Ionospheric Convection Pattern Research Articles

Theoretical models of polar-cap convection under the influence of a northward interplanetary magnetic field

The unexpected patterns of high-latitude auroral luminosity and ionospheric convection that are observed when the interplanetary magnetic field (IMF) has a northward orientation have inspired a variety of theoretical interpretations. The existing models, all referring to steady-state conditions, can be classified according to the topology of the polar magnetic field lines and of the polar-cap convection streamlines. The classes of model include: (1) a closed magnetosphere model, (2) a conventional open model with a distorted, but topologically unchanged, polar-cap boundary, (3) a conventional open model with distorted, but topologically unchanged, polar-cap convection cells, (4) a modified open model with ‘lobe convection cells’ contained wholly on open magnetic-field lines, and (5) a modified open model with a bifurcated polar cap. The third and fourth types require significant regions of sunward flow on open polar-cap field lines, a concept that presents serious theoretical difficulties. The other three types appear equally viable from a theoretical point of view, and the comparison against observations is an ongoing enterprise. Outstanding theoretical questions include (a) how do observed structures in the polar ionosphere map along magnetic field lines into the magnetosphere?, (b) what is the mechanism that drives the observed sunward convection at highest latitudes on the day side?, and (c) what role does time dependence play in the observed phenomena?

Correlated observations of the boundary between polar cap and nightside auroral zone by HF radars and the DMSP satellite

Simultaneous observations from HF radars and the DMSP satellite have been used to relate the ionospheric convection pattern and the nightside precipitation at the polar cap‐auroral zone boundary. The convection is eastward in the poleward part of the radars field of view and predominantly westward at lower latitudes. The DMSP‐F9 particle spectrograms exhibit energetic precipitating protons at the poleward edge of an “inverted V”. The downward H+ flow displays spatial filtering effects with a clear energy‐latitude dispersion. Single‐particle trajectory simulations in appropriate field models demonstrate that these ions were ejected from the neutral sheet near 120 RE geocentric distance. Most notably, the simulations reveal significant azimuthal dispersion during transport, and the source region in the magnetotail is found to extend over several Earth radii in the dawn‐dusk direction, less energetic particles originating duskward of the high‐energy ones. The longitudinal spread of the tail source suggests particle precipitation in the ionosphere over at least one hour magnetic local time. Also, as the downflowing protons travel near the lobeward edge of the plasma sheet, the observations suggest that the convection reversal lies inside the boundary layer plasma sheet (PSBL).

Magnetopause plasma transients: signatures in the auroral ionosphere

The magnetopause and adjacent boundary layers of the Earth's magnetosphere play important roles in transferring momentum and energy from the solar wind to the magnetosphere-ionosphere system. The details of the different boundary processes, their ionospheric signatures and relative importance are not well known at present. Particle precipitation, field-aligned current, auroral emission, ionospheric ion drift and ground magnetic perturbations are among the low-altitude parameters that show signatures of various plasma processes in the LLBL and the magnetopause current layer. Magnetic merging events, Kelvin-Helmholtz waves, and pressure pulses excited by the variable solar wind/magnetosheath plasma are examples of boundary phenomena that may be coupled to the ionosphere via field-aligned currents. In this paper, attention is focussed on a specific category of auroral activity occurring in the cusp/cleft region predominantly during the southward directed interplanetary magnetic field (IMF). Co-ordinated observations from the ground and satellites in polar orbit have been used to study the temporal/spatial development of the events in relation to the background patterns of particle precipitation and ionospheric convection as well as the field-aligned current and ion drift characteristics of the individual events. The auroral phenomenon is characterized by a sequence of elongated forms moving laterally into the polar cap. Spatial scales of major events repeating every 5–10 min are ∼200 km (N-S) times 300–1000 km (E-W). Smaller scale auroral structures with more irregular occurrence rates are observed at times. The preliminary evidence suggests that the motion pattern is regulated by the IMF orientation, that is, the direction of longitudinal motion along the polar cap boundary is determined by the IMF B Y polarity. The examples reported here occurred within 1000–1400 MLT, near the zero point potential line separating the morning and post-noon convection cells. During nonzero IMF B Y the auroral structures are associated with channels of enhanced zonal ionospheric ion flow and Birkeland current sheets of opposite polarity, imbedded within the larger scale IMF B Y -related cusp-mantle current system. These characteristics are discussed in relation to model predictions of ionospheric signatures of magnetopause plasma transients, with particular emphasis placed on impulsive magnetic merging events.

