Study of the Long-Lasting Daytime Field-Aligned Irregularities in the Low-Latitude F-Region on 13 June 2022

Abstract

The unusual daytime F-region Field-Aligned Irregularities (FAIs) were observed by the HCOPAR and the satellites at low latitudes on 13 June 2022. These irregularities survived from night-time to the following afternoon at 15:00 LT. During daytime, they appeared as fossil structures with low Doppler velocities and narrow spectral widths. These characteristics indicated that they drifted along the magnetic field lines without apparent zonal velocity to low latitudes. Combining the observations of the ICON satellite and the Hainan Digisonde, we derived the movement trails of these daytime irregularities. We attributed their generation to the rapid ascent of the F-layer due to the fluctuation of IMF Bz during the quiet geomagnetic conditions. Subsequently, the influence of the substorm on the low-latitude ionosphere was investigated and simulated. The substorm caused the intense Joule heating that enhanced the southward neutral winds, carrying the neutral compositional disturbances to low latitudes and resulting in a negative storm effect in Southeast Asia. The negative storm formed a low-density circumstance and slowed the dissipation of the daytime FAIs. These results may provide new insights into the generation of post-midnight irregularities and their relationship with daytime fossil structures.

1. Introduction

Equatorial Plasma Bubbles (EPBs), known as Field-Aligned Irregularities (FAIs), have been considered as night-time phenomena. These low-density structures frequently occur in the equatorial ionospheric F-region and are important space weather topics. They have been widely investigated owing to their significant influence on satellite navigation and communication. It has been widely accepted that EPBs are generated through the generalized Rayleigh–Taylor (R-T) instability at the F-region bottom side, where a steep gradient of vertical electron density exists after sunset [1]. The growth of R-T instability could be increased via elevating the ionosphere to a higher altitude [2]. For example, the eastward Pre-Reversal Enhancement (PRE) and storm-time disturbed electric fields can cause ionospheric uplift via E × B drift [3,4,5]. And the thermospheric winds can similarly drive upward motion of ions near the equator through ion drag forces [6,7,8]. When some initial perturbations occur, the EPBs will be triggered by the generalized R-T instability and rapidly evolve to topside ionosphere via polarization electric fields within them [8,9,10,11]. As a result of the high conductivity along the geomagnetic field lines, the EPBs could extend to low, even middle latitudes [12,13].

After sunrise, most of these irregularities decay rapidly due to the processes of plasma refilling, and some of them still survive on the dayside and drift with the ambient plasma [14]. Using the Very High Frequency (VHF) radar over Jicamarca, Woodman et al. [15] and Chau and Woodman [16] analyzed these daytime FAIs and pointed out that the echoes of these irregularities featured small Doppler velocities (close to zero) and narrow Doppler spectral widths. Sekar et al. [17] also observed these inactive irregularities and named them “fossil bubbles”. He et al. [18] also reported the spatial distribution of daytime irregularities, suggesting that they were narrow band-like structures and extended hundreds of kilometers vertically according to Imaging Radar observations. The daytime irregularities could be divided into two types. One is the freshly generated irregularities that are usually as active as the night-time irregularities and induced by a variety of perturbations like magnetic storms and Traveling Ionosphere Disturbances (TIDs) [19,20]. Another is the fossil structures identified as the remnants of EPBs created on previous nights. Huang et al. [21] traced the movements of long-lasting daytime EPBs and proposed that the high altitudes may be essential for the long duration of bubbles. Later, Chen et al. [22] reported two daytime irregularities observed on geomagnetically quiet days and deduced that they were the remnants of EPBs that occurred on the previous night. Kil et al. [23,24] analyzed this phenomenon and emphasized the importance of the ionospheric fountain process in redistributing daytime irregularities. Xie et al. [25] connected the weak echoes observed by radar at daytime with irregularities and deduced the source of these irregularities. Gao et al. [26] studied the impact of non-storm-time substorms on the generation of irregularities and reveled that the disturbance dynamo electric fields denominate the generation (suppression) of post-midnight (post-sunset) irregularities during a substorm.

