Local Magnetic Noon Research Articles

Global frequency distributions of pulsations driven by sharp decrease of solar wind dynamic pressure

On 24 August 2005, an impulse of solar wind dynamic pressure ( P sw) hit the magnetosphere. Using the high resolution geomagnetic field data from 15 ground stations and the data from Geotail and TC-1, we studied the geomagnetic pulsations at auroral latitudes driven by the sharp decrease of P sw at the trailing edge of the impulse. The results show that the sharp decrease of P sw can excite a global pulsation in the frequency range 4.3–11.6 mHz. The pulsation has a reversal of polarization between two auroral latitude stations, a larger power spectral density (PSD) close to resonant latitude and increasing frequency with decreasing latitude. All these features indicate that the pulsations are associated with field line resonance (FLR). The fundamental resonant frequency (the peak frequency of PSD between 4.3 and 5.8 mHz) is dependent on magnetic local time and is largest around magnetic local noon. This feature is due to the fact that the size of magnetospheric cavity is dependent on local time and smallest at noon. A second harmonic wave at about 10 mHz is also observed, which is strongest in the daytime sector, and becomes heavily attenuated in the night sector. The comparison of the PSDs of the pulsations driven by sharp increase and sharp decrease of P sw shows that the frequency of pulsations is negatively proportional to the size of magnetopause. Since the FLR is excited by compressional cavity/waveguide waves, the above results indicate that the resonant frequency in the magnetospheric cavity/waveguide is controlled not only by solar wind parameters but also by magnetic local time of observation point.

Simultaneous ground-based optical and SuperDARN observations of the shock aurora at MLT noon

Using ground-based high temporal and spatial optical aurora observations, we investigated one fortuitous event to illustrate the direct responses of the fine structure auroral emission to interplanetary shock on 7 January 2005. During the shock impact to the magnetosphere, the Chinese Arctic Yellow River Station (YRS) equipped with all-sky imagers (ASIs) was situated at the magnetic local noon region (~1210 MLT) in the Northern Hemisphere, while the SuperDARN CUTLASS Finland HF radar covering the field of view (FOV) of the ASIs at YRS had fine ionospheric plasma convection measurement. We observed that an intensified red aurora manifesting as a discrete emission band at a higher latitude responds to the shock impact gradually, which results in a distinct broadening of the dayside auroral oval due to the equatorward shifting of its lower latitude boundary after the shock arrival. In contrast, the green diffuse aurora, manifesting as a relatively uniform luminosity structure, reacts immediately to the shock compression, displaying prompt appearance in the southern edge of the FOV and subsequent poleward propagation of its higher latitude boundary. Simultaneously, the CUTLASS Finland radar monitored enhanced backscatter echo power and increased echo number, which coincided with intensified discrete aurora in approximately the same latitudinal region. Doppler velocity measurement showed moving ionospheric irregularities with generally enhanced line-of-sight (LOS) speed, but with prominent sunward flow in the polar cap and antisunward flow in both the eastern and western regions. The SuperDARN global ionospheric convection pattern clearly presented a large-scale plasma flow divided in four circulation cells, with two reversed flow cells nested in the noon sector of the polar cap. These direct observations strongly suggest that the prompt shock compression intensified the wave-particle interaction in the inner magnetosphere and enhanced the lobe magnetic reconnection rate at magnetospheric high latitude.

Comparative studies on ionospheric climatological features of NmF2 among the Arctic and Antarctic stations

Climatological features of the maximum electron density of the ionospheric F2 region (NmF2) are investigated and compared among the Arctic and Antarctic stations using long-term observations from the dynasonde measurements at Tromso, the EISCAT Svalbard Radar (ESR) measurements at Longyearbyen, and the digisonde DPS-4 measurements at Zhongshan, Antarctica. The NmF2 data are sorted as monthly medians of NmF2 for each hour, month, and solar activity. Results are presented as follows: (1) On the diurnal variability, the maximum value of NmF2 mainly occurs at local noon at the auroral latitude station Tromso, but at magnetic noon at the cusp latitude station Longyearbyen and between local noon and magnetic noon at Zhongshan, indicating the importance of soft particle precipitation and ionospheric convection in the cusp region. (2) In addition to the daytime peak, there is another peak just before magnetic midnight at Longyearbyen in winter during solar maximum years, which is attributed to the cross-polar transport of EUV ionization from day side to night side. (3) An enhancement of NmF2 is detected at magnetic midnight in winter at Tromso. It may be caused by the nighttime substorms. (4) On the seasonal variability, there are semi-annual anomaly at Tromso and normal variability at both Longyearbyen and Zhongshan during solar minimum years. (5) During solar maximum years there is always semi-annual anomaly for all three stations. The well-known winter anomaly is evident at both Tromso and Zhongshan, but does not exist at Longyearbyen. These anomalies are mainly due to the differences in photoionization and chemical compositions between the three stations.

