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Accepted: 2016-01-17, 資料番号: SA1160024000

In the previous papers (Nagashima et al., 1992, 1993) the authors pointed out the existence of the local-time-dependent precursory decrease of cosmic rays in front of the shock wave of the interplanetary magnetic field (IMF) and interpreted it as being due to the IMF-collimated outward flow of the low-density cosmic rays across the shock from the inside of the Forbush decrease. In those papers, however, the physical properties of the collimated flow such as the direction, the collimation angle, and the rigidity spectrum of the constituent cosmic rays were only estimated qualitatively owing to the lack of the simulation of the precursory decrease produced by the flow. In the present paper these properties are quantitatively obtained by analyzing the hourly data of the precursory decreases observed on January 25 and 26, 1968, at the worldwide neutron monitor stations. The obtained direction and collimation angle of the flow approximately coincide respectively with those expected from the IMF. This fact gives evidence for the reconfirmation of the existence of the IMF-guided collimated flow, responsible for the precursory decrease.

[1] We present results of a systematic study of electron densities in the dayside Martian ionosphere measured by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on board the Mars Express spacecraft. There are two distinct regions controlled by different physical mechanisms. The first region is located at altitudes up to about 5 neutral scale heights above the altitude of peak electron density. Electron densities in this region are well described by the basic Chapman theory. The observed small deviations can be most probably explained by the neutral scale height and electron temperature increasing with altitude rather than being constant. The second region is located at altitudes higher than about 10 neutral scale heights above the altitude of peak electron density. It is controlled primarily by diffusion, and the observed electron densities decrease exponentially with increasing altitude. The corresponding diffusion scale height increases with increasing solar zenith angle, which can be probably explained by nearly horizontal magnetic fields in the ionosphere induced by interaction with the solar wind. The obtained dependencies can be used as a simple empirical model of the dayside Martian ionosphere.

[1] Analysis of spectra from the Clementine ultraviolet-visible and near-infrared cameras of small, immature craters and surface soils both on and adjacent to the lunar swirls at Mare Ingenii has yielded the following conclusions about space weathering at a magnetic anomaly. (1) Despite having spectral characteristics of immaturity, the lunar swirls are not freshly exposed surfaces. (2) The swirl surfaces are regions of retarded weathering, while immediately adjacent regions experience accelerated weathering. (3) Weathering in the off-swirl regions darkens and flattens the spectrum with little to no reddening, which suggests that the production of larger (>40 nm) nanophase iron dominates in these locations as a result of charged particle sorting by the magnetic field. Preliminary analysis of two other lunar swirl regions, Reiner Gamma and Mare Marginis, is consistent with our observations at Mare Ingenii. Our results indicate that sputtering/vapor deposition, implanted solar wind hydrogen, and agglutination share responsibility for creating the range in npFe0 particle sizes responsible for the spectral effects of space weathering.

[1] Analysis of two absorption features near 3 μm in the lunar reflectance spectrum, observed by the orbiting M3 spectrometer and interpreted as being due to OH and H2O, is presented, and the results are used to discuss the processes producing these molecules. This analysis focuses on the dependence of the absorptions on lunar physical properties, including composition, illumination, latitude, and temperature. Solar wind proton-induced hydroxylation is proposed as the creation process, and its products could be a source for other reported types of hydrogen-rich material and water. The irregular and damaged fine-grained lunar soil seems especially adapted for trapping solar wind protons and forming OH owing to abundant dangling oxygen bonds. The M3 data reveal that the strengths of the two absorptions are correlated and widespread, and both are correlated with lunar composition but in different ways. Feldspathic material seems richer in OH. These results seem to rule out water from the lunar interior and cometary infall as major sources. There appear to be correlations of apparent band strengths with time of day and lighting conditions. However, thermal emission from the Moon reduces the apparent strengths of the M3 absorptions, and its removal is not yet completely successful. Further, many of the lunar physical properties are themselves intercorrelated, and so separating these dependencies on the absorptions is difficult, due to the incomplete M3 data set. This process should also operate on other airless silicate surfaces, such as Mercury and Vesta, which will be visited by the Dawn spacecraft in mid-2011.

[1] We have used multispectral images from Clementine and data from Lunar Prospector's magnetometer to conduct a survey of lunar crustal magnetic anomalies, prominent lunar swirls, and lesser known swirl markings to provide new information on the nature of swirls and their association with magnetic anomalies. We find that all swirls and swirl-like albedo patterns are associated with areas of magnetized crust, but not all areas of magnetized crust are colocated with swirl-like albedo anomalies. All observed swirls exhibit spectral characteristics similar to immature material and generally have slightly lower FeO values compared with their surroundings as determined with a multispectral iron-mapping method. We discuss these results in relation to the various hypotheses for swirl formation. The comet impact hypothesis for lunar swirls would not predict a difference in the spectrally determined FeO content between swirls and nearby ordinary surfaces. The compositional difference could be explained as a consequence of (1) magnetic shielding of the surface from the solar wind, which could produce anomalous space weathering (little darkening with limited reddening) and potentially alter the predictions of the multispectral iron-mapping algorithm while the compositional contrast could be enhanced by delivery of lower-FeO ejecta from outside the swirl; and (2) accumulation of fine plagioclase-rich dust moving under the influence of electric fields induced by solar wind interactions with a magnetic anomaly. Therefore, we cannot at present clearly distinguish between the solar wind shielding and electrostatic dust accumulation models for swirl formation. We describe future measurements that could contribute to solution of the puzzle of swirl origin.

In September 1983 the International Sun Earth Explorer 3 (ISEE 3) International Cometary Explorer (ICE) spacecraft made a long traversal of the distant dawnside flank region of the Earth's magnetosphere and had many encounters with the low Mach number bow shock. These weak shocks excite plasma wave electric field turbulence with amplitudes comparable to those detected in the much stronger bow shock near the nose region. Downstream of quasi-perpendicular (quasi-parallel) shocks, the E field spectra exhibit a strong peak (plateau) at midfrequencies (1 - 3 kHz); the plateau shape is produced by a low-frequency (100 - 300 Hz) emission which is more intense behind downstream of two quasi-perpendicular shocks show that the low frequency signals are polarized parallel to the magnetic field, whereas the midfrequency emissions are unpolarized or only weakly polarized. A new high frequency (10 - 30 kHz) emission which is above the maximum Doppler shift exhibit a distinct peak at high frequencies; this peak is often blurred by the large amplitude fluctuations of the midfrequency waves. The high-frequency component is strongly polarized along the magnetic field and varies independently of the lower-frequency waves.

