Magnetic Impulse Events Research Articles

Magnetic impulse event: A detailed case study of extended ground and space observations

Analysis of conjugate data from extended magnetometer networks in northern and southern high latitudes is used to elucidate the initiation and the evolution of a magnetic impulse event (MIE) on June 6, 1997. In addition, data from all‐sky imagers, imaging riometers, and Super Dual Auroral Radar Network radars in Antarctica are investigated to confirm the energy content, motion, and electrical current structure of the MIE. The MIE was accompanied by traveling convection vortices (TCVs) that began at ∼10 MLT and moved eastward (toward dusk) and slightly equatorward at 1–3 km/s across the noon meridian with north‐south conjugacy. The MIE had upward field‐aligned currents with soft electron precipitation that was located near the trailing edge of the Hall current loop. During the MIE interval the interplanetary magnetic field (IMF) was directed strongly outward from the Sun (Bx = −5 nT), with a slightly positive (1–2 nT) Bz, and a nearly zero By. Since abrupt solar wind pressure changes are unlikely under this IMF orientation (and none was, in fact, observed), classical mechanisms for MIE generation, such as a pressure pulse or dayside reconnection, are excluded. It is speculated that an abrupt IMF cone angle change from 60° to 20°, ∼30 min prior to the MIE onset, may have been an indirect trigger of this event via the interaction between the solar wind and the bow shock.

Dayside optical and magnetic correlation events

Sudden, short‐lived dayside auroral activity is often observed equatorward of the quiescent auroral oval. Several cases from the 1996 South Pole and United States Automatic Geophysical Observatory data were examined. In all cases the optical events were accompanied by well‐correlated magnetic impulsive events. Most optical auroral activity was primarily in 630‐nm emission (soft electron precipitation presumably of plasma sheet or magnetosheath origin). Most optical events also show the presence of much shorter lived 427.8‐nm emission (harder electron precipitation with associated electron acceleration). In almost all events the keograms showed repeated poleward propagation, indicating that the event started at lower latitudes and propagated to higher latitudes. The optical emissions showed distinct periodicities, which usually correlated well with the magnetic signature. All the events began equatorward of the preexisting quiescent aurora, indicating that they initiated in the region of closed field lines. The interplanetary magnetic field Bz component prior to the events was either small or positive in most cases. The majority of the observed events were consistent with being triggered by interplanetary Bz or solar wind pressure change. For some events, no specific trigger was found.

A statistical study of transient event motion at geosynchronous orbit

The geosynchronous GOES 5 and GOES 6 satellites frequently observe transient events marked by magnetic field strength increases and bipolar magnetic field signatures lasting several minutes. In this study we report a survey of 87 events observed simultaneously by both GOES spacecraft (for a total of 174 individual observations) from August to December 1984. Events detected in the prenoon sector outnumbered those in the postnoon sector by about a 3 to 1 ratio. The distribution of the events versus local time exhibited a significant prenoon peak like the distribution of magnetic impulse events observed in high‐latitude ground magnetometers. A cross‐correlation analysis of the two GOES data sets indicated lags that range from 0 to over 2 min, with the majority of the events moving antisunward. The short lags correspond to azimuthal speeds of hundreds of kilometers per second, greater than flow speeds in the magnetosheath, but less than fast mode waves. The short lags may indicate that the events move primarily latitudinally and/or that transient events are seldom localized, but rather occur over extended, if not global, regions. Investigations of event occurrence versus interplanetary magnetic field (IMF) Bz, event motion versus IMF By, and correspondence between upstream plasma data and the events all indicate that pressure pulses are the likely source of many of the events. About 27% of the events with simultaneous solar wind data were preceded by sharp reversals in one or more IMF components, and nearly all of this particular group of events occurred in the dawn sector. This suggests that the pressure pulses may be commonly generated in the foreshock/bow shock region, since the prenoon magnetopause lies generally behind the quasi‐parallel bow shock where such pulses are thought to be triggered by IMF discontinuities. Finally, several events in the data set were also observed by the AMPTE/CCE. These are presented as case studies.

