IMF effect on ionospheric trough occurrence at equinoxes

Previous observations have shown that there is a relationship between the F region trough and both B_z and B_y components of the interplanetary magnetic field (IMF). Since IMF governs the polar cap convection, we investigate here if this relationship can be explained by means of polar cap convection. The study is limited to equinox seasons. The poleward and equatorward edges of the trough are determined from satellite tomographic observations and their locations are plotted in magnetic coordinates together with the convection pattern given by Papitashvili and Rich [Papitashvili, V.O., Rich, F.J. High-latitude ionospheric convection models derived from DMSP ion drift observations and parameterized by the IMF strength and direction. J. Geophys. Res. 107, 2002, doi:10.1029/2001JA000264] using IMF measurements coincident with trough observations. The results indicate a close relationship between the troughs and convection. Most of the troughs are seen within the dusk cell and the pattern of trough observations rotates with the convection pattern, when B_y changes its sign. More dayside troughs are observed when B_z is negative than in the opposite case, i.e. fast convective flow favours the dayside trough occurrence. Nightside troughs are observed more frequently when B_y is negative. In both evening and morning sectors the trough is situated close to the edges of convection cells, which partly contradicts previous results showing that the troughs are associated with the convection reversal. It is concluded that plasma convection has an important role in trough generation, although the effect of a strong electric field and other mechanisms like precipitation certainly have a role of their own.     

Plasma convection in the low-latitude mantle of the near-earth magnetosphere

A detailed analysis of Geotail observations on June 1, 1997 provides evidence for the appearance of the plasma mantle at low-latitudes, Z_GSM 3 R_E 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 |;B_Y|<|;B_Z| than for IMF |B_Y|>|B_Z|;. This polewardconvection can be interpreted as being driven by a J × B force operating on the field lines.     

INTERBALL statistical study of ion flow fluctuations in the plasma sheet

We use ion distribution measurements with CORALL instrument on-board the INTERBALL/Tail spacecraft to study plasma flows in the mid-tail (−9> X> −27 R_E) plasma sheet. Three velocity components computed every 2 minutes exhibit two types of velocity variations: Earthward bursty bulk flows (BBFs) and random flow fluctuations. Their properties are in a good agreement with the observations of the ISEE-2 spacecraft (Borovsky et al., 1997). The INTERBALL/Tail spacecraft configuration favors measurements of V_z component, in contrast to previous experiments in which only V_x and V_y were measured reliably. In the outer part of the plasma sheet V_y and V_z fluctuations were close to each other (variances σ(V_y) and σ(V_z) were about 160 and 110 km/s, respectively), but in the inner part at the dusk flank amplitude of V_y fluctuations increased and was 2 times higher than that of V_z component. This asymmetry of fluctuations should be taken into account during modern theoretical analysis and simulations.     

Inconsistency of magnetic field and plasma velocity variations in the distant plasma sheet: Violation of the “frozen-in” criterion?

Measurements of the magnetic field and low energy plasma by the GEOTAIL spacecraft have been used to study the relationship between variations of the plasma velocity and of the magnetic field in the distant (100–200 R_E) and middle (40–80 R_E) tail. The analysis was carried out separately for the tail lobes and the plasma sheet. It is shown that the absolute values of the magnetic field and plasma velocity, as well as their corresponding components (V_X and B_X, V_Y and B_Y, V_Z and B_Z), are linearly connected in the tail lobes. In the plasma sheet, however, the plasma velocity and the magnetic field do not seem to be related to one another. The distant plasma sheet seems to be in a regime of turbulence. The diffusion coefficients estimated from our data set of the velocity parameters in the plasma sheet are in good agreement with the theoretical predictions of Antonova and Ovchinnikov (1996, 1999).     

