Solar Wind Pressure Variations Research Articles

Spatial distribution of magnetic fluctuation power with period 40 to 600 s in the magnetosphere observed by THEMIS

AbstractUltralow frequency (ULF) fluctuations are ubiquitous in the magnetosphere and have significant influence on the energetic particle transport. We use Time History of Events and Macroscale Interactions during Substorms (THEMIS) data to give the spatial distribution of the Pi2/Pc4 and Pc5 band magnetic fluctuation amplitude near the magnetic equator in the magnetosphere. Statistical results can be summarized as follows: (1) strong ULF fluctuations are common in the magnetotail plasma sheet; the amplitude of all three components of magnetic fluctuations decreases with decreasing radial distance; (2) during periods of high AE index, fluctuations can propagate toward the Earth as far as the data cutoff in the nightside of the magnetosphere, and the amplitude of magnetic fluctuations is clearly stronger near the dusk sector of the synchronous orbit than that near the dawn sector, suggesting that the substorm particle injection has significant contribution to these fluctuations; (3) intense compressional Pc5 band magnetic fluctuations are a persistent feature near two flanks of the magnetosphere. Clear peaks of the compressional Pi2/Pc4 band magnetic fluctuation power near two flanks can be found during periods of fast solar wind, while the power of compressional Pi2/Pc4 band fluctuations is weak when the solar wind is slow. (4) Solar wind dynamic pressure and its variations can globally affect the ULF fluctuation power in the magnetosphere. Magnetic fluctuations near the noonside can penetrate from the magnetopause to the synchronous orbit or inner when solar wind pressure variations are large.

Geostationary magnetic field response to solar wind pressure variations: Time delay and local time variation

The relationship between solar wind dynamic pressure changes and geosynchronous magnetic field response is studied using 15 years of OMNI2 and GOES data at 1‐minute resolution. Significant magnetospheric response to solar wind‐forcing is found to be most frequent near noon (30% of all intervals), and virtually absent on the night‐side. The strongest response occurs when IMF Bz is strongly northward and the effect of reducing IMF Bz is most pronounced in the dusk sector. Approximately 25% of dayside Bz variance for related intervals can be attributed to direct response from solar wind dynamic pressure forcing. Time lag between changes in the solar wind at the bow shock nose and similar fluctuations in the magnetosphere at geosynchronous orbit (6.6 Re) is typically 2 to 4 minutes, with responses occurring first in the post‐noon sector and approximately 2 minutes later near dawn. The OMNI2 HRO time‐shifting algorithm appears to be quite effective, with a slight (2 minute) systematic increase in lag and no increased scatter for the most distant upstream solar wind satellite location.

Modeling radiation belt radial diffusion in ULF wave fields: 2. Estimating rates of radial diffusion using combined MHD and particle codes

Quantifying radial transport of radiation belt electrons in ULF wave fields is essential for understanding the variability of the trapped relativistic electrons. To estimate the radial diffusion coefficients (DLL), we follow MeV electrons in realistic magnetospheric configurations and wave fields calculated from a global MHD code. We create idealized pressure‐driven MHD simulations for controlled solar wind velocities (hereafter referred to as pressure‐driven Vx simulations) with ULF waves that are comparable to GOES data under similar conditions, by driving the MHD code with synthetic pressure profiles that mimic the pressure variations of a particular solar wind velocity. The ULF wave amplitude, in both magnetic and electric fields, increases at larger radial distance and during intervals with higher solar wind velocity and pressure fluctuations. To calculate DLL as a function of solar wind velocity (Vx = 400 and 600 km/s), we follow 90 degree pitch angle electrons in magnetic and electric fields of the pressure‐driven Vx simulations. DLL is higher at larger radial distance and for the case with higher solar wind velocity and pressure variations. Our simulated DLL values are relatively small compared to previous studies which used larger wave fields in their estimations. For comparison, we scale our DLL values to match the wave amplitudes of the previous studies with those of the idealized MHD simulations. After the scaling, our DLL values for Vx = 600 km/s are comparable to the DLL values derived from Polar measurements during nonstorm intervals. This demonstrates the use of MHD models to quantify the effect of pressure‐driven ULF waves on radiation belt electrons and thus to differentiate the radial diffusive process from other mechanisms.

