Duggirala Pallamraju

HIRISE: a ground-based high-resolution imaging spectrograph using echelle grating for measuring daytime airglow=auroral emissions

Abstract We discuss a new optical technique for measuring daytime optical emissions. We elucidate the method of analysis for retrieving the daytime airglow and auroral emissions using this instrument and explain the procedure used to account for the Ring effect contribution. Finally, we show sample results of thermospheric oxygen dayglow emissions obtained by this instrument.

A multiwavelength investigation of the ring effect in the day sky spectrum

Ring effect refers to the ‘filling-in‧ of the Fraunhofer absorption lines in the day sky spectrum as compared to the solar spectrum. Rotational Raman scattering is believed to be the main cause for this excess in the sky spectrum. Earlier measurements showed contradictory behavior of this effect with solar zenith angle and wavelength. It is important to take proper account of this effect as it otherwise results in overestimating the dayglow emission intensities and underestimating the number densities of atmospheric trace gases. The present study details the results obtained from a simultaneous 11-wavelength investigation carried out using a newly built daytime spectrograph. This data demonstrates that the absorption line strength (normalized depth × half width) has a major control on the Ring effect contribution irrespective of the solar zenith angle and the wavelength.

Vertical coupling of atmospheres: dependence on strength of sudden stratospheric warming and solar activity

Comprehensive behavior of the low-latitude upper atmosphere during sudden stratospheric warming (SSW) events at varying levels of solar activity has been investigated. The equatorial electrojet (EEJ) strength and the total electron content (TEC) data from low latitudes over Indian longitudes during the mid-winter season in the years 2005 to 2013 are used in this study. Five major and three minor SSW events occurred in the observation duration, wherein the solar activity had varied from minimum (almost no sunspots) to mini-maximum (approximately 50 sunspots of the solar cycle 24). Spectral powers of the large-scale planetary wave (PW) features in the EEJ and the TEC have been found to be varying with solar activity and SSW strengths. Specially, the spectral powers of quasi-16-day wave variations during the three very strong SSW events in the years 2006, 2009, and 2013 were found to be very high in comparison with those of other years. For these major events, the amplitudes of the semi-diurnal tides and quasi-16-day waves were found to be highly correlated and were maximum around the peak of SSW, suggesting a strong interaction between the two waves. However, this correlation was poor and the quasi-16-day spectral power was low for the minor events. A strong coupling of atmospheres was noted during a relatively high solar activity epoch of 2013 SSW, which was, however, explained to be due to the occurrence of a strong SSW event. These results suggest that the vertical coupling of atmospheres is stronger during strong major SSW events and these events play an important role in enabling the coupling even during high solar activity.

HiTIES: A High Throughput Imaging Echelle Spectrogragh for ground‐based visible airglow and auroral studies

We describe an imaging spectrograph with high spectral resolution (0.3 A) that uses an echelle grating without the conventional cross disperser. The spectrograph covers the entire visible region. However, only 5 – 20 wavelength regions can be observed simultaneously. The total wavelength coverage is limited by the number of available pixels on the detector and for a 1024 × 1024 pixels detector is ∼ 100 – 300 A. Order separation is achieved by a mosaic of interference filters located at the image plane.

Simultaneous ground based observations of an auroral arc in daytime/twilighttime O I 630.0 nm emission and by incoherent scatter radar

We present the ground-based oxygen 630.0 nm daytime optical measurements of a discrete auroral arc from Sondre Stromfjord, Greenland. The optical measurements were made using an imaging echelle spectrograph built at Boston University. We show that the auroral optical signature extracted from the blue-sky background agrees closely in both space and time with the aurorally enhanced electron densities at 200 km altitude obtained simultaneously by the incoherent scatter radar. The dayglow measurements are also in good agreement with the integrated emission rates modeled using the measured N e , T e , and T i profiles from the radar. The results reported in this paper demonstrate the potential of this spectrograph to observe aurora during daytime, and it promises to be a valuable complement to the existing tools for the investigation of upper atmospheric phenomena.

Short- and Long-Timescale Thermospheric Variability as Observed from OI 630.0 nm Dayglow Emissions from Low Latitudes

We carried out high-cadence (5 min) and high-spatial resolution (2deg magnetic latitude) observations of daytime OI 630.0 nm airglow emission brightness from a low-latitude station to understand the behavior of neutral dynamics in the daytime. The results indicate that the wave periodicities of 12.20 min, and 2 h exist over a wide spatial range of around 8deg-12deg magnetic latitudes. The 20.80 min periodicities in the dayglow seem to appear more often in the measurements closer to the magnetic equator and not at latitudes farther away. Further, periodicities in that range are found to be frequent in the variations of the equatorial electrojet (EEJ) strength as well. We show that wave periodicities due to the neutral dynamics, at least until around 8deg magnetic latitude, are influenced by those that affect the EEJ strength variation as well. Furthermore, the average daily OI 630.0 nm emission brightness over 3 months varied in consonance with that of the sunspot numbers indicating a strong solar influence on the magnitudes of dayglow emissions.

