Smitha V. Thampi

On the latitudinal changes in ionospheric electrodynamics and composition based on observations over the 76–77°E meridian from both hemispheres during a geomagnetic storm

The relative contributions of the composition disturbances and the disturbance electric fields in the redistribution of ionospheric plasma is investigated in detail by taking the case of a long-duration positive ionospheric storm that occurred during 18–21 February 2014. GPS total electron content (TEC) data from the Indian Antarctic station, Bharti (69.4°S, 76.2°E geographic), the northern midlatitude station Hanle (32.8°N, 78.9°E geographic), northern low-latitude station lying in the vicinity of the anomaly crest, Ahmedabad (23.04°N, 72.54°E geographic, dip latitude 17°N), and the geomagnetic equatorial station, Trivandrum (8.5°N, 77°E geographic, dip latitude 0.01°S) are used in the study. These are the first simultaneous observations of TEC from Bharti and Hanle during a geomagnetic storm. The impact of the intense geomagnetic storm (Dst~−130 nT) on the southern hemisphere high-latitude station was a drastic reduction in the TEC (negative ionospheric storm) starting from around 0330 Indian standard time (IST) on 19 February which continued till 21 February, the maximum reduction in TEC at Bharti being ~35 TEC units on 19 February. In the northern hemisphere midlatitude and equatorial stations, a positive ionospheric storm started on 19 February at around 0900 IST and lasted for 3 days. The maximum enhancement in TEC at Hanle was about ∼25 TECU on 19 February while over Trivandrum it was ~10 TECU. This long-duration positive ionospheric storm provided an opportunity to assess the relative contributions of disturbance electric fields and composition changes latitudinally. The results indicate that the negative ionospheric storm over Bharti and the positive ionospheric storm over Hanle are the effect of the changes in the global wind system and the storm-induced composition changes. At the equatorial latitudes, the positive ionospheric storm was due to the interplay of prompt penetration electric field and disturbance dynamo electric field.

Toward prediction of L band scintillations in the equatorial ionization anomaly region

[1] The first observations of the duration and spread of equatorial spread F (ESF) at the magnetic equator and their relationship with the L band scintillations in the equatorial ionization anomaly (EIA) region have been presented here. The analysis is done for the equinoctial months of low solar activity period 2005–2006 and the moderate solar activity year 2004. Ionosonde and CRABEX data from Trivandrum and GPS data from four stations in the EIA region centered around 77°E meridian have been used for the study. The results show that the maximum scintillation index (s4) in the EIA region is linearly dependent on the spread of ESF traces for both the equinoxes. The corresponding duration of L band scintillations is also found to be linearly dependent on the duration of ESF at the magnetic equator. Further, the study for the first time reveals the plausible use of the ESF prediction parameter during 1600–1845 IST period for predicting L band scintillations and its inverse relationship with F10.7 cm flux.

On the evening time exosphere of Mars: Result from MENCA aboard Mars Orbiter Mission

The Mars Exospheric Neutral Composition Analyser (MENCA) aboard the Indian Mars Orbiter Mission (MOM) is a quadrupole mass spectrometer which provides in situ measurement of the composition of the low-latitude Martian neutral exosphere. The altitude profiles of the three major constituents, i.e., amu 44 (CO2), amu 28 (N2 + CO), and amu 16 (O) in the Martian exosphere during evening (close to sunset terminator) hours are reported using MENCA observations from four orbits of MOM during late December 2014, when MOMs periapsis altitude was the lowest. The altitude range of the observation encompasses the diffusively separated region much above the well-mixed atmosphere. The transition from CO2 to O-dominated region is observed near 270 km. The mean exospheric temperature derived using these three mass numbers is 271 ± 5 K. These first observations corresponding to the Martian evening hours would help to provide constraints to the thermal escape models.

An ensemble average method to estimate absolute TEC using radio beacon‐based differential phase measurements: Applicability to regions of large latitudinal gradients in plasma density

A GNU Radio Beacon Receiver (GRBR) system for total electron content (TEC) measurements using 150 and 400 MHz transmissions from Low-Earth Orbiting Satellites (LEOS) is fabricated in house and made operational at Ahmedabad (23.04°N, 72.54°E geographic, dip latitude 17°N) since May 2013. This system receives the 150 and 400 MHz transmissions from high-inclination LEOS. The first few days of observations are presented in this work to bring out the efficacy of an ensemble average method to convert the relative TECs to absolute TECs. This method is a modified version of the differential Doppler-based method proposed by de Mendonca (1962) and suitable even for ionospheric regions with large spatial gradients. Comparison of TECs derived from a collocated GPS receiver shows that the absolute TECs estimated by this method are reliable estimates over regions with large spatial gradient. This method is useful even when only one receiving station is available. The differences between these observations are discussed to bring out the importance of the spatial differences between the ionospheric pierce points of these satellites. A few examples of the latitudinal variation of TEC during different local times using GRBR measurements are also presented, which demonstrates the potential of radio beacon measurements in capturing the large-scale plasma transport processes in the low-latitude ionosphere.

Direct observational evidence for disturbance dynamo on the daytime low‐latitude ionosphere: A case study based on the 28 June 2013 space weather event

A case of the westward disturbance dynamo (DD) electric field, influencing the daytime equatorial and low-latitude ionosphere, during a geomagnetic storm that occurred on 28–29 June 2013 is presented. The GPS total electron content (TEC) observations from a network of stations in the Indian equatorial, low and middle latitude regions along with the radio beacon TEC, ionosonde, and magnetic field observations are used to study the storm time behavior of the ionosphere. Negative ionospheric storm effects were seen over the low and middle latitudes during the storm time due to the presence of a westward DD electric field. Observations show that the suppression of the equatorial ionization anomaly (EIA) from the morning hours itself on 29 June 2013 took place due to the prevailing westward DD electric field, providing evidence for the model calculations by Balan et al. (2013). Simulations using the GITM model also agree well with our results. The present study gains importance as the direct observational evidences for disturbance dynamo effects on the daytime low-latitude ionosphere and the EIA are sparse, as it has been difficult to delineate it from the compositional disturbances.

GNU Radio Beacon Receiver (GRBR) observations of large-scale wave structure (LSWS) and equatorial spread F (ESF)

“GNU Radio Beacon Receiver (GRBR) is a new digital receiver based on GNU Radio and USRP (Universal Software Radio Peripheral) [Yamamoto, 2008]. In this paper, the GRBR observations of large-scale wave structure (LSWS) and the subsequent development of equatorial spread F (ESF) using total electron content (TEC) derived from the ground based reception of signals from the radio beacon on board C/NOFS (Communications/Navigation Outage Forecasting System) satellite are presented. The other evidences of LSWS include the ‘satellite traces’ observed in ionograms. These observations show that LSWS appears to play an important role in the development of ESF.