Timing analysis of the relationship between solar wind parameters and geosynchronous Pc5 amplitude

Solar wind‐driven magnetospheric ULF waves in the Pc5 band (1.7–6.7 mHz) have been suggested to mediate the radial transport of electrons in the outer radiation belt. To identify the wave generation mechanism and the impact of the waves on the electrons, we performed correlation and superposed epoch analyses of the solar wind control parameters measured by the ACE spacecraft and the amplitude of Pc5 waves and the flux of radiation belt electrons measured at the GOES 12 geosynchronous satellite. The analyses were carried out for solar minimum year 2006, when the solar wind exhibited recurrent co‐rotating interaction regions. We used hourly averages of solar wind velocity Vsw, the root‐mean‐square amplitudes of the solar wind dynamic pressure variations in the Pc5 band σPsw, the azimuthal (σBy) and compressional (σBz) magnetic field components of geosynchronous Pc5 waves, as well as relativistic (>2 MeV) electron flux Je at geosynchronous orbit. We found that σPsw has a higher correlation with σBy and σBz than Vsw and that the correlation between σPsw and the wave amplitudes peaks at or very near zero time shift. We conclude that the major driver of geosynchronous Pc5 waves is solar wind pressure variations rather than the Kelvin–Helmholtz instability on the magnetopause. Solar wind pressure pulses propagate into the inner magnetosphere at the MHD wave speed and it takes less than 1 hour for the signal to reach GEOS 12 from the bow shock nose. The pressure pulses also excite standing Alfvén waves that contribute to the σPsw–σBy correlation. There is a positive correlation between Vsw and Pc5 amplitude, but the correlation peaks at a time shift of about −1 day (the Pc5 peak occurs earlier than the Vsw peak). We attribute this shift to an intrinsic time shift between σPsw and Vsw. Je exhibits a minimum at the Dst minimum, coincident with the maximum of σPsw, σBy, and σBz, and increases by 2 to 3 orders of magnitude over a period of about one day during which σBy and σBz decrease by one order of magnitude and Vsw reaches a peak value. However there are storms that do not exhibit electron flux enhancement despite the presence of strong Pc5 activity. Therefore the correlation between Pc5 amplitude and electron flux at geosynchronous orbit is not unique. The difference in electron response implies either the existence of other competing acceleration/loss mechanisms or that electron response to the Pc5 driver is nonlinear: similar driving conditions produce different response.

The critical solar wind pressure for IMF penetration into the Venus ionosphere

Early observations and simulations have revealed that the occurrence of IMF penetration into the Venus ionosphere depends on the upstream solar wind pressure, and that IMF is transported into the ionosphere by the downward convection when the solar wind dynamic pressure is relatively large. In this paper, we investigated the critical solar wind pressure for the IMF penetration, by using a two‐dimensional global MHD model. The simulations showed that variations in the upstream solar wind pressure (PSW) affect plasma dynamics and chemistry in the subsolar ionosphere. The chemical plasma loss rate increases as PSW increases, and the amount of plasma loss exceeds that of plasma production when PSW exceeds a certain critical level (Pcrtc). On this occasion, the plasma convection in the ionosphere connects to the solar wind flow, and as a result the connected downward convection transports IMF into the ionosphere. The simulations also showed that IMF penetration occurs only when PSW exceeds Pcrtc, even if the solar wind pressure changes abruptly. The critical pressure is not necessarily equal to the ionospheric peak thermal pressure (Ppeak). Our simulations suggested that Pcrtc is smaller than Ppeak.

