Simulation of the polar cap potential during periods with northward interplanetary magnetic field (original) (raw)
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Simulations of the behaviour of the polar thermosphere and ionosphere for northward IMF
Journal of Atmospheric and Terrestrial Physics, 1994
The dynamics and structure of the polar thermosphere and ionosphere within the polar regions are strongly influenced by the magnetospheric electric field. The convection of ionospheric plasma imposed by this electric field generates a large-scale thermospheric circulation which tends to follow the pattern of the ionospheric circulation itself. The magnetospheric electric field pattern is strongly influenced by the magnitude and direction of the interplanetary magnetic field (IMF), and by the dynamic pressure of the solar wind. Previous numerical simulations of the thermospheric response to magnetospheric activity have used available models of aurora1 precipitation and magnetospheric electric fields appropriate for a southward-directed IMF. In this study, the UCL/Sheffield coupled thermosphere/ionosphere model has been used, including convection electric field models for a northward IMF configuration. During periods of persistent strong northward IMF B,, regions of sunward thermospheric winds (up to 200 m s-') may occur deep within the polar cap, reversing the generally anti-sunward polar cap winds driven by lowlatitude solar EUV heating and enhanced by geomagnetic forcing under all conditions of southward IMF BP The development of sunward polar cap winds requires persistent northward IMF and enhanced solar wind dynamic pressure for at least 24 h, and the magnitude of the northward IMF component should exceed approximately 5 nT. Sunward winds will occur preferentially on the dawn (dusk) side of the polar cap for IMF B, negative (positive) in the northern hemisphere (reverse in the southern hemisphere). The magnitude of sunward polar cap winds will be significantly modulated by UT and season, reflecting Eand F-region plasma densities. For example, in northern midwinter , sunward polar cap winds will tend to be a factor of two stronger around 1800 UT, when the geomagnetic polar cusp is sunlit, then at 0600 UT, when the entire polar cap is in darkness.
Journal of Geophysical Research, 1993
Through a global three-dimensional MHD simulation, which makes it possible to reveal the physical processes and causalities of the global interaction between the solar wind and the magnetosphere (Watanabe and , the effect of the interplanetary magnetic field (IMF), both northward and southward, on Earth's magnetosphere has been investigated. A southward IMF, upon reconnecting at the dayside magnetopause, sweeps the Earth's magnetic field toward the nightside and drapes the magnetotail. This gives rise to a plasma sheet cross-tail current increase and explosive magnetic reconnection at around 15 Re from the Earth. This reconnection results in the formation of a large plasmoid which grows much faster than for the simulation with no IMF, thus supporting previous notions that a southward IMF is a driving mechanism of plasma sheet reconnection. A northward IMF is observed to reconnect with the magnetosphere along the cusp shoulder, stripping magnetic field lines away and weakening compression of the plasma sheet, thus inhibiting plasma sheet reconnection. In order to accommodate a balance of magnetic to dynamic pressure along the magnetopause, the magnetosphere changes from its usual cometlike shape to that rese•rnbling a tadpole as it attempts to return to a dipolaf structure. Plasma sheet reconnection is thereby inhibited.
Geophysical Research Letters, 2003
The behavior of the cross polar cap potential, È PC , under strong solar wind conditions is studied using global MHD simulations. Simulations using two typical values of the ionospheric Pedersen conductance in agreement with others show that the cross polar cap potential is reduced compared to the corresponding potential in the solar wind due to the stagnation of the magnetosheath flow and the existence of parallel potentials. However, it is the ionospheric conductance that affects the value of È PC the most: the transpolar potential saturates only for high enough ionospheric conductance. A mechanism in which the ionospheric conductance changes the properties of the magnetosheath flow is proposed. This mechanism assumes mapping of the electrostatic potential in the ideal MHD system and yields a self-consistent response of the reconnection and transpolar potentials to changes in the ionospheric conductance.
Geophysical Research Letters, 1999
3-D MHD simulations were used to investigate the behavior of the high-latitude convection and the polar cap variations during two events characterized by sudden southward IMF turnings. In agreement with recent observations the simulation results indicate that the convection pattern across the entire polar cap begins to change a few minutes after the arrival of the southward IMF. In contrast, the onset of the equatorward motion of the open closed fieldline boundary depends on the local time, with equatorward motion of the midnight boundary delayed by about 20 minutes relative to the the onset of the boundary motion at noon. We interpret this delay as the time required to convect newly merged flux from the dayside to the nightside. We belive that these two different responses can reconcile apparent contradictions in studies of ionospheric reconfigurations in response to changes in the IMF.
