## 1. Introduction

[2] The process of linear mode conversion (LMC) when the wave energy transferred from one mode to the other in an inhomogeneous magnetized plasma [*Stix*, 1965; *Budden*, 1961; *Ginzburg*, 1970] is of great importance in laboratory and space plasmas. In the magnetized F-region plasmas, LMC of electromagnetic (EM) waves into electrostatic (ES) or electron plasma (Langmuir) waves is very distinctive from the cases of unmagnetized and weakly magnetized plasmas. The resonant/collisional absorption due to LMC greatly affects the linear and non-linear plasma processes occurred in the F region of the ionosphere modified by high-frequency radio waves.

[3] The conversion of a fast EM wave propagating in an inhomogeneous plasma into a slow ES mode and a collision-free absorption of the slow wave have been first demonstrated by *Stix* [1965]. The absorption coefficient in a warm plasma for various angles of incidence has been first obtained by *Piliya* [1966]. Analytical solutions for the mode conversion problem in a cold unmagnetized collisionless plasma for oblique incidence have been derived by *Speziale and Catto* [1977]. The reflection and mode-conversion coefficients with finite temperature effects have been obtained analytically for the linear density profile [*Hinkel-Lipsker et al.*, 1989] and for a cold/warm plasma at the peak of the F layer [*Hinkel-Lipsker et al.*, 1991]. The theory and combined analytical and numerical simulations of linear conversion of HF electromagnetic waves into ES waves in a magnetized ionospheric plasma have been discussed by *Mjølhus and Flå* [1984] and *Mjølhus* [1984, 1990]. The characteristics of the LMC process relevant to ionospheric heating experiments at Arecibo have been studied by *Muldrew* [1993] using ray tracing techniques.

[4] In a magnetized plasma, there are two characteristic waves, the extraordinary (*X* mode) and the ordinary (*O* mode). These two EM waves propagating with different velocities are in general elliptically polarized for propagation at an arbitrary angle to the magnetic field. When the *O* mode is incident on the F layer of the ionosphere it can be partially reflected at cutoff, the critical layer where the wave frequency *ω* is equal to the electron plasma frequency *ω*_{pe}, and converted (or absorbed) at resonances [*Budden*, 1961; *Ginzburg*, 1970]. We consider the conversion near the reflection height of the *O* mode incident on the F layer of slowly varying plasma (*k*_{0}*L* ≫ 1), *k*_{0} = *ω*/*c*, *L* is the density scale length, and *c* is the velocity of light. The study of LMC in an inhomogeneous magnetized warm plasma is a complex problem whose analytical treatment becomes cumbersome, and the solution of the full wave equations by numerical simulations is required. The resonant absorption by conversion of EM waves into ES waves has been simulated for a hot unmagnetized laboratory plasma by *Forslund et al.* [1975]. Most recently, the LMC process has been numerically studied in magnetized cold ionospheric plasmas by *Gondarenko et al.* [2003].

[5] In this paper, we numerically investigate linear conversion of EM waves into ES/plasma waves in cold/warm plasmas, using a full-wave model for the propagation of the HF wave incident normally/obliquely on the layer of an inhomogeneous magnetized plasma [*Gondarenko et al.*, 2003, 2004]. For typical F-region plasma parameters of the high-latitude ionosphere, we compute the collisional/resonance absorption coefficient and its dependence on the angle of incidence for a cold/warm plasma model. To demonstrate a strong effect of the external magnetic field arbitrarily oriented in the plane of incidence on the absorption coefficient, the simulations are performed for a wide range of parameters relevant to weakly and strongly magnetized plasmas.