In this paper, we investigate unusual daytime FAIs observed by a VHF radar system and satellites around Southeast Asia, which survived into the afternoon. Combined with the density and vertical velocity of ions, the movement of these irregularities has been studied. We also considered the influence of the Interplanetary Magnetic Field (IMF) Bz component on the equatorial electric field and deduced the generation of these irregularities. In addition, based on the observations of the ICON satellite and ground-based receivers, the reasons for the long duration of these daytime irregularities have been reported and discussed. These results will deepen our understanding of the internal relationship between these daytime irregularities and the background ionosphere.

2. Observation Instruments and Data Processing

The Hainan Coherent Scatter Phased Array Radar (HCOAPR) at Fuke (19.5°N, 109.1°E), supported by the Chinese Meridian Project [27], is a VHF radar used to investigate FAIs at the 3 m scale, with operating frequencies of 47 MHz and peak powers of 54 kW. Its antenna array consists of 72 linearly polarized Yagi antennas arranged in an 18 × 4 matrix, forming seven beams distributed between −22.5° and 22.5° at ~7.5° intervals, with the central beam (Beam 4) pointing towards geographical north. A more detailed description of the HCOPAR can be found in [28]. The geographical locations of the HCOAPR and the Hainan Digisonde are marked by the red square in Figure 1. The red solid lines denote the distribution of beams, and the corresponding beam numbers and azimuths are also presented. The altitudes projected onto the longitude and latitude planes at 100 km intervals are shown around Beam 1.

The slant Total Electron Content (TEC) is derived from the Global Navigation Satellite System (GNSS) measurements from the Crustal Movement Observation Network of China (CMONOC) and the Ionospheric Observational Network for Irregularity and Scintillation in East/Southeast Asia (IONISE) (more information can be accessed in [29,30]). The vertical TEC has been calculated with the help of the GNSS research center at Wuhan University. The time resolution is 30 s, and only data with an elevation angle larger than 30° are considered to mitigate multipath effects in this study. The vertical TEC (vTEC) is converted from the slant TEC (sTEC) by the thin-layer model (350 km in this study), which assumes the ionospheric electron densities concentrate on a spherical shell. The sTEC is measured from pseudoranges and carrier phase measurements of two L-band frequency GNSS receivers. Only vTEC is used in this study. Therefore, the “TEC” refers to the “vTEC” when mentioned here.

Launched on 11 October 2019, NASA’s Ionospheric Connection Explorer (ICON) satellite has an inclination of 27° with respect to the equator. The satellite is in a nearly circular orbit at a typical altitude of ~580 km, and its onboard Ion Velocity Meter (IVM) can obtain the ion parameters at a sampling rate of 1 Hz [31]. In the present study, the ion density and velocity data were used. Kil et al. [23] estimated the daytime irregularities by the parameter S, defined as

$$\mathrm{S}=\sqrt{{\displaystyle \frac{1}{\mathrm{n}-1}}\sum _{\mathrm{i}=0}^{\mathrm{n}-1}{({\log }_{10}\mathrm{N}\mathrm{i}-\mathrm{L}\mathrm{i})}^2}$$

(1)

Here, N_i is the ion density measured by the ICON IVM, L_i is the result ${\log }_{10}\mathrm{N}\mathrm{i}$ after liner fitting. n is the data point and has been chosen for 10 in this study. The logarithm of the density can be used to minimize the dependence of irregularity detection on ion density. The value of 0.002 is employed to identify daytime (06:00–18:00 LT) irregularity as thresholds of S [32]. In addition, we can also use the Michelson Interferometer for Global High-Resolution Thermospheric Imaging (MIGHTI) instrument to measure the thermospheric neutral wind [33].

