ORIGINAL PAPER Stability of magnetosonic waves in an anti-loss cone plasma C Venugopal 1 *, S George 1 , V R Rajeev 1 , R Jayapal 1 , M J Kurian 2 and C P Anilkumar 3 1 School of Pure and Applied Physics, Mahatma Gandhi University, Priyadarshini Hills, Kottayam 686 560, Kerala, India 2 Department of Physics, Catholicate College, Mahatma Gandhi University, Pathnamthitta 689 645, Kerala, India 3 Equatorial Geophysical Laboratory, Indian Institute of Geomagnetism, Krishnapuram, Tirunelveli 627 011, Tamil Nadu, India Received: 06 February 2013 / Accepted: 03 May 2013 Abstract: We have studied the stability of magnetosonic wave in a plasma, where the ions and electrons are described by anti-loss cone (ALC) distributions. Our studies indicate that the magnetosonic waves produced by ions and electrons with ALC distributions are in the higher frequency end within the range of frequencies, as observed by the Combined Release and Radiation Effects Satellite spacecraft. They are weakly damped and can, therefore, travel long distances. These waves are expected to play an important role in the acceleration of radiation belt electrons. Keywords: Magnetosonic wave; Dispersion relation; Stability; Anti-loss cone plasma; Radiation belt electrons PACS Nos.: 52.27.Aj; 52.35.-g; 52.35.Qz 1. Introduction Field fluctuations, with frequencies close to the proton gyrofrequency and its harmonics and up to the lower hybrid frequency, have been observed at radial distances of 2–8 R E around the geomagnetic equator in the magnetosphere [1, 2]. These waves, observed mainly in the afternoon and pre- midnight sectors, propagate nearly perpendicular to the magnetic field. Observational [3, 4] and theoretical [5–7] studies indicate that the wave is driven by energetic (of the order of tens of keV) protons with ring like distributions. More recent observations are made by the CLUSTER satellite in the plasma sheet boundary layer (PSBL) [8]. These waves, initially called ‘‘equatorial noise’’ [1] are now referred to as ‘‘magnetosonic waves’’. In contrast to their generation mechanism in the PSBL, magnetosonic waves in the inner magnetosphere are driven by a tenuous, energetic (of tens of keV) protons, in the presence of a cold background plasma (electrons and ions with energies of *1 eV) [3, 9]. Other studies on these waves, as observed by the Combined Release and Radiation Effects Satellite (CRRES) spacecraft from the proton gyrofrequency up to the upper hybrid wave frequency, have been restricted to a frequency range between 0.5 f LHR and f LHR (f LHR is the lower hybrid frequency). Emissions are observed to occur most of the time outside plasmapause; the most intense being in region of L = 3–4 [4]. These waves are generally believed to contribute to the transverse heating of protons [10, 11] and the acceleration of radiation belt electrons [12–14]. Other proposed mechanism is a new variant of the theory of magnetospheric resonator for magnetosonic waves [15] and the numerical investigations of temporal evolution of cylindrical magnetoacoustic waves in plane- tary magnetospheres [16]. It has also been shown that the slow magnetosonic perturbations generated in vicinity of magnetopause can be transformed into fast magnetosonic wave, which can then propagate into the magnetosheath [17]. The association of Pc5 pulsations with the Alfven and magnetosonic waves has also been studied [18]. The dis- persion characteristics of low frequency waves in multi-ion plasmas have also been investigated recently [19]. Other relevant studies are the investigations of effect of an electric field on electromagnetic ion cyclotron (EMIC) wave [20] where it is found that the electric field control the growth rate of these waves, while the steep loss cone distribution enhance the growth rate and perpendicular heating of the ions. Wave propagation around the electron cyclotron frequency has also been investigated [21]. In magnetopause, the loss cone cannot be completely empty since a fraction of the loss cone particles is scattered *Corresponding author, E-mail: cvgmgphys@yahoo.co.in Indian J Phys DOI 10.1007/s12648-013-0315-3 Ó 2013 IACS