Dependence of polar cap potential drop on interplanetary parameters
Journal of Geophysical Research, 1981
We have computed the convection correlation is most easily explained in terms of potential drop accoss the polar cap from data the open magnetosphere model [Dungey, 1961], in obtained on high-inclination low-altitude satel-which the polar cap convection is driven by maglites (AE-C, AE-D, S3-3) and correlated these netic stress transmitted along 'open' or 'interpotential measurements with various combinations connected' magnetic field lines connecting the of parameters measured simultaneously in the geomagnetic and interplanetary magnetic fields upstream solar wind. These combinations of solar (see, for example, the discussion by Russell and wind parameters consist of predictions based on Atkinson [1973]). Another unavoidable prediction magnetic merging theory and suggestions based on of the open-model hypothesis is that the total earlier empirical work. We find that the bulk of strength of the convection system, as measured the potential drop, and its variation with inter-by the polar cap potential drop, should be an planetary magnetic field (IMF) parameters, are incceasing function of the southward component of successfully predicted by merging theory (to the the IMF, this component being most favorable for accuracy with which they can presently be mea-the 'merging' process whereby geomagnetic flux sured), but that a significant 'background' becomes interconnected with interplanetary flux potential drop (~ 35 kV) does not depend on IMF on the dayside magnetopause. To our knowledge, parameters and may thus be attributed to an this prediction has never been directly tested, unknown process other than merging. Our results and such a test is the primary purpose of this indicate that small values of the IMF are ampli-paper. (A lack of such correlation was tentafled by a factor of 5-10 at the dayside magneto-tively reported by Heppner [1972] on the basis of pause as a combined effect of bow shock compres-the very limited data set then available.) sion and the Zwan-Wolf depletion layer effect; Several studies have shown a strong positive correlations between IMF parameters and the polar correlation between the southward IMF component cap potential drop are dramatically improved when and a variety of geomagnetic and auroral activity this amplification is taken into account. The indices [e.g., Arnoldy, 1971; Foster et al., potential drop is better correlated wtth IMF 1971; Rostoker et al., 1972; Murayama and parameters than with geomagnetic activity Hakamada, 1975; Garrett et al., 1974; Burch, indices, presumably because the latter are 1974]. In view of the widespread belief that affected by nonlinear reponses of the magneto-magnetospheric convection powers most of the geosphere to the polar cap input. magnetic disturbances that are reflected in the activity indices, it is plausible (and perhaps the reviews by Burch [1974], Stern [1977], and between •1 and •2, for two values of the Cowley [1981], and references therein). This ratio • = B1/B 2. The solid and dashed curves follow from different possible assumptions Copyright 1981 by the American Geophysical Union. regarding the dependence of the merging rate on Paper number 1A1042.
Magnetopause mapping to the ionosphere for northward IMF
Annales Geophysicae, 2008
We study the topological structure of the magnetosphere for northward IMF. Using a magnetospheric magnetic field model we study the high-latitude response to prolonged periods of northward IMF. For forced solar wind conditions we investigate the location of the polar cap region, the polar cap potential drop, and the field-aligned acceleration potentials, depending on the solar wind pressure and IMF B y and B x changes. The open field line bundles, which connect the Earth's polar ionosphere with interplanetary space, are calculated. The locations of the magnetospheric plasma domains relative to the polar ionosphere are studied. The specific features of the open field line regions arising when IMF is northward are demonstrated. The coefficients of attenuation of the solar wind magnetic and electric fields which penetrate into the magnetosphere are determined.
