Polar Cap Convection 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?

Mapping ionospheric convection response to IMF By negative and Bz positive conditions

We have used the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure to derive electric potential patterns for a period of strong and slowly varying northward Interplanetary Magnetic Field (IMF). Using these patterns we offer some insight into high-latitude convection response to differing IMF conditions between the hours of 0850 and 1720 UT on 14 January 1988. We introduce a newly assimilated data set, Defense Meteorological Satellite Program (DMSP) ion drift measurements, as a means of mapping electric fields in polar regions. Also, for the first time we map both southern and northern hemisphere convection flows. We show that significantly different convection patterns are simultaneously possible in the two polar caps when B z is positive, and that asymmetries in these patterns are influenced by the strength of B y and B x . When the ratio B y B z| exceeds unity the polar cap convection increases and changes from sunward to anti-sunward for B z positive conditions.

A rotating, midday auroral event with northward interplanetary magnetic field

We present ground‐based observations of an isolated, daytime auroral event that was detected above Svalbard by means of an all‐sky TV camera and a multichannel, meridian scanning photometer. This 5‐min event occurred near magnetic noon, poleward of a stable cusp aurora. It emitted ∼20 kR of 557.7‐nm light, with little if any admixture of 630.0‐nm radiation. The structure rotated rapidly then underwent a sudden breakup. The spectral distribution of emitted light indicates that electrons responsible for this auroral structure were accelerated though several kilovolts between the magnetosheath and the ionosphere. Such a potential structure requires that the precipitating, arc electrons constitute a field‐aligned current out of the ionosphere. Current continuity and Ohm's law require radial electric fields within the arc. From the structure's rotational speed, we estimate that the accelerated electrons carried an upward field‐aligned current of order 100µA/m². Satellite observations of polar cap convection patterns suggest that the interplanetary magnetic field had turned northward prior to the event. The auroral structure may be explained as resulting from a transitory magnetic merging of interplanetary and lobe magnetic field lines at and/or a penetration of plasma across the high‐latitude magnetopause.

Imaging riometer and HF radar measurements of drifting F region electron density structures in the polar cap

A study of riometer, photometer, and ionosonde data from South Pole Station has revealed a class of unusual dayside absorption events which occur near local magnetic noon when South Pole is located poleward of the dayside cusp. These events, which can reach values in excess of 1 dB in narrow‐beam 38.2‐MHz riometer measurements, are accompanied by significant enhancement of the O(¹D2) 630.0‐nm emission. However, because there is no corresponding change in the N2+ 427.8‐nm emission, it is unlikely that these events are caused by the conventional D or lower E region ionization increases usually attributed to the precipitation of auroral electrons of keV energy. Rather, the comparison of South Pole imaging riometer and ionosonde data with simultaneous Halley PACE HF radar data suggests that these unusual absorption events are related to F region electron density structures drifting from the dayside oval into the polar cap. If further work sustains this interpretation, then new prospects will be opened up for using imaging riometers to examine aspects of polar cap convection.

Digisonde measurements of polar cap convection for northward interplanetary magnetic field

Controversy still exists regarding even the average convection pattern when the interplanetary magnetic field (IMF) has a northward component. Using two years of convection data from a Digisonde located at Qaanaaq only 3° from the corrected geomagnetic pole we have examined the diurnal convection flow direction variation in the central polar cap when the IMF is particularly stable. We find that when Bz is positive, and when By positive and By negative data are treated independently, each exhibits a clear diurnal pattern. The patterns are most nearly consistent with a multicell convection model, e.g. Potemra et al. (1984); there are, however, two anomalies. Our synthesized polar cap convection patterns exhibit a polar cap cell centered on 10 corrected geomagnetic local time (CGLT) when By > 1 nT and 13 CGLT when By < −1 nT in contrast to 06 and 18 CGLT predicted by the multicell models. Furthermore, in contrast to the simple multicell models the convection flow patterns for opposite By polarities are not simple mirror images of each other. When By < −1 nT the convection is directed across the central polar cap toward 02 CGLT for much of the day but when By > 1 nT the flow is tangential to the Qaanaaq geomagnetic latitude for much of the day.

