Lakha Singh | University of Texas at San Antonio (original) (raw)

Papers by Lakha Singh

Research paper thumbnail of Variation of TEC during 21st solar cycle over a low latitude station Hyderabad

The TEC and variations of the TEC are examined at a low-latitude location with data from the anal... more The TEC and variations of the TEC are examined at a low-latitude location with data from the analysis of Faraday rotation-angle records from 136-MHz satellite beacons. The general diurnal behavior of TEC is delineated as it occurred during the 21st solar cycle; comparisons are made with the mean diurnal plots of the TEC during winter, summer, and the equinoxes. No seasonal anomaly is identified at the low-latitude station during the period of low solar activity, but the anomaly appears during the high-activity period. The TEC values vary directly with 10.7-cm solar flux except during the diurnal summer maximum where it reaches higher values. It is concluded that TEC occurrences relate directly to solar activity at the location of the low-latitude station. The seasonal anomaly is also found to be pronounced at this latitude during the solar maximum, a phenomenon which is documented at other latitudes.

Research paper thumbnail of Ionospheric electron content variation observed at Delhi during totalsolar eclipse of 16 February 1980

Research paper thumbnail of Clock synchronization experiment in India using Symphonie satellite

A recent clock synchronization experiment between the National Physical Laboratory (NPL), New Del... more A recent clock synchronization experiment between the National Physical Laboratory (NPL), New Delhi, and Space Applications Center (SAC), Ahmedabed, via the geostationary satellite Symphonie-II stationed at 49 deg E longitude, is reported. Since only one satellite transponder was available for this experiment, the two-way transmission of the clock pulses was carried out by switching the transmit-receive roles at the two stations at 5 minute intervals to achieve a nearly simultaneous two-way transmission. Taking into account all additional delays, the results demonstrated a clock-synchronization accuracy of better than 0.5 microsecond. A crystal-based portable clock flown aboard an aircraft confirmed this clock-synchronization to within a microsecond.

Research paper thumbnail of Long term ionospheric electron content variations over Delhi

Annales Geophysicae, 2001

Research paper thumbnail of Faraday polarization fluctuations and their dependence on post sunset secondary maximum and amplitude scintillations at Delhi

Annales Geophysicae, 2002

VHF Faraday rotation (FR) and amplitude scintillation data recorded simultaneously during May 197... more VHF Faraday rotation (FR) and amplitude scintillation data recorded simultaneously during May 1978 December 1980 at Delhi (28.63° N, 77.22° E; Dip 42.44° N) is analyzed in order to study the Faraday polarization fluctuations (FPFs) and their dependence on the occurrence of post sunset secondary maximum (PSSM) and amplitude scintillations. It is noted that FPFs are observed only when both PSSM and scintillations also occur simultaneously. FPFs are observed only during winter and the equinoctial months of high sunspot years. FPFs events are associated with intense scintillation activity, which is characterized by sudden onsets and abrupt endings, and are observed one to three hours after the local sunset. When FPFs and scintillation data from Delhi is compared with the corresponding data from a still lower latitude station, Hyderabad (17.35° N, 78.45° E), it is found that the occurrence of FPFs and scintillations at Delhi is conditional to their prior occurrence at Hyderabad, which indicates their production by a plasma bubble and the as-sociated irregularities generated initially over the magnetic equator. In addition, FPFs and scintillation data for October 1979, when their occurrence was maximum, is also examined in relation to daytime (11:00 LT) electrojet strength (EEj) values and evening hour h’F from an equatorial location, Kodaikanal (10.3° N, 77.5° E). It is interesting to note that FPFs and scintillations are most likely observed when the EEj was 100 nT or more and h’F reaches around 500 km. These results show that EEj and evening hours h’F values over the magnetic equator are important parameters for predicting FPFs and scintillation activity at locations such as Delhi, where scintillation activity is much more intense as compared to the equatorial region due to the enhanced back-ground ionization due to the occurrence of PSSM.

Research paper thumbnail of Role of neutral wind and storm time electric fields inferred from the storm time ionization distribution at low latitudes: in-situ measurements by Indian satellite SROSS-C2

Annales Geophysicae, 2005

Recently, there has been a renewal of interest in the study of the effects of solar weather event... more Recently, there has been a renewal of interest in the study of the effects of solar weather events on the ionization redistribution and irregularity generation. The observed changes at low and equatorial latitudes are rather complex and are noted to be a function of location, the time of the storm onset and its intensity, and various other characteristics of the geomagnetic storms triggered by solar weather events. At these latitudes, the effects of geomagnetic storms are basically due to (a) direct penetration of the magnetospheric electric fields to low latitudes, (b) development of disturbance dynamo, (c) changes in atmospheric neutral winds at ionospheric level and (d) changes in neutral composition triggered by the storm time atmospheric heating. <P style="line-height: 20px;"> In the present study an attempt is made to further understand some of the observed storm time effects in terms of storm time changes in zonal electric fields and meridional neutral winds. For this purpose, observations made by the Retarding Potential Analyzer (RPA) payload on board the Indian satellite SROSS-C2 are examined for four prominent geomagnetic storm events that occurred during the high solar activity period of 1997-2000. Available simultaneous observations, from the GPS satellite network, are also used. The daytime passes of SROSS-C2 have been selected to examine the redistribution of ionization in the equatorial ionization anomaly (EIA) region. In general, EIA is observed to be weakened 12-24 h after the main phase onset (MPO) of the storm. The storm time behaviour inferred by SROSS-C2 and the GPS satellite network during the geomagnetic storm of 13 November 1998, for which simultaneous observations are available, is found to be consistent. Storm time changes in the delay of received GPS signals are noted to be ~1-3 m, which is a significant component of the total delay observed on a quiet day. <P style="line-height: 20px;"> An attempt is made to identify and delineate the effects of a) meridional neutral winds, b) the development of the ring currents and c) the disturbance dynamo electric fields on the low latitude ionization distribution. The weakening of the EIA is noted to be primarily due to the decrease in the eastward electric fields driving the equatorial fountain during the daytime. The meridional neutral winds are also noted to play an important role in redistribution of ionization in the EIA region. The present results demonstrate that storm time latitudinal distribution of ionization in this region can be better understood by taking into account the meridional winds in addition to E×B drifts.

