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This paper summarizes the current status of the BepiColombo Mercury magnetospheric orbiter (MMO) spacecraft design. The MMO is a spinning spacecraft of 223 kg mass whose spin axis is nearly perpendicular to the Mercury orbital plane. The current status of the overall MMO system and subsystems such as thermal control, communication, power, etc., are described. The critical technologies are also outlined. Furthermore, the outline of the international cooperation between Japan Aerospace Exploration Agency and European Space Agency is also presented.

We have successfully detected solar neutrons at ground level in association with the X17.0 solar flare that occurred on 2005 September 7. Observations were made with the solar neutron telescopes and neutron monitors located in Bolivia and Mexico. In this flare, large fluxes of hard X-rays and γ-rays were observed by the GEOTAIL and the INTEGRAL satellites. The INTEGRAL observations include the 4.4 MeV line γ-rays of 12 C. The data suggest that solar neutrons were produced at the same time as these hard electromagnetic radiations. We have however found an apparent discrepancy between the observed and the expected time profiles. This fact suggests a possible extended neutron emission.

Accepted: 2005-02-17, 資料番号: SA1002568000

Mariner 10 has been the only spacecraft to visit the innermost planet Mercury. Its three flybys, more than 25 years ago, yielded the first view of this little-understood world. With advances in spacecraft technology and a growing realization of how important Mercury is to our understanding of the solar system and its formation, two missions are now in development for more intensive Mercury exploration. The first is the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission, competitively selected under the NASA Discovery Program, that will send a spacecraft to fly by Mercury in 2007 and 2008 and to orbit Mercury for one Earth year beginning in April 2009. The second is the more comprehensive BepiColombo mission, consisting of three elements: the Mercury Planetary Orbiter (MPO), the Mercury Magnetospheric Orbiter (MMO), and the Mercury Surface Element (MSE). Still in final definition stage, BepiColombo is a partnership between the European Space Agency (ESA) and the Japanese Institute of Space and Astronautical Science (ISAS). With one or two launches (depending upon the final mission architecture) BepiColombo will use solar electric propulsion to send two orbiters (MPO from ESA and MMO from ISAS) and a lander (MSE) to Mercury as early as 2011. The BepiColombo orbiters, in orbits complementary to that of MESSENGER, will extend geochemical, spectral, and photometric mapping of the planet. With its factor-of-ten larger downlink, BepiColombo will complete the intensive study of Mercury begun with the exploration by MESSENGER. Synergistic strategies of exploration will enable efficient use of BepiColombo resources in a more detailed study of the planet than can be accomplished by MESSENGER alone. For example, the earlier MESSENGER mission can help identify appropriate landing sites for the MSE, while BepiColombo can provide complementary orbital measurements of surface features from different phase angles and exploit MESSENGER observations to target high-resolution measurements. Possible joint activities include cooperative use of ground stations and extension of the temporal baseline for geophysical measurements by combining data from all spacecraft. Were MESSENGER to be approved for an extended orbital phase, there is also the possibility of simultaneous dual-spacecraft measurements in the magnetosphere.

Accepted: 2003-04-22, 資料番号: SA1002461000

Accepted: 2003-04-24, 資料番号: SA1001213000

We present an analysis of sporadic and recurrent injections of magnetospheric ions in the midnight auroral oval during substorms and of the associated ionospheric ion outflows. The source of plasma sheet precipitating ions is determined using a simple method, based on the measured relation between the ion inverse velocity and time ( l = v × t ). This method is applied here to two typical passes of the Interball-Auroral (IA) satellite at distances of ∼ 3 R E above the auroral regions. Substorm related ion injections are shown to be mainly due to time of flight effects. In contrast with particle trajectory computations (Sauvaud et al. , 1999), the inverse velocity method does not require magnetic and electric field models and can thus be used systematically for the detection of time of flight dispersed ion structures (TDIS). This allowed us to build a large database of TDIS events and to perform a statistical analysis of their spatial distribution. For the cases presented here the source region of the injected ions is found at radial distances from 18 to 30 R E near the equatorial magnetosphere. At Interball altitudes (∼ 3 R E ), ion injections detected at the poleward boundary of the nighside auroral oval are associated with shear Alfven waves superimposed over large-scale quasi-static current structures. We show that the most poleward TDIS are collocated with a large outflow of ionospheric H + and O + displaying pitch-angle distributions peaked in the pitch-angle range 90°–120°. These ions are thus accelerated perpendicularly to the magnetic field not only in the main auroral acceleration region but also up to at least 3 R E . The expanding auroral bulge thus constitutes a significant source of H + and O + ions for the mid-tail magnetosphere.