The Thermosphere, Ionosphere, Mesosphere Energetics, and Dynamics (TIMED) satellite launched in 2001 is in a 630 km circular polar orbit with an inclination of 74.1°. The Global Ultraviolet Imager (GUVI) is one of the instruments that constitute the TIMED satellite to investigate the spectral radiance of the Earth’s Far Ultraviolet (FUV) airglow in the spectral region from 120 to 180 nm using a cross-track scanning spectrometer design [34]. The column density ratio of O to N₂ (∑O/N₂) is deduced from O (135.6) and LBHS channel data for disk viewing [34,35]. It can be used to demonstrate the variations in the neutral composition and its effect on the ionosphere.

3. Observation Results

3.1. Geomagnetic Conditions

Figure 2 illustrates the geomagnetic condition on 12–13 June 2022. As shown in the yellow area of Figure 2a, an abrupt increase in IMF Bz appeared at 18:53 UT with the value of 12.53 nT, when the Kp index was 2 − The northward turning of the IMF Bz could result in an “overshielding” phenomenon [36,37,38]. The dawnward shielding electric field may have penetrated the mid- and low-latitude ionosphere and remained eastward on the nightside [39]. The variations in the equatorial electric field (EEF) proposed by [40] and the vertical velocity of the F-region ions around 110°E could confirm the influence of these penetrating electric fields at low and equatorial latitudes. The drastic increase in IMF Bz caused the first enhancement of eastward EEF and the upward motion of the F-region several minutes later with the value of 29.5 m/s by E × B drift (Figure 2e,f). Then, the fluctuation of IMF Bz during 19:08–20:04 UT seemed to cause a second enhancement of EEF and upward vertical velocity with a maximum of 56.93 m/s at 21:21 UT. Although the maximum of the second eastward EEF was smaller than that of the first, the closer time interval between the two enhancements and the possibility of the lifting effect being superimposed on the low-latitude F-layer may have resulted in the upward vertical velocity of the second being greater than that of the first. It is worth noting that both enhancements were very short-lived, corresponding to the change in value of IMF Bz over a short period. After that, the vertical velocity of the F-region decreased rapidly, and the EEF turned negative when the IMF Bz turned southward at 21:00 UT. The plausible interpretation is that the southward turning of the IMF Bz created a westward “undershielding” penetration electric field to low latitudes at night, resulting in the negative EEF and larger downward motion of plasma in several minutes [41]. Then, the IMF Bz turned southward after rapidly rising at ~01:00 UT on 13 June, and the Kp index also increased. At the same time, the SuperMAG Auroral Electrojet index started to increase [42,43]. They exceeded 1000 nT after two hours, which indicated that a substorm occurred before the daytime irregularities observed by the HCOPAR (gray block). It triggered intense Joule heating on both hemispheres, as indicated by the enhancement of the Polar Cap (PC) index shown in Figure 2d [44]. The increase in the PC index was almost simultaneous with the SME index and rapidly reached 8.71 mV/m at 03:24 UT. Both indices gradually declined after a brief enhancement period and continued fluctuating throughout the day.

3.2. HCOPAR Observations

During the daytime on 13 June 2022, an unusual ionospheric FAI event was observed by the HCOPAR. Figure 3 depicts the Signal-to-Noise Ratio (SNR) and Doppler velocities of these daytime FAIs in the left and right columns, respectively. The daytime irregularities’ echoes occurred around 11:00 LT (04:00 UT, UT = LT + 7 h) in Beams 1–3 and 6. Furthermore, several minutes later, the other three beams also observed more dispersed echoes with exact altitudes (~500–600 km), indicating that they had suffered more severe dissipation before entering the Field of View (FoV) of the HCOPAR. These echoes appeared at different times in different beams, possibly due to the zonal inhomogeneities of the same irregularities. The second echoes occurred at around 12:00 LT (05:00 UT) above ~440 km, with stronger SNRs, especially in Figure 3d–f, than the first echoes and similar patterns in all beams. Continuously, the third echo moved into Beams 4–6 at ~12:48 LT (05:48 UT) and was distributed over 430–600 km. It only lasted 38 min and was connected with the front part of the echoes in Figure 3e,f, which could be judged as the second irregularity due to its zonal inhomogeneities. A similar scenario also occurred in the fourth echo. They appeared at ~13:30 LT and connected with the fifth echo in Beams 5–6. We speculate that this phenomenon may be due to the close onset of these FAIs and their continuous dissipation after sunrise, resulting in the preservation of only some of the heavily depleted portions. For the fifth echo, they first emerged at ~14:08 LT, as judged by Figure 3a–c, and were different from the previous irregularities, as their altitude distribution extended to around 400 km. Hu et al. [45] also reported unusual daytime irregularities detected by the HCOPAR. The observations also indicate that the latter parts of irregularities are stronger and extend to a broader altitude than the front parts. They suggest that this is because the daytime irregularities suffer more severe dissipation at the front edge during the drift, thus protecting the latter part, which is a plausible interpretation of the phenomenon shown in Figure 3.