Some magnetic impulse events observed in the polar region are related to vortices associated with plasma convection in the ionosphere. Recent analyses of satellite and ground data suggest that the interaction of solar wind dynamic pressure pulses and the magnetosphere may lead to the formation of velocity vortices in the magnetopause boundary layer region. This can in turn lead to the presence of vortices in the polar ionosphere. However, before reaching the Earth's magnetopause, these interplanetary pressure pulses must interact with and pass through the bow shock. A variation of the solar wind dynamic pressure may be associated with shocks, magnetic holes, or tangential discontinuities (TDs) in the interplanetary medium. We study the interaction of interplanetary TDs with the Earth's bow shock (BS) using both theoretical analysis and MHD computer simulations. It is found that as a result of the collision between a TD and the BS, the jump in the solar wind dynamic pressure associated with the TD is significantly modified, the bow shock moves, and a new fast shock or fast rarefaction wave, which propagates in the downstream direction, is excited. Our theoretical analysis shows that the change in the plasma density across the interplanetary TD plays the most important role in the collision process. In the case with an enhanced dynamic pressure behind the interplanetary TD, the bow shock is intensified in strength and moves in the earthward direction. The dynamic pressure jump associated with the transmitted TD is generally reduced from the value before the interaction. A fast compressional shock is excited ahead of the transmitted TD and propagates toward the Earth's magnetosphere. For the case in which the dynamic pressure is reduced behind the interplanetary TD, the pressure jump across the transmitted TD is substantially weakened, the bow shock moves in the sunward direction, and a rarefaction wave which propagates downstream is excited. We also simulate and discuss the interaction of a pair of tangential discontinuities, which may correspond to a magnetic hole, with the BS.

During a 40-day period in 1983, International Sun Earth Explorer 3 (ISEE 3) was located about 225 R(sub E) behind the Earth and remained within 12 R(sub E) of the nominal tail axis. During this time the spacecraft spent at least 70% of its time in the magnetotail with occasional excursions into the magnetosheath. However, during five geomagnetically distrubed intervals of 1 - 3 days duration during this period, ISEE 3 remained within the magnetosheath for extended intervals, even when it was very near the center of an average tail. Simultaneous observations of the solar wind direction and thermal pressure suggest that nonradial solar wind flow associated with interacting solar wind streams moves a compressed tail away from the nominal position at these times and explains most of these observations. However, during several few-hour intervals of strongly northward interplanetary magnetic field (IMF) within these periods, the solar wind is more radial and cannot explain the residence of the spacecraft in the magnetosheath. At these times ISEE 3 seems to be moving back and forth between two regions, one a higher-density, lower-temperature magnetosheathlike region but with density somewhat lower than the normal magnetosheath, the other a lower-density, higher-temperature taillike region but with density higher than the normal tail. Both regions have larger B(sub z) components and B(sub x) components that tend to vary as if the spacecraft were moving from one hemisphere of the tail to the other. It is suggested that the magnetotail at these times of northward IMF consists mostly of field lines that close Earthward of the spacecraft with a narrow remaining tail at 225 R(sub E) waving back and forth across the spacecraft. If relatively rare intervals of long-duration, very northward IMF can eliminate the extended tail, it seems likely that more common, less northward IMF might well have very important, though less drastic, effects on the tail configurations.

The solar wind plasma experiment aboard the Ulysses spacecraft, including separate ion and electron instruments, measured the plasma properties of the Jovian magnetosheath during the February 1992 encounter with Jupiter. Seven separate magnetosheath intervals were observed, as well as four bow shock crossings and numerous encounters with the magnetopause and its boundary layer. We present an overview of ion and electron bulk parameters and a sampling of distribution shapes for the magnetosheath and adjacent plasma regions. Plasma flows are generally appropriate for slowing and deflection of the solar wind flow about a relatively stationary obstacle, with the notable exception of the first inbound sheath transit, when an expanding magnetosphere resulted in sunward flow just above the magnetopause. The existence of a planetary depletion layer is suggested by trends in plasma density for some magnetopause encounters. The magnetopause boundary layer is characterized by a combination of sheathlike and magnetospheric distributions of both ions and electrons. The ion population in the sheath is observed to include a significant population of suprathermal protons. Electron distributions have a distinctive shape previously observed in the terrestrial magnetosheath, with fluxes parallel to the magnetic field dominating at thermal energies and perpendicular fluxes dominating at higher energies. Trends in electron temperature near the bow shock indicate that shock motion plays an important role in heating the electrons. In general, the plasma characteristics of the Jovian magnetosheath are quite similar to those in its terrestrial counterpart, but the compressible nature of the Jovian magnetosphere accentuates the importance of boundary motions.

The three-dimensional electron spectrometer of the Reme plasma analyzer-complete positive ion, electron and ram negative ion measurements near comet Halley (RPA-COPERNIC) experiment aboard the Giotto spacecraft, although damaged during the comet Halley encounter in March 1986, has provided very new results during the encounter on July 10, 1992, with the weakly active comet Grigg-Skjellerup (G-S). The main characteristic features of the highly structured interaction region extending from approximately 26,500 km inbound to approximately 37,200 km outbound are presented. These results are compared to the results obtained by the same instrument during the Giotto comet Halley fly-by. Despite the large difference in the size of the interaction regions (approximately 60,000 km for G-S, approximately 2000,000 km for Halley) due to 2 orders of magnitude difference in cometary neutral gas production rate, there are striking similarities in the solar wind interactions with the two comets.

We compare model predictions of cometary water group ion densities and the solar wind slow down with measurements made by the Giotto Johnstone plasma analyzer implanted ion sensor at the encounter with comet Grigg-Skjellerup (G-S) on July 10, 1992. The observed slope of the ion density profile on approach to the comet is unexpectedly steep. Possible explanations for this are discussed. We present also a preliminary investigation of the quasilinear velocity-space diffusion of the implanted heavy ion population at G-S using a transport equation including souce, convection, adiabatic compression, and velocity diffusion terms. Resulting distributions are anisotropic, in agreement with observations. We consider theoretically the waves that may be generated by the diffusion process for the observed solar wind conditions. At initial ion injections, waves are generated at omega approximately Omega(sub i) the ion gyrofrequency, and lower frequencies are predicted for diffusion toward a bispherical shell.

