Dip Lat Research Articles

Role of gravity wavelike seed perturbations on the triggering of ESF – a case study from unique dayglow observations

Abstract. First observational evidence, from the Indian longitudes, for the presence of gravity wavelike perturbations with periods of 20–30 min, acting as probable seeds for Equatorial Spread F (ESF) irregularities is described. The study is based on the daytime optical measurements of the mesopause temperature and the intensity of the thermospheric O(1D) 630.0 nm dayglow emissions using the unique MultiWavelength Dayglow PhotoMeter from Trivandrum (8.5° N; 77° E; dip lat ~0.5° N), a dip equatorial station. Measurements during the equinoctial months of a solar maximum (2001) and a solar minimum year (2006) have been used in this study. It is shown that under identical background ionospheric conditions within a solar epoch, the power of the gravity waves have a deterministic role in the generation of ESF. The mesopause temperature simultaneously observed, indicate that possible source regions for these perturbations lie in the lower atmosphere.

Correlations between Strong Range Spread-F and GPS L-Band Scintillations Observed in Hainan in 2004

Data from the DPS-4 digisonde and the GPS L-band ionospheric scintillation monitor are employed to study the correlations between strong range spread-F (SSF) and GPS L-band scintillations observed in the ionosphere over Hainan Island, China (19.5°N, 109.1°E geogr., dip lat. 9°N) in 2004. The SSF in the ionogram is different from the general range spread-F because it extends in frequency well beyond FoF2 and makes FoF2 difficult to be determined. The observations show that the SSF phenomenon is frequently accompanied by the occurrence of GPS L-band scintillations. The SSF and GPS L-band scintillations occur frequently in the equinoctial months (March, April, September, and October), but rarely in the winter (January, February, November, and December) and summer (May–August); especially, occurrence variations of the SSF and GPS L-band scintillations nearly have a same trend. The SSF and scintillations may be associated with the occurrence of topside plasma bubbles and could be explained by the generalized Rayleigh–Taylor instability.

Coherent backscatter radar imaging in Brazil: large-scale waves in the bottomside F-region at the onset of equatorial spread <I>F</I>

Abstract. The 30 MHz coherent backscatter radar located at the equatorial observatory in São Luís, Brazil (2.59° S, 44.21° W, −2.35° dip lat) has been upgraded to perform coherent backscatter radar imaging. The wide field-of-view of this radar makes it well suited for radar imaging studies of ionospheric irregularities. Radar imaging observations were made in support to the spread F Experiment (SpreadFEx) campaign. This paper describes the system and imaging technique and presents results from a bottom-type layer that preceded fully-developed radar plumes on 25 October 2005. The radar imaging technique was able to resolve decakilometric structures within the bottom-type layer. These structures indicate the presence of large-scale waves (~35 km) in the bottomside F-region with phases that are alternately stable and unstable to wind-driven gradient drift instabilities. The observations suggest that these waves can also cause the initial perturbation necessary to initiate the Generalized Rayleigh-Taylor instability leading to spread F. The electrodynamic conditions and the scale length of the bottom-type layer structures suggest that the waves were generated by the collisional shear instability. These results indicate that monitoring bottom-type layers may provide helpful diagnostics for spread F forecasting.

Equinox transition at the magnetic equator in Africa: analysis of ESF ionograms

Abstract. We study equatorial night-time F layer behaviour from quarter-hourly ionograms at Korhogo/Ivory Coast (9.2° N, 5° W, dip lat. −2.4°) during local Spring March–April 1995, declining solar flux period, according to the magnetic activity. The height and thickness of the F-layer are found to vary intensely with time and from one day to the next. At time of the equinox transition, by the end of March, a net change of the nightly height-time variation is observed. The regime of a single height peak phase before 22 March changes to up to three main F-layer height phases after 30 March, each associated to a dominant mechanism. The first phase is identified to the post-sunset E×B pulse, the second phase associated to a change in the wind circulation phenomenon and the third one attributed to pre-sunrise phenomena. The influence of the magnetic activity is identified by the increase in the second peak amplitude. After the 21 April magnetic-equinox period, the height-time morphology becomes more irregular suggesting meridional wind abatement. The initiation, the growth and the maintenance of ESF are explored in relation to these nightly variations. The Rayleigh-Taylor instability is clearly identified as main precursor phenomenon. This is followed by the P-type (F-layer peak spread) structures, the whole with no specific dependence on the magnetic activity and on the F-layer phases, in contrast to further I and F-type (Inside and Frequency spread) ESFs. We discuss our results in the light of recent advanced experiments in Peru and the pacific.