Does sudden stratospheric warming induce meridional circulation in the mesosphere thermosphere system

Oxygen dayglow emissions at multiple wavelengths that emanate from different heights (from around 130 km to peak altitude of the ionospheric F region) over a low-latitude location showed systematic enhancements in intensities throughout the daytime hours during four sudden stratospheric warming (SSW) events that occurred in the years 2010–2013. The lower thermospheric temperatures at 120 km obtained from the Sounding of the Atmosphere using Broadband Emission Radiometry instrument are found to be enhanced during SSW events at arctic latitudes and show a gradient with a decrease toward low-latitudes. During these events, the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics Doppler Interferometer measurements showed equatorward winds in the mesosphere lower thermosphere (MLT) altitudes over high-latitudes. Both the high-latitude lower thermospheric temperature enhancements and the MLT region equatorward winds occur nearly simultaneously with the observed enhancements in the atomic oxygen dayglow emission intensities at all the wavelengths over low-latitudes. Based on these measurements and other supporting information, it is proposed that a new cell of meridional circulation in the MLT winds is set up during SSW events, which enables transport of atomic oxygen from high-to-low latitudes. Such an additional contribution of oxygen density over low-latitudes interacts with lower thermospheric daytime dynamics in that region and is attributed to be the cause for the observed enhancement in the oxygen daytime optical emission intensities over low-latitudes. The results presented here provide experimental evidence to such circulation alluded to by earlier simulation studies.

Role of the equatorial ionization anomaly in the development of the evening prereversal enhancement of the equatorial zonal electric field

[1] During the evening prereversal enhancement of the zonal electric field (EPRE) that begins around 1700 LT when the F region neutral winds turn eastward, as assumed here, and continues till the postsunset zonal electric field reversal time, an overall positive feedback is shown to occur between the eastward electric field in the lower side of the flux tube integrated (LSFTI) F region and the increased flux tube integrated Pedersen conductivity (FTIC) of the tropical F region. The increase in this FTIC can take place because of the increase in electron density through the increase in solar flux and the intensification of the equatorial ionization anomaly (EIA). While the influence of EIA on EPRE is immediate, the growth time for EIA is 2 to 3 h. Therefore, for a strong EPRE to occur, a fairly strong EIA is required at 1700 LT which is then sustained by the electric field associated with EPRE during its growth period. This study suggests that the postsunset eastward electric field is due to the combined currents in the equatorial electrojet and the LSFTI F regions that get diverted from the daytime Sq current system and flow from the presunset region toward the postsunset zonal electric field reversal region. Thereafter these currents turn and flow poleward to meet the current continuity requirement of the F region dynamo followed by a westward turn to rejoin the daytime Sq current system in midlatitudes. Thus the currents responsible for EPRE are an extension of the daytime Sq current system.

Daytime wave characteristics in the mesosphere lower thermosphere region: Results from the Balloon‐borne Investigations of Regional‐atmospheric Dynamics experiment

Results obtained from a joint INDO-US experiment on the investigations of mesosphere/lower thermosphere wave dynamics using balloon-borne optical dayglow measurements in combination with ground-based optical, radio, and magnetometer data are presented. Ultraviolet OI 297.2 nm dayglow emissions that originate at ~ 120 km were measured from low-magnetic latitudes from onboard a balloon on 8 March 2010. This paper describes the details of a new spectrograph that is capable of making high spectral resolution (0.2 nm at 297.2 nm) and large (80°) field of view ultraviolet dayglow emission measurements and presents the first results obtained from its operation onboard a high-altitude balloon. Waves of scale sizes ranging from 40 to 80 km in the zonal direction were observed in OI 297.2 nm emissions. Meridional scale sizes of similar waves were found to be 200 km as observed in the OI 557.7 nm emissions that originate from ~ 100 km. Periodicities were also derived from the variations of equatorial electrojet strength and ionospheric height on that day. Common periodicities of waves (in optical, magnetic, and radio measurements) were in the range of 16 to 30 min, which result in intrinsic horizontal wave speeds in the range of 21 to 77 m s−1. It is argued that gravity waves of such scale sizes and speeds at these heights are capable of propagating well into the thermosphere because the background wind directions were favorable. These waves were potentially capable of forming the seeds for the generation of equatorial plasma irregularities which did occur on that night.

On the latitudinal distribution of mesospheric temperatures during sudden stratospheric warming events

Mesospheric rotational temperatures from O2(0-1) and OH(6-2) band nightglow emissions that originate from 94 and 87 km altitudes, respectively, were obtained from a low-latitude location, Mount Abu (24.6°N, 72.8°E), in India using a high spectral resolution grating spectrograph, which showed significant enhancements during the major sudden stratospheric warming (SSW) event of January 2013. To investigate the relationship of these enhancements in the context of SSW occurrences, a detailed study was carried out for 11 SSW events that occurred during 2004–2013 using SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) data. In addition to SABER, Optical Spectrograph and Infrared Imaging System and Solar Occultation For Ice Experiment mesospheric temperatures were also used which showed similar latitudinal behavior as obtained by SABER. The longitudinal mean mesospheric temperatures at different latitudes of Northern and Southern Hemispheres have been derived. It is found that during SSW events the well-known mesospheric cooling over the Northern Hemispheric high latitudes turns to heating over midlatitudes and then reverts to cooling closer to equatorial regions. This trend continues into the Southern Hemisphere as well. These variations in the mesospheric temperatures at different latitudes have been characterized based on northern hemispheric stratospheric temperature enhancements at high latitudes during SSW periods. In comparison with the COSPAR International Reference Atmosphere-86-derived temperatures, the SABER temperatures show an increase/decrease in Southern/Northern Hemisphere. Such a characterization in mesospheric temperatures with respect to latitudes reveals a hitherto unknown intriguing nature of the latitudinal coupling in the mesosphere that gets set up during the SSW events.