S. Sarkhel

Simultaneous sodium airglow and lidar measurements over India: A case study

[1] In order to understand the substantial variation of sodium (Na) airglow intensity from one night to another, a case study is performed using simultaneous, campaign-based measurements of Na airglow and Na lidar during March 2007 from Gadanki (13.5°N, 79.2°E), India, during postmidnight hours. The altitude profiles of mesospheric ozone, temperature, and pressure available for the nearest location during the local postmidnight hours are also obtained from the SABER instrument on board the TIMED satellite and are used in conjunction with the above measurements. The average Na airglow intensity level on 20 March 2007 is found to be less compared to that on the next night despite average Na concentration being larger by at least a factor of three. In order to explain the observation, volume emission rates of Na airglow are calculated for both of the nights using the measured parameters. The enhanced quenching due to the ambient gas is suggested to be responsible for the reduced Na airglow intensity level on 20 March 2007 despite higher Na concentration.

A case study on occurrence of an unusual structure in the sodium layer over Gadanki, India

The height-time-concentration map of neutral sodium (Na) atoms measured by a Na lidar during the night of 18 to 19 March 2007 over Gadanki, India (13.5° N, 79.2° E) reveals an unusual structure in the Na layer for around 30 min in the altitude range of 92 to 98 km which is similar to the usual ‘C’ type structures observed at other locations. In order to understand the physical mechanism behind the generation of this unusual event, an investigation is carried out combining the data from multiple instruments that include the meteor wind radar over Thiruvananthapuram, India (8.5° N, 77° E) and the SABER instrument onboard the TIMED satellite. The temperature and wind profiles from the data set provided by these instruments allow us to infer the Richardson number which is found to be noticeably less than the canonical threshold of 0.25 above 92 km over Thiruvananthapuram suggesting the plausible generation of Kelvin-Helmholtz (KH) billows over southwestern part of the Indian subcontinent. Based on the average wind speed and direction over Thiruvananthapuram, it is proposed that the KH-billow structure was modified due to the background wind and was advected with it in nearly ‘frozen-in’ condition (without significant decay) in the northeastward direction reaching the Na lidar location (Gadanki). This case study, therefore, presents a scenario wherein the initially deformed KH-billow structure survived for a few hours (instead of a few minutes or tens of minutes as reported in earlier works) in an apparently ‘frozen-in’ condition under favorable background conditions. In this communication, we suggest a hypothesis where this deformed KH-billow structure plays crucial role in creating the abovementioned unusual structure observed in the Na layer over Gadanki.

Erratum to: A case study on occurrence of an unusual structure in the sodium layer over Gadanki, India

Erratum After publication, it was found in the text that the words ‘Figure 6’ and ‘Figure 7’ were printed in the reverse order in several places of the article by Sarkhel et al. (2015). The details are as follows: Page 7, Right column, Last paragraph, Line 8: ‘Figure 6a’ should be read as ‘Figure 7a’ Page 9, Left column, Last paragraph, Line 11: ‘Figure 7’ should be read as ‘Figure 6’ Page 9, Right column, Last paragraph, Line 1: ‘Figure 6’ should be read as ‘Figure 7’ Page 10, Left column, Line 6: ‘Figure 6a’ should be read as ‘Figure 7a’ Page 10, Left column, Line 25: ‘Figure 6a’ should be read as ‘Figure 7a’ Page 10, Left column, Line 33: ‘Figure 6a’ should be read as ‘Figure 7a’ Page 10, Left column, Line 37: ‘Figure 6b’ should be read as ‘Figure 6’ Page 10, Right column, Line 15: ‘Figure 6a’ should be read as ‘Figure 7a’ Page 10, Right column, Line 19: ‘Figure 6b’ should be read as ‘Figure 7b’ Page 11, Left column, Section: Role of sporadic-E activity, Line 12: ‘Figure 6a’ should be read as ‘Figure 7a’

The Ionospheric Impact of an ICME-Driven Sheath Region Over Indian and American Sectors in the Absence of a Typical Geomagnetic Storm: ICME SHEATH REGION AND PP ELECTRIC FIELD

On 13 April 2013, the ACE spacecraft detected arrival of an interplanetary shock at 2250 UT, which is followed by the passage of the sheath region of an interplanetary coronal mass ejection (ICME) for a prolonged (18-hr) period. The polarity of interplanetary magnetic field Bz was northward inside the magnetic cloud region of the ICME. The ring current (SYM-H) index did not go below −7 nT during this event suggesting the absence of a typical geomagnetic storm. The responses of the global ionospheric electric field associated with the passage of the ICME sheath region have been investigated using incoherent scatter radar measurements of Jicamarca and Arecibo (postmidnight sector) along with the variations of equatorial electrojet strength over India (day sector). It is found that westward and eastward prompt penetration (PP) electric fields affected ionosphere over Jicamarca/Arecibo and Indian sectors, respectively, during 0545–0800 UT. The polarities of the PP electric field perturbations over the day/night sectors are consistent with model predictions. In fact, DP2-type electric field perturbations with ∼40-min periodicity are found to affect the ionosphere over both the sectors for about 2.25 hr during the passage of the ICME sheath region. This result shows that SYM-H index may not capture the full geoeffectivenss of the ICME sheath-driven storms and suggests that the PP electric field perturbations should be evaluated for geoeffectiveness of ICME when the polarity of interplanetary magnetic field Bz is northward inside the magnetic cloud region of the ICME.

Erratum to: Dependence of mesospheric Na and Fe distributions on electron density at Arecibo

Author details Space and Atmospheric Sciences, Arecibo Observatory, SRI International, Arecibo, PR, USA. Electrical and Computer Engineering Department, Miami University, Oxford, OH, USA. Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India. Radar Space Sciences Laboratory, The Pennsylvania State University, 323 Electrical Engineering East, University Park, PA, USA. Radar Systems Engineering Department Radar, Intelligence ASELSAN Inc, Ankara, Turkey.