Nozomi Spacecraft Research Articles

UV optical measurements of the Nozomi spacecraft interpreted with a two-component LIC-flow model(Corrigendum)

1 Department of Geophysics, University of Tohoku, 980-8577 Miyagi, Sendai, Japan e-mail: rom@pat.gp.tohoku.ac.jp 2 Space Research Centre PAS, Bartycka 18A, 00-716 Warsaw, Poland e-mail: bzowski@cbk.waw.pl 3 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 229-8510 Kanagawa, Sagamihara, Japan 4 Japan Society for the Promotion of Science, 100084 Beijing, PR China 5 Department of Earth and Planetary Science, University of Hokkaido, 001-0021 Hokkaido, Japan 6 Department of Physics, College of Science, Rikkyo University, 171-8501 Tokyo, Japan 7 Department of Earth and Planetary Science, University of Tokyo, 113-0033 Tokyo, Japan 8 Earth Observation Research Center, Japan Aerospace Exploration Agency, 305-8505 Ibaraki, Tsukuba, Japan

Two-Station Interplanetary Scintillation Measurements of Solar Wind Speed near the Sun Using the X-band Radio Signal of the Nozomi Spacecraft

Interplanetary scintillation (IPS) measurements of the solar wind speed for the distance range between 13 and 37 R S were carried out during the solar conjunction of the Nozomi spacecraft in 2000 – 2001 using the X-band radio signal. Two large-aperture antennas were employed in this study, and the baseline between the two antennas was several times longer than the Fresnel scale for the X-band. We successfully detected a positive correlation of IPS from the cross-correlation analysis of received signal data during ingress, and estimated the solar wind speed from the time lag corresponding to the maximum correlation by assuming that the solar wind flows radially. The speed estimates range between 200 and 540 km s−1 with the majority below 400 km s−1. We examined the radial variation in the solar wind speed along the same streamline by comparing the Nozomi data with data obtained at larger distances. Here, we used solar wind speed data taken from 327 MHz IPS observations of the Solar-Terrestrial Environment Laboratory (STEL), Nagoya University, and in situ measurements by the Advanced Composition Explorer (ACE) for the comparison, and we considered the effect of the line-of-sight integration inherent to IPS observations for the comparison. As a result, Nozomi speed data were proven to belong to the slow component of the solar wind. Speed estimates within 30 R S were found to be systematically slower by 10 – 15 % than the terminal speeds, suggesting that the slow solar wind is accelerated between 13 and 30 R S.

Properties of solar wind turbulence from radio occultation experiments with the NOZOMI spacecraft

Radio-sounding experiments using signals from the Japanese NOZOMI spacecraft to probe the circum solar plasma were performed from December 2000 through January 2001. They can be used to obtain information about the properties of the solar wind plasma in the region where it is accelerated at heliocentric distances of 12.8–36.9R s (where R s is the radius of the Sun). Measurements of the intensity and frequency of the received signals were carried out with high time resolution (∼0.05 s for the frequency and ∼0.0064 s for the intensity), making it possible to investigate the anisotropy of inhomogeneities and the spatial spectrum of the turbulence of the circum solar plasma. Analysis of these radio-sounding data has shown that the scintillation index and intensity of the frequency fluctuations decrease approximately according to a power law with increasing distance of the line of sight from the Sun. Measurements of the amplitude fluctuations and estimates of the solar wind velocity derived from spatially separated observations indicate the presence of small-scale inhomogeneities with sizes of the order of 50 km at heliocentric distances less than 25R s , which are elongated in the radial direction with anisotropy coefficients from 2.3 to 3.0. The inhomogeneities at heliocentric distances exceeding 30R s become close to isotropic.

Latitudinal dependence of the solar wind density derived from remote sensing measurements using interplanetary Lyman α emission from 1999 to 2002

Abstract The interplanetary Lyman α backscattered emission is an effective tool for remote sensing of the global structure of the solar wind proton flux. This paper reports an attempt to derive the latitudinal dependence of the solar wind density by combining the interplanetary Lyman α measurements of the Nozomi spacecraft for the period 1999–2002 with the solar wind speed data derived from interplanetary scintillation measurements. This approach successfully revealed the slow and dense solar wind over the poles during the period of the solar maximum. Data on the polar solar wind density indicate a significant growth from the middle of 2000, and the polar values of solar wind density are close to those of the equatorial values as a result of the disappearance of the coronal hole. A marked density depletion occurred in the middle of 2001, which can be ascribed to the development of fast winds from the polar coronal hole. To evaluate the remote sensing method, we considered solar wind density data from in situ measurements obtained by the Ulysses spacecraft. We conclude that our method basically agrees with in situ measurements, although we found a significant (a factor of 2) difference between these in the middle of 2001.

