Upward Currents Research Articles

Electron heat flow in the auroral ionosphere inferred from EISCAT‐VHF observations

At auroral latitudes the electron heat flow resulting from solar EUV heating, as well as magnetospheric energy input, are important parameters which drive the thermal electron structure of the upper ionosphere. A method is presented which allows a determination from EISCAT‐VHF observations of the heat flow at two altitudes, typical of the collision‐dominated F2 region and of the topside ionosphere. EISCAT midwinter and near‐summer observations are used to illustrate the method: diurnal and seasonal variations of solar origin are evidenced, with peculiar enhancements at sunrise and sunset that are interpreted as a delay between the F2 level and topside terminator passage. Perturbations of magnetospheric origin are then discussed. One of them is characteristic of low‐altitude energy inputs. Others correspond to high‐altitude energy inputs through heat flow variations and are interpreted as upward currents of the order of 14 µA m−2.

Effect of high‐latitude ionospheric convection on Sun‐aligned polar caps

A coupled magnetospheric‐ionospheric (M‐I) MHD model has been used to simulate the formation of Sun‐aligned polar cap arcs for a variety of interplanetary magnetic field (IMF) dependent polar cap convection fields. The formation process involves launching an Alfvén shear wave from the magnetosphere to the ionosphere where the ionospheric conductance can react self‐consistently to changes in the upward currents. We assume that the initial Alfvén shear wave is the result of solar wind‐magnetosphere interactions. The simulations show how the E region density is affected by the changes in the electron precipitation that are associated with the upward currents. These changes in conductance lead to both a modified Alfvén wave reflection at the ionosphere and the generation of secondary Alfvén waves in the ionosphere. The ensuing bouncing of the Alfvén waves between the ionosphere and magnetosphere is followed until an asymptotic solution is obtained. At the magnetosphere the Alfvén waves reflect at a fixed boundary. The coupled M‐I Sun‐aligned polar cap arc model of Zhu et al. (1993a) is used to carry out the simulations. This study focuses on the dependence of the polar cap arc formation on the background (global) convection pattern. Since the polar cap arcs occur for northward and strong By IMF conditions, a variety of background convection patterns can exist when the arcs are present. The study shows that polar cap arcs can be formed for all these convection patterns; however, the arc features are dramatically different for the different patterns. For weak sunward convection a relatively confined single pair of current sheets is associated with the imposed Alfvén shear wave structure. However, when the electric field exceeds a threshold, the arc structure intensifies, and the conductance increases as does the local Joule heating rate. These increases are faster than a linear dependence on the background electric field strength. Furthermore, above the threshold, the single current sheet pair splits into multiple current sheet pairs. For the fixed initial ionospheric and magnetospheric conditions used in this study, the separation distance between the current pairs was found to be almost independent of the background electric field strength. For either three‐cell or distorted two‐cell background convection patterns the arc formation favored the positive By case in the northern hemisphere.

Combined measurements of EISCAT and the EISCAT magnetometer cross to study Ω bands

A combination of incoherent scatter radar data and data from a magnetometer array complement each other in an ideal way. The magnetometer results help to distinguish between temporal and spatial fluctuations in the radar data, and the radar results provide a means to separate magnetometer‐inferred equivalent currents into their Hall and Pedersen components. The technique is used to study Ω bands and leads to a better understanding of the ionospheric current structure associated with this phenomenon. In extending previous models we propose a pair of counterrotating source free ionospheric current vortices driven by field‐aligned currents, an upward current centered in the luminous part of the Ω band and a downward current in the dark part with its center about 400 km west of the upward current.

Auroral ionospheric signatures of the plasma sheet boundary layer in the evening sector

We report on particles and fields observed during Defense Meteorological Satellite Program (DMSP) F9 and DE 2 crossings of the polar cap/auroral oval boundary in the evening MLT sector. Season‐dependent, latitudinally narrow regions of rapid, eastward plasma flows were encountered by DMSP near the poleward boundary of auroral electron precipitation. Ten DE 2 orbits exhibiting electric field spikes that drive these plasma flows were chosen for detailed analysis. The boundary region is characterized by pairs of oppositely‐directed, field‐aligned current sheets. The more poleward of the two current sheets is directed into the ionosphere. Within this downward current sheet, precipitating electrons either had average energies of a few hundred eV or were below polar rain flux levels. Near the transition to upward currents, DE 2 generally detected intense fluxes of accelerated electrons and weak fluxes of ions, both with average energies between 5 and 12 keV. In two instances, precipitating ions with energies >5 keV spanned both current sheets. Comparisons with satellite measurements at higher altitudes suggest that the particles and fields originated in the magnetotail inside the distant reconnection region and propagated to Earth through the plasma sheet boundary layer. Auroral electrons are accelerated by parallel electric fields produced by the different pitch angle distributions of protons and electrons in this layer interacting with the near‐Earth magnetic mirror. Electric field spikes driving rapid plasma flows along the poleward boundaries of intense, keV electron precipitation represent ionospheric responses to the field‐aligned currents and conductivity gradients. The generation of field‐aligned currents in the boundary layer may be understood qualitatively as resulting from the different rates of earthward drift for electrons and protons in the magnetotail's current sheet.