A detailed analysis of Geotail observations on June 1, 1997 provides evidence for the appearance of the plasma mantle at low-latitudes, ZGSM ∼3 RE for southward interplanetary magnetic field (IMF), and clarifies the characteristics of the plasma flow in the low-latitude mantle. The tailward flow speed in the mantle is 80 ∼ 160 km/s, smaller than the flow speed (≳300 km/s) in the adjacent magnetosheath. The component of the velocity perpendicular to the magnetic field lines is predominantly poleward at speeds up to 60 km/s, and the convection tends to be stronger for |;BY| |BZ|;. This polewardconvection can be interpreted as being driven by a J × B force operating on the field lines.

This paper shows the latest results of the feasibility study of the BepiColombo MMO (Mercury Magnetospheric Orbiter) Spacecraft, which was conducted by the ISAS (Institute of Space and Astronautical Science) Mercury Exploration Working Group.

Data measured in the magnetosheath by the Geotail satellite are used to test for the existence of a proportional relation between the temporal changes in magnetic field, ΔB, and the temporal changes in plasma velocity, ΔV. We have compared the ratio |ΔV/ΔB| with that which would be anticipated from magneto-hydrodynamic wave theory. When the plasma's β exceeds 1, the ratio often significantly differs from that expected from any wave mode in isotropic plasmas. The existence of anisotropies in plasma distribution functions and consequent modifications of |ΔV/ΔB| is strongly suggested. Under the assumption that the modifications of |ΔV/ΔB| are predominantly caused by such temperature anisotropies, β || / β ⊥ is evaluated over the range from 1 to 3. These anisotropies of plasma temperature may be caused by the adiabatic expansion of magnetic flux tubes in the magnetosheath.

We performed a multi-instrument ground-based and satellite study of substorm-related phenomena during auroral breakup. Our study is limited to the short (a few minutes) time interval around the breakup onset. A mapping made on the basis of event-oriented mangnetospheric model places the pre-breakup arc into the near-Earth region ( r ≈ 10 R E ) where thin and intense current sheet develops. Disruption of the current sheet was observed in the same region. The plasmoid was observed in the mid-tail. Backtracing the plasmoid onset time and location confirmed that reconnection related to the substorm onset could operate in the near-Earth thin current sheet. These observations are in agreement with the idea that the reconnection and current disruption signatures, as well as auroral breakup are different manifestations of the one process.

We have found clear evidence for intensification of the cross-tail current and associated occurrence of magnetic reconnection starting from several minutes before a substorm onset at a distance of ∼17 Re down the tail. Following a gradual increase in the growth phase, the current density was intensified stepwise at ∼5 minutes before the onset, and then gradually increased on the average while fluctuating. It is noted that the intensification of the cross-tail current density was due to enhancement of the dawnward electron velocity. The increasing current density accompanied a gradual increase of the tailward flow velocity and small enhancements of the electron and ion temperatures. A few minutes later, the magnetic Bz field dropped down to very small values, and shortly thereafter (around the substorm onset) the rate of change of the tailward velocity was enhanced in association with negative Bz field, significant increasing of the ion and electron temperatures, and decreasing of the plasma density. These signatures can be well understood in terms of temporal evolution of magnetic reconnections associated with local thinning of the current sheet and intensification of the current density.

Over a wide range of tailward distances in the lobe/mantle regions of the Earth's magnetotail (

Utilizing the GEOTAIL data obtained in the distant (X −1 4 power law that is theoretically expected when the total magnetic flux in the tail does not depend on the solar wind pressure. On the other hand, during southward interplanetary magnetic field, the magnetic flux is often much larger than that during northward case. A likely cause of this feature is the pile up of the dayside-eroded magnetic flux.

