A New Type Neutral Mass and Velocity Measurement Instrument

Atmospheric escape is commonly observed in terestrial planets. The mechanisms working for the escape is not identified yet. To understand the escape mechanism, it is important to measure neutral particle velocity distribution as well as the ion distribution. However traditional type of neutral mass spectrometers onboarded spacecraft are focused to measure neutral particles with high mass resolution. For that purpose neutral particles are accelerated after ionization and the velocity distribution information of the neutral gasses are lost. A new type neutral mass and velocity measurement instrument on board ePOP has developed different concept from the traditional neutral mass spectrometers. This instrument sacrifice mass resolution but keep velocity distribution in a plane. In this instrument, ionized particles are accelarated vertically to the ram direction and detected with 2-dimensional position sensitive detector. Mass information is derived by measuring time interval between start of acceleration and detection. Position information gives 2 dimentional velocity distribution and low resolution mass information. In this paper, we will present the concept of this new type mass and velocity measurement instrument and basic specification.