The ionospheric plasma flow and current patterns of travelling convection vortices: a case study

Following a short duration density enhancement in the solar wind, observed by the AMPTE/IRM spacecraft, transient disturbances appeared in the polar ionosphere in the prenoon local time sector which were identified as Travelling Convection Vortices (TCV). This event has been studied intensively by combining radar and magnetometer observations. EISCAT radar was operated in the special programme U.K.-POLAR which provides F-region plasma parameters from invariant latitudes around 72° at a rate of one sample per 15 s. The combined data set provides a detailed picture of the drift pattern of the plasma and the three-dimensional current distribution. There are two Hall current eddies drifting westward at a speed of 0.15° s −1. The leading one circulating clockwise is associated with a downward field-aligned current and the oppositely circulating eddy with an upward current. The ionospheric conductivity seems to be enhanced in the leading vortex compared to the trailing, although the latter is connected to an upward field-aligned current. Still unexplained is the mechanism generating the electric field which drives the vortices. The direction of the electric field observed in the ionosphere is opposite to that expected if the source were a compression of the magnetosphere.

Magnetosheath‐ionospheric plasma interactions in the cusp/cleft: 2, Mesoscale particle simulations

Ionospheric plasma flowing out from the cusp can be an important source of plasma to the magnetosphere. One source of free energy that can drive this outflow is the injection of magnetosheath plasma into the cusp. Two‐dimensional (three velocity) mesoscale particle simulations are used to investigate the particle dynamics in the cusp during southward interplanetary magnetic field. This mesoscale model self‐consistently incorporates (1) global influences such as the convection of plasma across the cusp, the action of the mirror force, and the injection of the magnetosheath plasma, and (2) wave‐particle interactions which produce the actual coupling between the magnetosheath and ionospheric plasmas. It is shown that, because the thermal speed of the electrons is higher than the bulk motion of the magnetosheath plasma, an upward current is formed on the equator ward edge of the injection region with return currents on either side. However, the poleward return currents are the stronger due to the convection and mirroring of many of the magnetosheath electrons. The electron distribution in this latter region evolves from upward directed streams to single‐sided loss cones or possibly electron conics. The ion distribution also shows a variety of distinct features that are produced by spatial and/or temporal effects associated with varying convection patterns and wave‐particle interactions. On the equatorward edge the distribution has a downflowing magnetosheath component and an upflowing cold ionospheric component due to continuous convection of ionospheric plasma into the region. In the center of the magnetosheath region, heating from the development of an ion‐ion streaming instability causes the suppression of the cold ionospheric component and the formation of downward ionospheric streams. Further poleward there is velocity filtering of ions with low pitch angles, so that the magnetosheath ions develop a ring‐beam distribution and the ensuing wave instabilities generate downward ionospheric conics. These downward ionospheric components are eventually turned by the mirror force, leading to the production of upward conies at elevated energies throughout the region.

The development of initial substorm expansive phase disturbance due to generation of localized electric fields in the region of maximum upward field-aligned current

The analysis of the plasma sheet plasma stability for the small scale electrostatic disturbances in the simple approximation of one fluid magnetohydrodynamics show that the region of upward field-aligned current near midnight where fast radial plasma flow take place can become unstable to localized electrostatic disturbances during the growth phase of substorms. Such instability can explain the onset of substorm deep in the magnetosphere near the Earth's border of the tail current sheet.