The diffusion of water group cometary ions in velocity space at comet Grigg-Skjellerup was measured during the Giotto spacecraft encounter. The evolution of the collapsed pitch angle and energy distributions during the inbound and outbound passes shows that the timescale for energy diffusion may be similar to that for pitch angle diffusion. Fully isotropic pitch angle distributions were never seen. Also the bulk parameters of the three-dimensional distributions are examined. Transformation of these parameters into a field-aligned solar wind frame allows us to test the gyrotropy of the distributions. The observations imply that there were deviations from gyrotropy throughout the encounter becoming most important near to closest approach.

The observations made during the encounter with comet Giacobini Zinner show that the character of MHD turbulence is governed by the magnetosonic (MS) waves generated by the pickup ions via a resonant cyclotron instability. The interaction of cometary ions in the mass loaded solar wind with MS waves propagating away from the comet and oblique to the interplanetary magnetic field (IMF) is investigated using the test particle approach. Ion trajectories, distribution functions, widths of pitch angle scattering and energy diffusion are obtained. We have also investigated the relationship between pitch angle scattering and the three parameters, namely, alpha, the angle between the solar wind flow direction and the ambient magnetic field, theta(sub BK), the angle between the ambient magnetic field and the wave propagation vector, and the ion injection velocity.

The pickup of newborn cometary ions in the solar wind excites strong wave activity. At comet P/Grigg-Skjellerup the interplanetary magnetic field is nearly perpendicular to the solar wind velocity; thus the pickup ions should form a ring distribution. In opposition to P/Halley, comet P/Grigg-Skjellerup is rather small, and its inhomogeneity scale is even smaller than the pickup scale. Therefore the ring will be filled incompletely, and the distribution becomes nongyrotropic. The dispersion of a nongyrotropic cometary pickup distribution in a cold proton-electron background plasma is studied. Nongyrotropy couples the gyrotropic eigenmodes and drives an instability. Left-hand polarized waves close to the cometary ion cyclotron frequency are excited.

The propagation and polarization characteristic of low-frequency electromagnetic wave fields near comet P/Grigg-Skjellerup (P/GS) are analyzed using magnetic field and plasma observations obtained by the Giotto magnetometer experiment and the Johnstone plasma analyzer during the encounter at the comet on July 10, 1992. The results have been physically interpreted.

On July 10, 1992, the Giotto spacecraft encountered a second comet, namely, P/Grigg-Skjellerup. Despite the small production rate of this comet (approximately 2 orders of magnitude smaller than that of P/Halley and 1 order of magnitude smaller than that of P/Giacobini-Zinner) the plasma flow showed several distinct flow transitions. We have applied theoretical models, previously developed to explain the existence and location of the flow boundaries at comets P/Giacobini-Zinner and P/Halley to this new data set for comet P/Grigg-Skjellerup. The observed bow shock crossings are used to evaluate the production rate of the cometary neutrals. Using this production rate, the position of the collisionopause and the ionopause are estimated. The observed location of the magnetic pile-up boundary corresponds well to the theoretical calculations. We also find the reason that a magnetic field-free region was not encountered was because the ionopause, while being sharp, was inside the spacecraft trajectory which had an estimated closest approach distance greater than 120 km. Finally we have used the observed spatial profile of the solar wind magnetic field together with a simple one-dimemsional, multispecies chemical model to calculate the radial density profiles of the H2O group ions outside the bow shock. Comparsion of the calculated total H2O group ion density profile with observations indicates a good fit in the inner region where the H2O ion dominates. While the corresponding fit in the outer region, where the O(+) ion dominates, becomes progressively less satisfactory with increasing cometocentric distance, it is noted that the observations themselves are believed to be less reliable at theses large distances.

We consider the effect of accelerated magnetopause motions arising from the arrival of a modified-pressure region of the solar wind with a sharp forefront upon the growth rate of the velocity shear layer instability. The range of values of relative pressure variation on the front was taken to lie within 1.25-2, and the front thickness ranged from 200 km to 1RE (6400 km). At different phases of motion of the boundary, the instability growth rate can change in either direction as a consequence of the Rayleigh-Taylor effect, in accordance with a change of sign of acceleration g of the boundary. The typical time during which of the boundary remains inside the new equilibrium position (≤ 1 min) is sufficient for the instability to reach the nonlinear regime. It is possible that plasma flutes on the boundary penetrate the magnetosphere. Generation of disturbances can be modulated by boundary oscillations inside the new equilibrium position. We discuss the interplanetary magnetic field influence upon the growth rate of the MHD instability under consideration. An analysis is made of a number of daytime geophysical conditions which cannot be understood when interpreted in terms of the instability on a stationary boundary.

A ground-based survey of Pc 1/2 (0.1-0.4 Hz) and Pc 1 micropulsations throughout 1986 has provided evidence for the location of the Pc 1/2 source region. Data were taken from three high-latitude stations, located at South Pole (−75° geomagnetic latitude; 1530 UT local noon), Sondre Stromfjord (+74°, 1330 UT LN) and Siple (−61°, 1700 UT LN). The study revealed a diurnal occurrence pattern for waves in the 0.1-0.4 Hz band (Pc 1/2) and showed that the pattern was not due to the effects of sunlight on the ionosphere but instead from a postnoon magnetospheric source region. On the basis of the latitudinal occurrence patterns of the waves above and below 0.4 Hz, it is concluded that the waves observed on the ground above 0.4 Hz come primarily from plasmapause latitudes, while the source of the Pc 1/2 lies between the plasmapause and the magnetopause. The estimate of source locations for waves above and below this frequency, combined with the typically sharp upper frequency limit of waves in the 0.1-0.4 Hz band (Pc 1/2) are interpreted as evidence that He+ ions in the outer magnetosphere influence propagation and possibly wave growth. These results are compared with those of Anderson et al. [1990, 1992a,b], who showed with a spacecraft study that Pc 1 are more commonly observed beyond L = 7 than in regions closer to the Earth. It is concluded that many of the waves above the He+ gyrofrequency from the outer magnetosphere do not always reach the ground. An extensive search for correlations between Pc 1/2 occurrence and solar wind pressure and magnetic field orientation showed no direct connection between solar wind parameters and Pc 1/2 generation. They may instead be amplified by plasma sheet ions that drift sunward on the dusk side of the magnetosphere [Kaye and Kivelson, 1979; Anderson and Hamilton, 1993] and undergo ion-cyclotron resonance in the afternoon sector. This mechanism is consistent with the diurnal pattern and apparent source location of the Pc 1/2.

