Cusp Latitudes Research Articles

Pairs of solar wind‐magnetosphere coupling functions: Combining a merging term with a viscous term works best

We investigated the behavior of 10 different characterizations of the magnetosphere, including traditional geomagnetic indices such as Kp and AE. We also used satellite based data such as global auroral power from Polar UVI, cusp latitude from DMSP, and magnetotail stretching from GOES. Multiyear data (typically a solar cycle) was studied at relatively high cadence (usually 1 h) to provide better statistical consistency. Simple two parameters best fits to a wide variety of candidate solar wind coupling functions were considered, with no hidden variables or adjustable parameters. Previously we showed that the best performing solar wind coupling functions all proved to be various estimators of the global merging rate, with the best results from dΦMP/dt = v4/3BT2/3sin8/3(θc/2). Here we investigate the best performing viscous candidates, and the best performing pairs of solar wind coupling functions, in predicting these same 10 characterizations of the magnetosphere. The top viscous functions all are closely related to the solar wind pressure, but n1/2v2 performs best, accounting for 22.3% of the variance, versus 14.7% for v and 12.5% for p. Altogether we considered 32 different candidate solar wind magnetosphere coupling functions, and all 496 unique pairs of functions. Because of the large number of function pairs, some statistical fluctuations are expected, and indeed observed, in predicting individual indices. Nonetheless, certain patterns emerged. The best performing overall pair (predicting 61.0% of variance across all indices) was dΦMP/dt coupled with n1/2v2, i.e., the best individual merging and best viscous terms make the best combination. All the top pairs consisted of one merging and one viscous term. Combining two distinct estimators of the merging rate always has less predictive power than combining a viscous and merging term. However, any merging term, such as Bs, vBs, or EKL, when coupled with almost any viscous term, such as v, p, n1/3v2, p1/2, etc, performs reasonably well, with the merging term invariably accounting for much the greater fraction of variance.

Cusp for high and low merging rates

The traditional distinction between the cusp for northward and southward interplanetary magnetic field (IMF) conditions is deficient in that many, if not most, “northward” IMF cusp encounters closely resemble southward IMF encounters. Partly for that reason, “southward” IMF cusps have been far more often examined than “northward. ” Recently, Newell et al. (2007) showed that the magnetopause merging rate, dΦMP/dt, much better predicts cusp latitude than does Bz (or Bs). Here, we investigate the extent to which high and low merging rate conditions better separates cusp encounters into mutually distinct classes. Indeed, high merging rate cusps (those with dΦMP/dt > 2* 〈dΦMP/dt〉) differ from low merging rate cusps (those with dΦMP/dt < 0.5〈dΦMP/dt〉) in several quantitative and even qualitative ways. High‐energy (tens of keV) ions, apparently of magnetospheric origin, do not extend into the high merging rate cusp but frequently do for low merging rates. However, the dispersion curve for ions, apparently of magnetosheath origin, extends to several keV for high merging rates but only to 1–2 keV for low merging rates. The local time extent of the cusp is 2.33 hours MLT for low merging rate conditions and 3.45 hours MLT for high merging rate conditions. All high merging rate cusps show clear forward dispersion (declining energy with increasing latitude) with low‐energy ion cutoffs within that dispersion. Low merging rates cusps rarely show forward dispersion, but about half, or slightly less, show reverse dispersion (declining energy with decreasing latitude) at the poleward edge of the cusp. Low‐energy ion cutoffs in the low‐merging rate cusp primarily occur only within that reverse dispersion. “Double” cusps, some with very clear latitudinal separation, occur in some high merging rate cases, but no low merging rate cases. The high merging rates cases also typically have a “shadow” region of electron‐only precipitation at roughly polar rain energies and intensities, lying immediately equatorward of the main cusp. For low‐merging rate conditions, the region immediately equatorward of the cusp (main magnetosheath ion population) contains ions as well as electrons, and forms a boundary layer.

