Influence of geomagnetic storms on the mid latitude D and F 2 regions

The signal amplitude of 22.1 kHz Very Low Frequency (VLF) waves, transmitted from the radio station Skelton, UK (GQD at 54.7 0 N, 2.8 0 W) and received at South France station (SID, Sudden Ionospheric Disturbance monitoring station at 46 0 N, 2 0 E) is studied for the effect of geomagnetic storms on the lower ionosphere. The VLF parameters, D-Layer Preparation Time (DLPT) depth and Mid-Day Peak (MDP) have been used to study the response of D-region to the geomagnetic storms occurred during the equinox months of 2012-2015. The two parameters recorded enhancement on the storm day and subsequent days. A shift in the sunrise Terminator Time is observed for the geomagnetic storms for which Sudden Commencement (SC) occurred during the day lit hours on the previous day. The observed effect of the geomagnetic storms on the mid latitude D-region is due to the precipitation of high energy particles. The response of F 2 -region to geomagnetic storms is studied using the f o F 2 data at Dourbes (50.1 0 N, 4.6 0 E), Belgium which is near the mid-point of the GQD-South France VLF path. The percentage of deviation of f o F 2 from the quiet day values, Df o F 2 is found to undergo positive and negative changes. The positive storm effect during daytime is mainly due to the penetration electric field and strong negative phase during both day time and night time is due to depleted [O]/[N 2 ] ratios.

During the geomagnetic storms, the solar wind interacts with the ionospheric plasma at high latitudes and electric fields are developed. These fields are mapped to mid and low latitudes and modify the existing electric fields [Uma et al., 2012 and references therein]. From the study of the phase variations of VLF signals transmitted from OMEGA-ALDRA (at 13.6 kHz), GBR (at 16.0 kHz) and OMEGA-NORTH DAKOTA (at 13.6 kHz) and received at Inubo, Japan, Kikuchi and Evans (1983) concluded that the high energy (>300 Kev) electron precipitation into the atmosphere during magnetically disturbed days is the main source of D-region ionization responsible for the observed phase anomalies. Lastovicka [1996] observed that the lower ionosphere at high latitudes readily respond to geomagnetic storms and the mid-latitudes show a delayed effect. The study also revealed that there is a correlation between geomagnetic storms and total ozone density under special conditions. Cummer et al. [1996] observed VLF phase and amplitude perturbations at the edge of auroral zone due to enhanced electron density caused by high energy precipitating electrons. From the study of VLF amplitude data from Holographic Array for Ionosphereic Lightning Research (HAIL) stations in United States and another station from Antarctica with L shell contours 2-3, Peter et al. [2006] observed a reduction in night time VLF amplitudes during geomagnetic storms. Clilverd et al. [2010] showed that VLF amplitude variations can be used to estimate the flux of energetic electrons entering the upper atmosphere. Using the amplitude of 24 kHz VLF waves transmitted from NAA, Maine, USA and received at sodankyla, Finland, they found that electron density varies by three orders during geomagnetic storms. Sokolov [2011] studied the variability of D-region electron density and the precipitation electron flux at L shell contours 3-8 during the different types of geomagnetic storms. This study revealed that during the recovery phase of intense magnetic storms, the electron precipitation extends over long latitudinal interval and VLF phase anomalies are observed at mid-latitudes due to this effect. Choudhury et al. [2015] found that the change in the VLF amplitude at the sunrise time decrease during the geomagnetic storms indicating the reduction in day-night asymmetry. Many investigators [Kumar and Kumar, 2014;Kumar et al., 2015;Nwankwo et al., 2016] reported anomalies in VLF amplitude during geomagnetic storms associated with solar flares. The enhancement in the electron density at D-region altitudes due to the high energy electron precipitation during geomagnetic storms is responsible for the signatures observed in VLF parameters.
Studies on the response of E-region to geomagnetic storms at mid latitudes are very scarce. The ionospheric E-region electron density reduces slightly after a geomagnetic storm leading to reduction in f o E (critical frequency of E-layer) at auroral latitudes and this response is very weak at mid latitudes [Danilov and Lastovicka, 2001]. The effect of geomagnetic storm on the ionospheric F 2 region is indirect and is through the disturbance dynamo electric fields (DDEF), prompt penetration electric field (PPEF) and neutral composition changes [Uma et al., 2012;Danilov, 2013]. The variations of electron densities and also the dynamics at the ionospheric F 2region altitudes are due to the mapping of PPEF, DDEF and the storm time thermospheric wind circulations which can generate Travelling Atmospheric Disturbances (TADs) and Traveling Ionospheric Disturbances (TIDs) [Kumar and Kumar, 2019 and references therein]. The critical frequency of the F 2 layer, f o F 2 and Total Electron Content (TEC) have been studied widely for the response of this region to the geomagnetic storms [Venkatesh et al., 2017;Kumar and Kumar, 2019]. The effect of geomagnetic storm on f o F 2 known as ionospheric storm has positive and negative phases. The positive and negative phases of f o F 2 variation refer to enhancement and depletion of maximum electron density respectively during the storm. Mansilla [2014] observed the negative storm effects at equatorial anomaly crest regions and positive storm effects at equatorial and low latitudes which are attributed to the DDEF developed due to the disturbance winds. The positive storm effect observed at mid latitudes during daytime can be due to abundance in [O]/[N 2 ] ratio [Klimenko et al., 2015]. The longduration negative storm effect at southern hemisphere mid latitude station, Hobart during the St. Patrick storms of 2012, 2013 and 2015 were observed by Kumar and Kumar [2019]. They indicated that these negative phases are caused by the storm induced DDEF, depleted [O]/[N 2 ] ratios and TIDs of high latitude origin penetrating to mid latitudes. In a detailed study of the geomagnetic storm effect on the total atmosphere and ionosphere, Danilov and Lastovicka [2001] observed that the effect of geomagnetic storms is through the strong Joule heating leading to disturbance winds in F-region and the enhancement of particle precipitation resulting in enhancement of electron densities at the D-region altitudes. The aim of the present work is to study the response of mid latitude D and F regions to a few geomagnetic storms which occurred during the equinoxial months of February, March and August months of 2012-2015.

