Substorm-associated injections of energetic ions observed by GEOS-1 and ATS-6 in and near synchronous altitude

Energetic ion measurements of GEOS-1 and ATS-6 are analysed for the period of geomagnetic activity following the arrival of a solar wind shock at 0027 UT on July 29, 1977. GEOS crossed the magnetopause at 6.9 R _E and 0027 UT (1312 LT). Although the difference in local time to ATS at 6.6 R _E is only 2 h ATS seems to remain well inside the magnetopause. During the second orbital pass on this day GEOS crossed the geostationary orbit at the onset time of a major substorm developing at 1120 UT. At this time the local time difference of GEOS and ATS was 12 h. The considerably different energy dispersions are discussed. An azimuthal anisotropy of approximately 20% observed in the GEOS data is interpreted to be the result of a particle density gradient.NOAA-SEL, Boulder, Colo., U.S.A.     

Modelling of the electromagnetic field in the interplanetary space and in the Earth's magnetosphere

A magnetohydrodynamic model of the solar wind flow is constructed using a kinematic approach. It is shown that a phenomenological conductivity of the solar wind plasma plays a key role in the forming of the interplanetary magnetic field (IMF) component normal to the ecliptic plane. This component is mostly important for the magnetospheric dynamics which is controlled by the solar wind electric field. A simple analytical solution for the problem of the solar wind flow past the magnetosphere is presented. In this approach the magnetopause and the Earth's bow shock are approximated by the paraboloids of revolution. Superposition of the effects of the bulk solar wind plasma motion and the magnetic field diffusion results in an incomplete screening of the IMF by the magnetopause. It is shown that the normal to the magnetopause component of the solar wind magnetic field and the tangential component of the electric field penetrated into the magnetosphere are determined by the quarter square of the magnetic Reynolds number. In final, a dynamic model of the magnetospheric magnetic field is constructed. This model can describe the magnetosphere in the course of the severe magnetic storm. The conditions under which the magnetospheric magnetic flux structure is unstable and can drive the magnetospheric substorm are discussed. The model calculations are compared with the observational data for September 24–26, 1998 magnetic storm (Dst _min=−205 nT) and substorm occurred at 02:30 UT on January 10, 1997. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Unusually Distant Bow Shock Encounters at Mars: Analysis of March 24, 1989 Event

Detailed analysis of disturbances observed on 24 March, 1989 far upstream of the usual Martian bow shock position was completed with the use of the planetary obstacle and bow shock models relevant for the period of Phobos 2 observations and for low Mach numbers, respectively. It is proven that the system of discontinuities observed in the solar wind between 18:42 and 19:36 UT was the consequence of unusually distant planetary bow shock excursions. The cause was unusually small ρV ² and M _a values in the solar wind flow.     

Latitudinal variation of solar wind velocity

Single station solar wind velocity measurements using the Ooty Radio Telescope (ORT) in India (operating at 327 MHz) are reported for the period August 1992 to August 1993. Interplanetary scintillation (IPS) observations on a large number of compact radio sources covering a latitudinal range of ±80° were used to derive solar wind velocities using the method of fitting a power law model to the observed IPS spectra. The data shows a velocity versus heliographic latitude pattern which is similar to that reported by Rickett and Coles (1991) for the 1981–1982 period. However, the average of the measured equatorial velocities are higher, being about 470 km s^–1 compared to their value of 400 km s^–1. The distribution of electron density variations (N _e ) between 50R and 90R was also determined and it was found that N _e was about 30% less at the poles as compared to the equator.     

Interdisciplinary summary of solar/interplanetary events during August 1972

The solar/interplanetary events in early August 1972 are summarized in Section 1 (Introduction), Section 2 (August 1972 Events in the Solar Cycle 20), Section 3 (Evolution of Solar Active Region: McMath region No. 11976 and its flare-activity), Section 4 (Radio, X-ray, and Proton Characteristics of Four Major Solar Flares: F-1 at 0316 UT on 2 August, F-2 at 1958 UT on 2 August, F-3 at 0626 UT on 4 August, and F-4 at 1522 UT on 7 August), Section 5 (Interplanetary Shock Waves: observations of the shock waves generated from the four major solar flares at several points in interplanetary space, the Earth, Pioneer-9, Pioneer-10, etc.; interplanetary scintillations; shock trajectories in the heliosphere), Section 6 (Variations of Solar and Galactic Cosmic Rays: four solar proton events observed in the vicinity of the earth and at the Pioneer-9 location in the course of interplanetary disturbances; Forbush decreases of cosmic ray intensity; the spikeshaped variation in solar and galactic cosmic rays on 5 August), and Section 7 (Conclusions).     

