[1] This paper provides the first observations of EIA asymmetry by receiving beacon transmissions onboard low earth orbiting satellites from a single station ground-based receiver. The EIA strength and asymmetry are derived from the latitudinal profiles of TEC obtained from a radio beacon receiver at Trivandrum (8.5°N, 77°E, diplat $0.5°N). These two parameters, obtained well ahead of the onset time of ESF, are shown to have a definite role on the subsequent ESF activity. In the present paper, both these factors are combined to define a new 'threshold parameter' for the generation of ESF. It has been shown that this parameter can define the state of the 'background ionosphere' conducive for the generation of ESF irregularities much prior to its onset.

The altitude profiles of argon‐40 (Ar) in the Martian exosphere are reported using Mars Exospheric Neutral Composition Analyser aboard Indian Mars Orbiter Mission (MOM) from four orbits during December 2014 (Ls = 250°–257°), when MOM's periapsis altitude was the lowest. The upper limit of Ar number density corresponding to this period is ∼5 × 105 cm−3 (∼250 km), and the typical scale height is ∼16 km, corresponding to an exospheric temperature of ∼275 K. However, on two orbits, the scale height over this altitude region is found to increase significantly making the effective temperature >400 K. Neutral Gas and Ion Mass Spectrometer observations on the Mars Atmosphere and Volatile Evolution mission also indicate that the change in slope in Ar density occurs near the upper exosphere (around 230–260 km). These observations indicate significant suprathermal CO2 and Ar populations in the Martian exosphere. Significant wave‐like perturbations are observed but only on certain days when suprathermal population is seen. Pickup ion‐induced heating is discussed as the other viable source.

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 MOM's 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.

[1] The paper presents the observations and modeling of an additional layer in the low-latitude ionosphere in Indian longitudes. The signatures of the additional layer are observed as ledges or humps between the equatorial ionization anomaly trough and crest (EIA) in the latitudinal profiles of total electron content (TEC), measured using a single ground-based beacon receiver located at Trivandrum (8.5°N, 77°E, dip 0.5°N) in India. The ground-based ionograms also show the presence of the so-called F 3 layer for a short duration corresponding to these signatures, and the layer is found to drift upward to the topside ionosphere. The study provides first observational evidence that the so-called ''humps'' in the latitudinal variation of TEC are nothing but the upward propagating F 3 layer. This conclusion is supported by theoretical modeling using the Sheffield University Plasmasphere Ionosphere Model. It is shown that upward ExB drift and strong equatorward neutral wind (perturbed by atmospheric waves) can produce the humps in the latitudinal variation of TEC through the reduction in the downward diffusion of ionization along geomagnetic field lines. The model results also show that the F 3 layer drifts to the topside and forms topside ledges.

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Ionization ledges in the topside ionosphere have been observed from Trivandrum (8.5°N, 77°E, 0.5°N dip) using single station Total Electron Content (TEC) measurements. These measurements are made using a ground based radio beacon receiver. The receiver measures the relative phase between 150 MHz and 400 MHz beacon transmissions from a Low Earth Orbiting Satellite (LEOS). The ionization ledges, which could be observed, using only topside sounders so far, are seen to have their clear signatures in relative TECs derived using differential Doppler measurements. Simulations were also carried out to understand how the ionospheric topside ledges of various intensities would manifest in the latitudinal profiles of the relative TEC. The magnetometer observations indicate the presence of the Equatorial Counter Electrojet (CEJ) on the days when the ledge is observed. The study demonstrates the potential of radio beacon measurements for observing topside ionization ledges, and phenomena similar to it.

The present work pertains to the eclipse-induced gravity wave modulations in the ionosphere-thermosphere region over Trivandrum (8.5°N, 77°E, dip 2°N) during the annular solar eclipse of 15 January 2010. Electron density and neutral wind rocket payload measured horizontal winds and electron densities at E region altitudes, and ionosonde-derived f o F 1 and f o F 2 parameters are used to analyze the characteristics of the eclipse-induced gravity waves. The analysis reveals an intensification of gravity waves with periods around 30-100 min during the peak phase of the eclipse. The vertical wavelength of the prevalent wave is found to be around 2 km. The role of gravity wave-induced winds in generating blanketing E s over the equator is also examined.