Plasma flow reversals at the dayside magnetopause and the origin of asymmetric polar cap convection

A number of events have been observed in the Los Alamos/Garching fast plasma experiment data from ISEE 2 within ±3 hours of noon wherein the y component of the plasma flow within the low latitude boundary layer and magnetopause current layer is oppositely directed to that in the adjacent magnetosheath. When the y component, By, of the local magnetosheath magnetic field is positive, events of this nature are found preferentially in the northern dusk and southern dawn quadrants, and when By is negative, they are found preferentially in the opposite two quadrants. Plasma flows in these events are always also poleward, that is they are always directed away from the GSE equatorial plane. The observations are qualitatively and quantitatively consistent with previous observations of accelerated flows at the magnetopause and with models of magnetic reconnection, with the reconnection occurring at low latitudes near the GSE XY plane independent of the magnitude or the sign of the y component of the local magnetosheath magnetic field. Local magnetic shears at the magnetopause for these events range from 60 to 180 degs, which, together with the occurrence of these events at low latitudes, does not tend to support the antiparallel merging hypothesis. These observations of By‐dependent flow reversals provide a graphic demonstration of how asymmetric polar cap convection and related phenomena, such as the Svalgaard‐Mansurov effect, originate in magnetic reconnection at the dayside magnetopause.

Relation of sun-aligned arcs to polar cap convection and magnetic disturbances

The relationship between the sun-aligned arcs and the polar cap convection is examined on the basis of the auroral data from an all-sky camera at Vostok station and the magnetic variation data from the meridional chain of stations in Antarctica. It is shown that the polar cap arcs associated with the transpolar band at θ-auroras are in a region of the sunward convection whereas the arcs occurring in the evening or morning limbs of θ-auroras lie in a region of the antisunward convection. There is a good agreement between the locations of the polar cap arcs observed simultaneously by a spacecraft imager and by a ground all-sky camera. A rare case is also analysed in the paper when the hook-shaped sun-aligned arcs appear near the border of the night-time oval at the beginning of a magnetic substorm.

Dynamics of Mars' magnetosphere

If Mars has a small intrinsic magnetic moment, Mars' magnetosphere could vary on time scales of a few minutes due to reconnection with the solar wind magnetic field. The day‐side magnetopause will be one or two reflected‐ion Larmor radii from the bow shock. Substorms will have scale‐times of about six minutes. Mars' high ionospheric conductance will virtually stop polar cap convection, and create a magnetic “topological crisis” unless convecting magnetic flux finds a dissipative way to return to the day‐side. The strong magnetic shear induced by magnetospheric convection above the ionosphere could be tearing unstable. The magnetic field might diffusively “percolate” through the tearing layer. This shearing also draws field aligned currents from the ionosphere which could inject few KeV heavy ionospheric ions into the magnetotail.

Mapping the merging potential from the magnetopause to the ionosphere through the dayside cusp

The pattern in which the potential drop generated at the magnetopause is applied across the polar cap is constructed with the use of two models: the new antiparallel merging model of the dayside magnetopause and the expanding polar cap convection model. The results yield a new set of model convection patterns. Flow into the polar cap begins at a distended, wedge‐shaped cusp, which rotates around the Chapman‐Ferarro cusp position in response to rotation of the IMF in the plane perpendicular to the Earth‐sun line. The length of the wedge base, and the potential drop across it, mapped from the merging line on the magnetopause, decrease as the IMF turns northward. For large IMF y component, rotational flow into the polar cap is distributed along one side of the wedge and feeds into a region of strong east–west flow, some of which turns sharply into the polar cap at the base of the other side of the wedge, at noon. Two convection cells driven by merging with closed geomagnetic field lines exist for all IMF orientations. The cells are confined to a circular polar cap and become distorted with rotation of the cusp, into patterns deduced independently from observations.