Seasonal dependence of the "forecast parameter" based on the EIA characteristics for the prediction of Equatorial Spread F (ESF)

Abstract. In an earlier study, Thampi et al. (2006) have shown that the strength and asymmetry of Equatorial Ionization Anomaly (EIA), obtained well ahead of the onset time of Equatorial Spread F (ESF) have a definite role on the subsequent ESF activity, and a new "forecast parameter" has been identified for the prediction of ESF. This paper presents the observations of EIA strength and asymmetry from the Indian longitudes during the period from August 2005–March 2007. These observations are made using the line of sight Total Electron Content (TEC) measured by a ground-based beacon receiver located at Trivandrum (8.5° N, 77° E, 0.5° N dip lat) in India. It is seen that the seasonal variability of EIA strength and asymmetry are manifested in the latitudinal gradients obtained using the relative TEC measurements. As a consequence, the "forecast parameter" also displays a definite seasonal pattern. The seasonal variability of the EIA strength and asymmetry, and the "forecast parameter" are discussed in the present paper and a critical value for has been identified for each month/season. The likely "skill factor" of the new parameter is assessed using the data for a total of 122 days, and it is seen that when the estimated value of the "forecast parameter" exceeds the critical value, the ESF is seen to occur on more than 95% of cases.

Investigation of the response of equatorial MLTI region during a partial solar eclipse through ground‐based daytime optical technique

First experimental evidence for solar eclipse induced changes in the equatorial mesopause and the thermosphere ionosphere regions using daytime optical photometer is presented. The photometer was operated in scanning mode to probe the thermosphere and mesopause regions simultaneously in North‐South direction over Trivandrum (8.5°N; 77°E; dip lat. 0.5°N) during a partial solar eclipse event on 03 October 2005. This provided a unique data set of thermosphere and mesopause regions over a horizontal distance of 600 and 240 km respectively from zenith during this event. The striking feature observed during the eclipse is the enhancement in the thermospheric O(1D) 630 nm dayglow intensity and its equatorward movement. Another noteworthy observation is an overall enhancement in the mesopause temperature, more pronounced (∼25–30 K) over zenith. These observations are discussed in context of the vertical coupling of the mesopause with thermosphere‐ionosphere region.

Highly localized cooling in daytime mesopause temperature over the dip equator during counter electrojet events: First results

The first observations of lowering of mesopause temperature during Counter Electrojet (CEJ) events over a narrow region of ∼ ±150 km centered at around magnetic equator are presented. The daytime mesopause temperature is measured over Trivandrum (8.5°N; 77°E; dip lat. 0.5°N), India using the ground based Multiwavelength Dayglow Photometer. The unique meridional scanning capability of the instrument is extensively used in this study. A lowering of temperature by as much as ∼25 K has been observed during certain CEJ events, which includes a few partial CEJs. The gravity wave tidal interaction through vertically upward wind is proposed to be manifesting as lowering in the mesopause temperature and also as CEJ. These observations are ‘new’ and address to the issues concerning the vertical coupling processes prevailing in the equatorial Mesosphere Lower Thermosphere Ionosphere (MLTI) region.

Unusual ionospheric effects observed during the intense 28 October 2003 solar flare in the Brazilian sector

Abstract. The 28 October 2003 solar flare (X-ray Class X17.2) was one of the most intense solar flares observed in the recent past. In the present investigation we show the unusual ionospheric effects observed in the Brazilian sector during this solar flare, using both the ionospheric sounding observations obtained at the UNIVAP stations: Palmas (7–10.2° S, 48.2° W, dip lat. 5.5° S) and Sao Jose dos Campos (23.2° S, 45.9° W, dip lat. 17.6° S), Brazil; and ground-based global positioning system (GPS) data obtained at the "Instituto Brasileiro de Geografia e Estatística" (IBGE) stations: Imperatriz (5.5° S, 47.5° W, dip lat. 2.9° S), Brasilia (15.9° S, 47.9° W, dip lat. 11.7° S), Presidente Prudente (22.3° S, 51.4° W, dip lat. 14.9° S), and Porto Alegre (30.1° S, 51.1° W, dip lat. 20.7° S), Brazil; on two consecutive days, viz., 27 (without solar flare) and 28 (with solar flare) October 2003. It should be mentioned that the vertical total electron content (VTEC) from the GPS observations obtained during the solar flare showed an unusual simultaneous increase in the VTEC values at about 11:00 UT at all four stations associated with the solar flare EUV enhancements and lasted for about 3 h. However, no ionograms were obtained at any of the two UNIVAP stations for a period of about 1 h between about 11:00 to 12:00 UT. Before 11:00 UT (from about 10:45 UT) and after 12:00 UT (to about 16:00 UT), the ionograms were only partial, with the low frequency end missing. During this intense solar flare, hard X-rays (1 to 10 A), as observed by the GOES 12 satellite, were ejected by the Sun during a long period (several hours), with peak radiation at about 11:10 UT. These hard X-ray radiations can penetrate further into the ionosphere, causing an increase in ionization in the lower part of ionosphere (D-region). In this way, the lack of ionograms or partial ionograms, which indicates no echoes or partial echoes of the transmitted digital ionosonde signals, are related to intense absorption, resulting in complete or partial fade-out of radio signals at the lower ionospheric heights. The partial and complete radio fade-out observed in the Brazilian sector for more than six hours must have resulted in considerable difficulties to the radio station operating agencies using ionospheric radio transmissions.