MESSENGER: Exploring Mercury’s Magnetosphere

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission to Mercury offers our first opportunity to explore this planet’s miniature magnetosphere since the brief flybys of Mariner 10. Mercury’s magnetosphere is unique in many respects. The magnetosphere of Mercury is among the smallest in the solar system; its magnetic field typically stands off the solar wind only ~1000 to 2000 km above the surface. For this reason there are no closed drift paths for energetic particles and, hence, no radiation belts. Magnetic reconnection at the day side magnetopause may erode the subsolar magnetosphere, allowing solar wind ions to impact directly the regolith. Inductive currents in Mercury’s interior may act to modify the solar wind interaction by resisting changes due to solar wind pressure variations. Indeed, observations of these induction effects may be an important source of information on the state of Mercury’s interior. In addition, Mercury’s magnetosphere is the only one with its defining magnetic flux tubes rooted beneath the solid surface as opposed to an atmosphere with a conductive ionospheric layer. This lack of an ionosphere is probably the underlying reason for the brevity of the very intense, but short-lived, ~l-2 min, substorm-like energetic particle events observed by Mariner 10 during its first traversal of Mercury’s magnetic tail. Because of Mercury’s proximity to the sun, 0.3-0.5 AU, this magnetosphere experiences the most extreme driving forces in the solar system. All of these factors are expected to produce complicated interactions involving the exchange and recycling of neutrals and ions among the solar wind, magnetosphere, and regolith. The electrodynamics of Mercury’s magnetosphere are expected to be equally complex, with strong forcing by the solar wind, magnetic reconnection, and pick-up of planetary ions all playing roles in the generation of field-aligned electric currents. However, these field-aligned currents do not close in an ionosphere, but in some other manner. In addition to the insights into magnetospheric physics offered by study of the solar wind-Mercury system, quantitative specification of the “external” magnetic field generated by magnetospheric currents is necessary for accurate determination of the strength and multipolar decomposition of Mercury’s intrinsic magnetic field. MESSENGER’S highly capable instrumentation and broad orbital coverage will greatly advance our understanding of both the origin of Mercury’s magnetic field and the acceleration of charged particles in small magnetospheres. In this article, we review what is known about Mercury’s magnetosphere and describe the MESSENGER science team’s strategy for obtaining answers to the outstanding science questions surrounding the interaction of the solar wind with Mercury and its small, but dynamic, magnetosphere.

Effects of continuous solar wind pressure variations on the long-lasting penetration of the interplanetary electric field during southward interplanetary magnetic field

It is believed that the interplanetary electric field (IEF) can penetrate into the mid- or low-latitude ionosphere. For the penetration of the IEF, the relationship between the solar wind pressure and the penetrated electric field in the equatorial ionosphere is yet an unsolved topic. With observations of the IEF, solar wind pressure obtained from the Wind satellite, and the geomagnetic field on four magnetometers from high latitudes to magnetic equator, this paper has present an event of a long-lasting penetration of the IEF into the dayside equatorial ionosphere during the main phase of a magnetic storm on April 17, 1999. In the event, variations of the geomagnetic field both at high latitudes and at magnetic equator show coherence with those of the solar wind pressure, indicating that a series of enhancements of solar wind pressures can drive those coherent increases of the penetrated electric field in dayside equatorial ionosphere during stable and southward interplanetary magnetic field (IMF). Therefore, not only the dawn-dusk component of the IEF, but also the solar wind pressure can strongly control the long-lasting penetration of the magnetospheric electric field into low-latitude or equatorial ionosphere during southward IMF. In addition, our result suggests that a combination of equatorial and low-latitude ground magnetograms may be used to monitor the solar wind dynamic pressure variation not only in the northward IMF, but also in the southward IMF.

