C. J. Farrugia

Modeling ring current proton precipitation by electromagnetic ion cyclotron waves during the May 14–16, 1997, storm

We study mechanisms contributing to proton precipitation from the ring current during the May 14–16, 1997, geomagnetic storm. This storm was caused partly by Bz< 0 fields in the sheath region behind an interplanetary shock and partly by the magnetic cloud driving the shock. The storm was characterized by a maximum Kp=7− and a minimum Dst=−115 nT and had a distinctive two-phase decay related to the passage of the ejection at the Earth. We model the ring current development caused by adiabatic drifts and losses due to charge exchange, Coulomb collisions, wave-particle interactions, and atmospheric collisions at low altitudes. The nightside magnetospheric inflow is simulated using geosynchronous Los Alamos National Laboratory data, whereas the dayside free outflow corresponds to losses through the dayside magnetopause. We calculate the equatorial growth rate of electromagnetic ion cyclotron waves with frequencies between the oxygen and helium gyrofrequencies and their integrated wave gain as the storm progresses. The regions of maximum wave amplification compare reasonably well to satellite observations. A time-dependent global wave model is constructed, and the spatial and temporal evolution of precipitating proton fluxes during different storm phases is determined. We find that the global patterns of proton precipitation are very dynamic: located at larger L shells during prestorm conditions, moving to lower L shells as geomagnetic activity increases during storm main phase, and receding back toward larger L shells with storm recovery. However, the most intense fluxes are observed along the duskside plasmapause during the main and early recovery phase of the storm and are caused by plasma wave scattering. This study is relevant to the analysis of the anticipated new data sets from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) and Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) missions.

A magnetic cloud containing prominence material: January 1997

This work discusses the relations among (1) an interplanetary force-free magnetic cloud containing a plug of cold high-density material with unusual composition, (2) a coronal mass ejection (CME), (3) an eruptive prominence, and (4) a model of prominence material supported by a force-free magnetic flux rope in a coronal streamer. The magnetic cloud moved past the Wind spacecraft located in the solar wind upstream of Earth on January 10 and 11, 1997. The magnetic field configuration in the magnetic cloud was approximately a constant-α, force-free flux rope. The 4He++/H+ abundance in the most of the magnetic cloud was similar to that of the streamer belt material, suggesting an association between the magnetic cloud and a helmet streamer. A very cold region of exceptionally high density was detected at the rear of the magnetic cloud. This dense region had an unusual composition, including (1) a relatively high (10%) 4He++/He+ abundance (indicating a source near the photosphere), and (2) 4He+, with an abundance relative to 4He++ of ∼1%, and the unusual charge states of O5+ and Fe5+ (indicating a freezing-in temperature of (1.6–4.0) × 105 °K, which is unusually low, but consistent with that expected for prominence material). Thus we suggest that the high-density region might be prominence material. The CME was seen in the solar corona on January 6, 1997, by the large angle and spectrometric coronagraph (LASCO) instrument on SOHO shortly after an eruptive prominence. A helmet streamer was observed near the latitude of the eruptive prominence a quarter of a solar rotation before and after the eruptive prominence. These observations are consistent with recent models, including the conceptual model of Low and Hundhausen [1995] for a quasi-static helmet streamer containing a force-free flux rope which supports prominence material and the dynamical model of Wu et al. [1997] for CMEs produced by the disruption of such a configuration.

Global energy deposition during the January 1997 magnetic cloud event

The passage of an interplanetary magnetic cloud at Earth on January 10–11, 1997, induced significant geomagnetic disturbances, with a maximum AE in excess of 2000 nT and a minimum Dst of about −85 nT. We use a comprehensive set of data collected from space-borne instruments and from ground-based facilities to estimate the energy deposition associated with the three major magnetospheric sinks during the event. It is found that averaged over the 2-day period, the total magnetospheric energy deposition rate is about 400 GW, with 190 GW going into Joule heating rate, 120 GW into ring current injection, and 90 GW into auroral precipitation. By comparison, the average solar wind electromagnetic energy transfer rate as represented by the e parameter is estimated to be 460 GW, and the average available solar wind kinetic power USW is about 11,000 GW. A good linear correlation is found between the AE index and various ionospheric parameters such as the cross-polar-cap potential drop, hemisphere-integrated Joule heating rate, and hemisphere-integrated auroral precipitation. In the northern hemisphere where the data coverage is extensive, the proportionality factor is 0.06 kV/nT between the potential drop and AE, 0.25 GW/nT between Joule heating rate and AE, and 0.13 GW/nT between auroral precipitation and AE. However, different studies have resulted in different proportionality factors. One should therefore be cautious when using empirical formulas to estimate the ionospheric energy deposition. There is an evident saturation of the cross-polar-cap potential drop for large AE (>1000 nT), but further studies are needed to confirm this.