Research paper thumbnail of Average response of IEC, T and NmF2 to geomagnetic storms at Delhi

Ionospheric electron content (IEC) peak density (NmF2) and slab thickness (T) obtained at Delhi (... more Ionospheric electron content (IEC) peak density (NmF2) and slab thickness (T) obtained at Delhi (28.6 deg N, 77.2 deg E) with the help of geostationary satellite radio beacon during the ascending half of the solar cycle 21 (1975-80) have been used to investigate the total response of the F-region ionization to geomagnetic storms. The average stormtime (Dst) and local time (SD) behavior patterns were constructed by analyzing 36 storms of various intensities and duration. In general, IEC and NmF2 show enhancements during the course of a storm except for negative phase of short durations. Simultaneously, T remains below the average level for most of the storm period. Maximum enhancements in IEC and NmF2 have been observed in the forenoon hours of second day. The corresponding average equatorial Dst (H) index shows maximum negative excursions in the same period. There are distinct seasonal biases in the Dst and SD variations of all the three parameters IEC, NmF2 and T. Vertical drifts caused by equatorward neutral winds coupled with fall or rise in neutral temperature are believed to be the mechanisms responsible for enhancement and depletion of ionization.

Research paper thumbnail of IEC variations during geomagnetic storms at Delhi

Ionospheric electron content (IEC) measurements made at Delhi with the help of geostationary sate... more Ionospheric electron content (IEC) measurements made at Delhi with the help of geostationary satellite radio beacon during the ascending half of the solar cycle (1975-80) have been used to investigate the response of F-region ionization to geomagnetic storms. A total of 35 storms have been used to obtain the average storm time (Dst) and local time (SD) behavior pattern of IEC. In general, IEC shows enhancements during the course of a storm except for negative phases of short duration during nighttime. Maximum enhancements have been observed in the prenoon hours of the second day or after 40-48 hr of storm commencement. The corresponding average equatorial Dst(H) index in these periods shows maximum negative excursions. The average pattern seems to have distinct seasonal characteristics. Also, the response of IEC to increased geomagnetic activity has distinct local time bias.

Research paper thumbnail of Clock synchronisation experiment in India using symphonie satellite

A recent clock synchronization experiment between the National Physical Laboratory (NPL), New Del... more A recent clock synchronization experiment between the National Physical Laboratory (NPL), New Delhi and Space Applications Center (SAC), Ahemedabad, in India via geostationary satellite symphonie 2, stationed at 49 E longitude, is reported. A two-way transmission using a microwave transponder considered to provide the greatest precision in synchronization of two remote clocks is described.

Research paper thumbnail of Morphological study of post sunset secondary maximum in ionospheric electron content and its correlation with Faraday polarisation fluctuations and VHF amplitude scintillations at Delhi

Continuous data on the ionospheric electron content (IEC) over Delhi are reported with special at... more Continuous data on the ionospheric electron content (IEC) over Delhi are reported with special attention given to the nocturnal anomalous enhancements. The morphology of the post-sunset secondary maximum (PSSM) is isolated to determine whether there exist any correlative behavior with amplitude scintillations (AS) and/or Faraday polarization fluctuations (FPFs). Data from the periods Nov. 1975 - Aug. 1976 and Apr. 1978 - Dec. 1980 are reviewed to identify PSSMs, the time of PSSM peak, duration, and peak amplitude. The PSSM is found to have both seasonal and solar-cycle dependence including: (1) more frequent occurrence in winter; (2) an average IEC during the winter similar to the noontime maximum around the autumnal equinox; and (3) no maximum during the months of the equinox. FPFs occur only on days when both amplitude scintillations and a secondary maximum are present.

Research paper thumbnail of Growth and decay of a post-sunset equatorial anomaly at low latitudes: Control of E×B, neutral winds and daytime electrojet strength

Journal of Atmospheric and Solar-terrestrial Physics, 2006

Analyzing the 2 months ionospheric electron content (IEC) data acquired during a high solar activ... more Analyzing the 2 months ionospheric electron content (IEC) data acquired during a high solar activity period from a network of a stations whose sub-ionospheric points at 420 km were located within ± 1° of 84° meridian and covering a latitude belt of 3 21°N (magnetic), it has been shown that following sunset a fresh equatorial anomaly develops in continuation to the decaying daytime equatorial anomaly (DEA) in the low latitude belt. The occurrence of this post-sunset equatorial anomaly (PEA) is although a regular phenomenon but large day-to-day variations are seen in its strength (crest to trough ratio) and the latitude extent, PEA develops fully within 2 3 h of sunset and then decays. During its development crest of anomaly clearly shows a pole-ward movement and during decay it shows substantial equator-ward movement. While the trough of PEA is always seen over the magnetic equator, the crest of fully developed anomaly may be seen to lie at latitudes anywhere between 12°N and 21°N magnetic or more on different days. Thus the development and decay of PEA produce large latitudinal disturbance of F-region plasma in the lower latitude belt in the post-sunset hours and the occurrence of various low latitude phenomenon observed in IEC, e.g. post-sunset IEC decreases at equatorial latitudes, post-sunset IEC enhancements at 12°N or higher latitudes, pre-midnight IEC enhancements at equatorial latitudes, etc., have been attributed to the systematic development and decay of PEA. Also, the occurrence and development of PEA has been found to play an important role in the onset of equatorial plasma bubble associated ionospheric irregularities and their latitudinal growth. The equatorial ionization anomaly (EIA) is primarily caused by the ExB drifts whereas meridional winds in association with E×B drifts also seems to play some role in determining the crest of PEA.