Intermittency analysis of geomagnetic storm time-series observed in Brazil

In this paper, we present and discuss the results of the statistical analysis applied to the geomagnetic field H-component (temporal resolution 1 min) observed at Vassouras (22.4°S, 43.6°W; dip lat. 18.5°S), Brazil, during the months of July 2000, March 2001, April 2001, April 2002 and May 2002. Each month studied had intense or very intense geomagnetic storm. For |Dst| max between 100 and 200 nT, the geomagnetic storms have been considered as intense, whereas for |Dst| max>200 nT, the geomagnetic storms have been considered very intense. The statistical parameter kurtosis (a measure of intermittency) shows that, the higher values in kurtosis cause the probability density function (PDF) more spiky. Furthermore, the higher values in kurtosis are possibly more efficient for the disturbed geomagnetic system to return to the steady-state conditions. In order to evaluate the importance of this feature in detail, we used the approach of the Hölder exponent applied to one of the periods studied, July 2000, representing respectively the interval of the quiet and post geomagnetically disturbed conditions. The results indicated that the part after the recovery phase had higher Hölder exponent than during quiet conditions. Thus, indicating that the intermittency introduced by disturbance acts more efficiently for return to the steady-state conditions. Analysis performed with Morlet wavelet transform and study of anisotropy in geomagnetic storm time-series show that the disturbances are important source of intermittency having small scales (with periods ≪1 day) and are much more dominant compared with the diurnal variations in geomagnetic field H-component. The present work indicates the importance of the intermittency in the recovery phase of geomagnetic storms.

Measurement of rotational temperature at Kolhapur, India

Abstract. Measurements of the hydroxyl rotational temperature for the (8,3) Meinel band have been reported from the observations of the ratio of the relative intensities of P1(2) and P1(4) lines of the OH(8,3) band at Kolhapur (16.8° N, 74.2° E, dip lat. 10.6° N) in India during the period 1 November 2002-29 April 2003 using tilting-filter photometers. Mean values of rotational temperature have been computed for 60 nights. The monthly mean value of temperature lies in the range 194(±11)-208(±18)K. The mean rotational temperature obtained from all the measurements was found to be 202±15K. The results agree with other low-latitude measurements of rotational temperature using photometric airglow techniques. Quasi-periodic fluctuations with a period of about one to two hours have been prominent on many nights. Furthermore, the results show the general agreement between observations and model (MSIS-86) predictions.

Wavelet analysis of long period oscillations in geomagnetic field over the magnetic equator

In this paper the complex Morlet Wavelet Transform is used to identify long period oscillations in the horizontal component ( H) of the geomagnetic field over the magnetic equatorial location of Trivandrum (8.5°N; 77°E; dip lat. 0.5°N) during the solar maximum period 1990/1991 and solar minimum period 1995/1996. The Morlet WT of the geomagnetic data set indicates the presence of multiple timescales, which are localized in both frequency and time. Extra long period oscillations are observed in the period range of 40–80, and 80–130 days during 1995/1996 (October–April), and 30–60, 130–180 days during 1990/1991 (October–April). This is one of the first such observations in the nature of long period oscillations in the earth's magnetic field. These observations on the existence of different long wave periods in the geomagnetic field are consistent with their findings in the lower and middle atmosphere (J. Atmos. Sol. Terr. Phys. 63 (2001) 835; J. Atmos. Sci. 29 (1972) 1109). This analysis also brings out clearly the merging of long period oscillations with short period oscillations during conditions of geomagnetic disturbance.