Pc2 EMIC waves generated high off the equator in the dayside outer magnetosphere

It is generally accepted that electromagnetic ion cyclotron (EMIC) waves are generated around the equatorial regions and propagate toward the high latitude ionospheres in both hemispheres. Here we describe a prolonged EMIC wave event in the Pc2 (0.1–0.2 Hz) frequency band above the He+ cyclotron frequency detected by the four Cluster satellites as they traversed sunward from L∼ 13 in the outer magnetosphere to the magnetopause, over 13°–20° magnetic latitude north of the equator and across the high latitude cusp region near local magnetic noon. Wave packet energy propagated dominantly along the geomagnetic field direction, confirming this was a traveling EMIC wave rather than a toroidal field line resonance. The energy packets propagated in alternating directions rather than uni‐directionally from the equator, implying the wave source was located in a high latitude region away from the equator, where a minimum in the B field is located. The CIS‐CODIF H+ion data provided evidence that the waves were generated locally via the ion cyclotron instability. We believe the off‐equatorial minimum magnetic field regions may be important source regions for these waves in the outer magnetosphere.

Transverse‐scale size of Pc3 ULF waves near the exterior cusp

The concept of a geomagnetic field line has been widely used in the study of magnetospheric physical phenomena. For example, the mode in which ULF waves propagate relates to the direction of the field, with the Alfven wave mode propagating along the field direction. Very little is known about the perpendicular extent of the propagating wave. In this paper, wave coherency methods are utilized to analyze ULF waves in the Pc3 band that were simultaneously observed by the Cluster satellites near the exterior cusp and by ground stations at local magnetic noon near the footprint of the cusp. The results show that the coherency of waves observed at the ground on the H component is much larger than that on the D component, which is opposite to that seen by Cluster in space. The coherency between the H component on the ground and the y component in space was higher than the other combination of pairs, with the coherency between the satellite and the Daneborg (DNB) station having the maximum value. These results suggest that the polarization of the waves are rotated by 90° after propagating through the ionosphere, and the magnetic footprint of Cluster is closest to the DNB station at this time. The coherency of the Pc3 waves between the satellites is highly related to the alignment of satellite pairs with respect to the geomagnetic field direction. This alignment may provide a transverse‐scale size of the geomagnetic Pc3 ULF waves near the exterior cusp at ∼900 km with a coherency of 0.65.

The Response of High‐Latitude Ionospheric Convection During a Southward IMF Turning Event

AbstractConvection data from SuperDARN radars in the northern hemisphere are utilized to investigate the response of high‐latitude ionospheric convection during a southward IMF turning event, focused on the response timescale. In this event, IMF Bz component changed from +7 nT to –8 nT in 1.5 min and had been relatively stable for 40 min before and after the IMF turning, respectively. The initial ionospheric convection response started near magnetic local noon about 3 min after the IMF turning. The response was delayed by increasing amounts away from noon. The delay of the convection response is about 6 min around dawn and dusk, and 12 min around midnight. There is no clear MLT dependence for the reconfiguration time of the convection response and such timescale is 10~14 min.

The coherence between the IMF and high-latitude ionospheric flows: The dayside magnetosphere–ionosphere low-pass filter

This study seeks to establish a new system characteristic describing dayside convective flows in the coupled magnetosphere–ionosphere: the low-pass filter function through which interplanetary magnetic field (IMF) fluctuations are processed as they are communicated from the magnetopause to the high-latitude ionosphere near local noon. In doing so, this study confirms that variations in the ionospheric flows at high-latitudes near local noon are well correlated with variations in the IMF orientation and magnitude on short timescales. We construct the filter function by comparing time series of the ionospheric equivalent flows at a fixed location at magnetic local noon and 80° latitude with time series of the IMF. The coherence spectra of these two parameters—averaged over 330 h of comparison—indicate that there is a low-pass cutoff in the ionospheric response to IMF driving at a periods shorter than 20 min (frequencies higher than 0.8 mHz). When there is sufficient power in the IMF fluctuations, this cutoff is relatively sharp—the coherence drops by roughly a factor of three between the periods 32 and 21 min (0.5 and 0.8 mHz). The results also show that on average the coherence between the east–west component of the equivalent flows and IMF B y tends to be less than the coherence between the north–south component of the equivalent flows and IMF B z .