Magnetic impulse events and related Pc1 bursts observed by the Automatic Geophysical Observatories network in Antarctica

Magnetic field data obtained by fluxgate and search coil magnetometers installed at four Automatic Geophysical Observatories (AGO Pl, P2, P3, and P4) and at South Pole and McMurdo in Antarctica and at Iqaluit on Baffin Island are used to study the relationship between magnetic impulse events (MIEs) and Pc1 bursts at high geomagnetic latitudes. The spatial scale sizes of MIEs are determined in the investigation of the sources of these events in the magnetosphere. Four MIE events with simultaneous Pc1 bursts are studied. Contour plots of MIE amplitudes show the scale sizes of the examined MIEs to be 5° to 7° in the magnetic latitude direction and 40° to 60° in the magnetic longitude direction. Temporal changes of the ionospheric currents calculated from the fluxgate H‐ and D‐component data indicate that the “convection vortices” associated with the MIEs traveled westward and decayed rapidly. An important finding is that MIE‐related Pc1 bursts were observed at higher‐latitude (Pl, P4 near 80°) and lower‐latitude (P2, P3 near 70°) AGO stations and South Pole Station at 74° with different spectral structures, although the Pc1 spectral power was always a maximum at South Pole Station. These results imply that Pc1 bursts are excited not only in the dayside outer magnetosphere but also in the low‐latitude boundary layer region.

Coupling between field‐aligned current impulses and Pi1 noise bursts

The estimated densities of local field‐aligned currents related to nighttime substorm onset or magnetic impulse events at cusp latitudes may exceed the threshold for the excitation of high‐frequency turbulence in the topside ionosphere. The evolution of an ion‐acoustic instability is shown to lead to quasi‐oscillations around the saturation level. Consequent variations in anomalous conductivity will result in pulsed electron precipitation and noise generation in the Pi1 band. This proposed mechanism gives a natural interpretation to the experimentally observed coupling between localized magnetic disturbances, electron precipitation/acceleration, and bursts of high‐frequency ULF noise. The alternative interpretations based on mechanisms of ion cyclotron instability stimulation and ionospheric Alfven resonator excitation can be applied only under specific plasma conditions.

Cosmic noise absorption at South Pole Station during magnetic impulse events

We report the results of an examination of the records of cosmic radio noise absorption in the ionosphere during times of previously identified magnetic impulse events (MIEs) at South Pole (SP) Station, Antarctica. Approximately 80% of the 153 MIEs that occurred in the interval from January to December 1986 were accompanied by impulsive increases in riometer absorption; a much smaller fraction (∼4%) showed temporary decreases from a prevailing enhanced level. As is the case for high‐latitude MIE events, generally, those associated with absorption exhibit a large morning peak at ∼1000 magnetic local time (MLT) and a smaller afternoon peak at ∼1400 MLT and occur most commonly at the equinoxes. Approximately 20% of the MIE events were not accompanied by an absorption response. These cases occurred preferentially in the afternoon hours. MIEs in the no response class may have occurred farther from SP Station, and thus out of the range of the riometer measurement, than those which show absorption. This is suggested by the distribution of their H component values which peaks at 40–70 nT, the lowest‐amplitude range considered. The high percentage of absorption/MIE coincidences indicates that the MIE mechanism often leads to the precipitation of energetic electrons but that on rare occasions may result in the temporary reduction or cessation of preexisting precipitation.

Morphological features of bipolar magnetic impulsive events and associated interplanetary medium signatures

Morphological features of magnetic impulsive events and the dynamics of associated ionospheric perturbances are examined from the magnetic records of high‐latitude stations. Most impulses chosen for investigation display a bipolar variation in all the magnetic field components. Event occurrence patterns versus magnetic local time for different latitudes reveal the existence of large‐scale and small‐scale events. The large‐scale magnetic events were observed typically throughout the wide latitude range from 67° to 77° corrected geomagnetic latitude (CGL). The occurrence pattern of events versus magnetic local time (MLT) shows a single maximum near 0900 MLT. Only about 12% of the impulsive perturbations were generated in the afternoon sector from 1200 to 1500 MLT. These events first moved westward as well as sunward up to noon and then tailward. The small‐scale impulsive magnetic perturbations are generally registered in the midday sector from 70° to 74° CGL. The small‐scale events, like the large‐scale traveling convectin vortices (TCVs), move westward with an average velocity of about 3.7 km/s. IMP‐8 observations of interplanetary medium conditions were investigated for the large‐scale events. The events occur mainly during periods when the Bz component of the interplanetary magnetic field (IMF) was northward. The events do not tend to occur for the highspeed solar wind flows or the flows with unusually high plasma densities. The IMF was frequently oriented close to the ecliptic plane and was almost radial along the Sun‐Earth direction. Magnetic events were caused generally by the solar wind dynamic pressure variations with increases in amplitude from 20 to 140%. We suggest that pressure impulses may be amplified by a factor of 2 by the simultaneous change of the foreshock geometry from lying upstream of the prenoon magnetosphere toward dawn, when the IMF cone angle changes from <30° to >30°.

