E-region Field-aligned Irregularities Research Articles

High-Resolution Observation of Ionospheric E-Layer Irregularities Using Multi-Frequency Range Imaging Technology

E-region field-aligned irregularities (FAIs) are a hot topic in space research, since electromagnetic signal propagation through ionospheric irregularities can undergo sporadic enhancements and fading known as ionospheric scintillation, which could severely affect communication, navigation, and radar systems. However, the range resolution of very-high-frequency (VHF) radars, which is widely used to observe E-region FAIs, is limited due to its bandwidth. As a technology that is widely used in atmosphere radars to improve the range resolution of pulsed radars by transmitting multiple frequencies, this paper employed the multifrequency radar imaging (RIM) technique in a Wuhan VHF radar. The results showed that the range resolution of E-region FAIs greatly improved when compared with the results in traditional single-frequency mode, and that finer structures of E-region FAIs can be obtained. Specifically, the imaging results in multifrequency mode show that E-region FAIs demonstrate an overall descending trend at night, and it could be related to the tides or gravity waves due to their downward phase velocities or even driven by downwind shear. In addition, typical quasi-periodic (QP) echoes with a time period of around 10 min could be clearly seen using the RIM technique, and the features of the echoes suggest that they could be modulated by gravity waves. Furthermore, the RIM technique can be used to obtain the fine structure of irregularities within a short time period, and the hierarchical structure of E-region FAIs can be easily found. Therefore, the multifrequency imaging RIM technique is suitable for observing E-region FAIs and their evolution, as well as for identifying the different layers of E-region FAIs. Combined with the RIM technique, a VHF radar provides an effective and promising way to observe the structure of E-region FAIs in more detail to study the physical mechanism behind the formation and evolution of ionospheric E-region irregularities.

E-Region Field-Aligned Irregularities in the Middle of a Solar Eclipse Observed by a Bistatic Radar

The Wuhan Ionospheric Oblique Backscatter Sounding System (WIOBSS) was applied as a bistatic radar to record the ionospheric E-region responses to a solar eclipse on 22 July 2009. The transmitter was located in Wuhan and the receiver was located in Huaian. The receiver observed anomalous echoes with larger Doppler shifts at the farther ranges compared with the echoes reflected by Es. According to the simulated ray propagation paths of the reflected and scattered waves, we considered that the anomalous echoes were scattered by E-region field-aligned irregularities (FAIs). The locations of the FAIs recorded by the WIOBSS were estimated with the International Geomagnetic Reference Field (IGRF) and the observed propagation parameters. These irregularities occurred at around the eclipse maximum and lasted for ~20–40 min. The steep plasma density gradient induced by the fast drop photo ionization under the lunar shadow was beneficial to the occurrence of gradient drift instability to generate the FAIs. They were different from the gravity wave-induced irregularities occurring in the recovery phase of the solar eclipse.

The First Report on the Afternoon E -Region Plasma Density Irregularities in Middle Latitude

We report, for the first time, the afternoon (i.e., from noon to sunset time) observations of the northern mid-latitude E-region field-aligned irregularities (FAIs) made by the very high frequency (VHF) coherent backscatter radar operated continuously since 29 December 2009 at Daejeon (36.18°N, 127.14°E, 26.7°N dip latitude) in South Korea. We present the statistical characteristics of the mid-latitude afternoon E-region FAIs based on the continuous radar observations. Echo signal-to-noise ratio (SNR) of the afternoon E-region FAIs is found to be as high as 35 dB, mostly occurring around 100–135 km altitudes. Most spectral widths of the afternoon echoes are close to zero, indicating that the irregularities during the afternoon time are not related to turbulent plasma motions. The occurrence of afternoon E-regional FAI is observed with significant seasonal variation, with a maximum in summer and a minimum in winter. Furthermore, to investigate the afternoon E-region FAIs- Sporadic E (Es) relationship, the FAIs have also been compared with Es parameters based on observations made from an ionosonde located at Icheon (37.14°N, 127.54°E, 27.7°N dip latitude), which is 100 km north of Daejeon. The virtual height of Es (h’Es) is mainly in the height range of 105 km to 110 km, which is 5 km to 10 km greater than the bottom of the FAI. There is no relationship between the FAI SNR and the highest frequencies (ftEs) (or blanket frequencies (fbEs)). SNR of FAIs, however, is found to be related well with (ftEs–fbEs).