The periodic passage of a high-speed solar wind sector and the related corotating interaction region (CIR) in January and February 1994 resulted in two well-defined periods of sustained geoactivity. During this time interval GEOTAIL happened to be in quasi-elliptical orbits with apogees at about x = −90 R E and with orbital periods close to the solar rotation of 27 days. These orbits provided consecutive radial cuts through the magnetosphere from a mid-tail position (x = −90 R E ) to the dayside magnetopause. The energetic particle spectrometer HEP-LD onboard GEOTAIL was used to identify oxygen bursts with beam-like angular distributions. A significant majority of these beams were moving in a tailward direction. All of these events are closely related to substorm activity. The formation of near-Earth neutral lines (NENLs) are a possible source for particle acceleration. Low charge states of the oxygen ions are strong evidence for an ionospheric origin. The main focus of this study is to provide a coarse picture of oxygen events and their distribution in the regions of geospace visited by GEOTAIL in the time interval of interest. Details are obviously beyond the scope of this report.

Non-Parker type interplanetary objects, planar magnetic structures, have been examined by using magnetic field and low-energy particle data obtained by GEOTAIL. Bi-directional electrons characteristic of closed field lines were not found in planar magnetic structures. Jumps in magnetic field in planar magnetic structures were tangential discontinuities across which there was no mass flux, with a few exception. It suggests that a planar magnetic structure is made of layers of plasmas bounded by several tangential discontinuities, and in a few cases, there is an interaction between the layers.

Several types of discrete ion signatures have been found in different regions of the Earth's magnetosphere. The discovery of ionospheric-origin ion flows in the distant magnetotail by Geotail has been one of the most remarkable findings by ISTP satellites. In an event study, flow velocity and density variations are compared with geomagnetic field data and auroral indices AU, AL, and AE. Following a sudden increase of the geomagnetic fields observed on the ground and by Geotail, clear double-component ion flows consisting of H+ and O+ were detected in the southern lobe/mantle region. The auroral indices were well correlated with the tailward velocities of the multi-composition ion flows after the sudden compression of the magnetosphere was recorded by magnetometers at ground stations and Geotail and the geomagnetic activity was enhanced. Another recent finding of Geotail has been distinctly energy-dispersed ion signatures in the equatorial plasma sheet and the dayside magnetosphere near the magnetosphere-magnetosheath boundary. Similar ion dispersions have been reported from satellite observations at low altitudes. The ion energies ranged from a few tens of eV/q to ∼ 10 keV/q, and pitch angle distributions showed the properties of bidirectional field-aligned cold flows which are multiple at different energies. We propose that the origin of the ion beams observed near the nightside and dayside equatorial regions is to be found in upward flowing ion beams or conics generated at low altitudes. These could contribute 30–60 percent of the ion population in the dayside magnetosphere. Although some time-dependent effects might be involved, the primary process causing energy-dispersed signatures would discriminate ion energies spatially, for example, the velocity filter effect due to the E×B. On the basis of these observations by Geotail and Akebono, we discuss the process of the acceleration and transport of the ionospheric ions supplied to the magnetosphere.

We obtained the occurrence rates and characteristics of fast dawnward and duskward ion flows whose speed exceeded 400 km/s and which are observed in the near and mid-distant magnetotail by analyzing data of plasma, magnetic field and electric field obtained by the GEOTAIL spacecraft. In both dawnward and duskward flows, the ion density was low, the ion temperature was high, and the ion pressure was low compared to those in a nominal state. Such fast dawnward ion flows are possibly due to E × B drifts with the electric fields directed to the center of magnetic field line curvature.

GEOTAIL observations have revealed that ion velocity distribution functions in plasmoids are in general non-Maxwellian, and often contain beams, which represent the memory of their source and route associated with magnetic reconnection processes. Cold ions of the lobe origin flow into the plasmoid along magnetic field lines, are accelerated and heated through slow shocks at the boundary, and finally merge with hot components inside the plasmoid. Deep inside the plasmoid, however, their remnants are still discernible and the ion distribution functions often show the existence of counterstreaming ion beams. In this region both ends of the field lines are most likely connected to IMF through the northern and southern lobes with short distances, so that ions of the lobe origin have not had time enough for thermalization. The presence of a long-duration post-plasmoid plasma sheet (PPPS) with high tailward speeds shows the continuing occurrence of reconnections on the earthward side of the spacecraft location. The ion velocity distributions contain beams almost everywhere in the PPPS region, but their appearance is highly variable and at times non-gyrotropic. In the near-Earth region, plasma flow properties are also highly variable and bursty after the plasmoid traversal. The high variability of plasma distribution functions and flow properties may represent a consequence of multiple and bursty reconnections, and coalescence of a number of vortices may constitute the structure of the post-plasmoid plasma sheet.

