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On the equinoctial asymmetry in the threshold height for the occurrence of equatorial spread F
Journal of Atmospheric and Solar-Terrestrial Physics 124 (2015) 59–62
Contents lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Short Communication
On the equinoctial asymmetry in the threshold height for the occurrence of equatorial spread F G. Manju n, M.K. Madhav Haridas Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695022, Kerala, India
art ic l e i nf o
a b s t r a c t
Article history: Received 30 April 2014 Received in revised form 20 October 2014 Accepted 20 January 2015 Available online 21 January 2015
In the present study, the threshold height (h′Fc) for ESF occurrence irrespective of the polarity of the meridional winds during vernal and autumnal equinoxes is investigated using a large database spread over 1993–2008. The characteristics of the thermosphere during the two equinoxes have also been examined using TIMED/GUVI O/N2 data. The major aspects that have emerged are (i) presence of significant equinoctial asymmetry in h′Fc for the years examined, (ii) increase in the equinoctial asymmetry of h′Fc with solar activity, (iii) presence of significant asymmetry in O/N2 and TIEGCM simulations of normalized atomic oxygen mass density, between the two equinoxes. & 2015 Published by Elsevier Ltd.
Keywords: Threshold height O/N2 Equinoctial asymmetry Solar flux
1. Introduction Collisional Rayleigh–Taylor (CRT) instability mechanism operating in the post sunset bottom side F region is found to be primarily responsible for equatorial spread F (ESF) irregularities which manifest in the equatorial F region during nighttime (Haerendel, 1973). ESF variability with season, longitude, solar cycle, and magnetic activity has been extensively investigated (Kelley and McClure, 1981; Abdu et al., 1981; Abdu, 2001). The meridional neutral winds also play a critical role in controlling ESF occurrence in addition to seed perturbations (Whitehead, 1971; Fejer et al., 1999). The importance of thermospheric neutral winds over and above the E B drifts for ESF onset is also shown (Maruyama and Matuura, 1984; Maruyama, 1988). Recent studies have shown that there is a threshold height (h′Fc) of the base of the F region above which ESF occurs irrespective of meridional wind polarity, while below this height ESF occurs only if the wind is equatorward (Devasia et al., 2002; Jyoti et al., 2004). The seasonal, solar and magnetic activity variability of h′Fc have also been brought out (Manju et al., 2007; Madhav Haridas et al., 2013). The phenomenon of ESF is one of the most important challenges faced by the scientific community working in the field. This phenomenon needs to be examined from diverse angles to get a definitive understanding which is a prelude to the deterministic prediction. In the light of this, the present results which show clear asymmetry in threshold height for occurrence of n
Corresponding author. Fax: þ 91 471 2706535. E-mail address: [email protected] (G. Manju).
http://dx.doi.org/10.1016/j.jastp.2015.01.008 1364-6826/& 2015 Published by Elsevier Ltd.
ESF during the two equinoxes and the possible role of asymmetric neutral densities in the same, assumes significant scientific relevance.
2. Data and method of analysis The ionosonde data obtained from Trivandrum (8.5°N, 76.9°E, dip latitude 0.5°N) and SHAR (13.7°N, 80.2°E, dip latitude 5.5°N), for the period 1993–2008, in the Indian longitude sector are used for this investigation. A very huge database, of around 600 days of ionosonde data from each of the two stations during vernal and autumnal equinoxes, has been processed and analyzed to determine the threshold heights. The method given by Krishna Murthy et al. (1990) is used in the present study for deriving the meridional winds. The meridional winds and the post sunset F region heights (h′F at Trivandrum) just before the onset of ESF are obtained for each day. Only those events for which ESF occurs before 21:00 IST (Indian Standard Time, UT þ5.5 h) are considered as ESF days for this study in order to avoid drifted irregularities from other locations. Another point to be noted is that even on ESF days we have taken the heights and meridional winds just prior to ESF occurrence to avoid the errors due to ESF contamination. The criterion of ESF occurrence irrespective of the polarity of meridional winds is used to determine h′Fc (Devasia et al., 2002; Jyoti et al., 2004; Manju et al., 2007; Madhav Haridas et al., 2013). For non-ESF days, the meridional wind and h′F at 19:00 IST are taken to be representative of the post sunset conditions.