A nearly universal solar wind‐magnetosphere coupling function inferred from 10 magnetospheric state variables

We investigated whether one or a few coupling functions can represent best the interaction between the solar wind and the magnetosphere over a wide variety of magnetospheric activity. Ten variables which characterize the state of the magnetosphere were studied. Five indices from ground‐based magnetometers were selected, namely Dst, Kp, AE, AU, and AL, and five from other sources, namely auroral power (Polar UVI), cusp latitude (sin(Λc)), b2i (both DMSP), geosynchronous magnetic inclination angle (GOES), and polar cap size (SuperDARN). These indices were correlated with more than 20 candidate solar wind coupling functions. One function, representing the rate magnetic flux is opened at the magnetopause, correlated best with 9 out of 10 indices of magnetospheric activity. This is dΦMP/dt = v4/3BT2/3sin8/3(θc/2), calculated from (rate IMF field lines approach the magnetopause, ∼v)(% of IMF lines which merge, sin8/3(θc/2))(interplanetary field magnitude, BT)(merging line length, ∼(BMP/BT)1/3). The merging line length is based on flux matching between the solar wind and a dipole field and agrees with a superposed IMF on a vacuum dipole. The IMF clock angle dependence matches the merging rate reported (albeit with limited statistics) at high altitude. The nonlinearities of the magnetospheric response to BT and v are evident when the mean values of indices are plotted, in scatterplots, and in the superior correlations from dΦMP/dt. Our results show that a wide variety of magnetospheric phenomena can be predicted with reasonable accuracy (r > 0.80 in several cases) ab initio, that is without the time history of the target index, by a single function, estimating the dayside merging rate. Across all state variables studied (including AL, which is hard to predict, and polar cap size, which is hard to measure), dΦMP/dt accounts for about 57.2% of the variance, compared to 50.9% for EKL and 48.8% for vBs. All data sets included at least thousands of points over many years, up to two solar cycles, with just two parameter fits, and the correlations are thus robust. The sole index which does not correlate best with dΦMP/dt is Dst, which correlates best (r = 0.87) with p1/2dΦMP/dt. If dΦMP/dt were credited with this success, its average score would be even higher.

EISCAT observations of plasma patches at sub-auroral cusp latitudes

Abstract. A sequence of 3 patches of high-density (1012 m−3) cold plasma on a horizontal scale-size of 300–700 km was observed near magnetic noon by the EISCAT VHF radar above Svalbard on 17 December 2001. The patches followed a trajectory towards the cusp inflow region. The combination of radar and all-sky observations demonstrates that the patches must have been segmented equatorward of the cusp/cleft auroral display, and hence their properties had not yet been influenced by cusp particle showers and electrodynamics on open flux tubes. The last patch in the sequence was intersected by radio tomography observations, and was found to be located adjacent to a broader region of the same high electron density further south. The patches occurred under moderately active conditions (Kp=3) and the total electron content (TEC) of the high-density plasma was 45 TEC units. The train of patches appeared as a segmentation of the tongue of ionization. The sequence of patches occurred in association with a sequence of flow bursts in the dusk cell return flow. It is proposed that reconnection driven pulsed convection is able to create sub-auroral patches in the region where high density mid-latitude plasma is diverted poleward toward the cusp. It is the downward Birkeland current sheet located at the equatorward boundary of the flow disturbance that represents the actual cutting mechanism.

Auroral electrojets and boundaries of plasma domains in the magnetosphere during magnetically disturbed intervals

