Sporadic Layers Research Articles

Origin of high-frequency TEC disturbances observed by GPS over the European mid-latitude region

High-frequency variability of the ionospheric Total Electron Content (TEC) can strongly affect precise positioning with GNSS. The occurrence rate as well as the amplitude of such disturbances has been extensively studied over the last decade. Mainly, one can distinguish disturbances due to space-weather events and the others, qualified as “quiet-time” as they are observed during quiet geomagnetic conditions. The latter, which represent more than 75% of the total number of disturbances over mid-latitudes, are then divided into two categories: the Winter Daytime (WD) and the Summer Nighttime (SN). The first category, representing the bulk of quiet-time disturbances, corresponds to classical Medium-Scale Traveling Ionospheric Disturbances (MSTIDs), that are the result of the interaction of gravity waves and the ionospheric plasma. On the other hand, SN disturbances are generally understood as non-classical MSTIDs of electrical origin. The paper investigates the origin of these two types of disturbance based on GPS measurements, ionospheric soundings and wind speed data at a tropospheric level. If one cannot exclude the solar terminator as a potential source of gravity waves responsible for WD events, it is thought that the major contribution comes from the lower atmosphere. More precisely, tropospheric jetstream is considered as the favorite candidate for daytime MSTIDs. Turning to SN disturbances, our analysis reveals that they are related to spread-F phenomenon, linked to the appearance of sporadic E-layers. The related instabilities are responsible for field-aligned irregularities in the F-region, which are thought to be responsible for noise-like fluctuations of the GPS TEC observed during SN events.

Coupling between parameters of Es layer and planetary waves during SSW 2008, 2010

We report the features of the critical frequency foEs and probability of the sporadic E layer occurrence P(Es) estimated at the Kaliningrad ionosonde station (54°N, 20°E) during the sudden stratosphere warming (SSW) in winter during the years 2008 and 2010. Additionally we use the neutral atmosphere (temperature, wind circulation data at 10hPa) data for the same temporal interval. Considered periods were characterized by the low solar and geomagnetic activities. We investigate the indirect relation of the neutral temperature in the stratosphere with the long-term variability in ionospheric Es layer parameters during SSW. We found a rather good correlation between the Es layer parameters and the stratospheric temperature increase during the SSW events. We used the continuous wavelet transforms in order to demonstrate the relationship between planetary wave (PW) activity and temporal variability of the sporadic E-layer parameters during SSW 2008 and SSW 2010. We detect a significant wave-like oscillation with periods covering a planetary oscillation in both Es layer parameters. Generally, the oscillations were observed in two bands that corresponded to the periods of 8–10days before SSW and 12–16 and more days’ band during SSW.

Coherent structures in the Es layer and neutral middle atmosphere

Abstract The present paper shows results from the summer campaign performed during geomagnetically quiet period from June 1 to August 31, 2009. Within time-series of stratospheric and mesospheric temperatures at pressure levels 10–0.1 hPa, mesospheric winds measured in Collm, Germany, and the sporadic E-layer parameters foEs and hEs measured at the Pruhonice station we detected specific coherent wave-bursts in planetary wave domain. Permanent wave-like activity is observed in all analyzed data sets. However, the number of wave-like structures persistent in large range of height from the stratosphere to lower ionosphere is limited. The only coherent modes that are detected on consequent levels of the atmosphere are those corresponding to eigenmodes of planetary waves.

Signal absorption effects on HF radio paths near Sodankyla observatory (Finland)