Research paper thumbnail of Multistation study of nighttime scintillations in low latitudes - Evidence of control by equatorial F region irregularities

Radio Science, 1984

VHF nighttime scintillations, recorded during a high solar activity period at a meridian chain of... more VHF nighttime scintillations, recorded during a high solar activity period at a meridian chain of stations covering a magnetic latitude belt of 3ø-21øN (420 km subionospheric points) are analyzed to investigate the influence of equatorial spread F irregularities on the occurrence of scintillation at latitudes away from the equator. Observations show that saturated amplitude scintillations start abruptly about one and a half hours after ground sunset and their onset is almost simultaneous at stations whose subionospheric points are within 12øN latitude of the magnetic equator, but is delayed at a station whose subionospheric point is at 21øN magnetic latitude by 15 min to 4 hours. In addition, the occurrence of postsunset scintillations at all the stations is found to be conditional on their prior occurrence at the equatorial station. If no postsunset scintillation activity is seen at the equatorial station, no scintillations are seen at other stations also. The occurrence of scintillations is explained as caused by rising plasma bubbles and associated irregularities over the magnetic equator and the subsequent mapping of these irregularities down the magnetic field lines to the F region of higher latitudes through some instantaneous mechanism; and hence an equatorial control is established on the generation of postsunset scintillation-producing irregularities in the entire low-latitude belt.

Research paper thumbnail of Observation of Fresnel-type fadings at Delhi, a low midlatitude station, over half a solar cycle

An investigation is presented of the Fresnel-type fading patterns observed on recordings of orbit... more An investigation is presented of the Fresnel-type fading patterns observed on recordings of orbiting as well as geostationary satellites made at Delhi for over half a solar cycle. It is found that the occurrence of the Fresnel-type fading patterns is most frequent during low solar activity but less during high solar activity. These patterns were most frequently recorded at times between the local sunset and midnight during the summer, while the patterns occur mostly in groups with an average periodicity which ranges from 2-10 min. The results of the orbiting satellite observations show the tendency of the patterns to occur at zenith angles greater than 45 degrees, but no azimuth preference is found in the occurrence of these irregularities. The height of the ionospheric irregularities responsible for such patterns is determined to correspond to the E-region height. The occurrence of these patterns is found to be associated with the occurrence of a long-lived or a short-lived blanketing type sporadic-E at Delhi with f(0)Es values normally greater than 5 MHz, while no correlation is observed with the occurrence of spread-F. In addition, it is found that the diffused nature of the sporadic-E layer, as indicated by the range spread on ionograms, is one of the important factors associated with the production of these diffraction patterns.

Research paper thumbnail of Results of foF2 and Ne-h profiles at low latitude using recent digital ionosonde observations and their comparison with IRI2000

Journal of Atmospheric and Solar-terrestrial Physics, 2003

Using digital ionosonde observations at low-latitude station, Delhi (28.6 N, 77.2 E, mag. dip 42.... more Using digital ionosonde observations at low-latitude station, Delhi (28.6 N, 77.2 E, mag. dip 42.4 N), the diurnal and seasonal variations of the critical frequency of F2 layer (foF2) are analyzed from August 2000 to July 2001 during a high solar activity period. Also, noontime bottomside electron density (Ne-h) profiles, below the F2-peak, are derived from ionogram, using the POLAN (Report UAG-93, WDC-A, for Solar Terrestrial Physics, Boulder, Co.) program during the same period, and these profiles are then normalized to the peak height and density (hmF2, NmF2) of the F2-region. These observations are used to assess the predictability of the International Reference Ionosphere, IRI-2000 model (Radio Sc. 36(2) (2001) 261). Results show in general, a large variability, (/1sigma, /sigma is standard deviation), in foF2 during nighttime than daytime during winter and equinox, the variability of foF2 about the mean is about /+/-25% by night and /+/-15% by day. The IRI model shows a fairly good agreement with foF2 observations during daytime, however during nighttime, the discrepancies between the two exist. Comparative studies of the normalized observed profiles with those obtained with the IRI model (Bilitza, 2001) using both the options namely: Gulyaeva's (Adv. Space Res. 7 (1987) 39) model and /B0-Table (Adv. Space Res. 25(1) (2000) 89), show that during all the seasons, in general, the /B0-Tab option, reveals a better agreement with the observations, while the IRI model using Gulyaeva's option, overestimates the electron density distribution during summer and equinox, however, during winter, the model is close to the observations. The comparisons of average profile shape parameters /(B0,B1) derived from noontime observed profiles, with those obtained, using /B0-Tab option, in the IRI model, show a good agreement during all the seasons. However, /B0, /B1 obtained, using Gulyaeva's option in the IRI model, show a disagreement with the derived /B0, /B1 values during all the seasons, except during winter, for /B0 parameter.