Ground-based measurements of an arc-associated electric field

The electrodynamics of a pre-midnight auroral arc, situated close to the equatorward border of the convection reversal, are studied by ground-based measurements. The optical intensity and the location of the arc is determined by a scanning photometer and an all-sky camera. Ionospheric electric fields around the arc are measured by the STARE radar. The EISCAT radar provides measurements of electric fields and conductivities equatorward of the arc. The EISCAT magnetometer chain is used to observe the development of ionospheric currents. An enhanced northward component of the electric field is observed equatorward of the arc. The width of this arc-associated electric field is estimated to be less than, or equal to, 45 km. A quite new result is the simultaneous intensification of the arc-associated electric field and the optical brightening of the auroral arc. The brightening of the arc is related to an intensification of the upward current from the arc, carried by energetic electrons. Previous satellite and rocket measurements have shown that auroral arcs are associated with matched pairs of currents with the upward current flowing from the arc and the downward current, carried by cold ionospheric electrons, flowing on the equatorward (poleward) side of the arc in the evening (morning) sector. The field-aligned currents are connected by a Pedersen current flowing in the direction of the electric field in the ionosphere. If the magnetospheric generator region acts as a current generator, the ionospheric electric field has to modify itself in a way that the current continuity in the ionosphere is preserved. When the ionospheric conductivity is low adjacent to the arc, as the EISCAT measurement indicates, the meridional electric field has to increase in order to produce an enhanced Pedersen current and an enhanced downward current (by the gradient of the Pedersen current) in response to the increased upward current. Thus we suggest that the enhanced electric field adjacent to the arc is a result of the ionosphere-magnetosphere coupling, influenced by the low ionospheric conductivity.

Characteristics of Suprathermal Electron Bursts Observed by the DMSP-F6/F7 Satellites in the Diffuse Aurora Region.

The particle data of the DMSP-F6 and -F7 satellites show the presence of suprathermal electron bursts in the diffuse aurora region. These bursts are characterized by intense (∼108-109 (cm2sr sec)-1), low energy (∼100-500 eV), and localized (∼20-30 km at satellite altitude h ≅ 830 km) electron precipitation. Simultaneous magnetic field data from the DMSP-F7 satellite show that these bursts are often, but not always, accompanied with intense (few μAm-2), small-scale (only few tens of km at satellite altitude) upward field-aligned currents. The bursts are observed from 19 h to 11 h MLT through midnight during the maximum to recovery phase of substorm. These bursts are less likely to be observed during geomagnetic storms than during times of isolated substorms. These results suggest that suprathermal electron bursts are generated by localized heating of ionospheric electrons which flow out from the ionosphere toward the magnetosphere during substorm.

Diodelike response of high‐latitude plasma in magnetosphere‐ionosphere coupling in the presence of field‐aligned currents

The dynamic processes in the plasma along high‐latitude field lines plays an important role in ionosphere‐magnetosphere coupling process. We have created a time‐dependent, large‐scale simulation of these dynamics parallel to the geomagnetic field lines from the ionosphere well into the magnetosphere. The plasma consists of hot e− and H+ of magnetospheric origin and low‐energy e−, H+, and O+ of ionospheric origin. Including multiple electron species, a major improvement to the model, has allowed us for the first time to simulate the upward current region properly and to dynamically simulate the diodelike response of the field‐line plasma to the parallel currents coupling the ionosphere and magnetosphere. It is shown that return currents flow with small resistance, while upward currents produce kilovolt‐sized potential drops along the field, as concluded from satellite observations. The kilovolt potential drops are due to the effect of the converging magnetic field on the high‐energy magnetospheric electrons.

Plasma dynamics on current‐carrying magnetic flux tubes, 2, Low potential simulation

The evolution of plasma in a current‐carrying magnetic flux tube of variable cross section is investigated using a one‐dimensional numerical simulation. The flux tube is narrow at the two ends and broad in the middle. The middle part of the flux tube is loaded with a hot, magnetically trapped population, and the two ends have a more dense, gravitationally bound population. A potential difference larger than the gravitational potential but less than the energy of the hot population is applied across the domain. The general result is that the potential change becomes distributed along the anode half of the domain, with negligible potential change on the cathode half. The potential is supported by the minor force of magnetically trapped particles. The simulations show a steady depletion of plasma on the anode side of the flux tube. The current steadily decreases on a time scale of an ion transit time. The results may provide an explanation for the observed plasma depletions on auroral field lines carrying upward currents.