Ionospheric signatures of the low-latitude boundary layer under conditions of northward IMF and small clock angle

Abstract. A case study is presented that concerns the footprints of the low-latitude boundary layer in the high-latitude ionosphere. The measurements were made near local magnetic noon in summertime under conditions of Bz>0 and small clock angle. Of particular interest are particle fluxes measured in the region by the NOAA-12 satellite that revealed energetic (>30 keV) electrons, characteristic of trapped particles, together with a population of softer precipitating magnetosheath particles. The particle energy-distribution was distinct from those identifying the central plasma sheet at lower latitudes. On its poleward side the layer extended to at least the latitude of the polar cap boundary as identified in ion flows and electron densities measured by the EISCAT Svalbard radar. It is proposed that the particles of the low-latitude boundary layer occurred on newly-closed magnetic field lines, which were formed by the closure of open polar cap field by lobe reconnection in both Northern and Southern Hemispheres.

ULF geomagnetic pulsations in the southern polar cap: Simultaneous measurements near the cusp and the geomagnetic pole

During November 2003, a 1‐week test campaign of ULF geomagnetic field measurements was conducted at the new Italian/French base of Concordia (Dome C, Antarctic plateau), close to the geomagnetic pole, at a corrected geomagnetic latitude of ∼89°S. An analysis of these measurements is presented, together with a comparison with simultaneous measurements conducted at the Italian “Mario Zucchelli” base, at Terra Nova Bay, also located in the polar cap, but at lower latitude (∼80°S). The variable solar wind and interplanetary magnetic field conditions made it possible to monitor the Pc 3–4 ULF pulsation activity under different magnetospheric conditions. For quiet conditions, the pulsation power does not show evidence for a diurnal modulation at Dome C, while at Terra Nova Bay it maximizes around local magnetic noon. This result can be interpreted in terms of the different latitude of the two stations which might be linked to different magnetospheric regions: Dome C deep in the polar cap, and Terra Nova Bay approaching the cusp around noon. Conversely, during a strong geomagnetic storm, the wave activity at the two stations becomes very similar, and major ground manifestations are driven by strong solar wind pressure pulses. The clear dependence of the pulsation frequency with the interplanetary magnetic field strength suggests that upstream waves can be the source of pulsations not only at Terra Nova Bay but also at Dome C, in the deep polar cap. We also find a clear relationship between the pulsation power and the solar wind speed which is more pronounced at Dome C, where cusp‐related phenomena are not present.

Initial results of high‐latitude magnetopause and low‐latitude flank flux transfer events from 3 years of Cluster observations

We present initial results from a statistical study of Cluster multispacecraft flux transfer event (FTE) observations at the high‐latitude magnetopause and low‐latitude flanks from February 2001 to June 2003. Cluster FTEs are observed at both the high‐latitude magnetopause and low‐latitude flanks for both southward and northward IMF. Among the 1222 FTEs, 36%, 20%, 14%, and 30% are seen by one, two, three, and four Cluster satellites, respectively. There are 73% (27%) of the FTEs observed outside (inside) the magnetopause, which might be caused by the motion of FTEs toward the magnetosheath when they propagate from subsolar magnetopause to the midlatitude and high‐latitude magnetopause and low‐latitude flanks. We obtain an average FTE separation time of 7.09 min, which is at the lower end of the previous results. The mean BN peak‐peak magnitude of Cluster FTEs is significantly larger than that from low‐latitude FTE studies. FTE BN peak‐peak magnitude clearly increases with increasing absolute magnetic latitude (MLAT), it has a weaker dependence on magnetic local time (MLT) with a peak near the magnetic local noon, and it has a complex dependence on Earth dipole tilt with a peak at around zero. FTE periodic behavior is found to be controlled by MLT, with a general increase of FTE separation time with increasing MLT, and by Earth dipole tilt, with a peak FTE separation time at around zero Earth dipole tilt. There is no clear dependence of FTE separation time on MLAT. There is a weak increase of FTE BN peak‐peak magnitude with increasing FTE separation time, and we see no clear dependence of it on FTE BN peak‐peak time. When no FTE identification thresholds are used, more accurate calculations of some FTE statistical parameters, including the mean BN peak‐peak time, can be obtained. Further, comparing results with different thresholds can help obtain useful information about FTEs.

