Evidence of changes in the low-latitude plasma drift under IMF $B_{z}$ coupling: a TIEGCM simulation approach

The occurrence of geomagnetic storms is associated with input of the energy of the solar wind and subsequent coupling of the same with the Magnetosphere-Ionosphere (MI) system. In general, these storms occur when the north-south component ($B_{z}$) of the frozen-in Interplanetary Magnetic Field (IMF), emanating from the Sun and carried by the solar wind into the heliosphere, turns completely southward (or negative $B_{z}$ by convention) and remains in this state for several hours while reconnecting with the geomagnetic field [1-2]. These phenomena occur mainly following a Coronal Mass Ejection (CME) or successive CMEs and the type of magnetic storms driven by this solar transient are generally refereed to as CME-driven geomagnetic storms. Additionally, these type of storms occur from components like the strong magnetic field in the ejecta, the strong magnetic field of the sheath and the interplanetary shocks. It is to be noted that these storms mainly occur in the ascending phase and solar maximum phase of solar cycles (or solar activity cycle of $\approx$ 11 years duration). The Storm Sudden Commencements (SSC), before the onset of the main phase of these type of storms, are associated with the interplanetary shocks that are driven by the CMEs while the recovery phase lasts for only about one-two days [3-4]. A geomagnetic storm’s beginning is noted with large variations of $B_{z}$ in addition to the transmission of magnetospheric convection electric field (with an associated equivalent current system having average periods of the order of few minutes to about 3 hours) from the higher to the lower-equatorial latitudes, also called as the Prompt Penetration Electric Fields (PPEF and eastward during dawn-to-dusk) which cause severe perturbations to the low-latitude electrodynamics [5-10].

The ionosphere of the low-latitude region is characterized by a phenomenon known as the Equatorial Ionization Anomaly (EIA), that starts around 09 Local Time (LT) and moves towards higher latitudes up to 15-20^∘ magnetic dip latitude around 16 LT and returns back to the magnetic equator around 24 LT. This phenomena is caused as a result of the two plasma motions: one perpendicular to the geomagnetic field and generating drifts upward the zonal electric field (eastward during daytime and westward during nighttime) produced by E region dynamo and the geomagnetic field and the other parallel to the geomagnetic field, that cause the plasma to diffuse down following the geomagnetic field line under influence of ambipolar diffusion drift related to gravity and pressure gradients [11-16].

The present work investigates the changes in the plasma drift, near the dip equator and the EIA regions, that were caused due to the influence of the CME-driven geomagnetic storm of October 13, 2016 by utilizing simulations from the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) developed by NCAR. Firstly, the analysis is performed by simulating the vertical plasma drifts at Indore:IDR (a location near the EIA, magnetic dip of 32.23^∘N) and Tirunelveli: TIR (a dip equatorial location, magnetic dip of 0.2^∘N) under no variations of the IMF $B_{z}$ (i.e: $B_{z}$ = 0 nT) and then by simulating the same for these two locations, with observed values of $B_{z}$ during the entire day of October 13, 2016. The motivation of the work comes from the need of understanding these physics-based models and their performance such that these models can be reliably run across locations where actual data from ionosondes or GNSS receivers are not present. The paper is divided starting with a general discussion of the NCAR TIEGCM model followed by the results, the corresponding discussions and the summary sections.

The National Center for Atmospheric Research (NCAR) TIE-GCM (https://www.hao.ucar.edu/modeling/tgcm/tie.php) is a 3D, non-linear and first principles representation of the coupled ionosphere and thermosphere system. It consists of self-consistent solution of the mid-and low-latitude dynamo field. It solves energy, continuity and momentum equations for ion and neutral species at every time-step (typically 120 s) using a semi-implicit, $\nth{4}$-order, finite difference method on each pressure surface in a vertical grid. The standard parameters in this model are: latitudinal and longitudinal extent from -87.5^∘ to +87.5^∘ and -180^∘ to +180^∘ respectively, in steps of 5^∘, altitude-wise pressure level from -7 to +7 in steps of $\frac{H}{2}$ with lower boundary $\approx$97 km and upper boundary $\approx$500-700 km (depending on the solar activity condition). The inputs to this model are: daily solar radio flux (F10.7) and its 81-day centered mean (F10.7$\_$81) while the output parameters (specified in 3D spatial and time dimensions) are: Height of pressure surfaces in cm, electron, ion and neutral temperatures in K, compositions like N2+, NO+, N+, Ne, NO, N(4S), N(2D), O, O2, O+, O2+, meridional, vertical and zonal drifts in m/s and potentials both geographic and geomagnetic coordinates. Several researchers [17-22] have shown the capability of the TIEGCM to understand geomagnetically disturbed as well as quiet-time conditions’ ionospheric electrodynamic processes thus making it an ideal simulation platform that closely reproduces and resembles the same.

The low-latitude ionosphere is dynamic in nature, especially in and around the EIA, where almost two-thirds of the global ionization gets concentrated. In addition, the perturbations might increase or decrease when there is an external driver such as geomagnetic storms as a result of solar outburst and subsequent strong coupling of the solar wind and MI system. There arises a need to understand the effect of the IMF parameters (especially $B_{z}$) during such events on the plasma motion in the form of drift from equatorial to off-equatorial locations. This study compares the TIEGCM simulation results of the vertical plasma drift over both EIA and magnetic equator in the Indian sector under the scenario of no coupling of the solar wind with MI system ($B_{z}$ = 0 nT) and a scenario with actual observations of $B_{z}$ fed to this model. Observations brought forward the fact that there had been significant difference in the variation of the vertical plasma drift, in the form of suppression of the entire profile towards westward, when the model was fed with actual observations of IMF $B_{z}$. Further studies with variations of the IMF conditions, under geomagnetically disturbed as well as quiet-time conditions, fed to the TIEGCM model could bring out new physical mechanisms that are taking place at such highly geosensitive locations over low-to-equatorial latitudes, especially in and around the EIA and the magnetic equator. This initial work will be followed up by thorough investigation of the effects of the other IMF components on the global plasma drift motions over these dynamic locations.

SC acknowledges the World Data Center (WDC) for Geomagnetism, Kyoto for the Dst index value. The solar wind and interplanetary magnetic field and the SYM-H index data are obtained from NASA’s SPDF omniweb service https://omniweb.gsfc.nasa.gov/form/omni_min.html. This work is supported by the Department of Space, Government of India.