Polar convection and Birkeland Currents during strongly positive IMF By

The TRIAD satellite passed directly over the field of view of the Scandinavian Twin Auroral Radar Experiment (STARE) radar on May 1, 1978, at approximately 0618 UT when the By component of the interplanetary magnetic field (IMF) was unusually large in comparison to the other two components. The average IMF values in the interval from 0600 to 0700 UT on this day were Bx = −1.8 nT, By = 8.7 nT, and Bz = −0.2 nT (King, 1979). The TRIAD magnetic field perturbations revealed the presence of a very intense (ΔB ∼ 500 nT) region 1 downward‐flowing Birkeland current near 0930 MLT and poleward of a much weaker upward‐flowing region 2 current. This “net” (unbalanced) Birkeland current was observed at a 67° magnetic latitude in this morning sector (near 0930 MLT). The net Birkeland current was located equatorward of a region of intense westward (antisunward) ionospheric convection flow detected by the STARE radar. We conclude that the polar cap convection flow is distorted and displaced to low latitudes (< 67° magnetic latitude) in the morning sector during this period of strongly positive By and almost negligible Bx and Bz. The large net region 1 Birkeland current is associated with the convection flow reversal. Convection velocity data acquired by the AE‐C satellite on October 29, 1978, near 1723 UT when Bx = −0.5 nT, By = 8.3 nT, and Bz = −2.7 nT (average values during the 1700 to 1800 UT interval) show the convection reversal near 67° invariant latitude and 0800 MLT (King, 1979). These two examples support previous suggestions for the important influence that the By component of IMF has on the intensity and location of high‐latitude convection and Birkeland currents.

IMF By and day‐night conductivity effects in the expanding polar cap convection model

During southward Bz periods the open field line region in the ionosphere (polar cap) expands due to increased dayside merging. Ionospheric plasma flow patterns result which can be classified by the sign of the interplanetary magnetic field (IMF) By component. In this paper we construct a time‐dependent ionospheric convection model to simulate these flows. The model consists of a spiral boundary with a gap in it. The sign of the IMF By component determines the geometry of the gap. A potential is applied across the gap and distributed around the boundary. A flow results which enters the polar cap through the gap and uniformly pushes the boundary outward. Results of the model show that By effects are greatest near the gap and virtually unnoticeable on the nightside of the polar cap. Adding a day‐night ionospheric conductivity gradient concentrates the polar cap electric field toward dawn. The resulting flow curvature gives a sunward component that is independent of By. These patterns are shown to be consistent with published observations.

Triggering of substorm expansion by the IMF directional discontinuities: Time delay analysis

Time delays in the response of polar cap (PC) convection and triggered substorm onsets to the sharp and isolated directional discontinuities (DD) of IMF are studied taking into account the DD type and orientation of its normal. An onset of increase (decrease) of PC convection follows with 10 min delay after NS (SN) IMF turning, referred to the subsolar point of bow shock (at 14 R E). This delay does not depend on DD orientation and agrees with predictions of merging theories. The times of triggered expansion onsets depend clearly on DD orientation and occur with average 8–10 min delay after the first contact of DD with the bow shock. This indicates that triggering is caused immediately by the disturbance launched by DD interaction with the magnetosphere, which must propagate in the magnetosphere with a speed of a fast magnetosonic wave. The triggering efficiency and time delays do not depend on the type of DD (tangential or rotational DDs). The triggered substorm onsets usually occur before the onset of a corresponding convection decrease, caused by the same positive jump of IMF B z-component. The results show that substorm expansion phase triggering by IMF directional discontinuities is a real and frequent phenomenon.

Interplanetary control of high latitude electrodynamics.

This paper reviews recent observational developments concerning the influence of the solar wind (SW) and the interplanetary magnetic (IMF) field on magnetospheric processes. Examples are chosen to demonstrate that the influence of the SW/IMF is global in scope. Subsequent to sudden compressions of the magnetosphere observed by the Dynamics Explorer satellite the entire auroral oval is bombarded by previously trapped particles. Whether this is followed by a main phase storm or a theta aurora depends on the north-south polarity of the IMF. The potential drop across the polar cap is shown to correlate with the interplanetary electric field. However the potential across the polar cap is much less than the potential across the magnetosphere in the solar wind. The influence of the azimuthal component and ionospheric conductivity on the polar cap convection pattern is discussed in terms of recent observations from the Dynamics Explorer satellite. During periods of northward IMF, regions of sunward convection, with an attendant field-aligned current system, are found in the sunlit ionosphere on the day side of the magnetic dawn-dusk meridian. On the night side of this meridian and in the darkened polar cap, electric fields and field-aligned currents show filamentary structures. Electron precipitation in the polar cap varies from nearly uniform polar rain during periods of southward IMF to structured polar rain and sun-aligned arcs when the IMF has a northward component. The theta aurora detected by the imaging system on Dynamics Explorer appears in regions of sunward convection accompanied by downcoming O+ ions. This suggests that the lobes of the magnetotail are bifurcated by closed, plasma sheet field lines. At lower magnetic latitudes the systematics of the equatorward boundary of auroral electron precipitation are indirectly controlled by the interplanetary electric field. For low to moderate levels of Kp, reasonable estimates of the polar cap potential may be derived from the positions of this boundary.