Large geomagnetic storms of extreme solar event periods in solar cycle 23

During extreme solar events such as big flares or/and energetic coronal mass ejections (CMEs) high energy particles are accelerated by the shocks formed in front of fast interplanetary coronal mass ejections (ICMEs). The ICMEs (and their sheaths) also give rise to large geomagnetic storms which have significant effects on the Earth’s environment and human life. Around 14 solar cosmic ray ground level enhancement (GLE) events in solar cycle 23 we examined the cosmic ray variation, solar wind speed, ions density, interplanetary magnetic field, and geomagnetic disturbance storm time index (D_st). We found that all but one of GLEs are always followed by a geomagnetic storm with D_st  −50 nT within 1–5 days later. Most(10/14) geomagnetic storms have D_st index  −100  nT therefore generally belong to strong geomagnetic storms. This suggests that GLE event prediction of geomagnetic storms is 93% for moderate storms and 71% for large storms when geomagnetic storms preceded by GLEs. All D_st depressions are associated with cosmic ray decreases which occur nearly simultaneously with geomagnetic storms. We also investigated the interplanetary plasma features. Most geomagnetic storm correspond significant periods of southward B_z and in close to 80% of the cases that the B_z was first northward then turning southward after storm sudden commencement (SSC). Plasma flow speed, ion number density and interplanetary plasma temperature near 1 AU also have a peak at interplanetary shock arrival. Solar cause and energetic particle signatures of large geomagnetic storms and a possible prediction scheme are discussed.     

Collisionless reconnection in the magnetotail

Recent developments regarding collisionless reconnection in current sheets with a finite normal magnetic field component (B_z) are reviewed. In 2-D x, z configurations the ion tearing mode is stabilized by the electron compressibility. When the y dependence is included, cross-field current instabilities can be excited. Of these, the drift kink mode appears to be particularly important. 3-D electromagnetic particle simulations indicate that this mode can act as the precursor to the growth of tearing modes and subsequent reconnection.     

Auroral signatures of transient processes in the outer magnetosphere

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 Alfvén 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.     

Magnetopause poleward of the cusp: Comparison of plasma and magnetic signature of the boundary for southward and northward directed interplanetary magnetic field

A coherent data set of high-latitude dayside magnetopause encounters by old (Heos 2, Hawkeye, Prognoz 7, 8) and new (Polar, Interball Tail, Cluster) spacecraft is needed to build a realistic model of the magnetopause (MP) including an indentation in the cusp. In building such a coherent data set a caution is necessary as the dayside magnetopause at high-latitudes may be less clearly defined than in the case of observations at low latitudes. It is due to expected presence of bundles of newly-reconnected magnetic field lines forming an extended boundary layer on the magnetosheath (MS) side of the magnetopause in the cusp region. Moreover, numerical magnetohydrodynamic (MHD) models of the solar wind-magnetosphere interaction predict that under northward interplanetary magnetic field (IMF) an additional thin current sheet should form inside the magnetopause at high latitudes on the dayside (e.g., Wu, 1983; Palmroth et al., 2001). Such a thin currect sheet is absent in empirical magnetosphere models. This internal current sheet, if a real one, may be mistaken for the magnetopause if magnetic field data are only taken into account and/or plasma data are unavailable. The Interball-Tail orbit allows for a full transition of magnetopause boundary layers at high-latitudes. We compare plasma and magnetic field signatures of the magnetopause poleward of the cusp for southward and northward IMF. The distance between the magnetic signature of the magnetopause (the current layer) and a cold and laminarly antisunward flowing MS plasma (so called free-flow MS) was found to be 0.5 to 1 R_E, at least. These observations were made under nominal solar wind of v350 km/s and p_dyn=1 to 4 nPa. We also observed several transient magnetic field reversals in the cusp related to pulses of solar wind dynamic pressure and/or the IMF discontinuity arrival. These transient reversals occurred at the same distance to the model MP as well defined full MP crossing, so most probably they represent just short encounters with the magnetopause current layer. Our analysis suggests that an indentation of the magnetopause with a subtle structure dependent on the local magnetic shear would explain and allow to predict the magnetic configuration in the high-altitude cusp.     

Interconnection of high-latitude and low-latitude boundary layers when IMF BY is dominant

The antiparallel merging model places the location of the reconnection region for a dominant interplanetary magnetic field (IMF) B_Y at high latitudes at the dayside magnetopause and predicts that the low-latitude boundary layer (LLBL) is located on open field lines of the magnetospheric flanks. Interball-1 data obtained in the wide local time range near the low-latitude magnetopause makes it possible to analyze the LLBL plasma population and to find a link between possible reconnection at high latitudes and LLBL occurrence. We found that no boundary layer was observed in the regions which have no topological connection with the merging site. All cases of LLBL observations are located downstream from a specific boundary. This boundary coincides with the first magnetospheric field line touching the reconnection region and can be located in a wide local time region depending on the instant IMF direction. Even the LLBL on closed field lines shows the tendency to be concentrated in the vicinity of this boundary. Thus we show that all types of observed LLBLs are linked to reconnection sites predicted by the antiparallel merging model.     