Studies were performed on three oblique ionospheric sounding (OIS) paths: Gor’kovskaya-Lovozero (St. Petersburg) with a length of 890 km, Sodankyla-Gor’kovskaya (800 km), and Sodankyla-Lovozero (360 km). The data for March 17 and April 14, 2012, the days during the recovery phase of the corresponding magnetic storms, have been analyzed. According to the observations performed at Sodankyla, riometer absorption in the morning-daytime hours was high against a background of very weak magnetic disturbances registered with a magnetometer; a high absorption level was also typical of the second day but during a substantial magnetic disturbance. The signal propagation mode structure and intensity on different paths were compared for the indicated days. The main results achieved are as follows. The OIS signal mode structure at weak (for April 14, 2012) and strong (for March 17, 2012) absorption substantially differed when magnetic disturbances were weak at the same instant. Diffuse reflections from the F2 layer were observed on the first two paths during a magnetic disturbance at night of April 14, 2012. At the same time, diffusivity was absent on the same paths under quiet magnetic conditions in the morning and daytime hours on March 17, 2012. A short-term abrupt increase in the maximum observed frequency of the Es layer (MOF Es) by 30–80% was registered half an hour before a substantial absorption burst on the first path. Signal reflections from the sporadic Es layer were observed only on the first path on March 17, 2012, and April 14, 2012, during the absorption maximum (A = 6 dB) according to the Sodankyla data, and the signal characteristics differed on those days.

Analysis of the ionosphere irregularities in E region based on COSMIC occultation data

Sporadic-E layer is a usual phenomenon in ionosphere which has an important impact on the satellite navigation, communication, radar system and so on. In this paper, by using a wavelet decomposition and reconstruction technique, fluctuation of slant total electron content have been obtained which shows a strong enhancement at the Es layer altitude. The COSMIC data during year 2007 have been calculated by using this method; global distribution and seasonal variation both show well agreement with the Es layer feature which confirm the validity of the method. Linear and non-linear empirical formulas between TEC fluctuation and Es critical frequency are built and these formulas indicate that it may be better in inversing the strong Es.

Variability of Sporadic-e (es) layer at two equatorial stations: Fortaleza (3os, 38ow) and Ilorin (8.5on, 4.5oe)

The day time variability of sporadic-E (Es) layer at Fortaleza (3oS, 38oW) and Ilorin (8.5oN, 4.5oE) during the solstice and equinox periods have been investigated using hourly daily Digisonde ionograms. The result show that during the equinox period, the critical frequency (foEs) of the Es-layer at Fortaleza dropped to minimum values before rising to its first peack values by 0900 hrs LT and 1000 hrs LT in March and September respectively. On the other hand, a continuous rise in the value of foEs from sunrise till the first peack values of 6.0MHz by 0800 hrs LT and 5.5MHz by 0700 hrs LT was observed during the June and December solstices respectively. At Ilorin, the foEs reached its first peack values of 5.5MHz by 1200 hrs LT and 6.0MHz by 1000 hrs LT during the March and September equinoxes respectively. A double peack of about 5.4MHz by 0800 hrs LT and 6.5MHz by 1300 hrs LT was observed during the June Solstice as compared to the single peack of about 7.00MHz during the December solstice. The height (hmEs) of Es was lower at Fortaleza than at Ilorin for all the seasons (with a difference of between 2 and 4 km, depending on the month). For all the seasons and months investigated, the E-layer was never observed earlier than 0900 hrs LT at Fortaleza. At Ilorin, the E-layer was formed right from sun rise with frequencies lower than at Fortaleza. These results comfirm the latitudinal variation of the sporadic-E layer at the equatorial region. It also showed that the E-layer is formed earlier in Ilorin than at Fortaleza with a time lag of about 3 hours. Keywords: Sporadic-E (Es) layer, Critical frequency (foEs), virtual height (hmE), Equinox, Solstice

Observation of electron biteout regions below sporadic E layers at polar latitudes

Abstract. The descent of a narrow sporadic E layer near 95 km altitude over Poker Flat Research Range in Alaska was observed with electron probes on two consecutive sounding rockets and with incoherent scatter radar during a 2 h period near magnetic midnight. A series of four trimethyl aluminum chemical releases demonstrated that the Es layer remained just slightly above the zonal wind node, which was slowly descending due to propagating long-period gravity waves. The location of the layer is consistent with the equilibrium position due to combined action of the wind shear and electric fields. Although the horizontal electric field could not be measured directly, we estimate that it was ~ 2 mV m−1 southward, consistent with modeling the vertical ion drift, and compatible with extremely quiet conditions. Both electron probes observed deep biteout regions just below the Es enhancements, which also descended with the sporadic layers. We discuss several possibilities for the cause of these depletions; one possibility is the presence of negatively charged, nanometer-sized mesospheric smoke particles. Such particles have recently been detected in the upper mesosphere, but not yet in immediate connection with sporadic E. Our observations of electron depletions suggest a new process associated with sporadic E.