60
G. Manju, M.K. Madhav Haridas / Journal of Atmospheric and Solar-Terrestrial Physics 124 (2015) 59–62
Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED)/Global Ultra Violet Imager (GUVI) O/N2 data for the period 2002–2007 are also used. The TIMED satellite orbit is at 630 km and the inclination is 74°. Daily averaged column number density ratios O/N2 are delineated for the latitude–longitude grid covering latitudes 4.5°N to 12.5°N and longitudes 72°E to 82°E for the period from 10 LT to 16.5 LT. GUVI disk measurements of column O/N2, are used for this study. The sunlit disk emission measurements of two channels (atomic oxygen emission at 135.6 nm and N2 LBHS: 140–160 nm bands), are used to derive O/N2. This ratio is derived for an altitude where the N2 column density is 1017 cm 2 (approximately 140 km) (Strickland et al., 2004; Zhang et al., 2004; Qian et al., 2009). The estimated error in O/N2 is 5% at low latitudes and for low O/N2 (Strickland et al., 2004). Only magnetically quiet days with Ap o18 are considered for the present study. The number of days analyzed to estimate h′Fc for each season is constrained by the availability of two station ionosonde data which is a pre-requisite for the above methodology of wind estimation. The average O/N2 information is derived from a database of around 50 days for each season.
3. Results There is a critical level of h′F called the threshold height, h′Fc for a season, below which equatorward winds are essential for ESF occurrence while above this, the wind direction does not matter. In the latter domain, the role of electrodynamics manifested through the vertical drift is the controlling factor where as in the former it is the polarity of the meridional neutral wind. This concept of threshold height was first introduced by Devasia et al. (2002). Later on, in the past decade many workers have examined different aspects of this very significant parameter which has a critical control on ESF day to day variability (Jyoti et al., 2004; Manju et al., 2007; Madhav Haridas et al., 2013). The concept has been extensively discussed in these works and the readers may refer these for detailed information. In all these studies the h′Fc had been determined only for equinox in general considering the behavior to be identical during the two equinoxes. In the present study, we have looked into the behavior of h′Fc for the two equinoxes separately. Accordingly, the h′Fc is estimated for all the years between 1993 and 2008 for which data are available. Fig. 1 shows the yearly variation of h′Fc for the vernal and autumnal equinoxes along with the corresponding mean F10.7 cm fluxes. The error values depicted by the error bars are estimated as discussed in Madhav Haridas et al., 2013 . The pattern
Fig. 1. Yearly variation of h’Fc for the vernal and autumnal equinoxes along with the corresponding mean F10.7 cm fluxes.
of variation of F10.7 cm fluxes for both equinoxes is similar. On the other hand, in the case of h′Fc, the higher values in vernal equinox compared to autumnal equinox are evident from the figure for all the years examined. It is also observed that the magnitude of the asymmetry in h’Fc increases with increasing solar activity. To understand the concurrent behavior of the thermosphere, the O/N2 distribution is examined. Fig. 2a shows the mean O/N2 pattern over the Indian peninsula for the vernal (left panels) and autumnal (right panels) equinoxes of 2004. The white bins indicate erroneous points which have been removed. As is evident, there is significant asymmetry in the O/N2 ratios between the two equinoxes. The TIEGCM model simulations of atomic oxygen mass mixing ratio (which is the normalized atomic oxygen mass density with respect to that of air) for the vernal (left panels) and autumnal (right panels) equinoxes of 2004 are shown in Fig. 2b. The model simulations confirm the higher values of normalized mass density in vernal equinox. The variability of the simulated parameter for the three years (2003, 2004 and 2005) for which the simulations and h′Fc are available for both equinoxes also follow the pattern shown by the respective O/N2 and h′Fc parameters. Since sufficient number of points are not available to examine the O/N2 variation between the two equinoxes with respect to the corresponding h′Fc variations, we have looked into the inter equinoctial variability of the two parameters with respect to F10.7 cm flux. It is logically expected that as the solar flux increases the thermosphere expands proportionately but differently for the two equinoxes and this will correspondingly modulate the h′Fc values for the two equinoxes. Fig. 3 shows the scatter plot of F10.7 cm flux vs. δh′Fc (bottom panel) and F10.7 cm flux vs. δ(O/N2) (top panel) for the years for which the respective data are available. Here, δh’Fc corresponds to the difference in h′Fc between the two equinoxes for a particular year, while δ(O/N2) represents the corresponding O/N2 difference. Fig. 3 reveals the increasing asymmetry in O/N2 as solar activity increases. The correlation coefficient is 0.81. The relationship is statistically significant (495%). Similarly the asymmetry in h′Fc also increases with solar flux. In this case also the correlation coefficient of 0.8 is statistically significant (4 95%).