The Doppler velocity and spectral width were also calculated and shown in Figure 3h–n and Figure 4, respectively. Compared to the large Doppler velocity of typical night-time irregularities [46,47], all these daytime FAIs exhibited low Doppler velocities (~±10 m/s), indicating that these irregularities were moving perpendicular to the beams and along the geomagnetic field lines without apparent zonal drift velocities. The narrow spectral width shown in Figure 4a,b indicates that these irregularities were less turbulent and not in evolutionary stages [22,25]. These features suggest that these echoes were produced by the typical daytime irregularities.

3.3. ICON Observations

Figure 5 demonstrates the simultaneous observations of the ICON satellite of the daytime irregularities and the ion flow velocity along the satellite trajectory. As mentioned in Section 2, the perturbation index S proposed by [23] can be used to identify these daytime irregularities. Values of 0.002 and 0.02 are chosen as the thresholds of S to identify the daytime and night-time irregularities (denoted by the pink dotted line in the middle column), respectively. The depleted regions of ion density are denoted by the red block in the left column and numbers in the middle column of Figure 5. As shown in Figure 5b, a severe depletion (D1) of ion density first occurred around 123°E at 21:19 UT, and the value of S exceeded the night-time threshold to 0.08. Then, the second depletion of ion density (D2) was detected in the equatorial region several minutes later. Unlike the D1, the D2 had a lower value but a broader range in the longitudinal direction. As the satellite moved northward, only the D1 was observed at 120°E in the Northern Hemisphere, which could be interpreted as the phenomenon of bubbles extending from the magnetic equator to the poles. The lower value of D1 in Figure 5e did not represent the weaker irregularities. Then, their altitudes gradually exceeded the constant detection altitude of the ICON satellite over time, which could also be interpreted as the disappearance of these irregularities in Figure 5e,k. Based on their gradual northward and westward movement, the depletion regions generated at predawn were most likely the source of the daytime irregularities on 13 June. The vertical velocity of the ion flows suddenly turned to positive with a maximum of 55 m/s around the longitudes of the HCOPAR after 21:12 UT, which was consistent with the time of enhancement in the vertical velocity of the F-layer observed by the Hainan Digisonde in Figure 2f. A similar upward motion of ions was observed when the ICON satellite passed around 89 to 138°E, as shown in Figure 5f. Then, it stayed positive in Figure 5i,l as the electric field changed eastward after sunrise. The horizontal velocities continued westward after the depletion had occurred.