CME-Driven Solar Wind Disturbances at High Heliographic Latitudes

We have identified 20 coronal mass ejections, or CMEs, in the solar wind in the Ulysses data obtained between S30° and S75° during the second polar orbit. Unlike CME-driven disturbances observed at high latitudes during Ulysses’ first polar orbit, these disturbances had plasma and magnetic field characteristics similar to those observed in the ecliptic plane near 1 AU when one allows for evolution with heliocentric distance. Here we provide a brief overview of CME observations at high latitudes both close to and far from the Sun, with emphasis on the recent Ulysses measurements on the rising portion of solar cycle 23. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ion Acoustic Waves in the Heliosphere

Observations of ion acoustic waves in the solar wind during the first and second orbit of the Ulysses spacecraft are presented. The observations show variations of the wave activity with the heliolatitude and with the phase of the solar cycle. The interrelationships between the wave intensity and the electron heat flux and the ratio of electron to proton temperature, T _e/T _p, are examined. This revised version was published online in August 2006 with corrections to the Cover Date.     

Magnetic turbulence at the magnetopause, a key problem for understanding the solar wind/ magnetosphere exchanges

According to ideal MHD, the magnetopause boundary should split the terrestrial environment in two disconnected domains: outside, the solar wind (including its shocked part, the magnetosheath), and inside, the magnetosphere. This view is at variance with the experimental data, which show that the magnetopause is not tight and that a net transfer of matter exists from the solar wind to the magnetosphere; it implies that the frozen-in condition must break down on the magnetopause, either over the whole boundary or at some points. In the absence of ordinary collisions, only short scale phenomena (temporal and/or spatial) can be invoked to explain this breakdown, and the best candidates in this respect appear to be the ULF magnetic fluctuations which show very strong amplitudes in the vicinity of the magnetopause boundary. It has been shown that these fluctuations are likely to originate in the magnetosheath, probably downstream of the quasi-parallel shock region, and that they can get amplified by a propagation effect when crossing the magnetopause. When studying the propagation across the magnetopause boundary, several effects are to be taken into account simultaneously to get reliable results: the magnetopause density gradient, the temperature effects, and the magnetic field rotation can be introduced while remaining in the framework of ideal MHD. In these conditions, the magnetopause amplification has been interpreted in term of Alfvén and slow resonances occurring in the layer. When, in addition, one takes the ion inertia effects into account, by the way of the Hall-MHD equations, the result appears drastically different: no resonance occurs, but a strong Alfvén wave can be trapped in the boundary between the point where it is converted from the incident wave and the point where it stops propagating back, i.e., the point where k _\|=0, which can exist thanks to the magnetic field rotation. This effect can bring about a new interpretation to the magnetopause transfers, since the Hall effect can allow reconnection near this particular point. The plasma transfer through the magnetopause could then be interpreted in terms of a reconnection mechanism directly driven by the magnetosheath turbulence, which is permanent, rather than due to any local instability of the boundary, for instance of the tearing type, which should be subject to an instability threshold and thus, as far as it exists, more sporadic.     

Neutral points on the boundary of the closed magnetosphere

Theoretical studies of a field-free plasma incident upon a magnetic dipole lead to a closed magnetosphere with two neutral points in the noon magnetic meridian, at a latitude of ± 70°–75° and a geocentric distance of approximately 10 R_E. The position of the neutral points with respect to the dipole axis is not greatly affected by the angle of incidence of the solar wind. Although the field magnitude near the neutral points is only a fraction of the dipole field, the direction is seen to reverse on opposite sides of the neutral point. Near the boundary the field direction is parallel to the boundary and tends to point towards the neutral point in the Northern hemisphere.     

