Post-midnight Hours Research Articles

Effects of the 2016 February minor sudden stratospheric warming on the MLT and ionosphere over Eastern Siberia

We present the results of studying the behavior of parameters of the mesosphere and lower thermosphere (MLT) and the ionosphere over the Eastern Siberia region during a minor winter sudden stratospheric warming (SSW) in early February 2016. We used the data from spectrometric measurements of OH ((6-2), 834.0 nm, ∼87 km) and O2 At ((0–1), 864.5 nm, ∼94 km), emissions from the Geophysical Observatory at the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences (51.8°N, 103.1°E, Tory). These emissions originate at the MLT heights. We also used the vertical sounding data on the peak electron density (NmF2), and on the peak height (hmF2). These data were obtained with the DPS-4 Irkutsk ionosonde (52.3°N, 104.3°E). For the analysis, we also involved the MLS Aura satellite data of measuring vertical temperature profiles and the MERRA reanalysis data.We found the MLT and ionospheric signatures for the 2016 February SSW. At the MLT heights, the OH and O2 emissions intensities increased by a factor of ∼2 and ∼3, respectively, and the temperature fell by ∼20 K. Analyzing the mesopause temperature variability and comparing the mean seasonal values showed an essential increase in the wave activity at the MLT. At the F2-region ionospheric heights, we revealed significant (up to ∼80%) NmF2 positive disturbances in the postmidnight hours and an essential increase (up to ∼ 2 times relative to root-mean-square values) in the amplitudes of the tidal component of the NmF2 disturbance. The results of the study show that a minor SSW may significantly impact the state of the MLT and the ionosphere at midlatitudes.

Climatology of ionospheric scintillation over the Vietnam low-latitude region for the period 2006–2014

This paper presents the characteristics of the occurrence of ionospheric scintillations at low-latitude, over Vietnam, by using continuous data of three GSV4004 receivers located at PHUT/Hanoï (105.9°E, 21.0°N; magnetic latitude 14.4°N), HUES/Hue (107.6°E, 16.5°N; magnetic latitude 9.5°N) and HOCM/Ho Chi Minh city (106.6°E, 10.8°N; magnetic latitude 3.3°N) for the period 2006–2014. The results show that the scintillation activity is maximum during equinox months for all the years and depends on solar activity as expected. The correlations between the monthly percentage scintillation occurrence and the F10.7 flux are of 0.40, 0.52 and 0.67 for PHUT, HUES and HOCM respectively. The distribution of scintillation occurrences is dominant in the pre-midnight sector and around the northern crest of the equatorial ionization anomaly (EIA), from the 15°N to 20°N geographic latitude with a maximum at 16°N. The results obtained from the directional analysis show higher distributions of scintillations in the southern sky of PHUT and in the northern sky of HUES and HOCM, and in the elevation angles smaller than 40°. The correlation between ROTI and S4 is low and rather good at PHUT (under EIA) than HOCM (near equator). We found better correlation in the post-midnight hours and less correlation in the pre-midnight hours for all stations. When all satellites are considered during the period of 2009–2011, the range of variation of the ration between ROTI and S4 is from 1 to 7 for PHUT, from 0.3 to 6 for HUES and from 0.7 to 6 for HOCM.

A high-altitude balloon experiment to probe stratospheric electric fields from low latitudes