Effects of sporadic E-layer characteristics on spread-F generation in the nighttime ionosphere near a northern equatorial anomaly crest during solar minimum

This study is to know how the characteristics of sporadic E-layer (Es-layer) affect the generation of spread-F in the nighttime ionosphere near the crest of equatorial ionization anomaly during solar minimum. The data of Es-layer parameters and spread-F are obtained from the Chungli ionograms of 1996. The Es-layer parameters include foEs (critical frequency of Es-layer), fbEs (blanketing frequency of Es-layer), and Δf (≡foEs−fbEs). Results show that the nighttime variations of foEs and fbEs medians (Δf medians) are different from (similar to) that of the occurrence probabilities of spread-F. Because the total number of Es-layer events is greater than that of spread-F events, the comparison between the medians of Es-layer parameters and the occurrence probabilities of spread-F might have a shortfall. Further, we categorize the Es-layer and spread-F events into each frequency interval of Es-layer parameters. For the occurrence probabilities of spread-F versus foEs, an increasing trend is found in post-midnight of all three seasons. The increasing trend also exists in pre-midnight of the J-months and in post-midnight of all seasons, for the occurrence probabilities of spread-F versus Δf. These demonstrate that the spread-F occurrence increases with increasing foEs and/or Δf. Moreover, the increasing trends indicate that polarization electric fields generated in Es-layer assist to produce spread-F, through the electrodynamical coupling of Es-layer and F-region. Regarding the occurrence probabilities of spread-F versus fbEs, the significant trend only appears in post-midnight of the E-months. This implies that fbEs might not be a major factor for the spread-F formation.

On the Diurnal Dependence of <i>f<sub>b</sub>E<sub>s</sub></i>-Variations Due to Earthquakes

In the present work, disturbances of the half-transparency frequency fbEs of the ionospheric sporadic E-layer are investigated in connection with earthquakes. The fbEs-frequency is proportional to the square root of the maximum ionisation density of the sporadic E-layer. In this work, it is shown that in 2/3 of the cases, two days before a seismic shock with magnitude M > 5.5 and on the day of the shock, an increase of the fbEs-frequency is obtained at sunset hours at distances from the epicenter R km. In contrast, before sunrise, the fbEs-value decreases. The data analysed are obtained by the three vertical ionospheric sounding stations“Kokubunji”, “Yamagawa”, and “Wakkanai” during some tens of years.

Geomagnetic cutoff effect as an evidence for the corpuscular nature of anomalous VLF disturbances

The effect of geomagnetic cutoff with respect to the primary ionizing flux, which caused anomalous VLF disturbances on a completely auroral radio path with a moderate length (∼900 km), has been proved. Abnormality manifested itself in a qualitative similarity of amplitude and phase variations at all three working frequencies of this radio path. This should not be observed if the contribution of the second ionospheric ray to the received field is substantial. This similarity is still the only signature of ultrarelativistic electron precipitation. The VLF data for the auroral and long partially auroral radio paths with a common receiver point (10–16 kHz) in the auroral zone have been qualitatively compared in order to prove the geomagnetic cutoff effect. The experimental database on disturbances occupies six years in the vicinity of the solar activity minimum (1982–1987). The manifestation of the magnetic cutoff in the above experimental data indicates that the primary ionizing flux is of the corpuscular nature. This flux generates X-ray bremsstrahlung, which produces the ionized sporadic DS layer in the middle auroral atmosphere.

