THE DYNAMICS OF ANY PLANETARY ATMOSPHERE IS GOVERNED BY ITS ENERGY SOURCES LOSSES AND TRANSPORT CHANNELS. MOST PLANETARY ATMOSPHERES AND IONOSPHERES ARE PROTECTED FROM SOLAR WIND FLOW BY LARGE INTRINSIC MAGNETIC FIELDS THUS THE MAJOR SOURCE OF EXTERNAL ENERGY IS THE SUN. PLANETS THAT DO NOT POSSESS LARGE INTRINSIC MAGNETIC FIELDS (VENUS AND MARS) NOT ONLY HAVE THE SOLAR ENERGY SOURCE BUT THE INCOMING SOLAR WIND AS A SOURCE OF ENERGY AND A VEHICLE OF ENERGETIC LOSS VIA JEAN S ESCAPE SPUTTERING AND ION PICKUP. .TITAN OFFERS AN INTERESTING LIMITING CASE FOR EXAMINING AN ATMOSPHERE S INTERACTION WITH THESE EXTERNAL SOURCES. WHILE TITAN IS NOT A PLANET IT HAS AN EXTENSIVE AND HEAVY ATMOSPHERE. FURTHER IT IS FAR FROM THE SUN AND THE BALANCE OF ENERGY SOURCES (MAGNETOSPHERIC FLOW AND FIELDS VS. SOLAR) IS MUCH CLOSER TO EVEN. RADIO OCCULTATION OBSERVATIONS OF TITAN S IONOSPHERE TYPICALLY HAVE FOUND NOT A MAIN IONOSPHERIC PEAK NEAR 1200 KM ALTITUDE AS EXPECTED FROM SOLAR DRIVEN CHEMICAL MODELING HOWEVER IN SOME CASES A SIGNIFICANT (~3 X 10^3 CM-3) LAYER BETWEEN 500-600 KM ALTITUDE IS MEASURED. THE DENSITY PEAK IN THIS SECOND LAYER WAS OBSERVED TO VARY BETWEEN 0.7-3 X 10^3 CM-3 ALONG WITH CHANGES IN THE SLOPE OF ITS DENSITY PROFILE SUGGESTING A VARIABLE PRODUCTION SOURCE. CRAVENS ET AL. [2008] SUGGESTED THAT THE IONIZATION SOURCE IS ENERGETIC H+ AND O+ IONS FROM SATURN S MAGNETOSPHERE. THIS IDEA IS FURTHER BOLSTERED BY ENA OBSERVATIONS OF TITAN BY THE INCA INSTRUMENT THAT SHOW ENERGETIC ION PRECIPITATING INTO TITAN S IONOSPHERE (CF. MITCHELL ET AL. 2005). THE LOWER IONOSPHERIC LAYER IS BELOW THE CLOSEST APPROACH OF CASSINI (~950 KM) SO IT WAS NOT POSSIBLE TO STUDY THIS LAYER WITH IN-SITU OBSERVATIONS. .THE VARIABLE NATURE OF THE LOWER IONOSPHERIC LAYER SUGGESTS SEVERAL QUESTIONS. TO WHAT EXTENT ARE THE PROPERTIES OF THIS LAYER CONTROLLED BY THE FLUX OF THE INCIDENT IONS? IS THE LAYER SPHERICALLY SYMMETRIC ABOUT THE MOON? DOES TITAN S INDUCED MAGNETOSPHERE HAVE ANY INFLUENCE ON THIS LAYER? HOW DOES THE INFLUX OF MAGNETOSPHERIC IONS INTERACTING WITH THE NEUTRAL ATMOSPHERE CREATE THIS LAYER? HOW MUCH DOES SCATTERING THE INCIDENT IONS BY THE NEUTRAL ATMOSPHERE AFFECT THESE EXPECTED RESULTS? WHAT EFFECTS MIGHT THIS LOWER IONOSPHERIC LAYER HAVE ON TITAN S PLASMA INTERACTION? THIS IS A COMPLEX PROBLEM THAT WILL REQUIRE A SOPHISTICATED SET OF TOOLS TO TACKLE IT. .WE PROPOSE TO ADDRESS THESE QUESTIONS USING A THREE-DIMENSIONAL (3-D) PARTICLE CODE THAT INCLUDES ELECTRIC AND MAGNETIC FIELDS FROM SIMULATIONS OF TITAN S INDUCED MAGNETOSPHERE A SIMPLIFIED ION-NEUTRAL CHEMICAL SCHEME THAT INCLUDES 10 NEUTRAL SPECIES 9 ION SPECIES OVER 40 ION-NEUTRAL CHEMICAL REACTIONS NEUTRAL DENSITIES AND FLOWS FROM THE 3-D TGITM ATMOSPHERIC MODEL. THE PATH OF THE ENERGETIC IONS WILL SOLVED USING THE LORENTZ FORCE WHILE SIMULTANEOUSLY MODELING INTERACTION PROCESSES (CHARGE-EXCHANGE ION-STRIPPING SCATTERING AND MOLECULAR DESTRUCTION) WITH THE ATMOSPHERIC NEUTRAL SPECIES. THE RESULTING PRODUCT WILL BE 3-D MAPS (LAT. LONG. AND ALTITUDE) OF IONIZATION RATES ION DENSITIES ATMOSPHERIC HEATING AND NEUTRAL SPECIES ALTERATION AS FUNCTIONS OF THE INCIDENT ION FLUX AND INDUCED MAGNETOSPHERIC CONDITIONS. .ONE OF THE PRIMARY GOALS OF THE CASSINI MISSION WAS TO STUDY TITAN S ATMOSPHERE AND ITS INTERACTION WITH SATURN S MAGNETOSPHERE. THE PROPOSED RESEARCH WILL SIGNIFICANTLY ADVANCE OUR UNDERSTANDING OF HOW SATURN S MAGNETOSPHERE INFLUENCES AND DRIVES PROCESSES IN TITAN S ATMOSPHERE/IONOSPHERE IN A REGION THAT WAS NOT DIRECTLY SAMPLED BY CASSINI. TO ACHIEVE THE RESEARCH OBJECTIVES CASSINI DATA FROM MIMI MAG INMS CAPS AND RSS WILL BE USED AS INPUTS AND VALIDATION OF THE SIMULATIONS AND TO IDENTIFY SIGNALS FOUND IN THE DATA THAT RESULTS FROM THE ENERGETIC ION DEPOSITION. THE PROPOSED RESEARCH WILL IMPROVE OUR UNDERSTANDING OF TITAN S LOWER IONOSPHERE AND WILL SIGNIFICANTLY ENHANCE THE SCIENTIFIC RETURN FROM THE CASSINI MISSION. .