Abstract. The Earth's electrical environment hosts a giant electrical circuit, often referred to as the global electric circuit (GEC), linking the various sources of electrical generators located in the lower atmosphere, the ionosphere and the magnetosphere. The middle atmosphere (stratosphere and mesosphere) has been traditionally believed to be passively transmitting electric fields generated elsewhere. Some observations have reported anomalously large electric fields at these altitudes, and the scientific community has had to revisit the earlier hypothesis time and again. At stratospheric altitudes and especially at low latitudes, horizontal electric fields are believed to be of ionospheric origin. Though measurements of these fields from a balloon platform are challenging because of their small magnitudes (around a few mV m−1), a suitably designed long-duration balloon experiment capable of detecting such small fields can provide useful information on the time evolution of ionospheric electric fields, which is otherwise possible only using radar or satellite in situ measurements. We present herein details of one such experiment, BEENS (Balloon Experiment on the Electrodynamics of Near Space), carried out from a low-latitude site in India. The instrument package for this experiment is comprised of four deployable booms for measurements of horizontal electric fields and one inclined boom for vertical electric field measurements, all equipped with conducting spheres at the tip. The experiment was conducted from Hyderabad (17.5° N, 78.6° E) during the post-midnight hours on 14 December 2013. In spite of a few shortcomings we report herein, a noticeable feature of the observations has been the detection of horizontal electric fields of ∼ 5 mV m−1 at the stratospheric altitudes of ∼ 35 km.

Equatorial ionospheric plasma drifts velocities using radar observations

This study presents the experimental results of the equatorial ionospheric plasma drift zonal velocity obtained from Incoherent Scatter Radar (ISR) observations for 6 selective days in 2011 from Jicamarca, Peru. Our results indicate that the daytime drifts are westward with peak values mostly below ~50 m/s, while the night time drifts velocities are eastward, with a maximum value up to 120 m/s at around local midnight hours. The drift velocity decreases during post-midnight hours and starts to reverse westward in early morning hours. Our plasma drifts results are in good agreement with results from previous radar studies and other measurement techniques. 
 BIBECHANA 14 (2017)1-8

Ionospheric scintillation observations over Kenyan region – Preliminary results

Ionospheric scintillation refers to the rapid fluctuations in the amplitude and phase of a satellite signal as it passes through small-scale plasma density irregularities in the ionosphere. By analyzing ionospheric scintillation observation datasets from satellite signals such as GPS signals we can study the morphology of ionospheric bubbles. At low latitudes, the diurnal behavior of scintillation is driven by the formation of large-scale equatorial density depletions which form one to two hours after sunset via the Rayleigh–Taylor instability mechanism near the magnetic equator. In this work we present ionospheric scintillation activity over Kenya using data derived from a newly installed scintillation monitor developed by CRIRP at Pwani University (39.78°E, 3.24°S) during the period August to December, 2014. The results reveal the scintillation activity mainly occurs from post-sunset to post-midnight hours, and ceases around 04:00LT. We also found that the ionospheric scintillation tends to appear at the southwest and northwest of the station. These locations coincide with the southern part of the Equatorial Ionization Anomaly crest over Kenya region. The occurrence of post-midnight L-band scintillation events which are not linked to pre-midnight scintillation observations raises fundamental question on the mechanism and source of electric fields driving the plasma depletion under conditions of very low background electron density.

Low latitude ionospheric scintillation and zonal plasma irregularity drifts climatology around the equatorial anomaly crest over Kenya

In this study we have used a VHF and GPS-SCINDA receiver located at Nairobi (36.8°E, 1.3°S, dip −24.1°) in Kenya to investigate the climatology of ionospheric L-band scintillation occurrences for the period 2009 to 2012; and seasonal variation of the zonal plasma drift irregularities derived from a VHF receiver for the period 2011. The annual and diurnal variations of L-band scintillation indicate occurrence at post sunset hours and peaks in the equinoctial months. However VHF scintillation occurs at all seasons around the year and is characterized by longer duration of activity and a slow fading that continues till early morning hours unlike in the L-band where they cease after midnight hours. A directional analysis has shown that the spatial distribution of scintillation events is mainly on the Southern and Western part of the sky over Nairobi station closer to the edges of the crest of the Equatorial Ionization Anomaly. The distribution of zonal drift velocities of the VHF related scintillation structures indicates that they move at velocities in the range of 20–160m/s and their dimension in the East–West direction is in the range of 100–00km. The December solstice is associated with the largest plasma bubbles in the range of 600–900km. The most significant observation from this study is the occurrence of post-midnight scintillation without pre-midnight scintillations during magnetically quiet periods. The mechanism leading to the formation of the plasma density irregularity causing scintillation is believed to be via the Rayleigh Tailor Instability; it is however not clear whether we can also attribute the post-midnight plasma bubbles during magnetic quiet times to the same mechanism. From our observations in this study, we suggest that a more likely cause of the east ward zonal electric fields at post-midnight hours is the coupling of the ionosphere with the lower atmosphere during nighttime. This however needs a further investigation based on relevant data.