Ionospheric convection response to slow, strong variations in a northward interplanetary magnetic field: A case study for January 14, 1988

We analyze ionospheric convection patterns over the polar regions during the passage of an interplanetary magnetic cloud on January 14, 1988, when the interplanetary magnetic field (IMF) rotated slowly in direction and had a large amplitude. Using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure, we combine simultaneous observations of ionospheric drifts and magnetic perturbations from many different instruments into consistent patterns of high‐latitude electrodynamics, focusing on the period of northward IMF. By combining satellite data with ground‐based observations, we have generated one of the most comprehensive data sets yet assembled and used it to produce convection maps for both hemispheres. We present evidence that a lobe convection cell was embedded within normal merging convection during a period when the IMF By and Bz components were large and positive. As the IMF became predominantly northward, a strong reversed convection pattern (afternoon‐to‐morning potential drop of around 100 kV) appeared in the southern (summer) polar cap, while convection in the northern (winter) hemisphere became weak and disordered with a dawn‐to‐dusk potential drop of the order of 30 kV. These patterns persisted for about 3 hours, until the IMF rotated significantly toward the west. We interpret this behavior in terms of a recently proposed merging model for northward IMF under solstice conditions, for which lobe field lines from the hemisphere tilted toward the Sun (summer hemisphere) drape over the dayside magnetosphere, producing reverse convection in the summer hemisphere and impeding direct contact between the solar wind and field lines connected to the winter polar cap. The positive IMF Bx component present at this time could have contributed to the observed hemispheric asymmetry. Reverse convection in the summer hemisphere broke down rapidly after the ratio |By/Bz| exceeded unity, while convection in the winter hemisphere strengthened. A dominant dawn‐to‐dusk potential drop was established in both hemispheres when the magnitude of By exceeded that of Bz, with potential drops of the order of 100 kV, even while Bz remained northward. The later transition to southward Bz produced a gradual intensification of the convection, but a greater qualitative change occurred at the transition through |By/Bz| = 1 than at the transition through Bz = 0. The various convection patterns we derive under northward IMF conditions illustrate all possibilities previously discussed in the literature: nearly single‐cell and multicell, distorted and symmetric, ordered and unordered, and sunward and antisunward.

Response of the ionospheric convection pattern to a rotation of the interplanetary magnetic field on January 14, 1988

Ionospheric convection signatures observed over the polar regions are provided by the DMSP F8 satellite. We consider five passes over the southern summer hemisphere during a time when the z component of the interplanetary magnetic field was stable and positive and the y component changed slowly from positive to negative. Large‐scale regions of sunward flow are observed at very high latitudes consistent with a strong z component. When By and Bz are positive, but By is greater than Bz, strong evidence exists for dayside merging in a manner similar to that expected when Bz is negative. This signature is diminished as By decreases and becomes smaller than Bz resulting in a four‐cell convection pattern displaced toward the sunward side of the dawn‐dusk meridian. In this case the sign of By affects the relative sizes of the two highest‐latitude cells. In the southern hemisphere the duskside high‐latitude cell is dominant for By positive and the dawnside high‐latitude cell is dominant for By negative. The relative importance of possible electric field sources in the low‐latitude boundary layer, the dayside cusp, and the lobe all need to be considered to adequately explain the observed evolution of the convection pattern.