Abstract. We investigate variations in the location and intensity of the auroral electrojets during magnetic storms and substorms using a numerical method for estimating the equivalent ionospheric currents based on data from meridian chains of magnetic observatories. Special attention was paid to the complex structure of the electrojets and their interrelationship with diffuse and discrete particle precipitation and field-aligned currents in the dusk sector. During magnetospheric substorms the eastward electrojet (EE) location in the evening sector changes with local time from cusp latitudes (Φ~77°) during early afternoon to latitudes of diffuse auroral precipitation (Φ~65°) equatorward of the auroral oval before midnight. During the main phase of an intense magnetic storm the eastward currents in the noon-early evening sector adjoin to the cusp at Φ~65° and in the pre-midnight sector are located at subauroral latitude Φ~57°. The westward electrojet (WE) is located along the auroral oval from evening through night to the morning sector and adjoins to the polar electrojet (PE) located at cusp latitudes in the dayside sector. The integrated values of the eastward (westward) equivalent ionospheric current during the intense substorm are ~0.5 MA (~1.5 MA), whereas they are 0.7 MA (3.0 MA) during the storm main phase maximum. The latitudes of auroral particle precipitation in the dusk sector are identical with those of both electrojets. The EE in the evening sector is accompanied by particle precipitation mainly from the Alfvén layer but also from the near-Earth part of the central plasma sheet. In the lower-latitude part of the EE the field-aligned currents (FACs) flow into the ionosphere (Region 2 FAC), and at its higher-latitude part the FACs flow out of the ionosphere (Region 1 FAC). During intense disturbances, in addition to the Region 2 FAC and the Region 1 FAC, a Region 3 FAC with the downward current was identified. This FAC is accompanied by diffuse electron precipitation from the plasma sheet boundary layer. Actually, the triple system of FAC is observed in the evening sector and, as a consequence, the WE and the EE overlap. The WE in the evening sector comprises only the high-latitude periphery of the plasma precipitation region and corresponds to the Hall current between the Region 1 FAC and Region 3 FAC. During the September 1998 magnetic storm, two velocity bursts (~2–4 km/s) in the magnetospheric convection were observed at the latitudes of particle precipitation from the central plasma sheet and at subauroral latitudes near the ionospheric trough. These kind of bursts are known as subauroral polarization streams (SAPS). In the evening sector the Alfvén layer equatorial boundary for precipitating ions is located more equatorward than that for electrons. This may favour northward electric field generation between these boundaries and may cause high speed westward ions drift visualized as SAPS. Meanwhile, high speed ion drifts cover a wider range of latitudes than the distance between the equatorward boundaries of ions and electrons precipitation. To summarize the results obtained a new scheme of 3-D currents in the magnetosphere-ionosphere system and a clarified view of interrelated 3-D currents and magnetospheric plasma domains are proposed.

Cusp latitude and the optimal solar wind coupling function

Previous work has established that the linear correlation of the low‐altitude particle cusp latitude with the southward component of the IMF is about 0.70. Several possibly better candidate functions for determining the coupling between the magnetosphere and the solar wind have been advanced, but none have been evaluated in terms of the cusp, which is a site of direct solar wind–magnetosphere interaction. On the basis of 11 years of DMSP satellite particle data from 1984–1994 (with verification from the subsequent 11 years, 1995–2005), we find that the best solar wind–magnetosphere coupling function involves electric field dimensions, such as the half wave rectifier (vBs) and the Kan‐Lee electric field (EKL = vBTsin2(θc/2), where θc is the IMF clock angle). Both the half wave rectifier (r = 0.77) and the Kan‐Lee (r = 0.78) functions have a linear correlation with cusp latitude which is noticeably better than the Bz function used in previous work, and also better than the ɛ parameter (ɛ = vB2sin4(θc/2)). However, the best correlation is with a function whose clock angle dependence is intermediate between the pure half wave rectifier (which implies no merging for Bz > 0) and the Kan‐Lee function. Namely, EWAV = vBTsin4(θc/2) correlates with cusp latitude at the r = 0.81 level. This latter clock angle dependence has been previously suggested at various times by J. R. Wygant, by S.‐I. Akasofu, and by V. M. Vasyliunas. The improved result holds for both the equatorward and poleward edge of the cusp, and regardless of how the IMF is propagated. Earlier work on cross polar cap potentials and on nightside auroral luminosity also favored the EWAV function, which in combination with our findings suggests a widely applicable result. Dayside merging is thus clearly not purely component driven, as the half wave rectifier formula implies. These results also suggest, albeit less convincingly, that merging shuts down for increasingly northward IMF more rapidly than the Kan‐Lee electric field implies.

Sporadic E at cusp latitudes

This is a study of the sporadic E (Es) that is observed over a ‘cusp latitude’ station, Cambridge Bay (77° CGM lat.). Es is extremely common in late evening hours at this location, reaching ∼80% occurrence rate in some months. The Es appears to form near the time when the convection reversal passes over the station. We explain our observations by a two-step formation mechanism involving horizontal convergence of ionization by the electric fields of the afternoon convection reversal, and then vertical convergence of this ionization by the electric fields in the polar cap. We show that several properties of the Es are consistent with this two-step mechanism.

