Observation of the interstellar helium cone by the NOZOMI spacecraft

‘The Japanese Mars probe, NOZOMI, is staying in the interplanetary space (1–1.5 AU) until its Mars’ orbit insertion scheduled in early 2004. Every 16 months on this interplanetary orbit the spacecraft crosses around 1 AU the ‘gravitational focusing cone’ of the interstellar helium, which are penetrating into the inner heliosphere under the solar gravity. During the first crossing of the cone in the season of March–May 2000, we observed these helium particles after the solar wind pickup process with an E/q type ion detector aboard NOZOMI. We have estimated the original temperature of the interstellar helium as 11 000 K. This revised version was published online in August 2006 with corrections to the Cover Date.     

A New Morphology of Solar Activity and Recurrent Geomagnetic Disturbances: The Late-Declining Phase of the Sunspot Cycle

Certain aspects of the Sun and resulting geomagnetic disturbances can be studied better on the source surface, an imaginary spherical surface of 3.5 solar radii, than on the photospheric surface. This paper presents evidence that the Sun exhibits one of the most fundamental aspects of activities most clearly during the late-declining phase of the sunspot cycle. It is the period when 27-day average values of the solar wind speed and of geomagnetic disturbances tend to be highest during the sunspot cycle. Important findings of this study on the late-declining phase of the sunspot cycle are the following:

1. By introducing a new coordinate system, modifying the Carrington coordinates, it is shown that various solar activity phenomena, solar flares, the brightest coronal regions, and also the lowest solar wind speed region, tend to concentrate in two quadrants, one around 90^° in longitude in the northern hemisphere (NE) and the other around 270^° in longitude in the southern hemisphere (SW). For this reason, the new coordinate system is referred to as the NESW coordinate system.
2. It is shown that the above results are closely related to the fact that the neutral line exhibits a single wave (sinusoidal or rectangular) in both the Carrington coordinates and the NESW coordinate system during the late-declining phase. The shift of the neutral line configuration during successive solar rotations during the late-declining phase causes longitudinal scatter of the location of solar flares with respect to the neutral line in a statistical study. The NESW coordinate system is designed to suppress the shift, so that the single wave location is fixed and thus a ‘nest’ of solar flares emerges in the NE and SW quadrants.
3. It is also shown that the single wave is the source of the double peak of the solar wind speed and two series of recurrent geomagnetic disturbances in each solar rotation, making the 27-day average solar wind and geomagnetic disturbances highest during the sunspot cycle. The double peak is a basic feature during the late-declining phase, but is obscured by several complexities which we identified in this paper; see item 8.
4. The single wave of the neutral line configuration can be approximated by three dipole fields, one which can be represented by a central dipole (parallel or anti-parallel to the rotation axis) and two hypothetical dipoles on the photosphere. This configuration is referred to as the triple dipole model.
5. The location of the two hypothetical photospheric dipoles coincide with the two active regions (solar flares, the brightest coronal region) and also the lowest solar wind speed region in the NESW coordinate system; the lowest solar wind regions are the cause of the valleys of the double peak of the solar wind speed.
6. The two hypothetical dipole fields actually do exist at the location of the two active regions in a coarse magnetic map (5 × 5^°). The two dipoles follow the Hale–Nicholson polarity law. Thus, they are real physical entities.
7. The apparent meridional rotation of the dipolar field on the source surface during the sunspot cycle results from combined changes of both the central dipole field and of the two photospheric dipoles, although the central dipole remains axially parallel or anti-parallel. Thus, the Sun has a general field that can be represented by an axially aligned dipole located at the center of the Sun throughout the sunspot cycle, except for the sunspot maximum period when the polarization reversal occurs.
8. The complexity of recurrent geomagnetic disturbances can also be understood by having the NESW coordinate system for various solar phenomena and the relative location of the earth with respect to the solar equatorial plane.
9. As the intensity of the two dipoles decreases toward the end of the sunspot cycle, the amplitude of the single wave decreases, and the neutral line tends to align with the heliographic equator.
10. The neutral line shows a double wave structure during certain epochs of the sunspot cycle. In such a situation, it can be considered that two NESW coordinate systems are present in one Carrington coordinate, resulting in four active regions.
11. The so-called classical “sector boundary” arises when the peaks (top and bottom) of the single wave reached 90^° in latitude in both hemispheres.
12. In summary: A study of the late-declining period of the sunspot cycle is very important compared with the sunspot maximum period. In the late-declining period, the Sun shows its activities in the simplest form. It is suggested that some of the basic features of solar activities and recurrent geomagnetic disturbances that have been studied by many researchers in the past can be synthesized in a simplest way by introducing the NESW coordinate system and the triple dipole model. There is a possibility that the basic results we learned during the late phase of the sunspot cycle can be applicable to the rest of the sunspot cycle.

     

Formation of bioorganic compounds in planetary atmospheres by cosmic radiation.