Magnetopause plasma transients: mapping into the auroral ionosphere

Attention is focused on a specific category of auroral event occurring predominantly during a southward directed interplanetary magnetic field (IMF). Coordinated observations from the ground and satellites in polar orbit have been used to study the temporal/spatial development of the events in relation to the background patterns of particle precipitation and ionospheric convection as well as the field-aligned current and ion drift characteristics of the individual events. Both prenoon and postnoon sector events are reported. In one case the auroral event was observed near the zero point potential line separating the morning and postnoon convection cells. The available data indicate that this auroral event sequence is a signature of quasi-periodic bursts of particle entry from the magnetosheath along flux tubes convecting into the polar cap, representing structures of enhanced IMF B/sub Y/-related convection poleward of the persistent cleft aurora. Such events may be initiated by localized patches of enhanced merging rate at the dayside magnetopause.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ionospheric convection response to changing IMF direction

By combining ground‐based and satellite‐based measurements of ionospheric electric fields, conductivities and magnetic perturbations, we are able to examine the characteristics of instantaneous, ionospheric convection patterns associated with changing directions of the Interplanetary Magnetic Field (IMF). In response to a rapid southward‐to‐northward turning of the IMF on 23 July 1983, the ionospheric convection reconfigured over a period of 40 minutes. The configuration changed from a conventional two‐cell pattern to a contracted four‐cell pattern, with reversed convection cells in the high‐latitude dayside, associated with a strong potential drop of about 75 kV. Later, in response to a gradual rotation of the IMF from the +Z through the −Y. toward the −Z direction, the nightside cells disappeared and the dawn cell in the reversed pair wrapped around and displaced the dusk cell until a conventional two‐cell pattern was reestablished, largely in accord with the qualitative model of Crooker [1988]. Our results suggest that multiple cells can arise as a result of strong southward to northward transitions in the IMF. They appear to persist for sometime thereafter.

The High Latitude Ionospheric Convection Pattern

Simple examination of models of the magnetic and electric field indicate that both direct connection with the interplanetary field and connection to the low-latitude boundary layer flow are required to explain many of the repeatable features in the observations of high latitude ionospheric motions. During times of southward IMF, these two sources of the electric field may not be easily discernible and during time of northward IMF the details of the magnetic field geometry are important in determining the geometry of the ionospheric convection cells. When both these sources of the electric field are considered, a transition from southward to northward IMF leads to distorted two-cell convection patterns that can contain multiple cell configurations.

Equivalent ionospheric currents above Antarctica during the austral summer

A regression analysis was carried out between the ground-based geomagnetic data from the southern polar regions (above −55° corrected geomagnetic latitude) and the interplanetary magnetic field (IMF) components in the geocentric solar-magnetospheric co-ordinate system and solar wind (SW) parameters. Hourly mean values of the geomagnetic field horizontal components H and D from 20 Antarctic observatories and automatic stations of 1978–80 and 1983–84 were examined. Regression coefficients were used as geomagnetic perturbation vectors which were rotated 90° clockwise to plot the equivalent current vector patterns in the “corrected geomagnetic latitude-magnetic local time” co-ordinates. The results which are described in the paper reflect geomagnetic phenomena associated with the IMF and SW parameters for the austral summer season only (November, December, January, February). It was found that, in general, global characteristics of the ionospheric convection patterns agree well for both hemispheres. Geomagnetic variations, which are generated by the interaction of the SW plasma and frozen-in IMF with Earth's magnetosphere, represent three types of equivalent current systems: a) two-vortex system with transpolar current from nightside to dayside, controlled by the “quasi-viscous” interaction and southward IMF; b) zonal current system, controlled by the azimuthal IMF; and c) two-vortex system with transpolar current from noon to midnight controlled by the northward IMF. The southern polar cap (above −75°), which was more densely packed with automatic magnetometers than the northern cap, permit us to investigate the fine structure of the high-latitude current systems in detail.

Model of the high‐latitude ionospheric convection pattern during southward interplanetary magnetic field using DE 2 data

Data from the polar‐orbiting satellite DE 2 are used to calculate one‐dimensional electrostatic potential distributions across the polar cap region. Using passes that lie within ±3 hours MLT of the dawn‐dusk line, various parameters of the polar potential distribution (location and magnitude of the maxima and minima, location of the zero potential point, etc.) are analyzed in relation to each other and to the interplanetary magnetic field (IMF). The resulting dependences are used to derive a two‐dimensional model of the distribution of the electrostatic potential in the high‐latitude ionosphere during times of southward IMF. This model can be generated using as inputs either the ionospheric potential parameters or, based on the relationships analyzed here, the IMF conditions. The capabilities of the resulting mathematical model are illustrated, and the importance of retaining a flexibility in the model to accommodate individual observations is emphasized.

