No meridional plasma flow in the heliosheath transition region

Over a two-year period, Voyager 1 observed a gradual slowing-down of radial plasma flow in the heliosheath to near-zero velocity after April 2010 at a distance of 113.5 astronomical units from the Sun (1 astronomical unit equals 1.5?×?10(8) kilometres). Voyager 1 was then about 20 astronomical units beyond the shock that terminates the free expansion of the solar wind and was immersed in the heated non-thermal plasma region called the heliosheath. The expectation from contemporary simulations was that the heliosheath plasma would be deflected from radial flow to meridional flow (in solar heliospheric coordinates), which at Voyager?1 would lie mainly on the (locally spherical) surface called the heliopause. This surface is supposed to separate the heliosheath plasma, which is of solar origin, from the interstellar plasma, which is of local Galactic origin. In 2011, the Voyager project began occasional temporary re-orientations of the spacecraft (totalling about 10-25 hours every 2 months) to re-align the Low-Energy Charged Particle instrument on board Voyager?1 so that it could measure meridional flow. Here we report that, contrary to expectations, these observations yielded a meridional flow velocity of +3?±?11?km?s(-1), that is, one consistent with zero within statistical uncertainties.     

Cool heliosheath plasma and deceleration of the upstream solar wind at the termination shock

The solar wind blows outward from the Sun and forms a bubble of solar material in the interstellar medium. The termination shock occurs where the solar wind changes from being supersonic (with respect to the surrounding interstellar medium) to being subsonic. The shock was crossed by Voyager 1 at a heliocentric radius of 94 au (1 au is the Earth-Sun distance) in December 2004 (refs 1-3). The Voyager 2 plasma experiment observed a decrease in solar wind speed commencing on about 9 June 2007, which culminated in several crossings of the termination shock between 30 August and 1 September 2007 (refs 4-7). Since then, Voyager 2 has remained in the heliosheath, the region of shocked solar wind. Here we report observations of plasma at and near the termination shock and in the heliosheath. The heliosphere is asymmetric, pushed inward in the Voyager 2 direction relative to the Voyager 1 direction. The termination shock is a weak, quasi-perpendicular shock that heats the thermal plasma very little. An unexpected finding is that the flow is still supersonic with respect to the thermal ions downstream of the termination shock. Most of the solar wind energy is transferred to the pickup ions or other energetic particles both upstream of and at the termination shock.     

Mediation of the solar wind termination shock by non-thermal ions

Broad regions on both sides of the solar wind termination shock are populated by high intensities of non-thermal ions and electrons. The pre-shock particles in the solar wind have been measured by the spacecraft Voyager 1 (refs 1-5) and Voyager 2 (refs 3, 6). The post-shock particles in the heliosheath have also been measured by Voyager 1 (refs 3-5). It was not clear, however, what effect these particles might have on the physics of the shock transition until Voyager 2 crossed the shock on 31 August-1 September 2007 (refs 7-9). Unlike Voyager 1, Voyager 2 is making plasma measurements. Data from the plasma and magnetic field instruments on Voyager 2 indicate that non-thermal ion distributions probably have key roles in mediating dynamical processes at the termination shock and in the heliosheath. Here we report that intensities of low-energy ions measured by Voyager 2 produce non-thermal partial ion pressures in the heliosheath that are comparable to (or exceed) both the thermal plasma pressures and the scalar magnetic field pressures. We conclude that these ions are the >0.028 MeV portion of the non-thermal ion distribution that determines the termination shock structure and the acceleration of which extracts a large fraction of bulk-flow kinetic energy from the incident solar wind.     

Domination of heliosheath pressure by shock-accelerated pickup ions from observations of neutral atoms

The solar wind blows an immense magnetic bubble, the heliosphere, in the local interstellar medium (mostly neutral gas) flowing by the Sun. Recent measurements by Voyager 2 across the termination shock, where the solar wind is slowed to subsonic speeds before entering the heliosheath, found that the shocked solar wind plasma contains only approximately 20 per cent of the energy released by the termination shock, whereas energetic particles above approximately 28 keV contain only approximately 10 per cent; approximately 70 per cent of the energy is unaccounted for, leading to speculation that the unmeasured pickup ions or energetic particles below 28 keV contain the missing energy. Here we report the detection and mapping of heliosheath energetic ( approximately 4-20 keV) neutral atoms produced by charge exchange of suprathermal ions with interstellar neutral atoms. The energetic neutral atoms come from a source approximately 60 degrees wide in longitude straddling the direction of the local interstellar medium. Their energy spectra resemble those of solar wind pickup ions, but with a knee at approximately 11 keV instead of approximately 4 keV, indicating that their parent ions are pickup ions energized by the termination shock. These termination-shock-energized pickup ions contain the missing approximately 70 per cent of the energy dissipated in the termination shock, and they dominate the pressure in the heliosheath.     

