Negative Interplanetary Magnetic Field Research Articles

Auroral event sequences at the dayside polar cap boundary for positive and negative interplanetary magnetic field BY

The quasi‐periodic sequence of moving auroral forms at the dayside polar cap boundary observed during southward directed interplanetary magnetic field (IMF) is one of the most interesting features of dayside auroral activity. Examples from the prenoon and postnoon sectors, corresponding to positive and negative IMF BY, respectively, are presented. Dual‐site ground‐based optical observations of one of the auroral events on January 12, 1991, are combined with “snapshot” information on the particle precipitation environment and ionospheric ion flow obtained from the polar‐orbiting satellite DMSP F9. A characteristic sequence of auroral forms observed within 08‐10 MLT expanded northwestward in a region of strong westward ion flow component, during intervals of strongly negative IMF BZ and positive BY. The optical events faded out near the 08 MLT meridian, within the regime of mantle precipitation (magnetosheath origin). The fading phase of the optical event traversed by the satellite along the 09 MLT meridian is characterized by structured mantle precipitation and associated ion flow shears within 72°‐78° magnetic latitude (MLAT). The integrated potential drop across this event in the north‐south direction, corresponding to the westward ion drift component within 72°‐78° MLAT, is 25 kV. IMP 8 observations of solar wind dynamic pressure outside the bow shock do not show good correlation between pressure variations and major optical event occurrence in the present case. Negative IMF BY events observed near the noon meridian and in the postnoon sector are moving eastward, in the same direction as the ionospheric convection. The optical, particle, and ion drift observations in combination with IMF and solar wind plasma data indicate that the events represent dynamical structures of merging cell convection over the polar magnetosphere, possibly initiated by pulses of enhanced merging rate (Bn ≠ 0) at the dayside magnetopause. The fading of the optical signature and the structured ionospheric ion flow/field‐aligned current at mantle latitudes are consistent with a high degree of spatial structure in the high‐latitude boundary layer and of the associated solar wind‐magnetosphere dynamo action (E · j < 0), provided the field line mapping implied by the present understanding of the particle precipitation characteristics is correct. The contribution from these events to the transpolar voltage (ϕpc) may be estimated on the basis of luminosity area and event repetition time (≃10 min). A wide range of spatial scales of optical events as well as their internal structure is observed. This study indicates that a nonnegligible fraction of the transpolar voltage may at times be generated by pulsed magnetopause merging. Discrete auroral activity and irregular magnetic pulsations with long periods are observed at cleft latitudes in the vicinity of the prenoon convection reversal and further north during the event sequence taking place.

Auroral event sequence at the dayside polar cap boundary: Signature of time-varying solar wind-magnetosphere-ionosphere coupling

A specific category of auroral event sequence taking place at the dayside polar cap boundary during southward-directed interplanetary magnetic field (IMF) is exemplified by three cases of combined ground and satellite observations. The detailed temporal and spatial evolution of these optical events were monitored by all-sky TV imagery mapped onto a geographic coordinate system. Profiles along the satellite track of particle precipitation and ionospheric convection in the vicinity of the optical events were important for the determination of the solar wind - magnetosphere coupling mode involved. The observed north-westward event motion in the prenoon sector is typical for periods of positive IMF B Y. Auroral events associated with negative IMF B Y were moving northeastward in the midday and early post-noon sector. The optical events mostly appear as a brightening of the 630.0 nm aurora near the poleward boundary of the persistent cleft/LLBL precipitation at the boundary of the field-of-view of the TV camera. A continuous motion into the polar cap (mantle precipitation) was observed before the optical emission faded out after 10–15 minutes, typically ∼500 km poleward of the continuous cleft arc. The auroral forms typically cover ∼500 km in the east - west direction and ∼200 km in the north - south direction. A glow of 630.0 nm emission spanning ∼500 km in latitudinal extent was observed during the decay phase of the Jan. 12, 1991 case presented here. The optical, particle and ion drift observations in combination with the available IMF and solar wind plasma data indicate that the events represent dynamical structures of IMF B Y-related Dungey cell convection over the polar magnetosphere, initiated by pulses of enhanced merging rate (B n ≠ 0) at the dayside magnetopause.