A study of an expanding interplanetary magnetic cloud and its interaction with the Earth's magnetosphere: The interplanetary aspect

In a series of three interlinked papers we present a study of an interplanetary magnetic cloud and its interaction with the Earths magnetosphere on January 14/15, 1988. This first paper is divided into three parts describing the principal results concerning the magnetic cloud. First, by applying the cylindrically symmetric, magnetic flux rope model to the high time resolution magnetic field and plasma data obtained by the IMP-8 spacecraft, we show that the axis of the magnetic cloud in question is approximately in the ecliptic and orthogonal to the Earth-Sun line. We note the presence of pulsations of ∼5-hour period in the bulk flow speed which are superimposed on an otherwise monotonically falling bulk speed profile. Second, we apply ideal MHD to model the self-similar, radial expansion of a magnetic cloud of cylindrical geometry. As initial condition for the magnetic field we choose a constant-α, force-free magnetic configuration. We demonstrate that the theoretical velocity profile for the free expansion of a magnetic cloud is consistent with observations made during the January 14/15, 1988, magnetic cloud encounter. Comparing model with data, we infer that prior to the start of observations at 1 AU the magnetic cloud had been expanding for 65.4 hours; the radius of the magnetic cloud at the time it arrived at Earth was 0.18 AU; and its expansion speed at 1 AU was ∼114 km/s. Third, we discuss energetic (∼1 MeV) ion data, also from instrumentation on IMP-8. We highlight the appearance of a sharp enhancement in the intensity of ∼0.5-MeV ions while IMP-8 was inside the cloud. These ions travel as a collimated, field-aligned beam from the west of the Sun. This is an “impulsive” solar event in which particles accelerated at a magnetically well-connected solar flare arrive promptly at the spacecraft. The observation of solar flare particles inside the cloud suggests that field lines within the magnetic cloud remained connected to the Sun. The observation is, however, inconsistent with the supposition that the cloud is formed of closed magnetic field loops disconnected from the Sun.

The Wind magnetic cloud and events of October 18–20, 1995: Interplanetary properties and as triggers for geomagnetic activity

Late on October 18, 1995, a magnetic cloud arrived at the Wind spacecraft ≈ 175 RE upstream of the Earth. The cloud had an intense interplanetary magnetic field that varied slowly in direction, from being strongly southward to strongly northward during its ≈ 30 hours duration, and a low proton temperature throughout. From a linear force free field model the cloud was shown to have a flux rope magnetic field line geometry, an estimated diameter of about 0.27 AU, and an axis that was aligned with the Y axis(GSE) within about 25°. A corotating stream, in which large amplitude Alfven waves of about 0.5 hour period were observed, was overtaking the cloud and intensifying the fields in the rear of the cloud. The prolonged southward magnetic field observed in the early part of the cloud produced a geomagnetic storm of Kp = 7 and considerable auroral activity late on October 18. About 8 hours in front of the cloud an interplanetary shock occurred. About three-fourths the way into the cloud another apparent interplanetary shock was observed. It had an unusual propagation direction, differing by only 21° from alignment with the cloud axis. It may have been the result of the interaction with the postcloud stream, compressing the cloud, or was possibly due to an independent solar event. It is shown that the front and rear boundaries of the cloud and the upstream driven shock had surface normals in good agreement with the cloud axis in the ecliptic plane. The integrated Poynting flux into the magnetosphere, which correlated well with geomagnetic indices, jumped abruptly to a high value upon entry into the magnetic cloud, slowly decreased to zero near its middle, and again reached substantial but sporadic values in the cloud-stream interface region. This report aims to support a variety of ISTP studies ranging from the solar origins of these events to resulting magnetospheric responses.