An asymmetric solar wind termination shock

Voyager 2 crossed the solar wind termination shock at 83.7 au in the southern hemisphere, approximately 10 au closer to the Sun than found by Voyager 1 in the north. This asymmetry could indicate an asymmetric pressure from an interstellar magnetic field, from transient-induced shock motion, or from the solar wind dynamic pressure. Here we report that the intensity of 4-5 MeV protons accelerated by the shock near Voyager 2 was three times that observed concurrently by Voyager 1, indicating differences in the shock at the two locations. (Companion papers report on the plasma, magnetic field, plasma-wave and lower energy particle observations at the shock.) Voyager 2 did not find the source of anomalous cosmic rays at the shock, suggesting that the source is elsewhere on the shock or in the heliosheath. The small intensity gradient of Galactic cosmic ray helium indicates that either the gradient is further out in the heliosheath or the local interstellar Galactic cosmic ray intensity is lower than expected.     

Magnetic fields at the solar wind termination shock

A transition between the supersonic solar wind and the subsonic heliosheath was observed by Voyager 1, but the expected termination shock was not seen owing to a gap in the telemetry. Here we report observations of the magnetic field structure and dynamics of the termination shock, made by Voyager 2 on 31 August-1 September 2007 at a distance of 83.7 au from the Sun (1 au is the Earth-Sun distance). A single crossing of the shock was expected, with a boundary that was stable on a timescale of several days. But the data reveal a complex, rippled, quasi-perpendicular supercritical magnetohydrodynamic shock of moderate strength undergoing reformation on a scale of a few hours. The observed structure suggests the importance of ionized interstellar atoms ('pickup protons') at the shock.     

Voyager 1 exited the solar wind at a distance of approximately 85 Au from the Sun

The outer limit of the Solar System is often considered to be at the distance from the Sun where the solar wind changes from supersonic to subsonic flow. Theory predicts that a termination shock marks this boundary, with locations ranging from a few to over 100 au (1 Au approximately 1.5 x 10(8) km, the distance from Earth to the Sun). 'Pick-up ions' that originate as interstellar neutral atoms should be accelerated to tens of MeV at the termination shock, generating anomalous cosmic rays. Here we report a large increase in the intensity of energetic particles in the outer heliosphere, as measured by an instrument on the Voyager 1 spacecraft. We argue that the spacecraft exited the supersonic solar wind and passed into the subsonic region (possibly beyond the termination shock) on about 1 August 2002 at a distance of approximately 85 Au (heliolatitude approximately 34 degrees N), then re-entered the supersonic solar wind about 200 days later at approximately 87 au from the Sun. We show that the composition of the ions accelerated at the putative termination shock is that of anomalous cosmic rays and of interstellar pick-up ions.     

Intense plasma waves at and near the solar wind termination shock

Plasma waves are a characteristic feature of shocks in plasmas, and are produced by non-thermal particle distributions that develop in the shock transition layer. The electric fields of these waves have a key role in dissipating energy in the shock and driving the particle distributions back towards thermal equilibrium. Here we report the detection of intense plasma-wave electric fields at the solar wind termination shock. The observations were obtained from the plasma-wave instrument on the Voyager 2 spacecraft. The first evidence of the approach to the shock was the detection of upstream electron plasma oscillations on 1 August 2007 at a heliocentric radial distance of 83.4 au (1 au is the Earth-Sun distance). These narrowband oscillations continued intermittently for about a month until, starting on 31 August 2007 and ending on 1 September 2007, a series of intense bursts of broadband electrostatic waves signalled a series of crossings of the termination shock at a heliocentric radial distance of 83.7 au. The spectrum of these waves is quantitatively similar to those observed at bow shocks upstream of Jupiter, Saturn, Uranus and Neptune.     