Substorms during the 10–11 August 2000 sawtooth event

Sawtooth events have been identified at geosynchronous orbit as large‐amplitude quasiperiodic (2–4 hour period) modulations of the energetic electron and ion fluxes. They are called sawtooth events because the shape of the flux versus time profiles are composed of rapid increases followed by gradual decreases that resemble the teeth on a saw blade. Although much of the phenomenology associated with sawtooth events is substorm‐like, there is still debate as to whether the individual teeth are substorms or not. Here we examine each of the teeth associated with the 10–11 August 2000 sawtooth event in detail. We find that all but one of the teeth were associated with injections at geosynchronous orbit and that most of the teeth were consistent with the hypothesis that they are predominantly caused by unusually large and longitudinally extended substorms. A few were unclear or complex, and the final flux enhancement at 1845:36 UT was not a substorm but a solar wind shock‐associated disturbance. In addition, the presence of numerous dispersionless flux perturbations in the LANL SOPA data provides support for the hypothesis that solar wind pressure variations can modulate the flux profiles to some extent. For the substorm events we find that the geosynchronous particle injections were neither globally simultaneous nor globally dispersionless but were instead consistent with a nightside/duskside source in most cases. Similarly, we show that the field dipolarizations were also not global and simultaneous. Each of the substorms was also associated with high‐latitude negative H bays, middle‐ and low‐latitude positive H bays, a partial recovery in Sym‐H, and the onset of Pi2 ULF pulsations. In addition, we show that the auroral distribution develops in a systematic way during each cycle of a sawtooth substorm event. Specifically, a localized auroral onset develops on the lower branch of a thinned double‐oval distribution. The location of onset is typically premidnight and often occurs to the west of intense omega band forms. This is followed by westward, eastward, and poleward expansion and the copious production of auroral streamers which can develop in complex patterns including a “spoke‐like” morphology. The double‐oval configuration thins again during the stretching phase until the next onset occurs and the cycle repeats. A schematic representation of the auroral dynamics associated with sawtooth substorms is also presented.

Rotationally driven quasi‐periodic radio emissions in the Jovian magnetosphere

The occurrence characteristics of Jovian multiple quasi‐periodic (QP) bursts are investigated based on Galileo spacecraft observations. Multiple QP bursts recurrently appeared in a group with a planetary spin period of 10 hours. Their appearance was synchronized with the planetary rotation, i.e., they were preferably excited when Jupiter had a particular spin phase angle with respect to the Sun (at a subsolar longitude of System III around 260–320°). We also found that the burst groups were activated during magnetospheric disturbances probably caused by solar wind pressure variations. The rotationally driven polar disturbances accompanying quasi‐periodic particle accelerations and radio wave bursts are discussed as being one of the daily unloading processes of planetary rotational energy.

Long-period magnetic disturbances or Pc5 events aboard INTERBALL-Auroral and POLAR

This paper discusses observations of dynamic magnetic field variations on auroral field lines at mid and high altitudes, utilizing a conjunction of POLAR and INTERBALL-Auroral spacecraft in the dusk magnetosphere on January 11, 1997. Both spacecraft indicate the presence of periodic magnetic variations in the Pc5 frequency range in the field-aligned current region 1. The satellites are believed to be closely on the same field line or within a flux tube (radius 130 km of its projection in the ionosphere). The polarization analysis shows an interesting event: a rotation of the main polarization axis by about 90° between POLAR and INTERBALL. When the two spacecraft are very close to a common field line, the difference between their observed magnetic fields might arise from a rotation of the field between the two altitudes. We cannot exclude different polarization properties arising from different horizontal positions (although small) of both satellite with respect to local plasma boundary. Frequency analysis of both solar wind density (WIND) and INTERBALL magnetic field showed that the solar wind pressure variation contains the same spectrum (2.4–4.4 mHz). It appears that the solar wind pressure variations are sufficient for a parametric mechanism of excitation of compressional disturbances propagating along the magnetopause and within the magnetospheric cavity.

Estimates of magnetotail reconnection rate based on IMAGE FUV and EISCAT measurements