Research paper thumbnail of Evolution and dynamics of equatorial plasma bubbles: Relationships to ExB drift, postsunset total electron content enhancements, and equatorial electrojet strength

Research paper thumbnail of Spatial estimates of Rayleigh-Taylor instability at geomagnetic equator from satellite scintillation observations at Delhi

An attempt is made to estimate the longitudinal extent of Rayleigh-Taylor instability (RTI) over ... more An attempt is made to estimate the longitudinal extent of Rayleigh-Taylor instability (RTI) over the equator from data obtained from two satellites simultaneously at Delhi. When the growth of RTI is sudden and encompassing a large longitudinal range, it is concluded that it is in response to a strong geophysical mechanism, such as an acoustic gravity wave or a severe magnetic storm. At any particular time, the irregularity scale sizes responsible for the scintillations seem to be of almost the same order over the entire RTI extent. The data suggest that increased magnetic activity may inhibit the growth of RTI and hence scintillation production, especially during high solar activity.

Research paper thumbnail of Role of neutral wind and storm time electric fields inferred from the storm time ionization distribution at low latitudes: in-situ measurements by Indian satellite SROSS-C2

Annales Geophysicae, 2005

Recently, there has been a renewal of interest in the study of the effects of solar weather event... more Recently, there has been a renewal of interest in the study of the effects of solar weather events on the ionization redistribution and irregularity generation. The observed changes at low and equatorial latitudes are rather complex and are noted to be a function of location, the time of the storm onset and its intensity, and various other characteristics of the geomagnetic storms triggered by solar weather events. At these latitudes, the effects of geomagnetic storms are basically due to (a) direct penetration of the magnetospheric electric fields to low latitudes, (b) development of disturbance dynamo, (c) changes in atmospheric neutral winds at ionospheric level and (d) changes in neutral composition triggered by the storm time atmospheric heating.

In the present study an attempt is made to further understand some of the observed storm time effects in terms of storm time changes in zonal electric fields and meridional neutral winds. For this purpose, observations made by the Retarding Potential Analyzer (RPA) payload on board the Indian satellite SROSS-C2 are examined for four prominent geomagnetic storm events that occurred during the high solar activity period of 1997-2000. Available simultaneous observations, from the GPS satellite network, are also used. The daytime passes of SROSS-C2 have been selected to examine the redistribution of ionization in the equatorial ionization anomaly (EIA) region. In general, EIA is observed to be weakened 12-24 h after the main phase onset (MPO) of the storm. The storm time behaviour inferred by SROSS-C2 and the GPS satellite network during the geomagnetic storm of 13 November 1998, for which simultaneous observations are available, is found to be consistent. Storm time changes in the delay of received GPS signals are noted to be ~1-3 m, which is a significant component of the total delay observed on a quiet day.

An attempt is made to identify and delineate the effects of a) meridional neutral winds, b) the development of the ring currents and c) the disturbance dynamo electric fields on the low latitude ionization distribution. The weakening of the EIA is noted to be primarily due to the decrease in the eastward electric fields driving the equatorial fountain during the daytime. The meridional neutral winds are also noted to play an important role in redistribution of ionization in the EIA region. The present results demonstrate that storm time latitudinal distribution of ionization in this region can be better understood by taking into account the meridional winds in addition to E×B drifts.

Research paper thumbnail of Growth and decay of a post-sunset equatorial anomaly at low latitudes: Control of E×B, neutral winds and daytime electrojet strength

Journal of Atmospheric and Solar-terrestrial Physics, 2006

Analyzing the 2 months ionospheric electron content (IEC) data acquired during a high solar activ... more Analyzing the 2 months ionospheric electron content (IEC) data acquired during a high solar activity period from a network of a stations whose sub-ionospheric points at 420 km were located within ± 1° of 84° meridian and covering a latitude belt of 3–21°N (magnetic), it has been ...

Research paper thumbnail of Ionospheric Electron Content Variation observed at Delhi during Total Solar Eclipse of 16 February 1980

Research paper thumbnail of Faraday polarization fluctuations and their dependence on post sunset secondary maximum and amplitude scintillations at Delhi

Annales Geophysicae, 2002

VHF Faraday rotation (FR) and amplitude scintillation data recorded simultaneously during May 197... more VHF Faraday rotation (FR) and amplitude scintillation data recorded simultaneously during May 1978-December 1980 Dip 42.44 • N) is analyzed in order to study the Faraday polarization fluctuations (FPFs) and their dependence on the occurrence of post sunset secondary maximum (PSSM) and amplitude scintillations. It is noted that FPFs are observed only when both PSSM and scintillations also occur simultaneously. FPFs are observed only during winter and the equinoctial months of high sunspot years. FPFs events are associated with intense scintillation activity, which is characterized by sudden onsets and abrupt endings, and are observed one to three hours after the local sunset. When FPFs and scintillation data from Delhi is compared with the corresponding data from a still lower latitude station, Hyderabad (17.35 • N, 78.45 • E), it is found that the occurrence of FPFs and scintillations at Delhi is conditional to their prior occurrence at Hyderabad, which indicates their production by a plasma bubble and the associated irregularities generated initially over the magnetic equator. In addition, FPFs and scintillation data for October 1979, when their occurrence was maximum, is also examined in relation to daytime (11:00 LT) electrojet strength (EEj) values and evening hour h'F from an equatorial location, Kodaikanal (10.3 • N, 77.5 • E). It is interesting to note that FPFs and scintillations are most likely observed when the EEj was 100 nT or more and h'F reaches around 500 km. These results show that EEj and evening hours h'F values over the magnetic equator are important parameters for predicting FPFs and scintillation activity at locations such as Delhi, where scintillation activity is much more intense as compared to the equatorial region due to the enhanced background ionization due to the occurrence of PSSM.

