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Thunderstorm-Associated Responses in the Vertical Motion of the Mid-Latitude F-Region Ionosphere

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Thunderstorm-Associated Responses in the Vertical Motion of the Mid-Latitude F-Region Ionosphere ARTICLE IN PRESS Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 787–793 Contents lists available at ScienceDirect Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp Short Communication Thunderstorm-associated responses in the vertical motion of the mid-latitude F-region ionosphere V.V. Kumar a,Ã,1, M.L. Parkinson a,1,2, P.L. Dyson a,3, G.B. Burns b,4 a Department of Physics, La Trobe University, Melbourne, Victoria 3086, Australia b Australian Antarctic Division, Australian Government, Kingston 7050, Australia article info abstract Article history: Mid-latitude Digisonde Doppler velocities, auroral electrojet (AE) indices and cloud-to-ground (CG) Received 21 July 2008 lightning strokes during August 2003–2004 were used to study the perturbations in the F-region Received in revised form vertical drift associated with terrestrial thunderstorms. A superposed epoch analysis (SEA) showed that 4 March 2009 À1 the F-region vertical drifts Vz had a net descent of 0.6 m s peaking 3 h after lightning. Stronger Accepted 25 March 2009 downward perturbations of up to 0.9 m sÀ1 were observed in the afternoon on the day prior to Available online 5 April 2009 lightning days. The perturbations were less significant on the day after and insignificant during the Keywords: remaining intervals up to 144 h on either side of the lightning. The stronger responses on the day before Mid-latitude ionosphere are consistent with causality because the lightning times were merely proxies for the physical Thunderstorm–ionosphere coupling mechanisms involved. The actual causes are unclear, but we discuss the possible roles of lightning- Superposed epoch analysis induced ionisation enhancements, intense electric fields penetrating upward from electrified clouds, and atmospheric gravity waves (AGWs) radiated from thunderstorms or from the accompanying tropospheric fronts. There is no doubt that the behaviour of the mid-latitude F-region is controlled by the thermospheric winds and the solar wind-magnetosphere electrical generators, but our results suggest that electrified clouds also account for a significant, albeit relatively small component of the ionospheric variability. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction storms on the F-region electric fields, as this is one of the least understood aspects of the global electrical circuit (Kartalev et al., An electrodynamic coupling between tropospheric thunder- 1998). storm generators and the ionosphere has been realized for nearly Conceptually, vertical mass exchange between the lower a century (Rodger, 1999) but there is still inadequate evidence atmosphere and ionosphere is not plausible beyond 100 km, but revealing the causal pathways. Recent optical observations of electric fields are not bounded by these conditions (Rishbeth, ‘sprites’ initiated near the base of the ionosphere and the 2006), as the ionosphere is not a perfect conductor. Several development of ‘blue jets’ propagating upwards from the top of mechanisms help to mediate coupling between thunderstorms thunderstorms (Pasko, 2007) combined with statistical evidence and the ionosphere: enhanced conductivity above thunderstorms for enhanced electron concentrations in ionospheric sporadic-E (Rodger, 1999); quasi-electrostatic fields above thunderstorms, (Es) layer associated with negative cloud-to-ground (CG) lightning and associated heating of the lower ionosphere (Pasko et al., strokes (Davis and Lo, 2008) have established an electrodynamic 1998); lightning associated AC phenomena (Rycroft, 2006), and link between the troposphere, mesosphere, and ionosphere. Latest wave dynamics including infrasonic and acoustic gravity waves reviews of atmospheric electricity (Siingh et al., 2005; Rycroft, (AGWs) (Johnson and Davis, 2006). 2006) encourage researchers to identify the signatures of thunder- Previous work suggests that storm systems influence the F-region ionosphere through pressure changes and the AGWs they generate (e.g., Sˇauli and Bosˇka, 2001), irrespective of the à Corresponding author. Tel.: +610 449751712, +610 400019618. occurrence of lightning. It is important to note that whilst this E-mail addresses: [email protected] (V.V. Kumar), paper identifies significant responses of the F-region ionosphere [email protected] (M.L. Parkinson), [email protected] occurring in association with lightning, the lightning may or may (P.L. Dyson), [email protected] (G.B. Burns). not be the direct physical cause or trigger for the observed 1 These authors contributed equally to this work. responses. The passage of thunderstorm cells is a complicated 2 Tel.: +613 9479 1433; fax: +613 9479 1552. 3 Tel.: +619479 2735; fax: +613 9479 1552. phenomena entailing many concurrent atmospheric processes 4 Tel.: +613 62323381; fax: +613 62323496. (pressure fronts, intense winds, electrified clouds, lightning, etc.). 