A simple model for polar cap convection patterns and generation of θ auroras

The simple addition of a uniform interplanetary magnetic field and the Earth's dipole magnetic field is used to evaluate electric field convection patterns over the polar caps that result from solar wind flow across open geomagnetic field lines. This model is found to account for observed polar‐cap convection patterns as a function of the interplanetary magnetic field components By and Bz. In particular, the model offers an explanation for sunward and antisunward convection over the polar caps for Bz > 0. Observed field‐aligned current patterns within the polar cap and observed auroral arcs across the polar cap are also explained by the model. In addition, the model gives several predictions concerning the polar cap that should be testable. Effects of solar wind pressure and magnetospheric currents on magnetospheric electric and magnetic fields are neglected. That observed polar cap features are reproduced suggests that the neglected effects do not modify the large‐scale topology of magnetospheric electric and magnetic fields along open polar cap field lines. Of course, the neglected effects significantly modify the magnetic field geometry, so that the results of this paper are not quantitatively realistic and many details may be incorrect. Nevertheless, the model provides a simple explanation for many qualitative features of polar cap convection.

The interplanetary electric field, cleft currents and plasma convection in the polar caps

This report investigates the suggestion that the pattern of plasma convection in the polar cleft region is directly determined by the interplanetary electric field (IEF). Owing to the geometrical properties of the magnetosphere, the East-West component of the IEF will drive field-aligned currents which connect to the ionosphere at points lying on either side of noon, while currents associated with the North-South component of the IEF will connect the two polar caps as sheet currents centered at noon. The effects of the hypothesized IEF driven cleft current systems on polar cap ionospheric plasma convection are investigated through a series of numerical simulations. The simulations demonstrate that this simple electrodynamic model can account for the narrow “throats” of strong dayside antisunward convection observed during periods of southward interplanetary magnetic field (IMF) as well as the sunward convection observed during periods of strongly northward IMF. Thedawn-dusk shift of polar cap convection which is related to the B y component of the IMF is also accounted for by the model.

High latitude electrodynamics: Observations from S3-2

The high spatial-temporal resolution of instrumentation on the polar-orbiting S3-2 satellite has allowed a wide variety of measurements of the electrodynamic characteristics of both large- and small-scale structures at high latitudes. Analyses of large scale features observed by S3-2 have shown that: (i) The IMF B ydependence of polar cap convection, first observed in June 1969 by OGO-6 persists in other seasons. During periods of northward IMF B zextensive regions of sunward convection may be found in the sunlit polar cap. (ii) In the dawn and dusk MLT sectors >90% of the region 1 currents lie equatorward of the convection reversal line. Potentials across the ionospheric projection of the low-latitude boundary layer are typically a few kV. (iii) The location of ‘extra’ field-aligned currents, near the dayside cusp and poleward of the region 1 current sheet is dependent on the IMF B ycomponent. (iv) Simultaneous observations by TRIAD and S3-2 show that sheets of field-aligned current extend uniformly for several hours in MLT, but may have an altitude dependence in the 1000–8000 km range. (v) During magnetic storms ionospheric irregularities occur in regions of poleward density gradients and downward field-aligned currents near the equatorward boundary of diffuse auroral precipitation. In the winter polar cap, density irregularities were also found in regions of highly structured electric fields and soft electron precipitation. (vi) During an intense magnetic storm the auroral zone height-integrated Pederson conductivity was calculated to be in the range 10–30 mho and downcoming energetic electron fluxes accounted for between 50% and 70% of the upward Birkeland currents.