October 1995 magnetic cloud and accompanying storm activity: Ring current evolution

The passage at Earth of the October 1995 magnetic cloud and the high-speed corotating stream overtaking it, monitored by the Global Geospace Science (GGS) spacecraft Wind, caused two consecutive geomagnetic storms: a major one during the strong Bz < 0 nT phase of cloud passage and a moderate one during the intermittent Bz < 0 activity in the fast corotating stream. Large dynamic pressure changes were observed in the sheath region ahead of the cloud and in the cloud-stream interface region at its rear, resulting in substantial corrections to the measured Dst index. A burst of superdense plasma sheet extending over ∼2 hours in local time was observed at geostationary orbit during the second storm. We simulate the ring current development during this storm period using our kinetic model and calculate the magnetic field perturbation caused by the ring current. The plasma inflow on the nightside is modeled throughout the investigated period using data measured at geosynchronous orbit. The modeled Dst index is compared with the observed Dst values corrected for magnetopause and telluric currents. The temporal evolution of the ring current H+ and O+ distribution functions is computed, considering losses due to charge exchange, Coulomb collisions, and ion precipitation. We find that (1) the storm time enhancement of the plasma sheet ion population contributed significantly to the ring current buildup; (2) an additional ∼12 nT decrease in Dst is achieved when the symmetry line of the plasma convection paths is rotated eastward from the dawn-dusk direction with 3 hours during the first storm; (3) the major loss process is charge exchange, followed by Coulomb collisions and ion precipitation; (4) however, the energy losses due to ion precipitation increase monotonically during the more active periods, reaching the level of Coulomb losses at peak storm intensity. We argue that the losses due to ion precipitation considered in this study are closely related to the enhanced convection electric field, which in our model is parameterized with the planetary Kp index. Correspondingly, we find that (5) there is a very good correlation between the variations in time of this index and the magnitude of the ion precipitation losses.

Magnetic flux rope versus the Spheromak as models for interplanetary magnetic clouds

Magnetic clouds form a subset of interplanetary ejecta with well-defined magnetic and thermodynamic properties. Observationally, it is well established that magnetic clouds expand as they propagate antisunward. The aim of this paper is to compare and contrast two models which have been proposed for the global magnetic field line topology of magnetic clouds: a magnetic flux tube geometry, on the one hand, and a spheromak geometry (including possible higher multiples), on the other. Traditionally, the magnetic structure of magnetic clouds has been modeled by force-free configurations. In a first step, we therefore analyze the ability of static force-free models to account for the asymmetries observed in the magnetic field profiles of magnetic clouds. For a cylindrical flux tube the magnetic field remains symmetric about closest approach to the magnetic axis on all spacecraft orbits intersecting it, whereas in a spheromak geometry one can have asymmetries in the magnetic field signatures along some spacecraft trajectories. The duration of typical magnetic cloud encounters at 1 AU (1 to 2 days) is comparable to their travel time from the Sun to 1 AU and thus magnetic clouds should be treated as strongly nonstationary objects. In a second step, therefore, we abandon the static approach and model magnetic clouds as self-similarly evolving MHD configurations. In our theory, the interaction of the expanding magnetic cloud with the ambient plasma is taken into account by a drag force proportional to the density and the velocity of expansion. Solving rigorously the full set of MHD equations, we demonstrate that the asymmetry in the magnetic signature may arise solely as a result of expansion. Using asymptotic solutions of the MHD equations, we least squares fit both theoretical models to interplanetary data. We find that while the central part of the magnetic cloud is adequately described by both models, the “edges” of the cloud data are modeled better by the magnetic flux tube. Further comparisons of the two models necessarily involve thermodynamic properties, since real magnetic configurations are never exactly force-free and gas pressure plays an essential role. We consider a poly tropic gas. Our theoretical analysis shows that the self-similar expansion of a magnetic flux tube requires the poly tropic index γ to be less than unity. For the spheromak, however, self-similar, radially expanding solutions are known only for γ equal to 4/3. This difference, therefore, yields a good way of distinguishing between the two geometries. It has been shown recently (Osherovich et al., 1993a) that the polytropic relationship is applicable to magnetic clouds and that the corresponding polytropic index is ∼0.5. This observational result is consistent with the self-similar model of the magnetic flux rope but is in conflict with the self-similar spheromak model.