Polar cap influx

Abstract. This study uses digital ionosonde data from a cusp latitude station (Cambridge Bay, 77° CGM lat.) to study the convection into the polar cap. Days when the IMF magnetic field was relatively steady were used. On many days it was possible to distinguish an interval near noon MLT when the ionosonde data had a different character from that at earlier and later times. Based on our data, and other published measurements, we used the interval 10:00-13:00 MLT as the cusp interval and calculated the convection into the polar cap in this interval. The integrated convection accounted for only ~1/3 of the open polar cap flux. If the convection through the prenoon/postnoon regions on either side of the cusp was calculated the remaining 2/3 of the flux could be accounted for. The characteristics of the prenoon/postnoon regions were different from the cusp region, and we attribute this to transient flank merging versus more steady frontside merging for the cusp. Keywords. Ionosphere (Plasma convection) Magnetospheric physics (Polar cap phenomenon)

A statistical analysis of low‐frequency magnetic pulsations at cusp and cap latitudes in Antarctica

We report a statistical analysis of low‐frequency magnetic variations (magnetic pulsations, 0.8–7 mHz) at South Pole (74°S corrected geomagnetic latitude) and Terra Nova Bay (80°S) during 1996. The results show that at South Pole (at cusp latitudes) the pulsation power exhibits two maxima during the day, one in the local premidnight and the other in the morning. The first maximum disappears when the analysis is restricted to northward interplanetary magnetic field conditions (Bz > 1 nT), suggesting that it might be associated to substorm phenomena. During closed magnetospheric conditions, when the cusp is expected to be located poleward with respect to the station, the spectral and polarization characteristics of pulsations between 1 and 3 mHz suggest that resonant oscillations of the outermost closed field lines commonly occur at South Pole in the local morning. At Terra Nova Bay, in the polar cap, the pulsation power is much lower and its diurnal variation is characterized by a single maximum, which occurs around local noon, when the station approaches the cusp. The corresponding polarization pattern indicates that Terra Nova Bay is always located poleward with respect to resonant field lines.

Coordinated observations of Pc3 pulsations near cusp latitudes

In spite of accumulated observations of ULF pulsations, there are no satisfactory explanations for the generation and transmission mechanisms of Pc3 pulsations at very high latitudes. We studied the characteristics of the high‐latitude Pc3 pulsations by using data acquired on the ground by magnetometer arrays, in the ionosphere by the Super Dual Auroral Radar Network (SuperDARN) of HF radars, and in space by the Geotail satellite. The observations revealed that narrowband Pc3 pulsations are occasionally observed simultaneously in magnetic field data on the ground and in Doppler velocity data acquired by the HF radars in the intervals of small interplanetary magnetic field (IMF) cone angles. We find that there is a clear peak of the Pc3 power near the cusp latitudes and that the power of pulsations decreases very steeply with latitude. At times, there are periods of high coherence between the pulsations in the dawn magnetosheath and on the ground, although the spectra of the magnetosheath magnetic field variations exhibit broadband signatures. The Poynting flux in the Pc3 band increases in the magnetosheath when narrowband Pc3 pulsations are observed in the cusp region. The results suggest that the driving source is located in the magnetosheath and that part of the MHD energy in the magnetosheath propagates to the ionospheric cusp. We suggest that the transmission of Pc3 pulsations in the cusp region requires not only a non‐MHD mechanism as suggested (e.g., modulated precipitation of trapped electrons) but also an MHD wave incident into the ionosphere.