In the framework of IACG Campaign #2, we study a case in which the GEOTAIL spacecraft was close to the mid-tail magnetopause when three substorms took place. GEOTAIL was mainly located within the dawnside plasmasheet, and its average speed normal to the nominal magnetopause was very small (∼0.2 R E /hr) because it was near its apogee. It exited to the LLBL∼magnetosheath during the expansion to recovery phases of each substorm, stayed there for a while, and then returned to the plasmasheet near the end of the recovery phase. This pattern suggests a shrinkage of the mid-tail magnetosphere after the expansion onset so that the tail radius becomes smaller than that in the quiet time. We interpret it as a result of strong tailward flows from the near-Earth neutral line which transports plasma in the mid-tail region to the distant tail and causes a rarefaction in the mid-tail region.

Following the suggestion that the low-latitude boundary layer (LLBL) in the near-Earth magnetotail can be the site of capturing magnetosheath plasma into the plasma sheet during extended northward IMF periods, we have made a case study of the plasma sheet using data from WIND, Geotail and Akebono. On Feb. 9–10, 1995, IMF was northward for more than 24 hours. The plasma sheet at (Xgsm, Ygsm) = (−30 ∼ −15, 0 ∼ −10)Re observed by Geotail changed its status from hot-tenuous to cold-dense in this period. The ions in the cold-dense plasma sheet have two characteristic energies (temperatures) just as those in the LLBL. The colder component had a temperature as low as a few hundreds eV. From Akebono observations during the same time interval, the convection pattern in the high-latitude ionosphere was found to show a clear four-cell structure. Although these facts are well-known on statistical basis, the simultaneous observations definitely put a constraint that the plasma transport which produces the cold-dense plasma sheet should be in accord with the four-cell convection pattern. Inspection on the Akebono particle data shows that a few keV ion/soft (< 1 keV) electron precipitation are seen in the sunward convection zone located adjacent to the trapped particle region at invariant latitudes of 75° ∼ 80°. Discussion on the formation of the cold-dense plasma sheet during northward IMF period is made on the basis of these observations.

Based on modern satellite observations in the earth's magnetotail and advanced computer simulation studies, it becomes possible to discuss demonstratively a kinetic magnetic reconnection process beyond ion inertia scale. One of the important findings is the observation of non-Maxwellian/non-gyrotropic ion distribution functions in the magnetotail. We discuss that a specific kind of non-gyrotropic ion distribution can be observed in the vicinity of an X-type neutral region.

Based on about two years of Geotail data, we use a superposed epoch approach to study the average behavior of plasma and magnetic fields at different radial distances during 66 substorms in the premidnight sector. Magnetic field dipolarization is first seen inside of 16 R E around substorm onset and subsequently moves tailward at a rate of 0.4 R E /min. Fast earthward and tailward ion bulk flows in the central plasma sheet indicate that the near-Earth neutral line is first located around 21 R E , but later moves a bit tailward. About 30 min after onset, the tailward moving dipolarization front reaches the near-Earth neutral line at around 26 R E . Thereafter the near-Earth neutral line suddenly disappears beyond 31 R E . This may be the beginning of the recovery phase.

Multipoint observations on ground-based stations and a fleet of ISTP satellites allowed us to study the plasma processes in different regions of space for multiple (4) substorm event on December 22–23, 1996 which was characterized by almost constant solar wind pressure ∼ 1.2 nPa and southward IMF B z . Comparison of measurements in the plasma sheet at opposite flanks showed (1) the similar loading/unloading processes in the tail, correlated with substorm development, and (2) a strong bursty convection concentrated in the narrow ( ΔY ∼ 15 R E ) channel.

For substorm onsets, earthward flowing ions appear as an initial signal of the onset in the near-Earth plasma sheet in the onset sector. The flow activity in the onset sector is followed by a step-like increase in the Bz field in the equatorial plane, which results in a dipolarization in the field. The field change is usually associated with energization in ions and electrons. Outside of the onset sector the field dipolarization takes place. However, it may not be preceded by earthward flowing ions, and ions usually have the duskward flows in the evening sector and the dawnward flows in the morning sector. The onset signatures at synchronous orbit are delayed relative to those beyond 10 R E .