Characteristics of postmidnight L band scintillation in the transition region from the equatorial to midlatitudes over the Indian longitude sector using COSMIC, C/NOFS, and GPS measurements

Occurrence of L band scintillations around midnight and postmidnight hours have not been well studied and reported from the higher equatorial latitudes in the transition region from the equatorial to midlatitudes over the Indian longitude sector. The present paper reports cases of postmidnight L band scintillation observations by COSMIC during March 2014 over the Indian longitude sector. GPS S4 measurements from the International Global Navigation Satellite Systems Service station at Lucknow (26.91°N, 80.96°E geographic; magnetic dip: 39.75°N) corroborate occurrence of postmidnight scintillations. The F region vertical upward velocities around the magnetic equator during evening hours have been used to understand the possibility of impact of irregularities generated over the magnetic equator at latitudes north of 30°N. Postmidnight L band scintillations at latitudes greater than 30°N without corresponding premidnight scintillations present interesting scientific scenario and give rise to suggestions of (1) any coupling mechanism between the equatorial and midlatitudes through which irregularities seeded in the midlatitudes may affect transionospheric satellite links at low latitudes or (2) irregularity generation at midlatitudes not connected with equatorial instabilities. Long-term analysis of S4 at L band measured by COSMIC over the Indian longitudes during March 2007–2014 exhibits a well-defined longitude swath around 75–83°E of reduced (0.2 < S4 < 0.4) or no scintillations which may be attributed to the longitudinal variability of scintillation occurrence following the global four-cell pattern of ionospheric activity.

Care for the care givers – The importance of supporting the health of humanitarian workers

In this study we have used a VHF and GPS-SCINDA receiver located at Nairobi (36.8°E, 1.3°S, dip −24.1°) in Kenya to investigate the climatology of ionospheric L-band scintillation occurrences for the period 2009 to 2012; and seasonal variation of the zonal plasma drift irregularities derived from a VHF receiver for the period 2011. The annual and diurnal variations of L-band scintillation indicate occurrence at post sunset hours and peaks in the equinoctial months. However VHF scintillation occurs at all seasons around the year and is characterized by longer duration of activity and a slow fading that continues till early morning hours unlike in the L-band where they cease after midnight hours. A directional analysis has shown that the spatial distribution of scintillation events is mainly on the Southern and Western part of the sky over Nairobi station closer to the edges of the crest of the Equatorial Ionization Anomaly. The distribution of zonal drift velocities of the VHF related scintillation structures indicates that they move at velocities in the range of 20–160 m/s and their dimension in the East–West direction is in the range of 100–00 km. The December solstice is associated with the largest plasma bubbles in the range of 600–900 km. The most significant observation from this study is the occurrence of post-midnight scintillation without pre-midnight scintillations during magnetically quiet periods. The mechanism leading to the formation of the plasma density irregularity causing scintillation is believed to be via the Rayleigh Tailor Instability; it is however not clear whether we can also attribute the post-midnight plasma bubbles during magnetic quiet times to the same mechanism. From our observations in this study, we suggest that a more likely cause of the east ward zonal electric fields at post-midnight hours is the coupling of the ionosphere with the lower atmosphere during nighttime. This however needs a further investigation based on relevant data.

