C. G. Maclennan

Cusp latitude magnetic impulse events: 1. Occurrence statistics

Magnetic impulse events were selected by a computer algorithm procedure from magnetic records obtained at the near cusp latitude conjugate stations Iqaluit, Northwest Territories, Canada, and South Pole Station, Antarctica. The algorithm was constructed to select large (≳ 50 nT in the vertical component of the magnetic field), short lived (6 to 12 min) events. These events were found to be highly localized in the 06 to 18 LT sector at the two stations. A strong minimum in occurrence was found during hour 11 LT. The field changes associated with these events can be interpreted as due to an approximately half-cycle, odd-mode, Alfven wave along a near-magnetopause flux tube. From the vertical magnetic deflections of the impulse events the directions of field-aligned currents into the conjugate ionospheres were inferred. In the morning LT sector, field-aligned currents were directed into the ionospheres, while in the afternoon LT sector, field-aligned currents were directed out of the ionospheres. These findings are comparable with the statistical results for quasi-stationary field-aligned currents and suggest that at the times of these events, Iqaluit and South Pole are at a higher effective magnetic latitude. The average deflection in the vertical component for the events was measured to be ∼ 95 nT. From this the magnitude of the average field-aligned currents was calculated to be J∥ ∼ 2 × 10−7 A/m².

Seasonal and diurnal variations of the latitude of the westward auroral electrojet in the nightside polar cap

Simultaneous measurements of magnetic field variations at the nominally conjugate sites of South Pole station, Antarctica, and Iqaluit, Canada, have been used to study latitude differences in the location of corresponding westward electrojets in the nightside polar cap. Substorm-related auroral events were selected from 1986 data with the criterion that the magnitude of the negative bay exceed 100 nT in at least one hemisphere and that the electrojet, oriented mainly in the magnetic east–west direction, pass overhead of this station. The resulting data base comprised 60 events distributed seasonally as follows: austral summer (17); fall (6); winter (18); spring (19). The interhemispheric latitude difference in the locations of the electrojets was obtained by examining the ratio of the vertical and horizontal component magnetic variations. Seasonal and diurnal variations of the latitude of westward electrojets were evident during solstice periods. In particular, it is noted that the substorm westward electrojet flows at higher latitudes in the winter hemisphere than in the summer hemisphere, after 2000 MLT. The latitude difference between the two hemispheres maximizes at ∼4° near local midnight. The latitude difference becomes smaller away from midnight and reverses for local times prior to ∼2000 MLT. The observed seasonal and diurnal variations are generally consistent with predictions of the effect of the magnetic dipole tilt on the location of last closed field lines. However, because on the nightside the field lines originating from South Pole station and Iqaluit reach deep into the magnetotail, they are likely to be influenced by the plasma sheet. Thus the implications of an asymmetric distribution of the plasma sheet with respect to the neutral sheet have also been considered. Such a postulated distribution can account for the seasonal variations while conserving magnetic flux and maintaining pressure balance across the tail.

Penetration of hydromagnetic energy deep into the magnetosphere

Magnetometer data, acquired on spacecraft and simultaneously at high and low latitudes on the ground, are compared in order to study the propagation characteristics of hydromagnetic energy deep into the magnetosphere. Single events provide evidence that wave energy at L ∼ 3 can at times be only one order of magnitude lower than at L ∼ 13. In addition, statistical analyses of the H-component groundbased data obtained during local daytime hours of 17 July-3 August 1985 show that wave amplitudes at L ∼ 3 are generally 10-30 times lower than at L ∼ 13. The L-dependence of near-equator magnetic field fluctuations measured on ISEE-2 show a sharp drop in energy near the magnetopause and a more gradual fall-off of energy deeper inside the magnetosphere. Such high levels of wave power deep in the magnetosphere have not been quantitatively understood previously. Our initial attempt is to calculate the decay length of an evanescent wave generated at a thick magnetopause boundary. Numerical calculations show that fast magnetosonic modes (called magnetopause and inner mode) can be generated under very restrictive conditions for the field and plasma parameters. These fast compressional modes may have their energy reduced by only one order of magnitude over a penetration depth of about 8RE. More realistic numerical simulations need to be carried out to see whether better agreement with the data can be attained.

Hydromagnetic field line resonances and modulation of particle precipitation

Abstract Hydromagnetic wave and modulated particle precipitation data are reported from conjugate areas near the particledrift shell L ∼ 4. A modulation of electrons precipitating from the magnetosphere is observed in the conjugate regions when the accompanying hydromagnetic wave period is ~ 90 s and the wave polarization is linear. When the wave period changes abruptly to ~ 30 s and the polarizations at the observing stations are no longer linear, the modulation of the precipitating electrons is no longer observed. The change in hydromagnetic wave characteristics does not appear to be related to interplanetary plasma and magnetic field conditions. Rather, it is proposed to arise from a change in the wave generation mechanism from an internal magnetospheric source near the inner edge of the plasmapause (lower frequency) to an externally driven source outside the magnetosphere (higher frequency). This observation of a change in the wave characteristics (frequency and polarization) associated with modulated electron precipitation appears to be related to two previous examples wherein modulated electron precipitation was reported to be closely associated with the existence of a wave resonance region near the observing site.

Remarks on the intrinsic timescale for reconnection on the dayside magnetopause

We show that with reasonable values of anomalous resistivity, surface-wave-induced magnetic reconnection at an interface between two plasma regions (such as the magnetosheath and the magnetopause) has an intrinsic timescale that can explain observations of a delay time between the southward turning of the interplanetary magnetic field and the onset of a flow transfer event at the magnetopause.

Hydromagnetic (ULF) power at separated sites (ΔΦ ∼ 90°) at mid- to low-latitudes

Abstract The power spectra of geomagnetic field fluctuations in the hydromagnetic wave band of periods 20–600 s have been systematically evaluated from data acquired at LAquila, Italy (geomagnetic invariant longitude Φ = 94.4°; invariant latitude Λ ∼ 42.5°; L ∼ 1.6) and Green Hill, R.I. (Φ = 5°; Λ ∼ 52°; L ∼ 2.8) during July 1985. The average levels of the ULF power in different frequency bands have been compared. Autocorrelations of power levels for the data at the two stations show a clear 24 h recurrence period at LAquila in the bands 20–30 s and 30–45 s, while the same periodicity is clearly observed at Green Hill for the 30–45 s and 45–150 s bands. The results of the cross correlation analysis of the data from the two stations indicate that there is a clear U.T. dependence for geomagnetic power for periods between 45 and 600 s, suggesting global source influences on the power levels. The power in the period band 30–45 s appears to be better correlated on a L.T. basis suggesting a similar diurnal pattern at the two stations of the 24 h modulation in this range of frequency.