Three‐dimensional MHD simulations of interplanetary rotational discontinuities impacting the Earth's bow shock and magnetosheath

The first ever fully three‐dimensional magnetohydrodynamic (MHD) simulations of the detailed effects of MHD discontinuities on the magnetosheath have been performed. The simulation results predict that the interaction between a rotational discontinuity (RD) and the bow shock produces an MHD wave pulse that propagates downstream from the bow shock to the magnetopause. The main components of this pulse are two slow shocks sandwiched between two time‐dependent intermediate shocks; in perfectly ideal MHD, these shocks would presumably resolve into slow shocks sandwiched between two RDs. Inside the pulse, the plasma density, thermal pressure, dynamic pressure (ρυ2), and total pressure all increase, while the magnetic field magnitude decreases. The pulse convects with the downstream flow through the magnetosheath at a speed that is markedly slower than that of the original RD. It comes to rest on the magnetopause, where it raises the total pressure by as much as 75% in some locations. The pulse eventually disappears, as it is convected away by the solar wind flow around the magnetopause. The pulse remains in the magnetosheath for a few Alfvén times (i.e., solar wind Alfvén velocity divided by the Earth's radius, perhaps 1–3 min depending on the actual solar wind parameters) after the initial RD has propagated downstream past the magnetopause. Comparisons are made with previous one‐dimensional and two‐dimensional studies of this problem. We conclude that the pulses seen in these simulations are possible causes of magnetic impulse events observed in the ionosphere and slow mode structures observed in the inner subsolar magnetosheath.

Simulation of pressure pulses in the bow shock and magnetosheath driven by variations in interplanetary magnetic field direction

Two‐dimensional (2‐D) hybrid simulations are carried out to study the effects of the variation in the interplanetary magnetic field (IMF) direction on the bow shock, magnetosheath, and magnetosphere. A curvilinear coordinate system is used in the simulation. The 2‐D simulation is also compared with our one‐dimensional simulation results. It is found that pressure pulses are generated as a result of the interaction between the bow shock (BS) and an interplanetary rotational discontinuity (RD). First, a structure consisting of a rotational discontinuity and two slow shocks are present downstream of the bow shock after the BS/RD interaction. The magnetic field and plasma density are anticorrelated in this structure. The dynamic pressure increases in the structure, leading to a pressure pulse in the magnetosheath. Second, a pressure pulse associated with reflected ions at the bow shock may be generated in the foreshock when the IMF changes its direction, especially when a local quasi‐parallel bow shock becomes a quasi‐perpendicular shock. The magnetic field, plasma density, and dynamic pressure are positively correlated in the upstream pressure pulse. This pressure pulse convects through and interacts with the bow shock, producing a pressure pulse in the downstream region. The downstream pressure pulses propagate to the magnetopause. The amplitude of the downstream pressure pulses can be up to 100% of the background magnetosheath value. It is suggested that the pressure pulses impinging on the magnetopause may lead to the magnetic impulse events observed in the high‐latitude ionosphere.

