Yoichi Kazama

Low energy neutral atom imaging on the Moon with the SARA instrument aboard Chandrayaan-1 mission

This paper reports on the Sub-keV Atom Reflecting Analyzer (SARA) experiment that will be flown on the first Indian lunar mission Chandrayaan-1. The SARA is a low energy neutral atom (LENA) imaging mass spectrometer, which will perform remote sensing of the lunar surface via detection of neutral atoms in the energy range from 10 eV to 3 keV from a 100km polar orbit. In this report we present the basic design of the SARA experiment and discuss various scientific issues that will be addressed. The SARA instrument consists of three major subsystems: a LENA sensor (CENA), a solar wind monitor (SWIM), and a digital processing unit (DPU). SARA will be used to image the solar wind-surface interaction to study primarily the surface composition and surface magnetic anomalies and associated mini-magnetospheres. Studies of lunar exosphere sources and space weathering on the Moon will also be attempted. SARA is the first LENA imaging mass spectrometer of its kind to be flown on a space mission. A replica of SARA is planned to fly to Mercury onboard the BepiColombo mission.

Geospace exploration project ERG

The Exploration of energization and Radiation in Geospace (ERG) project explores the acceleration, transport, and loss of relativistic electrons in the radiation belts and the dynamics for geospace storms. This project consists of three research teams for satellite observation, ground-based network observation, and integrated data analysis/simulation. This synergetic approach is essential for obtaining a comprehensive understanding of the relativistic electron generation/loss processes of the radiation belts as well as geospace storms through cross-energy/cross-regional couplings, in which different plasma/particle populations and regions are strongly coupled with each other. This paper gives an overview of the ERG project and presents the initial results from the ERG (Arase) satellite.

Pulsating aurora from electron scattering by chorus waves

Auroral substorms, dynamic phenomena that occur in the upper atmosphere at night, are caused by global reconfiguration of the magnetosphere, which releases stored solar wind energy. These storms are characterized by auroral brightening from dusk to midnight, followed by violent motions of distinct auroral arcs that suddenly break up, and the subsequent emergence of diffuse, pulsating auroral patches at dawn. Pulsating aurorae, which are quasiperiodic, blinking patches of light tens to hundreds of kilometres across, appear at altitudes of about 100 kilometres in the high-latitude regions of both hemispheres, and multiple patches often cover the entire sky. This auroral pulsation, with periods of several to tens of seconds, is generated by the intermittent precipitation of energetic electrons (several to tens of kiloelectronvolts) arriving from the magnetosphere and colliding with the atoms and molecules of the upper atmosphere. A possible cause of this precipitation is the interaction between magnetospheric electrons and electromagnetic waves called whistler-mode chorus waves. However, no direct observational evidence of this interaction has been obtained so far. Here we report that energetic electrons are scattered by chorus waves, resulting in their precipitation. Our observations were made in March 2017 with a magnetospheric spacecraft equipped with a high-angular-resolution electron sensor and electromagnetic field instruments. The measured quasiperiodic precipitating electron flux was sufficiently intense to generate a pulsating aurora, which was indeed simultaneously observed by a ground auroral imager.

First measurement of ∼10 keV neutral atoms in the low‐latitude ionosphere

Energetic neutral atoms (ENAs) with energies of 4 to 35keV were measured at altitudes of 170 to 570km by a new ENA instrument on board a sounding rocket. The instrument measured particles precipitating into the ionosphere from the equatorial region of the magnetosphere at a magnetic local time of ∼1830. The geomagnetic activity was quiet for a prolonged period before the launch. The measured ENA flux was ∼10 2 (cm 2 s str keV) -1 at energies of ∼10 keV. The energy spectrum is in a good agreement with an expected spectrum of hydrogen atoms originating from the ring current region as reported by Milillo et al. [1996], The altitude profile is also discussed in terms of collisional interaction of ENAs with upper-atmospheric constituents.

Solar Wind Monitoring with SWIM-SARA Onboard Chandrayaan-1

The SARA experiment aboard the Indian lunar mission Chandrayaan-1 consists of two instruments: Chandrayaan-1 Energetic Neutral Analyzer (CENA) and the SolarWind Monitor (SWIM). CENA will provide measurements of low energy neutral atoms sputtered from lunar surface in the 0.01–3.3 keV energy range by the impact of solar wind ions. SWIM will monitor the solar wind flux precipitating onto the lunar surface and in the vicinity of moon. SWIM is basically an ion-mass analyzer providing energy-per-charge and number density of solar wind ions in the energy range 0.01–15 keV. It has sufficient mass resolution to resolve H+ , He++, He+, O++, O+, and >20 amu, with energy resolution 7% and angular resolution 4:5° × 22:5. The viewing angle of the instrument is 9° × 180°.Mechanically, SWIM consists of a sensor and an electronic board that includes high voltage supply and sensor electronics. The sensor part consists of an electrostatic deflector to analyze the arrival angle of the ions, cylindrical electrostatic analyzer for energy analysis, and the time-of-flight system for particle velocity determination. The total size of SWIM is slightly larger than a credit card and has a mass of 500 g.

A LENA Instrument onboard BepiColombo and Chandrayaan‐1

Low‐energy neutral atom (LENA) observations bring us important information on particle environments around planetary objects such as Mercury and the Moon. In this paper, we report on a new development of a LENA instrument of light weight (∼2 kg) for planetary explorations. The instrument is capable of energy and mass discrimination with a large sensitivity by utilizing surface ionization followed by an electrostatic analyzer and a time‐of‐flight velocity spectrometer. The performance of the instrument is investigated by numerical simulations. This enables us to obtain detailed performance characterization of LENA measurements by the instrument. We also made trajectory tracing of photons entering the instrument to examine photon rejection capability. The simulations show that the energy range is from ∼10 eV to >3.3 keV and the angular resolutions are 10 deg×25 deg for 25‐eV LENAs, which are sufficient for planetary LENA observations. Laboratory tests of a prototype model of the instruments developed with t...

Collisional interactions of precipitating energetic neutral atoms with upper-atmospheric particles in the low-latitude region

Altitude profiles of energetic neutral atom (ENA) fluxes precipitating into the upper atmosphere are simulated numerically by using a Monte Carlo method with source magnetospheric protons distributed between L shells 4 and 7, taking account of collisional interactions with atmospheric particles. Numerical results are in reasonable agreement with the profile of ∼10keV neutral atom fluxes recently measured under a geomagnetically quiet condition. Through collisional interactions, ENAs can be converted to ions and then the ionized ENAs can be reneutralized and repeat this cycle. Therefore the precipitating ENAs lose their original directionality because of gyromotions of the ionized ENAs. However, the source information is still seen in ENA data measured at low altitudes where collisions are significant, because their pitch angles are almost conserved with the forward scattering approximation for each collision in the case of parallel geomagnetic fields at low altitudes. On the other hand, a part of ENAs precipitating into the atmosphere returns to the magnetosphere. These reflected ENAs act as a secondary source that cannot be neglected in ENA data measured in the magnetosphere.