Very high latitude F‐region irregularities observed by HF‐radar backscatter

In February and March, 1982, a coherent scatter HF radar was operated from Cleary, Alaska to observe 7‐ to 15‐m wavelength F‐region plasma irregularities near the poleward edge of the auroral zone and in the polar cap. The radar operated for five days from February 25 to March 1 and produced approximately 700,000 Doppler spectra during that time. Of those nearly 700,000 spectra, approximately 10% showed backscattered power 3 dB or more above the noise level. A ray tracing technique using electron densities determined by the Chatanika incoherent scatter radar was used to predict locations where the HF waves were approximately normal to the magnetic field. If those locations were also to contain small scale electron density structure, then one would expect them to backscatter the HF waves. Several comparisons were made between predicted and observed locations of radiowave backscatter and excellent agreement was obtained. In addition, comparisons of the Doppler velocities observed by the coherent scatter HF radar and those observed by the Chatanika radar showed good agreement, suggesting that the plasma irregularities observed by the HF radar drift with the ambient plasma. In addition, average vector velocities calculated for the entire 5‐day period show a flow pattern consistent with polar cap convection models. This again indicates that the irregularities drift with the plasma, as is predicted by a number of theories of F‐region plasma irregularities. In the summer of 1983, the research program begun with those measurements will be continued with a steerable phased‐array HF radar located at Goose Bay, Labrador, that will view the same ionospheric region as does the Sondre Stromfjord incoherent scatter radar.

On the orientation of the polar cap electric field

Using realistic models of the ionospheric conductivity and the field‐aligned currents, we have determined how the distribution of the electric field in the polar cap ionosphere is controlled by the day‐night contrast in conductivity and by the relative strengths of the region 1 and region 2 field‐aligned currents. The sunward directed conductivity gradient acts to set up a space charge in the polar cap which crowds the equipotentials toward the dawn sector for current sources of both region 1 and region 2 polarity; this effectively shifts the polar cap convection pattern toward dawn. Our results show further that for a given conductivity distribution the orientation of the electric field in the central polar cap depends sensitively on the relative strengths of the Birkeland current pairs: for very weak region 2 currents (quiet times) the polar electric field is directed ≈ 60° east of noon, for equal region 1 and 2 currents (disturbed times) the direction is ≈ 10° east of noon, and for stronger region 2 than 1 currents (which may happen on occasion) the electric field points into the prenoon sector. These findings imply that the orientation of the polar cap electric field should serve as a measure or index of the ratio of region 1 to region 2 net current intensity and should possess correlations with geomagnetic activity similar to those of this ratio. This analysis does not include effects of possible source currents in the region of the polar cusp.

Dayside cleft aurora and its ionospheric effects

The existence of an opening on the dayside of the earth's magnetic field through which solar plasma could penetrate was noted by Chapman and Ferraro in 1931, and although observational evidence of its auroral and ionospheric effects accumulated steadily over the years, the dayside cleft (or cusp) was not accepted as a real magnetospheric feature until 1971. The history of this development is described, and some reasons for the long delay in recognizing this profound ionospheric influence are given. The entry of magnetosheath plasma to ionospheric levels provides an essentially permanent dayside energy source for the F region that generates a rather constant 1–2 kR of 6300‐Å atomic oxygen emission, a weak hydrogen emission, and some other emissions. It also produces ionospheric irregularities and enhanced electron temperatures that can be detected by a variety of methods, all of this in a region normally centered at noon and 78° in invariant coordinates, with a latitude extent of a few degrees and a longitude extent of a few hours. The cleft ionization is an important source for the polar cap ionosphere, and the detailed nature of its effect depends upon the interplanetary field and on magnetospheric influences on the polar cap convection pattern. Some questions for further study are given in conclusion.

Observations of differences between regions of current flowing into and out of the ionosphere

A correlative study of field‐aligned currents, upflowing ions and electrostatic shocks has been made using data from the S3‐3 satellite. Upflowing ions with pitch angle distributions that are peaked at an angle to the magnetic field ("conics") are observed in all auroral current regions, but upflowing ions with distributions peaked along the field ("beams") are seen only in upward current regions. Thus, indirect evidence for parallel electric fields is seen in regions of upward current, but not in regions of downward current. Electrostatic shocks are usually observed in the current sheet associated with the polar cap convection boundary. In the evening, when this current is upward, shocks are largest. Indirect evidence for parallel electric fields is seen in regions of upward current, but not in regions of downward current. The altitude and magnetic local time dependence of upflowing ions and electrostatic shocks may be explained by the physical consequences of the differing particle populations carrying current into and out of the ionosphere. The data suggest that current‐driven turbulence plays a crucial role in energizing particles in the auroral zone.