SCIFER‐Height measurements of the midmorning aurora

Dual‐site ground‐based optical observations were performed January 25, 1995 in support of the SCIFER rocket campaign (Sounding of the Cleft Ion Foundation Energization Region). Two Meridian Scanning Photometers (MSPs) were operative simultaneously at Ny‐Ålesund (NYA) and Nordlysstasjonen in Adventdalen (LYR) 118 km apart in the geomagnetic meridian. The photometers at each station scanned approximately along the same geomagnetic meridian plane and measured the intensity of auroral wavelengths as a function of zenith angle and time. The principal wavelengths used in the present analysis were the green 5577 Å and the red 6300 Å emission lines of atomic oxygen. For the magnetospheric cleft auroral activity studied here at 10 hr magnetic local time (MLT), the luminosity maximum for the green line varied in altitude between 125 and 160 km. The red line maximum was in the range 200 to 220 km. The absolute intensity ratio and the height‐luminosity profiles obtained using this method were consistent with the near simultaneous rocket measurement of the field‐aligned electrons having a characteristic energy of 230 eV by the SCIFER experiment [Lorentzen, et al., 1996]. Geomagnetic disturbance data obtained during the flight included an example of a magnetic impulse event (MIE) coinciding with a bright, discrete arc. The altitude of this arc was >140 km, and it is doubtful that it produced an increase of conductivity in the E Region or developed a current system which could be associated with the MIE. It is proposed that the transient magnetic events were associated with the energetic pulsating aurora which was occurring equatorward of the zenith. In addition, the altitude of three poleward‐moving auroral forms (PMAFs) was measured. Within the error of the observations, the PMAFs do not seem to change in altitude with increasing latitude. The sample is too small for a definitive conclusion, but the method shows promise.

Magnetic impulse events and associated Pc 1 bursts at dayside high latitudes

Ground measurements of bursts of Pc 1 pulsations (minutes duration) are a common dayside phenomenon at high latitudes. For the past several years, dayside ground measurements of similar‐duration bursts of Pc 5 period pulsations, magnetic impulse events (MIE), have attracted a great deal of interest concerning their role in dayside reconnection. From an analysis of several years of induction antenna data from southern and northern hemisphere sites in the Antarctic, Greenland, and Canada, it is found that about 70% of all isolated MIE events are associated with Pc 1 bursts, and about half of all events are conjugate between hemispheres. There appears to be no definitive phase relationship between the onset of the Pc 1 bursts and the MIE, suggesting that both might be produced by the same event on the dayside but have different source locations. Correlative data from ground and spacecraft measurements suggest this to be the case and also show that the Pc 1 are waves inside the magnetosphere propagating into the ionosphere. Finally, a timing study between ground sites separated by 2 hours local time shows that the isolated MIEs are measured first near the subsolar point and at later times down both flanks of the magnetosphere. This suggests that the source mechanism for these events has an E–W component of motion which when projected to the equatorial plane is of the order of the antisunward flow velocity in the magnetosheath. As this source moves down the magnetospheric flanks it creates well within the magnetosphere conditions (pressure pulses?) ripe for the ion cyclotron generation of the Pc 1 bursts associated with the MIE.

Generation of dynamic pressure pulses downstream of the bow shock by variations in the interplanetary magnetic field orientation

One‐dimensional resistive MHD and hybrid simulations are carried out to study the manner by which variations of the interplanetary magnetic field (IMF) direction generate dynamic pressure pulses in the magnetosheath. The reaction of the magnetosheath to the temporal IMF variation is modeled as the interaction between the bow shock (BS) and an interplanetary rotational discontinuity (RD), an Alfven wave pulse (AW), or an Alfven wave train. The resistive MHD simulation indicates that the arrival of an RD produces two time‐dependent intermediate shocks (TDISs) and two slow shocks downstream of the bow shock, which propagate through the magnetosheath toward the Earth's magnetopause. An enhancement of plasma density is present throughout the TDISs and slow shocks. A plasma dynamic pressure pulse is formed in this region. In the hybrid simulation, the two TDISs are replaced by rotational discontinuities. For a bow shock with a shock normal angle θBn > 45°, the pulse in the dynamic pressure ρV2 causes the total pressure (P + B2/2μ0 + ρV2) in the magnetosheath to increases by about 0–100% of the background value. The strength of the pressure pulse increases with the field rotation angle across the incident rotational discontinuity, while it decreases with the Mach number or upstream plasma beta of the bow shock. The pressure pulse propagates toward the magnetopause with nearly a constant amplitude. On the other hand, the BS/AW interaction leads to the generation of Alfven waves downstream of the bow shock, and large‐amplitude dynamic pressure pulses are generated in the downstream Alfven wave. Pressure pulses impinging on the magnetopause may produce magnetic impulse events (MIEs) observed in the high‐latitude ionosphere.

