1 URSI AP-RASC 2019, New Delhi, India, 09 - 15 March 2019 Plasma Wave Scenario in Comets Vipin K. Yadav Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala, 695022 Abstract Plasma waves have a universal presence in our solar system – planets, some of their satellites and interplanetary medium. The solar plasma environment of also supports the generation of plasma waves such as electron and ion cyclotron waves, the magnetosonic and Alfven waves which travels a long distance in heliosphere. The plasma environment around a comet is also capable of sustaining plasma waves, some of which are observed in comets Giacobini-Zinner, Halley, Grigg-Skjellerup and Borrelly. Some other comets such as Hyakutake and 67/PChuryumov- Gerasimenko are also supposed to sustain plasma waves but are yet to be observed. This paper presents a brief review on the plasma wave observations in cometary plasma environment carried out so far and some of the unresolved plasma wave issues in comets. 1. Introduction Plasma waves are omnipresent and thus are a unique feature of space plasmas. Electrostatic (ES) and electromagnetic (EM) plasma waves are observed in almost all the solar system objects, such as planets, some of the planetary satellites [1], Sun [2] and interplanetary medium (IPM) [3]. Plasma waves are predicted to exist in interstellar medium (ISM) [3] and are also believed to exist in many other natural plasma systems such as pulsars, quasars and galaxies. Plasma waves have significance as they propagate energy across various space regions and transport particles in the absence of collisions and accelerate them to attain higher energies. The study of plasma waves help in having a better understanding of the ionosphere of any planetary body. The plasma waves can be employed as a diagnostic tool for estimating local plasma parameters – density and temperature of the regions which are otherwise not accessible for in-situ measurements such as solar corona due to their hostile environment. A comet is a universal body, primarily made of dust, ice and gaseous matter. In the solar system, a comet can have a large elliptical orbit so that it become visible at Earth periodically such as Halley (76 years) or it can have a parabolic orbit where it passes the solar system once and then get immersed in the interstellar space. The solar radiation upon its interaction with cometary surroundings results in the formation of plasma environment by photoionization. This cometary plasma is modified by the solar wind but is still capable of sustaining basic plasma waves. In this paper such plasma waves, observed in four comets – Halley, Giacobini-Zinner (G-Z), Grigg- Skjellerup (G-S) and Borrelly are discussed. 2. Plasma and Magnetic Environment in Comets The plasma flow near a comet, where the solar wind encounters a shock front at distance of 10 6 km from the comet nucleus, reduced the supersonic solar wind speed from 400 km/s to about 50 km/s. On moving farther, another shock-transition allows the solar wind plasma to circumvent the cometary nucleus. The cometary plasma here has velocity can be as low as 3 km/s [4]. The cometary tail is the extension of comet in a direction opposite to its movement and has a different plasma environment than that of its coma. The activity of comet and the solar wind conditions decides the overall macroscopic structure of the interaction region as well as the plasma environment around the comet including the magnetic field. The magnetic fields of four comets named in section 1 are displayed in Figure 1 where CA represents the closest approach. The figure shows that at the CA the maximum magnetic field is about 60 nT for G- Z and Halley whereas for G-S and Halley it is 80 nT [5]. Figure 1: Magnetic field in comets G-Z, Halley, G-S and Borrelly [5].