We present Polar plasma wave data during cusp energetic particle (CEP) events at 6-9 R(sub E). These data suggest the presence of coherent electrostatic structures that are highly localized and that have typical velocities on the order of hundreds to thousands of kilometers per second along the ambient magnetic field. Some of the wave signatures are solitary waves and some are wave packets. The Polar wave instrument also provides evidence that some of the bursts of electromagnetic waves (with frequencies of a few hundred Hz and just below the electron cyclotron frequency around 800 Hz to 1-2 kHz) that are observed are coher&nt and propagating both up and down the field lines. Electron cyclotron harmonic (ECH) waves are often detected but their duration is usually short (less than 1 second). Low Frequency (less than 1 kHz), broadband, bursty electromagnetic waves are also present. The Polar wave data results are used to obtain a better understanding of the macro/microphysics during a CEP event that takes place on September 11, 1996, by correlating various Polar (approximately 7.0 R(sub E)) and Akebono (approximately 1.4 R(sub E)) data while both spacecraft are in or near the cusp/cleft region and nearly on the same field line, and magnetometer data from the Canadian Intermagnet and Canopus ground stations, which lie near the base of the magnetic footprint passing through Polar. Solar wind and magnetic field data from the interplanetary medium and magnetosheath are provided by the Geotail and IMP-8 satellites, respectively. Some of the cusp waves may be indicators of the reconnection process taking place through the cusp, the result of mixing of magnetosheath with magnetospheric plasma, and the consequence of an anisotropic electron population in a depressed magnetic field. The low frequency electromagnetic waves are still under study to determine their role, if any, in the heating and acceleration of the MeV He ions during CEP events.

In order to understand the processes of formation and motion of sun-aligned polar cap arcs as well as patches, we have analyzed simultaneous observational data from Akebono (EXOS D) satellite and the all-sky imager at Qaanaaq, Greenland. The results demonstrate the correspondence between the localized electron precipitation signatures observed from Akebono and the sun-aligned arcs seen from the all-sky imager at Qaanaaq, and confirm that sun-aligned arc correspond to divE

Structure of the distant magnetotail (x

The GEOTAIL spacecraft has made comprehensive magnetic field and plasma observations in the Earth's magnetotail at radial distances of 10–210 Re. Data from the GEOTAIL spacecraft have led to the following conclusions regarding the relationship between substorms and field and plasma behaviors. Substorms produce systematic and repeatable signatures in magnetic field and plasmas in various radial distances in the magnetotail. A repeatable signature for substorms is tailward plasma flows with a southward magnetic field structure. These are often associated with plasmoids, as seen in the local magnetic field variations. Examples are presented of signatures seen at different distances from the Earth. From a statistical survey of the data, it is shown that magnetic reconnection takes place in the expansion phase at radial distances of 20–30 Re. Plasmoids are created there and travel tailward at high speeds. Earthward flows in the recovery phase appear only inside 100 Re. Plasmoids are dominant substorm signatures beyond 100 Re.

Strength of the GEOTAIL mission derives from several factors: the optimal choice of the orbit, installation of the advanced instrumentation, use of numerical simulation as the basic intepretational tool, and formation of a satellite network with other IASTP missions. Representative scientific results which reflect these assets are reviewed.

Geotail has surveyed the dayside magnetopause in the equatorial plane and studied the dayside reconnection of the interplanetary magnetic field (IMF) with the magnetospheric field lines. When the IMF is directed southward leakage of ions from the low latitude boundary layer (LLBL) to the magnetosheath boundary layer (MSBL) are observed. Reconnection is shown to be responsible for the leakage. A variation in lower cut-off levels occurring in the velocity distribution function of the leakage ions is explained by the “velocity filter effect” model with a finite source region. In addition to such conditions, reconnection occurs also when the IMF has low-inclination angles, i.e., the condition between the southward and northward IMF conditions. Between the magnetosphere and the magnetosheath, two types of boundary region develop, i.e., the inner-LLBL and the outer-LLBL. The inner-LLBL is characterized by the bi-directional cold electrons and trapped cold ions coexisting with hot magnetospheric plasma. The field lines are regarded to be closed. The outer-LLBL is, on the other hand, open to the magnetosheath. It is characterized by the uni-directional cold electrons escaping to the magnetosheath. Newly penetrating solar wind ions are overtaking the trapped cold ions. The formation of the outer-LLBL is explained by the high-latitude reconnection at the equatorward regions of the cusp.