The response of dayside ionospheric convection to the Y-component of the magnetosheath magnetic field: A case study

High resolution ground magnetometer data are used in a case study to investigate three effects : (i) the large-scale ionospheric convection pattern appropriate to the two polarities of magnetosheath B y ; (ii) the temporal evolution of the ionospheric flow from one pattern to another following a sudden switch in the polarity of B y ; (iii) the imprint on the ionosphere of reconnection processes at the dayside magnctopause, both of the “quasi-steady” (QSR) and the flux transfer event (FTE) variety. The results obtained on these topics are as follows, (i) Asymmetries in the ionospheric convection pattern are observed, and these are different for the two polarities of B y . These asymmetries are in qualitative agreement with the results obtained earlier from statistical studies, (ii) We report for the first time observations which show the ionospheric flows to evolve smoothly from one asymmetrical pattern to another, triggered by a single. large, sudden change in the polarity of the magnetosheath B y . The flow change is estimated to commence (1.9 ± 0.7) min after the signal reaches ionospheric heights. The time scale associated with the reorientation of the whole ionospheric flow pattern is found to be 15–18 min, which we ascribe to the time for the ionospheric end of a reconnected flux tube to lose its memory of the B y sense under which it was reconnected. (iii) During the time when reconnection was observed by the ISEE spacecraft at the magnetopause (both QSR and FTEs), large ionospheric flows were present at high-latitude stations. Superposed on this large background flow clear and substantial (∼ 20–40% of the total disturbance) fluctuations were recorded at the cusp region stations. These increased and variable flows may constitute the ionospheric imprint of reconnection processes occurring at the higher altitudes, though another plausible explanation is that the transient flow fluctuations may have been caused by solar wind pressure changes.

Electrodynamic patterns for September 19, 1984

Variations of the northern high‐latitude ionospheric convection pattern are estimated from combined ground‐based and satellite‐based measurements for the disturbed day of September 19, 1984, during the Equinox Transition Study (ETS) campaign period. The electric potential distribution is determined from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure of Richmond and Kamide (1988) on the basis of available electric field, conductance, and magnetic perturbation observations. As an improvement over previous data inversion studies of high‐latitude electrodynamics, we have incorporated satellite measurements of precipitating particles to help determine variations of the conductances in the auroral oval. These measurements are found to cause substantial modifications to the initial empirical conductance model. A large substorm commenced about 0920 UT and peaked about 1050 UT, followed by smaller substorms starting about 1430 UT and 1800 UT. The polar‐cap potential drop varied between about 30 kV and 160 kV during the day, and the integrated Joule heating over the northern polar region varied between 3×109 W and 8.6×1011 W. During the substorms the derived convection potential was considerably enhanced and distorted with respect to the empirical model of Foster et al. (1986). Strong eastward convection in the midnight sector of the auroral oval during the first large substorm was associated with a marked westward rotation of the two‐cell convection pattern at the peak of the substorm. These variations can be expected to have influenced significantly the dynamics of the high‐latitude ionosphere and thermosphere during the ETS period.

