High-latitude Sporadic-E and other Thin Layers – the Role of Magnetospheric Electric Fields

Observations of high-latitude sporadic-E (Es) layers and theories of their formation are reviewed. The layers are found to be composed of metallic ions, they are at times formed by tidal wind shear, and they are more common in summer than in winter. All of these properties are common to Es layers at mid-latitudes. However, the high-latitude layers are rather often formed, modified or transported by the action of magnetospheric electric fields. Taking into account the action of both tides and electric fields leads to an understanding of the daily variation of Es occurrence, the daily variation of Es heights and the occasional appearance of upward migrating Es layers. Correlations between Es and neutral metallic layers at low altitudes can be explained by neutralisation of the metallic ions in the Es layers, but joint Es and neutral layers at higher altitudes are still unexplained. The action of electric fields and the interaction with neutral layers can explain the formation of multiple Es layers and may provide an explanation for the seasonal variation of Es occurrence. Uncertainties remain as to whether the narrowness of Es layers is fully compatible with formation by electric fields, to whether neutralisation at low altitudes can provide a sufficient explanation of the seasonal variation, and to whether the quasi-periodic fine structure observed in mid-latitude Es also appears at high latitudes. The understanding of ion transport by electric fields which results from the study of Es layers leads to new insights and new questions related to other plasma layers (polar mesosphere summer echoes and winter ion layers) which appear in the high-latitude winter ionosphere.     

竖直平板Blasius流层流边界层流动与传热耦合解

对考虑辐射传热、流体黏度随温度变化、有和无滑移边界条件下的可渗透竖直平板Blasius流层流边界层的无量纲速度场与温度场进行了深入研究.经相似变换将描述速度场与温度场耦合的偏微分方程组转换成非线性常微分方程组,用Runge-Kutta法对常微分方程组进行了数值求解.研究了无量纲参数对无量纲速度场及温度场的影响,着重分析了滑移边界条件下速度和温度随无量纲参数的变化规律.结果表明:吸入时边界层变薄,喷注时边界层加厚;对比于无滑移边界条件,滑移边界条件下速度、温度边界层变薄;随着变黏度参数a或喷注与吸入参数的增大,壁面摩擦因数、局部努塞尔数Nu增大,速度和温度边界层变薄;随着普朗特数Pr、热辐射参数R的增加,或毕渥数Bi,布林克曼数Br的减小, 温度边界层变薄.      

Interplanetary Origin of Intense,Superintense and Extreme Geomagnetic Storms

We present a review on the interplanetary causes of intense geomagnetic storms (Dst≤−100 nT), that occurred during solar cycle 23 (1997–2005). It was reported that the most common interplanetary structures leading to the development of intense storms were: magnetic clouds, sheath fields, sheath fields followed by a magnetic cloud and corotating interaction regions at the leading fronts of high speed streams. However, the relative importance of each of those driving structures has been shown to vary with the solar cycle phase. Superintense storms (Dst≤−250 nT) have been also studied in more detail for solar cycle 23, confirming initial studies done about their main interplanetary causes. The storms are associated with magnetic clouds and sheath fields following interplanetary shocks, although they frequently involve consecutive and complex ICME structures. Concerning extreme storms (Dst≤−400 nT), due to the poor statistics of their occurrence during the space era, only some indications about their main interplanetary causes are known. For the most extreme events, we review the Carrington event and also discuss the distribution of historical and space era extreme events in the context of the sunspot and Gleissberg solar activity cycles, highlighting a discussion about the eventual occurrence of more Carrington-type storms.     

The Freja wave and plasma density experiment

The Wave Experiment, F4, on the Swedish/German satelliteFreja, is designed to measure the electric wave fields up to 4 MHz, the magnetic wave fields up to 16 kHz and the plasma density and its relative variations up to 2 kHz. Six wave signals and four density probe signals can be measured simultaneously. The wave forms of all signals are transmitted to ground without any analysis onboard. The limited TM allocation does not allow continuous sampling of the wave signals, so normally the measurements are made in snapshots of various lengths dependent on sampling frequency, etc. Continuous sampling can be made for shorter time periods by using a 6 Mbyte memory as a buffer.     