Research paper thumbnail of Variation of TEC during 21st solar cycle over a low latitude station Hyderabad

The TEC and variations of the TEC are examined at a low-latitude location with data from the anal... more The TEC and variations of the TEC are examined at a low-latitude location with data from the analysis of Faraday rotation-angle records from 136-MHz satellite beacons. The general diurnal behavior of TEC is delineated as it occurred during the 21st solar cycle; comparisons are made with the mean diurnal plots of the TEC during winter, summer, and the equinoxes. No seasonal anomaly is identified at the low-latitude station during the period of low solar activity, but the anomaly appears during the high-activity period. The TEC values vary directly with 10.7-cm solar flux except during the diurnal summer maximum where it reaches higher values. It is concluded that TEC occurrences relate directly to solar activity at the location of the low-latitude station. The seasonal anomaly is also found to be pronounced at this latitude during the solar maximum, a phenomenon which is documented at other latitudes.

Research paper thumbnail of Ionospheric electron content variation observed at Delhi during totalsolar eclipse of 16 February 1980

Research paper thumbnail of Clock synchronization experiment in India using Symphonie satellite

A recent clock synchronization experiment between the National Physical Laboratory (NPL), New Del... more A recent clock synchronization experiment between the National Physical Laboratory (NPL), New Delhi, and Space Applications Center (SAC), Ahmedabed, via the geostationary satellite Symphonie-II stationed at 49 deg E longitude, is reported. Since only one satellite transponder was available for this experiment, the two-way transmission of the clock pulses was carried out by switching the transmit-receive roles at the two stations at 5 minute intervals to achieve a nearly simultaneous two-way transmission. Taking into account all additional delays, the results demonstrated a clock-synchronization accuracy of better than 0.5 microsecond. A crystal-based portable clock flown aboard an aircraft confirmed this clock-synchronization to within a microsecond.

Research paper thumbnail of Long term ionospheric electron content variations over Delhi

Annales Geophysicae, 2001

Research paper thumbnail of Faraday polarization fluctuations and their dependence on post sunset secondary maximum and amplitude scintillations at Delhi

Annales Geophysicae, 2002

VHF Faraday rotation (FR) and amplitude scintillation data recorded simultaneously during May 197... more VHF Faraday rotation (FR) and amplitude scintillation data recorded simultaneously during May 1978 December 1980 at Delhi (28.63° N, 77.22° E; Dip 42.44° N) is analyzed in order to study the Faraday polarization fluctuations (FPFs) and their dependence on the occurrence of post sunset secondary maximum (PSSM) and amplitude scintillations. It is noted that FPFs are observed only when both PSSM and scintillations also occur simultaneously. FPFs are observed only during winter and the equinoctial months of high sunspot years. FPFs events are associated with intense scintillation activity, which is characterized by sudden onsets and abrupt endings, and are observed one to three hours after the local sunset. When FPFs and scintillation data from Delhi is compared with the corresponding data from a still lower latitude station, Hyderabad (17.35° N, 78.45° E), it is found that the occurrence of FPFs and scintillations at Delhi is conditional to their prior occurrence at Hyderabad, which indicates their production by a plasma bubble and the as-sociated irregularities generated initially over the magnetic equator. In addition, FPFs and scintillation data for October 1979, when their occurrence was maximum, is also examined in relation to daytime (11:00 LT) electrojet strength (EEj) values and evening hour h’F from an equatorial location, Kodaikanal (10.3° N, 77.5° E). It is interesting to note that FPFs and scintillations are most likely observed when the EEj was 100 nT or more and h’F reaches around 500 km. These results show that EEj and evening hours h’F values over the magnetic equator are important parameters for predicting FPFs and scintillation activity at locations such as Delhi, where scintillation activity is much more intense as compared to the equatorial region due to the enhanced back-ground ionization due to the occurrence of PSSM.

Research paper thumbnail of Role of neutral wind and storm time electric fields inferred from the storm time ionization distribution at low latitudes: in-situ measurements by Indian satellite SROSS-C2

Annales Geophysicae, 2005

Recently, there has been a renewal of interest in the study of the effects of solar weather event... more Recently, there has been a renewal of interest in the study of the effects of solar weather events on the ionization redistribution and irregularity generation. The observed changes at low and equatorial latitudes are rather complex and are noted to be a function of location, the time of the storm onset and its intensity, and various other characteristics of the geomagnetic storms triggered by solar weather events. At these latitudes, the effects of geomagnetic storms are basically due to (a) direct penetration of the magnetospheric electric fields to low latitudes, (b) development of disturbance dynamo, (c) changes in atmospheric neutral winds at ionospheric level and (d) changes in neutral composition triggered by the storm time atmospheric heating. <P style="line-height: 20px;"> In the present study an attempt is made to further understand some of the observed storm time effects in terms of storm time changes in zonal electric fields and meridional neutral winds. For this purpose, observations made by the Retarding Potential Analyzer (RPA) payload on board the Indian satellite SROSS-C2 are examined for four prominent geomagnetic storm events that occurred during the high solar activity period of 1997-2000. Available simultaneous observations, from the GPS satellite network, are also used. The daytime passes of SROSS-C2 have been selected to examine the redistribution of ionization in the equatorial ionization anomaly (EIA) region. In general, EIA is observed to be weakened 12-24 h after the main phase onset (MPO) of the storm. The storm time behaviour inferred by SROSS-C2 and the GPS satellite network during the geomagnetic storm of 13 November 1998, for which simultaneous observations are available, is found to be consistent. Storm time changes in the delay of received GPS signals are noted to be ~1-3 m, which is a significant component of the total delay observed on a quiet day. <P style="line-height: 20px;"> An attempt is made to identify and delineate the effects of a) meridional neutral winds, b) the development of the ring currents and c) the disturbance dynamo electric fields on the low latitude ionization distribution. The weakening of the EIA is noted to be primarily due to the decrease in the eastward electric fields driving the equatorial fountain during the daytime. The meridional neutral winds are also noted to play an important role in redistribution of ionization in the EIA region. The present results demonstrate that storm time latitudinal distribution of ionization in this region can be better understood by taking into account the meridional winds in addition to E×B drifts.