4. Discussion

Using the observations from the HCOPAR and the ICON satellite, the characteristics and movement of the unusual daytime FAIs that occurred on 13 June 2022 were studied, which helped us to speculate on the origin of these daytime irregularities. In the past years, most irregularities relating to the post-midnight and predawn periods occurred on magnetically disturbed days [48,49]. The International Service of Geomagnetic Indices (https://isgi.unistra.fr/events_qdays.php, accessed on 26 July 2024) classifies Kp as an indicator of quiet days, and on 12 June 2022, Kp was smaller than 3, indicating that the geomagnetic condition was quiet. According to the description of geomagnetic conditions shown in Section 3, the variation in IMF Bz after 18:00 UT led to the disturbance of the equatorial electric field and resulted in the fluctuation of the F-layer’s vertical motion. Based on the newly developed Multiscale Atmosphere–Geospace Environment (MAGE) model, Wu et al. [50] simulated the response of the penetrating electric field in the equatorial region to the variation in the IMF Bz. The results suggested that the equatorial dawn–dusk potential drop could change nearly instantaneously with the variation in IMF Bz, altering the vertical motion of ions. In this study, two peaks in the vertical drift velocities with values of 29.5 m/s and 56.93 m/s coincided with the variations in the EEF and IMF Bz. Moreover, Fejer et al. [51] indicate that the strong irregularities are observed most often and have the longest lifetimes when the peak upward drift velocities reach the value of ~40–45 m/s, which is similar to our results.

However, the statistics on daytime irregularities show that the maximum occurrence rate appeared at around 09:00 LT [25], which is much earlier than the period of irregularities in this study. These results suggest that the event is highly unusual and cannot be explained simply by the ionospheric fountain effect on account of its long duration. Although this may be related to their initial strength, it does not fully interpret how they could survive for almost 7 h in the sunlight. It is worth mentioning that a substorm occurred at 02:00 UT on 13 June and triggered intense Joule heating in both hemispheres, as indicated by the PC indices, which may disturb the thermosphere and transport the neutral composition from high latitudes to the equator and even the opposite hemisphere [52]. It might affect the living environment and the duration of these daytime irregularities. On account of this, we calculated the storm-to-quiet values of the ion density observed by the ICON satellite and depict the distribution of thermospheric ∑O/N₂ measured by the GUVI on board the TIMED in Figure 6. As shown in Figure 6a, a low-ion-density region gradually developed over time, first being observed within 110°–160°E longitudes during 03:55–04:19 UT. As the trajectory of the ICON satellite progressively moved southward, the value of ion density decayed further and eventually reached a minimum in the longitude range of 90°–130°, where the HCOPAR observed daytime irregularities. And in a similar region, the ∑O/N2 decrease was also observed by the GUVI, as shown in Figure 6c. Compared to the previous day (Figure 6b), a significant decrease in ∑O/N2 was observed on 13 June with a value close to 0.4. The low ∑O/N2 region covered the area where the HCOAPR was located and extended south of the magnetic equator.

A rapid southward shift in the IMF Bz can generate the duskward convection electric field that will penetrate into the low-latitude regions with eastward polarity during daytime. Then, when the IMF Bz turns northward after a prolonged southward orientation, it will generate a dawnward shielding electric field that is westward on the dayside [39]. These disturbed electric fields will stimulate [53,54] or inhibit [55,56,57] the generation of FAIs by enhancing or suppressing the ionospheric vertical drift. In this event, it can be deduced that during the geomagnetically quiet time, the fluctuation of the IMF Bz pushes the penetrating electric field to low and equatorial latitudes and leads to variation in EEF, which induces the drastically upward motion of the F-layer and created fresh irregularities by R-T instability at around 21:10 UT. These fresh irregularities quickly reach high altitudes and are captured by the ICON satellite. After sunrise, the ionospheric fountain effect elevated these strong irregularities to a higher altitude, where the photoionization rate was low, and then carries them to higher latitudes and lower altitudes [21,23]. According to the velocities measured by the ICON satellites, the motion of the background F-region ionosphere is westward. Therefore, the irregularities detected by the ICON satellites move westward with the background ionosphere and are observed by the HCOPAR, as simulated by [25]. This process explains most of the daytime irregularities and is widely accepted.

