Cusp Latitudes Research Articles

Cusp latitude magnetic impulse events: 2. Interplanetary magnetic field and solar wind conditions

The interplanetary magnetic field (IMF) conditions and solar wind plasma parameters prevailing during the magnetic impulse events identified by Lanzerotti et al. (1991) at the near cusp latitude stations Iqaluit, Northwest Territories, Canada, and South Pole Station, Antarctica, are examined. The impulse events are found to occur during periods of high IMF variability. The prevailing IMF orientation, averaged over 11‐min periods during the events, shows no distinct anisotropies. However, by examining the IMF orientation when the IMF variability is small a south‐east anisotropy is found. Examining the IMF orientation on timescales of less than 1 min reveals that the impulse events are characterized by a more southward oriented IMF than is to be expected statistically. The duration of the events is found to be controlled by both solar wind speed and by the IMF By. The IMF By control of the duration of nonconjugate events is in accordance with the effect expected from the presence of magnetic tension in a reconnection setting. The amplitude of the events is found to be strongly related to solar wind speed. These findings suggest that sporadic reconnection is among the mechanisms producing the events. On the basis of the amplitudes of the impulse events, it is argued that the fractional changes in dynamic pressure must be of the order of 1 to be able to associate the impulse events with changes in the dynamic pressure. Such variability in the dynamic pressure at the time of the magnetic impulse events is not observed. It is concluded that a change in the solar wind dynamic pressure, while not among the primary mechanisms generating the magnetic impulse events, may account for between 15% and 30% of them. On the other hand, reconnection processes are able to produce impulse events of the required magnitude. It is argued then that magnetic reconnection is likely responsible for generating a minimum of 50%‐70% to a maximum of 90% of the events. However, this conclusion leaves parts of the picture unclear. Lanzerotti et al. (1991) found that magnetic impulse events with a unipolar deflection in the vertical component of the magnetic field possessed a local time dependence; positive deflections occurred in local morning, while negative deflections occurred in local afternoon. This dependence can be explained by postulating that the magnetic impulse events are associated with vortices generated by the Kelvin‐Helmholtz instability in the low‐latitude boundary layer. As such, it is suggested that the Kelvin‐Helmholtz instability may mediate the process by which sporadic reconnection produces a ground signature.

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, Alfvén 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.

A conjugate area study of Pc 3 pulsations near cusp latitudes

Pc 3 pulsations are examined from conjugate studies at Iqaluit (Frobisher Bay), Northwest Territories, Canada, and at the geographic south pole, Antarctica (L ∼ 14). These Pc 3 (f ∼ 20–50 mHz) events are quantitatively selected from hourly power spectra of the H component magnetometer time series for local daytime hours. Above a cutoff frequency of 29 mHz we find that the frequency of these events has a linear relationship with hourly averages of the interplanetary magnetic field (IMF) magnitude as measured by the IMP 8 spacecraft while in the upstream region. These events are narrow‐banded and occur mostly in local morning and for IMF conditions with Bx > 0, By < 0, and Bz > 0. Further, the frequency of the selected events is found to be independent of the IMF below the 29‐mHz cutoff. These characteristics of Pc 3 agree well with our previously reported Pc 3 events observed at South Pole (Slawinski et al., 1988). However, few simultaneous events are observed at Iqaluit and South Pole, indicating a lack of conjugacy on an event by event basis. We conclude that upstream ion cyclotron waves are a source of these Pc 3 events above 29‐mHz frequencies in both hemispheres. We speculate that the lack of conjugacy may be due to the station(s) being on open field lines during these events or that field line resonance may not be excited at Pc 3 frequencies for such high latitudes.

Resonant absorption of Alfven Waves and the Associated Phenomenon of Magnetic Reconnection

The mathematical analysis of the Alfven Wave equation in inhomogeneous magnetic fields which explain the resonance absorption of Alfven surface waves near a resonant layer can also be used to show that magnetic reconnection process can arise near the zero frequency resonant layer driven by very low frequency Alfven surface waves. The associated phenomena of resonant absorption and magnetic reconnection in inhomogeneous plasmas can explain the recent obsrevations of intense magnetic activity in the long period geomagnetic micropulsations range, at magnetospheric cusp latitudes, during the time of occurence of flux transfer events.