4. Discussion Devasia et al. (2002) for the first time introduced the now well known concept of threshold height as a critical parameter controlling the day to day variability of ESF. They demonstrated that converging/diverging thermospheric meridional winds significantly modulate ESF occurrence pattern with the equatorward wind being a pre-requisite when h′F is below the threshold height (h′Fc), for the R–T instability to get triggered. Above the critical height the polarity of the meridional winds did not matter. The seasonal/solar/magnetic activity dependence of h′Fc, for the occurrence of ESF irrespective of the magnitude and polarity of the meridional winds has been revealed in past studies (Jyoti et al., 2004; Manju et al., 2007; Madhav Haridas et al., 2013). Further, the modulation of h′Fc by neutral density changes is also presented (Madhav Haridas et al., 2013). In the past the behavior of the ionosphere–thermosphere during the two equinoxes was believed to be identical. Off late, several studies have brought out significant equinoctial asymmetry in different ionospheric parameters. Equinoctial asymmetry in ESF occurrence has been investigated in the Indonesian sector wherein they have shown that meridional wind asymmetry is a possible mechanism (Maruyama et al., 2009). Sripathi et al. (2011) examined the equinoctial asymmetry in scintillation occurrence in the Indian sector and proposed the asymmetry in the electron density distribution and meridional winds as possible causative
G. Manju, M.K. Madhav Haridas / Journal of Atmospheric and Solar-Terrestrial Physics 124 (2015) 59–62
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Fig. 2. (a) Seasonal mean O/N2 pattern for the vernal (left panels) and autumnal (right panels) equinoxes of 2004. (b) TIEGCM simulations of atomic oxygen mass mixing ratio for the vernal (left panels) and autumnal (right panels) equinoxes of 2004.
ionosphere have been reported by several others also (Ren et al., 2011; Liu et al., 2010; Balan et al., 1997). The present study highlights a new aspect of the asymmetry between the two equinoxes. In this study we observe significant asymmetry in the threshold height h′Fc between the two equinoxes with consistently higher threshold heights being observed in vernal equinox compared to autumnal equinox. This is an important new finding which underlines the distinct differences in the role of neutral dynamics in ESF triggering during the two equinoxes. It is also seen that the threshold heights for the two equinoxes show direct relation with the F10.7 cm flux. Further, the difference in h′Fc between the two equinoxes is observed to be higher during high solar activity period compared to solar minimum. At or above the threshold height, the ESF occurrence on a particular day can be wholly ascribed to the CRT instability mechanism (Manju et al., 2007). The growth rate of the collisional Rayleigh–Taylor (CRT) instability is given by Fig. 3. Scatter plot of F10.7 cm flux vs. δh’Fc (bottom panel) and F10.7 cm flux vs. δ(O/N2) (top panel).
mechanisms. Manju et al. (2013) also reported equinoctial asymmetry in ESF occurrence and discussed the possible role of asymmetric meridional winds. Dasgupta et al. (1983) studied the equinoctial asymmetry in equatorial and low latitude F region ionization distribution and attributed it to neutral composition changes. Aspects related to equinoctial asymmetry in the
ϒ = 1/L [g /υ in ]
(1)
Where, υin is the ion-neutral collision frequency, g is the acceleration due to gravity and L is the electron density gradient scale length. The seasonal changes in neutral density in the upper atmosphere have been studied using GUVI O/N2 ratio by Qian et al. (2009). Modulations in neutral composition and consequent changes in neutral density have been discussed in their paper. They suggested that neutral composition significantly affects neutral density in the upper thermosphere due to the difference in
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atomic/ molecular weight between the two principal thermospheric constituents, O and N2. According to them, the change in O/N2 ratio has the effect of changing mean molecular mass and, therefore, the density scale height, which in turn modifies the mass density at a given altitude in the upper thermosphere. They have also shown good correspondence between satellite drag derived neutral densities and GUVI O/N2 ratio. In the light of this result, we are using GUVI-O/N2 as a proxy for neutral density in this study. O/N2 shows substantial equinoctial asymmetry with higher values in vernal equinox compared to autumnal equinox. This implies higher neutral density in vernal equinox than autumnal equinox. The seasonal variations in thermospheric atomic oxygen densities (from Jacchia model) and their impact on post sunset peak vertical drift are reported by Batista et al. (1986). They showed that at equatorial station Fortaleza, a lower neutral density value resulted in lower peak Vd amplitude. This result dwells on the importance of vertical drift in controlling the occurrence of ESF. Our study brings out the point that even for lower mean vertical drifts the minimum requisite height in a season for ESF to occur is determined by the neutral density prevalent during that season. That is in autumnal equinox, the overall drifts themselves are lower than in vernal equinox, but the neutral density being lower the consequent favorable υin factor facilitates occurrence of ESF at lower altitudes. Qian et al. (2009) have also shown TIEGCM simulations depicting increased neutral density in vernal than autumnal equinox. This increased neutral density in vernal equinox results in increased υin which in turn causes a reduced growth rate compared to autumnal equinox at a given altitude. This means that optimum growth rate is achieved only at a higher altitude in vernal equinox compared to autumnal equinox thereby resulting in a higher h′Fc value in vernal equinox. We have also obtained the TIEGCM simulated normalized atomic oxygen density and found it to be following the pattern shown by h′Fc. The direct relation between F10.7 cm flux and δO/N2 as well as δh′Fc are brought out in this study. As the solar flux increases, the atmosphere expands and the scale height increases leading to increased neutral densities at all levels. It is seen in the present study that as solar activity increases the increase in neutral density is less for autumnal equinox compared to vernal equinox. Accordingly, the increase in h′Fc is also correspondingly lower in autumnal equinox. These aspects point towards the role of neutral density asymmetry in producing the observed h′Fc asymmetry. Apart from υin, the electron density scale length L (Eq. (1)), seed perturbations etc. also modulate the growth rate of CRT instability. These aspects are not addressed in the present work.