Large-amplitude electromagnetic plasma waves are one of the dominant features of the solar wind-comet interaction. Wave characteristics strongly depend on parameters such as the solar wind flow and Alfven velocities and the angle between flow and interplanetary magnetic field as well as the production rate. With respect to the latter the flyby of the spacecraft Giotto at comet P/Griff-Skjellerup provides a unique possibility to study such waves in further detail. Pickup ion-related wave signatures have been observed up to a distance of 600,000 km from the nucleus. Peak spectral power in the spacecraft frame of reference occurs at frequencies mainly somewhat below the water group ion gyrofrequency. From this the waves are determined to be mainly left-hand polarized waves, causing one-sided pitch angle diffusion outbound. The wave activity strongly increases close to the comet; upstream it exhibits a quadratic dependence on the water group pickup ion free energy. Furthermore, a phenomenological study of the wave characteristics provides a unique description of the fine-structure of the interaction region. Indications of steepened magnetosonic waves have been found in the outbound magnetosheath region.

We have analysed 14 cases of magnetic clouds identified by R. M. Lepping et al. [1990], who have fitted them with the cylindrical model. We treated cloud magnetic field profiles and compared them with the spheroidal models. We argue that all cases exhibit features of spheroidal clouds, namely, the complete sinusoidal profile of magnetic field components inside the cloud, double-peak and/or plateau-type magnetic field magnitude profiles.

We present a new method for determining the shape of the magnetopause as a bivariate function of the hourly averaged solar wind dynamic pressure (p) and the north-south component of the interplanetary magnetic field (IMF) B(sub z). We represent the magnetopause (for X(sub GSE) greater than -40 R(sub E)) as an ellipsoid of revolution in solar-wind-aberrated coordinates and express the (p, B(sub z)) dependence of each of the three ellipsoid parameters as a second-order (6-term) bivariate expansion in Inp and B(sub z). We define 12 overlapping bins in a normalized dimensionless (p, B(sub z)) `control space' and fit an ellipsoid to those magnetopause crossings having (p, B(sub z)) values within each bin. We also calculate the bivariate (Inp, B(sub z)) moments to second order over each bin in control space. We can then calculate the six control-space expansion coefficients for each of the three ellipsoid parameters in configuration space. From these coefficients we can derive useful diagnosis of the magnetopause shape as joint functions of p and B(sub z): the aspect ratio of the ellipsoid's minor-to-major axes; the flank distance, radius of curvature, and flaring angle (at X(sub GSE) = 0); and the subsolar distance and radius of curvature. We confirm and quantify previous results that during periods of southward B(sub z) the subsolar magnetopause moves inward, while at X(sub GSE) = 0 the flank magnetopause moves outward and the flaring angle increases.

First from a one-dimensional MHD model we derive estimates for position and shape of the bow shock applicable for arbitrary magnetosonic Mach numbers > 1 in the solar wind. This generalization is important in the case of the Giotto encounter with P/Grigg-Skjellerup with the unusual interplanetary magnetic field (IMF) of 18 nT. We compare three-dimensional model calculations with observations from six cometary missions and derive new corrected estimates for the gas production of P/Grigg-Skjellerup, which turn out to be substantially lower than those hitherto proposed assuming a large magnetosonic Mach number in the solar wind. Furthermore, we use global MHD models for this comet to determine from the measured magnetic field the position of the Giotto trajectory with respect to the comet. As a measure of the goodness of fit we use the mean square of the difference of the observed and the model field. We fit segments of the measured data of three different lengths centered near closest approach and for three different values of the unperturbed IMF. For all parameters considered, we find a unique best approximating trajectory which depends only little on the parameters. In any case, our best fitting trajectory passes through the comet at the side of the nucleus nearly straightly eastward (within ±15°) at a distance of 400 km. It has closest approach at about 1518∶59 space craft event time (SCET).

The electromagnetic turbulence and associated frequencies observed far upstream of comet P/Grigg-Skjellerup during the recent Giotto spacecraft fly by can be attributed to relative streaming between the cometary ions and the solar wind, and the hypothesis that unstable parallel modes are responsible is used to deduce the wavelengths involved. Such a self-consistent approach yields resonant instabilities with wavelengths of 7000 ∼ 8000 km, phase velocities which are sub-Alfvenic and polarizations which are left-handed in the spacecraft frame.

[1] Three new models of the global internal magnetic field of the Moon based on Lunar Prospector (LP) fluxgate magnetometer observations are developed for use in understanding the origin of the Moon's crustal magnetic field and for modeling its interaction with the solar wind. The models are at spherical harmonic degree 170, corresponding to 64 km wavelength resolution, from 30 km mean altitude LP observations. Coverage is complete except for a few areas near each pole. Original signal amplitudes are best preserved in the sequential approach map, whereas feature recognition is superior in the coestimation and harmonic wave number correlation maps. Spherical harmonic degrees less than 15, corresponding to 666 km wavelength, are largely absent from the Moon's internal magnetic field. We interpret this bound in terms of the Moon's impact history. A derived magnetization map suggests magnetizations may locally exceed 0.2 A/m in the lunar crust at the survey resolution if the magnetic crust is as thick as 40 km.

Periodicities in the area of coronal hole (CH) regions on the solar disk and solar wind (SW) high-speed streams (HSSs) have been studied, for the first time, during complete solar cycle 23 (SC 23) from 1996 to 2009 using solar EUV image data from the Solar and Heliospheric Observatory and STEREO satellites and ACE solar wind-magnetic data. Both the SW velocity and the area of CH regions consistently exhibit large values during the declining phase and the minimum of SC23 (2003–2009) due to equatorward extended holes and/or low-latitude isolated holes. Further, the SW velocity and CH area exhibit a greater tendency for subharmonic (13.5 and 9 day) periodicities during the declining phase and solar minimum. The response of Earths' geomagnetic and ionospheric properties to these periodicities associated with corotating interaction regions in SW HSSs is studied, with a focus on the extremely low-solar-activity period of 2008. Subharmonic oscillations in both day- and nightside ionospheric electron density are found to correlate well with oscillations in SW and Kp during 2008. The topside ionospheric response (above 350 km) appears to be dominated by changes in the plasma temperature and/or scale height and exhibits concurrent enhancements with the oscillations in geomagnetic activity during both day- and nighttime. However, the electron density response at altitudes between 200 and 350 km is dominated by changes in the neutral composition and exhibits significant latitudinal, local time, and seasonal variations. The results are discussed in light of equatorward wind perturbations during enhanced geomagnetic activity and summer-to-winter transequatorial neutral wind patterns.