A statistical investigation of dayside magnetosphere erosion showing saturation of response

We present a statistical investigation of dayside magnetospheric erosion based on a survey of 316 cases in 1996–2004. We monitor erosion through the depression in the strength of the terrestrial magnetic field at geostationary heights using magnetic field observations acquired by four GOES spacecraft 2 hours on either side of local magnetic noon. The southward component of the interplanetary magnetic field (IMF) and the solar wind dynamic pressure, quantities which are responsible for opposing effects on the geostationary field, are obtained from the Wind and ACE spacecraft. We extend our previous work to encompass the Bz range [0, −35] nT. We find that dayside erosion saturates when IMF Bz is in the range [−12, −16] nT. This result is consistent with global simulations. With the measured solar wind velocity, this corresponds to an interplanetary electric field (IEF) of 6.2 ± 1.6 mV m−1. We discover that the maximum depression of the geostationary field at saturated erosion is ∼26 nT. In addition, we reveal a direct relation between the saturation of flux erosion of the dayside magnetosphere and that of the cross‐polar cap potential (CPCP), both of which occur at approximately the same electric field. For the latter, we apply the Hill‐Siscoe model for the calculation of CPCP to our data set. We find the CPCP to deviate from a linear increase with IEF at ∼4 and 7 mV/m, which correspond closely to the IEF range at which we find saturation of dayside erosion to start to manifest itself.

Interannual consistency of bi‐monthly differences in diurnal variations of the ground‐level, vertical electric field

Interyear consistency is demonstrated in the bi‐monthly average diurnal vertical electric field measured over ∼720 “fair‐weather” days collected during a 5‐year interval (1998 to 2002) at Vostok (78.5°S, 107°E; magnetic latitude 83.6°S), Antarctica. After correcting for the influence of polar‐cap convection, seasonal peaks in the average electric field values occur around July‐August with a diurnal maximum at ∼ 2050 UT while lowest average magnitudes are measured near November‐December when the associated diurnal maximum occurs at ∼1830 UT. These variations are consistent with expected seasonal changes in global thunderstorm activity. Comparisons of ground‐level vertical electric field measurements (Vm−1) with Weimer‐1996 model cross‐polar cap potentials above Vostok (kV) for individual UT hours yield significant correlations over all hours but with reduced standard errors around local magnetic noon (∼1300 UT). This implies a more direct linkage between solar wind parameters and the cross‐polar cap potential near magnetic noon, for this site (magnetic latitude: 83.6°S). An all hours–all seasons linkage factor of 0.76 ± 0.06 Vm−1 per kV is determined, broadly consistent with an average ionosphere‐ground potential difference of ∼250 kV and a measured average vertical electric field of 185 Vm−1. Evidence is presented supporting a seasonal variation in this linkage factor, with generally lower magnitudes in the austral winter (May to August).

Observing the open‐closed boundary using cusp‐latitude magnetometers

A new ground‐based diagnostic of reconnection at the magnetopause using ULF waves is presented, and we give an example day in which the dynamics of the open‐closed boundary (OCB) are clearly observed. The diagnostic is based on data from two close (∼110 km), magnetic azimuthally spaced magnetometer sites (Davis and Zhongshan, Antarctica). During average IMF conditions Davis passes just equatorward of the last closed field lines in the dayside magnetosphere, and cross‐phase measurements indicate propagation away from local magnetic noon (LMN) as expected for the Kelvin‐Helmholtz Instability (KHI) acting at the magnetopause. During times when IMF Bz < −2 nT intervals of broadband activity appear, with the azimuthal component of propagation directed towards noon. Statistical analysis shows a correlation between the negative magnitude of IMF Bz and cross‐phase measurements ±2–6 hours from LMN. We conclude that under the right conditions, reconnection signatures can be identified in magnetometer data superimposed on the KHI‐like propagation signatures of the last closed field lines.