Early results from ISEE-1 electric field measurements

The double probe, floating potential instrumentation on ISEE-1 is producing reliable direct measurements of the ambient DC electric field at the bow shock, at the magnetopause, and throughout the magnetosheath, tail plasma sheet and plasmasphere. In the solar wind and in middle latitude regions of the magnetosphere spacecraft sheath fields obscure the ambient field under low plasma flux conditions such that valid measurements are confined to periods of moderately intense flux. Initial results show: (a) that the DC electric field is enhanced by roughly a factor of two in a narrow region at the front, increasing B, edge of the bow shock, (b) that scale lengths for large changes in E at the sub-solar magnetopause are considerably shorter than scale lengths associated with the magnetic structure of the magnetopause, and (c) that the transverse distribution of B-aligned E-fields between the outer magnetosphere and ionospheric levels must be highly complex to account for the random turbulent appearance of the magnetospheric fields and the lack of corresponding time-space variations at ionospheric levels. Spike-like, non-oscillatory, fields lasting <0.2 s are occasionally seen at the bow shock and at the magnetopause and also intermittently appear in magnetosheath and plasma sheet regions under highly variable field conditions. These suggest the existence of field phenomena occurring over dimensions comparable to the probe separation and c/_pe (the characteristic electron cyclotron radius) where _pe is the electron plasma frequency.     

Magnetic field measurements in space

Conclusions The magnetosphere boundary has been penetrated in several places, conflicting evidence about the ring current location has been found, and the field exterior to the boundary has revealed some unexpected features. Pronouncements about the structure of the geomagnetic and interplanetary magnetic fields are still based on scanty evidence but the experimental basis of such estimates is more adequate than in 1958.The boundary between the geomagnetic field and the interplanetary medium has been found, by Explorer XII, to be located at approximately 10 R _E on the sunlit side of the earth near the equator. It has been observed to fluctuate between 8 and 12 R _E during August, September and October of 1961. During several days in March, 1961, the boundary, on the dark side of the earth, was penetrated repeatedly by Explorer X on an outbound pass near 135° from the earth-sun line. Several interpretations are possible; the most reasonable one at present is that the boundary was fluctuating in this period, placing the satellite alternately inside the geomagnetic field and outside in a region of turbulent magnetic fields and plasma flow.A region of turbulent magnetic fields was also observed by Pioneer I, Pioneer V, and Explorer XII between 10 and 15 R _E on the sunlit side of the earth. Pioneer V observed also a steady field 2 to 5 gammas in magnitude beyond 20 R _E. It appears that there exists a region of turbulent magnetic fields between the geomagnetic field boundary near 10 R _E, and another boundary, located near 14–15 R _E near the earth-sun line. This second boundary was seen only by Pioneer I and Pioneer V; Explorer XII and Explorer X apparently did not reach it. This boundary has been tentatively identified as a shock front in the flow of solar plasma about the magnetosphere (see Figure 5).^{41, 42} The geomagnetic field inside the boundary is relatively quiet. An abrupt transition in the magnitude of fluctuations occurs at the boundary surface. The ratio of fluctuation amplitude, B, to average field, B, decreases from 1 to 0.1 on a passage through the boundary on 13 September 1961.⁴³ The boundary is not unstable in the solar wind but fluctuations in solar wind pressure do cause changes in boundary location.^42,43 The ring current location appears to be above 1.4 R _E and below 5 R _E on the basis of Pioneer I, Vanguard III, and Explorer XII data. Lunik I and II records indicate that it is located between 3 and 4 R _E. Explorer VI data indicates that it must be at distances greater than 4 R _E on the dark side of the earth. Some variation in altitude of a ring current with time appears likely, but the bulk of present evidence limits a possible ring current to a distance of 3 to 5 R _E.The interplanetary field during quiet times is of the order of 2 to 5 gammas. The direction indicated for this field, with a significant component perpendicular to the earth-sun line, is puzzling in view of solar cosmic ray transit times. Solar disturbances with resultant plasma flow past the satellite produce increases in the field magnitude. Field increases at the satellite are sometimes correlated with disturbances observed at the earth.Further investigations are needed to map the magnetosphere and boundary more completely, to investigate the postulated shock front and the turbulent region inside, to refute or confirm the ring current theory, and to measure the interplanetary field direction and magnitude more completely. Theoretical studies are needed to support these experiments and to suggest new avenues of investigations. Particularly needed are theoretical investigations of collisionless shock fronts in plasma flow and of characteristics of the flow between the shock front and the obstacle.     