Abstract. Dayside merging between the interplanetary and terrestrial magnetic fields couples the solar wind electric field to the Earth's magnetosphere, increases the magnetospheric convection and results in efficient transport of solar wind energy into the magnetosphere. Subsequent reconnection of the lobe magnetic field in the magnetotail transports energy into the closed magnetic field region. Combining global imaging and ground-based radar measurements, we estimate the reconnection rate in the magnetotail during two days of an EISCAT campaign in November-December 2000. Global images from the IMAGE FUV system guide us to identify ionospheric signatures of the open-closed field line boundary observed by the two EISCAT radars in Tromsø (VHF) and on Svalbard (ESR). Continuous radar and optical monitoring of the open-closed field line boundary is used to determine the location, orientation and velocity of the open-closed boundary and the ion flow velocity perpendicular to this boundary. The magnetotail reconnection electric field is found to be a bursty process that oscillates between 0mV/m and 1mV/m with ~10-15min periods. These ULF oscillations are mainly due to the motion of the open-closed boundary. In situ measurements earthward of the reconnection site in the magnetotail by Geotail show similar oscillations in the duskward electric field. We also find that bursts of increased magnetotail reconnection do not necessarily have any associated auroral signatures. Finally, we find that the reconnection rate correlates poorly with the solar wind electric field. This indicates that the magnetotail reconnection is not directly driven, but is an internal magnetospheric process. Estimates of a coupling efficiency between the solar wind electric field and magnetotail reconnection only seem to be relevant as averages over long time intervals. The oscillation mode at 1mHz corresponds to the internal cavity mode with additional lower frequencies, 0.5 and 0.8mHz, that might be modulated by solar wind pressure variations.

Some aspects of the geomagnetic response to solar wind pressure variations: a case study at low and middle latitudes

Abstract. We examined geomagnetic field observations at low and middle latitudes in the Northern Hemisphere during a 50-min interval (12 May 1999), characterized by a complex behaviour of the solar wind dynamic pressure. For the entire interval, the aspects of the geomagnetic response can be organized into four groups of events which show common characteristics for the H and D components, respectively. The correspondence between the magnetospheric field and the ground components reveals different aspects of the geomagnetic response in different magnetic local time (MLT) sectors. For the H component, the correspondence is highly significant in the dusk and night sectors; in the dawn and prenoon sectors it shows a dramatic change across a separation line that extends approximately between (6 MLT, 35°) and (13 MLT, 60°). For the D component, the correspondence has significant values in the dawn and prenoon regions. We propose a new approach to the experimental data analysis which reveals that, at each station, the magnetospheric field has a close correspondence with the geomagnetic field projection along an axis (M1) that progressively rotates from north/south (night events) to east/west orientation (dawn events). When projected along M1, the geomagnetic signals can be interpreted in terms of a one-dimensional pattern that mostly reflects the field behaviour observed at geostationary orbit. Several features appear more evident in this perspective, and the global geomagnetic response to the SW pressure variations appears much clearer than in other representations. In particular, the MLT dependence of the geomagnetic response is much smaller than that one estimated by previous investigations. A clear latitudinal dependence emerges in the dusk sector. The occurrence of low frequency waves at ~2.8mHz can be interpreted in terms of global magnetospheric modes driven by the SW pulse. This event occurred in the recovery phase after the day the SW almost disappeared (11 May 1999): in this sense our results suggest a rapid recovery of almost typical magnetospheric conditions soon after a huge expansion. Overshoot amplitudes, greater than in other cases, are consistent with a significant reduction of the ring current.

Case study of solar wind pressure variations and neutral atom emissions observed by IMAGE/LENA

The Low Energy Neutral Atom (LENA) imager on the IMAGE spacecraft was designed to observe neutral atom emissions at low energy (10 eV–∼4 keV) from the high‐latitude ionosphere. In this study we present data from an ion outflow event on 24 June 2000 showing increases in neutral flux that appear to be associated with dynamic pressure variations in the solar wind. Results from a correlation analysis show the neutral flux to be well correlated (∼0.64) with the solar wind dynamic pressure. The neutral atom data are most consistent with a source region at a radial distance of 2.2–2.3 RE, interpreted as the region over which the bulk ion outflow escapes the ionosphere and is partly converted to neutral atoms. Very short time delays (∼2 min) were calculated between the solar wind pressure increase and neutral atom response, indicating a prompt ionospheric response to the solar wind input.

