A statistical study of satellite traces and evolution of equatorial spread F
© Narayanan et al.; licensee Springer. 2014
Received: 30 April 2014
Accepted: 14 November 2014
Published: 3 December 2014
The ionosonde observations made at 5-min intervals at the Indian dip equatorial station Tirunelveli (8.7°N, 77.8°E geographic; 1.1°N dip latitude) from March 2008 to February 2009 during the extended solar minimum period are used to study the interlink between equatorial spread F (ESF) and satellite traces (STs) which are assumed to represent tilts in the bottomside iso-electron density surfaces probably caused by large-scale wave-like structures (LSWS). The data show different patterns of ESF onset in the bottomside F region, which are illustrated through examples. In addition, the statistics of occurrence of ST and its relation to the formation of ESF are studied. The results indicate that (1) the zonally drifting ESF irregularities can be differentiated from those forming over the observing station. (2) Nearly half of the ESF events were preceded by ST. (3) In about 30% of the cases of occurrence of ST, ESF was not formed afterwards implying that LSWS may not always lead to ESF. (4) The percentage of ESF following ST was high in summer and increased with the time of the night. (5) Following the first occurrence of ST, the ESF onset was delayed by about 30 min on the average suggesting that ST may be used as a precursor of ESF. (6) Pre-reversal enhancement (PRE) of upward plasma drift was found insignificant during the period of study. The trapping of high-frequency radio waves between the E and F regions during intense sporadic E is also illustrated.
The spreading of the F region trace in nighttime ionograms at low latitudes was first noticed in 1930s (Booker and Wells ). Since then, different aspects of the phenomenon known as equatorial spread F (ESF) have been studied using ionosondes, VHF radars, airglow observations, VHF and GPS scintillations, in situ rocket and satellite measurements, GPS-TEC map, etc. (e.g., Woodman and LaHoz ; Rastogi ; Weber et al. ; Basu and Basu ; Prakash et al. ; Aggson et al. ; Valladares et al. ; Park et al. ). Those experimental observations and theoretical investigations (e.g., Ossakow and Chaturvedi ; Zalesak et al. ; Kelley and Maruyama ; Sekar et al. ; Huba et al. ) together seem to establish that the Rayleigh-Taylor instability (RTI) is the causative mechanism behind the formation of ESF.
Under favorable conditions in the evening/nighttime ionosphere, bottomside density perturbations lead to the growth of magnetic field-aligned plasma-depleted regions through the RTI mechanism. The plasma-depleted regions known as plasma bubbles often extend well into the topside ionosphere (Woodman and LaHoz ). The plasma bubbles host different scale sizes of electron density irregularities that reflect radio waves in a random manner hindering trans-ionospheric communications. The phenomena have therefore been studied widely with a view to reach a prediction capability. The seasonal dependence of ESF and plasma bubble occurrence is reasonably well understood based on the alignment of solar terminator with the magnetic meridian and trans-equatorial meridional winds (Abdu et al. ; Maruyama and Matuura ; Tsunoda ), though their day-to-day variability is not yet understood.
The generation of plasma bubbles requires favorable background ionospheric conditions and a seed perturbation triggering the instabilities. Earlier studies indicate that favorable background ionospheric condition is brought about by the pre-reversal enhancement (PRE)-induced height rise of the ionosphere (Farley et al. ; Abdu et al. ; Jayachandran et al. ; Fejer et al. ). Gravity waves are believed to provide the seed perturbations (Kelley et al. ; Balachandran Nair et al. ; Abdu et al. ; Takahashi et al. ; Tsunoda ; Narayanan et al. ; Patra et al. ), though other theories also discuss the generation of the irregularities (e.g., Kudeki and Bhattacharyya ). Another important likely source for the seeding discussed in recent years is the large-scale wave-like structures (LSWS) detected by azimuthal scanning VHF radars (Tsunoda , ), TEC measurements (Thampi et al. ), and airglow images (Narayanan et al. ). Though the source and nature of LSWS are not understood well, they are recognized as wave-like structures in the bottomside ionosphere with scale sizes of a few hundred kilometers.
The ionograms often reveal the presence of additional traces situated close to the F region trace (1F trace) and its higher order reflections, which are known as satellite traces (STs) (Lyon et al. ; Abdu et al. ). Tsunoda () suggested that the STs are probably signatures of LSWS. In the ionograms obtained at close time intervals, ST often appears before the onset of ESF implying that ST can be regarded as a precursor signature of ESF. In addition, Tsunoda () identified the presence of highly tilted traces in ionograms known as multi-reflected echoes (MRE) and suggested them as probable signatures of larger scale tilts than those that could have led to the formation of ST. More recently, Thampi et al. () and Abdu et al. () studied the relationship between ST and ESF onset.