Data and Analysis
The amplitude of VLF signals at 22.1 kHz transmitted from Skelton (Station code GQD), UK (54.7 0 N, 2.8 0 W) and received at SID monitoring station in South France (46 0 N, 2 0 E) is obtained from the website https://sidstation.loudet.org/data-en.xhtml. The great circle distance between GQD and the receiving station is 1024 km. The magnetic storms considered in the present study are listed in Table 1.
The VLF amplitude for seven days (3 days prior to and after the event day) has been analysed for calculating the VLF parameters, D-Layer Preparation Time (DLPT) depth and Mid-Day Peak (MDP). At the time of sun rise and sun set, the VLF amplitude undergoes drastic changes. The first minimum in VLF amplitude graph during sun rise time is denoted by sunrise terminator. The difference in night time VLF amplitude at the time where the sun rise effect starts and that at sunrise terminator time is taken as DLPT depth. The average of day time signal amplitude from 1200 UT to 0200 UT is designated as MDP. The sun rise and sun set terminator times are important parameters since the lower ionospheric condition can be monitored by these Terminator Times [Maekawa and Hayakawa, 2006].
The effect of the geomagnetic storm on F 2 region can be studied using the critical frequency f o F 2 . The mid latitude station, Dourbes (50.1 0 N, 4.6 0 E) is in the same time zone of Skelton (GQD) and has been selected for studying the response of mid latitude F 2 region to the geomagnetic storms. The f o F 2 at Dourbes is obtained from the website http://ulcar.uml.edu/DIDBase/ . Df o F 2 indicates the deviation of foF2 from the quiet day values and is given by The quietest day of the corresponding month is taken from website http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.html.
The geomagnetic indices Dst and AE are taken from the website https://omniweb.gsfc.nasa.gov/form/dx1.html.

