J. F. Watermann

Predicting interplanetary magnetic field (IMF) propagation delay times using the minimum variance technique

[1]xa0It has been known that the fluctuations in the interplanetary magnetic field (IMF) may be oriented in approximately planar structures that are tilted with respect to the solar wind propagation direction along the Sun-Earth line. This tilting causes the IMF propagating from a point of measurement to arrive at other locations with a timing that may be significantly different from what would be expected. The differences between expected and actual arrival times may exceed an hour, and the tilt angles and subsequent delays may have substantial changes in just a few minutes. A consequence of the tilting of phase planes is that predictions of the effects of the IMF at the Earth, on the basis of IMF measurements far upstream in the solar wind, will suffer from reduced accuracy in the timing of events. It has recently been shown how the tilt angles may be determined using multiple satellite measurements. However, since the multiple satellite technique cannot be used with real-time data from a single sentry satellite, then an alternative method is required to derive the phase front angles, which can then be used for more accurate predictions. In this paper we show that the minimum variance analysis (MVA) technique can be used to adequately determine the variable tilt of the plane of propagation. The number of points that is required to compute the variance matrix has been found to be much higher than expected, corresponding to a time period in the range of 7 to 30 min. The optimal parameters for the MVA were determined by a comparison of simultaneous IMF measurements from four satellites. With use of the optimized parameters it is shown that the MVA method performs reasonably well for predicting the actual time lags in the propagation between multiple spacecraft, as well as to the Earth. Application of this technique can correct for errors, on the order of 30 min or more, in the timing of predictions of geomagnetic effects on the ground.

Pressure‐pulse interaction with the magnetosphere and ionosphere

[1]xa0We 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 Earths 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 negative sudden impulses: Interplanetary causes and polarization distribution

[1]xa0We made a study of the characteristics of geomagnetic negative sudden impulses (SI−s) identified in the midlatitude geomagnetic SYM indices and the causative structures in the solar wind using data from the Wind and ACE spacecraft. A total of 28 SI−s with an amplitude larger than 20 nT in the H component SYM index were found over the period 1995 through 1999, with 50% of them occurring in conjunction with a positive sudden impulse, SI+ (i.e., SI pair). In the SI pairs the amplitude of SI− was almost always larger than that of the preceding SI+. We attempted for the first time a classification of structures in the solar wind associated with SI−s. It is found that reverse shocks are not responsible for SI−s. Instead, SI−s are associated with varied structures such as tangential discontinuities at high-low speed stream interfaces, front boundaries of interplanetary magnetic clouds, and trailing edges of heliospheric plasma sheets. There is no preferential association of SI−s in our sample with any particular type of solar wind structure. We investigated statistically the polarization characteristics of SI−s at high latitude. The sense of the polarization in the auroral zone tended to be clockwise in the afternoon and counterclockwise in the morning. The rotational sense reversed in the polar cap. The latitudinal reversal occurred in the range from 65° to 80°. Thus the polarization distribution of SI− is not opposite to but is consistent with that of SI+. We suggest that the contribution from the longitudinal movement of a twin vortex ionospheric current system is dominant to produce the polarization of SC and SI−.

A traveling convection vortex event study: Instantaneous ionospheric equivalent currents, estimation of field‐aligned currents, and the role of induced currents