Ionospheric Time-delay over Akure Using Global Positioning System Observations

Ionospheric time delay (VΔt) variability using Global Positioning System (GPS) data over Akure (7.15°N, 5.12°E), Nigeria, has been studied. The observed variability of VΔt in comparison to older results of vertical total electron content (TEC) across similar regions has shown equivalent signatures. Higher monthly mean values of VΔt (MVΔt) were observed during daytime as compared to nighttime (pre- and post-midnight) hours in all months. The highest MVΔt observed in September during daytime hours range between ~6 and ~21 ns (~1.80 and ~6.30 m) and at post-midnight, they are in the range of ~1 to ~6 ns (~0.3 to ~1.80 m). The possible mechanisms responsible for this variability were discussed. Seasonal VΔt were investigated as well.

Explicit characteristics of evolutionary‐type plasma bubbles observed from Equatorial Atmosphere Radar during the low to moderate solar activity years 2010–2012

AbstractUsing the fan sector backscatter maps of 47 MHz Equatorial Atmosphere Radar (EAR) at Kototabang (0.2°S geographic latitude, 100.3°E geographic longitude, and 10.4°S geomagnetic latitude), Indonesia, the spatial and temporal evolution of equatorial plasma bubbles (EPBs) were examined to classify the evolutionary‐type EPBs from those which formed elsewhere and drifted into the field of view of radar. A total of 535 EPBs were observed during the low to moderate solar activity years 2010–2012, out of which about 210 (~39%) are of evolving type and the remaining 325 (~61%) are drifting‐in EPBs. In general, both the evolving‐type and drifting‐in EPBs exhibit predominance during the postsunset hours of equinoxes and December solstices. Interestingly, a large number of EPBs were found to develop even a few minutes prior to the apex sunset during equinoxes. Further, the occurrence of evolving‐type EPBs exhibits a clear secondary peak around midnight (2300–0100 LT), primarily, due to higher rate of occurrence during the postmidnight hours of June solstices. A significant number (~33%) of postmidnight EPBs generated during June solstices did not exhibited any clear zonal drift, while about 14% of EPBs drifted westward. Also, the westward drifting EPBs are confined only to June solstices. The responsible mechanisms for the genesis of fresh EPBs during postmidnight hours were discussed in light of equatorward meridional winds in the presence of weak westward electric fields.

Observation of ionospheric irregularities around midnight and post-midnight near the northern crest of the Equatorial Ionization Anomaly in the Indian longitude sector: Case studies

Multistation and multi-technique observation of equatorial ionospheric irregularities from locations over the magnetic equator through the northern crest of the Equatorial Ionization Anomaly (EIA) and beyond is one of the important objectives of the Indian CAWSES program. For this purpose, multistation observational campaign was conducted during September 2011, April 2012 and September 2012 involving GPS TEC and S4 from Calcutta situated virtually underneath the northern crest of the EIA, and GPS S4 measurements from Siliguri, located beyond the northern crest of the EIA. Intense amplitude scintillation observations have been noted around midnight and post-midnight hours on certain GPS links located north of Siliguri with no scintillation patches during pre-midnight period. Satellite links experiencing scintillations from Siliguri were unaffected at Calcutta. Day-to-day variability has been noted in the maximum northern observation of GPS scintillations at different S4 levels from both Calcutta and Siliguri. Simultaneous observations from Siliguri and Calcutta indicate that post-midnight GPS amplitude scintillations are possibly associated with interaction of traveling ionospheric disturbances from mid-latitudes towards the equator, with transionospheric GPS links contrary to early evening hours when such movement are usually from over the magnetic equator towards the northern and southern crests of EIA. The present paper reports three cases of ionospheric irregularity tracking using the above stations on September 25, 2012, April 12, 2012 and September 11, 2011, all magnetically quiet days. Information related to the variabilities of latitudinal extent of scintillation observations along 88°E meridian at different times during 13–19UT and different levels of intensities of scintillation could provide useful information for transionospheric satellite link design and Satellite-based Augmentation System (SBAS).