However, a detailed statistical relationship between ST (or LSWS) and ESF is not yet reported to our knowledge. We present such a study using the ionosonde observations described in the ‘Database’ section. Since the ionosonde probes only the bottomside ionosphere, in this article, by ESF we refer to those bottomside irregularities associated with ESF which caused the spreading of the F region traces observed in the ionograms. The results are presented and discussed in the ‘Results and discussions’ section which contains cases of different types of onset of ESF.
We use the data from the ionospheric sounding experiment made with a Canadian Advanced Digital Ionosonde (CADI) installed at the Indian dip equatorial station, Tirunelveli (8.7°N, 77.8°E geographic; 1.1°N dip latitude). The observations were made at 5-min intervals from 19 March 2008 to 12 February 2009 during the extended solar minimum period when magnetic activity was generally quiet (AP <15). However, there were some data gaps during 17 October to 10 November 2008, 9 to 11 December 2008, and 17 to 19 December 2008. The ionosonde comprises a delta-type dipole transmitter antenna with a peak power of 600 W. The four center-fed dipole receivers are arranged in a square configuration to receive the reflected echoes from the ionosphere. The ionosonde was swept in the frequency range from 2 to 16 MHz in 295 steps. Though the ionograms were noisy in this site, the F region traces were clear enough for the study. Since this work discusses nighttime phenomena, we note the nights using dates of pre-midnight and post-midnight hours. For example, the night of 1 April 2008 will be noted as 1/2 April 2008. Further, all the data are presented in Indian Standard Time (IST) which is 19 min behind the local solar time at Tirunelveli (for example, 18:00 IST = 17:41 LST).
Results and discussion
Examples of onset of ESF with and without ST
When the patch of ESF was first seen at 22:35 IST in Figure 5, it was situated between 6- and 7-MHz frequencies while the critical frequency of overhead ionosphere was around 5.5 MHz (see Figure 5). As the ESF patch moved to the overhead sky and continued to approach the 1F trace, its maximum frequency did not decrease but rather stayed nearly stable at approximately 7 MHz. This illustrates that the maximum frequency within the spread F is dependent on the electron densities in the region of irregularities and it does not decrease like the virtual range when the irregularities move towards the overhead sky. In a uniformly stratified ionosphere (within the area covered by ionosonde beam), the electron content can increase in oblique ray paths, but the electron densities will be the same as that for a vertical incidence ray path. Since the plasma frequency is proportional to the electron density, oblique reflections do not show any pseudo enhancement or reduction in the plasma frequency.
However, since plasma density depletions correspond to ESF irregularities, the frequencies of ESF echoes are not expected to be higher than the critical frequency of the F region ionosphere. Recently, Abdu et al. () discussed the mechanisms of radio wave echoes due to ESF irregularities and concluded that a significant portion arises out of coherent backscattering perpendicular to the magnetic field lines in addition to the total reflections. On the other hand, previous studies indicate that total reflection mechanism explains the observed ionogram patterns even during ESF times (King ; Wright et al. ). Further, earlier studies have shown the existence of enhanced electron density regions known as plasma blobs often situated around the electron density depletions (Le et al. ; Park et al. ; Pimenta et al. ). Since ionosonde beam covers a large area, such enhanced electron density regions, if present between adjacent depletions, will also be noticed in the ionograms. We believe that the ESF reflections at frequencies higher than the critical frequency of overhead ionosphere might be due to the presence of plasma blobs in between adjacent plasma depletions hosting the electron density irregularities.
Example of ST without ESF
The examples shown above illustrated the development of ESF with or without ST. Recently, Abdu et al. () studied the relationship between the ST and ESF using the digisonde measurements over the dip equator and two magnetically conjugate locations in the Brazilian sector. In their study, STs were observed prior to the onset of ESF over the dip equator, and later, STs were detected in the conjugate sites nearly simultaneously prior to the occurrence of ESF. With such observations during 66 days of solar maximum period, Abdu et al. () suggested that ST might represent early phase of evolution of ESF. However, studies by Narayanan et al. () and Li et al. () have shown cases in which STs were not followed by ESF. The following example illustrates such an observation.