5. Conclusions In the present study, the equinoctial asymmetry of the threshold height (h′Fc), during the period 1993–2008 is investigated for the first time. The asymmetric behavior of the thermospheric neutral density is also brought out. The important new findings are i) consistently higher h′Fc for vernal equinox compared to autumnal equinox; ii) higher O/N2 in vernal equinox compared to autumnal equinox; iii) TIEGCM simulations reveal higher normalized atomic oxygen mass density in vernal equinox than in autumnal equinox; iv) clear solar flux dependence of δh′Fc and δO/N2.
Acknowledgments This work is supported by Indian Space Research Organization. One of the authors Mr. M. K. Madhav Haridas thanks Indian Space Research Organization (ISRO) for the financial support provided. The present study made use of the thermospheric O/N2 ratios obtained from TIMED/GUVI instrument. The authors acknowledge the use of TIE-GCM model runs which were obtained from the website http://ccmc.gsfc.nasa.gov.
References Abdu, M.A., et al., 1981. Some characteristics of spread F at the magnetic equatorial station Fortaleza. J. Geophys. Res. 86 (A8), 6836–6842. Abdu, M.A., 2001. Outstanding problems in equatorial ionosphere-thermosphere electrodynamics relevant to spread F. J. Atmos. Sol. Terr. Phys. 63, 869–884. Balan, N., Otsuka, Y., Fukao, S., 1997. New aspects in the annual variation of the ionosphere observed by the MU Radar. Geophys. Res. Lett. 24 (18), 2287–2290. Batista, I.S., Abdu, M.A., Bittencourt, J.A., 1986. Equatorial F-region vertical plasma drifts: seasonal and longitudinal asymmetries in the American sector. J. Geophys. Res. 91, 12055–12064. Dasgupta, A., Anderson, D.N., Klobuchar, J.A., 1983. Equatorial F-region ionization differences between March and September, 1979. Adv. Space Res. 10, 199–202. Devasia, C.V., et al., 2002. On the plausible linkage of thermospheric meridional winds with equatorial spread F. J. Atmos. Sol. Terr. Phys 64, 1–12. Fejer, B.G., et al., 1999. Effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread F. J. Geophys. Res. 104, 19859–19869. Haerendel, G., 1973. Theory of equatorial spread F, Report, Max-Plank inst. fur Phys. and Astrophys., Munich. Jyoti, N., et al., 2004. Threshold height (h′Fc) for the meridional wind to play a deterministic role in the bottom side equatorial spread F and its dependence on solar activity. Geophys. Res. Lett. 31, L12809. http://dx.doi.org/10.1029/ 2004GL01945. Kelley, M.C., McClure, J.P., 1981. Equatorial spread-F: a review of recent experimental results. J. Atmos. Terr. Phys. 43, 427–435. Krishna Murthy, B.V., et al., 1990. Night time equatorial thermospheric meridional winds from h’F data. J. Geophys. Res. 95, 4307–4310. Liu, L., He, M., Yue, X., Ning, B., Wan, W., 2010. Ionosphere around equinoxes during low solar activity. J. Geophys. Res. 115, A09307. http://dx.doi.org/10.1029/ 2010JA015318. Madhav Harida, M.K., Manju, G., Pant, T.K., 2013. First observational evidence of the modulation of the threshold height h’Fc for the occurrence of equatorial spread F by neutral composition changes. J. Geophys. Res. Space Phys. 118, 3540–3545. http://dx.doi.org/10.1002/jgra.50331. Manju, G., et al., 2007. On the seasonal variations of the threshold height for the occurrence of equatorial spread F during solar minimum and maximum years. Ann. Geophys. 25, 855–861. Manju, G., et al., 2013. Equinoctial asymmetry in the occurrence of equatorial spread-F over Indian longitudes during moderate to low solar activity period. Indian J. Radio Space Phys. 41, 240–246. Maruyama, T., 1988. A diagnostic model for equatorial spread F 1. Model description and applications to the electric field and neutral wind effects. J. Geophys. Res. 93, 14611–14622. Maruyama, T., et al., 2009. Equinoctial asymmetry of a low-latitude ionospherethermosphere system and equatorial irregularities: Evidence for meridional wind control. Ann. Geophys. 