1364-6826/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2009.03.021 ARTICLE IN PRESS 788 V.V. Kumar et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 787–793 Lightning may be the cause or merely a proxy for the causes. 10-min. During the intervening 8-min, Doppler sorted interfer- Extremely comprehensive, multi-instrument studies of all the ometery (DSI) (Parkinson et al., 1997) measurements were made inter-connected atmospheric layers may ultimately be required to using ordinary mode polarization at licensed fixed frequencies. unambiguously identify the actual physical causes. A relatively small number of echoes with virtual heights below In this work, we use part of a 5.5-year (February 1999–August 180 km were classified as E-region data and excluded. The 2004) digital ionosonde (DPS-4) database complied at a southern standard Digisonde Data Analysis (DDA) software was used to mid-latitude station, Bundoora, Melbourne (145.11E, 37.71S) estimate 3D drift velocity in a Cartesian reference frame. The use (Kumar et al., 2008), 1-min auroral electrojet (AE) index and of a long integration time (40.96-s) and thus high Doppler World Wide Lightning Location Network (WWLLN) (Dowden resolution meant that our estimates of vertical velocity Vz were et al., 2002) data, to determine thunderstorm-associated re- very accurate (Parkinson et al., 2001). Kumar et al. (2008) sponses in the overhead ionosphere. The chosen study interval analysed the entire DPS-4 F-region database and found that the was August 2003–2004, the first year WWLLN was fully opera- velocities exhibited well-defined diurnal and seasonal variations tional. The DPS-4 measurements were also continuous during this with strong sensitivity to AE. The results were consistent with 1-year interval. earlier studies using incoherent scatter radar (e.g. Buonsanto The mid-latitude F-region is controlled by the E–(day) and et al., 1993). Any relatively weak responses in the F-region drifts to F–(night) region dynamos during geomagnetically quiet times, electrified cloud activity may be masked by the normal Sq and ionospheric disturbance dynamo and prompt penetration dynamo, the disturbance dynamo, and prompt penetration fields. fields during disturbed times (Kumar et al., 2008 and references We addressed this problem by creating a statistical model using therein). In the method adopted here, the local time, seasonal, and the 5.5-year F-region database, with the results separated magnetic activity-dependent average velocities were removed into two representative seasons, the spring–summer months from the measurements. The residuals were then analysed with (September–February) and autumn–winter months (March– respect to the lightning times using a superposed epoch analysis August). Also, it was found the drift magnitudes were generally (SEA). SEA has been frequently employed to identify relatively larger than usual in the year 2003 (Kumar et al., 2008) due to the weak responses swamped by other competing influences operat- high-speed solar wind streams known to re-occur during the ing over similar time scales (Prager and Hoenig, 1989; Davis and declining phase of solar activity (e.g. Cliver et al., 1996). ‘‘DC Lo, 2008). offsets’’ associated with year-to-year variations were incorporated In this paper, we report statistically significant, reproducible in the model. responses in the vertical motion of the F-region ionosphere The model, hereafter the LT–AE model, consisted of average associated with cloud-to-ground lightning strokes located within velocity components calculated using 10-min bins in local 600 km of Melbourne. Although these responses may be directly time (LT) and 100 nT bins of the AE. The AE data of 1-min related to lightning, it is important to recall that lightning may be resolution was downloaded from the World Data Centre, a proxy for some other atmospheric process. This paper Kyoto (http://swdcwww.kugi.kyoto-u.ac.jp/aeasy/index.html). concentrates on the analysis of the vertical velocity component An average velocity was calculated provided there were at least because it is the most accurate velocity component measured by 500 samples per bin, thereby maintaining the statistical sig- the digital ionosonde (Parkinson et al., 2001), and it readily nificance of the model. The upper limit for AE in the LT–AE model reveals significant responses. However, preliminary analysis was 1000 nT. In situations when AE 41000 nT, the model suggests small but significant responses in the horizontal motions gives values corresponding to 1000 nT and the local time of which will be reported elsewhere. measurement. The validity of the LT–AE model was checked using all 31,376 drift velocities recorded during the interval August 2003–2004. 2. Instruments and data analysis The results for the vertical velocity Vz component are summarised in Fig. 1. Part (a) (top) shows the LT variation of raw Vz and 2.1. F-region data indicates that the mean Vz (white curve) was strongly downwards up to 11m sÀ1 at dawn, reverting to gradually upwards up to À1 À1 The Bundoora DPS-4 records ionograms by scanning between 2 6ms by dusk. The mean Vz was near to 0 m s in the post- and 12 MHz in steps of 100 kHz for less than 2-min, once every midnight sector.
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