Simulated planetary atmospheres (mixtures of simple gases) were irradiated with high energy particles to simulate an action of cosmic rays. When a mixture of carbon monoxide, nitrogen and water was irradiated with 2.8-40 MeV protons, a wide variety of bioorganic compounds including amino acids, imidazole, and uracil were identified in the products. The amount of amino acids was proportional to the energy deposit to the system. Various kinds of simulated planetary atmospheres, such as "Titan type" and "Jovian type", were also irradiated with high energy protons, and gave amino acids in the hydrolyzed products. Since cosmic rays are a universal energy source in space, it was suggested that formation of bioorganic compounds in planetary atmospheres is inevitable in the course of cosmic evolution.     

Communication system and operation for lunar probes under lunarsurface

In the Japanese LUNAR-A mission, penetrators will be deployed to the Moon for global seismic measurement. The unique communication system between the subsurface probes under the lunar surface and the lunar orbiter is described. Radiowave propagation through a crater which is formed at the penetration is investigated by means of scaled measurements in a simulating environment. Acquisition and tracking sequence is optimized within limited power capacity of the probe to maximize contact time between the probe and the spacecraft     

In-flight Performance and Initial Results of Plasma Energy Angle and Composition Experiment (PACE) on SELENE (Kaguya)

MAP-PACE (MAgnetic field and Plasma experiment—Plasma energy Angle and Composition Experiment) on SELENE (Kaguya) has completed its ～1.5-year observation of low-energy charged particles around the Moon. MAP-PACE consists of 4 sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). ESA-S1 and S2 measured the distribution function of low-energy electrons in the energy range 6 eV–9 keV and 9 eV–16 keV, respectively. IMA and IEA measured the distribution function of low-energy ions in the energy ranges 7 eV/q–28 keV/q and 7 eV/q–29 keV/q. All the sensors performed quite well as expected from the laboratory experiment carried out before launch. Since each sensor has a hemispherical field of view, two electron sensors and two ion sensors installed on the spacecraft panels opposite each other could cover the full 3-dimensional phase space of low-energy electrons and ions. One of the ion sensors IMA is an energy mass spectrometer. IMA measured mass-specific ion energy spectra that have never before been obtained at a 100 km altitude polar orbit around the Moon. The newly observed data show characteristic ion populations around the Moon. Besides the solar wind, MAP-PACE-IMA found four clearly distinguishable ion populations on the dayside of the Moon: (1) Solar wind protons backscattered at the lunar surface, (2) Solar wind protons reflected by magnetic anomalies on the lunar surface, (3) Reflected/backscattered protons picked-up by the solar wind, and (4) Ions originating from the lunar surface/lunar exosphere.     

The Mercury Electron Analyzers for the Bepi Colombo mission

Bepi Colombo is a joint mission between ESA and JAXA that is scheduled for launch in 2014 and arrival at Mercury in 2020. A comprehensive set of particle sensors will be flown onboard the two probes that form Bepi Colombo. These sensors will allow a detailed investigation of the structure and dynamics of the charged particle environment at Mercury. Onboard the Mercury Magnetospheric Orbiter (MMO) the Mercury Electron Analyzers (MEA) sensors constitute the experiment dedicated to fast electron measurements between 3 and 25,500 eV. They consist of two top-hat electrostatic analyzers for angle-energy analysis followed by microchannel plate multipliers and collecting anodes. A notable and new feature of MEA is that the transmission factor of each analyzer can be varied in-flight electronically by a factor reaching up to 100, thus allowing to largely increasing the dynamical range of the experiment. This capability is of importance at Mercury where large changes of electron fluxes are expected from the solar wind to the various regions of the Mercury magnetosphere. While the first models are being delivered to JAXA, an engineering model has been tested and proven to fulfill the expectations about geometrical factor reduction and energy-angular transmission characteristics. Taking advantage of the spacecraft rotation with a 4 s period, MEA will provide fast three-dimensional distribution functions of magnetospheric electrons, from energies of the solar wind and exospheric populations (a few eVs) up to the plasma sheet energy range (some tens of keV). The use of two sensors viewing perpendicular planes allows reaching a ¼ spin period time resolution, i.e., 1 s, to obtain a full 3D distribution.     

Wave dynamics in the thermosphere: I: Tidal motion

The atmospheric tides, their transmission and excitation in the thermosphere, are discussed in reviewing various investigations in this field. We are still fairly ignorant on the subjects and facing various unsolved problems although there is an established link between the theory and the observation in the dynamo region, the lower boundary of the thermosphere. As to the middle and upper thermosphere the observed data are scanty and only those obtained by satellite drag are available; the theoretical approach is very complicated because of viscosity, thermal conduction, hydromagnetic forces and non-linearity, all of which are effective above certain heights. Moreover, the thermosphere couples mainly with the lower atmosphere, this coupling having been considered only in a very simplified way. Another coupling is between thermal excitation and the resultant motion, the coupling of which has never been considered; thermal excitation has been discussed on a given input. International cooperation in the observations is of vital importance for future studies. New developments of observation techniques are desirable.