C. Vineeth

A comparative study of daytime mesopause temperatures obtained using unique ground based optical and meteor wind radar techniques over the magnetic equator

[1] This paper presents the first ever comparison of daytime Mesopause OH rotational temperatures as determined using Meinel (8-3) dayglow emissions with those obtained using the recently established and collocated, meteor radar (SKiYMET) over the magnetic equator in India. The measurements were made over Thiruvananthapuram (8.5°N, 76.5°E, 0.5°N diplat.) for January-May 2005 period. Overall agreement had been good (±20 K) with all the gross features coming out fairly well. Further, the temperatures estimated using these two techniques, exhibit significant wavelike modulations with periods spanning ∼3-65 days with varying phase differences, presumably due to their interaction with the longer period waves like the semiannual oscillation (SAO). The consistency of the temperature variabilities at two different altitudes viz. 87 and 94 km enables investigation of the mesosphere lower thermosphere dynamic coupling.

Planetary wave-tidal interactions over the equatorial mesosphere-lower thermosphere region and their possible implications for the equatorial electrojet

[1] Optically measured daylight mean mesopause temperatures over a dip equatorial station, Trivandrum (8.5°N; 77°E; dip lat. 0.5°N), have been analyzed in conjunction with simultaneously measured equatorial electrojet (EEJ)-produced magnetic field at the surface. The signature of planetary wave-tidal interactions in the mesosphere-lower thermosphere (MLT) region has been observed for the first time in the day-to-day variability in the EEJ, i.e., the time of its peaking and the duration, as inferred from the EEJ-produced magnetic field on the ground. The present study shows that the planetary wave of quasi 16 day periodicity plays an important role in causing these variabilities, especially during the winter months. The quasi 16 day wave is found to be modulating the mesopause temperature (MT), duration, and time of the maximum EEJ intensity (D EEJ and T EEJ ). During positive excursions of the planetary wave, T EEJ showed a shift toward evening, while the MT showed an increase and D EEJ showed a broadening. Similarly, all these parameters exhibited an opposite trend during negative excursions. The planetary wave-tidal interactions and subsequent modification of the tidal components have been shown to be responsible for the observed variations. This study presents a new perspective addressing the day-to-day variability of the EEJ.

Impact of Sudden Stratospheric Warming of 2009 on the Equatorial and Low‐Latitude Ionosphere of the Indian Longitudes: A Case Study

Using the Equatorial Electrojet (EEJ) induced surface magnetic field and Total Electron Content (TEC) measurements, we investigated the impact of the Sudden Stratospheric Warming (SSW) of January 2009 on the equatorial electrodynamics and low-latitude ionosphere over the Indian longitudes. Results indicate that the intensity of EEJ and the TEC over low-latitudes (extending up to 30°N) exhibit significant perturbations during and after the SSW peak. One of the interesting features is the deviation of EEJ and TEC from the normal quiet time behaviour well before the onset of the SSW. This is found to be coincided with the beginning of enhanced planetary wave (PW) activity over high-latitudes. The substantial amplification of the semidiurnal perturbation after the SSW peak is seen to be coinciding with the onset of new and full moon. The response of TEC to SSW is found to be latitude dependent as the near-equatorial (NE) stations show the semidiurnal perturbation only after the SSW peak. Another notable feature is the presence of reduced ionization in the night sector over the NE and low-latitude regions, appearing as an ‘ionization hole’, well after the SSW peak. The investigation revealed the existence of a quasi-16 day wave in the TEC over low-latitudes similar to the one present in the EEJ strength. These results have been discussed in the light of changes in the dynamical background because of enhanced PW activity during SSW, which creates favourable conditions for the amplification of lunar tides, and their subsequent interaction with the lower thermospheric tidal fields.

Unusual depletion of OI 630.0 nm dayglow and simultaneous mesopause heating during the penetration of interplanetary electric field over dip equator

Evidence for simultaneous changes in the thermosphere and mesopause associated with penetration electric fields using the OI 630.0 nm and OH (8, 3) dayglow measurements are presented. An unusual decrease in the thermospheric OI 630.0 nm dayglow and an intense heating (~30 K) at OH emission altitudes over a magnetic dip equatorial station, Trivandrum, India, were observed on 9 April 2006, coincident with the penetration of a noontime westward interplanetary electric field. The Sounding of the Atmosphere using Broadband Emission Radiometry observations on board TIMED satellite also revealed heating at the mesopause heights during this period. In addition, ionosonde and magnetometer observations indicated the presence of strong penetration electric fields. We propose that strong heating at the mesopause resulted in the intrusion of additional neutrals, like N2, into the emission altitudes and quenched the O(1D) atoms therein. The reduced Cowling conductivity at the E region inferred using magnetometer observations further corroborates this proposed mechanism.