Polar cusp topology and position as a function of interplanetary magnetic field and magnetic activity: Comparison of a model with Viking and other observations

A family of quantitative magnetospheric models is used to determine the cusp position in the ionosphere and to analyze the field topology around the outer cusp. It is demonstrated that the “cusp proper” (∼1° × 2 hours), observed in particle precipitation patterns, can be defined as the footprint of the magnetopause current layer with field lines connected to the interplanetary medium via a normal magnetic field component. A wider magnetic local time extent of the cleft (∼8–16 hours MLT) can be associated with field lines connected to the low‐latitude boundary layer between XGSM = 0 and 20 RE downtail and extending 1–2 RE inward from the magnetopause layer. It is suggested that the finite‐sized physical cusp originates as a result of gyroviscous erosion at the magnetopause current layer that enlarges the topologically singular magnetic cusp. The model‐predicted magnetic latitude (MLAT) of the ionospheric footprint of the cusp is 79° during quiet conditions and 74° during active periods. The latitude of the cusp varies also with universal time and with season, due to variation of the dipole tilt angle. The corrected magnetic coordinates are found to exhibit the smallest amplitude of diurnal variation of the cusp position and are recommended for mapping of auroral phenomena. Other coordinate systems produce artificial diurnal variations of the cusp position. These artificial effects have an amplitude of ±2.5 (±1) hours MLT and ±4° (±2°) MLAT for dipole (eccentric dipole) coordinates, respectively. An interplanetary magnetic field (IMF) By component linearly superposed with the magnetospheric field can shift the magnetic local time of the cusp by a maximum of 0.07–0.15 hour/nT. A negative (positive) IMF Bz component can shift equatorward (poleward) the position of the cusp by a maximum of 0.1°–0.2° /nT. A similar effect is obtained with a positive (negative) Bz component. These results imply that the instantaneous effect of a negative Bz component on the equatorward shift of the cusp is smaller than an indirect effect due to increased magnetic activity. Some of the model predictions are compared with cusp observations made on board the Swedish Viking satellite. Both the Viking and other observations can be quantitatively explained by the model predictions.

Magnetic properties of the high‐latitude tail boundary: Draping of magnetosheath field lines and tail‐aligned current

Magnetic properties of the high‐latitude tail boundary are examined with IMP 8 magnetometer data. The high‐latitude tail boundary separates the tail lobe from the magnetosheath. Magnetic fields are stable in the tail lobe, but very irregular in the magnetosheath. Boundary crossings are marked by the rotation of magnetic components parallel to the boundary plane. The magnetic component normal to the boundary, if any, is very small in comparison to this rotational change. Despite large magnetic fluctuations in the magnetosheath, the magnetosheath‐side field orientation is consistent with the draping of the interplanetary magnetic field (IMF) against the magnetotail. The boundary current has a component parallel to the lobe field (tail‐aligned current), as well as a circumferential component. The IMF orientation controls the sheath‐side BY, while the lobe‐side field has a more rigid configuration flaring antisunward. Accordingly, the polarity of the tail‐aligned current is primarily determined by the sign of IMF BY; in other words, the tail‐aligned current shields the magnetotail against the IMF Y component. The current polarity is antisunward for positive IMF BY and sunward for negative IMF BY in the northern hemisphere. The tail‐aligned current is also discussed in terms of the cusp (mantle) current system.

Relationship between four‐cell and distorted two‐cell convection patterns during northward IMF

Four‐cell convection pattern in the ionosphere during a northward IMF (interplanetary magnetic field) was widely accepted until it was challenged by the distorted two‐cell convection pattern proposed by Heppner and Maynard [1987]. We propose that a four‐cell convection pattern, imposed on the magnetosphere by the solar wind, can be distorted into a wrapped two‐cell pattern by the nonuniform and anisotropic conductance in the ionosphere. This is demonstrated by extending the magnetosphere‐ionosphere coupling model [Kan et al., 1988] to the northward IMF condition. The resulting wrapped two‐cell patterns for positive and negative IMF By are similar to the empirical two‐cell patterns obtained by Heppner and Maynard [1987]. The distributions of the resulting field‐aligned currents are also characteristically consistent with the observed current distributions, including the NBZ currents, during northward IMF.