Mapping of the solar wind electric field to the Earth's polar caps

In this paper we describe a quantitative model of a magnetically interconnected (open) magnetosphere, developed as a perturbation to Voigt's closed magnetosphere model with a given magnetopause shape. The “interconnection” (perturbation) field is obtained as a solution to a Neumann boundary value problem, with the magnetopause normal component distribution as a boundary condition. The normal component at the magnetopause is required to be time independent, and is specified in accordance with one of two hypotheses: the subsolar point merging hypothesis and Crooker's antiparallel merging hypothesis. The resulting open magnetospheric configuration is used to map the magnetopause electric field down to the polar cap ionosphere. We present ionospheric convection patterns derived from three representative interplanetary magnetic field (IMF) orientations for each of the two dayside merging geometries. Both merging geometries reproduce the observed convergence of convection streamlines near noon in a convection “throat,” and the east‐west deflection of these streamlines in response to the east‐west IMF component. The major difference between the two dayside merging geometries occurs for nonsouthward IMF, and consists of a Sun‐aligned convection gap that bifurcates the polar cap in the case of the antiparallel merging geometry but not in the subsolar point merging geometry. This convection gap may plausibly be associated with the “theta aurora” structure observed when the IMF has a northward component.

Studies of ionospheric plasma and electrodynamics and their application to ionosphere‐magnetosphere coupling

A brief description of the contributions of the Dynamics Explorer (DE) program to an understanding of ionospheric plasma dynamics is given. In addition to being used in some further study of small‐scale structure in the equatorial ionospheric number density, the DE 2 intrumentation has been extensively used to study the bulk motion of the plasma in the equatorial ionosphere. Variations in the east‐west plasma drift, at night as a function of magnetic flux tube apex height, show a decrease in magnitude with increasing height above the altitude of the peak F region concentration. The role of both E region and F region winds in producing this behavior and the differences between DE measurements taken at solar maximum and similar observations at solar minimum have been discussed. The work described here shows a general convergence of opinion about the behavior of the ionospheric convection pattern at high latitudes during periods of southward interplanetary magnetic field (IMF). Progress has been made in describing the magnetosphere‐solar wind interaction that may be involved in the production of the convection pattern. In particular, the expected magnetic field topology and the applicability of different convection drivers have been examined. The characteristics of the high‐latitude ionospheric plasma motion during periods of northward IMF have been extensively studied. Under such conditions the existence of small‐scale (1–10 km) and medium‐scale (10–200 km) structures in the electric field and plasma motion is quite evident. Its relationship to auroral forms and in particular to the theta aurora has been examined in some detail. The interpretation of larger‐scale electric field and plasma drift features in terms of closed loop convection trajectories and the applicability of different theoretical models to the data have also been discussed.

Ionospheric convection signatures and magnetic field topology

We present here a statistical study of signatures of the high‐latitude ionospheric convection pattern and the simultaneously observed energetic electron precipitation. We most often find convection cells in which the sunward flowing region contains auroral particle precipitation but the antisunward flowing region does not. However, our observations also show the frequent occurrence of convection cells in which neither the antisunward nor the sunward flowing plasma region contains auroral particle precipitation. These findings may appear within the dawnside or duskside convection pattern and strongly suggest that such convection cells may be associated with open magnetic field lines that thread the magnetotail lobes. Examination of the interplanetary magnetic field (IMF) data shows that this “lobe cell” convection signature is most likely to be accompanied by the signature of dayside merging when the IMF has a significant y component but is directed southward. A lobe convection cell has a location and sense of circulation that are dependent on the sign of By. For the northern hemisphere, clockwise circulation displaced to the duskside appears roughly 35% of the time when By is positive, and anticlockwise circulation displaced to the dawnside appears when By is negative. The same circulation sense and location exist in the southern hemisphere for the opposite polarity of By. At times of northward IMF, the circulation within the polar cap can be at least partially on closed field lines and cannot be easily reconciled with merely a distortion of the standard “two‐cell” convection pattern. The significance of these results to several models of the solar wind/magnetosphere interaction is discussed.

Joint STARE and SABRE radar auroral observations of the high-latitude ionospheric convection pattern

The projection of the magnetospheric convection electric field into the ionosphere drives an asymmetric two-cell pattern of plasma circulation in the auroral zone. The Scandinavian Twin Auroral Radar Experiment (STARE) and the Sweden And Britain auroral Radar Experiment (SABRE) are two similar and independent coherent radar systems which can estimate E-region electron drift velocities at auroral latitudes. By combining their measurements, we have monitored simultaneously, for the first time, the convection flow pattern over 8 degrees of latitude. We show here how this allows a more extensive study of the nightside (Harang discontinuity) and dayside (cusp) boundaries between the cells, and enables spatial and temporal features of the flow pattern to be resolved.