27 (2009), 2027–2034. Maruyama, T., Matuura, N., 1984. Longitudinal variability of annual changes in activity of ESF and plasma depletions. J. Geophys. Res. 89, 10903–109012. Qian, L., et al., 2009. Seasonal variation of thermospheric density and composition. J. Geophys. Res. 114, A01312. http://dx.doi.org/10.1029/2008JA013643. Ren, Z., Wan, W., Liu, L., Chen, Y., Le, H., 2011. Equinoctial asymmetry of ionospheric vertical plasma drifts and its effect on F-region plasma density. J. Geophys. Res. 116, A02308. http://dx.doi.org/10.1029/2010JA016081. Sripathi, S., et al., 2011. Study of equinoctial asymmetry in the Equatorial Spread F (ESF) irregularities over Indian region using multi-instrument observations in the descending phase of solar cycle 23. J. Geophys. Res. 116, A11302. http://dx. doi.org/10.1029/2011JA016625. Strickland, D.J., et al., 2004. Quiet-time seasonal behavior of the thermosphere seen in the far ultraviolet dayglow. J. Geophys. Res. 109 (A1), A01302. http://dx.doi. org/10.1029/2003JA010220. Whitehead, J., 1971. Ionization disturbances caused by gravity waves in the presence of an electrostatic field and background wind. J. Geophys. Res. 76, 238–241. Zhang, Y., et al., 2004. O/N2 changes during 1–4 October 2002 storms: IMAGE SI-13 and TIMED/GUVI observations. J. Geophys. Res. 109, A10308. http://dx.doi.org/ 10.1029/2004JA010441.
Contents lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Short Communication
On the equinoctial asymmetry in the threshold height for the occurrence of equatorial spread F G. Manju n, M.K. Madhav Haridas Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695022, Kerala, India
art ic l e i nf o
a b s t r a c t
Article history: Received 30 April 2014 Received in revised form 20 October 2014 Accepted 20 January 2015 Available online 21 January 2015
In the present study, the threshold height (h′Fc) for ESF occurrence irrespective of the polarity of the meridional winds during vernal and autumnal equinoxes is investigated using a large database spread over 1993–2008. The characteristics of the thermosphere during the two equinoxes have also been examined using TIMED/GUVI O/N2 data. The major aspects that have emerged are (i) presence of significant equinoctial asymmetry in h′Fc for the years examined, (ii) increase in the equinoctial asymmetry of h′Fc with solar activity, (iii) presence of significant asymmetry in O/N2 and TIEGCM simulations of normalized atomic oxygen mass density, between the two equinoxes. & 2015 Published by Elsevier Ltd.
Keywords: Threshold height O/N2 Equinoctial asymmetry Solar flux
1. Introduction Collisional Rayleigh–Taylor (CRT) instability mechanism operating in the post sunset bottom side F region is found to be primarily responsible for equatorial spread F (ESF) irregularities which manifest in the equatorial F region during nighttime (Haerendel, 1973). ESF variability with season, longitude, solar cycle, and magnetic activity has been extensively investigated (Kelley and McClure, 1981; Abdu et al., 1981; Abdu, 2001). The meridional neutral winds also play a critical role in controlling ESF occurrence in addition to seed perturbations (Whitehead, 1971; Fejer et al., 1999). The importance of thermospheric neutral winds over and above the E B drifts for ESF onset is also shown (Maruyama and Matuura, 1984; Maruyama, 1988). Recent studies have shown that there is a threshold height (h′Fc) of the base of the F region above which ESF occurs irrespective of meridional wind polarity, while below this height ESF occurs only if the wind is equatorward (Devasia et al., 2002; Jyoti et al., 2004). The seasonal, solar and magnetic activity variability of h′Fc have also been brought out (Manju et al., 2007; Madhav Haridas et al., 2013). The phenomenon of ESF is one of the most important challenges faced by the scientific community working in the field. This phenomenon needs to be examined from diverse angles to get a definitive understanding which is a prelude to the deterministic prediction. In the light of this, the present results which show clear asymmetry in threshold height for occurrence of n
Corresponding author. Fax: þ 91 471 2706535. E-mail address: [email protected] (G. Manju).
