The induced radio frequency magnetic fields of a low-frequency inductively coupled plasma are measured and modeled. The fields penetrate deep into the discharge, in contrast with existing predictions of field decay within a thin skin layer. Fluid calculations show that the enhanced penetration is due to a reduction of the plasma conductivity by the induced magnetic fields.[S0031-9007(96)00766-1] PACS numbers: 52.40.Db, 52.50.Gj, 52.80.Pi Inductively coupled plasmas (ICPs) consist of a dielectric chamber surrounded by a conducting coil [1]. Radio frequency (rf) power continuously applied to the coil induces electric fields that partially ionize a gas inside the chamber and sustain a discharge. ICPs have in the past been operated with high (1-760 Torr) gas pressures for thermal [2] and lighting [3] applications. Recently, ICPs have been rediscovered by the semiconductor and other industries as an important class of high-density ͑10 11 10 12 cm 23 ͒, low-pressure (1-10 mTorr) plasma sources [4]. Present low-pressure ICP development [5-10] and modeling [11][12][13][14][15][16] are among the most active areas of plasma research.The understanding of rf power absorption is fundamental to ICP heating and transport properties. These properties must be accurately described in ICP models to predict plasma radial uniformity, a crucial issue for future large-area processes. In present models that consider induced rf electric (but not magnetic) fields, the rf power is transferred to plasma electrons within a skin layer near the plasma surface [2][3][4][11][12][13][14][15][16]. The electron heating is either collisional (Ohmic) or collisionless (stochastic) [12,16]. For typical rf frequencies of 0.1-13.56 MHz, the skin depths are a few cm while device dimensions are 10 -30 cm. The ICPs can be cylindrical [2,3,5], planar [6-9], or hemispherical [10]. The measured skin depths are consistent with the above models [2,3,7].In this Letter, data are presented that show deep rf penetration into argon ICP discharges. For these data, stochastic effects are unimportant and collisional skin depths are small. Hence, a new mechanism must be invoked to explain the enhanced rf field penetration. Fluid calculations suggest that this mechanism is a reduction of the plasma conductivity by the induced rf magnetic fields when the electron cyclotron angular frequency v c exceeds the rf angular frequency v and the electron-neutral collision frequency n. This condition is satisfied for many low-pressure ICPs. Hence, the induced rf magnetic fields must be included in ICP models to predict electron heating and the resulting plasma transport. The influence of rf magnetic fields on ICP density profiles (the ponderomotive effect) has already been identified [17][18][19].