Cusp-latitude Pc3 spectra: band-limited and power-law components

Abstract. This work attempts to fill a gap in comparative studies of upstream-generated Pc3–4 waves and broad band ULF noise observed at cusp latitudes. We performed a statistical analysis of the spectral properties of three years of cusp-latitude ground magnetometer data, finding that the average daytime Pc3–4 spectra are characterized by two principal components: an upstream-related band-limited enhancement (‘signal’) and a power-law background (‘noise’) with S(f ) a f -4 . Based on this information we developed an algorithm allowing for the deconvolution of these two components in the spectral domain. The frequency of the signal enhancement increases linearly with IMF magnitude as f [mHz] ~ 4.4 | BIMF | [nT], and its power maximizes around IMF cone angles qxB ~ 20 and 160° and at 10:30–11:00 MLT. Both spectral components exhibit similar semiannual variations with equinoctial maxima. The back-ground noise power grows with increasing southward Bz and remains nearly constant for northward Bz . Its diurnal variation resembles that of Pc5 field-line resonance power, with a maximum near 09:00 MLT. Both the band-limited signal and broad band noise components show power-law growth with solar wind velocity a V 5.71sw and a V 4.12sw, respectively. Thus, the effective signal-to-noise ratio increases with in-creasing Vsw. The observations suggest that the noise generation is associated with reconnection processes.Key words. Magnetospheric physics (magnetopause, cusp, and boundary layers; MHD waves and instabilities; solar wind magnetosphere interactions)

A statistical study of traveling convection vortices using the Magnetometer Array for Cusp and Cleft Studies

The Magnetometer Array for Cusp and Cleft Studies (MACCS) is primarily a longitudinal magnetometer array located in the eastern Canadian arctic at cusp latitudes. We present a statistical study of traveling convection vortices (TCVs) as observed by MACCS from the first year of operations, August 1992 to July 1993. For this period, data are routinely available from three stations, extending more than 2 hours in magnetic local time (MLT) and located roughly on a line of constant geomagnetic latitude at 75°. With the selection criteria used, we find an average of 38 TCV events per month. We characterize the events as “isolated” and “nonisolated”. The isolated events have the typical, accepted signature of a TCV while the nonisolated ones are events that satisfy the selection criteria but occur among relatively strong magnetic activity. The diurnal distribution of these events shows a prenoon‐afternoon asymmetry, but the distribution has a single peak at 10 MLT and then falls off smoothly in the afternoon hours. It does not show the previously reported minimum of event occurrence around magnetic local noon except for isolated events occurring during higher Kp values. The most likely explanation for this discrepancy is the wide longitudinal extent of MACCS. The majority of the events have propagation velocities between 3 and 11 km/s, which can be a factor of 2 larger than the events reported in previous studies. The majority of the events move away from noon, as is expected, with a small fraction moving toward noon. Most (80%) of the studied events exhibit all the characteristics of a typical TCV, while the remaining 20% show some form of irregularity. We find four different types of irregularities. The isolated and nonisolated events have many similar morphological and propagation characteristics. However, quantitative comparison of the statistical distributions of the two types of TCVs indicates that they belong to different populations. We also find that nonisolated TCVs may occur for somewhat higher Kp values. We therefore suggest that the two classes of TCVs have different source mechanisms.

Statistical characteristics of Doppler spectral width as observed by the conjugate SuperDARN radars

Abstract. We performed a statistical analysis of the occurrence distribution of Doppler spectral width around the day-side high-latitude ionosphere using data from the conjugate radar pair composed of the CUTLASS Iceland-East radar in the Northern Hemisphere and the SENSU Syowa-East radar in the Southern Hemisphere. Three types of spectral width distribution were identified: (1) an exponential-like distribution in the lower magnetic latitudes (below 72°), (2) a Gaussian-like distribution around a few degrees magnetic latitude, centered on 78°, and (3) another type of distribution in the higher magnetic latitudes (above 80°). The first two are considered to represent the geophysical regimes such as the LLBL and the cusp, respectively, because they are similar to the spectral width distributions within the LLBL and the cusp, as classified by Baker et al. (1995). The distribution found above 80° magnetic latitude has been clarified for the first time in this study. This distribution has similarities to the exponential-like distribution in the lower latitude part, although clear differences also exist in their characteristics. These three spectral width distributions are commonly identified in conjugate hemispheres. The latitudinal transition from one distribution to another exhibits basically the same trend between two hemispheres. There is, however, an interhemispheric difference in the form of the distribution around the cusp latitudes, such that spectral width values obtained from Syowa-East are larger than those from Iceland-East. On the basis of the spectral width characteristics, the average locations of the cusp and the open/closed field line boundary are estimated statistically.Key words. Ionosphere (ionosphere-magnetosphere inter-actions; plasma convection) – Magnetospheric physics (magnetopause, cusp, and boundary layers)