Figure 7 exhibits the zonal and meridional wind observed by the ICON satellite on 12 June and 13 June. As can be seen, the meridional neutral wind observed by Path 5 turned southward at an altitude of about 250 km around 110°E. Its altitude distribution was expanding and the wind speeds were accelerating as the satellite trajectory gradually moved eastward. The contrast between Path 3 and Path 6 is even more significant. On 13 June, the southward neutral winds were firstly observed at 90°E, and the maximum wind speed exceeded 150 m/s. It should be noted that the time of the enhanced equatorward wind onset cannot be precisely determined because the satellite trajectory is constantly changing and cannot provide continuous observations of the same region. This significantly enhanced equatorward neutral wind was observed after the onset of substorm and close to location of the HCOPAR, almost coinciding with the region of low ion density observed by the ICON satellite in Figure 6. The increased PCN index was measured after 03:00 UT and indicated intensified Joule heating in the Northern Hemisphere, which represents the inducement of the enhanced southward neutral winds shown in Figure 7.

The extent to which the ionosphere is affected by substorms is reflected by the changes in TEC values shown in Figure 8. The red dotted lines are the dividing line between the high and low values of TEC. It can be seen that the TEC was already showing a clear westerly high and eastly low at around 02:20 UT. The observations of the ICON satellite shown in Figure 6a also support it. This phenomenon occurred because the effect of Joule heating on the ionosphere has already started with the first increases in the AE and PC indices (shown in Figure 2c,d at 01:10 UT) and has transmitted to the middle and lower latitudes. As time went by, the dividing line gradually moved west–south, and the TEC values west of that also gradually increased. It is worth mentioning that although the TEC was increasing west of the dividing line, its values were still lower than the daytime norm. Moreover, the crest of electrons due to the fountain effect after sunrise that should have appeared east of the dividing line did not appear and was replaced by an environment of low electron density, which could also be attributed to the enhanced southward meridional winds on the electron distribution.

In brief, the substorm caused intense Joule heating and created oxygen-depleted or nitrogen-rich air and enhanced meridional winds indicated by the observations of the ICON satellite and the GUVI [58]. The enhanced southward neutral winds transported these thermospheric compositional disturbances towards low latitudes, resulting in a negative storm effect by increasing the neutral molecular concentration and losing ionospheric ion density [59]. It was concentrated in Southeast Asia, where the ICON satellite and the HCOPAR observed the daytime irregularities. The negative storm formed a low-density circumstance that was conducive to the survival of the daytime irregularities at low altitudes, prolonged their lifetimes by slowing the process of replenishing the depleted fossil structure when they were drifting to higher latitudes, and allowed them to sustain higher SNR and more complete structures even at noon.

5. Conclusions

This study investigates an unusual event of the daytime F-region FAIs observed on 12–13 June 2022. The fluctuation of the IMF Bz generated a penetrating electric field. It influenced the equatorial electric potential to raise the F-layer and generate the irregularities at predawn. These irregularities rapidly extended to higher altitudes and were recorded by the ICON satellite, and then they gradually moved westward with the background ionosphere and extended to higher latitudes. While observed by the HCOPAR in the daytime, they presented low Doppler velocities and narrow spectral widths. Based on the changes in ion density and thermospheric ∑O/N₂ observed by the GUVI, it is speculated that these long-lasting daytime irregularities are related to the substorm, as indicated by the TEC maps and the observation of the ICON MIGHTI. The enhanced southward neutral winds due to the Joule heating carried the thermospheric compositional disturbance to low latitudes and created a low-ion-density environment, which prolonged the duration of the irregularities by slowing the process of refilling the depleted areas. These results suggest that the disturbances of the IMF Bz can penetrate the equatorial region to induce irregularities after midnight, even when the geomagnetic condition is quiet. These observations emphasize that not only gravity waves and geomagnetic storms but also the fluctuation of the IMF Bz could be essential initiators for generating post-midnight irregularities. It should also be noted that the significant influence of the substorm can prolong the lifetime of the daytime irregularities, which is different from the previous reports. It suggests that we should take more factors into account when analyzing the generation and subsequent evolution of the F-layer irregularities, especially the FAIs that occur after midnight and last into daytime.