A user's guide to locating flux transfer events in low‐altitude satellite measurements: An S3‐2 case study

Although flux transfer events (FTEs) have been observed frequently at high altitudes near the magnetopause their ionospheric signatures remain elusive. Vestiges of FTEs and other magnetopause phenomena have possibly been detected in transient magnetic field perturbations measured on the ground at cusp latitudes. Direct observations of FTEs by instrumentation on polar orbiting satellites have not yet been forthcoming. We have developed a methodology for use by investigators based on expected, simultaneous measurements of plasma drifts (electric fields), field‐aligned currents (magnetic fields) and precipitating particle fluxes. To test and illustrate this methodology we present the results of a search through the available S3‐2 data for electric and magnetic field signatures from the mid‐day sector displaying irregular structures of scales 100 to 200 km, consistent with ionospheric mapping of FTEs. Out of 36 orbits between December 1975 and April 1976 we found five possible events. An event selected for detailed analysis was observed approximately at 2219 UT on January 5, 1976 during an equatorward crossing of the northern dayside auroral region at about 1030 MLT. The qualitative electric and magnetic field fluctuations expected during a satellite encounter with an FTE at ionospheric altitudes are compared with single component measurements for S3‐2. Although an encounter with a fast stream in the plasma flow poleward of the cusp cannot be excluded, the data are consistent with expectations for the S3‐2 spacecraft having made a near center crossing of an ionospheric footprint of an FTE with a diameter of about 150 km moving eastward along the polar cap boundary with a velocity of 1.3 km/s. Criteria for distinguishing between FTEs and fast streams from satellite measurements of two components of the electric and magnetic field are presented.

Impulsive electric and magnetic field perturbations observed over South Pole: Flux transfer events?

Electric and magnetic field data from the cusp latitude station at South Pole were searched for impulsive perturbations. These events may be evidence of the ionospheric signature of flux transfer events (FTE's). In a single X‐line reconnection model, the field aligned current system associated with the convection of the flux tube across the polar cap will produce two convection vortices around the flux tube. The multiple X‐line reconnection model predicts additional co‐axial field‐aligned currents in helical flux tubes. The electric field signatures of these two models are different. The electric field of the towing system attenuates less than the electric field of the twisting system as the field is mapped down to balloon altitude. Combining electric and magnetic field data may determine the relative importance of the two systems. Near local magnetic noon on 7 January 1986, at least three candidate events have been found. One event is shown to approximate the signature expected for reconnected flux tubes passing overhead. The paper brings a new data set to bear on an important phenomenon.

Pc3 magnetic pulsations at Davis, Antarctica

A brief morphological study of Pc3 (10–45 s) ULF magnetic pulsations observed at the polar cusp latitude station, Davis, Antarctica (Λ ∼ 74.5°) is presented. A significant observation of these deeply amplitude-modulated near monochromatic Pc3 magnetic pulsations is their large amplitudes, i.e. ⩽ 32 nT, which may provide a ground diagnostic for the location of the last closed magnetic field lines, adjacent to the dayside polar cusp. Observational analyses include both diurnal and seasonal amplitude and occurrence distributions, power spectral analyses, dynamic sonagrams and polarization studies.

ULF geomagnetic power at cusp latitudes in response to upstream solar wind conditions

Energy densities are computed for geomagnetic field fluctuations in the Pc 3–4 range using H component magnetometer data recorded at South Pole station over a three‐month period from June 3 to September 4, 1982. Hourly values of the energy densities in the Pc 3 and Pc 4 period bands are found to be highly correlated during geomagnetic local daytime hours. The results of multivariate analyses between geomagnetic energies and upstream solar wind quantities show that the most important quantity in controlling the magnitude of the field fluctuations is the solar wind speed, with the IMF Bz component being of next importance. We conclude that the Kelvin‐Helmholtz instability at the dayside magnetopause is, statistically, the dominant energy source that contributes to ULF hydromagnetic wave activity at cusp latitudes.

Correlation of cusp latitude with Bz and AE (12) using nearly one year's data

This investigation reports the statistical correlation of cusp latitude with the interplanetary magnetic field (IMF) Bz and the AE (12) index. Eleven months of data from 1979 were collected, subject only to the criteria that the IMP 8 spacecraft be outside the magnetopause to measure the IMF and that the daily average of AE (12) exceed 200 nT. In general, the latitudes of the poleward cusp boundaries display higher correlation coefficients than do the latitudes of the equatorward boundaries. The best correlations of cusp latitudes with AE involve no time delay, while the best correlations with Bz involve a 30‐min time delay. The strongest correlation (R = 0.77) occurs between the 30‐min time‐delayed Bz and the latitude of the poleward cusp boundary. The latitude of the equatorward cusp boundary correlates best with the time‐retarded southward Bz (R = 0.75). The best correlations of the cusp latitudes with AE (R = 0.68–0.71) are generally not as good as those with the time‐delayed Bz.