On diurnal dependence and spatial scales of seismo-ionospheric effects in the E-layer

In the present work, disturbances of the fbEs-frequency of the sporadic E-layer of the ionosphere are investigated in connection with earthquakes. The fbEs-frequency is proportional to the square root of the maximum ionisation density of the sporadic E-layer. In this work, it is shown that two days before a seismic shock with magnitude M>5.5, and during the shock, an increase of the fbEs-frequency is obtained around midnight at distances from the epicentre R<expM+100 km in the case that the focus of the shock was situated at depths smaller than 60km. Data obtained by the three ionospheric sounding stations “Kokubunji”, “Akita” and “Yamagawa” are analysed, which were recorded during a total time of 42years. The superimposed epoches method is applied for a few tens of earthquakes.

Dynamics of the auroral Es layer during weak and strong disturbances in the magnetosphere

The paper is dedicated to studying the dynamics of the auroral ionosphere at the level of the sporadic Es layer during magnetospheric disturbances. A new approach to this problem, proposed in the paper, uses the geomagnetic PC index, which is calculated using the magnetic data in the polar caps of the northern and southern hemispheres and manifests the geoefficiency of the interplanetary electric field. It is shown that variations in the sporadic electron concentration in the auroral Es layer could be related to changes in the PC index with a high degree of statistical reliability. However, the character of precipitations of sporadic particles into the ionosphere under high (PC > 2 mV/m) and low (PC < 2 mV/m) magnetic activity differs substantially. During strong magnetic disturbances and under intensified electric fields in the interplanetary environment, the intensity of particle precipitation from the magnetosphere into the E region of the high-latitude ionosphere is governed by the values of the PC magnetic index. During weak magnetic disturbances, short-time pulses of an increase in the PC values, caused by the variability in electric field in the magnetosphere, are the main factor in the occurrence of sporadic ionization in the Es layer.

Recent investigation on the coupling between the ionosphere and upper atmosphere

Scientific attention has recently been focused on the coupling of the earth’s upper atmosphere and ionosphere. In the present work, we review the advances in this field, emphasizing the studies and contributions of Chinese scholars. This work first introduces new developments in the observation instruments of the upper atmosphere. Two kinds of instruments are involved: optical instruments (lidars, FP interferometers and all-sky airglow imagers) and radio instruments (MST radars and all-sky meteor radars). Based on the data from these instruments and satellites, the researches on climatology and wave disturbances in the upper atmosphere are then introduced. The studies on both the sporadic sodium layer and sporadic E-layer are presented as the main works concerning the coupling of the upper atmosphere and the low ionosphere. We then review the investigations on the ionospheric longitudinal structure and the causative atmospheric non-migrating tide as the main progress of the coupling between the atmosphere and the ionospheric F2-region. Regarding the ionosphere-thermosphere coupling, we introduce studies on the equatorial thermospheric anomaly, as well as the influence of the thermospheric winds and gravity waves to the ionospheric F2-region. Chinese scholars have made much advancement on the coupling of the ionosphere and upper atmosphere, including the observation instruments, data precession, and modeling, as well as the mechanism analysis.

First report of sporadic Na layers at Qingdao (36° N, 120° E), China

Abstract. This paper reports, for the first time, observational results of mesopause sporadic Na (Nas) layers by a ground-based lidar at Qingdao (36° N, 120° E), China. Based on ~ 430 h of observational data on 95 nights from December 2007 to June 2012, we have selected a total of 53 Nas layer events. It is found that characteristics of Nas layers over Qingdao have general similarity with those over nearby sites, Wuhan (30° N, 114° E) and Hefei (32° N, 117° E), but not those over the site Hachioji (35° N, 139° E) at nearly the same latitude as Qingdao. At the same time, parameters of sporadic E (Es) layers were recorded by an ionosonde. The fact that Es layer occurrence probabilities of 19, 22, and 18% in time intervals before, during, and after the Nas layers are very close to the average occurrence ratio of the nocturnal Es layer (21%), may reveal a general independence between Nas and Es layers over Qingdao. Only those strong Nas layers above the peak altitude of the main Na layer might have a significant correlation with Es layers. In addition, a total of 11 high-altitude (above 105 km) Nas layer events have been surveyed specially. It is found that these high-altitude Nas layers were usually weak. And they possessed long-duration (&gt; 147 min) and broad-layer width (4.0 km) compared with Nas layers below 105 km (&gt; 96 min and 2.4 km). These characteristics are in accord with observational results at Wuhan. It is suggested that there is little correlation between this kind of Nas layers and Es layers. Finally, the summer topside enhancement phenomenon of Na atoms observed at Qingdao is in accord with several earlier observational results at different sites (18, 30, and 54° N) in the Northern Hemisphere.