The possibility of using all-sky meteor radar to observe ionospheric E-region field-aligned irregularities

All-sky meteor radars are primarily used for meteor observations. This paper reports the first observations of ionospheric E-region field-aligned irregularities (FAIs) from a conventional all-sky meteor radar located at Wuhan (31°N, 114°E) for the period of March–June 2018. E-region FAI echoes evident in range-time intensity (RTI) maps show quasiperiodic striations with positive and negative slopes, which are consistent with multiple FAI structures moving across the wide beam of the meteor radar. A statistical analysis shows that out of a total of 111 d, there are 73 d with E-region FAI echoes detected by the meteor radar. The FAI events correspond well with the presence of sporadic-E layers which provide the necessary plasma density gradient for the development of gradient drift instability producing FAIs. The results demonstrate the capability of conventional meteor radars to make simultaneous routine observations of meteors and ionospheric E-region FAIs through incorporating RTI and spectral analysis into the online realtime data processing. Meteor radar observations could potentially address the limitations of ionospheric radars, which cannot provide simultaneous measurements of neutral winds and irregularity structures, and thereby contribute to better understanding of the dynamical processes producing E-region irregularities.

Investigation on the Occurrence of Mid-Latitude E-Region Irregularity by Wuhan VHF Radar and Its Relationship With Sporadic E layer

By using the 2015–2016 data set of newly built Wuhan very high-frequency (VHF) radar, we investigate the occurrence of E-region field-aligned irregularities (FAIs) in mid-latitude China region for the first time. Wuhan E-region FAIs present the strongest occurrence of quasi-periodic (QP) echoes at postsunset period in the height range of 100–120 km and moderate occurrence of continuous echoes at postsunrise period in the height range of 90–100 km. Characteristics including temporal and altitudinal distribution, Doppler spectra and period of QP echoes are presented from a statistical point of view. We find that E-region FAIs observed by Wuhan VHF radar show strong linkage with the concurrent sporadic E layer observed by ionosonde. In general, Doppler spectra present type 2 feature with velocity in the range of ±50 m/s and width less than 50 m/s. Our results also indicate that mid-latitude E-region FAIs are closely associated with Es layer. Several mechanisms were proposed to interpret the mid-latitude E-region FAIs; however, further investigation is required to understand the generation with different ionospheric background and irregularity types.

Case study of simultaneous observations of sporadic sodium layer, E-region field-aligned irregularities and sporadic E layer at low latitude of China

Using the Na lidar at Haikou (20.0°N, 110.3°E), the VHF coherent radar and the digital ionosonde both at Sanya (18.4°N, 109.6°E), cases of simultaneous observations of sporadic sodium layer (SSL), E-region field-aligned irregularities (FAI) and sporadic E layer (Es) in the mesosphere and lower thermosphere (MLT) region at low latitude of China are studied. It is found that SSL occurs simultaneously or follows the enhancement of Es and FAI. The Es, FAI and SSL descend slowly with time which is mostly controlled by the diurnal tide (DT). Besides, the interaction of gravity wave (GW) with tides can cause oscillations in FAI and SSL. Our observations support the neutralization of ions for SSL formation: when the metallic ions layer descents to the altitudes where models predict, the sodium ions convert rapidly to atomic Na that may form an SSL event. Moreover, the SSL peak density will increase (decrease) in the convergence (divergence) vertical shear region of zonal wind.