Enhancements of energetic particles near the heliospheric termination shock

The spacecraft Voyager 1 is at a distance greater than 85 au from the Sun, in the vicinity of the termination shock that marks the abrupt slowing of the supersonic solar wind and the beginning of the extended and unexplored distant heliosphere. This shock is expected to accelerate 'anomalous cosmic rays', as well as to re-accelerate Galactic cosmic rays and low-energy particles from the inner Solar System. Here we report a significant increase in the numbers of energetic ions and electrons that persisted for seven months beginning in mid-2002. This increase differs from any previously observed in that there was a simultaneous increase in Galactic cosmic ray ions and electrons, anomalous cosmic rays and low-energy ions. The low-intensity level and spectral energy distribution of the anomalous cosmic rays, however, indicates that Voyager 1 still has not reached the termination shock. Rather, the observed increase is an expected precursor event. We argue that the radial anisotropy of the cosmic rays is expected to be small in the foreshock region, as is observed.     

The vertical profile of winds on Titan

One of Titan's most intriguing attributes is its copious but featureless atmosphere. The Voyager 1 fly-by and occultation in 1980 provided the first radial survey of Titan's atmospheric pressure and temperature and evidence for the presence of strong zonal winds. It was realized that the motion of an atmospheric probe could be used to study the winds, which led to the inclusion of the Doppler Wind Experiment on the Huygens probe. Here we report a high resolution vertical profile of Titan's winds, with an estimated accuracy of better than 1 m s(-1). The zonal winds were prograde during most of the atmospheric descent, providing in situ confirmation of superrotation on Titan. A layer with surprisingly slow wind, where the velocity decreased to near zero, was detected at altitudes between 60 and 100 km. Generally weak winds (approximately 1 m s(-1)) were seen in the lowest 5 km of descent.     

Identification of comet Hyakutake's extremely long ion tail from magnetic field signatures

Observations of the varying orientations of comet tails led to the suggestion of the existence of the solar wind--a continuous outflow of ionized material from the Sun. It is now well established that gas from comets is ionized by several processes and joins the solar wind, forming an ion (plasma) tail that points away from the Sun. The plasma environments of three comets have been measured in situ, but only in the upstream direction or less than 8,000 km downstream of the nucleus. Here we report a fortuitous crossing by a spacecraft of the plasma tail of comet Hyakutake (C/1996 B2), at a distance of more than 3.8 astronomical units (550 million kilometres) from its nucleus. This surpasses the tail length of 2 AU determined for the Great March Comet of 1843 (C/1843 D1). Our measurements reveal that, at this distance, the tail of comet Hyakutake was a structured entity at least 7 million kilometres in diameter.     

Continuing Colorado plateau uplift by delamination-style convective lithospheric downwelling

The Colorado plateau is a large, tectonically intact, physiographic province in the southwestern North American Cordillera that stands at ～1,800-2,000?m elevation and has long been thought to be in isostatic equilibrium. The origin of these high elevations is unclear because unlike the surrounding provinces, which have undergone significant Cretaceous-Palaeogene compressional deformation followed by Neogene extensional deformation, the Colorado plateau is largely internally undeformed. Here we combine new seismic tomography and receiver function images to resolve a vertical high-seismic-velocity anomaly beneath the west-central plateau that extends more than 200?km in depth. The upper surface of this anomaly is seismically defined by a dipping interface extending from the lower crust to depths of 70-90?km. The base of the continental crust above the anomaly has a similar shape, with an elevated Moho. We interpret these seismic structures as a continuing regional, delamination-style foundering of lower crust and continental lithosphere. This implies that Pliocene (2.6-5.3?Myr ago) uplift of the plateau and the magmatism on its margins are intimately tied to continuing deep lithospheric processes. Petrologic and geochemical observations indicate that late Cretaceous-Palaeogene (～90-40?Myr ago) low-angle subduction hydrated and probably weakened much of the Proterozoic tectospheric mantle beneath the Colorado plateau. We suggest that mid-Cenozoic (～35-25?Myr ago) to Recent magmatic infiltration subsequently imparted negative compositional buoyancy to the base and sides of the Colorado plateau upper mantle, triggering downwelling. The patterns of magmatic activity suggest that previous such events have progressively removed the Colorado plateau lithosphere inward from its margins, and have driven uplift. Using Grand Canyon incision rates and Pliocene basaltic volcanism patterns, we suggest that this particular event has been active over the past ～6?Myr.     