Dayside high latitude magnetic impulsive events: their characteristics and relationship to sudden impulses

On 17 December 1990 a series magnetic impulsive events (MIEs) were observed at high latitudes near local noon. EISCAT, situated some 5 hours of MLT away from the noon sector, detected simultaneous impulsive electron density enhancements at heights between 90 and 120 km. The MIEs at noon were also associated with riometer absorption spikes. The correlated EISCAT and riometer observations indicate that there was an elongated electron precipitation region some 3000 km wide stretching from local noon to morning. In close association with the impulsive electron precipitation, VLF emissions were observed by groundbased stations in the morning side. We interpret the large scale electron precipitation and VLF emissions as signatures of a global compression of the Earth's magnetosphere. This is confirmed by the specific type of magnetic variations simultaneously recorded at the worldwide network of magnetometers. We conclude that the small scale MIEs with their drifting ionospheric current vortex structures can (but do not necessarily have to) occur in conjunction with large scale SIs. Moreover, MIEs and SIs have a common origin: the interaction of solar wind inhomogeneities with the Earth's magnetosphere. They do, however, represent different effects of the same primary agent.

Cusp latitude magnetic impulse events: 3. Associated low‐latitude signatures

Low‐latitude magnetic perturbations at San Juan, Puerto Rico, occurring simultaneously with the set of South Pole magnetic impulse events (MIEs) discussed by Lanzerotti et al. (1991) and Konik et al. (1994) are examined. The amplitudes of the magnetic deflections at the times of the MIEs at South Pole and at San Juan are compared to the magnetic deflections seen at these two stations during storm sudden commencements (SSCs). At the South Pole, we find the mean deflections in the magnetic H and V components to be larger for the MIEs than for the SSCs. At San Juan, we find the opposite to be true; the mean amplitudes of the deflections in the H and V components of the field are much smaller during the MIEs than during the SSCs. The interplanetary plasma conditions prevailing at the times of the MIEs are also examined and contrasted to those found during SSCs. We find that while the MIEs are characterized by changes in the solar wind dynamic pressure at the noise level, the SSCs are concurrent with large changes in the dynamic pressure. Examining the correlations between MIE amplitudes and dynamic pressure changes, we find no correlation for the MIE deflections at the South Pole, but a strong correlation for the concurrent deflections seen at San Juan. In contrast, we find strong correlations between SSC amplitudes and dynamic pressure changes at both the South Pole and San Juan. We conclude that the MIE signatures at the South Pole are not predominantly generated by dynamic pressure fluctuations, and that the concurrent ground magnetic signatures at San Juan are unrelated to those at the South Pole. We also examine our data in order to deduce any amplitude dependence upon local time and in particular whether larger ground signatures are found in local morning than in local afternoon. We find that for both the sets of MIEs and SSCs, the amplitudes of deflections at the South Pole exhibit no significant local time pattern.

Coincident bursts of auroral kilometric radiation and VLF emissions associated with a type III solar radio noise event

This paper examines an isolated magnetospheric VLF/radio noise event that is highly suggestive of the triggering of terrestrial auroral kilometric radiation (AKR) by solar type III radio emission and of a close relation between AKR and broadband hiss. The solar type III burst was measured on polar HF riometers and was coincident with local dayside VLF/LF noise emission bursts at South Pole station. It was also coincident with AKR bursts detected on the AMPTE/IRM satellite, at the same magnetic local time as South Pole. On the basis of the close association of AKR and VLF bursts, and from geometrical considerations relating to wave propagation, it is likely that the AKR source was on the dayside and on field lines near South Pole station. The general level of geomagnetic activity was very low. However, an isolated magnetic impulse event (MIE) accompanied by a riometer absorption pulse was in progress when all of the VLF/radio noise bursts occurred. The very close association of the type III burst at HF with the AKR is consistent with external stimulation of the AKR, if a different, more immediate, triggering process than that implied by Calvert (1981) is invoked. It is suggested here that some of the HF solar radiant energy may decay into waves with frequencies comparable to those of the AKR by parametric excitation or some other process, thus providing the few background photons required for the generation of AKR by the Wu and Lee (1979) cyclotron maser instability. The AKR, perhaps by modifying the magnetospheric electron velocity distribution, might have produced the observed VLF emissions. Alternatively, the VLF emissions may have arisen from the same anisotropic and unstable electron distribution function responsible for the AKR. While the relationship of these emission features to the occurrence of the magnetic/absorption impulse may have been coincidental, the MIE could be a preconditioning agent for the ionosphere/magnetosphere plasma, making it susceptible to the external stimulation of the AKR.