Southern high-latitude Digisonde observations of ionosphere E-region Bragg scatter during intense lacuna conditions

Abstract. During summer months at solar cycle minimum, F-region lacuna and slant-Es conditions (SEC) are common features of daytime ionograms recorded around local magnetic noon at Casey, Antarctica. Digisonde measurements of drift velocity height profiles show that the occurrence of lacuna prevents the determination of F-region drift velocities and also affects E-region drift velocity measurements. Unique E-region spectral features revealed as intervals of Bragg scatter superimposed on typical background E-region reflection were observed in Digisonde Doppler spectra during intense lacuna conditions. Daytime E-region Doppler spectra recorded at carrier frequencies from 1.5 to 2.7MHz, below the E-region critical frequency foE, have two side-peaks corresponding to Bragg scatter at approximately ±1-2Hz symmetrically located on each side of a central-peak corresponding to near-zenith total reflections. Angle-of-arrival information and ray-tracing simulations show that echo returns are coming from oblique directions most likely resulting from direct backscatter from just below the total reflection height for each sounding frequency. The Bragg backscatter events are shown to manifest during polar lacuna conditions, and to affect the determination of E-region background drift velocities, and as such must be considered when using standard Doppler-sorted interferometry (DSI) techniques to estimate ionospheric drift velocities. Given the Doppler and spatial separation of the echoes determined from high-resolution Doppler measurements, we are able to estimate the Bragg scatter phase velocity independently from the bulk E-region motion. The phase velocity coincides with the ExB direction derived from in situ fluxgate magnetometer records. When ionospheric refraction is considered, the phase velocity amplitudes deduced from DSI are comparable to the ion-acoustic speed expected in the E-region. We briefly consider the plausibility that these previously unreported polar cap E-region Bragg scatter Doppler spectral signatures, observed at Casey in December 1996 during SEC/lacuna conditions may be linked to ionosphere irregularities. These irregularities may possibly be generated by primary plasma waves triggered by current-driven instabilities, that is to say, a hybrid of the "modified two-stream" and "gradient drift" instability mechanisms.

Evidence for solar-production as a source of polar-cap plasma

Abstract. The focus of the study is a region of enhanced ionospheric densities observed by the EISCAT Svalbard radar in the polar F-region near local magnetic noon under conditions of IMF Bz<0. Multi-instrument observations, using optical, spacecraft and radar instrumentation, together with radio tomographic imaging, have been used to identify the source of the enhancement and establish the background ionospheric conditions. Soft-particle precipitation was ruled out as a candidate for the production. Tomographic observations identified a latitudinally restricted region of enhanced densities at sub-auroral latitudes, distinct from the normal mid-latitude ionosphere, which was likely to be the source. The evidence suggested that the increased sub-auroral densities were photoionisation produced at the equatorward edge of the afternoon high-latitude cell, where the plasma is exposed to sunlight for an extended period as it flows slowly sunward toward magnetic noon. It is proposed that this plasma, once in the noon sector, was drawn antisunward by the high-latitude convection toward polar latitudes where it was identified by the EISCAT Svalbard radar. The observations are discussed in terms of earlier modelling studies of polar patch densities. Key words. Ionosphere (polar ionosphere; plasma temerature; plasma convection)

Plasmaspheric mass loss and refilling as a result of a magnetic storm

Using the sounding measurements from the radio plasma imager on IMAGE and a plasma density inversion algorithm, we derive the plasma density profiles along the magnetic field in a few L shells every 14 hours at magnetic local noon before, during, and after the 31 March 2001 magnetic storm. An empirical model of the plasmaspheric plasma density distribution is derived as a reference using the measurements before the storm. During the storm the equatorial plasma was substantially depleted in a range of L shells. The flux tubes were refilled after the storm. The filling ratio, the equatorial plasma density normalized by its quiet time value before the storm, is introduced to assess the time evolution of the depletion and refilling processes. The depletion, more than two thirds of the quiet time content, appeared to occur rather quickly after the storm onset, as determined by the limited temporal resolution of the measurements. The refilling proceeded, although more slowly than the depletion process, significantly faster than the theoretical prediction of a 3‐day timescale. Dynamic structures are observed in situ and confirmed by the extreme ultraviolet imager (EUV) measurements.