The Magnetic Field of Mercury

The magnetic field strength of Mercury at the planet’s surface is approximately 1% that of Earth’s surface field. This comparatively low field strength presents a number of challenges, both theoretically to understand how it is generated and observationally to distinguish the internal field from that due to the solar wind interaction. Conversely, the small field also means that Mercury offers an important opportunity to advance our understanding both of planetary magnetic field generation and magnetosphere-solar wind interactions. The observations from the Mariner 10 magnetometer in 1974 and 1975, and the MESSENGER Magnetometer and plasma instruments during the probe’s first two flybys of Mercury on 14 January and 6 October 2008, provide the basis for our current knowledge of the internal field. The external field arising from the interaction of the magnetosphere with the solar wind is more prominent near Mercury than for any other magnetized planet in the Solar System, and particular attention is therefore paid to indications in the observations of deficiencies in our understanding of the external field. The second MESSENGER flyby occurred over the opposite hemisphere from the other flybys, and these newest data constrain the tilt of the planetary moment from the planet’s spin axis to be less than 5°. Considered as a dipole field, the moment is in the range 240 to 270 nT-R _M ³ , where R _M is Mercury’s radius. Multipole solutions for the planetary field yield a smaller dipole term, 180 to 220 nT-R _M ³ , and higher-order terms that together yield an equatorial surface field from 250 to 290 nT. From the spatial distribution of the fit residuals, the equatorial data are seen to reflect a weaker northward field and a strongly radial field, neither of which can be explained by a centered-dipole matched to the field measured near the pole by Mariner 10. This disparity is a major factor controlling the higher-order terms in the multipole solutions. The residuals are not largest close to the planet, and when considered in magnetospheric coordinates the residuals indicate the presence of a cross-tail current extending to within 0.5R _M altitude on the nightside. A near-tail current with a density of 0.1 μA/m² could account for the low field intensities recorded near the equator. In addition, the MESSENGER flybys include the first plasma observations from Mercury and demonstrate that solar wind plasma is present at low altitudes, below 500 km. Although we can be confident in the dipole-only moment estimates, the data in hand remain subject to ambiguities for distinguishing internal from external contributions. The anticipated observations from orbit at Mercury, first from MESSENGER beginning in March 2011 and later from the dual-spacecraft BepiColombo mission, will be essential to elucidate the higher-order structure in the magnetic field of Mercury that will reveal the telltale signatures of the physics responsible for its generation.     

Magnetic Reconnection Phenomena In Interplanetary Space

Interplanetary magnetic reconnection(IMR) phenomena are explored based on the observational data with various time resolutions from Helios, IMP-8, ISEE3, Wind, etc. We discover that the observational evidence of the magnetic reconnection may be found in the various solar wind structures, such as at the boundary of magnetic cloud, near the current sheet, and small-scale turbulence structures, etc. We have developed a third order accuracy upwind compact difference scheme to numerically study the magnetic reconnection phenomena with high-magnetic Reynolds number (R _M=2000–10000) in interplanetary space. The simulated results show that the magnetic reconnection process could occur under the typical interplanetary conditions. These obtained magnetic reconnection processes own basic characteristics of the high R _M reconnection in interplanetary space, including multiple X-line reconnection, vortex velocity structures, filament current systems, splitting, collapse of plasma bulk, merging and evolving of magnetic islands, and lifetime in the range from minutes to hours, etc. These results could be helpful for further understanding the interplanetary basic physical processes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Observations of the Acceleration of Polar Solar Wind Near Solar Maximum

In a previous paper (Tappin et al., 1999) we used cross-correlation analysis of high-cadence observations with the LASCO coronagraphs to trace the acceleration of the solar wind at low latitudes. In this paper we present a similar analysis carried out over the North pole of the Sun. The observations which were made in March 2000 with the C3 coronagraph show low bulk flow speeds (comparable to or lower than those seen at the equator in early 1998). We observe the acceleration continuing to the edge of the C3 field of view at about 30 R _⊙. We also observe, as at low latitude, a high-speed tail but now reaching out well beyond 2000 km s⁻¹. We do not see a clear signature of a fast polar bulk flow. We therefore conclude that at this phase of the solar cycle, any fast bulk flow occupies only a small part of the line of sight and is therefore overwhelmed by the denser slow solar wind in these observations. We also show that the fast component is consistent with observed solar wind speeds at 1 AU. This revised version was published online in August 2006 with corrections to the Cover Date.     