Ionospheric variability over Indian low latitude linked with the 2009 sudden stratospheric warming

AbstractIn this paper, we analyze radar observations of E × B drift and plasma irregularities, ionosonde observations of E and F layer parameters including spread F, and magnetic field observations made from Indian low latitudes linked with the 2009 sudden stratospheric warming (SSW) event. E × B drift variations presented here are the first of their kind from the Indian sector as far as the effect of SSW is concerned. Difference of magnetic fields observed from the equator and low‐latitude (∆H) and E × B drift show linear relation, and both show remarkably large positive values in the morning and negative values in the afternoon exhibiting semidiurnal behavior. Remarkable changing patterns in the critical frequency of F2 layer (foF2) and F3 layer (foF3) were observed after the occurrence of SSW. Large variations with quasi 16 day periodicity were observed in ∆H, foF2, and foF3. Both semidiurnal and quasi 16 day wave modulation observed after the 2009 SSW event are consistent with those reported earlier. We also noted quasi 6 day variations in ∆H and foF2 soon after the SSW commencement, not much reported before. During the counterelectrojet events linked with the SSW event, while equatorial Es (Esq) disappeared as expected, there were no blanketing Es (Esb), a finding not reported and discussed earlier. Esb was also not formed at the off‐equatorial location, indicating the absence of required vertical wind shear, but E region plasma irregularities were observed by the ionosonde and radar with a close relationship between the two. Weak F region irregularities were observed in the postmidnight hours, and case studies suggest the possible role of SSW‐related background electric field in the manifestation of postmidnight F region irregularities.

A statistical analysis of occurrence characteristics of Spread-F irregularities over Indian region

We investigate the regularities of a change in Spread-F F probability during day-to-day, under varying solar variability, latitudinal behavior and their response to geomagnetic storm in equatorial and low-mid latitude stations. The occurrence characteristics of Spread-F irregularities, is obtained from daily hourly ionosonde data from a low-mid latitude station, Delhi (28.6°N, 77.2°E), for more than half a solar cycle (2001 to 2007). The latitudinal behavior of Spread-F is studied using ionosonde data from anomaly crest station, Ahmedabad (23.01°N, 72.36°E) and equatorial station, Kodaikanal (10.2°N, 77.5°E) for low, moderate and high solar activity periods. The maximum percentage occurrences of Spread-F were observed during the low solar activity year 2007, we believe, the low plasma and neutral density during 23/24 solar cycle minimum could be an important factor leading to the generation and propagation of TIDs and gravity waves. An anti-solar activity correlation to Spread-F occurrence is reported during all the seasons at different stations which are because of instability generated by the trans-equatorial meridional winds. There is a substantial variation during pre and post midnight hours in F region height from equatorial to low latitudes in response to magnetic disturbances. Concurrence was observed in the occurrence time of Spread-F to different storm events during different storm phases. The established irregularities and their behavior in Indian region are qualitatively interpreted and discussed.

Postmidnight bubbles and scintillations in the quiet‐time June solstice

While the mechanism for producing plasma irregularities in the dusk sector is believed to be fairly well understood, the cause of the formation of irregularities and bubbles during the postmidnight sector is still unknown, especially for magnetically quiet periods. This paper presents a case study of the strong postmidnight bubbles that often occur during magnetically quiet periods primarily in June solstice, along with a 4 year (2009–2012) statistical study that shows strong occurrence peak during June solstice predominantly in the African sector. We also confirm, for the first time, the presence of Rayleigh‐Taylor (RT) instability during postmidnight hours by using the physics‐based model for plasma densities and RT growth rates. Finally, we consider several possible sources of the eastward electric fields that permit the RT instability to develop and form bubbles in the postmidnight local time sector.