http://dx.doi.org/10.1016/j.jastp.2015.01.008 1364-6826/& 2015 Published by Elsevier Ltd.
ESF during the two equinoxes and the possible role of asymmetric neutral densities in the same, assumes significant scientific relevance.
2. Data and method of analysis The ionosonde data obtained from Trivandrum (8.5°N, 76.9°E, dip latitude 0.5°N) and SHAR (13.7°N, 80.2°E, dip latitude 5.5°N), for the period 1993–2008, in the Indian longitude sector are used for this investigation. A very huge database, of around 600 days of ionosonde data from each of the two stations during vernal and autumnal equinoxes, has been processed and analyzed to determine the threshold heights. The method given by Krishna Murthy et al. (1990) is used in the present study for deriving the meridional winds. The meridional winds and the post sunset F region heights (h′F at Trivandrum) just before the onset of ESF are obtained for each day. Only those events for which ESF occurs before 21:00 IST (Indian Standard Time, UT þ5.5 h) are considered as ESF days for this study in order to avoid drifted irregularities from other locations. Another point to be noted is that even on ESF days we have taken the heights and meridional winds just prior to ESF occurrence to avoid the errors due to ESF contamination. The criterion of ESF occurrence irrespective of the polarity of meridional winds is used to determine h′Fc (Devasia et al., 2002; Jyoti et al., 2004; Manju et al., 2007; Madhav Haridas et al., 2013). For non-ESF days, the meridional wind and h′F at 19:00 IST are taken to be representative of the post sunset conditions.
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G. Manju, M.K. Madhav Haridas / Journal of Atmospheric and Solar-Terrestrial Physics 124 (2015) 59–62
Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED)/Global Ultra Violet Imager (GUVI) O/N2 data for the period 2002–2007 are also used. The TIMED satellite orbit is at 630 km and the inclination is 74°. Daily averaged column number density ratios O/N2 are delineated for the latitude–longitude grid covering latitudes 4.5°N to 12.5°N and longitudes 72°E to 82°E for the period from 10 LT to 16.5 LT. GUVI disk measurements of column O/N2, are used for this study. The sunlit disk emission measurements of two channels (atomic oxygen emission at 135.6 nm and N2 LBHS: 140–160 nm bands), are used to derive O/N2. This ratio is derived for an altitude where the N2 column density is 1017 cm 2 (approximately 140 km) (Strickland et al., 2004; Zhang et al., 2004; Qian et al., 2009). The estimated error in O/N2 is 5% at low latitudes and for low O/N2 (Strickland et al., 2004). Only magnetically quiet days with Ap o18 are considered for the present study. The number of days analyzed to estimate h′Fc for each season is constrained by the availability of two station ionosonde data which is a pre-requisite for the above methodology of wind estimation. The average O/N2 information is derived from a database of around 50 days for each season.
3. Results There is a critical level of h′F called the threshold height, h′Fc for a season, below which equatorward winds are essential for ESF occurrence while above this, the wind direction does not matter. In the latter domain, the role of electrodynamics manifested through the vertical drift is the controlling factor where as in the former it is the polarity of the meridional neutral wind. This concept of threshold height was first introduced by Devasia et al. (2002). Later on, in the past decade many workers have examined different aspects of this very significant parameter which has a critical control on ESF day to day variability (Jyoti et al., 2004; Manju et al., 2007; Madhav Haridas et al., 2013). The concept has been extensively discussed in these works and the readers may refer these for detailed information. In all these studies the h′Fc had been determined only for equinox in general considering the behavior to be identical during the two equinoxes. In the present study, we have looked into the behavior of h′Fc for the two equinoxes separately. Accordingly, the h′Fc is estimated for all the years between 1993 and 2008 for which data are available. Fig. 1 shows the yearly variation of h′Fc for the vernal and autumnal equinoxes along with the corresponding mean F10.7 cm fluxes. The error values depicted by the error bars are estimated as discussed in Madhav Haridas et al., 2013 . The pattern
Fig. 1. Yearly variation of h’Fc for the vernal and autumnal equinoxes along with the corresponding mean F10.7 cm fluxes.