Associating the Solar Wind Measured by Ulysses with its Source at the sun

Radio occultation, ultraviolet, and white-light measurements have expanded our knowledge of the morphology of density and velocity in polar coronal holes, and made it possible to carry out the first systematic comparisons between the Ulysses solar wind measurements and quantitative white-light observations of the solar corona. This paper summarizes the rationale and salient features of this new approach which has been used to relate the solar wind observed by Ulysses in 1993–1995 to the inner corona. The statistical characteristics (average, standard deviation, and autocorrelation function) of the Ulysses density measurements of the fast wind are found to be mirrored in those of polarized brightness measurements of path-integrated density made by the High Altitude Observatory (HAO) Mauna Loa K-coronagraph at 1.15 R _⊙. These results reinforce the conclusions from comparisons between measurements of the outer and inner corona. They show that the polar coronal hole extends radially into the solar wind, and that sources of the fast wind are not limited to coronal holes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interaction of non-perpendicular/parallel solar wind shock waves with the earth's magnetosphere

The interaction of travelling interplanetary shock waves with the bow shock-magnetosphere system is considered. We consider the general case when the interplanetary magnetic field is oblique to the Sun-planetary axis, thus, the interplanetary shock is neither parallel nor perpendicular. We find that an ensemble of shocks are produced after the interaction for a representative range of shock Mach numbers. First, we find that the system S ₊ R _– CS _– S ₊ appears after the collision of travelling fast shock waves S ₊ (Mach number M = 2 to 7) with the bow shock. Here, S _– and R _– represent the slow shock wave and slow rarefaction wave, and C represents the contact surface. It is shown that in the presence of an interplanetary field that is inclined by 45° to the radial solar wind velocity vector, the waves R _– and S _– are weak waves and, to the first degree of approximation, the situation is similar to the previously studied normal perpendicular case. The configuration, R ₊ C _m S _– S ₊ or R ₊ C _m R _– S ₊ where C _m is the magnetopause, appears as the result of the fast shock wave's collision with the magnetopause. In this case the waves S _– and R _– are weak. The fast rarefaction wave reflected from the magnetosphere is developed similar to the case for the collision of a perpendicular shock. The shock wave intensity is varied for Mach numbers from 2 to 10. Thus, in the limits of the first approximation, the validity of the one-dimensional consideration of the nonstationary interaction of travelling interplanetary shock waves with the bow shock-magnetosphere system is proved. The appearance of the fast rarefaction wave, R ₄, decreasing the pressure on the magnetosphere of the Earth after the abrupt shock-like contraction, is proved. A possible geomagnetic effect during the global perturbation of the SSC or SI⁺ type is discussed.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

Source Region of High and Low Speed Wind during the Spartan 201-05 Flight

The large-scale coronal magnetic fields of the Sun are believed to play an important role in organizing the coronal plasma and channeling the high and low speed solar wind along the open magnetic field lines of the polar coronal holes and the rapidly diverging field lines close to the current sheet regions, as has been observed by the instruments aboard the Ulysses spacecraft from March 1992 to March 1997. We have performed a study of this phenomena within the framework of a semi-empirical model of the coronal expansion and solar wind using Spartan, SOHO, and Ulysses observations during the quiescent phase of the solar cycle. Key to this understanding is the demonstration that the white light coronagraph data can be used to trace out the topology of the coronal magnetic field and then using the Ulysses data to fix the strength of the surface magnetic field of the Sun. As a consequence, it is possible to utilize this semi-empirical model with remote sensing observation of the shape and density of the solar corona and in situ data of magnetic field and mass flux to predict values of the solar wind at all latitudes through out the solar system. We have applied this technique to the observations of Spartan 201-05 on 1–2 November, 1998, SOHO and Ulysses during the rising phase of this solar cycle and speculate on what solar wind velocities Ulysses will observe during its polar passes over the south and the north poles during September of 2000 and 2001. In order to do this the model has been generalized to include multiple streamer belts and co-located current sheets. The model shows some interesting new results. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ultra low frequency waves in the magnetosphere