Energy build-up and release mechanisms in solar and auroral flares

Flare phenomena in the solar atmosphere and in the terrestrial magnetosphere exhibit many similarities. The mechanical energy of enhanced photospheric motion is converted and stored in the form of magnetic potential energy in sunspot fields, which is analogous to the case of the growth phase of magnetospheric substorms. The energy release during the explosive phase is initiated by a sudden collapse in the magnetic field topology and the X-type magnetic neutral point is created in the corona. Subsequent electrical discharge takes place in the form of an intense electrojet current flowing in the base of the chromosphere at the altitude where the Cowling conductivity is a maximum. It is suggested that the acceleration of particles by field-aligned electric fields and the Ohmic heating in the chromosphere result in major features of solar flares.This article also appears inSolar Physics 40 (1975) 217–226. By way of exception this paper is reproduced here for the sake of completeness.     

The FAST Satellite Fields Instrument

We describe the electric field sensors and electric and magnetic field signal processing on the FAST (Fast Auroral SnapshoT) satellite. The FAST satellite was designed to make high time resolution observations of particles and electromagnetic fields in the auroral zone to study small-scale plasma interactions in the auroral acceleration region. The DC and AC electric fields are measured with three-axis dipole antennas with 56 m, 8 m, and 5 m baselines. A three-axis flux-gate magnetometer measures the DC magnetic field and a three-axis search coil measures the AC magnetic field. A central signal processing system receives all signals from the electric and magnetic field sensors. Spectral coverage is from DC to 4 MHz. There are several types of processed data. Survey data are continuous over the auroral zone and have full-orbit coverage for fluxgate magnetometer data. Burst data include a few minutes of a selected region of the auroral zone at the highest time resolution. A subset of the burst data, high speed burst memory data, are waveform data at 2×10⁶ sample s^–1. Electric field and magnetic field data are primarily waveforms and power spectral density as a function of frequency and time. There are also various types of focused data processing, including cross-spectral analysis, fine-frequency plasma wave tracking, high-frequency polarity measurement, and wave-particle correlations.     

Potential experiments with superconducting magnets in Earth orbit

Several experiments that can be performed in Earth orbit with a superconducting magnet are discussed. They are divided into 2 classes, pure plasma physics experiments that can be performed in near Earth orbit and planetary magnetosphere simulation experiments that are best conducted in weak background fields distant from the Earth. The later are all based on the Minimag concept where plasma is directed toward a large dipolar magnet in Earth orbit to form a model miniature magnetosphere. Several experiments that cannot be performed in ground based laboratories and tests that cannot be made in the real magnetosphere can be carried out in Earth orbit. The creation of a miniature model of the magnetosphere (Minimag) forms the basis for several of these experiments.     

旋转状态下方形通道内部流场特性热线实验

为了解决旋转条件下热线技术应用问题并且在此基础上精确测量旋转方形通道内部流场特性,搭建了用于旋转通道流场测试实验平台,采用了两种连线方式对热线进行了标定实验,获得了热线测量旋转通道内部平均速度的相对误差为±6%,对雷诺数和旋转数范围分别是5000~10000和0~0.222的旋转通道流场进行了测量,结果表明:旋转导致速度型整体向后缘面（Y/D=-0.5）偏转,X/D和旋转数越大,速度型偏转越明显;旋转数为0.222时,后缘面附近边界层速度型出现了一个拐点,可能与由哥氏力不稳定性引起的二次流有关.      