Shadowed locations ncar the lunar poles arc almost certainly electrically complex regions. At these locations near the terminator, the local solar wind flows nearly tangential to the surface and interacts with large-scale topographic features such as mountains and deep large craters, In this work, we study the solar wind orographic effects from topographic obstructions along a rough lunar surface, On the leeward side of large obstructions, plasma voids are formed in the solar wind because of the absorption of plasma on the upstream surface of these obstacles, Solar wind plasma expands into such voids) producing an ambipolar potential that diverts ion flow into the void region. A surface potential is established on these leeward surfaces in order to balance the currents from the expansion-limited electron and ion populations, Wc find that there arc regions ncar the leeward wall of the craters and leeward mountain faces where solar wind ions cannot access the surface, leaving an electron-rich plasma previously identified as an "electron cloud." In this case, some new current is required to complete the closure for current balance at the surface, and we propose herein that lofted negatively charged dust is one possible (nonunique) compensating current source. Given models for both ambipolar and surface plasma processes, we consider the electrical environment around the large topographic features of the south pole (including Shoemaker crater and the highly varied terrain near Nobile crater), as derived from Goldstone radar data, We also apply our model to moving and stationary objects of differing compositions located on the surface and consider the impact of the deflected ion flow on possible hydrogen resources within the craters

[1] The night side ionosphere of Mars is known to be highly variable: essentially nonexistent in certain geographic locations, while occasionally nearly as strong as the photoionization-produced dayside ionosphere in others. The factors controlling its structure include thermospheric densities, temperatures and winds, day-night plasma transport, plasma temperatures, current systems, solar particle events, crustal magnetic fields, and electron precipitation, none of which are adequately understood at present. Using a kinetic Monte Carlo approach called Mars Monte Carlo Electron Transport (MarMCET), we model the dynamics of precipitating solar wind electrons on the nightside ionosphere of Mars to study the effects of these last two factors on ionospheric density and structure. We calculate ionization rate profiles and, using simple assumptions concerning atmospheric chemistry, also calculate electron density profiles, total electron content, and equivalent ionosphere slab thickness. We present the first model investigation of the coupled effects of crustal magnetic field gradients and precipitating electron pitch angle distributions (PADs). Including such effects, particularly in cases of nonisotropic PADs, is found to be essential in accurately predicting ionization rate and electron density profiles: peak ionization rates can vary by a factor of 20 or more when these effects are included.

[1] The global dynamics of the flow of energetic particles through the Martian upper atmosphere is studied for different cases reflecting variations in solar cycle, seasons, and epochs over history. In this study, the combination of the new 3-D Direct Simulation Monte Carlo kinetic model and the modern 3-D Mars Thermosphere General Circulation Model is employed to describe self-consistently the Martian upper atmosphere (i.e., the thermosphere/ionosphere and the exosphere). The variations in the Martian upper atmosphere over long-term (seasons and solar cycle) and evolutionary (Martian history) time scales are presented and discussed using the equinox solar low case extensively described in the work of Valeille et al. (2009c) as reference throughout. These characteristic conditions lead to significant variations in the thermosphere/ionosphere temperatures, dynamical heating, winds, and ion/neutral density distributions, which, in turn, affect the exosphere general structure, the hot corona shape, and the escape rate and have important implications for the study of the ion loss, atmospheric sputtering, and interaction with the solar wind in general. Calculations for present conditions are performed for three characteristic seasons (aphelion, equinox, and perihelion), while solar activity is either fixed to low or high conditions. Calculations for past conditions are related to a solar EUV flux enhancement of 1, 3, and 6 times the present values. Spatial-, seasonal-, solar cycle–, and evolutionary-driven variations, although exhibiting very different time scales, are all shown to exert an influence of the same order. Models of Mars upper atmosphere should address them accordingly.

important reaction, the dissociative recombination of O2 is responsible for most of the production of hot atomic oxygen deep in the dayside thermosphere/ionosphere. The investigation of the Martian upper atmosphere is therefore complicated by the change in the flow regime from a collisional to a collisionless domain. Past studies, which used simple extrapolations of 1-D thermospheric/ionospheric parameters, could not account for the full effects of realistic conditions, which are shown to be of significant influence on the exosphere both close to and far away from the exobase. In this work, the combination of the new 3-D Direct Simulation Monte Carlo kinetic model and the modern 3-D Mars Thermosphere General Circulation Model is employed to describe selfconsistently the Martian upper atmosphere at equinox for solar low conditions. For the first time, a 3-D analysis and shape of the Martian hot corona is provided, along with density and temperature profiles of cold and hot constituents as functions of position on the planet. Atmospheric loss and ion production (found to be more than an order of magnitude lower than the neutral escape), calculated locally all around the planet, provide valuable information for plasma models, refining the understanding of the ion loss, atmospheric sputtering, and interaction with the solar wind, in general.

Figure 9. Simulation results in the terminator plane, i.e., looking from behind on Venus. The solar wind is orientated out of the plane (+x axis), the convective electric field is antiparallel to the (+z) axis and points away from the planet; thus, the background magnetic field completes the right-handed system (+y axis). Figure 10 displays (left) the solar wind density (nsw), (middle) the heavy ion density (nhi), and (right) the background magnetic field. An asymmetric bow shock, exhibiting a shocklet structure, is formed in front of the obstacle owing to the supersonic solar wind (left plot). The interplanetary magnetic field is draping around Venus generating an induced magnetotail on the nightside of the planet (right plot). See text for details. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, E00B98, doi:10.1029/2009JE003377, 2009

[1] Venus Express is well and healthy and has now been providing exciting new data from Venus, our nearby twin planet, for over 2 years. Many of the new results are presented and discussed in the subsequent papers in this special section. The overall scientific objective of Venus Express is to carry out a detailed study of the atmosphere of Venus, including the interaction of the upper atmosphere with the solar wind and the interaction of the lowest part of the atmosphere with the surface of the planet. In addition, the plasma environment and magnetic fields as well as some aspects of the surface of the planet are addressed. For the first time, investigations make systematic use of the transparent infrared spectral windows in order to probe the atmosphere in four dimensions: three spatial dimensions plus time. The spacecraft design is taken from Mars Express with some modifications necessary owing to the specific environment around Venus. The payload is composed of three spectrometers, a camera, a magnetometer, an instrument for detecting energetic particles, and a radio science package. The orbit is polar and highly elliptic, with a pericenter altitude of about 200 km over the northern polar region and an apocenter altitude of 66,000 km. Presently, the coverage of the southern hemisphere is very good, but important gaps still do exist. The coverage of the northern hemisphere is much less dense. Venus Express is a part of the European Space Agency's program for the exploration of the inner solar system, which includes missions to study the Sun, Mercury, Venus, the Moon, Mars, and comets and asteroids.