UV optical measurements of the Nozomi spacecraft interpreted with a two-component LIC-flow model

Aims. Following recent reports on spectroscopic observations by SWAN/SOHO suggesting that the flows of neutral interstellar helium and hydrogen in the inner heliosphere are slightly divergent, we tried to verify them on the basis of simultaneous photometric observations of heliospheric hydrogen and helium glows performed by a spacecraft located on an orbit between the Earth and Mars (which differs from the orbit of SWAN/SOHO). The observations were interpreted with the use of various independent models of interstellar hydrogen and helium in the inner heliosphere, evaluated over a mesh of parameters. Methods. The data might suggest that the upwind and downwind directions of interstellar H may differ by less than 180 ◦ ,w hich we interpret as due to a side shift of the secondary population of interstellar hydrogen, which might be due to a deformation of the outer heliosheath e.g. because of the action of interstellar magnetic field. The simulations we performed do not support the idea that the secondary population is significantly shifted to the side. Results. The upwind/downwind direction of interstellar hydrogen as derived from our observations agrees within the error bars with the upwind/downwind direction of interstellar helium and the error bars include both the upwind direction of interstellar helium, derived from in-situ observations of GAS/Ulysses, and the upwind direction of interstellar hydrogen, derived from observations of SWAN/SOHO.

Reconstructed global feature of an interplanetary disturbance for the full-halo coronal mass ejection event on 1999 September 20

Traveling interplanetary (IP) disturbances associated with the full-halo coronal mass ejection (CME) which occurred on September 20, 1999, were observed clearly with the 327-MHz interplanetary scintillation (IPS) system of the Solar-Terrestrial Environment Laboratory of Nagoya University. In this study, we retrieved the global features of the IP disturbances by fitting a three-dimensional model to our IPS data. We obtained a best fit model, which had a bubble-like structure with a center axis shifted south with respect to the Sun–Earth line and a nearly isotropic angular span. We believe this feature represents the compression region between the IP shock and the CME. The reconstructed global feature was found to be in good agreement with in situ measurements by ACE and the Nozomi spacecraft, which were situated near the earth and distant from the Earth, respectively, at the time of the full-halo CME event.

Phase scintillation observation during coronal sounding experiments with NOZOMI spacecraft

Radio occultation observations of the solar corona at solar offset distances of 12.8-36.9 RS were performed from December of 2000 to January of 2001, using the Nozomi spacecraft (Planet-B) of Japan. Phase scintillation spectra up to the frequency of ∼10 Hz were obtained in two-way mode with S -band uplink and X-band downlink. The spectra cannot be represented by a single power law especially for small offset distances. That is, at low frequencies (large scales) the spectra show slopes indicative of Kolmogorov, a local flattening occurs from ∼0.5 Hz (scale ∼600 km) to ∼ 3H z (∼100 km), and a steepening occurs again at higher frequencies. This three-component nature of the spectrum was observed with a single method for the first time, although the spectral shape is variable and the three-component feature is not always evident.

Summary of the orbit determination of NOZOMI spacecraft for all the mission period

Japanese first Mars explorer NOZOMI, which was launched in July 1998, suffered several problems during the operation period of more than five years. It could have reached near Mars at the end of 2003, but it was not put into the orbit around Mars. Although NOZOMI was not able to execute its main mission, it provided us a lot of good experiences from the point of the orbit determination of spacecraft. One of the most difficult works was the orbit determination for the period without the telemetry. In this period, for the most of the time the high gain antenna did not point to the earth because of a constraint of the attitude. Therefore, the quality of the tracking data was not good, and for some period it was impossible to get the tracking data at all. Under such critical condition, we managed to get the solution of the orbit, and in a near-miraculous way, we were able to control NOZOMI and execute two earth swingbys successfully. Other issues related to the orbit determination are the spin modulation, the solar radiation pressure, the small force related to the attitude change, and the solar conjunction. We tried to solve these issues by the conventional way using range and Doppler data. However, we also tried the new method, that is the orbit determination by using the Delta-VLBI method (VLBI: Very Long Baseline Interferometry). In addition to this, we tried optical observations of NOZOMI at the earth swingbys.