Pressure‐pulse interaction with the magnetosphere and ionosphere

We reexamine traveling convection vortices (TCVs) seen by the Magnetometer Array for Cusp and Cleft Studies on 9 November 1993. IMP‐8 energetic ion observations confirm that the solar wind pressure variations previously associated with these TCVs were generated by kinetic processes within the Earth's foreshock. As expected during this interval of spiral IMF orientation, fast mode waves launched by the pressure variations first arrived in the equatorial ionosphere near dusk and propagated dawnward. We derive a model for the field‐aligned currents generated by transient compressions of the magnetopause and show that it accounts for the number of TCVs seen in the prenoon ionosphere, their sense of rotation, the latitude at which they occur, and their absence in the postnoon ionosphere.

Geomagnetic field fluctuations during the passage at the Earth’s orbit of the tail of the 15–16 July 2000 ejecta

Abstract. In this work we present the analysis of the geomagnetic field fluctuations observed at different ground stations (approximately along two latitudinal arrays, separated by several hours in local time) during the passage at the Earth’s orbit of the tail of the 15–16 July 2000 coronal ejecta. The time interval of interest is characterized by northward interplanetary magnetic field conditions and several changes in the solar wind dynamic pressure. We found at all stations, both in the local morning and in the local evening, simultaneous and highly coherent waves at the same discrete frequencies (~ 1.8 and ~ 3.6 mHz) and suggest a possible interpretation in terms of global compressional modes driven by an impulsive variation of the solar wind pressure. Along the array situated in the morning sector, at the highest latitudes, the higher frequency mode seems to couple with the local field line resonance; on the other hand, along the array situated in the evening sector, the characteristics of the observed fluctuations suggest that the highest latitude station could be located at the footprint of open field lines. Our results also show that solar wind pressure variations observed during the recovery phase of the storm do not find correspondence in the geomagnetic field variations, regardless of local time and latitude; conversely, some hours later continuous solar wind pressure variations find a close correspondence in the geomagnetic field variations at all stations.Key words. Magnetospheric physics (solar wind-magnetosphere interaction; MHD waves and instabilities)

Effects of solar wind variations on the midlatitude ionosphere

It is known that the solar wind has significant effect on the magnetospheric convection and that magnetospheric electric fields can penetrate into the midlatitude and low‐latitude ionosphere. In this paper, we present observations of midlatitude ionospheric plasma perturbations during two events. In one event, the interplanetary magnetic field (IMF) oscillated between southward and northward with a period of ∼2 hours, while the solar wind pressure was relatively stable. In the other event the solar wind dynamic pressure oscillated with a period of ∼1 hour, while the IMF remained strongly northward and stable. In both events the Millstone Hill incoherent scatter radar detected quasiperiodic perturbations in all F region plasma parameters (electron density, ion velocity, electron temperature, and ion temperature) in the midlatitude dayside ionosphere. The midlatitude ionospheric perturbations were well correlated with the IMF or solar wind pressure variations. Near the F peak altitude the electron density could be increased or decreased by ∼30% within 30–70 min. We suggest that the midlatitude ionospheric electron density perturbations were caused by the penetration of magnetospheric electric fields which were controlled or modulated by the oscillations in the IMF or solar wind pressure.

Solar wind control of magnetospheric energy content: Substorm quenching and multiple onsets

In this paper we report coordinated multispacecraft and ground‐based observations of a double substorm onset close to Scandinavia on November 17, 1996. The Wind and the Geotail spacecraft, which were located in the solar wind and the subsolar magnetosheath, respectively, recorded two periods of southward directed interplanetary magnetic field (IMF). These periods were separated by a short northward IMF excursion associated with a solar wind pressure pulse, which compressed the magnetosphere to such a degree that Geotail for a short period was located outside the bow shock. The first period of southward IMF initiated a substorm growth phase, which was clearly detected by an array of ground‐based instrumentation and by Interball in the northern tail lobe. A first substorm onset occurred in close relation to the solar wind pressure pulse impinging on the magnetopause and almost simultaneously with the northward turning of the IMF. However, this substorm did not fully develop. In clear association with the expansion of the magnetosphere at the end of the pressure pulse, the auroral expansion was stopped, and the northern sky cleared. We will present evidence that the change in the solar wind dynamic pressure actively quenched the energy available for any further substorm expansion. Directly after this period, the magnetometer network detected signatures of a renewed substorm growth phase, which was initiated by the second southward turning of the IMF and which finally lead to a second, and this time complete, substorm intensification. We have used our multipoint observations in order to understand the solar wind control of the substorm onset and substorm quenching. The relative timings between the observations on the various satellites and on the ground were used to infer a possible causal relationship between the solar wind pressure variations and consequent substorm development. Furthermore, using a relatively simple algorithm to model the tail lobe field and the total tail flux, we show that there indeed exists a close relationship between the relaxation of a solar wind pressure pulse, the reduction of the tail lobe field, and the quenching of the initial substorm.