Aircraft measurements of the atmospheric electrical global circuit during the period 1971–1984

This report will update an investigation of the global circuit conducted over the last 14 years through aircraft measurements of the variation of ionospheric potential and associated parameters. The data base included electric field, conductivity, and air‐earth current density profiles from the tropics (25°N) to the Arctic (79°N). Almost all of the data have been obtained over the ocean to reduce noise associated with local generators, aerosols, and convection. Recently, two aircraft have been utilized to obtain, for the first time, quasi‐periodic sets of simultaneous ionospheric potential (VI) soundings at remoted locations and extending over time spans sufficiently long so that the universal time diurnal variation (Carnegie curve) could be observed. In addition, these measurements provided the first detection of the modulation of electric fields in the troposphere caused by the double vortex ionospheric convection pattern. Beside summarizing these measurements and comparing them to similar data obtained by other groups, this report will discuss meteorological sources of error and criteria for determining if the global circuit is being measured rather than variations caused by local meteorological processes.

Plasma convection in the vicinity of the dayside cleft

Experiments conducted with the Chatanika, Alaska, incoherent scatter radar observed the ionospheric convection pattern equatorward of the noontime cleft. There plasma transport is characterized by low‐speed convergence toward a meridian centered somewhat before noon and rotation of the flow into a poleward direction at higher latitudes. For undisturbed conditions there is only weak convection at noon at these latitudes, but the eastward electric field reaches a maximum of 15 mV/m 2–4 hours before and after noon at latitudes between 70°Λ and 74°Λ. During active conditions at the peak of the solar cycle the ionospheric cleft was located within the Chatanika field of view on several occasions. A longitudinally narrow region of rapid poleward convection near noon was not observed; rather, poleward flow occurred over a 3–4 hour spread of local time. For disturbed conditions, the prenoon and postnoon regions of plasma entry into the polar cap were enhanced, and an eastward electric field of 25 mV/m was seen across the 2 hours of local time around noon. High‐density F region plasma was observed convecting poleward through the cleft from a source at lower latitudes in the afternoon sector. Such plasma, seen at very high latitudes within the polar cap, serves as a tracer of the convection pattern away from the cleft.

Ionospheric signatures of magnetospheric convection

Convection plays a major role in determining the characteristics of the high‐latitude ionosphere and the processes that drive magnetospheric convection are reflected in the ionospheric configuration. Average patterns of ionospheric convection observed with the Chatanika incoherent scatter radar, when mapped along magnetic field lines to the equatorial plane, delineate regions of earthward convection within the magnetotail that either proceed sunward to the dayside magnetopause where reconnection with the IMF may occur or that join the antisunward circulation along the magnetospheric flanks at dusk and dawn. Summer and winter patterns of ionospheric convection and F region density are examined in terms of the corresponding patterns of magnetospheric circulation. It is found that ionospheric ionization troughs at dusk and dawn are associated with sunward convection of low‐density plasma from the night sector and that plasma in the vicinity of the dawn and dusk electric field reversals circulates at high latitudes away from the solar ionization source at noon. Convection into the cleft from low latitudes populates the polar cap with high density solar‐produced plasma. Winter densities in this region were three times those seen in summer. Plasma exiting the polar cap is convected away from midnight in the equatorward portion of the regions of sunward convection at auroral latitudes. Characteristic features of the polar cap plasma serve to identify plasma following such convection trajectories.

A model of the high‐latitude ionospheric convection pattern

Mathematical expressions have been constructed that allow the large‐scale global convection characteristics of the high‐latitude ionosphere to be reproduced. The model contains no discontinuities in the ion convection velocity and as such should be useful in F region chemical models. The number of variables in the model allow such features as the dayside throat and the Harang discontinuity to be modeled. The applicability of the model to magnetospheric physics is limited by the exclusion of large‐magnitude small‐scale flow features associated with discrete arcs and by the inability of the model to produce separate flow cells at the same local time.