[1] The global behavior and escape rate of hot oxygen atoms around Mars and their response to different solar wind dynamic pressure (PSW) conditions are investigated using a multidimensional time-dependent magnetosheath-ionosphere-exosphere (Msh-I-E) coupling model. Recently we reported that an increase in PSW leads to a short-term increase in the escape rate of nonthermal oxygen using a one-dimensional (1-D) Msh-I-E model. The model used in the present paper is a multidimensional version of our previous 1-D model. For the exosphere model, we adopt a three-dimensional Monte Carlo approach above a 250-km altitude, while a time-dependent two-stream approach is employed below 250 km. The exosphere model is coupled with a two-dimensional resistive magnetohydrodynamic model of the magnetosheath-ionosphere interaction, assuming axial symmetry with respect to the Sun-Mars line. The results of the present model are consistent with the results of the 1-D model. The escape rate of hot oxygen for PSW = 0.36 nPa is roughly twice that for PSW = 1.43 nPa under steady state conditions. For nonstationary conditions, where PSW is suddenly increased from 0.36 to 1.43 nPa, the escape rate is temporarily enhanced by a factor of 2.3 to 4.5 compared with that of the steady state case. The hot oxygen density is found to be less dependent on PSW than is the escape rate.

[1] Venus forms an obstacle in the streaming solar wind; inside the obstacle boundary (that is below the magnetic barrier) the ions of planetary origin dominate the plasma. The objective of this study is to investigate the properties of the O+ ions inside the obstacle boundary of Venus in the wake; we are especially interested in the characterization of the different plasma regions the O+ ions occupy. The study is based on the data collected by the ASPERA 4 plasma analyzer flying onboard of the Venus Express mission in a region never explored before experimentally. The obstacle boundary was approximately identified from the dropout of magnetospheric electrons and the sharp decrease of the proton speed; the entry point correlated well with the location of the magnetic barrier derived by eyes from magnetometer data. The most characteristic structures seen during the various flybys were (1) the tailward continuation of the mantle was evident; (2) in the mantle near Venus the O+ ion flow was significantly intense in low-energy counts; (3) the inbound and outbound crossings of the tailward boundary were sharp, characterized by less intense but higher-energy O+ beams; (4) the crossing of the central tail region (current sheet) was marked by the change of the sign of Bx and by an intense low-energy O+ ion flux; (5) it is remarkable that the O+ ion outflow was not confined to the central tail region; the intensity elsewhere was highly variable, resulting in a ray-like outflow pattern in most of the cases.

Recent research has shown that distinct physical regions in the Venusian induced magnetosphere are recognizable from the variations of strength of the magnetic field and its wave/fluctuation activity. In this paper the statistical properties of magnetic fluctuations are investigated in the Venusian magnetosheath and wake regions. The main goal is to identify the characteristic scaling features of fluctuations along Venus Express (VEX) trajectory and to understand the specific circumstances of the occurrence of different types of scalings. For the latter task we also use the results of measurements from the previous missions to Venus. Our main result is that the changing character of physical interactions between the solar wind and the planetary obstacle is leading to different types of spectral scaling in the near-Venusian space. Noisy fluctuations are observed in the magnetosheath, wavy structures near the terminator and in the nightside near-planet wake. Multi-scale turbulence is observed at the magnetosheath boundary layer and near the quasi-parallel bow shock. Magnetosheath boundary layer turbulence is associated with an average magnetic field which is nearly aligned with the Sun-Venus line. Noisy magnetic fluctuations are well described with the Gaussian statistics. Both magnetosheath boundary layer and near shock turbulence statistics exhibit non-Gaussian features and intermittency over small spatio-temporal scales. The occurrence of turbulence near magnetosheath boundaries can be responsible for the local heating of plasma observed by previous missions.

The ASPERA-3 experiment on Mars Express provides the first measurements of energetic neutral atoms (ENAs) from Mars. These measurements are used to study the global structure of the interaction of ...

[1] The solar wind interaction with Venus creates an induced magnetosphere around the planet. It is shown that within the space bound by Venus' bow shock and ionopause, there is a rich occurrence of mirror-mode-like structures in the magnetic field data. The dayside magnetosheath and nightside magnetosheath/wake regions are investigated separately. It is shown that the probability to observe mirror mode structures is much higher at the dayside, where it is also strongly dependent on the angle between the solar wind magnetic field and the bow shock normal. In Venus' wake the chance to observe these structures is low, most likely because of the fully developed turbulence in this region, which will decrease temperature anisotropies. The results stand in contrast to the very low occurrence rate claimed from data taken by the Pioneer Venus Orbiter mission.

[1] Statistically measurable responses of atmospheric circulation to solar wind dynamic pressure are found in the Northern Hemisphere (NH) zonal-mean zonal wind and temperature, and on the Northern Annular Mode (NAM) in winter and spring. When December to January solar wind dynamic pressure (PswDJ) is high, the circulation response is marked by a stronger polar vortex and weaker sub-tropical jet in the upper to middle stratosphere. As the winter progresses, the Arctic becomes colder and the jet anomalies shift poleward and downward. In spring, the polar stratosphere becomes anomalously warmer. At solar maxima, significant positive correlations are found between PswDJ and the middle to late winter NAM all the way from the surface to 20 hPa, implying a strengthened polar vortex, reduced Brewer–Dobson circulation and enhanced stratosphere-troposphere coupling. The combined effect of high solar UV irradiance and high solar wind dynamic pressure in the NH middle to late winter is enhanced westerlies in the extratropics and weaker westerlies in the subtropics, indicating that more planetary waves are refracted toward the equator. At solar minima, there is no correlation in the NH winter but negative correlations between PswDJ and the NAM are found only in the stratosphere during spring. These results suggest possible multiple solar inputs that may cause refraction/redistribution of upward wave propagation and result in projecting the solar wind signals onto the NAM. The route by which the effects of solar wind forcing might propagate to the lower atmosphere is yet to be understood.

