Occurrence climatology of equatorial plasma bubbles (EPBs) using optical observations over Kolhapur, India during solar cycle-24

In this paper, the occurrence characteristics of the equatorial plasma bubbles (EPBs) using OI 630.0 nm all sky imager (ASI) night airglow observations over Kolhapur (16.8 o N, 74.2 o E, 10.6 o dip. Lat.) during the solar cycle-24 are presented. These results are discussed in terms of season, solar and magnetic activity during years 2011 to 2018. The ASI observations were only carried out during January to May and October to December months due to unfavorable weather conditions. The results suggest that while January, February and December are the only months where EPBs were found to occur over Kolhapur in any year, but the percentage of occurrence of EPBs during these months suggests their low occurrence rate during solar minimum. A total of 683 nights of observations were carried, out of which, 93 nights are found to be magnetically disturbed nights having Ap>18. In addition, the ASI observations are also correlated with Pre-Reversal Enhancement of the vertical drift of the evening sector at Tirunelveli on few storm events for comparison. The important findings of this study are: 1) increase in the occurrence of EPBs with respect to the solar activity; 2) suppression of EPBs on 71 disturbed nights, while enhancement of EPBs on 22 nights under magnetic disturbance; 3) EPBs occurrence during equinox months is found to be higher than winter months during ascending phase of solar cycle-24.; and, 4) EPBs are mostly observed in the pre-midnight sector in the high solar activity (HSA) period, while they are seen in the post-midnight to dawn sector during the low solar activity (LSA) period. We also noticed non-occurrence of EPBs during equinox month in the year 2018 which seems to be peculiar and needs further investigations.


Introduction
The equatorial spread-F (ESF) is a nighttime phenomenon occurring at low and equatorial ionosphere. The optical signatures of the ESF are often named as equatorial plasma bubble (EPB) or the equatorial plasma depletion (EPD). Severe disturbances may result from such EPBs in trans-ionospheric radio propagation at Giga Hertz frequency range affecting the communication and navigation systems. The first observation of this magnetically North-South aligned structures within the low-intensity regions of OI 630.0 nm airglow images during the period of spread-F observations were observed by Weber et al. [1978]. The all-sky images of OI 630.0, 557.7 nm and 777.4 nm emissions were well appreciated with these low intensity regions. These optical signatures of all sky imager shows low electron density patches which are termed as EPBs. Many researchers have studied the different characteristics of EPBs such as zonal drift velocity [e.g. Mukherjee and Shetti 2008, Nade et al., 2013, Sripathi et al., 2016, occurrence [e.g. Sharma et al., 2014, Sharma et al., 2017a, generation mechanism and its morphological features [Sinha and Raizada 2000, Taori et al., 2011, Narayanan et al., 2014, Gurav et al., 2019 over Indian sector. Dabas et al. [2007] have studied the occurrence features of equatorial spread-F irregularities and their latitudinal variation using concurrent instruments such as digital ionosondes located at Trivandrum (8.21 Sharma et al., [2017b] studied occurrence of scintillation recorded in VHF scintillation data for the period of 2011 to 2015 over Kolhapur. They found that, the percentage occurrence of strong scintillations decreases with increase in solar activity. However, the moderate scintillations are found to be increased with solar activity. Also, Kelley et al., [2002], reported the observations of EPBs which reached to altitude of 1500 km over the magnetic equator and mapped magnetically to latitudes well north of Maui,Hawaii (20.71 o N,203.83 o E).
They found that the radio signals from all the Global Positioning System (GPS) satellites were severely disturbed in their field of view, whenever the corresponding line of sight of signals passed through one of the turbulent regions of the ionosphere. An interesting observations by Ray et al., [2003] from Calcutta (32 o N dip lat.) with 1.5 GHz L-band signals reveal that the scintillation occur in patches of duration changing from a few minutes to about several hours (48 hr 55 min). Also, Ray and Das Gupta [2007] reported statistical occurrence of