The interaction of a magnetic cloud with the Earth - Ionospheric convection in the Northern and Southern Hemispheres for a wide range of quasi-steady interplanetary magnetic field conditions

This is the second of three papers which study a large interplanetary magnetic cloud, and its interaction with the earths magnetosphere. Here the authors study flows within the ionosphere during the passage of the magnetic cloud on Jan 13-15, 1988. This is the first study of ionospheric convections during prolonged periods of stable and different IMF orientations, which result from the stable, but spatially varying field structure within the magnetic cloud. Data from IMP-8 and DMSP-F8 are analyzed for this work. This observation gave information on ionospheric responses to greater than 10 hour period of northward and southward IMF, with a gradual change from one to the other. Issues studied included strengths of peak flows for north and south IMF; changes in cross polar cap potential with IMF B[sub z]; types and variations of convective patterns vs IMF; variations in size of the polar cap; etc.

Anomalous magnetosheath properties during Earth passage of an interplanetary magnetic cloud

The aim of this paper is to model for the first time the variation of field and flow parameters in the magnetosheath during Earth passage of an interplanetary magnetic cloud. Under typical Solar wind conditions, magnetohydrodynamic (MHD) effects on the flow of plasma in the terrestrial magnetosheath are important only in a layer adjacent to the magnetopause which is a few thousand kilometers thick (“depletion layer” or “magnetic barrier”). During the passage of an interplanetary magnetic cloud, however, conditions upstream of the bow shock depart strongly from the norm. In this case, interplanetary parameters vary slowly over a wide range of values. Values of the upstream Alfven Mach number are much lower than those otherwise sampled (∼3 versus 8–10). Together with the magnetic shear across the magnetopause, this parameter plays a central role in determining the structure of the magnetosheath close to the magnetopause. As a consequence of sustained low values of the upstream Alfven Mach number, the magnetic field exerts a strong influence on the flow over a very substantial fraction of the magnetosheath throughout the duration of cloud passage, i.e., for a time period of the order of 1–2 days. We apply an algorithm to integrate the ideal MHD equations, using a boundary layer technique, and compute the variations of field and flow parameters along the stagnation streamline. We choose as our example the magnetic cloud which passed Earth on January 14–15, 1988. The interaction of this cloud with the magnetosphere, as regards the resulting ionospheric flow patterns and the substorm activity, has been the subject of various investigations. Using information from these studies, we obtain results on the magnetosheath when the magnetopause is modeled, first as a tangential discontinuity and then as a rotational discontinuity. Our results are in good general agreement with recent observations on the behavior of field and flow quantities in the magnetosheath region adjacent to the magnetopause. In addition, we predict the existence of a magnetic barrier when the upstream Alfven Mach number is low, irrespective of the magnetic shear across the magnetopause.

Effects of inner magnetospheric convection on ring current dynamics: March 10–12, 1998

A stream-stream interaction region observed by the Wind spacecraft from ∼1200 UT to ∼2400 UT, March 10, 1998, triggered a major geomagnetic storm which peaked within a few hours at Dst≈−130 nT and Kp=7+. During the main phase of this storm the north–south component of the interplanetary magnetic field (IMF) was large and negative (Bz≈−l5 nT), the solar wind velocity was ∼550 km/s, and the solar wind dynamic pressure increased to ∼10 nPa. We simulated the storm time injection and trapping of H+, O+, and He+ ring current ions using our global drift-loss model with initial and boundary conditions as specified by measurements from the Equator-S ion composition (ESIC) instrument, the HYDRA instrument on Polar, and the hot plasma instruments on geosynchronous spacecraft. We demonstrated effects of magnetospheric convection, comparing results derived from two inner magnetospheric convection models: (1) the 3 hour averaged Kp-dependent Volland-Stern model and (2) the Weimer [1996] IMF-driven model, where we input interplanetary data from the Magnetic Field Investigation (MFI) and Solar Wind Experiment (SWE) instruments on Wind at 30 min resolution. During the main phase of the storm the Volland-Stern model predicted a large-scale electric field of ∼1 mV/m at L=2 to L=5, whereas the Weimer model predicted a maximum electric field of ∼3.5 mV/m localized near dusk at L≈3. We found that both ring current simulations show reasonable agreement with ESIC and HYDRA data at larger L shells and on the nightside. However, the simulation using a Volland-Stern model predicted wider dips in the ion energy spectra than observed at low L shells in the postnoon local time sector. Ions followed paths at larger distances from Earth and experienced less collisional losses in the Weimer convection model. As a result, the agreement between model and data was significantly improved on the dayside when the IMF-driven convection model of Weimer was used.