Pc 3 pulsations observed near the south polar cusp

We have compared high time resolution search coil magnetometer data near local magnetic noon from South Pole Station, Antarctica, for January and June 1983 with energetic particle data from DMSP and DE 2 satellites and with interplanetary magnetic field data from the IMP 8 satellite. Broadband irregular magnetic pulsations with period range 0.5–40 s (Pi 1) observed at South Pole Station are found to be associated with the proximity of strong >10‐keV electron precipitation, with a somewhat weaker dependence on Kp and the proximity of the cusp. Large‐amplitude Pc 3 pulsations (up to 20 nT peak to peak) also appear frequently in the data and are distinguished by their packetlike structure and their nearly monochromatic character. Pc 3 pulsations show little or no correlation with latitudinal position of the cusp, with magnetic activity, or with the presence of >10‐keV electron precipitation, but their occurrence is strongly correlated with the X component of the interplanetary magnetic field, and their frequency is dependent on the magnitude of this field. Our results suggest that no local sources need be postulated to explain the occurrence of Pc 3 pulsations at cusp latitudes and may suggest that upstream wave energy in the Pc 3 frequency range can reach low altitudes on cusp field lines.

Relations between the interplanetary magnetic field Bz, AE index, and cusp latitude

The connection between the interplanetary magnetic field (IMF) Bz, AE index, and magnetospheric cusp location is examined qualitatively. Seven months during 1979–1980 were searched for periods of low auroral activity and/or periods when the IMF Bz component and the AE index varied inconsistently (i.e., southward Bz and quiescent AE). During quiet auroral conditions, the IMF Bz, AE index, and equatorward cusp boundary generally tend to act coherently, although during very quiet times the cusp may exhibit considerable variability that is apparently unassociated with either Bz or AE. We found a number of instances in which the cusp location clearly followed Bz more closely than AE, especially when a time delay of ≤ 1 hour was assumed. However, we cannot conclude that either Bz or AE exerts a dominant influence on the latitudinal position of the polar cusp. The effects of Bz and AE variations on the cusp are quite complicated and the observations simply do not indicate clear cusp response to one or the other. The largest equatorward cusp motions seem to occur as a result of prolonged southward Bz lasting 3 hours or more, and cusp latitude appears to depend strongly on time‐accumulated Bz.

Simultaneous electric field changes in the equatorial electrojet in phase with polar cusp latitude changes during a magnetic storm

The Doppler frequency of the VHF radar signals backscattered from the drifting ionization irregularities in the equatorial electrojet is proportional to the east‐west electric field Ey in the equatorial electrojet. Using the VHF backscatter radar observations at Thumba during the geomagnetic storm of February 16, 1980, it is shown that the decreases and increases of the storm‐time electric fields in the equatorial electrojet are completely in phase with the decreases and increases in the mean latitude of the polar cusp region. This observation has important implications for understanding the magnetosphere‐ionosphere interactions, particularly the effect of the magnetospheric electric fields on the low latitude ionosphere.

Effects of the dayside field-aligned currents in location and structure of polar cusps

We have studied the dayside magnetosphere structure and its K p , A E  and IMF-dependence using the magnetic data from IMP and HEOS satellites obtained during 1966–1972. An analysis of the field line configurations has been done on the basis of results of a least squares fitting of the model coefficients to the data subsets. The plots of the magnetopause subsolar point distance and of the polar cusp latitude vs K p and A E have been obtained. A detailed study of the model field distribution has revealed a substantial difference in the polar cusp field line geometry between the cases of weak and strong geomagnetic activity. We find that this results in a considerable longitudinal extension of the isointensity contours of particle precipitation at ionospheric heights during disturbed periods with K p ⩾ 3 or A E ≳ 300 nT. The same effect has been detected for the data subsets corresponding to the IMF B z < 0. In contrast, at quiet times the precipitation isolines are much closer to circles. We conclude therefore that the cleft-like structure of polar cusps pertains only to active periods and can be explained by a magnetic effect of enhanced Birkeland currents.