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.

Magnetospheric response to solar wind dynamic pressure variations: Interaction of interplanetary tangential discontinuities with the bow shock

Some magnetic impulse events observed in the polar region are related to vortices associated with plasma convection in the ionosphere. Recent analyses of satellite and ground data suggest that the interaction of solar wind dynamic pressure pulses and the magnetosphere may lead to the formation of velocity vortices in the magnetopause boundary layer region. This can in turn lead to the presence of vortices in the polar ionosphere. However, before reaching the Earth's magnetopause, these interplanetary pressure pulses must interact with and pass through the bow shock. A variation of the solar wind dynamic pressure (ΔρV²) may be associated with shocks, magnetic holes, or tangential discontinuities (TDs) in the interplanetary medium. We study the interaction of interplanetary TDs with the Earth's bow shock (BS) using both theoretical analysis and MHD computer simulations. It is found that as a result of the collision between a TD and the BS, the jump in the solar wind dynamic pressure associated with the TD is significantly modified, the bow shock moves, and a new fast shock or fast rarefaction wave, which propagates in the downstream direction, is excited. Our theoretical analysis shows that the change in the plasma density across the interplanetary TD plays the most important role in the collision process. In the case with an enhanced dynamic pressure behind the interplanetary TD, the bow shock is intensified in strength and moves in the earthward direction. The dynamic pressure jump associated with the transmitted TD is generally reduced from the value before the interaction. A fast compressional shock is excited ahead of the transmitted TD and propagates toward the Earth's magnetosphere. For the case in which the dynamic pressure is reduced behind the interplanetary TD, the pressure jump across the transmitted TD is substantially weakened, the bow shock moves in the sunward direction, and a rarefaction wave which propagates downstream is excited. We also simulate and discuss the interaction of a pair of tangential discontinuities, which may correspond to a magnetic hole, with the BS.

Interaction of solar wind with the magnetopause-boundary layer and generation of magnetic impulse events

The transport of mass, momentum, energy and waves from the solar wind to the Earth's magnetosphere takes place in the magnetopause-boundary layer region. Various plasma processes that may occur in this region have been proposed and studied. In this paper, we present a brief review of the plasma processes in the dayside magnetopause-boundary layer. These processes include 1. (1) flux transfer events at the dayside magnetopause, 2. (2) formation of plasma vortices in the low-latitude boundary layer by the Kelvin-Helmholtz instability and coupling to the polar ionosphere, 3. (3) the response of the magnetopause to the solar wind dynamic pressure pulses and 4. (4) the impulsive penetration of solar wind plasma filaments through the dayside magnetopause into the magnetospheric boundary layer. Through the coupling of the magnetopause-boundary layer to the polar ionosphere, those above processes may lead to occurrence of magnetic impulse events observed in the high-latitude stations.

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.

Magnetic impulses and associated optical signatures in the dayside aurora

We present five magnetic impulse events observed at South Pole and in the magnetically conjugate area at Iqaluit. The magnetic signatures recorded during these events appear to be similar to those discussed by several authors as possible ionospheric signatures of sporadic dayside reconnection via the flux transfer process. Coordinated ground‐based image‐intensified all sky camera data were acquired at South Pole during the events. The optical data show that prior to the onset of each of the events the dayside cusp aurora is several degrees poleward of the station, signifying event initiation on closed field lines. Overhead aurora appears at the onset of the magnetic event and precipitation seems to expand over a large region equatorward of the quiescent cusp aurora. As the event decays the aurora dies away and the quiescent dayside cusp aurora, in four out of five cases, returns poleward of the station. This morphology is inconsistent with the model in which the ground based signature of an FTE is caused by a field aligned current configuration moving over a ground station in the antisunward direction. The observations show temporal events which begin at or inside the boundary of closed field lines. These are consistent with sudden precipitation activity on flux tubes which were previously equatorward of the cusp/cleft boundary and which became temporarily disconnected (non‐dipolar).