Location of high‐altitude cusp during steady solar wind conditions

We have investigated the high‐altitude cusp location and its dependence on the interplanetary magnetic field (IMF) Bz, solar wind Ey, and solar wind dynamic pressure during steady solar wind conditions. We have adopted strict event selection criteria, where we have studied only events during stationary IMF Bz, when the cusp latitude is supposed to be stationary. Prior to this study, the statistical studies have shown a great deal of scatter between the cusp latitude and the IMF Bz; however, with the strict event selection criteria, we have been able to reduce the scatter. The strongest source of scatter comes from large and largely fluctuating IMF By. When the events are selected during small IMF By, the scattering between the cusp latitude and the IMF Bz is reduced. When IMF is southward, also the fluctuation of IMF By has strong influence on the cusp latitude, whereas for northward IMF, the IMF By fluctuation has less influence on the cusp latitude. For larger southward IMF the cusp appears further equatorward than with smaller southward IMF. When the scatter is reduced by introducing the IMF By control, the larger northward IMF brings the cusp to lower latitudes than the smaller northward IMF, i.e., the maximum cusp latitude is obtained for small positive IMF Bz. For increasing solar wind Ez the cusp moves equatorward. Furthermore, with increasing solar wind dynamic pressure, the cusp appears at lower latitudes. This behavior depends on the orientation of the IMF Bz, with southward IMF the cusp moves faster equatorward than with northward IMF.

Simulation Study of the Influence of the Precipitating Electrons on the Polar Ionosphere

AbstractSimulation study on the effects of precipitating electrons on the polar ionosphere is described. Electron precipitation is included in the polar ionospheric model. Improvements are made in applying finite differential scheme for solving the ionospheric model equations. When only soft electron precipitation ionization is considered, a typical ionospheric F layer can be obtained, while precipitating electrons with higher characteristic energy show maximum ionization rate at lower height to form distinct ionospheric E layer. The electron density of E layer becomes even greater than the F layer sometimes. Taking the statistical results of precipitating electrons at the cusp region latitudes as model inputs, the simulation results coincide well with observations. Comparing the simulation and statistical results of ionospheric F layer with statistical distribution of precipitating electrons at the cusp latitudes, we conclude that the so called magnetic noon anomaly at Zhongshan Station can be interpreted as the effects of precipitating electrons.

The effect of fluctuating ionospheric electric fields on Es-occurrence at cusp and polar cap latitudes

Theory predicts that in the high-latitude southern hemisphere, southwest (SW) electric fields will produce convergent ion flow and thereby create thin sporadic E (Es)-layers at node heights > 120 km, whilst northwest (NW) fields will produce downward ion flow and create thicker Es-layers at heights < 110 km. To investigate this theory, Digisonde ionograms (giving the Es-occurrence) and drift measurements (giving electric field estimates) at two Antarctic stations were statistically analyzed. As previously found for the polar cap station Casey (81 degreesS magnetic), more of the Es-traces were associated with SW fields than NW fields. However, new results for the cusp station Zhongshan (73 degreesS) show that fewer Es-layers occur there, and NW fields play a slightly more important role than SW fields, similar to the results found at auroral latitudes in the northern hemisphere. To further our understanding of the occurrence distributions, we study the fluctuating properties of the electric fields at the two stations. It is found that the electric fields at Zhongshan fluctuate more than those at Casey. Thus we suggest that the field fluctuation is also an important consideration helping to explain the differences in the Es-occurrence at the two stations. This suggestion is confirmed by our numerical simulations which show that Es-layers are more effectively formed by steady SW fields than by steady NW fields, and less effectively by fluctuating SW fields than by fluctuating NW fields. (C) 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

The distention of the magnetosphere on May 11, 1999: High latitude Antarctic observations and comparisons with low latitude magnetic and geopotential data

We examine the Earth's ionospheric response on May 11, 1999, to an unusually tenuous solar wind, focusing on magnetometer, riometer, and optical data from high geomagnetic latitudes in Antarctica. Comparisons are also made with POLAR satellite data during a perigee pass over Antarctica, and with geomagnetic data collected at low latitudes. It is shown that the southern hemisphere was geophysically active, even though the Kp index on May 11 ranged only from 0 to 0+. Furthermore, despite the fact that the IMF and solar wind conditions favored northern hemisphere polar rain, low energy electron precipitation did occur in the southern polar cap. Geomagnetic power levels at low, cusp, and polar cap latitudes were also lower on May 11 than on surrounding days. Although this might be expected, discrete millihertz peaks in ULF power were still evident, especially at cusp latitudes.

