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.by Fazlul I. Laskar, Duggirala Pallamraju and Bhaskara Veenadhar

The Extreme Space Weather Event in 1903 October/November: An Outburst from the Quiet Sun

While the Sun is generally more eruptive during its maximum and declining phases, observational evidence shows certain cases of powerful solar eruptions during the quiet phase of the solar activity. Occurring in the weak Solar Cycle 14 just after its minimum, the extreme space weather event in 1903 October -- November was one of these cases. Here, we reconstruct the time series of geomagnetic activity based on contemporary observational records. With the mid-latitude magnetograms, the 1903 magnetic storm is thought to be caused by a fast coronal mass ejection (~1500 km/s) and is regarded as an intense event with an estimated minimum Dst' of ~-513 nT The reconstructed time series has been compared with the equatorward extension of auroral oval (~44.1{\deg} in invariant latitude) and the time series of telegraphic disturbances. This case study shows that potential threats posed by extreme space weather events exist even during weak solar cycles or near their minima.Comment: 20 pages, 5 figures, 1 table, and accepted for publication in the ApJ

Characteristics of storm time ion composition in the near-Earth plasma sheet using Geotail and RBSP measurements

Abstract Solar wind particles and ionospheric O+ ions influence the near-Earth plasma sheet and inner magnetospheric composition. We studied the behavior of H+, O+ and He+ ions (9–210 keV) for intense and moderate geomagnetic storms of solar cycle 23 and 24. An average energy density of ions over a given interval and flux enhancement is estimated using observations from satellites at different L values, namely STICS sensor on-board Geotail spacecraft and HOPE spectrometer on-board Radiation Belt Storm Probes. It provides a comprehensive understanding of the energy density variation of H+, O+ and He+ ions with the strength of IMF Bz, Psw, intensity of storms and L value. Statistically, we observed that (1) In the plasma sheet region, during main phase of the intense geomagnetic storm, and enhances, (2) is well correlated with Psw (CC = 0.86) and IMF Bz (CC = 0.85), (3) shows higher correlation (CC = 0.73) with Kp than (CC = 0.65), indicating a fairly good dependence on the strength of geomagnetic activity, (4) and dependence on L value indicates that O+/H+ and He+/H+ is more pronounced near L = 3. It is a cumulative extension of the previous studies on ion composition change which is in accordance with the existing picture of the plasma sheet and inner magnetosphere

Gravity waves in the thermosphere: solar activity dependence

A statistical study of the thermospheric gravity waves has been carried out using multiwavelength daytime oxygen airglow emission intensity and equatorial electrojet (EEJ) strength data, which originate from four different altitude regions of the thermosphere. The thermospheric daytime oxygen airglow emission intensities at wavelengths 557.7, 630.0, and 777.4 nm, obtained during the January to March period in the three years 2011–2013, have been used. The percentage number of days in which waves with spectral periods in the gravity wave range have occurred are found to be greater for the relatively higher solar activity duration (in 2013) compared to that of low solar activity (in 2011). This observation is explained to be due to the altering background atmospheric density and temperature (that vary with solar activity), which, in turn, influences the propagation and dissipation of waves. Moreover, the higher frequency gravity waves (of periods Brunt–Väisälä to 30 min) have been found to be present in greater numbers in the thermosphere compared to that of low-and-moderate frequency gravity waves (of periods 30–60 min). This behavior in the frequency selection by ambient conditions at thermospheric altitude is in accordance with earlier theoretical and simulation works. The ratios of high- to low-frequency occurrences have also been found to be greater in higher solar activity period of 2013 compared to that of the relatively low solar activity period of 2011. These results thus provide experimental evidence to the earlier simulation works suggesting similar behavior, as found here, for thermospheric gravity waves.by Fazlul I. Laskar, Duggirala Pallamraju, Bhaskara Veenadhari, T. Vijaya Lakshmi, M. Anji Reddy and Supriya Chakrabart

Gravity waves in the thermosphere: solar activity dependence

A statistical study of the thermospheric gravity waves has been carried out using multiwavelength daytime oxygen airglow emission intensity and equatorial electrojet (EEJ) strength data, which originate from four different altitude regions of the thermosphere. The thermospheric daytime oxygen airglow emission intensities at wavelengths 557.7, 630.0, and 777.4 nm, obtained during the January to March period in the three years 2011–2013, have been used. The percentage number of days in which waves with spectral periods in the gravity wave range have occurred are found to be greater for the relatively higher solar activity duration (in 2013) compared to that of low solar activity (in 2011). This observation is explained to be due to the altering background atmospheric density and temperature (that vary with solar activity), which, in turn, influences the propagation and dissipation of waves. Moreover, the higher frequency gravity waves (of periods Brunt–Väisälä to 30 min) have been found to be present in greater numbers in the thermosphere compared to that of low-and-moderate frequency gravity waves (of periods 30–60 min). This behavior in the frequency selection by ambient conditions at thermospheric altitude is in accordance with earlier theoretical and simulation works. The ratios of high- to low-frequency occurrences have also been found to be greater in higher solar activity period of 2013 compared to that of the relatively low solar activity period of 2011. These results thus provide experimental evidence to the earlier simulation works suggesting similar behavior, as found here, for thermospheric gravity waves.by Fazlul I. Laskar, Duggirala Pallamraju, Bhaskara Veenadhari, T. Vijaya Lakshmi, M. Anji Reddy and Supriya Chakrabart