Response of the EIA ionosphere to the 7-8 May 2005 geomagnetic storm

In this paper, response of low latitude ionosphere to a moderate geomagnetic storm of 7–8 May 2005 (SSC: 1920UT on 7 May with Sym-H minimum, ∼−112nT around 1600UT on 8 May) has been investigated using the GPS measurements from a near EIA crest region, Rajkot (Geog. 22.29°N, 70.74°E, Geomag.14°), India. We found a decrease in total electron content (TEC) in 12h after the onset of the storm, an increase during and after 6h of Sym-H deep minimum with a decrease below its usual-day level on the second day during the recovery phase of the storm. On 8 May, an increase of TEC is observed after sunset and during post-midnight hours (maximum up to 170%) with the formation of ionospheric plasma bubbles followed by a nearly simultaneous onset of scintillations at L-band frequencies following the time of rapid decrease in Sym-H index (−30nT/h around 1300UT).

The day‐to‐day longitudinal variability of the global ionospheric density distribution at low latitudes during low solar activity

One important characteristic of longitudinal variability of the ionosphere is the global wavenumber‐4 signature. Recent investigations have focused mainly on the climatological pattern during daytime and evening sectors. We investigate the day‐to‐day variability of the wavenumber‐4 structure of the longitudinal ionospheric density distribution using the global total electron content (TEC) measurements from Global Positioning Systems receivers on the ground. The quiet time (Kp ≤ 3) day‐to‐day occurrence of the wavenumber‐4 is obtained during periods of low solar flux conditions for the years 2008 and 2009. We find that the wavenumber‐4 structure occurs at all local time sectors; however, the daytime TEC wavenumber‐4 structures are clearer and can persist until the midnight hours. The most significant occurrence is observed during the 1000–2400 LT sector while the minimum number of wavenumber‐4 structure is observed between the 0400 and 0600 LT sector. Around the nighttime sector, more wavenumber‐4 occurrence is observed during the premidnight sector than the postmidnight hours. The seasonal occurrence probability of the wavenumber‐4 pattern is at a maximum during the March–April equinox and June–July solstice. December–January is the period when the wavenumber‐4 occurrence is less dominant than the rest of the year.

Ionospheric Scintillation and Dynamics of Fresnel-Scale Irregularities in the Inner Region of the Equatorial Ionization Anomaly

This paper reports differences in the occurrence statistics of global positioning system (GPS) L-band scintillations at observational sites located in the inner regions of the northern and southern crests of the equatorial ionization anomaly. Ground-based GPS data acquired at the closed magnetically aligned stations of Manaus (3.1°S; 59.9°W; dip lat. 6.2°N) and Cuiaba (15.5°S; 56.1°W; dip. lat. 6.2°S), Brazil, from December 2001 to February 2007 are used in the analysis. The drift dynamics of Fresnel-scale ionospheric irregularities at the southern station of Cuiaba are also investigated. Only geomagnetically quiet days with the sum of daily Kp < 24 were used in the analysis statistics and in the irregularity drift studies. The results reveal a clear dependence of the scintillation occurrence with the solar activity, but there exists an asymmetry in the percentage of scintillation occurrence between the two stations throughout the period analyzed. The nocturnal occurrence of the scintillations over Cuiaba is predominantly larger than over Manaus, but this scenario seems to change with the decline in the solar activity (mainly during local post-midnight hours). A broad minimum and maximum in the scintillation occurrence appears to occur over both the stations, respectively, during the June solstice (winter) and December solstice (summer) months. The dynamics of the Fresnel-scale irregularities, as investigated from the estimations of the mean zonal drift velocities, reveals that the amplitude of the eastward drifts tends to reduce with the decline in the solar activity. The magnitude of the zonal drift velocities during the December solstice months is larger than during the equinoxes, with the differences being more pronounced at solar maximum years. Other relevant aspects of the observations, with complementary data from a low-latitude ionospheric model, are highlighted and discussed.