Lyman alpha imaging of solar activity on the interplanetary hydrogen screen for space weather forecasting

Interplanetary hydrogen atoms that penetrate into the heliosphere are illuminated by enhanced hydrogen Lyman alpha emission from solar active regions. Using the data obtained by the Solar Wind Anisotropies (SWAN) instrument on board the Solar and Heliospheric Observatory (SOHO), Bertaux et al. (2000) proposed a new method for earlier detection and monitoring of solar active regions on the far‐side hemisphere. However, a quantitative relationship between the sky map of interplanetary Lyman alpha intensity and the enhanced emission from a solar active region was not discussed. Here, we analyze the interplanetary Lyman alpha intensity data obtained by the Ultraviolet Imaging Spectrometer (UVS) on board the Nozomi spacecraft. We develop a method to map the distribution of interplanetary Lyman alpha intensity into the Carrington coordinate system by assuming an interplanetary hydrogen screen at the location of maximum volume emission rate. Analysis period is from January 2000 to March 2000 near solar maximum. The solar activity during this analysis period is much higher as compared with the case study by Bertaux et al. (2000). The sky map of the Lyman alpha emission obtained by the UVS is shown to exhibit a high correlation with the solar extreme ultraviolet (EUV) intensity map obtained by the EUV Imaging Telescope (EIT) on board the SOHO satellite. The highest value of two‐dimensional correlation coefficient is 0.80. An observed change in the periodicity of interplanetary Lyman alpha intensity is in good agreement with the growth of an active region on the Sun. It has been also confirmed that the far‐side observations of UVS detected the growth of an active region in advance. This case study demonstrates that the location and intensity variation of activity on the far side of the Sun can be detected by far‐side measurements of interplanetary Lyman alpha emission intensity. It is thus concluded that the developed method contributes to space weather forecasting on a scale of 2 weeks.

On the correlation of the solar wind observed at the L5 point and at the Earth

The L5 point is a promising location for forecasting co-rotating high-speed streams in the solar wind arriving at the Earth. We correlated the solar wind data obtained by the Nozomi spacecraft in interplanetary space and by the Advanced Composition Explorer (ACE) at the L1 point, and found that the correlation is significantly improved from that of the 27-day recurrence of ACE data. Based on the correlation between the two spacecraft observations, we estimated the correlation of the solar wind velocity between the L5 point and at the Earth, and found that the correlation coefficient was about 0.78 in late 1999, while that of the 27-day recurrence was 0.51. Eighty-eight percent of the velocity difference falls within 100 km/s between the L5 point and the Earth. This demonstrates the potential capability of solar wind monitoring at the L5 point to forecast the geomagnetic disturbances 4.5 days in advance.

Solar cycle dependence of interplanetary Lyman α emission and solar wind anisotropies derived from NOZOMI/UVS and SOHO/SWAN observations

We have investigated the distributions of neutral interplanetary hydrogen Lyman α emission and their solar cycle dependence from observations with the ultraviolet imaging spectrometer (UVS) onboard the NOZOMI spacecraft and the solar wind anisotropy instrument (SWAN) onboard the solar and heliospheric observatory (SOHO) spacecraft. The SOHO/SWAN observation covers the period from the solar minimum in 1996 to the solar maximum in 2001, while the NOZOMI/UVS observation covers the period of 1999–2001 near the solar maximum. The most distinct feature of the full sky Lyman α intensity maps near the solar maximum in 2000 is a structure with a single bright region extending across the ecliptic plane on the upwind side of the interstellar wind. This structure is entirely different from the structure observed by SWAN at the solar minimum in 1996, which shows an intensity depression called a “groove” and bright lobes above and below the groove centered near the ecliptic plane. From the fitting of model calculations to the observational data, it is confirmed that the latitudinal distribution of the ionization rate of the interplanetary hydrogen due to the charge exchange with solar wind protons becomes isotropic toward the solar maximum. However, an important result is that the condition for producing anisotropic ionization rate due to the slow, high‐density solar wind around the equator exists by October 1999, although the theoretical model predicts isotropic ionization rate throughout 1999.