Evidence of the influence of the interplanetary magnetic field azimuthal component on polar cusp configuration

A study is presented of the low‐altitude polar cusp regions using the precipitating electron data from DMSP F2 and F4 satellites during 54 quasi‐simultaneous traversals of the polar cusp region in the same hemisphere. Electron precipitation features are used to determine the cusp morphology and behavior at different magnetic local times for varying orientations of the interplanetary magnetic field (IMF). Indication is found of a systematic shift of the equatorward boundary of the morning cusp region toward the pole (or the equator) in the southern hemisphere for positive (or negative) IMF By with respect to its position at noon as detected nearly simultaneously by the other satellite.

Quantitative patterns of large‐scale field‐aligned currents in the auroral ionosphere

Quantitative patterns of the distribution of field‐aligned current (FAC) density have been derived from gradients of the average patterns of the Hall and Pedersen currents at high latitudes under the assumption that the total current is divergence‐free. The horizontal currents were calculated from empirical convection electric field models, derived from Millstone Hill radar observations, and the ionospheric Hall and Pedersen conductances, based on satellite observations of the precipitating particle energy flux and spectrum and including an average (equinox) solar contribution. These independent empirical models, and the resultant patterns of the field‐aligned currents, are keyed to an auroral precipitation index which quantifies the intensity and spatial extent of high‐latitude particle precipitation and which is determined from a single satellite crossing of the auroral precipitation pattern. The patterns detail the spatial distribution of the currents as a function of increasing disturbance level. The magnitudes of the total single‐hemisphere currents into or out of the ionosphere are closely balanced at each activity level and increase exponentially between 0.1 and 6 MA with increasing values of the precipitation index. The interplanetary magnetic field (IMF) sector dependence of the FAC patterns is investigated for disturbed conditions. A large portion of the FAC pattern is closed by local Pedersen currents (current into the ionosphere is balanced by an equal current out of the ionosphere at that local time). This locally balanced portion of the FAC system is enhanced in the prenoon (postnoon) sector for IMF By > +1 nT (By < −1 nT). In addition, there are net currents into the ionosphere postnoon and out of the ionosphere in the premidnight sector. Whereas the magnitude of these nighttime net FACs increases with increasing negative IMF Bz, the magnitude and distribution of the net currents appear to be independent of the IMF By orientation.

A model for multiple throat structures in the polar cap flow entry region

A two‐dimensional ionospheric convection model has been developed to produce convection patterns for southward interplanetary magnetic field (IMF) and a positive or negative IMF y component. The model consists of a movable, shear convection reversal boundary with a gap in it where flux enters the polar cap. The sign of IMF By determines the dayside gap geometry. We use this simple model to simulate measured ionospheric flows from the DE 2 satellite. Roughly 35% of DE 2 passes that cross the dayside between 0800 and 1400 hours MLT cannot be modeled with a single narrow flow entry region. By comparing model calculations and the measured ion flows, we show that the dayside flow entry region to the polar cap typically spans several hours in local time. The electric field can concentrate along portions of the polar cap entrance and weaken between the concentrated regions, thus forming multiple “throats.”

Auroral and magnetic variations in the polar cusp and cleft — Signatures of magnetopause boundary-layer dynamics

By combining continuous ground-based observations of polar cleft/cusp auroras and local magnetic variations with electromagnetic parameters obtained from satellites in polar orbit (low-altitude cleft/cusp) and in the magnetosheath/interplanetary space, different electrodynamic processes in the polar cleft/cusp have been investigated. One of the more controversial questions in this field is related to the observed shifts in latitude of cleft/cusp auroras and the relationship with the interplanetary magnetic field (IMF) orientation, local magnetic disturbances (DP2 and DPY modes) and magnetospheric substorms. A new approach which may contribute to clarifying these complicated relationships — simultaneous ground-based observations of the midday and evening-midnight sectors of the auroral oval — is illustrated. A related topic is the spatial relationship between the cleft/cusp auroras and the ionospheric convection currents. A characteristic feature of the polar cusp and cleft regions during negative IMF B Z is repeated occurrence of certain short-lived auroral structures which seem to move in accordance with the local convection pattern. Satellite measurements of particle precipitation, magnetic field and ion drift components permit detailed investigations of the electrodynamics of these cusp/cleft structures. Information on electric field components, Birkeland currents, Poynting flux, height-integrated Pedersen conductivity, and Joule heat dissipation rate has been derived. These observations are discussed in relation to existing models of temporal plasma injections from the magnetosheath.