The collective gyration of a heavy ion cloud in a magnetized plasma

In both the ionospheric barium injection experiments CRIT I and CRIT II, a long-duration oscillation was seen with a frequency close to the gyro frequency of barium and a time duration of about one second. A model for the phenomenon which was proposed for the CRIT 1 experiment is here compared to the results from CRIT II which made a much more complete set of measurements. The model follows the motion of a low-β ion cloud through a larger ambient plasma. The internal field of the model is close to antiparallel to the injection direction v_i but slightly tilted towards the self-polarization direction E_P = −v_ixB. As the ions move across the magnetic field, the space charge is continuously neutralized by magnetic-field aligned electron currents from the ambient ionosphere, drawn by the divergence in the perpendicular electric field. These currents give a perturbation of the magnetic field related to the electric field perturbation by ΔE/ΔB ≈ V_a. The model predictions agree quite well with the observed vector directions, field strengths, and decay times of the electric and magnetic fields in CRIT II. The possibility to extend the model to the active region, where the ions are produced in this type of self-ionizing injection experiments, is discussed.     

两种用于NS-DSMC耦合方法的连续失效参数对比

NS-DSMC（Navier Stokes-Direct Simulation Monte Carlo）耦合方法是计算连续-稀薄跨流域流动的主要方法,应用过程中如何确定连续流域和稀薄流域的界面是此方法的关键问题之一,界面位置通常通过连续失效参数来判定.为合理选择连续失效参数,对目前广泛使用的两种连续失效参数Kn_Q和B参数进行了理论上的分析和比较,表明虽然他们建立的出发点不同,但在数学形式上具有一定相似性.通过圆柱绕流问题的数值试验进一步验证了两种连续失效参数具有一定对应关系.Kn_Q和B两种连续失效参数在NS-DSMC耦合方法中应用效果相差不大,但Kn_Q 的阈值0.05适用范围较广,而B参数的阈值对不同流动问题会有变化.      

行星际磁场对磁尾场向电流发生率的影响

利用Cluster卫星的磁场和等离子体探测数据, 研究了行星际磁场(IMF)时钟角(clock angle) Φ和锥角(cone angle) θ对磁尾等离子体片边界层(PSBL)区场向电流发生率的影响. 当时钟角Φ >0时, 磁尾场向电流 的发生率较高, 这表明磁尾场向电流的发生与昏向太阳风条件更为密切; 当 90°<|Φ|<180°时, 场向电流的发生率较高, 这表明 场向电流的发生与南向IMF更为密切. 当锥角θ <30°时(即IMF与 日地连线夹角较小时)场向电流的发生率较低. 而当θ> 30°时, 场向电流在90°<|Φ|<180°的情况下发生率明显增大, 这说明南向IMF情况下, 场向电流发生率明显增大. 但是当|Φ|<90°时 (北向IMF情况下), 尽管θ很大, 场向电流的发生率并未明显增大. 当θ>70°时, 且在140°< < i>Φ<160°的行星 际磁场条件下, 磁尾等离子体片边界层区场向电流的发生率最大.      

AL index dependence on the solar wind input revisited

Estimates of the geomagnetic indices made with real-time solar wind measurements form the basis of many space weather forecast techniques. We analyze 20 years of hourly AL and OMNI solar wind data to determine geomagnetic importance of various solar wind and IMF parameters. Besides the solar wind driver of primary importance (VB_s), the first-order contributions, significantly increasing the quality of the model are: solar wind velocity, 2 h of solar wind prehistory and 1 h of AL history. The factors of secondary importance, marginally contributing to overall statistical quality, are IMF B_y, solar wind density, and IMF fluctuations. The dynamic pressure is geomagnetically effective only if the pressure is lower than the average. Modelling of the same data set with an artificial neural network (ANN) confirmed our selection of important factors. Statistically the ANN model was just marginally better than our analytic expression . The AU index dependence is principally different from AL in several respects; therefore modelling of the AE composite index is physically misleading.     