Transverse halo orbits about Mars?

We predict a new family of charged dust rings about Mars, transverse to the ecliptic plane. These orbits are stable to the perturbations of planetary oblateness, Mars' orbital motion, and the solar wind. Lifetimes of individual orbits are limited primarily by the Lorentz force and Poynting‐Robertson drag, and may exceed 1000 years. They may be populated via collisions of micrometeoroids with one of the martian moons. Some of these small (≲1 μm) grains may be detected by the dust detector on board the Nozomi spacecraft now en route to Mars.

Meteoric ions in the atmosphere of Mars

The effect of the metallic concentration in meteoric material in the Martian ionosphere has been investigated with the scope of exploring the possible existence of meteoric ionospheric layers as it occurs on Earth. We have calculated the ion and neutral deposition of the most abundant metals in meteoroids, iron, magnesium and silicon, and modeled their ion-neutral chemistry in order to estimate the concentration of neutral and ionic metals and electrons. We found that a low ionospheric metallic layer due to meteoric ablation is created at around 80 km , well below the main maximum at 125– 135 km produced during the daytime by photoionization and photoelectron impact. Daytime and nighttime models have been developed and the effect of solar activity, and seasonal variations has also been explored. This meteoric layer is mainly formed of Mg + and Fe + with a magnitude of the order of 10 4 cm −3 at noon and decreases by two orders of magnitude during the night. The study of this lower part of the Martian ionosphere is very important in the context of the future missions, especially regarding the HF radio wave sounding technique that will be used onboard Mars Express and Nozomi spacecraft in 2004–2005.

Moon‐related nonthermal ions observed by Nozomi: Species, sources, and generation mechanisms

The 3‐D velocity distribution functions of nonthermal ions observed in the vicinity of the Moon have been analyzed to understand the interaction between the solar wind and the Moon. The observation was made during the lunar swing‐by of the Nozomi spacecraft, and the closest approach was about 1.6 RL from the Moon's surface. Both Nozomi and the Moon were located in the solar wind, and nonthermal ions were observed only when the spacecraft was very close to the Moon. The nonthermal ions were measured by a Particle Spectrum Analyzer/Ion Spectrum Analyzer (PSA/ISA) on board the Nozomi. After analyses and careful examination of the 3‐D velocity distribution of the nonthermal ions, the characteristics of the nonthermal ions were revealed as follows: The nonthermal ions have a partial ring structure in the phase space, they are protons, their source location is the dayside of the Moon, and they have large velocities when they are generated. We propose the following scenario of a solar wind interaction with the Moon: The electromagnetic field in the vicinity of the Moon must have a dynamic structure, possibly a miniature bow shock associated with a local magnetic anomaly, where some of the solar wind protons are deflected violating their first adiabatic invariant. After the deflection, they move under the force of the convection electric field and gyrate around the magnetic field in the solar wind. This motion forms a partial ring structure with large initial velocities in the velocity phase space.

NOZOMI observation of the interplanetary magnetic field in 1998

The insertion of NOZOMI spacecraft into interplanetary space, where other spacecraft were operating, added a new array of multipoint observation that extended up to 1.6×10 6 km in y-direction of the GSE coordinate system. The coherency length of planar magnetic structures has been found to be less than 1.9×10 6 km in a direction tangential to the discontinuity surface. The region of low coherency seemed to coincide with region of high density plasma. An interplanetary magnetic flux rope which appeared on October 19, 1998, was identical over a distance of 1.9×10 6 km, while in a high-beta sheath preceding it, the magnetic field appeared differently at separate spacecraft.

Electron and ion spectrometer onboard the Nozomi spacecraft and its initial results in interplanetary space

The electron and ion spectrometer (EIS) is one of the fourteen instruments onboard the Nozomi spacecraft, a Mars orbiter, launched on July 4, 1998. EIS consists of two kinds of telescopes (TOF– E and ΔE– E telescopes) designed to measure the fluxes of electrons and ions in the energy range from ∼40 keV to a few MeV. The ΔE– E telescopes are used to measure electrons and protons, while the TOF– E telescope is mainly used to measure He, CNO-group, NeMgSi-group, and Fe-group. In this paper, (a) the objectives of this experiment, (b) the details of the instrument and (c) initial results of the observation in interplanetary space are described.