The role of the equatorial electrojet in the evening ionosphere

This paper focuses on the role of the equatorial E region in the electrodynamics of the evening ionosphere. The influence and reaction of the electrojet current on the equatorial ionosphere at sunset is investigated using a field line integrated, one‐dimensional, electrodynamic model. The one‐dimensional, time‐varying model predicts the divergence of the horizontal current of the equatorial electrojet for a given time variation of the horizontal electric field. The negative divergence of the horizontal current during the evening hours provides a net upward current out of the equatorial E region into the integrated ionosphere of higher equatorial altitudes (and equivalent latitudes). This upward current affects the vertical electric field magnitudes and subsequent horizontal plasma drifts of the overlying ionosphere. The model allows for chemical recombination and dynamic redistribution of ionization within the electrojet region under the assumption that the profile of the ionization density along a field line is proportional to the chemical equilibrium profile. The eastward horizontal electric field and the net upward current during the 2 hours after sunset combine to lift the ionization out of the E region resulting in ionization densities less than the equilibrium values. As the ionization densities (conductivities) are reduced, the electrodynamics of the equatorial ionosphere is altered. This model of the equatorial electrojet current divergence can be used as a lower boundary to global, two‐dimensional models of the equatorial electric fields.Finally, it is proposed that the equatorial electrojet current near sunset has a significant role in the determination of the postsunset enhancement of the horizontal electric field. The electrojet region provides the best avenue for current to be channeled from the dayside to meet the vertical current demands of the F region neutral wind dynamo after sunset. The conductivity reduction in the E region due to the recombination of ionization and the plasma uplift enhances the horizontal (eastward) electric field and thereby increases the speed of the uplift. Thus the dynamic adjustment of ionization has an unstable, feedback relationship with the electric fields which may explain the night to night variability of the horizontal electric field enhancement.

Dynamics Explorer Measurements of Particles, Fields, and Plasma Drifts Over a Horse-Collar Auroral Pattern.

As shown from ground-based measurements and satellite-borne imagers, one type of global auroral pattern characteristic of quiet (usually northward IMF) intervals is that of a contracted but thickened emission region of a pattern referred to as 'horse-collar' aurora (Hones et al., 1989). In this report we use the Dynamics Explorer data set to examine a case in which this horse-collar pattern was observed by the DE-1 auroral imager, while at the same time DE-2, at lower altitude, measured precipitating particles, electric and magnetic fields, and plasma drifts. Our analysis shows that, in general, there is close agreement between the optical signatures and the particle precipitation patterns. In many instances, over scales ranging from tens to a few hundred kilometers, electron precipitation features and upward field-aligned currents are observed at locations where the plasma flow gradients indicate negative V-average x E. The particle, plasma, and field measurements made along the satellite track and the 2D perspective of the imager provide a means of determining the configuration of convective flows in the high-latitude ionosphere during this interval of northward IMF. Recent mapping studies are used to relate the low-altitude observations to possible magnetospheric source regions.

DMSP F7 observations of a substorm field‐aligned current

In this paper we present observations of a substorm field‐aligned current (FAC) system that DMSP F7 traversed just after 0300 UT on April 25, 1985. Ground magnetometer data show that a major substorm was in progress at that time and that DMSP F7 flew through a region of predominantly upward FAC. The DMSP F7 magnetic field data are consistent with this interpretation. The precipitating particle data suggest that there were three distinct large‐scale FAC systems. In ascending latitude these were a downward current, an upward current, and a paired upward/downward current system. We identify the first current, which was coincident with the diffuse aurora, as region 2. The next (upward) FAC was coincident with a spatially unstructured region of energetic (∼12 keV) electron precipitation. This was the substorm‐associated FAC that made up part of the current wedge. The upward/downward current pair was coincident with a region of highly structured precipitation. We suggest that these currents may have been the duskside region 1 and, poleward of that, the extension of the dawnside region 1. The particle data show that the upward substorm current lay well equatorward of the boundary between open and closed field lines. In fact, using a model field, the equatorward boundary of the substorm FAC maps to the neutral sheet at 6.9 RE. While one should be cautious in stressing results obtained by mapping model field lines, our result is consistent with scenarios for substorms which postulate a disruption and diversion of the near‐Earth cross‐tail current.