Interplanetary shocks and geomagnetic activity during solar maximum (2000) and solar minimum (1995–1996)

Plasma and magnetic field parameter variations through fast forward interplanetary shocks were correlated with the peak geomagnetic activity index Dst in a period from 0 to 3 days after the shock, during solar maximum (2000) and solar minimum (1995–1996). Solar wind speed (V) and total magnetic field (B_t) were the parameters with higher correlations with peak Dst index. The correlation coefficients were higher during solar minimum (r² = 56% for V and 39% for B_t) than during solar maximum (r² = 15% for V and 12% for B_t). A statistical distribution of geomagnetic activity levels following interplanetary shocks was obtained. It was observed that during solar maximum, 36% and 28% of interplanetary shocks were followed by intense (Dst  −100 nT) and moderate (−50  Dst < −100 nT) geomagnetic activity, whereas during solar minimum 13% and 33% of the shocks were followed by intense and moderate geomagnetic activity. It can be concluded that the upstream/downstream variations of V and B_t through the shocks were the parameters better correlated with geomagnetic activity level, and during solar maximum a higher relative number of interplanetary shocks can be followed by intense geomagnetic activity than during solar minimum. One can extrapolate, for forecasting goals, that during a whole solar cycle a shock has a probability of around 50% to be followed by intense/moderate geomagnetic activity.     

行星际磁场与磁尾磁通量绳形成的关系

收集了Cluster卫星在2001-2005年间观测到的磁尾磁通量绳事件,并对磁通量绳(magnetic flux rope)形成及其内部磁场结构与行星际磁场(IMF)的关系作了统计研究.考虑磁通量绳被观测到时行星际磁场的条件,在所有73个磁通量绳事件中,行星际磁场By分量占有主导地位的事件有80%,且78%的事件具有与行星际磁场By分量相同方向的核心场.行星际磁场通过在磁层顶与地球磁场相互作用改变南北等离子体片内磁场相对方向,形成有利于磁通量绳形成的磁场位形,并且行星际磁场By分量的方向对磁通量绳内部核心场的方向具有决定性影响.从统计结果来看,磁通量绳的形成并不会依赖于行星际磁场Bz分量的方向.     

Coupling of the interstellar and interplanetary magnetic fields at the heliospheric interface: The effect of neutral hydrogen atoms

We present numerical results showing the effect of neutral hydrogen atoms on the solar wind (SW) interaction with the local interstellar medium (LISM), where the interstellar magnetic field (ISMF) is coupled to the interplanetary magnetic field (IMF) at the surface of the heliopause. The IMF on the inner boundary surrounding the Sun is specified in the form of a Parker spiral and self-consistently develops in accordance with the SW motion inside the heliopause. The model of the SW–LISM interaction involves both plasma and neutral components which are treated as fluids. The configuration is chosen where the ISMF is orthogonal to the LISM velocity and tilted 60° to the ecliptic plane. This orientation of the magnetic field is a possible explanation of the 2–3 kHz emission data which is believed to originate ahead of the heliopause. It is shown that the topology of the heliospheric current sheet is substantially affected by the ISMF. The interaction pattern dependence on the neutral hydrogen density is analyzed.     

Solar wind dependence of the Pc1 wave activity

There are a host of factors influencing the excitation of Pc1 geomagnetic pulsations, which are ULF waves in the frequency range between 0.2 and 5 Hz. We have studied carefully the dependence of the pearl-type Pc1 activity at Sodankylä, Finland (L = 5.1) on the plasma density N in front of the magnetosphere, the bulk velocity V of the solar wind, and the intensity B of the IMF. The result is as follows: high values of N and reduced values of V are favorable to appearance of Pc1, whereas the dependence of Pc1 activity on B is practically absent. We also show that the probability of Pc1 occurrence decreases with the interplanetary electric field, and increases with solar wind impact pressure and with the plasma to magnetic pressure ratio “beta”.     