of variation of F10.7 cm fluxes for both equinoxes is similar. On the other hand, in the case of h′Fc, the higher values in vernal equinox compared to autumnal equinox are evident from the figure for all the years examined. It is also observed that the magnitude of the asymmetry in h’Fc increases with increasing solar activity. To understand the concurrent behavior of the thermosphere, the O/N2 distribution is examined. Fig. 2a shows the mean O/N2 pattern over the Indian peninsula for the vernal (left panels) and autumnal (right panels) equinoxes of 2004. The white bins indicate erroneous points which have been removed. As is evident, there is significant asymmetry in the O/N2 ratios between the two equinoxes. The TIEGCM model simulations of atomic oxygen mass mixing ratio (which is the normalized atomic oxygen mass density with respect to that of air) for the vernal (left panels) and autumnal (right panels) equinoxes of 2004 are shown in Fig. 2b. The model simulations confirm the higher values of normalized mass density in vernal equinox. The variability of the simulated parameter for the three years (2003, 2004 and 2005) for which the simulations and h′Fc are available for both equinoxes also follow the pattern shown by the respective O/N2 and h′Fc parameters. Since sufficient number of points are not available to examine the O/N2 variation between the two equinoxes with respect to the corresponding h′Fc variations, we have looked into the inter equinoctial variability of the two parameters with respect to F10.7 cm flux. It is logically expected that as the solar flux increases the thermosphere expands proportionately but differently for the two equinoxes and this will correspondingly modulate the h′Fc values for the two equinoxes. Fig. 3 shows the scatter plot of F10.7 cm flux vs. δh′Fc (bottom panel) and F10.7 cm flux vs. δ(O/N2) (top panel) for the years for which the respective data are available. Here, δh’Fc corresponds to the difference in h′Fc between the two equinoxes for a particular year, while δ(O/N2) represents the corresponding O/N2 difference. Fig. 3 reveals the increasing asymmetry in O/N2 as solar activity increases. The correlation coefficient is 0.81. The relationship is statistically significant (495%). Similarly the asymmetry in h′Fc also increases with solar flux. In this case also the correlation coefficient of 0.8 is statistically significant (4 95%).
4. Discussion Devasia et al. (2002) for the first time introduced the now well known concept of threshold height as a critical parameter controlling the day to day variability of ESF. They demonstrated that converging/diverging thermospheric meridional winds significantly modulate ESF occurrence pattern with the equatorward wind being a pre-requisite when h′F is below the threshold height (h′Fc), for the R–T instability to get triggered. Above the critical height the polarity of the meridional winds did not matter. The seasonal/solar/magnetic activity dependence of h′Fc, for the occurrence of ESF irrespective of the magnitude and polarity of the meridional winds has been revealed in past studies (Jyoti et al., 2004; Manju et al., 2007; Madhav Haridas et al., 2013). Further, the modulation of h′Fc by neutral density changes is also presented (Madhav Haridas et al., 2013). In the past the behavior of the ionosphere–thermosphere during the two equinoxes was believed to be identical. Off late, several studies have brought out significant equinoctial asymmetry in different ionospheric parameters. Equinoctial asymmetry in ESF occurrence has been investigated in the Indonesian sector wherein they have shown that meridional wind asymmetry is a possible mechanism (Maruyama et al., 2009). Sripathi et al. (2011) examined the equinoctial asymmetry in scintillation occurrence in the Indian sector and proposed the asymmetry in the electron density distribution and meridional winds as possible causative
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Fig. 2. (a) Seasonal mean O/N2 pattern for the vernal (left panels) and autumnal (right panels) equinoxes of 2004. (b) TIEGCM simulations of atomic oxygen mass mixing ratio for the vernal (left panels) and autumnal (right panels) equinoxes of 2004.
ionosphere have been reported by several others also (Ren et al., 2011; Liu et al., 2010; Balan et al., 1997). The present study highlights a new aspect of the asymmetry between the two equinoxes. In this study we observe significant asymmetry in the threshold height h′Fc between the two equinoxes with consistently higher threshold heights being observed in vernal equinox compared to autumnal equinox. This is an important new finding which underlines the distinct differences in the role of neutral dynamics in ESF triggering during the two equinoxes. It is also seen that the threshold heights for the two equinoxes show direct relation with the F10.7 cm flux. Further, the difference in h′Fc between the two equinoxes is observed to be higher during high solar activity period compared to solar minimum. At or above the threshold height, the ESF occurrence on a particular day can be wholly ascribed to the CRT instability mechanism (Manju et al., 2007). The growth rate of the collisional Rayleigh–Taylor (CRT) instability is given by Fig. 3. Scatter plot of F10.7 cm flux vs. δh’Fc (bottom panel) and F10.7 cm flux vs. δ(O/N2) (top panel).