Studies of equatorial F-region depletions and dynamics using multiple wavelength nightglow imaging

A charge coupled device based all-sky imaging system with 180° field of view has been operating at Kolhapur (16.8°N, 74.2°E, dip lat. 10.6°N) in India on clear moonless nights to study the dynamics of equatorial ionospheric irregularities by examining the images of three oxygen lines at OI 630, 557.7 and 777.4 nm . The all-sky camera offers broad and instantaneous coverage of the movement of large-scale size ionospheric irregularities (depletions or bubbles) over a region of 1800 km at about 300 km altitude. The location of the station is important as the single image at a given instant maps the irregularities over a large area extending from the geomagnetic equator to the equatorial ionization anomaly region. Several characteristics of irregularities discussed in this report were observed during ISTEP campaign periods conducted in March–April of 1998 and 1999. One example of the signature of simultaneous depletions observed in OI 630, 557.7 and 777.4 nm images is reported. Also, the signature of midnight temperature maximum (MTM) in OI 630 nm airglow has been presented. The tilt of the MTM structure with magnetic meridian was found to change during the course of its movement towards north. The eastward plasma drift speeds ranging from 0 to 289 m/ s computed from bubble movement matches well with the drift speeds computed from VHF scintillation experiments and Jicamarca radar observations.

Effects of solar eclipse on the electrodynamical processes of the equatorial ionosphere: a case study during 11 August 1999 dusk time total solar eclipse over India

Abstract. The effects on the electrodynamics of the equatorial E- and F-regions of the ionosphere, due to the occurrence of the solar eclipse during sunset hours on 11 August 1999, were investigated in a unique observational campaign involving ground based ionosondes, VHF and HF radars from the equatorial location of Trivandrum (8.5° N; 77° E; dip lat. 0.5° N), India. The study revealed the nature of changes brought about by the eclipse in the evening time E- and F-regions in terms of (i) the sudden intensification of a weak blanketing ES-layer and the associated large enhancement of the VHF backscattered returns, (ii) significant increase in h' F immediately following the eclipse and (iii) distinctly different spatial and temporal structures in the spread-F irregularity drift velocities as observed by the HF radar. The significantly large enhancement of the backscattered returns from the E-region coincident with the onset of the eclipse is attributed to the generation of steep electron density gradients associated with the blanketing ES , possibly triggered by the eclipse phenomena. The increase in F-region base height immediately after the eclipse is explained as due to the reduction in the conductivity of the conjugate E-region in the path of totality connected to the F-region over the equator along the magnetic field lines, and this, with the peculiar local and regional conditions, seems to have reduced the E-region loading of the F-region dynamo, resulting in a larger post sunset F-region height (h' F) rise. These aspects of E-and F-region behaviour on the eclipse day are discussed in relation to those observed on the control day.Key words. Ionosphere (electric fields and currents; equatorial ionosphere; ionospheric irregularities)

First results of all-sky imaging from India

A CCD based all-sky (180° field view) airglow imaging system was operated from a low latitude station, Kolhapur (16.8° N, 74.2° E; 10.6° N dip lat.) in India to map the thermospheric motion and to study the low latitude ionospheric irregularities by measuring the nightglow signatures at 630 nm. In general, all-sky imaging techniques offer broad, instantaneous coverage (2.5 million sq km area at 150° field of view at 300 km height) of the spatial and temporal characteristics of the airglow features from the selected layers of the thermosphere and ionosphere system. In addition to the all-sky camera, photometer and radio scintillation observations together with ionospheric soundings were carried out from Kolhapur to verify the signature of small scale as well as large scale size F-region irregularities. Initial results from these measurements display good examples of north-south motion of the thermosphere with apparent drift speeds of 143–200 m/s during geomagnetically quiet nights. The north-south aligned (>1000 km) bands of plasma depletions or bubbles with east-west dimension of 50–250 km moving with drift speed of 140 m/s towards east have been observed. The zonal component of plasma drift speed matches well with the eastward component of the neutral speed computed using Hedin’s recent model. However, the meridional components of plasma drift speed are generally higher than the components of the neutral wind speed computed using the model. The results confirm the earlier quantitative investigations of equatorial depletion characteristics from Brazil and elsewhere.

First results from ground-based daytime optical investigation of the development of the equatorial ionization anomaly

Abstract. A meridional scanning OI 630.0-nm dayglow photometer was operated from Ahmedabad (17.2°N dip lat.) scanning a region towards the south in the upper atmosphere extending over ~5° in latitude from 10.2°N to 15.2°N dip latitude. From the spatial and temporal variabilities of the dayglow intensity in the scanning region we show for the first time, evidence for the passage of the crest of the equatorial ionization anomaly (EIA) in the daytime by means of a ground-based optical technique. The relationship between the daytime eastward electric field over the dip equator in the same longitude zone as inferred from the equatorial electrojet strength and the evolutionary pattern of EIA is clearly demonstrated. The latter as inferred from the dayglow measurements is shown to be consistent with our present understanding of the electrodynamical processes in the equatorial region. The present results reveal the potential of this ground-based optical technique for the investigation of ionospheric/thermospheric phenomena with unprecedented spatial and temporal resolution.