Alfvénic waves with sufficient energy to power the quiet solar corona and fast solar wind

Energy is required to heat the outer solar atmosphere to millions of degrees (refs 1, 2) and to accelerate the solar wind to hundreds of kilometres per second (refs 2-6). Alfvén waves (travelling oscillations of ions and magnetic field) have been invoked as a possible mechanism to transport magneto-convective energy upwards along the Sun's magnetic field lines into the corona. Previous observations of Alfvénic waves in the corona revealed amplitudes far too small (0.5?km?s(-1)) to supply the energy flux (100-200?W?m(-2)) required to drive the fast solar wind or balance the radiative losses of the quiet corona. Here we report observations of the transition region (between the chromosphere and the corona) and of the corona that reveal how Alfvénic motions permeate the dynamic and finely structured outer solar atmosphere. The ubiquitous outward-propagating Alfvénic motions observed have amplitudes of the order of 20?km?s(-1) and periods of the order of 100-500?s throughout the quiescent atmosphere (compatible with recent investigations), and are energetic enough to accelerate the fast solar wind and heat the quiet corona.     

纳秒脉冲气动激励无人机流动控制风洞试验

等离子体流动控制作为一种新型的主动流动控制技术,可显著提升飞行器的气动性能。采用纳秒脉冲气动激励进行了某型无人机流动分离控制实验。实验结果表明:纳秒放电和毫秒放电的激励电压几乎相等,但是纳秒放电产生的电流(30A)比毫秒放电电流(0.1A)大得多;纳秒脉冲气动激励在流场中诱导产生近似向上的冲击波,最大诱导速度不超过0.5m/s;纳秒放电的快速温升效应在静止空气中诱导产生冲击波,冲击波的持续时间约为80μs,传播速度约为380m/s;当激励电压大于一定阈值时,纳秒脉冲气动激励使得该型无人机上表面的流动分离得到抑制,临界失速迎角从20°提升至27°,最大升力系数增大11.24%。探究放电频率对流动控制效果的影响规律,结果表明:最佳激励频率是使得施特劳哈尔数为1的频率值;在附面层流动控制方面,纳秒脉冲气动激励较毫秒脉冲气动激励更加有效;纳秒脉冲等离子体流动控制的主要机制是冲击效应,在高速流动控制中,冲击效应比动力效应更加有效。     

Three-dimensional MHD simulation for the solar wind structure observed by Ulysses

Ulysses has been the first spacecraft to explore the high latitudinal regions of the heliosphere till now. During its first rapid pole-to-pole transit from September 1994 to June 1995, Ulysses observed a fast speed flow with magnitude reaching 700—800 km/s at high latitudinal region except 20°area near the ecliptic plane where the velocity is 300—400 km/s. The observations also showed a sudden jump of the velocity across the two regions. In this note, based on the characteristic and representative observations of the solar magnetic field and K-coronal polarized brightness, the large-scale solar wind structure mentioned above is reproduced by using a three-dimensional MHD model. The numerical results are basically consistent with those of Ulysses observations. Our results also show that the distributions of magnetic field and plasma number density on the solar source surface play an important role in governing this structure. Furthermore, the three-dimensional MHD model used here has a robust ability to simulate this kind of large-scale wind structure.     

Sound velocities of majorite garnet and the composition of the mantle transition region

The composition of the mantle transition region, characterized by anomalous seismic-wave velocity and density changes at depths of approximately 400 to 700 km, has remained controversial. Some have proposed that the mantle transition region has an olivine-rich 'pyrolite' composition, whereas others have inferred that it is characterized by pyroxene- and garnet-rich compositions ('piclogite'), because the sound velocities in pyrolite estimated from laboratory data are substantially higher than those seismologically observed. Although the velocities of the olivine polymorphs at these pressures (wadsleyite and ringwoodite) have been well documented, those of majorite (another significant high-pressure phase in the mantle transition region) with realistic mantle compositions have never been measured. Here we use combined in situ X-ray and ultrasonic measurements under the pressure and temperature conditions of the mantle transition region to show that majorite in a pyrolite composition has sound velocities substantially lower than those of earlier estimates, owing to strong nonlinear decreases at high temperature, particularly for shear-wave velocity. We found that pyrolite yields seismic velocities more consistent with typical seismological models than those of piclogite in the upper to middle parts of the region, except for the potentially larger velocity jumps in pyrolite relative to those observed at a depth of 410 km. In contrast, both of these compositions lead to significantly low shear-wave velocities in the lower part of the region, suggesting possible subadiabatic temperatures or the existence of a layer of harzburgite-rich material supplied by the subducted slabs stagnant at these depths.     