mechanisms. Manju et al. (2013) also reported equinoctial asymmetry in ESF occurrence and discussed the possible role of asymmetric meridional winds. Dasgupta et al. (1983) studied the equinoctial asymmetry in equatorial and low latitude F region ionization distribution and attributed it to neutral composition changes. Aspects related to equinoctial asymmetry in the
ϒ = 1/L [g /υ in ]
(1)
Where, υin is the ion-neutral collision frequency, g is the acceleration due to gravity and L is the electron density gradient scale length. The seasonal changes in neutral density in the upper atmosphere have been studied using GUVI O/N2 ratio by Qian et al. (2009). Modulations in neutral composition and consequent changes in neutral density have been discussed in their paper. They suggested that neutral composition significantly affects neutral density in the upper thermosphere due to the difference in
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atomic/ molecular weight between the two principal thermospheric constituents, O and N2. According to them, the change in O/N2 ratio has the effect of changing mean molecular mass and, therefore, the density scale height, which in turn modifies the mass density at a given altitude in the upper thermosphere. They have also shown good correspondence between satellite drag derived neutral densities and GUVI O/N2 ratio. In the light of this result, we are using GUVI-O/N2 as a proxy for neutral density in this study. O/N2 shows substantial equinoctial asymmetry with higher values in vernal equinox compared to autumnal equinox. This implies higher neutral density in vernal equinox than autumnal equinox. The seasonal variations in thermospheric atomic oxygen densities (from Jacchia model) and their impact on post sunset peak vertical drift are reported by Batista et al. (1986). They showed that at equatorial station Fortaleza, a lower neutral density value resulted in lower peak Vd amplitude. This result dwells on the importance of vertical drift in controlling the occurrence of ESF. Our study brings out the point that even for lower mean vertical drifts the minimum requisite height in a season for ESF to occur is determined by the neutral density prevalent during that season. That is in autumnal equinox, the overall drifts themselves are lower than in vernal equinox, but the neutral density being lower the consequent favorable υin factor facilitates occurrence of ESF at lower altitudes. Qian et al. (2009) have also shown TIEGCM simulations depicting increased neutral density in vernal than autumnal equinox. This increased neutral density in vernal equinox results in increased υin which in turn causes a reduced growth rate compared to autumnal equinox at a given altitude. This means that optimum growth rate is achieved only at a higher altitude in vernal equinox compared to autumnal equinox thereby resulting in a higher h′Fc value in vernal equinox. We have also obtained the TIEGCM simulated normalized atomic oxygen density and found it to be following the pattern shown by h′Fc. The direct relation between F10.7 cm flux and δO/N2 as well as δh′Fc are brought out in this study. As the solar flux increases, the atmosphere expands and the scale height increases leading to increased neutral densities at all levels. It is seen in the present study that as solar activity increases the increase in neutral density is less for autumnal equinox compared to vernal equinox. Accordingly, the increase in h′Fc is also correspondingly lower in autumnal equinox. These aspects point towards the role of neutral density asymmetry in producing the observed h′Fc asymmetry. Apart from υin, the electron density scale length L (Eq. (1)), seed perturbations etc. also modulate the growth rate of CRT instability. These aspects are not addressed in the present work.
5. Conclusions In the present study, the equinoctial asymmetry of the threshold height (h′Fc), during the period 1993–2008 is investigated for the first time. The asymmetric behavior of the thermospheric neutral density is also brought out. The important new findings are i) consistently higher h′Fc for vernal equinox compared to autumnal equinox; ii) higher O/N2 in vernal equinox compared to autumnal equinox; iii) TIEGCM simulations reveal higher normalized atomic oxygen mass density in vernal equinox than in autumnal equinox; iv) clear solar flux dependence of δh′Fc and δO/N2.
Acknowledgments This work is supported by Indian Space Research Organization. One of the authors Mr. M. K. Madhav Haridas thanks Indian Space Research Organization (ISRO) for the financial support provided. The present study made use of the thermospheric O/N2 ratios obtained from TIMED/GUVI instrument. The authors acknowledge the use of TIE-GCM model runs which were obtained from the website http://ccmc.gsfc.nasa.gov.
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