[1] On 10-11 September 2006 the Venus Express magnetometer detected a very strong Interplanetary Coronal Mass Ejection (ICME) event with an average field about 2 times higher than that of a typical ICME at 0.72 AU. While the effective obstacle to the solar wind is compressed to a smaller dimension during this ICME event, the bow shock is located far upstream of its nominal location. The observed shocks are weak and appear very dynamic. The location of the shock crossing can be found all along the Venus Express trajectory, which has an apocenter of 12 R v . We attribute the atypical distant bow shock location as an effect of the extremely low Mach number during the ICME.

[1] Magnetometer data from two Venus years of the Venus Express mission in orbit are investigated for the occurrence of ion cyclotron waves. Proton cyclotron waves were recently detected in the upstream region of Venus by the spacecraft, indicating pickup of planetary protons from Venus's exosphere by the solar wind and loss of hydrogen to interplanetary space. A study of representative cases illustrates the waveform, spectrum, duration, and higher-order resonances of the transverse waves with left-hand circular polarization and propagation nearly along the magnetic field; their properties in the magnetic field principal axes system are determined. A statistical approach studies the wave properties as a function of the angle between the solar wind and magnetic field direction, as a function of their occurrence in space, and with respect to the motional solar wind electric field. Proton cyclotron waves are found up to 9 Venus radii from the planet, for a large range of angles between the solar wind and magnetic field direction, independent from foreshock geometry and independent from the direction of the motional electric field. This reveals that cyclotron wave generation from local pickup of neutral hydrogen is efficient over a large volume of space upstream of the planet and imposes the existence of an extended reservoir of planetary neutral hydrogen at Venus.

[1] The solar wind interaction contributes to the loss of Venus atmospheric constituents, especially oxygen, by direct antisunward acceleration of planetary ions and possibly by related sputtering of neutrals. Both comet-like “ion pickup” and related sputtering processes may have played a key role in Venus' atmosphere evolution, but the significance of their effects, as well as other proposed escape mechanisms, is still uncertain. In particular, recent reports of only modest ion escape rates measured on Venus Express (VEX) during the current low-activity phase of the solar cycle make it important to reconsider the evidence seen in both Pioneer Venus Orbiter and VEX observations suggesting significantly enhanced escaping O+ ion fluxes during periods of disturbed interplanetary conditions. At present, the most extreme interplanetary conditions result from the effects of coronal mass ejections (CME), which may have been more prevalent in the first 1–2 billion years of the Sun's history. The Analyser of Space Plasmas and Energetic Atoms 4 (ASPERA-4) Ion Mass Analyzer and Magnetometer on Venus Express have now made detailed measurements during several periods when CME disturbances encountered Venus. The observations and models described in this report provide further insights into the possible response of oxygen ion escape to solar activity. In particular, they illustrate nuances of in situ sampling of large-gyroradius pickup ions, related to the atmospheric source properties, spacecraft orbit geometry, and the prevailing interplanetary conditions, that make the estimation of the variable global escape fluxes due to that process particularly challenging. In three of the four cases examined in some detail, ionospheric oxygen ions were either unobservable or below the limit of detectability for passes well within interplanetary coronal mass ejection (ICME) intervals. In the fourth example, where ionospheric ions were observed as expected from the model, O+ pickup ions were observed in greater abundance than is typical in undisturbed solar wind. Other escape processes are not considered here, although it is assumed the source population for the modeled pickup ions is a preaccelerated upper atmosphere component. Analysis of many more ICME events, expected to be obtained as the Sun becomes more active in future years, are necessary to resolve the question of the importance of ICME to Venus oxygen escape.

Processing and mapping of the Lunar Prospector magnetometer data is supported by the NASA Discovery Data Analysis Program.

We have analyzed some 807 Mars Global Surveyor electron density profiles that are confined to the northern high latitudes and thus are relatively free of the effects of crustal magnetic fields. These profiles have shown some anomalous features in the Martian ionosphere, and one of these is the noticeable variability in number density (N m ) and height (h m ) of the primary ionospheric peak on the same day when solar conditions and solar zenith angle have remained the same, a feature not expected from a photochemically controlled layer. We study this feature by generating longitudinal plots of N m and h m for the 807 profiles and by applying a least squares spectral fit consisting of wave number 1, 2, and 3 components to these data sets. We find some significant relationship between the two parameters, with the troughs in N m coinciding with the ridges in h m (and vice versa) on the longitudinal scale. An examination at fixed solar zenith angles shows a significant anticorrelation between the two parameters recorded over a period of about 3 months. However, theoretical considerations would support a positive correlation expected in response to changes in the EUV flux that occurred during this period. Further, we observe a large variability in electron density at 160 and 180 km, altitudes in the topside ionosphere, where photochemistry is expected to dominate. This is an additional anomalous feature. No such variability is observed in the topside ionosphere of Venus. We discuss plausible mechanisms like neutral atmosphere dynamics and solar wind interaction to explain some of the features.

[1] Space weather storms at the Earth are dominated by the magnetosphere's response to coronal mass ejections, or CMEs, whose disturbances propagate through the solar wind to 1 AU, and to a lesser extent by the pressure ridges associated with the interactions of solar wind streams with different properties. The interplanetary signatures of these events include high solar wind dynamic pressures and high interplanetary magnetic fields, the same parameters that cause compression and/or magnetization of the ionosphere at weakly magnetized Venus. The nature of Venus' response to these events is also expected to include increased atmosphere erosion by the solar wind interaction, a matter of potential interest for historical extrapolations of atmosphere escape. We consider the possible evidence for enhanced escape during these disturbances in the data from the Pioneer Venus Orbiter (PVO). Available magnetometer and plasma analyzer results are used to identify periods of interest and the interplanetary characteristics of the disturbed periods. It is found that the suprathermal (>36 eV) ion measurements from the Pioneer Venus neutral mass spectrometer (ONMS) provide convincing evidence of the related enhanced escape of mostly O+ suprathermal ions. The results provide the first direct demonstration that space weather has played an important role in Venus (and other weakly magnetized planet) atmosphere loss through time. It is important to pursue study of these effects with the new measurements from Venus Express and, in light of the approaching solar activity cycle rising phase, extremely timely.

