Keigo Ishisaka

The Energization and Radiation in Geospace (ERG) Project

The Energization and Radiation in Geospace (ERG) project for solar cycle 24 will explore how relativistic electrons in the radiation belts are generated during space storms. This geospace exploration project consists of three research teams: the ERG satellite observation team, the ground-based network observation team, and the integrated data analysis/simulation team. Satellite observation will provide in situ measurements of features such as the plasma distribution function, electric and magnetic fields, and plasma waves, whereas remote sensing by ground-based observations using, for example, HF radars, magnetometers, optical instruments, and radio wave receivers will provide the global state of the geospace. Various kinds of data will be integrated and compared with numerical simulations for quantitative understanding. Such a synergetic approach is essential for comprehensive understanding of relativistic electron generation/loss processes through crossenergy and cross-regional coupling in which different plasma populations and regions are dynamically coupled with each other. In addition, the ERG satellite will utilize a new and innovative measurement technique for wave-particle interactions that can directly measure the energy exchange process between particles and plasma waves. In this paper, we briefly review some of the profound problems regarding relativistic electron accelerations and losses that will be solved by the ERG project, and we provide an overview of the project.

Relationship between the Geotail spacecraft potential and the magnetospheric electron number density including the distant tail regions

The spacecraft potential has been used to derive the electron number density surrounding the spacecraft in the magnetosphere and solar wind. We have investigated the correlation between the spacecraft potential of the Geotail spacecraft and the electron number density derived from the plasma waves in the solar wind and almost all the regions of the magnetosphere, except for the high-density plasmasphere, and obtained an empirical formula to show their relation. The new formula is effective in the range of spacecraft potential from a few volts up to 90 V, corresponding to the electron number density from 0.001 to 50 cm−3. We compared the electron number density obtained by the empirical formula with the density obtained by the plasma wave and plasma particle measurements. On occasions the density determined by plasma wave measurements in the lobe region is different from that calculated by the empirical formula. Using the difference in the densities measured by two methods, we discuss whether or not the lower cutoff frequency of the plasma waves, such as continuum radiation, indicates the local electron density near the spacecraft. Then we applied the new relation to the spacecraft potential measured by the Geotail spacecraft during the period from October 1993 to December 1995, and obtained the electron spatial distribution in the solar wind and magnetosphere, including the distant tail region. Higher electron number density is clearly observed on the dawnside than on the duskside of the magnetosphere in the distant tail beyond 100 RE.

Low Frequency plasma wave Analyzer (LFA) onboard the PLANET-B spacecraft

The Low Frequency plasma wave Analyzer, LFA, on board the PLANET-B spacecraft has been developed to measure the Martian plasma waves. Two orthogonal electric dipole wire antennas, 50 m tip-to-tip, in the spacecraft spin plane are used to measure plasma waves, dc electric fields, and the spacecraft potential relative to the ambient plasma. The LFA has capability to measure the wave spectrum in the band from 10 Hz to 32 kHz, and to capture the signal waveform in the band from dc to 32 kHz by using a 4 MByte memory. The LFA scientific objectives are to explore the following: (1) Macroscopic plasma environment and boundaries from the solar wind to the ionosphere, (2) Microscopic plasma phenomena induced by the interaction between the solar wind and the Martian atmosphere and the moon Phobos, (3) Generation and propagation of electromagnetic waves, (4) Plasma densities and waves in the nightside ionosphere and tail, and (5) Comparison of Martian plasma waves with those of other planets such as non-magnetized Venus and magnetized Earth.

Electron temperature and density of magnetospheric plasma from GEOTAIL spacecraft potentials

The spacecraft potential is an index giving the density and temperature of electrons surrounding spacecraft, as understood from the single probe theory. We investigate the correlation between electron density derived from the plasma wave and spacecraft potential observations of the GEOTAIL spacecraft in the solar wind and magnetosphere. We have obtained the good correlation between them within the voltage range of spacecraft potential less than 35V which may mean that electron density is in the range above 0.02 cm−3. We could estimate the electron temperature in the regions of solar wind and electron foreshock, and we found the cases that the electron temperature is higher when the Langmuir wave appears than when no Langmuir wave appears. Then, accumulating many observations of electron density along the GEOTAIL trajectory, we obtained a map showing the global plasma population, in which the bowshock and magnetopause positions are found to be nearly same with the their average positions during the magnetically quiet times.

Investigation of electron density profile in the lower ionosphere by SRP-4 rocket experiment

The SRP-4 rocket was launched at 12:07 LT on 18 March 2002. The objective of this rocket experiment is the investigation of the electron density profile in the high-latitude D-region of the ionosphere at noon. The Low frequency and Medium frequency band radio Receiver (LMR) and the DC Probe System (DPS) were installed on-board the rocket to estimate the D-region electron density. The LMR measured the intensities of radio waves received from ground-based stations operating at 257 kHz, 660 kHz and 820 kHz, respectively. The DPS measured the electron current and the positive ion current using the biased electrodes. The electron density profile at altitudes below 90 km was estimated from the measured absorption of these radio waves. It was found that the electron density began to increase at the altitude of 52 km and was larger than that estimated by the International Reference Ionosphere model at altitudes from 74 km to 89 km.