Research paper thumbnail of Average response of IEC, T and NmF2 to geomagnetic storms at Delhi

Ionospheric electron content (IEC) peak density (NmF2) and slab thickness (T) obtained at Delhi (... more Ionospheric electron content (IEC) peak density (NmF2) and slab thickness (T) obtained at Delhi (28.6 deg N, 77.2 deg E) with the help of geostationary satellite radio beacon during the ascending half of the solar cycle 21 (1975-80) have been used to investigate the total response of the F-region ionization to geomagnetic storms. The average stormtime (Dst) and local time (SD) behavior patterns were constructed by analyzing 36 storms of various intensities and duration. In general, IEC and NmF2 show enhancements during the course of a storm except for negative phase of short durations. Simultaneously, T remains below the average level for most of the storm period. Maximum enhancements in IEC and NmF2 have been observed in the forenoon hours of second day. The corresponding average equatorial Dst (H) index shows maximum negative excursions in the same period. There are distinct seasonal biases in the Dst and SD variations of all the three parameters IEC, NmF2 and T. Vertical drifts caused by equatorward neutral winds coupled with fall or rise in neutral temperature are believed to be the mechanisms responsible for enhancement and depletion of ionization.

Research paper thumbnail of IEC variations during geomagnetic storms at Delhi

Ionospheric electron content (IEC) measurements made at Delhi with the help of geostationary sate... more Ionospheric electron content (IEC) measurements made at Delhi with the help of geostationary satellite radio beacon during the ascending half of the solar cycle (1975-80) have been used to investigate the response of F-region ionization to geomagnetic storms. A total of 35 storms have been used to obtain the average storm time (Dst) and local time (SD) behavior pattern of IEC. In general, IEC shows enhancements during the course of a storm except for negative phases of short duration during nighttime. Maximum enhancements have been observed in the prenoon hours of the second day or after 40-48 hr of storm commencement. The corresponding average equatorial Dst(H) index in these periods shows maximum negative excursions. The average pattern seems to have distinct seasonal characteristics. Also, the response of IEC to increased geomagnetic activity has distinct local time bias.

Research paper thumbnail of Clock synchronisation experiment in India using symphonie satellite

A recent clock synchronization experiment between the National Physical Laboratory (NPL), New Del... more A recent clock synchronization experiment between the National Physical Laboratory (NPL), New Delhi and Space Applications Center (SAC), Ahemedabad, in India via geostationary satellite symphonie 2, stationed at 49 E longitude, is reported. A two-way transmission using a microwave transponder considered to provide the greatest precision in synchronization of two remote clocks is described.

Research paper thumbnail of Morphological study of post sunset secondary maximum in ionospheric electron content and its correlation with Faraday polarisation fluctuations and VHF amplitude scintillations at Delhi

Continuous data on the ionospheric electron content (IEC) over Delhi are reported with special at... more Continuous data on the ionospheric electron content (IEC) over Delhi are reported with special attention given to the nocturnal anomalous enhancements. The morphology of the post-sunset secondary maximum (PSSM) is isolated to determine whether there exist any correlative behavior with amplitude scintillations (AS) and/or Faraday polarization fluctuations (FPFs). Data from the periods Nov. 1975 - Aug. 1976 and Apr. 1978 - Dec. 1980 are reviewed to identify PSSMs, the time of PSSM peak, duration, and peak amplitude. The PSSM is found to have both seasonal and solar-cycle dependence including: (1) more frequent occurrence in winter; (2) an average IEC during the winter similar to the noontime maximum around the autumnal equinox; and (3) no maximum during the months of the equinox. FPFs occur only on days when both amplitude scintillations and a secondary maximum are present.

Research paper thumbnail of Growth and decay of a post-sunset equatorial anomaly at low latitudes: Control of E×B, neutral winds and daytime electrojet strength

Journal of Atmospheric and Solar-terrestrial Physics, 2006

Analyzing the 2 months ionospheric electron content (IEC) data acquired during a high solar activ... more Analyzing the 2 months ionospheric electron content (IEC) data acquired during a high solar activity period from a network of a stations whose sub-ionospheric points at 420 km were located within ± 1° of 84° meridian and covering a latitude belt of 3 21°N (magnetic), it has been shown that following sunset a fresh equatorial anomaly develops in continuation to the decaying daytime equatorial anomaly (DEA) in the low latitude belt. The occurrence of this post-sunset equatorial anomaly (PEA) is although a regular phenomenon but large day-to-day variations are seen in its strength (crest to trough ratio) and the latitude extent, PEA develops fully within 2 3 h of sunset and then decays. During its development crest of anomaly clearly shows a pole-ward movement and during decay it shows substantial equator-ward movement. While the trough of PEA is always seen over the magnetic equator, the crest of fully developed anomaly may be seen to lie at latitudes anywhere between 12°N and 21°N magnetic or more on different days. Thus the development and decay of PEA produce large latitudinal disturbance of F-region plasma in the lower latitude belt in the post-sunset hours and the occurrence of various low latitude phenomenon observed in IEC, e.g. post-sunset IEC decreases at equatorial latitudes, post-sunset IEC enhancements at 12°N or higher latitudes, pre-midnight IEC enhancements at equatorial latitudes, etc., have been attributed to the systematic development and decay of PEA. Also, the occurrence and development of PEA has been found to play an important role in the onset of equatorial plasma bubble associated ionospheric irregularities and their latitudinal growth. The equatorial ionization anomaly (EIA) is primarily caused by the ExB drifts whereas meridional winds in association with E×B drifts also seems to play some role in determining the crest of PEA.