Case studies of the storm time variation of the polar cusp

The latitudinal variations of the polar cusp region were examined during three intense geomagnetic storms. The variations were compared with the intensity of storm time ring current inferred from the Dst index, with the magnitude of the north‐south component Bz of the interplanetary magnetic field and with substorm activity. The common feature is that the rapid equatorward shift occurred during the increase of the ring current growth and during the southward turning of the interplanetary magnetic field orientation. The equatorwardmost latitude of the cusp was reached before the peak of the ring current intensity, by a few to several hours, coinciding with the occurrence of the largest magnitude of the southward interplanetary magnetic field component. However, details of the polar cusp latitudinal movement differ from storm to storm. During the three storms studied, the poleward recovery commenced at the peak magnitude of the negative IMF Bz component, but the recovery proceeded without a clear relation to variations of the interplanetary Bz component, to the ring current intensity, or to the substorm activity. The lowest cusp latitude observed was at ∼61.7°, and the magnitude of this shift seems to be related to the magnitudes of −Bz. It is further observed that the approximate rates of the cusp macroscopic equatorward and poleward movements are about 3° and 1.5° per hour, respectively.

Latitudinal variation of the polar cusp during a geomagnetic storm

Large amplitude latitudinal variation of the polar cusp position was observed during the intense geomagnetic storm of 15‐16 February 1980. The observation of the polar cusp, identified as the region of intense but extremely soft electron precipitation, was made by two nearly noon‐midnight orbit DMSP satellites over both northern and southern hemispheres. The latitudinal shift of the polar cusp is observed to be related to the intensity variation of the ring current indicated by the hourly Dst values. The polar cusp region moved from its normal location at ∼ 76° gm lat down to ∼ 62° gm lat at the peak of this storm. This movement took about 5 hours and was detected over both hemispheres. A drastic variation in the width of the cusp region was also observed; it is very narrow (∼ 1°) during the equatorial shift and expands to ≳ 5° during the poleward recovery. Variation of the polar cusp latitude with that of the Dst index was also seen during the period before the intense storm.

Magnetic field variations in the polar region during magnetically quiet periods and interplanetary magnetic fields

The concepts of near-pole magnetic field variations during magnetically quiet periods are explored, with special emphasis on the relationships of these variations to interplanetary magnetic field components. Methods are proposed for relating the variations which have been observed to the fields from the various sources, based on a thorough selection of reference levels. We assume that the field variations in the summer polar cap during magnetically quiet periods consist of the following components: (i) the middle-latitude Sqvariation extended to the polar region; (ii) the DPC(By) single-cell current system with a polar electrojet in day-side cusp latitudes; (iii) the DMC(Bz) two-cell current system of magnetospheric convection, in the form of a homogeneous current sheet in the polar cap towards the sun, with return currents through lower latitudes; (iv) the DPC(Bz) single-cell counterclockwise current system with a focus in the day-side cusp region. Quantitative relations between the near-pole variation intensities and the value and sign of the IMF azimuthal component, with a 1 hr time resolution, have been obtained and used to suggest ways of diagnosing the interplanetary magnetic field on the basis of ground observations.

Rate of erosion of dayside magnetic flux based on a quantitative study of the dependence of polar cusp latitude on the interplanetary magnetic field

By considering only those periods when the delay time from the interplanetary observing position to the magnetosphere is less than about 5 min, it is found that irrespective of substorm activity: (a) the 45‐min average value of interplanetary Bz predicts the latitudes of the poleward and equatorward boundaries of polar cusp electron precipitation with rms errors of 1.34° and 1.16° respectively; (b) both boundaries move equatorward by about 5° as Bz varies from 0 to −6γ, the cusp remaining about 4° wide; (c) the amount of flux added to the polar cap is about 9.2% of the total southward flux impingent on the magnetosphere in the previous 45 min; and (d) as Bz becomes more positive, the equatorward boundary moves only slightly more poleward (½° between Bz = 2γ and Bz = 6γ), while the poleward boundary moves significantly toward higher latitudes, resulting in a cusp approximately 7° wide for Bz = 6γ. This dependence of the width of the cusp on positive Bz suggests that reconnection between the IMF (interplanetary magnetic field) and open geomagnetic field lines (which are southward near the dayside magnetopause) is facilitated when the IMF is northward, allowing deeper penetration of magnetosheath plasma into the high latitude magnetosphere.