The behaviour of continuous pulsations pc 2–5 observed on the ground has been known for some time. They seldom occur at night, their amplitudes generally increase towards the auroral zones and the sense of rotation of their polarisation often agrees with surface waves on the magnetopause. Recently ULF sonagrams for middle latitudes have shown systematic behaviour and dominant periods. Theoretical study of normal modes for symmetrical models is also well established. If the wave depends on longitude like e ^im , modes with large m are quasi-transverse and these are likely to be excited and will be emphasised.The Kelvin-Helmholtz instability has recently been studied in a general formulation. For given fields and plasma properties on both sides of the boundary, a plot of critical wind speed against the direction of the wave fronts shows a cusp, meaning that for most directions of the wind the onset of instability will correspond to the cusp and the nature of the waves can be predicted from this. Almost circularly polarised waves are predicted confirming an earlier heuristic suggestion.Magnetic data from Explorer 33 shows rather irregular disturbance near the magnetopause, but an integration designed to show the sense of rotation of the polarisation shows clear agreement. The disturbance outside the magnetopause also shows the predicted polarisation, indicating that a substantial part of it must be due to surface waves, whereas previously it was believed to be the turbulence of the magnetosheath.Bounce resonance has also been invoked to excite ULF waves, particularly those observed at the geostationary orbit, which may also correspond to pg at the ground. They are remarkably regular and quite strictly transverse, suggesting large m. Energetic particles may then see a higher frequency as a result of their drift. A simple picture of the exchange of energy is obtained using a frame rotating with the wave and it is seen that the wave can be driven by a spatial gradient in the energetic particles. The most important mechanism is due to the tilting of the field lines and the growth rate can be large. The reflection by the ionosphere requires further study.     

Magnetic Field Observations of Transient Events at Ulysses, 1996–2000

During the years 1996–2000 solar activity has been gradually rising and is now close to maximum. At the same time the Ulysses spacecraft has performed a north to south traverse of the low latitude regions of the heliosphere and is now once again travelling through high southerly latitudes. We show some examples and report on the occurence rates of transient solar wind disturbances which have been identified by their magnetic field signatures. ‘Magnetic clouds’ remain more common at low (compared to high) latitudes despite the rise in solar activity. However, more events were observed at high latitudes than at solar minimum. This revised version was published online in August 2006 with corrections to the Cover Date.     

Remote Sensing of H from Ulysses and Galileo

We model interplanetary H Lyman-α (Lα) observations from Galileo UVS (Ultraviolet Spectrometer) and EUVS (Extreme Ultraviolet Spectrometer) (Hord et al., 1992) and the Ulysses interstellar neutral gas (GAS) instrument (Witte et al., 1992). EUVS measurements near solar maximum (max) in 1990–1992 have a peaked brightness maximum upwind due to a rather isotropic solar wind charge-exchange ionization pattern (A=0–0.25). GAS measurements from solar minimum (min) in 1997 have a plateau in the upwind direction that we model using Ulysses SWOOPS (solar wind plasma experiment) solar min data on solar wind density and velocity at different heliographic latitudes. The isotropic ionization pattern deduced from EUVS at solar max may be consistent with recent SWOOPS results (McComas et al., 2000b, c) that high speed solar wind is absent at high latitudes during solar max. Galileo and Ulysses Lα data favor higher H temperatures (15 000–18 000 K) than previous models. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Strength and Structure of the Coronal Magnetic Field

I discuss a method for determining the strength and spatial structure of the coronal magnetic field by observations of the Faraday rotation of a radio galaxy which is in conjunction with the Sun. Given a knowledge of the plasma density in the outer corona, and the magnetic field sector structure (both independently available), the strength of the coronal field can be determined, as well as the magnitude of spatial variations on scales of 1000 km to several solar radii. Such knowledge is crucial for testing computational models of the solar corona, which are prominently featured in this meeting. Results are presented from observations with the Very Large Array radio telescope of the radio galaxy 3C228 on August 16, 2003, when the line of sight to the source was at heliocentic distances of 7.1−6.2R _⊙. The observations are consistent with a coronal magnetic field which is proportional to the inverse square of the distance in the range 6 ≤ r ≤ 10R _⊙, and has a value of 39 mG at 6.2R _⊙. The Faraday rotation is uniform across the source, indicating an absence of strong plasma inhomogeneity on spatial scales up to 35,000 km.