First results from ground-based daytime optical investigation of the development of the equatorial ionization anomaly

A meridional scanning OI 630.0-nm dayglow photometer was operated from Ahmedabad (17.2°N dip lat.) scanning a region towards the south in the upper atmosphere extending over ~5° in latitude from 10.2°N to 15.2°N dip latitude. From the spatial and temporal variabilities of the dayglow intensity in the scanning region we show for the first time, evidence for the passage of the crest of the equatorial ionization anomaly (EIA) in the daytime by means of a ground-based optical technique. The relationship between the daytime eastward electric field over the dip equator in the same longitude zone as inferred from the equatorial electrojet strength and the evolutionary pattern of EIA is clearly demonstrated. The latter as inferred from the dayglow measurements is shown to be consistent with our present understanding of the electrodynamical processes in the equatorial region. The present results reveal the potential of this ground-based optical technique for the investigation of ionospheric/thermospheric phenomena with unprecedented spatial and temporal resolution.

Precursor to equatorial spread‐F in OI 630.0 nm dayglow

A unique OI 630.0 nm dayglow photometer operated from Waltair (10.0° N dip lat.), a‐low‐latitude station in India, in a bidirectional mode, i.e., over zenith and at 20° elevation pointing north, has revealed features associated with the evolution of the Equatorial Ionization Anomaly (EIA) The estimated strength of the EIA on a particular day based on these features reveal that significant differences exist in the EIA contribution to OI 630.0 nm on equatorial spread‐F (ESF) and non‐ESF days. There exists a precursor in the OI 630.0 nm dayglow which enables the prediction of ESF at least 3 hours prior to its actual occurrence and hence points to the significant control of daytime EIA‐related processes in the triggering of the post‐sunset ESF.

Nightglow Measurements of 630.0 nm Line from Kolhapur, India and Their Association with TEC Enhancements.

Nightglow measurements of 630.0 nm emission line by high resolution tilting photometer have been compared with simultaneous Faraday rotation measurements of the ionosphere's total electron content (TEC) from ETS-II satellite (-130°E) at a low latitude station, KOLHAPUR (Geog. lat. 16.8°N, long. 74.2°E, dip lat. 10.6°N) in India. The preliminary observational data are presented and discussed for seven nights during March, 1989 and April, 1990. The comparative study shows that in general the airglow enhancements are associated with the night time enhancement in TEC. The airglow enhancement due to downward motion of the F layer during pre-dawn hours does not correlate with TEC fluctuations. Further, the time rate of change of content has been used to obtain rough estimate of the nightglow intensity level at the station. On comparisons 630 nm emission intensities computed from TEC values and those predicted by MSIS-86 (Hedin, 1987) and FAIM (Anderson et al., 1989) models show an accepted level of agreement. Results focus the coupling between airglow producing irregularities and TEC at low latitude ionosphere.

Equatorial night-time spread- F characteristics observed from stations in the Western Pacific

A comparative study of the ionospheric spread- F observed at an equatorial station. Port Moresby (9°24.5′S, 147°9.9′E; magn. dip lat. 18°S). Papua New Guinea, with corresponding data from the East Asian stations during a period 27 May–19 June 1983, as well as the same periods for 1981 and 1982, is presented. The Port Moresby spread- F condition is particularly outstanding and persistent at night and the occurrence of spread- F between 23 LT and 05 LT near June solstice is very high, reaching 100 % of all observed nights. The latitudinal variation of nocturnal spread- F, its duration and connection with sunspot number are analyzed. The relationship between long-term variation of night-time spread- F occurrence and solar activity during June and December solstices from 1954 up to 1984 at a typical lower magnetic latitudinal station, Wuhan (30°32.7′N, 114°21.5′E; magn. dip lat. 26°N). is also investigated.

Equatorial spread F instability conditions as determined from ionograms

Conditions of plasma irregularity (or Spread F) generation under the Rayleigh‐Taylor mechanism in the nighttime equatorial ionosphere are examined from ionogram data registered over Fortaleza (4°S, 38°W, mag. dip lat. −1.7°) Brazil for a one year period. The results show that local ionospheric conditions such as the bottomside F‐region electron density scale length and the ion‐neutral collision frequency at the base of the layer, at sunset hours, determine the generation of the spread F irregularities over the equator in the post sunset hours.