Counterstreaming electrons in the near vicinity of the Moon observed by plasma instruments on board NOZOMI

The NOZOMI spacecraft approached the Moon at an altitude of about 2800 km at 0734 UT on December 18, 1998. Around the time of closest approach, a plasma instrument Particle Spectrum Analyzer/Electron Spectrum Analyzer (PSA/ESA) detected non‐solar‐wind electrons in addition to the normal solar wind component. From the characteristics of the electron distribution function, we can categorize these events into two types: (1) backstreaming electrons exhibiting a velocity distribution similar to that of the solar wind electrons, but its phase space density ratio to the solar wind electrons decreases as a function of velocity; (2) backstreaming electrons that are thermalized and have a flux comparable to or dominating that of the solar wind electrons. We considered possible source locations as well as possible mechanisms that can produce these backstreaming electrons. After careful investigation of the velocity distribution function of the electrons and the magnetic field orientation, we concluded that their origins are (1) the lunar wake region, where the electrostatic potential drop associated with ambipolar plasma expansion reflects the solar wind electrons and produces backstreaming electrons of category 1, and (2) the terrestrial bow shock, where the electrons are thermalized downstream and escaping electrons are of category 2.

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.

First test of the NOZOMI radio science system in actual space flight

The Japanese NOZOMI spacecraft (S/C) launched on the 4 th of July, 1998 was equipped with a coherent dual frequency Radio Science System (RSS) to explore the Martian atmosphere and the solar wind plasma by the radio sounding method. Several communication operations were performed in order to test and calibrate the RSS during the cruise phase of the flight to Mars. S- and X- band signals emitted by the S/C were received and recorded at the Usuda tracking station (Japan). Analysis of the data processing results revealed the influence of the S/C spin on all signal parameters. In particular the S/C spin shifted the frequencies of both the S- and X- band signals, destroying their coherency. It was shown that the effect of the spin stabilizing system can be removed from the final occultation results and the Nozomi S/C can be successfully used in radio occultation experiments.

Upper limits for the Martian exospheric number density during the Planet B/Nozomi mission

Dissociative recombination (DR) of ionospheric O 2 + ions is an important source of suprathermal atomic oxygen in the exosphere as previous studies about the Martian upper atmosphere have shown. Because of the weaker gravitational attraction a hot oxygen corona on Mars should be denser than that observed on Venus. Since the most important mechanism for the production of the hot oxygen atoms in the Martian exosphere is DR, we investigated the variability of this production mechanism depending of solar activity. The Japanese Nozomi spacecraft will have the possibility to detect with the neutral mass spectrometer (NMS) for the first time in-situ the theoretically predicted hot oxygen corona on Mars, if the corona number density above the cold background atmosphere is of the order of 10,000 cm −3 . Due to a problem in the propulsion system Nozomi failed its planned arrival rendevouzs with Mars in October 1999 and will, therefore, arrive at the red planet not before January 2004. Solar activity will reach its maximum in 2001, so the related production rate of hot oxygen atoms will be in the medium range during the new arrival date of Nozomi. We used the ionospheric profiles from the Viking mission for low solar activity conditions ( F 10.7≈70) and the Mariner 9 mission with a solar activity of about 120 for medium solar wind activity. The latter is comparable to the level we expect for the Mars arrival of Nozomi. The resulting influence of the hot oxygen corona number density distribution was calculated with a Monte Carlo technique. This technique is used to compute a hot particle density distribution function. We studied the atomic diffusion process in the Martian atmosphere by simulating the collision probability, particle direction and energy loss after collisions by generating random numbers. Compared to previous studies we have improved the Monte Carlo model by using more and smaller altitude steps and more detailed treatment of particles with a temporary downward motion. This has resulted in an increased amount of collisions and a shift to lower energies in the energy spectrum. Our results show that the hot oxygen component should begin to dominate above the cold background atmosphere at an altitude of about 500 km above the Martian surface. The NMS instrument on board of Nozomi should detect the hot oxygen component after its arrival at Mars in January 2004, at an altitude of about 600 km above the Martian surface. Since the solar activity will decrease during the mission the measurements during the first orbits will be the most significant ones. The first in-situ measurements of the hot oxygen number density would be very important for adjusting atmospheric escape models by separating ballistic, satellite and escape trajectories of the hot oxygen atoms, which are significant for studies of the evolution and solar wind interaction of the Martian atmosphere.