Simulations of the behaviour of the polar thermosphere and ionosphere for northward IMF (original) (raw)
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Thermospheric winds in the geomagnetic polar cap for solar minimum conditions
Journal of Geophysical Research, 1988
A Fabry-Perot interferometer located at Thule, Greenland (A=86) has monitored the F region thermospheric neutral wind over the northern hemisphere geomagnetic polar cap during the 1985/1986 solar minimum, winter solstice period. The wind observation s were Obtained bY determining the Doppler shift of the (O I) 15,867-K (630.0-nm) emission. We present a subset of the measurements made during December 1985 to January 1986. Three factors m ake this data set unique and particularl y valuable fo r a StUdy of the effects of the deposition of energy and momentum from the magnetosphere into the high-latitude neutral thermosphere. These factors are (1) the proximity of the observing station to the geomagnetiC Pole ' (2) the continuous nature of the coverage due to the high geographic latitude and polar night conditions, and (3) the fact that the data set was obtained near solar minimum. The meaSured winds are compared with the simulations of the NCAR thermospheric general circulation model (TGCM). The results show that winds in the geomagnetic polar cap hav e a fundamental diurnal Character, in accord with model predictions, with typical speeds of-200 m/s, generally in an antisunward direction. A large degr ee of variability, however, in both the magnitude and direction of the winds is observed, including evidence for curvature in the neutral flow within the instrumental region of observation (-400 km diameter). Acceleration of the' meridional component across Thule is observed at times. This acceleration is ascribed to regions of ion-drag forcing associated with the magnetospheric input of energy and momentum. Characteristic asymmetric wind signatures were seen that Were well correlated with positive or negative changes in the By component of the interplanetary magnetic field. 1. INTRODUCTION derived from rocket experiments [e.g., Rees, 1971; During the last several years the dynamics of the Meriwether et al., 1973; Rees et al., 1980; Heppner and high-latitude thermosphere have received considerable Miller, 1982] and incoherent scatter radars [Wickwa r, experimental and theoretical attention (see detailed review in U.S.-IUGG report by Killeen [1987]). Results from the Dynamics Explorer (DE-2) satellite [e.g., Killeen
Journal of Atmospheric and Terrestrial Physics, 1988
The University College London Thermospheric Model and the Sheffield University Ionospheric Convection Model have been integrated and improved to produce a self-consistent coupled global thermospheric/high latitude ionospheric model. The neutral thertnospheric equations for wind velocity, composition, density and energy are solved, including their full interactions with the evolution of high latitude ion drift and plasma density, as these respond to convection, precipitation, solar photoionisation and changes of the thermosphere, particularly composition and wind velocity. Four 24 h Universal Time (UT) simulations have been performed. These correspond to positive and negative values of the IMF BY component at high solar activity, for a level of moderate geomagnetic activity, for each of the June and December solstices. In this paper we will describe the seasonal and IMF reponses of the coupled ionosphere/thermosphere system, as depicted by these simulations. In the winter polar region the diurnal migration of the polar convection pattern into and out of sunlight, together with ion transport, plays a major role in the plasma density structure at F-region altitudes. In the summer polar region an increase in the proportion of molecular to atomic species, created by the global seasonal therrnospheric circulation and augmented by the geomagnetic forcing, controls the plasma densities at all Universal Times. The increased destruction of F-region ions in the summer polar region reduces the mean level of ionization to similar mean levels seen in winter, despite the increased level of solar insolation. In the upper thermosphere in winter for BY negative, a tongue of plasma is transported anti-sunward over the dusk side of the polar cap. To effect this transport, co-rotation and plasma convection work in the same sense. For IMF BY positive, plasma convection and co-rotation tend to oppose so that, despite similar cross-polar cap electric fields, a smaller polar cap plasma tongue is produced, distributed more centrally across the polar cap. In the summer polar cap, the enhanced plasma destruction due to enhancement of neutral molecular species and thus a changed ionospheric composition, causes F-region plasma minima at the same locations where the polar cap plasma maxima are produced in winter.