Electric currents and H? emission in two active regions on the sun

We investigated the structure of magnetic field and vertical electric currents in two active regions through a comparison of the observed transverse field with the potential field, which was computed according to Neumann boundary-value problem for the Laplace equation using the observed H z -value. Electric currents were calculated from the observations of the transverse magnetic field. There exist two systems of vertical electric currents in active regions: a system of local currents and a global one. The global current is about 2 × 1012 A. In the leading part of the active regions it is directed upward, and in the tail downward. Flare activity is closely connected with the value and direction of both local and global currents: the flares tend to apear in places with upward currents. The luminosity of Hα flocculi is also connected with vertical electric currents; the brighter the flocculi, the more frequently they appear in places of upward electric currents. The sensitivity of Hα emission to the sign of the current suggests that charged particles accelerated in the upper parts of magnetic loops may be responsible for these formations. Joule heating might be important for flocculi, if plasma conductivity is about 5 × 108 c.g.s.e. A model of a flare is suggested based on current redistribution in a system of emerging loops due to changes of loop inductance.

A new numerical code for simulating current‐driven instabilities on auroral field lines

We have studied the microphysical processes that occur on auroral field lines carrying field‐aligned currents by means of a newly developed two‐dimensional electrostatic PIC (particle in cell) numerical code. We are, in particular, interested in the question of whether field‐aligned currents carried by obliquely propagating shear Alfvén waves can lead to the formation of quasi‐stationary field‐aligned electric fields and double layers in order to provide a detailed test of the shear Alfvén wave model for generating small‐scale field‐aligned currents and auroral particle acceleration. Our model includes upward and downward propagating oblique shear Alfvén waves in a phenomenological way through an impedance characterizing the source plasmas outside the simulation box. In this paper we concentrate on the description of the numerical scheme, the testing of it, and a few important results which are obtained from a number of runs. Some of the findings are as follows: the reflection of Alfvén waves at regions containing plasma waves coupling the electron and ion motion causes a reduction of the field‐aligned current and limits it to a critical value which is marginal for the excitation of electrostatic ion cyclotron waves at a height of 1–2 Re. Electron heating due to current‐driven plasma turbulence causes the overall plasma density in the system to decrease, because we do not require the number of particles to remain constant inside the system. This may be related to the formation of the auroral cavity. The plasma density in the region of upward current is enhanced relative to that in the region of downward current, and the current density in upward directed current channels (carried by precipitating electrons) can be larger than in downward directed current channels. We also observe a filamentation of the current in the upward directed current channel. We find upward propagating solitary potential structures in the upward directed current channel and intermittent double layers in both channels.

The physics of the Harang discontinuity

Absent a source of energetic ions at the flanks of the tail, the westward gradient/curvature drift of E×B‐convecting plasma results in the depletion of energetic ions from the dawnside of the plasma sheet. This dawnside depletion effect means that, on average, the duskside of the plasma sheet will have higher ion temperatures, pressures, and flux tube contents, and hence, stronger westward cross‐tail drift current than the dawnside. The resulting cross‐tail divergence of drift current must find closure by means of Birkeland currents connecting to the ionosphere. Tailward and poleward of the inner edge region, the divergence of cross‐tail current requires upward current from the ionosphere. In the ionosphere, current closure requires electric fields that are directed toward the center of the upward current, i.e., directed equatorward on the poleward side, poleward on the equatorward side. This is precisely the nature of the Harang discontinuity. The region poleward of the Harang discontinuity maps well out into the plasma sheet and provides an eastward component of E×B drift to oppose the westward gradient/curvature drift of the ions. This helps keep the flow of plasma sheet ions directed toward the inner plasma sheet, rather than toward the dusk flank of the tail, a point originally made by Atkinson. The region equatorward of the Harang discontinuity maps close to the inner edge of the plasma sheet and results in westward E×B drift, increasing the westward flow of plasma azimuthally around the duskside of the inner magnetosphere and toward the dayside magnetopause. Although this scenario can be understood qualitatively, runs were carried out using the Rice convection model (RCM) to examine the ionospheric‐magnetospheric coupling implications of this dawnside depletion effect. These runs confirm the above scenario, generally. They show that dawnside ion depletion results in a band of upward Birkeland current in the central auroral zone on the nightside, similar to what has been consistently observed by Iijima and Potemra and others. These currents modify the nightside, auroral electric field distribution to produce a strong reversal in the meridional electric field, similar to the classically observed Harang discontinuity. Inclusion of dawnside ion depletion also results in a major improvement in the agreement between the RCM and observations with regard to the latitudinal distribution of Birkeland currents. Finally, the dawnside depletion effect results in a reduction of plasma sheet pressures in the near‐Earth, midnight sector of the plasma sheet. However, this reduction is significantly less than that suggested by Kivelson and Spence.