近空间飞行环境泰氟隆烧蚀流场化学非平衡流数值算法及应用研究

本文通过数值求解考虑烧蚀产物在内的23种组分的化学非平衡Navier-Stokes方程,建立考虑泰氟隆材料烧蚀效应的化学非平衡流数值计算方法.理论预测了球锥体RAMC-II的等离子体鞘套电子数密度,并和飞行试验进行比较,验证了算法可靠性,揭示了电子数密度峰值结果在25–70 km中层近空间随飞行高度变化规律.算法应用于不同高度球锥体泰氟隆烧蚀流场计算,研究表明,在泰氟隆烧蚀现象比较明显的中低空,烧蚀产物对电子数密度的影响主要局限在边界层内;泰氟隆烧蚀导致边界层内流场温度降低,因而引起空气阴离子的增加,另一方面随着飞行高度降低,由亲电子产物引起的F?数量也不断增加,这些因素引起了边界层内的电子数密度降低.     

红河断裂以西大理‒永平地区上地壳各向异性分层特征

对中国地震科学台阵探测项目一期于2011—2013年布设在红河断裂以西大理永平地区的5个流动台站进行横波分裂研究, 分别得到18, 14, 7, 9 和5个横波分裂参数测量结果, 并使用更精确的实际横波路径, 通过过量归一化方法进行改正, 研究该区域各向异性分层特征。结果显示, 研究区上地壳10 km深度之上存在各向异性强度大小相间的3层各向异性层, 其中第2层各向异性强度最小, 厚度为2~2.4 km; 第1层各向异性强度稍强, 厚度为4.1~5.0 km; 第3层各向异性强度最强。各向异性分层特征与前人在该区域的大地电磁测深结果吻合。结合滇西地区地壳中的低速异常、低电阻率和低Q值现象, 认为第3层的强各向异性是地幔物质上涌造成裂隙发育以及热流上传所致。     

阵列桥箔电爆炸过程数值模拟

为提高金属桥箔电爆炸的能量利用效率,增加桥箔电爆炸等离子体作用范围,设计了金属阵列桥箔结构,采用有限元流体动力学方法对金属阵列桥箔电爆炸过程进行了数值模拟.利用相变分数和考虑电离度、粒子数目变化及粒子间库仑作用的等离子体状态方程,实现了金属导体在脉冲大电流作用下电爆炸产生等离子体及冲击波的数值模拟计算.对比分析了阵列桥箔在有加速膛通道和无加速膛自由场两种情况下的电爆炸等离子体流场特征及演化规律.计算结果表明,两种情况下等离子体束在叠加汇聚区压力较高,在无加速膛自由场中电爆炸初始冲击波速度和等离子体射流传播速度都较高;在有加速膛情况下,由于气体粘性作用,管壁附近存在一个高压低速边界层,加速膛通道内冲击结构较为复杂.     

某超燃冲压发动机尾喷管的三维流场特性

超燃冲压发动机结构简单,推重比大,应用前景广阔.但其流场结构十分复杂,研究超燃冲压发动机三维流场结构具有重要的意义.采用计算流体力学软件对某超燃冲压发动机尾喷管的三维流场进行数值模拟,先后得到了流场的密度、马赫数、涡量及速度矢量线图.结果表明:喷管内及侧壁面出口处存在膨胀波.各个壁面出口处附近的流场存在羽流激波和剪切层,内喷管出口周围的羽流激波和剪切层呈环状分布在流场周围,上壁面尾部受羽流激波的影响产生一道由压强决定的管内斜激波.内喷管出口及上壁面尾部处也均存在流向涡结构.