Joint observations of sporadic sodium and sporadic E layers at middle and low latitudes in China

Sporadic sodium layers (SSLs) were studied with sodium (Na) lidar at Beijing (40.5°N, 116.0°E) and Haikou (19.5°N, 109.1°E) in China. The altitude distribution, strength factors, and occurrence time of SSL and sporadic E (Es) layer events were statistically analyzed at both observing sites. SSL occurrence had a maximum near 22:00 local time, and Es layer occurrence had a maximum before midnight. The altitude distributions of SSL and Es layer events were better correlated at Haikou than at Beijing. All the average values of height and strength factor for SSL and Es layer events, as well as the average of the maximum frequency that can be reflected by the Es layer (foEs), at Beijing were higher than those at Haikou. A better correlation between SSL and Es layer events was also found at Haikou, and the formation of SSLs was considered to likely depend on the seasonal variability of chemical and dynamical processes.

A comparative sporadic-E layer study between two mid-latitude ionospheric stations

Hourly systematic measurements of the highest frequency reflected by the sporadic-E layer (foEs) recorded from January 1976 to June 2009 at the ionospheric stations of Rome (Italy, 41.8°N, 12.5°E) and Gibilmanna (Italy, 37.9°N, 14.0°E) were considered to carry out a comparative study between the sporadic E layer (Es) over Rome and Gibilmanna. Different statistical analysis were performed taking into account foEs observations near the periods of minimum and maximum solar activity. The results reveal that: (1) independently from the solar activity, Es develops concurrently over extended regions in space, instead of being a spatially limited layer which is transported horizontally by neutral winds over a larger area; especially during summer months, when an Es layer is present at Rome, there is a high probability that an Es layer is also present over Gibilmanna, and vice versa; (2) Es layer lifetimes of 1–5h were found; in particular, Es layers with lifetimes of 5h both over Gibilmanna and Rome are observed with highest percentages of occurrence in summer ranging between 80% and 90%, independently from the solar activity; (3) latitudinal effects on Es layer occurrence emerge mostly for low solar activity during winter, equinoctial, and summer months, when Es layers are detected more frequently over Gibilmanna rather than Rome; (4) when the presence of an Es layer over Rome and Gibilmanna is not simultaneous, Es layer appearance both over Rome and Gibilmanna confirms to be a locally confined event, because drifting phenomena from Rome to Gibilmanna or vice versa have not been emphasized.

Unusual Enhancements of NmF2 in Anyang Ionosonde Data

Sudden enhancements of daytime NmF2 appeared in Anyang ionosonde data during summer seasons in 2006-2007. In order to investigate the causes of this unusual enhancement, we compared Anyang NmF2's with the total electron contents (GPS TECs) observed at Daejeon, and also with ionosonde data at at mid-latitude stations. First, we found no similar increase in Daejeon GPS TEC when the sudden enhancements of Anyang NmF2 occurred. Second, we investigated NmF2's observed at other ionosonde stations that use the same ionosonde model and auto-scaling program as the Anyang ionosonde. We found similar enhancements of NmF2 at these ionosonde stations. Moreover, the analysis of ionograms from Athens and Rome showed that there were sporadic-E layers with high electron density during the enhancements in NmF2. The auto-scaling program (ARTIST 4.5) used seems to recognize sporadic-E layer echoes as a F2 layer trace, resulting in the erroneous critical frequency of F2 layer (foF2). Other versions of the ARTIST scaling program also seem to produce similar erroneous results. Therefore we conclude that the sudden enhancements of NmF2 in Anyang data were due to the misrecognition of sporadic-E echoes as a F-layer by the auto-scaling program. We also noticed that although the scaling program flagged confidence level (C-level) of an ionogram as uncertain when a sporadic-E layer occurs, it still automatically computed erroneous foF2's. Therefore one should check the confidence level before using long term ionosonde data that were produced by an auto-scaling program.