Estimation of spatial structure of lower ionosphere with two-dimensional FDTD simulations

We performed a series of FDTD simulations with different types of electron density profiles in the lower ionosphere, and then confirmed characteristics of MF wave propagations in the lower ionosphere. According to sounding rocket experiments, we can only obtain an altitude profile of wave intensity, especially magnetic field intensity, by rocket observations. In this study, therefore, we are going to try to estimate spatial structure in the lower ionosphere by analyzing altitude profiles of magnetic field intensities of waves with various frequencies. Simulation results indicate that spatial structure in the lower ionosphere can be estimated by analyzing altitude profiles of different waves emitted from different wave sources with various frequencies.

Current status and planning of the Plasma Wave Experiment (PWE) onboard the ERG satellite

The ERG (Exploration of energization and Radiation in Geospace) project is a mission to study acceleration and loss mechanisms of relativistic electrons around the Earth. To achieve comprehensive observations of plasma/particles, fields, and waves, the Plasma Wave Experiment (PWE) is installed onboard the ERG satellite to measure electric field in the frequency range from DC to 10 MHz, and magnetic field in the frequency range from a few Hz to 100 kHz. Two CPU boards, one for electric field and another for magnetic field, are installed for the PWE and a variety of operational modes can be implemented. In the present paper, we introduce unique specifications of the PWE to meet the scientific objects of the ERG mission.

Study of medium-scale traveling ionospheric disturbances (MSTID) with sounding rockets and ground observations

Medium-scale traveling ionospheric disturbance (MSTID) is an interesting phenomenon in the F-region. The MSTID is frequent in summer nighttime over Japan, showing wave structures with wavelengths of 100-200 km, periodicity of about 1 hour, and propagation toward the southwest. The phenomena are observed by the total electron content (TEC) from GEONET, Japanese dense network of GPS receivers, and 630 nm airglow imagers as horizontal pattern. It was also measured as Spread-F events of ionograms or as field-aligned echoes of the MU radar. MSTID was, in the past, explained by Perkins instability while its low growth rate was a problem [1]. Recently 3D simulation study by Yokoyama et al. [2] hypothesized a generation mechanism of the MSTID, which stands on electromagnetic E/F-region coupling of the ionosphere. The hypothesis is that the MSTID first grows with polarization electric fields from sporadic-E, then show spatial structures resembling to the Perkins instability. We recently conducted an observation campaign to check this hypothesis. We launched JASA ISAS sounding rockets S-310-42 and S-520-27 at 23:00 JST and 23:57JST on July 20, 2013 while an MSTID event was monitored in real-time by the GPS-TEC from GEONET. We found 1-5mV/m northeastward/eastward electric fields during the flight. Variation of electric fileds was associated with horizontal distribution of plasma density. Wind velocity was measured by the TME and Lithium releases from S-310-42 and S-520-27 rockets, respectively, showing southward wind near the sporadic-E layer heights. These results are consistent to the expected generation mechanism shown above. In the presentation we will discuss electric-field results and its relationship with plasma density variability together with preliminary results from the neutral-wind observations.

Application of Spacecraft Potential to Investigate the Distribution of Low-Energy Plasma in Magnetosphere

Abstract We have found that the spacecraft potential in the magnetosphere and the solar wind can be used to derive the electron number density of the plasma surrounding spacecraft. The relationship between the Geotail spacecraft potential and the electron number density as determined by the plasma wave observations in the solar wind and broader magnetosphere (except for the high-density plasmasphere) was investigated and an empirical formula correlating these values obtained. Using this empirical formula and plasma particle measurements, we have shown the distribution of low-energy plasma in the magnetosphere.

Study of VLF band electromagnetic waves in the Antarctica observed by polar patrol balloons

The two large scientific balloons (PPB: polar patrol balloons) were launched on January 13th, 2003 at Syowa Station in Antarctica. The balloons reached the altitude of 33 km, and observed various data for about two weeks. We developed the wideband electromagnetic wave receiver onboard PPB and observed ELF/VLF waves in Antarctica. VLF band waves observed in Antarctica are frequently modulated in frequency. Among these frequency modulations, the modulations of about 20 sec are considered to have relations with the compressional Alfven waves. By taking cross correlations between data observed by two balloons, we can identify the direction from which VLF waves propagate as well as the source region where these waves are excited. We compared one event observed by both balloons on January 17th, and found the time when the strong VLF wave observed by the 8th PPB was 3 seconds earlier than that observed by the 10th PPB. This result indicates that this VLF wave propagates with the velocity of about 67 km/s. The projection of this velocity on the equator plane is about 1800 km/s, which is almost the same order as the estimated velocity of the compressional Alfven waves in this region.