D. Chakrabarty

A multiwavelength daytime photometer—a new tool for the investigation of atmospheric processes

Technological innovations in optics in the form of a spiral mask system and in electronics in the form of on-line gate scanning of Fabry-Perot fringes and critical balancing of gate widths for complete background rejection have allowed unambiguous detection of faint dayglow emission features of multiple wavelengths buried in the bright daytime background continuum. The technical details of this unique multiwavelength daytime photometer (MWDPM) and its performance characteristics together with its potential application for the investigation of several geophysical phenomena are presented and discussed.

Evolution and Consequences of Interacting CMEs of 9 – 10 November 2012 Using STEREO/SECCHI and In Situ Observations

Understanding the kinematic evolution of coronal mass ejections (CMEs) in the heliosphere is important to estimate their arrival time at Earth. The kinematics of CMEs can change when they interact or collide with each other as they propagate in the heliosphere. In this article, we analyze the collision and post-interaction characteristics of two Earth-directed CMEs that were launched successively on 9 and 10 November 2012. To do this, we used white-light imaging observations from STEREO/SECCHI and in situ observations taken from the Wind spacecraft. We tracked two density-enhancement features associated with the leading and trailing edge of the 9 November CME and one density enhanced feature associated with the leading edges of the 10 November CME by constructing J-maps. We found that the leading edge of the 10 November CME interacted with the trailing edge of the 9 November CME. We also estimated the kinematics of these features of the CMEs and found a significant change in their dynamics after interaction. In in situ observations, we identified distinct structures associated with interacting CMEs and also observed heating and compression as signatures of their interaction. Our analysis shows an improvement in the arrival-time prediction of CMEs when their post-collision dynamics are used instead of the pre-collision dynamics. By estimating the true masses and speeds of these colliding CMEs, we investigated the nature of the observed collision, which is found to be almost perfectly inelastic. The investigation also places in perspective the geomagnetic consequences of the two CMEs and their interaction in terms of occurrence of geomagnetic storms and triggering of magnetospheric substorms.

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.

Conjugate hemisphere ionospheric response to the St. Patrick's Day storms of 2013 and 2015 in the 100°E longitude sector

The effects of the St. Patricks Day geomagnetic storms of 2013 and 2015 in the equatorial and low-latitude regions of both hemispheres in the 100°E longitude sector is investigated and compared with the response in the Indian sector at 77°E. The data from a chain of ionosondes and GPS/Global Navigation Satellite Systems receivers at magnetic conjugate locations in the 100°E sector have been used. The perturbation in the equatorial zonal electric field due to the prompt penetration of the magnetospheric convective under shielded electric field and the over shielding electric field gives rise to rapid fluctuations in the F2 layer parameters. The direction of IMF Bz and disturbance electric field perturbations in the sunset/sunrise period is found to play a crucial role in deciding the extent of prereversal enhancement which in turn affect the irregularity formation (equatorial spread F) in the equatorial region. The northward (southward) IMF Bz in the sunset period inhibited (supported) the irregularity formation in 2015 (2013) in the 100°E sector. Large height increase (hmF2) during sunrise produced short-duration irregularities during both the storms. The westward disturbance electric field on 18 March inhibited the equatorial ionization anomaly causing negative (positive) storm effect in low latitude (equatorial) region. The negative effect was amplified in low midlatitude by disturbed thermospheric composition which produced severe density/total electron content depletion. The longitudinal and hemispheric asymmetry of storm response is observed and attributed to electrodynamic and thermospheric differences.

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.

Significant upper thermospheric contribution to the O dayglow emission: first ground based evidence

Abstract Out of the two source regions of O ( 1 S ) green line 557.7 nm emission namely the one in the lower thermosphere and the other in the upper thermosphere, the upper thermospheric source is known to contribute only 15–20% of the total 557.7 nm volume emission during nighttime. On the other hand, successful, simultaneous ground based measurements of daytime 557.7 nm O ( 1 S ) and 630.0 nm O ( 1 D ) thermospheric emissions with the multiwavelength daytime photometer (MWDPM) reveal striking similarities in their variabilities, the inference being the dominance of thermospheric contribution in 557.7 nm O ( 1 S ) emission throughout the day, in conformity with the space borne measurements made else where. The first set of ground-based measurements are presented and discussed.

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.

Solar wind flow angle and geoeffectiveness of corotating interaction regions: First results

A total of 43 CIR-induced geomagnetic storms during the unusually deep solar minimum of solar cycle 23 (2006-2010) were identified using a superposed epoch analysis technique. Of these 43 events, a detailed cross spectrum analyses, between the variations in the Z-component of the interplanetary magnetic field (IMF Bz) and the equatorial electrojet (EEJ) strength, were performed for 22 events when the daytime EEJ strengths from Jicamarca were available. The analyses revealed that the ∼30 and ∼60 min periodic components in IMF Bz were causally related to the EEJ strength subject to the average solar wind flow being radial to within 6∘ at L1 during the interval for which EEJ strengths were considered. This investigation elicits the important role of average solar wind azimuthal flow angle in determining the geo-effectiveness of CIR events.

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’

Role of filament plasma remnants in ICMEs leading to geomagnetic storms

We studied three interplanetary coronal mass ejections associated with solar eruptive filaments. Filament plasma remnants embedded in these structures were identified using plasma, magnetic and compositional signatures. These features when impacted the Earth’s terrestrial magnetosphere ionosphere system, resulted in geomagnetic storms. During the main phase of associated storms, along with high density plasma structures, polarity reversals in the Y-component (dawn-to-dusk) of the interplanetary electric field seem to trigger major auroral substorms with concomitant changes in the polar ionospheric electric field. Here, we examine the cases where plasma dynamics and magnetic structuring in the presence of the prompt penetration of the electric field into the equatorial ionosphere affected the space weather while highlighting the complex geomagnetic storm-substorm relationship.