Ionospheric convection inferred from interplanetary magnetic field‐dependent Birkeland currents

It is fairly well‐established that sunward convection of the ionosphere exists in the central portion of the polar cap when the interplanetary magnetic field (IMF) is northward. What is unclear, however, is whether the sunward convection is created by additional cells within the polar cap or by a distortion of the normal two‐cell pattern. Computer simulations of ionospheric convection were conducted by combining empirical models of Birkeland currents with a model of ionospheric conductivity to study IMF‐dependent convection characteristics. Birkeland currents for three IMF configurations were available: (1) Bz southward, (2) Bz weakly northward, and (3) Bz more strongly northward. In all cases the currents used represented conditions in the northern polar cap of a negative IMF By component. Two different conclusions are possible given the limited dayside measurements of Birkeland currents. The first is that as the IMF turns northward, the morning cell becomes slightly elongated in roughly the sunward direction, and the evening cell begins to wrap around the morning cell by extending into the morning sector. This process continues as the IMF becomes more strongly northward, until what was originally the evening cell becomes severely distorted, almost completely surrounding the morning cell. What was originally the morning cell shifts into the polar cap and it is this cell along with the distorted evening cell which provide for sunward flow in the polar cap. With a different distribution of Birkeland currents on the dayside, it is also possible that when the IMF is strongly northward, a three‐cell convection pattern exists. However, this is different from the expected four‐cell pattern since the low‐latitude cell in the morning sector is missing.

Interplanetary magnetic field By effects on the magnetic field at synchronous orbit

GOES 3 magnetic field data are analyzed to reveal interplanetary magnetic field (IMF) By effects on the magnetic field configuration near the magnetic equatorial plane at synchronous orbit. The magnetic equatorial plane on the nightside moves upward (northward) in the evening sector and downward (southward) in the morning sector for the positive IMF By. The magnetic equatorial plane moves in the opposite sense for the negative IMF By. A perturbation appears in the azimuthal component of the magnetic field in the midnight sector (2100–0300 LT), and its magnitude is a fraction 0.3 of the IMF By near midnight. These changes are attributable to the IMF By‐induced asymmetry of the reconnection. Observations of substorm‐associated field‐aligned current effects depend strongly on the geometry of synchronous spacecraft relative to these currents so that the IMF By‐induced distortion of the magnetic equatorial plane produces apparent IMF By effects on D observations during substorms.

On the spatial relationship between auroral emissions and magnetic signatures of plasma convection in the midday polar cusp and cap ionospheres during negative and positive IMF Bz: A case study

The dynamics of midday auroras, including polar cusp and cap emissions, and their relation to the interplanetary magnetic field (IMF) have been investigated with optical ground‐based observations from Svalbard (75° geomagnetic latitude), Norway, and IMF data from spacecraft ISEE 2. One case is presented, showing the spatial relationship—along the magnetic meridian in the midday sector—between the cusp aurora and IMF By‐related convection currents (the DPY signature) for negative and positive values of IMF Bz.

IMF By‐dependent plasma flow and Birkeland currents in the dayside magnetosphere: 1. Dynamics Explorer observations

Plasma, magnetic‐field and dc electric‐field observations from Dynamics Explorers 1 and 2 are used to investigate the morphology of solar‐wind ion injection, Birkeland currents, and plasma convection in the morning sector for both positive and negative interplanetary magnetic field (IMF) By components. The results of the study are used to construct a By‐dependent global convection model for southward IMF. A significant element of the model is the coexistence of three types of convection cells (“merging cells,” “viscous cells,” and “lobe cells”). This model can account for observations of a nearly stationary (in local time) convection “throat,” a sunward‐antisunward convection reversal zone at the polar‐cap boundary in both the morning and afternoon quadrants, the morphology of solar‐wind ion injection and transport in the mid‐altitude polar cusp, and the By‐dependent dawn‐dusk asymmetry of polar‐cap electron fluxes.