The electrodynamic, thermal, and energetic character of intense Sun‐aligned arcs in the polar cap

The electrodynamic, thermal, and energetic character of stable Sun‐aligned arcs in the polar cap can be meaningfully diagnosed by an incoherent scatter radar, provided a suitable observing scheme is selected. We report here such measurements of two intense Sun‐aligned arcs. The two arcs were diagnosed on two different nights (February 26 and March 1, 1987) using the Sondre Stromfjord radar as a stand‐alone diagnostic. Repeatable patterns are found in mesoscale area (order 10³ km by 10³ km) maps of altitude profiles for observed electron and ion gas number densities, temperatures, and line‐of‐sight velocities, and projected mesoscale area maps of derived electric fields, Pedersen and Hall conductivities (Ne, Te, Ti, V, E, ΣP, ΣH), horizontal and field‐aligned currents, joule heating rate, and Poynting flux. We confirm, for the first time with continuous mesoscale area maps, that the arcs have the anticipated simple arc electrodynamics. That is, the visual and enhanced ionization signatures of the arc are produced by incoming energetic electrons carrying the outgoing current from the electric field convergence in the arc. Strong electron temperature enhancements (>2000 K) are found as expected within the sheets of ionizing particle precipitation. Dawn to dusk decreases in the antisunward plasma flow of order 1 km s−1, across order 100 km, correspond to peak electron densities of order 105 cm−3 down to altitudes as low as 120 km, and upward currents of order 1 µA m−2. These data also lead to important implications for the physics of polar cap arcs. The high‐velocity (antisunward flow on the dawnside) edge of the arc marks the location of strong persistent Joule heating driven by downward Poynting flux. There is a channel of strongly enhanced ion temperature (well above the electron temperature) along the high‐velocity edge of the arc, quantitatively accounted for by ion frictional heating as the strong electric field drags the ions through the neutral gas. The deposition rate into the atmosphere of the net electromagnetic energy well exceeds the net particle energy deposited by the ionizing energetic electron flux. This heating is a substantial source of heat into the polar thermosphere. As estimated by heat into the ions, heat lost by the ions to the neutrals, or energy available from the Poynting and energetic particle flux, several ergs cm−2 s−1 are deposited in channels of order 100 km for good fractions of an hour in these arcs; this contributes to resolving the problem of the missing polar thermosphere heat source. Finally, a reasonably simple yet self‐consistent, accurate, and comprehensive representation of stable intense Sun‐aligned arcs is presented, including the electrodynamic, thermal, and energetic character.

Sources of field‐aligned currents in the auroral plasma

Data from the Dynamics Explorer 1 High Altitude Plasma Instrument (HAPI) and magnetometer are used to investigate the sources of field‐aligned currents in the nightside auroral zone. It is found that the formula developed by S. Knight predicts the field‐aligned current density fairly accurately in regions where a significant potential drop can be inferred from the HAPI data; there are, however, regions in which the proportionality between potential drop and field‐aligned current does not hold. In particular, we note occurrences of strong upward field‐aligned current associated not with inverted‐V events but instead with supratherrnal bursts. In addition, upward field‐aligned currents are often observed to peak near the edges of inverted‐V events, rather than in the center as would be predicted by Knight.

A simulation study of the vortex structure in the low‐latitude boundary layer

Satellite observations indicate that the plasma density and the flow velocity are highly variable in the low‐latitude boundary layer. The thickness of the boundary layer is also highly variable and appears to increase with increasing longitudinal distance from the subsolar point. In this paper plasma dynamics in the low‐latitude boundary layer region is studied on the basis of a two‐dimensional incompressible hydrodynamic numerical model. In the simulation, plasma is driven into the boundary layer region by imposing a diffusion flux along the magnetopause. The vortex motions associated with the Kelvin‐Helmholtz instability are observed in the simulation. The resulting vortex structures in the plasma density and the flow velocity may coalesce as they are convected tailward, causing them to grow in size. The boundary layer thickness increases with increasing longitudinal distance from the subsolar point in accord with satellite observations. The plasma density and the flow velocity are positively correlated. A mixing region is formed where magnetosheath plasma and magnetospheric plasma mix due to the vortex motions. In the later stage of development, a density plateau is formed in the central part of the boundary layer. Many features of the satellite observations of the boundary layer can be explained using our numerical model. Our simulation results also predict that the vortices generated in the postnoon (prenoon) boundary layer lead to the presence of localized upward (downward) field‐aligned currents in both the northern and the southern polar ionospheres. The upward field‐aligned currents in turn may lead to the formation of dayside auroral patches observed in the postnoon region.