[1] The low-frequency radar, Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), on board the Mars Express spacecraft is used to sound electron densities in the topside Martian ionosphere. The radar records the delay times to echoes of reflected radio waves as a function of frequency, yielding spectrograms with traces of radar echoes. At times, two traces are present in spectrograms of the Martian ionosphere. One of these traces corresponds to reflections from the direction to nadir. The other trace originates in a localized reflector in the ionosphere. The local reflectors can be associated with the cusplike regions of near-vertical crustal magnetic fields. The apparent nadir angle of reflection can occasionally increase to 90°. This suggests that steep gradients of the altitude of the electron isodensity exist in the Martian ionosphere and indicates rapid horizontal spatial variations of vertical diffusion of Martian plasma. Such gradients may arise owing to preferential access of solar wind to the cusplike regions or to precipitation of energetic electrons from acceleration regions located on cusp magnetic field lines high above the ionosphere.

[1] We present a global map of the amplitude of Mars' crustal magnetic field at 170-km altitude based on electron reflectometry, using data from the Magnetometer/Electron Reflectometer (MAG/ER) experiment on board Mars Global Surveyor. This new map contains features that are about seven times weaker than those in previously published maps obtained with the MAG alone from the 400-km-altitude mapping orbit. The increased sensitivity and nearly complete sampling reveal numerous weak magnetic sources in the northern lowlands. A group of these sources forms a nearly complete ring surrounding the Utopia basin, coinciding with a ridge of thickened crust. A larger 5800-km-diameter ring is defined by a ∼120° arc of magnetic sources that extend to the north pole, supporting the hypothesis that part of the dichotomy boundary was established or modified by the Utopia impact. We also map the magnetic signatures of the Hellas, Argyre, and Isidis impact basins with increased dynamic range, confirming the large contrast in magnetic field strength between the basin interior and surroundings. Magnetic sources surround most of the Tharsis rise, except in the north, suggesting that the construction of Tharsis thermally demagnetized a large region of the northern lowlands. Thermal demagnetization is also suggested in the Elysium region, which intersects the inner Utopia magnetic ring, and south of the Hellas basin in the vicinity of Peneus Patera and Amphitrites Patera.

[1] Detailed observations of the plasma environment of Comet 19P/Borrelly were made by the Plasma Experiment for Planetary Exploration (PEPE) during the Deep Space 1 (DS1) flyby on 22 September 2001. DS1 flew from north to south relative to the plane of the ecliptic on the sunward side of the comet and all plasma boundaries (i.e., first and last pickup ions, bow shock, decelerated solar wind) were observed to be shifted northward. This surprising result was initially attributed to large well-collimated dayside jets directed ∼10° northward from the nucleus as observed by the DS1 MICAS camera. However, ground-based observations of Schleicher et al. (2003) indicate a symmetric gas cloud with respect to the comet-Sun line. In an attempt to resolve this conundrum, we have conducted a series of hybrid plasma simulations to investigate the effects of the large pickup ion gyroradius (i.e., comparable to the length scale of the cometary obstacle) at 1.36 AU. We find that that a maximum asymmetry, consistent with the observations, is generated when the pickup ion gyroradius is comparable to or less than the standoff distance of the bow shock (BIMF ∼ 4 nT). The degree of asymmetry is reduced for larger and smaller magnetic field strengths.

[1] We present numerical simulations of Alfven waves in steady state, leading to the formation of intense magnetic filaments when the nonlinearity arises due to the ponderomotive effects and Joule heating. When the plain Alfven wave is perturbed by a transverse perturbation and the magnitude of the pump Alfven wave changes, chaotic filamentary structures of magnetic field in kinetic Alfven wave (KAW) filamentation and quasiperiodic in inertial Alfven wave (IAW) filamentation have been observed. At higher KAW pump wave amplitude, the spectra approaches near the Kolmogorov k−5/3 scaling. The spectral index becomes k−3 scaling in IAW. Relevance of these studies in solar wind and magnetosphere for KAW and solar corona and auroral region for IAW has also been pointed out. Particle heating in these small-scale filamentary structures has also been calculated by using velocity space diffusion coefficient in Fokker-Planck equation.

[1] Numerical simulations of magnetic clouds (MCs) propagating through a structured solar wind suggest that MC-associated magnetic flux ropes are highly distorted by inhomogeneities in the ambient medium. In particular, a solar wind configuration of fast wind from high latitudes and slow wind at low latitudes, common at periods close to solar minimum, should distort the cross section of magnetic clouds into concave-outward structures. This phenomenon has been reported in observations of shock front orientations, but not in the body of magnetic clouds. In this study an analytical magnetic cloud model based upon a kinematically distorted flux rope is modified to simulate propagation through a structured medium. This new model is then used to identify specific time series signatures of the resulting concave-outward flux ropes. In situ observations of three well studied magnetic clouds are examined with comparison to the model, but the expected concave-outward signatures are not present. Indeed, the observations are better described by the convex-outward flux rope model. This may be due to a sharp latitudinal transition from fast to slow wind, resulting in a globally concave-outward flux rope, but with convex-outward signatures on a local scale.

[1] Substorms sometimes occur repetitively with a period of ∼1–4 hours. In this paper we examine repetitive substorms, identified using particle injections and positive H bays on the nightside, that we find to occur during corotating high-speed streams associated with coronal holes. The high-speed streams often last for several days and are accompanied by large amplitude Alfven waves of the interplanetary magnetic field (IMF). We find that repetitive substorms occur every ∼1–4 hours, regardless of the solar cycle phase, whenever the Earth's magnetosphere is impinged by these high-speed streams. We further find that a significant number of these substorms are associated with repetitive northward turnings of the Alfvenic IMF, each northward turning preceded by weakly-to-moderately southward IMF, i.e., Bz ∼ −3.6 nT for ∼29 min on the average. We present eight example intervals where most of the repetitive substorms were associated with a northward turning. Statistically, for 63.5% of 312 substorms we are able to identify a reasonable association with a northward turning. While limitations of the Weimer-mapped IMF used here and the spatial structure of the Alfvenic IMF prevent us from estimating a precise figure for the percentage of IMF triggered substorms, our results indicate that many of the repetitive substorms are likely due to repetitive triggering by the Alfvenic IMF.