[1]xa0We analyze a traveling convection vortex (TCV) event on 31 January 1997 using ground magnetometer data of the CANOPUS, MACCS, Geological Survey of Canada, Greenland, and IMAGE networks. For the first time, spatial and instantaneous distributions of the mesoscale ionospheric equivalent currents associated with a TCV are obtained. We apply the method of spherical elementary currents (SECS) to calculate these currents, as well as to infer the part of the ground magnetic signatures that is caused by internal currents induced in the Earth. The resulting ionospheric equivalent currents consist of a leading clockwise vortex centered at 71° CGM latitude which moves westward with ∼7.3 km s−1 and a trailing anticlockwise vortex at 77 ° latitude which moves southwestward away from noon at ∼3.0 km s−1. In an area of ∼200 km around the center of the twin vortices the derived equivalent current densities are less than 70 mA m−1 but reach 100–160 mA m−1 in a broad channel of equatorward currents between the vortices. Using the assumption of a uniform Hall to Pedersen conductance ratio and the assumption that conductance gradients perpendicular to the ionospheric electric field are vanishing, we can estimate the field-aligned current (FAC) associated with the TCV. The maxima of ∼1 μA m−2 of downward FAC in the leading western vortex, and of ∼0.4 μA m−2 of upward FAC in the trailing eastern vortex occur along the perimeter of the vortices, not in their centers, in contrast to the prediction of ionosphere-magnetosphere coupling theories for TCVs. The integrated FAC are not fully balanced between the two vortices but show an excess of downward FAC. While the ratio between the internally generated and the total horizontal ground magnetic field for most magnetometer sites in the TCV area amounts to around 20–40%, at some stations it can reach values of 50% and larger.

Field-aligned currents in the dayside cusp and polar cap region during northward IMF

[1]xa0The field-aligned currents in the dayside cusp and polar cap region are examined using magnetic data from the low-altitude polar-orbiting satellite Orsted. The study is confined to cases where the interplanetary magnetic field (IMF) has a steady northward component and to a rather narrow region spanning ∼4 hours around magnetic noon. We examine individual passes using a maximum variance analysis method, and we complement, for a single event, with ground-based data from the Greenland meridian chain of magnetometers. We suggest that when an east-west component By of the IMF exists for positive IMF Bz, the two NBZ (northward Bz) field-aligned currents that prevail over the polar region rotate to form the two field-aligned currents equatorward and poleward of the east-west flowing ionospheric DPY current in the dayside. The high accuracy of the Orsted data makes it possible to uncover details not previously described.

Modeling of equivalent ionospheric currents from meridian magnetometer chain data

In recent years, quantitative analysis of the magnetosphere-ionosphere coupling and electrodynamics of the polar ionosphere received much attention. Though remarkable progress has been made in this field by using a variety of magnetogram inversion techniques in order to infer the global ionospheric current distribution, there is still a need for modeling ionospheric currents locally, over a certain region, for comparison with other geophysical ground-based and satellite observations. This paper presents a simple method for estimating equivalent ionospheric currents using magnetic field observations along a meridian chain of ground-based vector magnetometers. The method can be applied in an automatic fashion to any available magnetometer chain data, for example, from the DMI Greenland west coast chain. We first describe how we separate contributions to the observed geomagnetic variations from external (ionospheric) and internal (induced) sources. We then model the ionospheric electrojet by a sequence of narrow current strips and apply the Biot-Savart law to formulate an inversion problem. Using a regularization technique, we find a stable distribution of the equivalent ionospheric currents crossing the magnetometer chain in eastward and westward direction. Simulation tests and a case study (20 March 1999) are discussed in order to illustrate properties of the solution to the inverse problem and to present a practical tool, which is accessible through the DMI World Wide Web site.

Observations of two complete substorm cycles during the Cassini Earth swing-by: Cassini magnetometer data in a global context

During the Earth swing-by of the Cassini spacecraft, a worldwide program of data-gathering was undertaken to define the prevailing interplanetary and geophysical conditions. This included observations of the interplanetary medium, outer magnetosphere, geostationary orbit, UV aurora, geomagnetic disturbance, and ionospheric flow. These data show that during the Cassini outbound passage through the geomagnetic tail the magnetosphere underwent two complete “classic” substorm cycles. The global data are used to set the Cassini data into a context which allows a much fuller interpretation. The pass took place through the dawn tail, where previous Geotail observations indicate that the plasma sheet usually remains ‘stationary’ at expansion phase onset. Reconnection and plasmoid formation are typically dusk and midnight phenomena. The Cassini observations nevertheless show a marked response of the plasma sheet both to the growth phase (thinning), and expansion phase onset (expansion without heating). Subsequent impulsive expansion of the substorm into the Cassini sector, however, resulted in the prompt appearance of reconnection-related phenomena (earthward flowing plasma and strongly disturbed fields), though the spacecraft footprint remained poleward of the intense UV auroras. Due to continuing strong southward-directed IMF during the expansion phases, a quasi-equilibrium appears to have formed between dayside and near-Earth nightside reconnection for ∼30 min after onset. Auroral zone recovery began about ∼10 min after a northward turn of the interplanetary magnetic field (IMF) to nearer-zero values in each case. A net closure of open flux then ensued, leading to deflation of the tail lobe field, ‘dipolarization’ of the near-Earth tail plasma sheet field, simultaneous reduction in the earthward plasma sheet flow and the flow in the nightside ionosphere, and displacement of the ground-based disturbance to high latitudes.