IMF polarity effects on the equatorial ionospheric F-region

An exploratory study is made of the influence, during the equinoxes, of the interplanetary magnetic field (IMF) sector structure on the ionospheric F-region using ionosonde data from several equatorial stations for a 3-yr period around the 19th sunspot cycle maximum. It is found that, compared with days having positive IMF polarity, the post-sunset increase of h'F near the dip equator and the depth of the equatorial ionization anomaly (EIA) are reduced during the vernal equinox and enhanced during the antumnal equinox on days with negative IMF polarity. Similar trends are also noted in the data for the 20th sunspot cycle maximum, but with reduced amplitude. The systematic changes in the F-region characteristics suggest a modification of the equatorial zonal electric fields in association with the IMF polarity-related changes in the semi-annual variation of geomagnetic activity.

Downstream properties of magnetic flux transfer events

We consider the downstream evolution of the tubes of field lines known as flux transfer events (FTE's) formed by the localized, dayside, sporadic merging of magnetosheath and magnetosphere magnetic field lines. FTE's have magnetic field and plasma properties distinct from those of the nearby unmerged magnetosheath and magnetosphere field lines. Tailward advection stretches these earth‐rooted flux tubes and orients them parallel to the field in the magnetotail lobes. Following dayside merging, magnetosheath plasma flows into the magnetosphere part of the FTE. This flow is modeled by a self‐similar expansion of a plasma into a vacuum. The magnetosheath part of the FTE empties, and the magnetosphere part fills with time. After the FTE has moved 200 RE down the tail, the drained portion of the FTE reaches about 25 RE radially outward from the tail boundary. We suggest that most multiple crossings of the tail boundary observed by spacecraft are encounters with tailward moving FTE's. This interpretation explains fully the way boundary normals behave during multiple crossings. The previously favored interpretation, tailward moving boundary waves, only partly explains the known behavior. The model explains how the sign of the interplanetary magnetic field (IMF) causes the observed dawn‐dusk asymmetries in the thickness of the magnetotail boundary layer and in the pattern of “lobe filling.” It also predicts that multiple boundary crossings should be correlated with negative IMF Bz.

The relation of the cusp precipitating electron flux to the solar wind and interplanetary magnetic field

The temporal variation of the precipitating electron flux in the 47‐eV to 1‐keV energy range, detected in the low‐altitude cusp, is studied as a function of the solar‐wind parameters and the Bz component of the interplanetary magnetic field (IMF). It is found that the electron flux intensity in the polar cusp region depends nonlinearly on the density of the solar‐wind plasma and that it is higher in the negative IMF Bz period than in the positive IMF Bz period. Both these facts are consistent with the predictions of the existing models of magnetic field‐line merging at the magnetopause; however, the possibility that non‐linear diffusion processes predominate cannot be eliminated.

The relationship between the IMF By and the distant tail (150‐238 Re) lobe and plasmasheet By fields

The relationships between the Solar Magnetospheric (SM) y‐ component of the interplanetary magnetic field (IMF) and the lobe and plasmasheet magnetic fields have been studied for the two ISEE‐3 deep tail passes. It is found that for positive sector IMFs, 13% of the interplanetary magnetic field penetrates into the aberrated north‐dawn and south‐dusk lobe quadrants, and about the same amount in the north‐dusk and south‐dawn lobe quadrants for negative sector IMFs. For the above cases, field penetration is significantly less for opposite polarity IMFs. The former results are generally consistent with open magnetospheric models, but the latter (the lack of response in certain quadrants) are unexplained by theory at this time. If the magnitude of the plasmasheet By fields are related to plasma pressure anisotropies, very small anisotropies of P⊥/P∥ ≃ 1.01 are expected.