Neutral and Electrodynamical coupling of the Atmosphere-Ionosphere System (NECLAS)
Chief Co-ordinator - Prof. S. Sripathi & Members
Definition of the problem
The atmospheric waves and tides which are generated in the troposphere-stratosphere region when propagated to the mesosphere-lower thermosphere (MLT) get amplified due to low density can cause disturbances in the ionosphere over equatorial region even under geomagnetic quiet conditions. Severe weather disturbances like tropical cyclones and Sudden Stratospheric Warmings (SSWs)produce gravity waves and planetary waves which when propagated to MLT region do cause disturbances in the ionosphere. While many of them dissipate their energy in the MLT region by depositing their energy and momentum to charged particles,few of them may also directly penetrate to ionosphere. These waves interact non-linearly with the background neutral medium to produce secondary waves. These atmospheric waves interacts with the ionized medium through collisions in the ionosphere and produces appreciable magnitude of short and long term variabilities in the eastward electric field. This in turn modifies the Equatorial Electrojet current, vertical E X B drift, Post-sunset Pre-Reversal Enhancement (PRE),Equatorial spread F (ESF) irregularities and anomaly crest regions. The large- and medium-scale travelling ionospheric disturbances (LSTIDs and MSTIDs) are another source of ionospheric variability and have been gathering attention significantly in recent years. Understanding the day-to-day variabilities of these equatorial processes is key areas of research in this program. Since IIG has a strong ground observational network of radio and optical instruments acrossIndian low latitudes,the data gathered from these observational network scan be utilized effectively along with the experience gained in the past on data processing and numerical modelling to address the atmosphere-ionosphere coupling. Understanding the equatorial ionosphere and ESF irregularities in particular are very important for radio wave propagation as they disrupt the GNSS satellite-ground communication link and thus degrades the overall accuracy of the positioning and navigation. Therefore, understanding the generation and dynamics of ESF irregularities are always of great importance. The new program is designed so as to advance our understanding of the coupling processes and further our predictive capabilities in certain areas like, the occurrence of equatorial plasma bubbles and the radio wave scintillation that is caused by a wide range of ionospheric irregularities. A systematic study on the dynamics (day to day, seasonal and annual variations) of the ESF irregularities causing L-band scintillation has not been performed so far in an extensive way. To understand the physical mechanisms responsible behind these velocity variations during quiet as well as geomagnetically disturbed period, a dedicated experiment containing two spaced SBAS enabled GNSS receivers has been initiated at the EGRL, Tirunelveli during 2015. Assuming that L-band scintillation occurs at typical height of ~350 km, the first Fresnel zone at L1 (λ = 19.05 cm) can be given as √2λh = 365 m. Thus, the receivers are placed in magnetic east-west with spatial distance of ~376 m. The receivers record signals transmitted by the GPS-aided GEO augmented navigation (GAGAN) geostationary satellites of GSAT-8 (PRN 127) and GSAT-10 (PRN 128). GAGAN is an Indian Satellite Based Augmentation System (SBAS). Under this program, these signals are further processed to estimate the zonal drifts of the ESF irregularities.
The Earth's ionosphere possesses different types of current systems during geomagnetically quiet and disturbed conditions. Electric currents associated with the ionospheric E-region dynamo such as equatorial electrojet (EEJ) and Sq are significant during day times, and dominate the magnetic field variations on ground. Other than these, the currents due to F-region dynamo, gravity-driven and pressure-gradient currents are also present in the ionosphere. Under this program, it is proposed to estimate the contribution due to these currents using Swarm satellite measurements.
Underthis program, it is also proposed to utilize the state-of-the-art artificial neural networks and machine learning algorithms to the long term data for data mining and advance our knowledge on data related science, besides investigations on the existing observational network and planned new experiments. In this science plan, it is proposed to utilize the radar observations from Tirunelveli and Kolhapur in a series of studies aimed at understanding the short-term variabilities of the mesosphere-lower thermosphere (MLT) region and how these variabilities couple to the ionosphere above. Gravity waves play a greater role in contributing to the shorter variabilities and hence, the focus would be on gravity waves and to some extent, on tides and planetary waves. Radio wave propagation in the ionosphere in the HF band and their ray tracing techniques shall be investigated. While coupled solar wind-magnetosphere-ionosphere system in the high latitudes shall be investigated under space weather and observations and modeling (SWOM) program, their signatures in the equatorial ionosphere shall also be investigated under this program using coupled magnetosphere-ionosphere current systems, Prompt Penetration Electric Fields (PPEF) and associated EEJ/CEJ fluctuations.
Objectives:
The scientific objectives of this program are classified into four sub-themes:
Atmosphere-Ionosphere Coupling through waves Understanding the role of Atmospheric Tides
Gravity Waves and Planetary Waves on the dynamical and electrodynamical coupling of atmosphere-ionosphere system through Radio and Optical remote sensing.
To identify sources of potential gravity waves propagating into the middle atmosphere using sophisticated ray tracing model aided by the observations.
Equatorial Spread-F, Ionospheric irregularities and Scintillation
To understand the gravity wave seeding and precursory signatures of Equatorial Plasma Bubblesusing radio and optical probing.
To study the vertical and zonal evolution of shorter scale field-aligned irregularities and its impact on spatial and temporal occurrence of GNSS/IRNSS signal scintillation.
Determining the zonal drift of equatorial spread-F (ESF) irregularities based on the spaced SBAS enabled GNSS receiver experiment over the dip equatorial Indian region
Characterization of equatorial low-latitude Ionosphere
To understand the equatorial and low-latitude ionospheric variability in response to extreme weather events like Stratospheric Sudden Warming and Tropical Cyclones.
Study of radio wave propagation and their characteristics using observations and modeling.
Development of equatorial vertical ExB drift model from the long term observations of Ionosonde and EEJ.
To study of 150-km echoes from the magnetic equatorial location and derive the vital equatorial zonal electric field from Doppler during quiet and disturbed periods.
Coupled Ionosphere-Magnetosphere current systems
To study the ionospheric currents such as equatorial electrojet, Sq, F-region dynamo currents, gravity-driven and pressure-gradient currents using polar orbiting LEO satellite magnetic field measurements.
Studies on the coupled magnetosphere-ionosphere current systems, Prompt Penetration Electric Fields (PPEF) and associated EEJ/CEJ signatures during storms and substorms using ground based and satellite observations.