IMF By‐related cusp currents observed from the Ørsted satellite and from ground

Orsted is the first satellite to conduct high-precision magnetometer observations from low-altitude noon-midnight orbits passing through the polar cusp regions. Field-aligned currents (FAC) derived from Orsted magnetic field measurements have been combined with ionospheric current patterns inferred from ground-based magnetic observations to define the structure and location of cusp currents and their dependencies on interplanetary magnetic field (IMF) conditions. Example cases illustrate the close relation between IMF By-related FAC and horizontal ionospheric currents in the cusp region. Our statistical analysis defines for the noon region the variations in FAC latitude with IMF Bz. Comparisons with the statistical cusp location indicate that the more equatorward region of IMF By-related FAC is located on field lines closing at the dayside, while the more poleward FAC are on “open” field lines. High-energy electron measurements from the satellite confirm this result.

Ionospheric footprint of magnetosheathlike particle precipitation observed by an incoherent scatter radar

The authors have examined Sondrestrom incoherent scatter radar observations of ionospheric plasma density and temperature distributions and measurements of F region ion drifts that were made during a prenoon pass of the DMSP-F7 satellite through the radar field of view. The spacecraft traversed a region of intense electron precipitation with a characteristic energy below approximately 200 eV. Particles with such low characteristic energies are believed to be directly or indirectly of magnetosheath origin. The precipitation region had a width of about 2 deg invariant latitude and covered the low-latitude boundary layer (LLBL), the cusp, and the equatorward section of the plasma mantle (PM). The corotating radar observed a patch of enhanced electron density and elevated electron temperature in the F2 region between about 10.5 and 12 magnetic local time in the same invariant latitude range where DMSP-F7 detected the soft-electron flux. The ion drift pattern, also obtained by radar, shows that it is unlikely that the plasma patch was produced by solar radiation and advected into the radar field of view. The authors suggest that the radar observed modifications of the ionospheric plasma distribution, which resulted from direct entry of magnetosheath electrons into the magnetosphere and down to ionospheric altitudes.morexa0» Model calculations of the ionospheric response to the observed electron precipitation support this interpretation. The spectral characteristics of the electron flux in the LLBL, cusp, and equatorward section of the PM were in this case too similar to allow to distinguish between them by using incoherent scatter radar measurements only.«xa0less

Magnetospheric signature of an ionospheric traveling convection vortex event

[1]xa0We analyze Geotail observations of magnetic field and flow oscillations in the dayside magnetosphere that occurred in direct association with an event of traveling convection vortices observed in the northern high-latitude ionosphere. Geotail was positioned on a magnetic flux tube that mapped very close to the centers of the driving field-aligned currents in the ionosphere. These are the first in situ observations from the source region of such events. The flow signature is seen clearly in the cold, dense plasma population, which the satellite encounters during the event, in addition to a hot magnetospheric (plasma sheet-like) population. This report will present the observations and briefly discuss their implications for our understanding of the traveling convection vortex (TCV) signature. We interpret the event in terms of a boundary wave at the magnetopause. Our analysis suggests that strong gradients in the magnetic field strength or plasma density in the inner magnetosphere should play an important role in this process.