The Earth's passage of the April 11, 1997 coronal ejecta: geomagnetic field fluctuations at high and low latitude during northward interplanetary magnetic field conditions

Abstract. An analysis of the low frequency geomagnetic field fluctuations at an Antarctic (Terra Nova Bay) and a low latitude (L'Aquila, Italy) station during the Earth's passage of a coronal ejecta on April 11, 1997 shows that major solar wind pressure variations were followed at both stations by a high fluctuation level. During northward interplanetary magnetic field conditions and when Terra Nova Bay is close to the local geomagnetic noon, coherent fluctuations, at the same frequency (3.6 mHz) and with polarization characteristics indicating an antisunward propagation, were observed simultaneously at the two stations. An analysis of simultaneous measurements from geosynchronous satellites shows evidence for pulsations at approximately the same frequencies also in the magnetospheric field. The observed waves might then be interpreted as oscillation modes, triggered by an external stimulation, extending to a major portion of the Earth's magnetosphere. Key words. Magnetospheric physics (MHD waves and instabilities; solar wind-magnetosphere interactions)

Geomagnetic response at low latitude to continuous solar wind pressure variations during northward interplanetary magnetic field

A study of the geomagnetic field response to a series of continuous solar wind pressure variations has been performed at a low‐latitude station during interplanetary events characterized by stable northward magnetic field conditions. The geomagnetic field H component responds well to the solar wind pressure changes on a timescale of few minutes. The amplitude of the geomagnetic field response is found to depend on local time, showing greater values around local noon and midnight.

Ionospheric observations of magnetospheric low‐latitude boundary layer waves on August 4, 1991

We report on a coordinated set of observations from Greenland magnetometers, Sondrestrom incoherent scatter radar, DMSP F9 and F10 satellite data, and IMP 8 measurements of the interplanetary magnetic field (IMF) and solar wind plasma. These data were acquired during an interval (August 4, 1991, 1100 ‐ 1400 UT) which is characterized by magnetic ULF pulsations observed on the ground which occur in a limited band between 73° and 78° magnetic latitude. The spatial characteristics of the ground ULF magnetic pulsation disturbances are consistent with a continuous train of ionospheric convection vortices moving westward across local magnetic noon sector with a phase velocity of 1.3 km/s. The vortices occur at the ionospheric convection reversal boundary. Defense Meterological Satellite Program (DMSP) drift meter data are consistent with the incoherent scatter convection measurements, and DMSP particle data indicate that the ionospheric reversal boundary maps to the low‐latitude boundary. Using magnetometer stations across Greenland from the east coast, central Greenland, and the west coast, we can determine that individual vortices are long lived, lasting over 15 min. Cross correlations of the pulsation wave forms at stations separated longitudinally by about 500 km produce coefficients between 0.65 and 0.75. Cross correlations between stations separated longitudinally by 1000 km produce coefficients of only 0.39. This indicates that while individual vortices are long lived, they are evolving significantly in shape, latitudinal location, and intensity, as they move distances of 1000 km across the ionosphere. Using a field‐aligned current filament model to fit the data, we find the strength (50 to 100 kA) of these currents to be larger than typical currents previously reported. We find solar wind pressure variations an unlikely source for the generation of these pulsations and suggest that they are the ionospheric signature of either a Kelvin‐Helmholtz mechanism or perhaps magnetic merging during an interval of IMF oriented predominantly in the By direction.