A C T A G E O P H Y S I C A P O L O N I C A Vol. 52, No. 2 2004 IONOSPHERIC IRREGULARITIES, SCINTILLATION AND ITS EFFECT ON SYSTEMS Andrzej W. WERNIK 1 , Lucilla ALFONSI 2 and Massimo MATERASSI 3 1 Space Research Center, Polish Academy of Sciences Bartycka 18a, 00-716 Warszawa, Poland e-mail: aww@cbk.waw.pl 2 Istituto Nazionale di Geofisica e Vulcanologia Via di Vigna Murata 605, 00143 Rome, Italy 3 Istituto di Fisica Applicata “Nello Carrara” (CNR) Via Panciatichi 64, 50127 Florence, Italy Abstract An essential component of the ionosphere is a small-scale electron density structure that causes scintillation of radio waves on transionospheric links. We briefly review the morphology of irregularities and physical mechanisms of their formation. Results of the scintillation theory relating statistical characteristics of irregularities and scintillation are also outlined. Examples of the scintillation ef- fect on the system performance are given. Finally, we will present recent at- tempts to model the scintillation. Key words: ionospheric irregularities, ionospheric scintillation. 1. INTRODUCTION One of the top-priority problems in the U.S. and European space weather programs is the understanding of: − the thermosphere-ionosphere-magnetosphere interactions that control the for- mation and evolution of 10-km to 50-m electron density irregularities that cause scin- tillation,