Range Imaging of E-Region Field-Aligned Irregularities by Using a Multifrequency Technique: Validation and Initial Results

This paper reports the first use of a multifrequency range imaging (RIM) technique for observing E-region field-aligned irregularities (FAIs) in the midlatitude ionosphere. The Middle and Upper atmosphere Radar (MUR; 34.85°N, 136.10°E) was used to conduct experiments with five equally spaced frequencies between 46.25 and 46.75 MHz. Three types of RIM data were examined: data with 13-element binary Barker codes, with 16-element binary complementary codes, and without phase codes. Moreover, two calibration approaches were adopted to validate the applicability of the RIM technique, which functioned as intended. Excellent RIM performance such as the ability to resolve several striations in an echo region of FAIs was demonstrated. However, sidelobe echoes caused by pulse coding mechanisms were occasionally observed at altitudes above and below the source regions in the coded data. Therefore, a procedure was developed according to one of the calibration approaches to identify and remove such kind of sidelobe echoes, which was shown to be applicable for the complementary-coded data. In addition to FAIs, a thin plasma layer with a thickness of approximately 1 km was identified as being structured with some tilted finer structures, which could not be observed in the original intensity images with a range resolution of 600 m. Preliminary estimates of the Doppler velocities indicated that a wind shear effect could be the cause of such tilted finer structures.

First simultaneous lidar observations of sodium layers and VHF radar observations of E-region field-aligned irregularities at the low-latitude station Gadanki

Abstract. Simultaneous observations of atmospheric sodium (Na) made by a resonance lidar and E-region field-aligned-irregularities (FAI) made by the Indian MST radar, both located at Gadanki (13.5° N, 79.2° E) and horizontal winds acquired by a SKiYMET meteor radar at Trivandrum (8.5° N, 77° E) are used to investigate the relationship among sodium layer, FAI and neutral winds in the mesosphere and lower thermosphere region. The altitudes and descent rates of higher altitude (~95 km) Na layer and FAI agree quite well. The descending structures of the higher altitude Na layer and FAI are found to be closely related to the diurnal tidal phase structure in zonal winds observed over Trivandrum. At lower altitudes, the descent rate of FAI is larger than that of Na layer and zonal tidal phase. These observations support the hypothesis that the metallic ion layers are formed by the zonal wind shear associated with tidal winds and subsequently get neutralized to manifest in the form of descending Na layers. The descending FAI echoing layers are manifestation of the instabilities setting in on the ionization layer. In the present observations, the altitudes of occurrence of Na layer and FAI echoes being low, we surmise that it is quite possible that the FAI echoes are due to the descent of already formed irregularities at higher altitudes.

Low latitude E-region irregularities studied using Gadanki radar, ionosonde and in situ measured electron density

In this paper, we attempt to identify the driving forces responsible for the generation of low-latitude E-region field-aligned irregularities (FAIs). It is evident that the low-latitude E-region FAIs occur both during the day and night with preferential occurrence being most during local sunrise period. Simultaneous measurements are made with Gadanki radar and nearby located Ionosonde for understanding the low-latitude Es–FAIs relationship. The observations suggest that the occurrence and SNR of FAIs have a close relationship with ftEs−−fbEs. Finally, using the past electron density profiles and reasonable values of electric field, we have shown that it is generally difficult with the gradient drift instability to explain low-latitude E-region FAIs when electric field alone is considered as driving agency. We hypothesize that neutral winds play important role for the generation of irregularities at low latitudes outside the electrojet belt either by forming sharp Es layers or by enhancing the differential drift of electrons and ions or by both.

Radar observations of artificial E-region field-aligned irregularities

Abstract. Artificial E region field aligned plasma density irregularities (FAIs) were generated using HAARP in four different experimental modes and observed with a coherent scatter radar imager located 450 km to the southwest where it could detect field-aligned backscatter. The experiments were conducted in July of 2008, during the Polar Aeronomy and Radio Science Summer School (PARS), during quiet conditions in the daytime when the E layer was dense and absorption was modest. The echoes observed during zenith and magnetic zenith heating experiments were deflected from their nominally anticipated horizontal positions toward the midpoint position. The occurrence of hysteresis when heating with amplitude modulated pulses implied the development of the resonance instability, although the threshold for the onset of instability appeared to be higher than what has been predicted theoretically. Heating experiments involving pump frequencies slightly above and below the second electron gyroharmonic frequency produced no significant differences in the observed echoes. Finally, heating with a pump frequency slightly above the E region critical frequency appears to have produced FAIs at two distinct altitudes where the upper-hybrid resonance condition could be satisfied.