控制体有限元方法中的一种高阶插值格式

针对控制体有限元方法对流动方向不平行于坐标轴方向的回流问题数值模拟采用各种迎风格式都容易产生假扩散的问题,本文给出了一种由插值单元及其迎风相邻单元所确定的高阶插值格式。通过对Re=1000时三种不同形状的二维四边形空穴流模拟,表明该格式与局部斜迎风格式的适当组合可有效地减小控制体有限元方法对回流问题数值模拟的假扩散。      

Cosmic Ray Isotopic Composition Studies with the Ulysses High Energy Telescope: Implications for Origin and Distribution in the Galaxy

The isotopic abundances of the Galactic cosmic radiation measured in the Heliosphere provide unique information on acceleration, propagation modes and containment times in the Galactic magnetic fields. Nuclear interactions with interstellar matter lead to observable γ-radiation production and, thus, to direct information on cosmic ray distribution throughout the Galaxy and its magnetic halo. The COSPIN High Energy Telescope (HET) has excellent isotopic resolution from hydrogen to nickel over the ten year period of Ulysses in space. Based on our recent work, we discuss the implications for modeling the acceleration and propagation of the cosmic radiation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Plasma transport across the heliopause

Existing heliopause models are critically rediscussed under the new aspect of possible plasma mixing between the solar wind and the ambient ionized component of the local interstellar medium (LISM). Based on current kinetic plasma theories, effective diffusion rates across the heliopause are evaluated for several models with turbulence caused by electrostatic or electromagnetic interactions that could be envisaged in this context. Some specific cases that may lead to high diffusion rates are investigated, especially in regard to their LISM magnetic field dependence.For weak fields (less than 10^–7 G), macroscopic hydrodynamic instabilities, such as of Rayleigh-Taylor or Kelvin-Helmholtz-types, can be excited. The resulting plasma mixing rates at the heliopause may amount to 20–30% of the impinging mass flow.Recently, an unconventional new approach to the problem for the case of tangential magnetic fields at the heliopause was published in which a continuous change of the plasma properties within an extended boundary layer is described by a complete set of two-fluid plasma equations including a hybrid MHD-formulation of wave-particle interaction effects. If a neutral sheet is assumed to exist within the boundary layer, the magnetic field direction is proven to be constant for a plane-parallel geometry. Considering the electric fields and currents in the layer, an interesting relationship between the field-reconnection probability and the electric conductivity can be derived, permitting a quantitative determination of either of these quantities.An actual value for the electrical conductivity is derived here on the basis of electron distribution functions given by a superposition of Maxwellians with different temperatures. Using two-stream instability theory and retaining only the most unstable modes, an exact solution for the density, velocity, and magnetic and electric fields can be obtained. The electrical conductivity is then shown to be six orders of magnitude lower than calculated by conventional formulas. Interestingly, this leads to an acceptable value of 0.1 for the reconnection coefficient.By analogy with the case of planetary magnetopauses, it is shown here for LISM magnetic fields of the order of 10^–6 G or larger that field reconnection processes may also play an important role for the plasma mixing at the heliopause. The resulting plasma mixing rate is estimated to amount to an average value of 10% of the incident mass flow. It is suggested here that the dependence of the cosmic-ray penetration into the heliosphere on the distribution of reconnecting areas at the heliopause may provide a means of deriving the strength and orientation of the LISM field.A series of observational implications for the expected plasma mixing at the heliopause is discussed in the last part of the paper. In particular, consequences are discussed for the generation of radio noise at the heliopause, for the penetration of LISM neutrals into the heliosphere, for the propagation of cosmic rays towards the inner part of the solar system and for convective electric field mergings into the heliosphere during the course of the solar cycle, depending on the solar cycle variations. With concern to a recent detection of electrostatic plasma waves by plasma receivers on Voyagers 1 and 2, we come to an interesting alternate explanation: the heliopause, rather than the heliospheric shock front, could be responsible for the generation of these waves.     

