play a more important role in the development of ESF than the post‐sunset rise (PSSR) of the F‐layer, and (2) LSWS can appear well before E region sunset. Other findings, that LSWS does not have significant zonal drift in the initial stages of growth, and can have zonal wavelengths of several hundred kilometers, corroborate earlier reports.

Satellite traces: An ionogram signature for large‐scale wave structure and a precursor for equatorial spread F

Although the source that controls day‐to‐day variability in the occurrence of equatorial plasma structure (i.e., equatorial spread F, or ESF) remains to be identified, progress is being made. There is evidence that the appearance of large‐scale wave structure (LSWS) in the bottomside F layer, around the time of its post‐sunset rise (PSSR), is a more‐direct precursor of ESF than the PSSR itself. The bulk of the evidence, however, is in the form of “satellite” F traces in ionograms, which may be viewed as less than convincing, because these signatures have not been shown to be causally related to LSWS. In this paper, incoherent‐scatter radar and ionosonde data, both collected on 24 July 1979 from the Kwajalein atoll, Marshall Islands, are used to show that this is indeed the case.

Seeding of equatorial plasma bubbles with electric fields from an Es‐layer instability

Evidence is presented that supports the suggestion by Tsunoda (2006) that a polarization electric field, if generated by a sporadic‐E (Es) layer instability (Cosgrove and Tsunoda, 2002), should map to the base of the F layer and seed equatorial plasma bubbles. Seeding, which leads to bubble development, seems to be a four‐step process operating in the bottomside F layer: (1) amplification of seed perturbations in a region of westward‐drifting plasma, which is found below the velocity‐shear node; (2) upward transference of this modulation through a rotational velocity shear to eastward‐drifting plasma; (3) further seed amplification, when the F layer begins to descend; and (4) bubble growth from the seed via the Rayleigh‐Taylor instability. The time available for interaction between F‐region plasma and elongated channels of enhanced polarization electric field (the latter formed by the mentioned instability) appears to be crucial for seed amplification. The need for a descending F layer to increase interaction time with the eastward‐drifting plasma is appealing because plasma bubbles display a propensity to appear after, and not during, the postsunset rise of the F layer. Although plausible, the need for Es presence, perhaps low solar activity, and a multistep process suggest that this mechanism may be a more occasional contributor than the collisional‐shear instability (Hysell and Kudeki, 2004).

On the post‐sunset rise of the equatorial F layer and superposed upwellings and bubbles

Usual practice has been to describe the post‐sunset rise (PSSR) of the equatorial F layer with the vertical drift velocity measured overhead, as a function of local time, by a ground‐based sensor. However, with the finding that large‐scale wave structure (LSWS) develops with its own eastward polarization electric field, during a PSSR, the measured upward velocity must now be thought of as containing two components, one associated with PSSR, and the other with LSWS. This distinction may hold the key to understanding the lack of correlation, on a day‐to‐day basis, between measured vertical drift and ensuing plasma structure in the form of plasma bubbles. Measurements of backscatter from the bottomside of the F layer, using a three‐beam radar at Pohnpei, are presented in this paper to further validate this interpretation.

Equatorial ionospheric response to isolated auroral substorms over a solar cycle (1980−85): evidence of longitudinal anomaly

Abstract. The equatorial ionospheric response to 228 isolated, rapid-onset auroral substorms (as defined from the auroral electrojet index AE) was found from enhancements of the virtual (minimum) height of the F-region (∆h$^\\prime$F) in the declining phase of a solar cycle (1980–85). The responses, found for three longitudinal sectors at the equator: Africa (Ouagadougou and Dakar), Asia (Manila) and America (Huancayo), were compared with the response close to the auroral source region at Yakutsk (northern Siberia). The auroral substorm onsets were centered at 17 and 15 UT at sunspot maximum (1980–82) and minimum (1983–85), preceding by 3–5 h the period of post-sunset height rise in the African sector whereas other sectors were in the early afternoon (Huancayo) and morning (Manila). The African response, particularly at Ouagadougou, was distinctly different from other sectors. In the sunspot maximum years (1980–81) the auroral surges were followed after about 3 h by a sharp depression (∆h$^\\prime$F<0) in the post-sunset height rise, with a period of little or no response (∆h$^\\prime$F=0) in 1982. A response polarity reversal (∆h$^\\prime$F>0) was noted in this sector for sunspot minimum (1983–85) when large h$^\\prime$F enhancements were observed at the sunset region. The responses in the Asian and American sector were positive except for a case in Huancayo when response was negative, following an auroral surge before the sunset at this station. It appears that the aurorally generated large-scale travelling ionospheric disturbances (LSTIDs), which first cause positive height enhancements in a sub-auroral location (Yakutsk), subsequently affect the unstable post-sunset ionosphere in the equatorial Africa.

Postsunset rise of F layer height in the equatorial region and its relation to the F layer dynamo polarization fields

The ionosonde data are studied for equatorial station, Thumba, to delineate various features of the evening height rise of F layer. Sharp increase of h′F and hpF2 is observed in the postsunset period for high solar activity. Seasonal variation is observed in this increase of h′F and it is maximum for equinox months. For summer months, there is a delay of about an hour in the time of occurrence of h′F (peak) as compared to winter and equinox months. This delay is shown to be associated with the delay in sunset times in the conjugate E regions. As for magnetic activity dependence, it is found that this height increase is less pronounced for disturbed days for winter and equinox whereas for summer it is marginally higher over the quiet day values. Further, it is observed that the value of h′F (peak) during disturbed periods is almost at the same value of 350 km for all the three seasons. Thus the seasonal variation of magnetic activity effects appears to be mainly governed by the average seasonal variation for quiet times. The increase in F layer height is due to zonal eastward electric fields developed after sunset which is believed to be due to F region dynamo fields. While the main driving force for these fields is the zonal neutral winds, the development of these fields depend on the ratio of the F region to E region conductivity and the longitudinal gradient in the E region conductivity. Experimental observations of both the neutral winds and ionospheric conductivities are examined for their variation with solar activity, season and magnetic activity as both these factors will contribute for the various observed features of the height rise. Through model calculations it is shown that the E region density (conductivity) gradient in the postsunset period is higher by a factor of 2 for high solar activity compared to low solar activity and hence it is partly responsible for the observed solar activity variation of the postsunset height rise of the F layer. The importance of the studies using available ionosonde data for understanding the F region dynamo electric fields is emphasized.

A fully analytic, low‐ and middle‐latitude ionospheric model

A fully analytic ionospheric model emphasizing the low‐latitude F region for various seasonal and solar cycle conditions is presented. Features such as the postsunset rise in the F layer peak height and the “equatorial anomaly” maxima in plasma density near ±15° geomagnetic latitude are designed to closely approximate those in the semiempirical low‐latitude ionospheric model (Anderson et al., 1987a). Significant improvements are thus obtained over previous comparable analytic models. Numerical computations are also presented which demonstrate the importance of these large‐scale plasma structures to the neutral dynamics of the low‐latitude thermosphere. Equally significant effects are anticipated for electrodynamic simulations of E region/F region coupling involving flux tube integrated electrical conductivities.