The origin of hot and high speed plasmas observed by the GEOTAIL spacecraft in the magnetotail is discussed in terms of slow shock acceleration and heating. We find that the bulk flow energy for the hot and high speed plasma in the tail plasma sheet is larger than the thermal energy, and there is an lower limit on the ratio of the thermal to bulk flow energy 2p ϱv 2 . The lower boundary of 2p ϱv 2 is about 0.2 – 0.4, and the ratio is independent of the plasma temperature in the range of several 100 eV to 10 keV. It is believed that the magnetic reconnection associated with a slow shock can produce the hot and high speed plasma, though the observed lower limit cannot be explained by a standard slow shock heating and acceleration process, because the lower limit of 2p ϱv 2 obtained by the standard slow shock Rankine-Hugoniot relation is 0.4 even for the strong slow shock limit. In order to explain the observed lower boundary of 2p ϱv 2 , we study the Rankine-Hugoniot relations by taking into account of non-standard MHD effects such as a temperature anisotropy and a heat flux. The observed lower limit can be explained by a slow shock including the temperature anisotropy and the heat flux effects.

An initial survey of distant tail energetic particle phenomena from Geotail shows that (1) energetic ions of both solar wind and ionospheric origin populate the distant tail, (2) the entry of energetic ions from the magnetosheath into the magnetotail can take place many tens of earth radii behind the Earth in a transient manner, (3) energetic molecular ions of ionospheric origin are present in the distant tail during very high geomagnetic activity, and (4) energetic ion flux enhancements and substorm intensifications are temporally correlated.

The structure of plasma sheet in the distant magnetotail observed by the GEOTAIL satellite is examined. We discuss that the structure of the plasma sheet can be expressed as a doubly humped current sheet, which is quite different from the standard Harris-type current sheet. Possible theoretical models to explain the formation of the doubly humped current sheet are shortly discussed.

A sounding rocket S-520-12 was launched from Andoya, Norway at 02:06:00 U.T. on 26 February 1990, into pulsating aurora. Electron energy spectra were observed with a quadrispherical electrostatic analyzer (QESA). The rocket flew from one pulsating patch to another, and we observed the spectral variation of precipitating electron flux following this transition. Pulsation of particle flux was observed in the precipitating electrons above 4 keV and the spectrum was fitted with a power-law distribution, although the electrons with energy less than 4 keV did not show significant pulsation. We found that the pulsation periods obtained through Fourier analysis for the auroral emission recorded by ground TV camera and for the in-situ energy flux data of the precipitating electrons agreed well. However, the one-to-one correlation between the electron energy flux and the auroral intensity was relatively poor. We attributed this to the spatial nonuniformity of the boundary region between two pulsating patches, and to the unstable phase relationship between the dominant Fourier components of the auroral emission and the electron energy flux. This might be caused by the propagating and streaming nature of the pulsating aurora during the time of the observation. We also found low-energy electron precipitation at the boundary region between the two pulsating patches, which can be attributed to the acceleration of the electrons at an altitude of several thousand kilometers by upward-propagating kinetic Alfven waves. That wave might be generated in association with the ionospheric conductivity change caused by the precipitation of the auroral electrons and the resultant enhancement of ionization of the upper atmosphere.

By analyzing measurements of the precipitating electrons and ions obtained by the low energy particle experiment on board a semi-polar orbiting satellite EXOS-C launched in 1984, we address two issues. The first is the source of electrons forming the precipitation events observed as spike-like flux enhancements in the polar cap. Information derived from the conjugacy and the temperature is used to examine the adequacy of two possible sources, namely the plasma sheet and the solar wind. The second issue is the acceleration mechanism of the inverted V electrons in the auroral region. The quantitative model of the field-aligned acceleration formulated by Lyons is compared with the observation and a fair agreement is found.

Proton temperature in the solar wind between 0.68 and 0.98 a.u. is investigated from the point of view of heat generation in the interplanetary space based on the solar wind data from the Japanese interplanetary spacecraft Suisei . The heliocentric radial gradient of proton tempeature tends to decrease in high-speed regions owing to heat generation accompanied by a velocity increase of more than 50 km s −1 per hour. Temperature measurements in the region with decreasing or nearly constant flow velocity reveal cooling of the form R −0.9 ≈−12, still slower than the adiabatic case. The remnant difference suggests the existence of another heating mechanism for protons, especially in a low-speed stream.