Research paper thumbnail of Multistation study of nighttime scintillations in low latitudes - Evidence of control by equatorial F region irregularities

Radio Science, 1984

VHF nighttime scintillations, recorded during a high solar activity period at a meridian chain of... more VHF nighttime scintillations, recorded during a high solar activity period at a meridian chain of stations covering a magnetic latitude belt of 3ø-21øN (420 km subionospheric points) are analyzed to investigate the influence of equatorial spread F irregularities on the occurrence of scintillation at latitudes away from the equator. Observations show that saturated amplitude scintillations start abruptly about one and a half hours after ground sunset and their onset is almost simultaneous at stations whose subionospheric points are within 12øN latitude of the magnetic equator, but is delayed at a station whose subionospheric point is at 21øN magnetic latitude by 15 min to 4 hours. In addition, the occurrence of postsunset scintillations at all the stations is found to be conditional on their prior occurrence at the equatorial station. If no postsunset scintillation activity is seen at the equatorial station, no scintillations are seen at other stations also. The occurrence of scintillations is explained as caused by rising plasma bubbles and associated irregularities over the magnetic equator and the subsequent mapping of these irregularities down the magnetic field lines to the F region of higher latitudes through some instantaneous mechanism; and hence an equatorial control is established on the generation of postsunset scintillation-producing irregularities in the entire low-latitude belt.

Research paper thumbnail of Observation of Fresnel-type fadings at Delhi, a low midlatitude station, over half a solar cycle

An investigation is presented of the Fresnel-type fading patterns observed on recordings of orbit... more An investigation is presented of the Fresnel-type fading patterns observed on recordings of orbiting as well as geostationary satellites made at Delhi for over half a solar cycle. It is found that the occurrence of the Fresnel-type fading patterns is most frequent during low solar activity but less during high solar activity. These patterns were most frequently recorded at times between the local sunset and midnight during the summer, while the patterns occur mostly in groups with an average periodicity which ranges from 2-10 min. The results of the orbiting satellite observations show the tendency of the patterns to occur at zenith angles greater than 45 degrees, but no azimuth preference is found in the occurrence of these irregularities. The height of the ionospheric irregularities responsible for such patterns is determined to correspond to the E-region height. The occurrence of these patterns is found to be associated with the occurrence of a long-lived or a short-lived blanketing type sporadic-E at Delhi with f(0)Es values normally greater than 5 MHz, while no correlation is observed with the occurrence of spread-F. In addition, it is found that the diffused nature of the sporadic-E layer, as indicated by the range spread on ionograms, is one of the important factors associated with the production of these diffraction patterns.

Research paper thumbnail of Results of foF2 and Ne-h profiles at low latitude using recent digital ionosonde observations and their comparison with IRI2000

Journal of Atmospheric and Solar-terrestrial Physics, 2003

Using digital ionosonde observations at low-latitude station, Delhi (28.6 N, 77.2 E, mag. dip 42.... more Using digital ionosonde observations at low-latitude station, Delhi (28.6 N, 77.2 E, mag. dip 42.4 N), the diurnal and seasonal variations of the critical frequency of F2 layer (foF2) are analyzed from August 2000 to July 2001 during a high solar activity period. Also, noontime bottomside electron density (Ne-h) profiles, below the F2-peak, are derived from ionogram, using the POLAN (Report UAG-93, WDC-A, for Solar Terrestrial Physics, Boulder, Co.) program during the same period, and these profiles are then normalized to the peak height and density (hmF2, NmF2) of the F2-region. These observations are used to assess the predictability of the International Reference Ionosphere, IRI-2000 model (Radio Sc. 36(2) (2001) 261). Results show in general, a large variability, (/1sigma, /sigma is standard deviation), in foF2 during nighttime than daytime during winter and equinox, the variability of foF2 about the mean is about /+/-25% by night and /+/-15% by day. The IRI model shows a fairly good agreement with foF2 observations during daytime, however during nighttime, the discrepancies between the two exist. Comparative studies of the normalized observed profiles with those obtained with the IRI model (Bilitza, 2001) using both the options namely: Gulyaeva's (Adv. Space Res. 7 (1987) 39) model and /B0-Table (Adv. Space Res. 25(1) (2000) 89), show that during all the seasons, in general, the /B0-Tab option, reveals a better agreement with the observations, while the IRI model using Gulyaeva's option, overestimates the electron density distribution during summer and equinox, however, during winter, the model is close to the observations. The comparisons of average profile shape parameters /(B0,B1) derived from noontime observed profiles, with those obtained, using /B0-Tab option, in the IRI model, show a good agreement during all the seasons. However, /B0, /B1 obtained, using Gulyaeva's option in the IRI model, show a disagreement with the derived /B0, /B1 values during all the seasons, except during winter, for /B0 parameter.