IMF By effects in the plasma flow at the polar cap boundary
Annales Geophysicae, 2011
We used the dataset obtained from the EISCAT Svalbard Radar during 2000-2008 to study statistically the ionospheric convection in a vicinity of the polar cap boundary as related to IMF B y conditions separately for northward and southward IMF. The effect of IMF B y is manifested in the intensity and direction of the azimuthal component of ionospheric flow. The most significant effect is observed on the day and night sides whereas on dawn and dusk the effect is essentially less prominent. However, there is an asymmetry with respect to the noon-midnight meridian. On the day side the intensity of B y -related azimuthal flow is maximal exactly at noon, whereas on the night side the maximum is shifted toward the post-midnight hours (∼03:00 MLT). On the dusk side the relative reduction of the azimuthal flow is much larger than that on the dawn side. Overall, the magnetospheric response to IMF B y seems to be stronger in the 00:00-12:00 MLT sector compared to the 12:00-24:00 MLTs. Quantitative characteristics of the IMF B y effect are presented and partly explained by the magnetospheric electric fields generated due to the solar wind and also by the position of open-closed boundary for different IMF orientation.
Simulation of the polar cap potential during periods with northward interplanetary magnetic field
Journal of Geophysical Research: Space Physics, 2012
In this paper we examine the response of the ionospheric cross-polar cap potential to steady, purely northward interplanetary magnetic field (IMF) using the Lyon-Fedder-Mobarry global magnetohydrodynamic simulation of the Earth's magnetosphere. The simulation produces the typical, high-latitude "reversed cell" convection that is associated with northward IMF, along with a two cell convection pattern at lower latitude that we interpret as being driven by the viscous interaction. The behavior of the potential can be divided into two basic regions: the viscous dominated region and the reconnection dominated region. The viscous dominated region is characterized by decreasing viscous potential with increasing northward IMF. The reconnection dominated region may be further subdivided into a linear region, where reconnection potential increases with increasing magnitude of northward IMF, and the saturation region, where the value of the reconnection potential is relatively insensitive to the magnitude of the northward IMF. The saturation of the cross-polar cap potential for northward IMF has recently been documented using observations and is here established as a feature of a global MHD simulation as well. The region at which the response of the potential transitions from the linear region to the saturation region is also the region in parameter space at which the magnetosheath transitions from being dominated by the plasma pressure to being dominated by the magnetic energy density. This result is supportive of the recent magnetosheath force balance model for the modulation of the reconnection potential. Within that framework, and including our current understanding of the viscous potential, we present a conceptual model for understanding the full variation of the polar cap potential for northward IMF, including the simulated dependencies of the potential on solar wind speed and ionospheric conductivity.
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
Journal of Geophysical Research, 2002
1] When plasma in the polar cap F region becomes highly structured, patches, irregularities, and scintillations of HF signals may be observed. The topic of this paper is not the mechanism for structuring or distributing the plasma but rather the source of the plasma. By understanding the plasma source we gain insight into the specification and forecasting of ionospheric structures and irregularities as required for space weather applications. The two major sources of polar cap F region plasma are the solar EUV radiation and the auroral precipitation. The region over which solar EUV production occurs is readily modeled. In contrast, the auroral precipitation is not subject to diurnal or seasonal dependences in the same predictable manner; the auroral precipitation can almost be viewed as stochastic within certain geomagnetic coordinate constraints. In this study we use a physical model to separate the effects of solar EUV and auroral precipitation. We find that the auroral contribution does provide a far-from-negligible ''baseline'' level of polar cap F region plasma, upon which is superimposed the UT and seasonally dependent TOI. This baseline level of ionization is very difficult to predict or forecast since it is determined by plasma flux tube histories through extended regions of the auroral oval over several hours. This result raises the need for more advanced auroral precipitation modeling in order to obtain improved space weather specification. The inclusion of soft auroral precipitation is especially important since it can be a significant source of F region plasma.
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.