Monthly averaged percentage occurrence of EPBs
The EPBs percentage occurrence on day-to-day basis is calculated by measuring the ratio of number of images in which EPBs are present to the total number of images logged during that night multiplied by 100. The ASI is usually operated in the months of January, February, March, April, May, October, November and December. The ASI is not operated in the months of June to September due to unfavorable sky conditions.   Figure 3, it is observed that the monthly percentage occurrence is positively correlated (Correlation Coefficient: 0.45) with corresponding solar flux. Mendillo et al., [1992], stated the conditions for growth of EPBs which are: (1) the alignment of the terminator with geomagnetic flux tubes as suggested by Tsunoda [1985]; (2) the lack of a strong trans-equatorial thermospheric wind that might decrease the growth rate as proposed by Maruyama and Matuura [1984]; (3) the quick rise of the F-layer just after sunset and (4) the presence of a seed perturbation. Also, it is well believed that the solar activity has direct impact on the generation of EPBs. Xiong et al. [2010] have investigated the characteristics of EPBs using multi-year data base i.e., 2001-2009 of CHAMP Planar Langmuir Probe (PLP) and GRACE K-Band Ranging (KBR1B) observations measured at the altitudes of CHAMP (300-400 km) and GRACE (∼500 km) respectively. They found that the occurrence seems to be linearly increasing with that of solar-flux at about the similar rate at CHAMP and GRACE. Further, they observed that the EPBs associated with higher/lower apex altitude exhibits strong/weak occurrence rates. In the present study, we observed lesser percentage occurrence of EPBs during the decreasing phase of solar-cycle 24 as can be seen in Figure 3.
Many researchers have investigated the occurrence statistics of EPBs and its generation/inhibition for several decades. Deng et al., [2013], reported the L-band scintillations observed in GPS data available from July 2008 to March 2012 at the northern crest of equatorial anomaly stations in Guangzhou and Shenzhen of South China.
They found that in equinox, scintillation is observed only prior to midnight hours during quiet solar-activity years. Also more scintillation were observed in the pre-midnight hours of both the equinox and the winter months during high solar-activity years. Also, GPS scintillations were noted in the after-midnight hours of summer and winter months. They found that, the period of scintillation patches were mostly less than 60 min and it is extended to about 180 min in low and high solar activity years respectively. Magdaleno et al., [2017], using GPS Onkar B. Gurav et al.
6 Their analysis shows that majority of the EPBs occur at the magnetic equator and in the South America-Africa sector, whereas their occurrence shrinks as the distance from the magnetic equator increases. Further they observed, the period of the TEC depletions and its depth also increases at the equator but they get decreases away from the equator. They also observed maximum number of EPBs during high solar activity. Smith and Heelis [2017], found that post-midnight plasma bubbles appear preferentially at solar minimum with no seasonlongitude dependence. Sun et al., [2016], by making use of ASI setups over Chinese longitude sector presented the statistical analysis of EPBs. They observed the EPBs percentage occurrence was 8.3% in 2013 and 17.2% in 2014. The statistical analysis presented in the current study has findings which are in agreement with the previously reported literature. The important thing to note is that the occurrence varies in both the increasing and decreasing phase of solar-cycle 24 over Kolhapur.

The month wise variation in EPBs occurrence
The . They reported that the probability for EPB growth is observed as maximum in the data of April (45%) and September (83%). Further, a wide maxima of occurrence is observed during June to October (62%). However in present study maximum occurrence is seen in the months of January, February and December from low to high solar activity. Recently, Sharma et al., [2017b] performed statistical analysis of scintillation activity using spaced VHF scintillation monitoring unit over Kolhapur, low latitude Indian region. They also found the maximum scintillation in the equinox months (March-April), moderate in winter months (January-February) and lower in summer months (May-September). The similar results were also observed in the past by various researchers over Indian zone (e.g. Vyas and Dayanandan [2011], Chatterjee and Chakraborty [2013]). So with this, it should be noted that the statistics regarding occurrence shown in this study is in agreement with the previously reported studies over India region.