Comparison of sporadic sodium layer characteristics observed at different time resolutions

Abstract. Sporadic sodium (Nas) layers, occurring in roughly the same height range as ionospheric sporadic-E layers, were first detected by lidar some 30 yr ago. Nas layers have a typical thickness of a few hundred meters to a few km, with peak atom concentrations several times that of the background layer. Despite a great deal of excellent work over the past decades, the source of Nas layers is still not altogether clear, partly as a result of our incomplete knowledge of Nas layer characteristics. In this paper we concentrate on some typical case studies chosen from the ~127 h of sporadic sodium layer observations made at a time resolution of 1.5 s at Yanqing (115.97° E, 40.47° N), Beijing, China. This is a much better time resolution than what has been employed in most earlier measurements. The results show that the Nas layer peak heights are dispersed at slightly different although adjacent heights. When averaged over several minutes, as has been the case with most earlier measurements, the height scatter results in an apparent layer thickness of a few km. We conclude, therefore, that these dispersed peaks at different but adjacent heights constitute the 5 min Nas layer. Similar to the observations of sporadic-E-ion (Es) layers and meteor rate, we observe quasi-periodic fluctuations on a timescale on the order of several minutes in the peak height and the peak density of sporadic layers, which is a universal feature but concealed by the lower temporal resolution previously adopted. Spatially localized multiple scatterers and multiple thin layers with similar apparent movement in Nas layers are also found. We discuss the possible formation mechanism by the direct deposition of large swarms of micrometeoroids and demonstrate a typical example of meteor trails evolving into a Nas layer, which suggests that this mechanism might indeed occur.

Ultrarelativistic electrons in the near cosmos and X‐ray aurora in the middle polar atmosphere

The rare phenomenon of ultrarelativistic electron precipitation into the middle polar atmosphere, prevalent under calm geophysical conditions, was established from ground‐based radio wave measurements during the period of 1982–1992. Precipitating electrons with energy ∼ 100 MeV and sufficient density to generate X‐ and gamma‐ray bremsstrahlung create a sporadic layer of ionization in the atmosphere under the regular D layer of the ionosphere. Very low frequency radio waves reflect from this sporadic layer with abnormal weakening and with an unusually low height of reflection. The layer has a horizontal linear scale of about several thousand kilometers, with a thickness in altitude of about 20–30 km, and persists for several hours. Due to this layer of electric conductivity, the effective height of this “ground‐ionized atmosphere” waveguide diminishes in exceptional cases by 2–2.5 times. The auroras of X‐ray bremsstrahlung have been detected by the reflection of radio waves with wavelengths of 30–20 km. This phenomenon may be termed “a polar cap absorption effect of the second kind” as an electron analog of proton precipitation.

Hemisphere-coupled modeling of nighttime medium-scale traveling ionospheric disturbances

Nighttime medium-scale traveling ionospheric disturbances (MSTIDs), which have tilted frontal structures in the midlatitude ionosphere, are investigated by the midlatitude ionosphere electrodynamics coupling (MIECO) model in this study. It has been proposed that the electrodynamic coupling between the E and F regions plays an important role in generating MSTIDs within a few hours. An intriguing aspect of MSTIDs is that they were simultaneously observed at magnetic conjugate locations in the Northern and Southern Hemispheres. In order to study the hemisphere-coupled electrodynamics, the MIECO model has been upgraded to consist of two simulation domains for both hemispheres in which the electrostatic potential is solved by considering electrodynamics in both hemispheres. The simultaneous occurrence of MSTIDs at the magnetic conjugate stations has clearly been reproduced when the F-region neutral wind satisfies the unstable condition in both hemispheres and a sporadic-E layer is given only at the Northern (summer) Hemisphere. Even if the unstable condition is satisfied in the summer hemisphere, an unfavorable F-region neutral wind in the winter hemisphere largely suppresses the growth of MSTIDs in both hemispheres.