A conjugate study of Pc3–4 pulsations at cusp latitudes: Is there a clock angle effect?

Pc3–4 (15–50 mHz) pulsations are often observed during daytime hours in the Earth's magnetosphere and on the ground. The ion foreshock region of the solar wind upstream of Earth is now understood to be a major source of these waves, and the general dependence of their occurrence in the magnetosphere on the cone angle of the interplanetary magnetic field (IMF) is well attested. However, there have also been suggestions that the IMF clock angle should introduce latitudinal or local time asymmetries in wave occurrence, and in particular, that Pc3–4 pulsations might preferentially reach northern versus southern high‐latitude regions. In order to test this latter suggestion, we have compared a full year's (1997) data from two cusp‐latitude stations, Sondrestromfjord, Greenland, and South Pole, Antarctica, both near 74° corrected geomagnetic latitude. Our data show the usual strong IMF cone angle control of Pc3–4 wave occurrence/amplitude but also indicate that occurrence and relative amplitude are essentially identical near both cusps; any clock angle control is at most a second‐order effect, not detectable in our analysis. We further find little seasonal dependence in the ratio of power in the two cusp regions other than that attributable to differences in solar illumination. These observations, in conjunction with the well‐known sharp cutoff in wave activity for cone angles above 45°, indicate that only those perturbations occurring at the subsolar bow shock can reach the magnetopause and propagate to any location in the dayside magnetosphere. In addition, the absence of a clock angle effect supports earlier suggestions that Pc3–4 signals in the magnetosheath do not cross streamlines and thus do not propagate as waves, but are rather convected as spatial structures in the highly turbulent, high‐beta downstream magnetosheath plasma.

Spatiotemporal characteristics of cusp latitude spectra

In this study we analyze the spatiotemporal characteristics of spectra generated from 754 days of cusp latitude magnetometer data (Longyearbyen and selected Magnetometer Array for Cusp and Cleft Studies (MACCS) stations). In order to distinguish between the presence of spatial (fixed in magnetic local time) and temporal (fixed in universal time) signatures in cusp latitude spectra we develop a simple test using trace power, polarization, and ellipticity spectra. On the basis of this test we find evidence for both spatial and temporal signatures in cusp latitude spectra. We find that the trace power spectrum is dominated by temporal information; however, the polarization and ellipticity spectra contain unambiguous spatial structure. Temporal information in cusp latitude spectra is carried primarily by broadband Pc3 (10–50 mHz) noise, while spatial information is carried by polarized Pc3 (1–10 mHz) pulsations. Additionally, we establish a state‐space measure as a quantitative means of discriminating the spatial passage of the cusp and boundary regions by ground‐based magnetic means. The measure is based on the difference between daily polarization spectra (centered on local magnetic noon) and the mean polarization spectra for a given station. This procedure replaces previous determinations which were made from spectra “by‐eye”.

Conjugate ULF field line resonances at cusp latitudes

Significant similarities have been identified in the dynamic power spectra of induction magnetometer data recorded at the conjugate cusp stations of Davis, Antarctica (mlat −74.3, mlong 101.5) and Longyearbyen, Svalbard (mlat 75.0, mlong 114.5). A 60 day period in 1993 (January–February) has been studied to characterise two common features, 1) Broadband Pc5 bursts which predominantly occur 2–4 hours before and after local magnetic noon, and which exhibit polarisations suggestive of a Kelvin-Helmholtz-like source mechanism, and 2) A resonance structure which has an arch-shaped local time dependence. Interhemisphere phase measurements indicate odd-mode toroidal standing-waves after field line resonance and azimuthal propagation effects are taken into account. Phase studies such as this may be used to study conjugacy at high latitudes and identify the location of the open and closed field line boundary, an important diagnostic parameter in space weather studies.