Application of an MHD simulation to the study of substorms

The substorm mechanism is studied by the numerical solutions obtained from a resistive magnetohydrodynamic (MHD) simulation. After a southward turning of the interplanetary magnetic field (IMF), the simulation results reproduce observed features of the growth phase. The numerical solutions show that the plasma sheet thinning during the growth phase is formed under the dynamic balance between the flux pileup from the midtail and the flux removal toward the dayside controlled by the convection in the magnetosphere-ionosphere (M-I) coupling system. After the growth phase, dipolarization is generated in the near-earth tail accompanied by a plasma injection into the inner magnetosphere, the formation of plasmoid in the midtail, and the enhancement of the nightside field-aligned currents (FACs). The direct cause of this onset is the state (phase space) transition of the convection system from a thinned state to a dipolarized state associated with a self-organization in the nonlinear system.     

LMXB luminosity function and connection to globular cluster in early type galaxies

We derive bias-corrected X-ray luminosity functions (XLFs) of LMXBs detected in 14 E and S0 galaxies observed with Chandra. After correcting for incompleteness, the individual XLFs are statistically consistent with a single power-law. A break at or near L_X,Eddington , as previously reported, is not required in any individual case. The combined XLF with a reduced error, however, suggests a possible break at L_X = 5 × 10³⁸ erg s⁻¹, which may be consistent with the Eddington luminosity of neutron stars with the largest possible mass (3 M), or of He-enriched neutron star binaries. We confirm that the total X-ray luminosity of LMXBs is correlated with the the near-IR luminosities, but the scatter exceeds that expected from measurement errors. The scatter in L_X(LMXB)/L_K appears to be correlated with the specific frequency of globular clusters, as reported earlier.

We cross-correlate X-ray binaries with globular clusters determined by ground-based optical and HST observations in 6 giant elliptical galaxies. With the largest sample reported so far (300 GC LMXBs with a 5:2 ratio between red and blue GCs), we compare their X-ray properties, such as X-ray hardness, XLF and L_X/L_B and find no statistically significance difference between different groups of LMXBs. Regardless of their association with GCs, both GC and field LMXBs appear to follow the radial profile of the optical halo light, rather than that of more extended GCs. This suggests that while metallicity is a primary factor in the formation of LMXBs in GCs, there may be a secondary factor (e.g., encounter rate) playing a non-negligible role.     

Substorms in the inner plasma sheet

Thin Current Sheets (TCS) are regularly formed prior to substorm breakup, even in the near-Earth plasma sheet, as close as the geostationary orbit. A self-consistent kinetic theory describing the response of the plasma sheet to an electromagnetic perturbation is given. This perturbation corresponds to an external forcing, for instance caused by the solar wind (not an internal instability). The equilibrium of the configuration of this TCS in the presence of a time varying perturbation is shown to produce a strong parallel thermal anisotropy (T T) of energetic electrons and ions (E>50keV) as well as an enhanced diamagnetic current carried by low energy ions (E<50keV). Both currents tend to enhance the confinement of this current sheet near the magnetic equator. These results are compared with data gathered by GEOS-2 at the geostationary orbit, where the magnetic signatures of TCS, and parallel anisotropics are regularly observed prior to breakup. By ensuring quasi-neutrality everywhere we find, when low frequency electromagnetic perturbations are applied, that although the magnetic field line remains an equipotential to the lowest order in T_e/T_i, a field-aligned potential drop exists to the next order in (T_e/T_i). Thus the development of a TCS implies the formation of a field-aligned potential drop ( few hundred volts) to ensure the quasi-neutrality everywhere. For an earthward directed pressure gradient, a field-aligned electric field, directed towards the ionosphere, is obtained, on the western edge of the perturbation (i.e. western edge of the current sheet). Thus field aligned beams of electrons are expected to flow towards the equatorial region on the western edge of the current sheet. We study the stability of these electron beams and show that they are unstable to “High Frequency” (HF) waves. These “HF” waves are regularly observed at frequencies of the order of the proton gyrofrequency (f_H+) just before, or at breakup. The amplitude of these HF waves is so large that they can produce a strong pitch-angle diffusion of energetic ions and a spatial diffusion that leads to a reduction of the diamagnetic current. The signature of a fast ion diffusion is indeed regularly observed during the early breakup; it coincides with the sudden development of large amplitude transient fluctuations, ballooning modes, observed at much lower frequencies (fH+). These results suggest that the HF waves, generated by field-aligned electron beams, provide the dissipation which is necessary to destabilize low frequency (ballooning) modes.