Coronal transient phenomena

Solar coronal transients, particularly those caused by flares and eruptive prominences, play a major role in the fields of solar-terrestrial physics and astrophysics. In the former field, coronal transients and their associated interplanetary disturbances are responsible for solar and galactic cosmic ray modulations, as well as planetary magnetospheric and ionospheric disturbances. In the latter field, supernovae remnants are scaled-up manifestations of such disturbances; that is they are stellar, rather than solar, coronal transients. Study of the more accessible solar transients is proving invaluable in both fields and is, therefore, selected for attention in this paper.A series of coronal transient observations is discussed in the spirit of a representative overview following some introductory remarks on the background solar wind. One of these observations is chosen because its interplanetary signature-the shock wave-was detected by two spacecraft at different heliocentric radii. Other cases are chosen because of the extended observations of embedded eruptive prominences. Progress is also being made in the interdisciplinary areas of optical imagery complemented with radio astronomical techniques.Finally, several recent theoretical models and MHD computer simulation studies are summarized. It is suggested that further comparison of specific events with such models promises a rich harvest of physical understanding of the origin, structure and interplanetary progeny of coronal transients.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

Particle acceleration and kinematics in solar flares – A Synthesis of Recent Observations and Theoretical Concepts (Invited Review)

We review the physical processes of particle acceleration, injection, propagation, trapping, and energy loss in solar flare conditions. An understanding of these basic physical processes is inexorable to interpret the detailed timing and spectral evolution of the radiative signatures caused by nonthermal particles in hard X-rays, gamma-rays, and radio wavelengths. In contrast to other more theoretically oriented reviews on particle acceleration processes, we aim here to capitalize on the numerous observations from recent spacecraft missions, such as from the Compton Gamma Ray Observatory (CGRO), the Yohkoh Hard X-Ray Telescope (HXT) and Soft X-Ray Telescope (SXT), and the Transition Region and Coronal Explorer (TRACE). High-precision energy-dependent time delay measurements from CGRO and spatial imaging with Yohkoh and TRACE provide invaluable observational constraints on the topology of the acceleration region, the reconstruction of magnetic reconnection processes, the resulting electromagnetic fields, and the kinematics of energized (nonthermal) particles. This revised version was published online in June 2006 with corrections to the Cover Date.     

Solar flares and particle acceleration

Energy release in solar flares occurs during the impulsive phase, which is a period of a few to about ten minutes, during which energy is injected into the flare region in bursts with durations of various time scales, from a few tens of seconds down to 0.1 s or even shorter. Non-thermal heating is observed during a short period, not longer than a few minutes, in the very first part of the impulsive phase; in average flares, with ambient particle densities not larger than a few times 10¹⁰ cm^–3 it is due to thick-target electron beam injection, causing chromospheric ablation followed by convection. In flares with larger densities the heating is due to thermal fronts (Section 1). The average energy released in chromospheric regions is a few times 10³⁰ erg, and an average number of 10³⁸ electrons with E 15 keV is accelerated. In subsecond pulses these values are about 10³⁵ electrons and about 10²⁷ erg per subsecond pulse. The total energy released in flares is larger than these values (Section 2). Energization occurs gradually, in a series of fast non-explosive flux-thread interactions, on the average at levels about 10⁴ km above the solar photosphere, a region permeated by a large number ( 10) of fluxthreads, each carrying electric currents of 10¹⁰–10¹¹ A. The energy is fed into the flare by differential motions of magnetic fields driven by photospheric-chromospheric movements (Section 3). In contrast to these are the high-energy flares, characterized by the emission of gamma-radiation and/or very high-frequency (millimeter) radiobursts. Observations of such flares, of the flare neutron emission, as well as the observation of ³He-rich interplanetary plasma clouds from flares all point to a common source, identified with shortlived ( 0.1 s) superhot ( 10⁸ K) flare knots, situated in chromospheric levels (Section 4). Pre-flare phenomena and the existence of homologous flares prove that flare energization can occur repeatedly in the same part of an active region: the consequent conclusions are that only seldom the full energy of an active region is exhausted in one flare, or that the flare energy is generated anew between homologous flares; this latter case looks more probable (Section 5). Flare energization requires the formation of direct electric fields, in value comparable with, or somewhat smaller than the Dreicer field (Section 6). Such fields originate by current-thread reconnection in a regime in which the current sheet is thin enough to let resistive instability originate (Section 7). Particle acceleration occurs by fast reconnection in magnetic fields 100 G and electric fields exceeding about 0.3 times the Dreicer field at fairly low particle densities ( 10¹⁰ cm^–3); for larger densities plasma heating is expected to occur (Section 8). Transport of accelerated particles towards interplanetary space demands a field-line configuration open to space. Such a configuration originates mainly after the gradual gamma-ray/proton flares, and particularly after two-ribbon flares; these flares belong to the dynamic flares in Sturrock and vestka's flare classification. Acceleration to GeV energies occurs subsequently in shock waves, probably by first-order Fermi acceleration (Section 9).     

