H. S. S. Sinha

Ionization hole campaign-a coordinated rocket and ground-based study at the onset of equatorial spread-F: first results

Abstract A comprehensive multi-technique campaign involving the launch of two high altitude RH-560 rockets was carried out from Sriharikota (SHAR), India, a near-equatorial rocket launching station at the onset of equatorial spread-F, along with a host of ground-based complementary experiments at other locations spread over the country. The main objectives were to obtain the background ionospheric and thermospheric conditions at the onset of equatorial spread-F, and to be able to evaluate the relative importance of the various agencies, the neutral dynamics in particular. Multiple barium cloud releases, in situ plasma diagnostic measurements, along with complementary optical and radio probing experiments were carried out as a part of this campaign. The presence of large scale gradients in the ambient electric fields at ~185 km altitude and also of vertical winds of significant magnitudes at higher altitudes were found. The presence of large scale irregularities in the ion densities at heights above 250 km in a region of negative background density gradient is one of the significant new results. The details of the Ionization hole campaign along with the first results are presented and discussed in the context of the present knowledge of the phenomenon of equatorial spread-F.

Role of the equatorial ionization anomaly in the development of the evening prereversal enhancement of the equatorial zonal electric field

[1] During the evening prereversal enhancement of the zonal electric field (EPRE) that begins around 1700 LT when the F region neutral winds turn eastward, as assumed here, and continues till the postsunset zonal electric field reversal time, an overall positive feedback is shown to occur between the eastward electric field in the lower side of the flux tube integrated (LSFTI) F region and the increased flux tube integrated Pedersen conductivity (FTIC) of the tropical F region. The increase in this FTIC can take place because of the increase in electron density through the increase in solar flux and the intensification of the equatorial ionization anomaly (EIA). While the influence of EIA on EPRE is immediate, the growth time for EIA is 2 to 3 h. Therefore, for a strong EPRE to occur, a fairly strong EIA is required at 1700 LT which is then sustained by the electric field associated with EPRE during its growth period. This study suggests that the postsunset eastward electric field is due to the combined currents in the equatorial electrojet and the LSFTI F regions that get diverted from the daytime Sq current system and flow from the presunset region toward the postsunset zonal electric field reversal region. Thereafter these currents turn and flow poleward to meet the current continuity requirement of the F region dynamo followed by a westward turn to rejoin the daytime Sq current system in midlatitudes. Thus the currents responsible for EPRE are an extension of the daytime Sq current system.

Equatorial spread-F campaign over SHAR

Abstract A RH-560 rocket instrumented with a Langmuir Probe and two pairs of double probes was launched from SHAR (dip 14 ° N) at 2130 hr LT (1ST) on 4 October 1988 during spread-F conditions. As a part of this campaign VHF scintillations at 136.1 MHz were also recorded, using the beacon onboard the geostationary satellite ETS-II (130 ° E). A new digital ionosonde (KEL) installed at SHAR during the first week of September 1988 provided the basic ionospheric information. Ionosonde and scintillation data during this campaign (15 September–4October), along with the Langmuir probe data, are described. The occurrence of scintillations and spread-F during the campaign period was very high, with maximum hourly percentage occurrences of about 80 and 70%, respectively. Generally the onset of spread-F occurred slightly before 1900 hr LT when scintillations in the satellite signal also start appearing. This is followed soon by the range spreading on the ionosonde trace. On days with spread-F the minimum virtual height of the F-layer, h′F, rose to about 400 km in the post-sunset period while on days without spread-F it rose to only about 300–320 km. The electric field reversal, as inferred from the h′F variations, occurred around 1930 hr LT on spread-F days and around 1900 hr LT on days without spread-F. The ionograms, taken at 5 min interval on several nights, were used to determine the vertical drift velocities and these exceeded 50 m/s on spread-F days in the post-sunset period. At the time of the rocket launch, a strong blanketing type of sporadic-E with several multiple reflections appeared around 105 km. The electron density profile obtained from the Langmuir probe data also showed an extremely sharp layer at 105 km and another prominent layer at 130 km, both during the ascent and the descent. The electron density irregularities associated with spread-F were seen in different altitude regions, the most prominent being from 210 to 250 km and from 300 to 320 km. The power spectra of the electron density fluctuations have been obtained both from the in situ probe data and the scintillation data.

Ionospheric scintillation observations from SHAR

Abstract VHF intensity scintillations associated with equatorial spread-F were recorded at SHAR (dip 13°N), India, using the 136.1 MHz beacon from the geostationary satellite ETS-2. The observations were made during the periods 12 March–26 April 1986 and later from 15 September to 5 October 1988. The maximum hourly occurrence frequency of scintillations was about 50% during March 1986 and 30% during April 1986. It increased to about 85% during September–October 1988. The onset time of scintillations was generally between 20 and 21 h IST (77.5°E) during 1986 and between 1900 and 1930 h IST during 1988. Scintillations lasted for a longer duration of night in 1988 than in 1986. Data for a few nights were analysed to study the temporal variation of the scintillation index S4, computed at close time intervals. Wave-like fluctuations with a dominant periodicity of about 1 h and another periodicity of about 20 min were seen. Power spectra were studied for 225 data samples of 100 s duration each for the night of 4–5 October 1988. The spectral index p[P(ƒ) ∞ ƒ −p ] ranged between 2 and 6.8 with a mean value of 4. The temporal variation of the spectral index (p) also showed a periodic fluctuation. The higher the S4 index the steeper were the spectral slopes.