Other signatures of LSWS
Statistics of ST and ESF
STs were often observed in more than one ionogram at 5-min intervals. All ST occurrences within 2 h are considered as one ST event. The ST signatures separated in excess of 2 h are considered as separate ST events. Similarly, we presume that the ST and ESF are related only if the ESF is observed within 2 h of the last observation of ST. If the time gap was more than 2 h, the ST and ESF are considered to be unrelated. Time gap of up to 2 h seems sufficient for the LSWS to trigger ESF.
In addition to the abovementioned criteria, we consider only those STs occurring near the 1F trace and well below the 2F trace. To study the role of LSWS on the generation of ESF, the STs are first identified and those ESFs that follow the STs are identified next. This makes the statistics more robust. Further, we consider only clear STs that are not affected by a spread. Sometimes, STs are observed with a spread (Lynn et al. ) which indicates the presence of ESF irregularities at off-zenith locations probably approaching the zenith of the observation site as discussed earlier (refer to Figure 5). The presence of ST without any spread indicates bottomside tilts associated with LSWS, as mentioned above. In this study, we make an attempt to understand the role of such tilts and how often do they lead to the formation of ESF.
Figure 10b shows the seasonal occurrences of ESF and ST along with the number of events in which ESF followed ST. The percentage occurrences of ESF following ST was small during spring equinox (58%) and winter (66%) compared to other seasons (76% and 75% during fall equinox and summer, respectively). This pattern is similar to the occurrence rates of ESF and ST separately. Further, the observations are separated into three time bins corresponding to 18 to 22 IST, 22 to 02 IST, and 02 to 06 IST as shown in Figure 10c. A significant number of ST occurrences are observed later in the night indicating that LSWS does not always occur around the post-sunset vertical motions associated with PRE. Figure 10c shows that the percentage of ESF following the ST increases with time. This indicates that LSWS plays comparatively a greater role in triggering ESF later in time when the ionosphere is less dense. It may be seen from Figure 10c that the number of events was very less during 02 to 06 IST. This observation would have instrumental bias in that the F region electron density is very much reduced resulting in smaller foF2 values below the lower frequency limit of the ionosonde.
Figure 11b shows the time delays (i) between the first occurrence of ST and the first occurrence of ESF (red dots) and (ii) between the last occurrence of ST and the first occurrence of ESF (black dots). On the average, the ESF occurred about 44 min after the first occurrence of ST, with a median value of approximately 30 min. This implies that ST can be used as a potential precursor signature, with an average warning time of 30 min. Our results (Figure 11b) indicate that on about half of the cases ESF was observed immediately following the last occurrence of ST, while there is a time delay between the last occurrence of ST and the formation of ESF in the remaining cases.
It may be rightly argued that the formation of ESF also depends on several other parameters such as PRE. Recent satellite observations show that the vertical drift due to PRE is generally absent at satellite altitudes during the extended solar minimum period of the present study (Stoneback et al. ). A detailed investigation on the intensity of PRE (if present), altitudes, and electron density gradients in the bottomside ionosphere is underway. Preliminary results relevant to the days with the occurrence of STs are discussed in the next section.
Role of PRE on the days with ST
The relationship between the occurrence of STs in ionograms and the formation of ESF is studied using the ionosonde observations made at 5-min intervals from an Indian dip equatorial station from March 2008 to February 2009 during the extended solar minimum conditions. The results reveal the following: (1) The zonally drifting ESF irregularities can be differentiated from those forming over the observing station. (2) Plasma blobs existing around the plasma depletions may be inferred from ionosonde measurements. (3) About 50% of the observed ESF events were preceded by ST. (4) ST occurred at later hours of the night as well implying that PRE is not the cause of ST at these hours. (5) STs were not followed by ESF in about 30% of the cases indicating that LSWS does not trigger ESF in all occasions. (6) About 70% of the STs were followed by ESF, and the percentage was high in summer solstice and during later hours of the night. (7) Following the occurrence of ST, the ESF onset was delayed by about 30 min on the average, indicating that ST may be used as a precursor of ESF. (8) PRE was almost non-existent in the Indian sector on most of the nights during the extended solar minimum.
The lead author V. L. N. acknowledges the support from the National Institute of Information and Communications Technology, Japan under its International Exchange Program. The observations presented in this work were carried out by the Indian Institute of Geomagnetism with the support from the Department of Science and Technology, Government of India.
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