Magnetic field variations in the polar region during magnetically quiet periods and interplanetary magnetic fields

The concepts of near-pole magnetic field variations during magnetically quiet periods are explored, with special emphasis on the relationships of these variations to interplanetary magnetic field components. Methods are proposed for relating the variations which have been observed to the fields from the various sources, based on a thorough selection of reference levels. We assume that the field variations in the summer polar cap during magnetically quiet periods consist of the following components: (i) the middle-latitude S _qvariation extended to the polar region; (ii) the DPC(B _y) single-cell current system with a polar electrojet in day-side cusp latitudes; (iii) the DMC(B _z) two-cell current system of magnetospheric convection, in the form of a homogeneous current sheet in the polar cap towards the sun, with return currents through lower latitudes; (iv) the DPC(B _z) single-cell counterclockwise current system with a focus in the day-side cusp region. Quantitative relations between the near-pole variation intensities and the value and sign of the IMF azimuthal component, with a 1 hr time resolution, have been obtained and used to suggest ways of diagnosing the interplanetary magnetic field on the basis of ground observations.     

Structure of the magnetopause: Observations and implications for reconnection

The Earth's magnetopause is the boundary between a hot tenuous plasma in the magnetosphere and a cooler denser plasma in the magnetosheath. Both of these plasmas contain magnetic fields whose directions are usually different but whose magnitudes are often comparable. Efforts to understand the structure of the magnetosphere have been hampered by the variability and complexity of this boundary. Waves on the magnetopause surface propagate toward the magnetotail and produce the multiple boundary crossings frequently seen by spacecraft. Boundary velocities are poorly known and range anywhere within an order of magnitude of 10 km s^–1. Typical thicknesses are probably on the order of a few hundred km which is a few times the gyroradius of a thermal proton. Although conclusive direct evidence for a field component, B _n , across the magnetopause has not been found, this lack of evidence may reflect the difficulty in determining B _n in the presence of magnetopause waves rather than the real absence of this component. Considerable indirect evidence exists for an open magnetosphere, but the importance of the reconnection process thought to produce open field lines has recently been questioned.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

Latitude and solar-cycle dependence of radial IMF intensity

I describe a simple procedure for extrapolating the observed solar magnetic field into the heliosphere, which averages the asymptotic fields computed using the standard source surface and current sheet models. The resultant field is characterized by strong latitudinal gradients (maintained by volume currents outside the source surface) and by abrupt reversals in direction at the current sheets. The model yields good agreement with the observed long-term variation of the radial IMF component in the ecliptic, and is used to predict the variation of |B _r | along the latitudinal trajectory of Ulysses during 1990–1994. As found in earlier studies, the magnitude ofB _r at any latitude is determined largely by the strength and relative orientation of the Sun's dipole moment.