Hourly seasonal occurrence of EPBs
For seasonal analysis, the months are categorized as Equinox (March-April-September-October), Summer (May-June-July-August) and Winter (January-February-November-December) [Vyas and Dayanandan 2011]. The occurrence is calculated on hourly basis for each month to get the seasonal variation as shown in Figure 5 (a). The black vertical bars depicts the occurrence for equinox months and the red vertical bars depicts the occurrence for winter months.
In the year 2014, the data for the months of January to April is not available. It is observed that the equinox months exhibits greater occurrence as compared to winter months for increasing phase of solar-cycle. However, the occurrence for equinox and winter months in descending phase is do not show any significant pattern. Also, we observed the zero occurrence in equinox months for the solar minimum year 2018. It is obvious that during equinox EPBs are more frequently observed, but here in the month of March 2018 no EPBs were seen. This is mainly because of low solar activity period.
We have analyzed the plots of equinox and winter months for increasing as well as decreasing phase by calculating the Gaussian parameters. Figure 5 (b) indicates the Gaussian fit for equinox and winter for the year 2011. These are only sample plots. The same fitting has been carried out for all the years which is not shown here but the parameters obtained by this analysis are shown in Table 2. The Gaussian fit gives the information about the time span and the peak time of occurrence EPBs throughout the night. From Table 2, it is seen that, the peak time      with the previously reported literature over Indian sector. As we also observed higher occurrence percentage during the equinox period when the solar activity is in increasing phase.

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Occurrence climatology of EPBs during solar cycle-24 in low latitude station, Kolhapur  In addition to this, they suggests that, the maxima in equinox differs from the seasonal dependence at other longitude sectors, endorsing the longitude dependence of seasonal EPB occurrence behavior. Abdu et al. [1992] also suggest that,

Category (Y/N)
the global occurrence of EPBs shows a strong seasonal variability that depends on longitude. Buhari et al. [2017] investigated the occurrence rate of the equatorial plasma bubble with season, solar activity, and geomagnetic conditions using long-term data sets of Malaysia Real-Time Kinematics Network (MyRTKnet) from 2008 to 2013.
Their results show that the EPB tends to occur successively over Malaysia in equinox season during high solar activity years. However, the occurrence day of EPBs was found to be relatively active only in equinoctial months during low solar activity. They suggest that the strong perturbation is essential for the development of the EPBs during low solar activity years. But in present study, we found lesser EPBs occurrence during equinoctial period for low solar activity.

Daily Averaged
The  EPBs presence in early storm nights. The EPBs occurrence is positively but weakly correlated (+0.14) with solar 10.7 cm flux and it is negatively correlated (-0.07) with Ap indices. We found no-EPBs on many disturbed nights.
However, we found EPBs occurrence on some nights despite magnetic activity as seen in bottom panel of Figure 6 (a). To understand the reason behind the EPBs suppression/occurrence during the magnetically disturbed nights, we have categorized all the disturbed nights as per Aarons [1991] criteria. Although we have used Ap index to identify the disturbed nights while Dst index is used to categorize storms. Aarons [1991] proposed three simple relations of the ring current with the EPBs after analyzing several storm cases which can be summarized as follows: 1) if the maximum excursion of Dst takes place during daytime hours and well before sunset, the normal height rise of the F layer is disturbed and irregularities are inhibited that night If the large excursion of Dst takes place after sunset and before midnight (i.e. 18 to 22 LT) then there is no effect on irregularity (EPB) occurrence. The grey colored bar denotes the hourly ap index for respective nights.
On the other hand we have seen occurrence of EPBs on previous night. It is noted that, in the upper panel of Figure   6 (b), Dst minima is seen as low as -80 nT. Further in Category II (Middle Panel), the large excursion of Dst occurs in the midnight to post-midnight time period (00 to 06 LT). Here we observe the generation in irregularities (EPBs) from post-midnight to the dawn periods. And hence this night satisfy category II. In Category III (Bottom Panel), the large excursion of Dst takes place after sunset and before midnight (i.e. 18 to 22 LT) then there is no effect on irregularity (EPBs) occurrence.
Geomagnetic disturbances are usually characterized by geomagnetic indices such as AP/Kp/AE/Dst. The most commonly used parameter for low latitude is the Dst index. The Dst index can be treated as a good measure of the overall strength of the near-Earth electric currents, especially the ring current. It is acquired from designated geomagnetic observatories operational at equatorial regions. We have measured the hourly Dst indices for the previous night, storm night and later night and measured the maximum Dst excursion time as shown in Table 4.