Digisonde measurements of polar cap convection for northward interplanetary magnetic field
Journal of Geophysical Research, 1992
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 ø 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 B z is positive, and when By positive and By negative data are treated independently, each exhibits a clear diurnal pattern. The pauerns are most nearly consistent with a multicell convection model, e.g. Petetara 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 B > 1 nT and 13 CGLT when B <-1 nT in Y Y 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 B polarities are not simple mirror images of each other. Y When By <-1 nT the convection is directed across the central polar cap toward 02 CGLT for much of the day but •hen By > 1 nT the flow is tangential to the Qaanaaq geomagnetic latitude for much of the day. 1. INTRODUCTION During periods when the interplanetary magnetic field (IMF) z component is negative (southward) it is widely accepted that the open magnetospheric model of Dungey [1961] is appropriate. This results in a two-cell ionospheric convection model such as that proposed by Heppner and Maynard [1987]. The IMF y component, and to lesser extent the IMF x component, are also recognized to play important roles in controlling this convection pattern [e.g. Heppner and Maynard, 1987; Cannon et al., 1991; Cowley et al., 1991]. When the IMF z component is positive (northward) the convection pattern is thought to be more complex than that for the negative (southward) configuration. Considerable controversy exists, however, regarding even the average or statistical pattern. Maezawa [1976], using ground-based magnetometers, recognized that the simple two-cell pattern is not appropriate to IMF northward conditions, and he deduced that sunward flow should occur in the central polar cap. Magnetospheric satellite electric field measurements using S3-2 and Magsat [Burke et al., 1979; lijima et al., 1984] confirmed the presence of sunward convection, and Burke et al. [1979] suggested a four-cell model of convection, although this pattern could only be clearly inferred on the dayside. Support for the multicell model was provided by Potemra et al. [1984] who suggested that a three-cell pattern could occur for Copyright 1992 by the American Geophysical Union. Paper number 92JA01190. 0148-0227/92/92JA-01190505.00 northward B z and furthermore, that the pattern was dependent upon the IMF By component. When By .-0 four cells were proposed, much the same as those in the Burke et al. [1979] model, but this reduced to three cells when IByl was greater than zero. In the northern hemisphere as By becomes more positive the polar cap dawn cell expands and the dusk cell shrinks. When By is particularly strong the expanded cell gives the appearance of a single convection cell in the polar cap region and the whole pattern appears to have three cells. When By < 0 the opposite occurs and the dusk cell expands while the dawn cell shrinks. The three-or four-cell model was challenged by Heppner and Maynard [1987] who interpreted sunward convection, measured by the DE 2 satellite in terms of a distorted two-cell pattern. The distortion proposed was that of a translational stretching of the evening cell focus toward and beyond noon, accompanied by a rotational twist of the stream lines around the foci. Progressively greater distortion was proposed as B z increased. Recently, Zhu and Kan [1990] have had some theoretical success in rationalizing the distorted two-cell and multicell models. In their theoretical approach they propose that a number of the differences may be accounted for by anisotropic conductance in the ionosphere. Zhu and Kan [1990] start with a four-cell magnetospheric model and show that an asymptotic ionospheric flow pattern develops (after 25 min) which is similar to the two-cell Heppner and Maynard [1987] model except that there are additional convection cells in the polar cap. The purpose of this paper is to report on Digisonde measurements of the convection flow directions in the central polar cap when B z is positive and to relate these measurements 16,877
IMF By effects in the plasma flow at the polar cap boundary
2010
We used the dataset obtained from the EISCAT Svalbard Radar during 2000-2008 to study statistically the ionospheric convection as related to IMF By conditions, separately for northward and southward IMF. The effects of IMF By are manifested in the intensity and direction of the East-West component of ionospheric flow. The most significant effects are observed near noon and also in the early morning around 03 MLT, whereas in the evening (at 18 MLT) the effect is essentially less prominent. The other feature is an anti-sunward flow across the polar cap, which shows increasing with the magnitude of IMF By. Quantitative characteristics of the IMF By effects are presented and explained in frame of the magnetospheric electric fields generated due to the solar wind, with taking into account position of the open-closed boundary for different IMF conditions. This work was supported by the Academy of Finland.
J. Geophys. Res, 2003
We have used the polar cap (PC) index, which is a measure of magnetic disturbance caused by the transpolar portion of ionospheric currents associated with global convection, to investigate the effect of sudden changes in the solar wind dynamic pressure (P SW). We find that during a period of steady interplanetary magnetic field (IMF), the PC index shows an enhancement that is a direct response to an enhancement in P SW. Examination of the response associated with the passage of interplanetary clouds accompanied by magnetic storms shows that a P SW pulse can cause changes in the PC index that are as large as the effects due to the changes in the interplanetary electric field. Responses of the PC index, AE index and low-latitude horizontal component of magnetic field are observed to be similar, though during a magnetic storm, the influence of a significant ring current can distort the dayside low-latitude response. Sudden increases in P SW lead to an initial negative spike in the PC index that is followed by the main increase. Sudden decreases in P SW are found to have the opposite effect, an initial positive spike that is followed by a decrease in the PC index. The spike of ~3 minutes duration is found to initiate simultaneously with the sudden impulse or commencement and an AE spike on the ground, and the same time that geosynchronous particle fluxes start to increase sharply in response to magnetic field compression. It likely reflects a temporal response of convection to an abrupt change in PSW that is initially of the opposite sign to the more prolonged response, and it may be associated with short time scale current vortices that have been observed in the ionosphere near noon. The above results show that both P SW and IMF play a significant role in controlling the strength of magnetospheric convection.