Results and Discussion
During the sunrise time, the D-layer undergoes significant change in its electrical conductivity due to the onset of photoionization. The DLPT depth being the index of the day-night asymmetry in VLF propagation, responds to the space weather events [Choudhury et al., 2015]. This parameter calculated for seven days for all the events has been plotted in Figure 1. There is an increase in the parameter by an amount of 2 to 4 dB on the storm day for all the events except for the event of August 27, 2015. There is a dip of 3.5 dB on August 27, 2015 when compared to the previous day. Using the amplitude data of 19.8 kHz VLF signal transmitted from Northwest Cape, Australia and received at Tripura, India, Choudhury et al., [2015] studied the variation of DLPT during the long duration storm days. They reported a dip in the parameter on the storm day and the subsequent day. They also observed a negative correlation between A p index and DLPT depth parameter. In the present study, only for one storm event, dip in the DLPT depth is observed on the storm day. From the rocket measurements of electron concentration in the altitude range 70 -110 km at mid latitude station, South Uist, Scotland (57.3 0 N, 7.3 0 W), Dickinson and Bennett [1978] observed that the electron density on the post storm days is 10 times the average quiet time value. They concluded that the enhancement occurs below 85 Km during the early morning hours. This results in the lowering of reflection height of the VLF waves and increase in the absorption of radio waves. The DLPT depth which depends on the amplitude of VLF waves at the time of night to day transition, increases due to decrease in amplitude of these waves. There exists minima during sunrise or sunset times in the diurnal variation of VLF amplitude. The generation mechanism for such minima is mainly the mode conversion at the sunrise/sunset terminator [Maekawa and Hayakawa, 2006]. From the full wave computation, Soloviev et al. [2004] concluded that any perturbation in the lower ionosphere results in significant change in the Terminator Time of the VLF propagation. The study of shift in Terminator Time known as TT method is applicable to East-West Propagation path. This method is useful in monitoring the lower ionospheric condition even for North South short paths [Maekawa and Hayakawa, 2006]. In the present study, the TT method is applied to the VLF path between GQD and South France station, being a North-South short path. The sunrise terminator timings for the corresponding months are shown in Figure 3  Circle Path [Ray and Chakrabarti, 2013;Sasmal et al., 2014;Latha et al., 2014]. No seismic events were reported near the propagation path during the period of study for all the events. The shift in morning terminator time is present when the Sudden Commencement (SC) occurred during day time on previous day. The morning Terminator

Geomagnetic storm effects on midlatitudes
Time is not deviating from the quiet time value when the SC occurred during sun rise period or pre sun rise period as in the case of other two events.
In the lower ionosphere mainly D-region, the electron concentration is considerably enhanced in the auroral zone during a geomagnetic storm [Lastovicka, 1996]. Danilov and Lastovicka [2001] opined that a delayed post storm effect at mid latitudes is due to the loss of energetic electrons from the trapped regions. Dickinson and Bennett     geomagnetic storms, Danilov [2013] suggested that long lasting negative storm effect can be due to the storm time equatorward disturbance meridional winds.  [Huang, 2013]. The DDEF exhibits delayed effects at mid and low latitudes which can last for more than a day after SC [Richmond et al., 2003]. These fields are westward during the day and eastward during the night at equatorial latitudes [Scherliess and Fejer, 1997].    [Mazaudier et al., 1987]. The disturbance winds from the auroral latitudes are equatorward and can result in negative Df o F 2 at the mid latitudes [Danilov, 2013].

Conclusions
The response of mid latitude D and F 2 regions to intense geomagnetic storms which occurred during equinox months of 2012-2015 has been studied using the VLF amplitude data of Skelton (GQD) U.K -South France SID station path and f o F 2 data at Dourbes. The results of the study are summarized below.
1. The DLPT depth evaluated using the VLF amplitude of 22.1 kHz waves propagating over the path of GQD (UK) and South France station showed an increase on the storm day (Dst minimum). This can be due to the reduction in the reflection height of VLF waves caused by the enhanced electron densities during the geomagnetic storm. But a dip in the parameter is observed for the event of August 27, 2015 for which the disturbed conditions prevailed for a period of 3 days from August 26, 2015 to August 28, 2015.