Magnetospheric Response To The Solar Wind Dynamic Pressure Inferred From Polar Cap Index
2002
Polar cap index (PC) is derived from geomagnetic data from a single near-pole station. It is the measure of magnetic disturbances caused by transpolar portion of ionospheric current associated with global convection system. Comparison of interplanetary mag- netic field and solar wind quantities with PC index shows an of polar cap magnetic activity in direct response to solar wind dynamic pressure enhancement during the pe- riod of stable IMF. Thirty strong magnetic storms occurred on 1998-2001 have been examined. We find that dynamic pressure has a major influence on polar cap currents throughout the period of interplanetary shock/magnetosphere interaction. It leads to a loss of linear dependence and a poor correlation of the PC index with the interplane- tary electric field. The magnitude of PC index response to a southward IMF and to a dynamic pressure pulse can be of the same value. Another notable feature of the PC index is 1-3 min negative spikes that occur just before the sharp increase of polar cap magnetic activity. Negative spike coincides with a spike in the AE index and a sharp jump of H-SYM index that indicates intensification of auroral electrojets and sharp compression of the dayside magnetopause. Particle injections of the same duration as spikes are observed simultaneously at geosychronous orbit. Possible explanation of negative PC spike is suggested in framework of simplified nonlinear electric circuit.
Solar Wind Control of the Magnetospheric and Auroral Dynamics
Space Science Reviews, 2006
A dependence of the polar cap magnetic flux on the interplanetary magnetic field and on the solar wind dynamic pressure is studied. The model calculations of the polar cap and auroral oval magnetic fluxes at the ionospheric level are presented. The obtained functions are based on the paraboloid magnetospheric model calculations. The scaling law for the polar cap diameter changing for different subsolar distances is demonstrated. Quiet conditions are used to compare theoretical results with the UV images of the Earth's polar region obtained onboard the Polar and IMAGE spacecrafts. The model calculations enable finding not only the average polar cap magnetic flux but also the extreme values of the polar cap and auroral oval magnetic fluxes. These values can be attained in the course of the severe magnetic storm. Spectacular aurora often can be seen at midlatitude during severe magnetic storm. In particularly, the Bastille Day storm of July 15-16, 2000, was a severe magnetic storm when auroral displays were reported at midlatitudes. Enhancement of global magnetospheric current systems (ring current and tail current) and corresponding reconstruction of the magnetospheric structure is a reason for the equatorward displacement of the auroral zone. But at the start of the studied event the contracted polar cap and auroral oval were observed. In this case, the sudden solar wind pressure pulse was associated with a simultaneous northward IMF turning. Such IMF and solar wind pressure behavior is a cause of the observed aurora dynamics.
Journal of Atmospheric and Terrestrial Physics, 1991
Long-tee averages of Fabry-Perot lnte~eromet~r (FPI) observations of the night-time UI ('0) emission at 630 nm from the F-region thern~ospbere at a high latitude site are presented here. The data base contains measurements of thermospher~c neutral winds for every winter period from November 1981 to April 1989 inclusive. This covers nearly one complete solar cycle in terms of the radio and UV/EUV &IX variation, from the last solar maximum to the present solar maximum. The instrument is located in Kiruna, Sweden, which is situated at the equatorward edge of the amoral oval at quiet to moderate levels of geomagnetic activity and beneath the oval at higher Levels of activity. From this location, the FPI has sampled the response of the upper the~osphere winds at night to a wide range of geamagneti~ and solar activity conditions. The data presented here show a significant correlation between the solar radio or EUV fluxes and the response of thermospheric neutral winds to geomagnetic conditions in the amoral oval. For K,, < 2 the amoral oval is polewards of Kiruna and neutral winds are similar at solar minimum and solar maximum. At higher levels of geomagnetic activity, for a given level of activity, the neutral winds are a factor of two greater at high levels of solar flux, than at low levels of solar flux. Alternately, the same winds are seen at Kp = 3 at high solar flux levels, as at K, = 5 for low flux levels. Comparing the situation for high and low solar activity, for a given ievel of geomagnetic activity, in the dusk aurora1 oval, the sunward or westward winds are a factor of two larger at solar maximum. In the midnight period, the equatorward wind is a factor of two larger at high solar activity, compared with low solar activity. In this paper, the observations will be discussed, with the implication that I$ is a rather poor indicator of momentum and energy coupling from the solar wind to the upper thermosphere. In a second paper, the salar-terrestrial processes which cause this phenomenon will be discussed and modelled.