GPS scintillation and TEC depletion near the northern crest of equatorial anomaly over South China

This study presents a statistical analysis of GPS L-band scintillation with data observed from July 2008 to March 2012 at the northern crest of equatorial anomaly stations in Guangzhou and Shenzhen of South China. The variations of the scintillation with local time, season, solar activity and duration of scintillation patches were investigated. The relationship between the scintillation and TEC depletion was also reported. Our results revealed that GPS scintillation occurred from 19:30 LT (pre-midnight) to 03:00 LT (post-midnight). During quiet solar activity years, the scintillation was only observed in pre-midnight hours of equinox months and patches durations were mostly less than 60min. During high solar activity years, more scintillation occurred in the pre-midnight hours of equinox and winter months; and GPS scintillation started to occur in the post-midnight hours of summer and winter. The duration of scintillation patches extended to 180min in high solar activity years. Solar activity had a larger effect to strong scintillations (S4>0.6) than to weak scintillations (0.6⩾S4>0.2). Strong scintillations were accompanied by TEC depletion especially in equinox months. We also discussed the relationship between TEC depletion and plasma bubble.

Solar and magnetic control on night-time enhancement in TEC near the crest of the Equatorial Ionization Anomaly

The ionospheric Total Electron Content (TECs), derived by dual frequency signals from the Global Positioning System (GPS) recorded near the Indian equatorial anomaly region, Bhopal (23.2°N, 77.4°E, Geomagnetic 14.2°N) were analyzed for the period of January, 2005 to February, 2008. The work deals with monthly, diurnal, solar and magnetic activity variations on night-time enhancement in TEC. From a total of 157 night-time enhancements, 75 occur during pre-midnight and 82 post-midnight hours. The occurrence of night-time enhancement in TEC is utmost during summer months, followed by equinox and winter months. The occurrence of night-time enhancement in TEC decreases with increase in solar and magnetic activities. We observed that peak size and half amplitude duration are positively correlated, while time of occurrence of night-time enhancement in TEC and time of peak enhancement are negatively correlated with solar activity. The peak size, half amplitude duration, time of peak enhancement and time of occurrence of night-time enhancement in TEC shows negative correlation with magnetic activity. The results have been compared with the earlier ones and discussed in terms of possible source mechanism responsible for the enhancement at anomaly crest region.

Morphological studies on ionospheric VHF scintillations over an Indian low latitude station during a solar cycle period (2001–2010)

The amplitude scintillations data recorded at 244MHz from the geostationary satellite, FLEETSAT (73°E) at a low latitude station, Waltair (17.7°N, 83.3°E) during the ten year period of high to low solar activity from 2001 to 2010 is considered to study the occurrence characteristics of the VHF scintillations. A close association between the intense scintillations on VHF signals during pre-midnight hours, associated with range type of spread-F on ionograms and a relatively weak and slow fading scintillations during post-midnight hours associated with frequency type of spread-F is observed during the relatively high sunspot years from 2001 to 2004, whereas during the low sunspot years from 2005 to 2010 the scintillation activity as well as spread-F activity are found to be minimum. During both the high and low sunspot years, it is observed that the maximum scintillation activity occurs during equinoctial months followed by winter with the minimum occurrence during summer months. The annual mean percentage occurrence of scintillations is found to be clearly associated with the variations in the annual mean sunspot number. The nocturnal variations in the occurrence of scintillations show the onset of scintillation activity starts from 19:00h LT with maximum of occurrence around 21:00h LT. A clear semiannual variation in the occurrence of scintillations is observed during pre-midnight hours with two peaks in equinoctial months of March/April and September/October. The number of scintillation patches observed is found to be more during pre-midnight hours compared to those during post-midnight hours. The most probable scintillation patch duration lies around 30min. Further, it is also found that the number of scintillation patches with durations of 60min and more decreases with the increase in the patch duration. It is also observed in general that the scintillation activity is inhibited during geomagnetic disturbed days.