The geostationary field during dayside erosion events 1996–2001: A joint Wind, ACE, and GOES study

We carry out a statistical investigation on dayside magnetosphere erosion during the period 1996–2001, using Wind, ACE, and GOES observations supplemented by ground‐based magnetograms. The inner magnetospheric signature of erosion that we focus on is the depression of the geostationary magnetic field strength (ΔBtot) within 2 hours of local magnetic noon. We relate this decrease to the southward component of the interplanetary magnetic field (IMF) in the range −16 ≤ Bz ≲ 0 nT and the solar wind dynamic pressure in the range ∼0–6 nPa. Thus both relevant interplanetary parameters extend to well beyond average values at 1 AU. The geostationary field strength (Btot) is derived from measurements by the GOES 8, 9, and 10 spacecraft. Our data set consists of 288 measurements. We adopt two approaches. In one approach, we first correct for the effect of dynamic pressure on the geostationary field strength by studying days with various dynamic pressure values but low IMF clock angles, θ (<45°). With the latter condition we mean to exclude dayside reconnection to a reasonable degree. We then use this pressure correction to infer the erosion contribution to Btot (ΔBtot). We find ΔBtot[nT] = −3.58 + 1.67Bz with a correlation coefficient of 0.59. This analysis is most reliable in giving the change of the geostationary field to a decrease in IMF Bz, for which we obtain ΔBtot/Bz = −1.67 nT per 1 nT decrease in Bz. In the second approach, we do a bivariate fit of Btot to the (Bz, Pdyn) data pairs and obtain Btot[nT] = 97.64 + 20.22 + 1.56Bz. Major sources of error are discussed, and the results are compared to previous work.

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.

An unusual geometry of the ionospheric signature of the cusp: implications for magnetopause merging sites

Abstract. The HF radar Doppler spectral width boundary (SWB) in the cusp represents a very good proxy for the equatorward edge of cusp ion precipitation in the dayside ionosphere. For intervals where the Interplanetary Magnetic Field (IMF) has a southward component (Bz < 0), the SWB is typically displaced poleward of the actual location of the open-closed field line boundary (or polar cap boundary, PCB). This is due to the poleward motion of newly-reconnected magnetic field lines during the cusp ion travel time from the reconnection X-line to the ionosphere. This paper presents observations of the dayside ionosphere from SuperDARN HF radars in Antarctica during an extended interval ( ~ 12 h) of quasi-steady IMF conditions (By ~ Bz < 0). The observations show a quasi-stationary feature in the SWB in the morning sector close to magnetic local noon which takes the form of a 2° poleward distortion of the boundary. We suggest that two separate reconnection sites exist on the magnetopause at this time, as predicted by the anti-parallel merging hypothesis for these IMF conditions. The observed cusp geometry is a consequence of different ion travel times from the reconnection X-lines to the southern ionosphere on either side of magnetic local noon. These observations provide strong evidence to support the anti-parallel merging hypothesis. This work also shows that mesoscale and small-scale structure in the SWB cannot always be interpreted as reflecting structure in the dayside PCB. Localised variations in the convection flow across the merging gap, or in the ion travel time from the reconnection X-line to the ionosphere, can lead to localised variations in the offset of the SWB from the PCB. These caveats should also be considered when working with other proxies for the dayside PCB which are associated with cusp particle precipitation, such as the 630 nm cusp auroral emission.Key words. Ionosphere (plasma convection) – Magnetospheric physics (magnetopause, cusp, and boundary layers) – Space plasma physics (magnetic reconnection)

Special features of the September 24–27, 1998 storm during high solar wind dynamic pressure and northward interplanetary magnetic field

The geomagnetic storm on September 24 – 27, 1998, was initiated by a sudden compression of the magnetosphere in response to the solar wind dynamic pressure pulse. Simultaneous with the pressure increase, the interplanetary magnetic field (IMF) became strongly northward. Several unexpected magnetospheric responses to this sudden impulse were observed. First, for ∼ 30 min following the sudden impulse, the entire auroral oval became active and thick, while the polar cap area decreased to less than 1/2 of its original size. The second unusual observation associated with the sudden impulse is the global magnetic perturbation measured by low‐latitude magnetic stations. The field shows an asymmetric increase in the axial component (parallel to the dipole axis) with the strongest enhancement measured on the night side and at local magnetic noon the perturbation is small or slightly negative. This is very unusual since sudden compressions are generally measured by low‐latitude stations to have the largest enhancement of the field on the dayside. The main phase of the geomagnetic storm begins in the second hour on September 25, 1998, following the southward turning of the IMF. The auroral oval becomes thin and moves equatorward increasing the polar cap area by more than a factor of 3. The theoretical analysis presented in this paper suggests that the response to the sudden impulse is an electrodynamic effect produced by a “transition” current system in response to the northward turning of the IMF.