Statistical characteristics of VHF radar observations of low latitude E-region field-aligned irregularities over Gadanki

The statistical characteristics of E-region field-aligned irregularities observed with the Gadanki MST radar are presented and compared with observations made with the Piura radar at a similar magnetic latitude. Echoes are observed day and night, with the nighttime echoes being more intense and covering a greater height extent than during the day. The most probable echo occurrence times are just after sunrise (95%) and just before sunset (80%), when the echoing regions were centered around 95 and 100 km, respectively. The lowest probability of echo occurrence is found to be during noontime hours. The spectra of these echoes over Gadanki were all type 2. The mean Doppler velocities for altitudes above 105 km were upward at typically 10 m s - 1 during the day and downward at about 10 m s - 1 during the night. Spectral widths of 10– 40 m s - 1 were found during the day and 30– 60 m s - 1 during the night. These characteristics of E-region irregularities over Gadanki are quite similar to those found over Piura, and are remarkably different from those at either equatorial or higher latitudes.

A role of neutral motions in formation of midlatitude E‐region field‐aligned irregularities

To explain observations of the E‐region 3.2‐m and 6.1‐m backscatter measured by the MU and FAR radars over Japan we propose that a source for generation of field‐aligned irregularities (FAI) is neutral gas motions operating over a wide altitude range. Due to an altitude dependence of ionospheric parameters these neutral motions may result in different dominating processes at different altitudes. On the basis of recent theories of neutral wind‐driven gradient drift and thermal instabilities [Kagan and Kelley, 1998; 1999] we show that thermal processes may both support the gradient drift instability and be responsible for sporadic‐E independent backscatter. In this scenario the time of FAI occurrence seems to follow a tidal mode behavior. The nighttime echo occurrence is most probably due to slower recombination.

Preliminary results from joint measurements of E-region field-aligned irregularities using the MU radar and the frequency-agile radar

Abstract In the period from 26 May to 26 July 1993, we have conducted a joint experiment of field-aligned irregularities (FAI) in the mid-latitude E-region by using the MU radar (MUR) and the Frequency-Agile radar (FAR). While the MUR in Shigaraki was utilized to conduct multi-beam observations of FAI, the FAR was operated in Taga, which is approximately 40 km north of the MUR site, to reveal the horizontal structure of the FAI echoes. The simultaneous observations with both radars were successful for 17 days. The multi-beam experiments with the MUR showed that the ‘quasi-periodic’ echoes had a monochromatic wave structure propagating westward. Moreover, the comparison of echoing regions between both radars showed that the wavefront of the FAI echoes aligned from northwest to southeast, and propagated toward southwest or west-southwest. The full-correlation technique was applied to cross-correlation functions between echo intensities from both radars to give an averaged motion of the FAI echoing regions, which showed similar results to the one-by-one comparison of the FAI echoing regions.

A morphological study on mid-latitude E-region field-aligned irregularities observed with the MU radar

The MU radar located at Shigaraki, Japan (34.9°N, 136.1°E) can observe field-aligned irregularities in the E-region by steering its antenna beam perpendicular to the geomagnetic field. First, this paper investigates daily and seasonal variations of occurrence frequency of echoes from the irregularities. The irregularities were most frequently observed in June, near the summer solstice; they appeared in the periods after sunset and sunrise of the E-region, and the variance of the signal intensity was larger for the echoes in the post-sunset period than those in the post-sunrise period. The frequency occurrence showed similar behavior both in June 1989 and June 1990. In November and December, near the winter solstice, the irregularities were least active in all seasons, and the echoes appear earlier in time of day. The limited number of data in October showed fairly active irregularities though they were less active than in summer months. Second, by using the data on 20–23 June 1990, the time-height distributions of the echoes were compared with the activity of sporadic- E layers observed by the ionosonde at the MU radar site. It was shown that the activity of the irregularity echoes was associated with that of the sporadic- E layers in the post-sunset period. In the post-sunrise period, on the other hand, the echoes did not show clear correlation with the sporadic- E layer. Although the sporadic- E layers were very active in the daytime, no irregularity echoes were found by the MU radar in the afternoon. Finally, as for other features of the irregularity echoes, it was found on 25 October 1989 that echo power showed fluctuations with a short period of 2–3 min, which was much shorter than the Brunt-Väisälä period in the E-region.