Research paper thumbnail of Evolution and dynamics of equatorial plasma bubbles: Relationships to ExB drift, postsunset total electron content enhancements, and equatorial electrojet strength

Research paper thumbnail of Spatial estimates of Rayleigh-Taylor instability at geomagnetic equator from satellite scintillation observations at Delhi

An attempt is made to estimate the longitudinal extent of Rayleigh-Taylor instability (RTI) over ... more An attempt is made to estimate the longitudinal extent of Rayleigh-Taylor instability (RTI) over the equator from data obtained from two satellites simultaneously at Delhi. When the growth of RTI is sudden and encompassing a large longitudinal range, it is concluded that it is in response to a strong geophysical mechanism, such as an acoustic gravity wave or a severe magnetic storm. At any particular time, the irregularity scale sizes responsible for the scintillations seem to be of almost the same order over the entire RTI extent. The data suggest that increased magnetic activity may inhibit the growth of RTI and hence scintillation production, especially during high solar activity.

Research paper thumbnail of Role of neutral wind and storm time electric fields inferred from the storm time ionization distribution at low latitudes: in-situ measurements by Indian satellite SROSS-C2

Annales Geophysicae, 2005

Recently, there has been a renewal of interest in the study of the effects of solar weather event... more Recently, there has been a renewal of interest in the study of the effects of solar weather events on the ionization redistribution and irregularity generation. The observed changes at low and equatorial latitudes are rather complex and are noted to be a function of location, the time of the storm onset and its intensity, and various other characteristics of the geomagnetic storms triggered by solar weather events. At these latitudes, the effects of geomagnetic storms are basically due to (a) direct penetration of the magnetospheric electric fields to low latitudes, (b) development of disturbance dynamo, (c) changes in atmospheric neutral winds at ionospheric level and (d) changes in neutral composition triggered by the storm time atmospheric heating.

In the present study an attempt is made to further understand some of the observed storm time effects in terms of storm time changes in zonal electric fields and meridional neutral winds. For this purpose, observations made by the Retarding Potential Analyzer (RPA) payload on board the Indian satellite SROSS-C2 are examined for four prominent geomagnetic storm events that occurred during the high solar activity period of 1997-2000. Available simultaneous observations, from the GPS satellite network, are also used. The daytime passes of SROSS-C2 have been selected to examine the redistribution of ionization in the equatorial ionization anomaly (EIA) region. In general, EIA is observed to be weakened 12-24 h after the main phase onset (MPO) of the storm. The storm time behaviour inferred by SROSS-C2 and the GPS satellite network during the geomagnetic storm of 13 November 1998, for which simultaneous observations are available, is found to be consistent. Storm time changes in the delay of received GPS signals are noted to be ~1-3 m, which is a significant component of the total delay observed on a quiet day.

An attempt is made to identify and delineate the effects of a) meridional neutral winds, b) the development of the ring currents and c) the disturbance dynamo electric fields on the low latitude ionization distribution. The weakening of the EIA is noted to be primarily due to the decrease in the eastward electric fields driving the equatorial fountain during the daytime. The meridional neutral winds are also noted to play an important role in redistribution of ionization in the EIA region. The present results demonstrate that storm time latitudinal distribution of ionization in this region can be better understood by taking into account the meridional winds in addition to E×B drifts.

Research paper thumbnail of Growth and decay of a post-sunset equatorial anomaly at low latitudes: Control of E×B, neutral winds and daytime electrojet strength

Journal of Atmospheric and Solar-terrestrial Physics, 2006

Analyzing the 2 months ionospheric electron content (IEC) data acquired during a high solar activ... more Analyzing the 2 months ionospheric electron content (IEC) data acquired during a high solar activity period from a network of a stations whose sub-ionospheric points at 420 km were located within ± 1° of 84° meridian and covering a latitude belt of 3–21°N (magnetic), it has been ...

Research paper thumbnail of Ionospheric Electron Content Variation observed at Delhi during Total Solar Eclipse of 16 February 1980

Research paper thumbnail of Faraday polarization fluctuations and their dependence on post sunset secondary maximum and amplitude scintillations at Delhi

Annales Geophysicae, 2002

VHF Faraday rotation (FR) and amplitude scintillation data recorded simultaneously during May 197... more VHF Faraday rotation (FR) and amplitude scintillation data recorded simultaneously during May 1978-December 1980 Dip 42.44 • N) is analyzed in order to study the Faraday polarization fluctuations (FPFs) and their dependence on the occurrence of post sunset secondary maximum (PSSM) and amplitude scintillations. It is noted that FPFs are observed only when both PSSM and scintillations also occur simultaneously. FPFs are observed only during winter and the equinoctial months of high sunspot years. FPFs events are associated with intense scintillation activity, which is characterized by sudden onsets and abrupt endings, and are observed one to three hours after the local sunset. When FPFs and scintillation data from Delhi is compared with the corresponding data from a still lower latitude station, Hyderabad (17.35 • N, 78.45 • E), it is found that the occurrence of FPFs and scintillations at Delhi is conditional to their prior occurrence at Hyderabad, which indicates their production by a plasma bubble and the associated irregularities generated initially over the magnetic equator. In addition, FPFs and scintillation data for October 1979, when their occurrence was maximum, is also examined in relation to daytime (11:00 LT) electrojet strength (EEj) values and evening hour h'F from an equatorial location, Kodaikanal (10.3 • N, 77.5 • E). It is interesting to note that FPFs and scintillations are most likely observed when the EEj was 100 nT or more and h'F reaches around 500 km. These results show that EEj and evening hours h'F values over the magnetic equator are important parameters for predicting FPFs and scintillation activity at locations such as Delhi, where scintillation activity is much more intense as compared to the equatorial region due to the enhanced background ionization due to the occurrence of PSSM.