Onkar B. Gurav et al.
14 Table 4 The maximum excursion time in Dst is a measure of maximum intensity of the globally symmetrical equatorial electrojet i.e. the "ring current". As per the Aarons [1991] criteria, we have categorized the disturbed nights into three categories using our observations by observing the time of Dst maximum excursion. The categories and corresponding % occurrence of EPBs are depicted in Table 4.
In a nutshell, as per Aarons [1991] criteria the % occurrence should be low or zero for category I due to expectations of EPB inhibition, whereas the % occurrence should be high for category II due to expectation of EPB generation. We found that minority of storms which fall under category II supports Aarons [1991] criteria because on those storm nights the EPB occurrence has enhanced (>50%). It is known that in addition to Dst index, the interplanetary magnetic field (IMF) Bz also plays very important role in prompt penetration electric fields (PPEFs).
The development of EPBs can be affected due to the geomagnetic disturbances and this is related to the variations in the zonal electric field strength near the equatorial regions that controls the vertical propagation of the F-layer.
The related variations in the equatorial electric field strength due to geomagnetic activity can be classified into  et al., 2002]. In the present study, we found occurrence of EPBs on 21 nights under magnetic disturbances (Table 4).
From Table 4 we see that out of 22 nights only 06 nights (i.e. 27 %) obeys Aaron's criteria. Few disturbed nights support the Aarons criteria but the large number of nights do not obey the same which needs detailed investigations.

Hourly Averaged:
In addition to daily averaged occurrence, the hourly variation in occurrence of EPBs on all the disturbed nights is also calculated as shown in Figure 6 (c). The blue background depicts zero occurrence. It is seen that the EPBs occurrence on magnetically disturbed nights shows different variation during increasing and decreasing phase of solar-cycle.

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Occurrence climatology of EPBs during solar cycle-24 in low latitude station, Kolhapur    It is observed that, pre mid-night occurrence of EPBs is maximum in increasing phase as compared to decreasing phase of the solar cycle. But the case is different for post mid-night EPBs. Post mid-night EPBs are minimum during solar maximum years (2014)(2015) and their occurrence is maximum towards solar minimum years. This suggests that post mid-night EPBs are expected during solar minimum period. Further, on the disturbed nights no significant variation is observed between pre and post mid-night EPB occurrence (bottom panel of Figure 7).

Conclusions
We 1) The EPBs on quiet nights occurs around 19:30 LT, peaks at or around mid-night and then its occurrence decreases towards remaining course of night. However, the onset timings are found to be delayed beyond 19:30 LT in solar minimum years.
2) It is seen that, the EPBs occurrence is higher during the ascending phase and comparatively lower in descending phase of the solar cycle. However, few disturbed nights show presence of EPBs and this occurrence is strongly dependent on the time of maximum Dst excursion as proposed by Aarons [1991].
3) The hourly occurrence of EPBs on disturbed night shows that the EPBs are restricted to pre-midnight sector in the low solar activity (LSA) period and are well spread towards dawn sector during the high solar activity (HAS) period. This peculiar feature could be linked to the F layer height rise at the equator. If height of F layer is very high in the high solar activity period, it may produce early and long duration spread F. But if F layer height didn't rise to very high altitudes during low solar activity period, it is possible that spread-F can be of short lived and also delayed. Also, the onset timings of EPBs on disturbed nights goes on increasing towards solar maximum but decreases towards solar minimum.
4) The EPBs occurrence during equinox months is found to be higher than winter months during ascending phase of solar cycle-24. Also, we unexpectedly observed the zero occurrence in equinox months during near solar minimum year. 5) The January, February, March and December are the only months where we can expect EPBs occurrence over Kolhapur in any year during solar cycle provided that during solar minimum the percentage of occurrence may be low. 6) During solar minimum years we found higher occurrence percentage of post mid-night EPBs.