IMF BY Component Research Articles

Ionospheric plasma flows associated with the formation of the distorted nightside end of a transpolar arc

Abstract. We investigate ionospheric flow patterns occurring on 28 January 2002 associated with the development of the nightside distorted end of a J-shaped transpolar arc (nightside distorted TPA). Based on the nightside ionospheric flows near to the TPA, detected by the SuperDARN (Super Dual Auroral Radar Network) radars, we discuss how the distortion of the nightside end toward the pre-midnight sector is produced. The J-shaped TPA was seen under southward interplanetary magnetic field (IMF) conditions, in the presence of a dominant dawnward IMF-By component. At the onset time of the nightside distorted TPA, particular equatorward plasma flows at the TPA growth point were observed in the post-midnight sector, flowing out of the polar cap and then turning toward the pre-midnight sector of the main auroral oval along the distorted nightside part of the TPA. We suggest that these plasma flows play a key role in causing the nightside distortion of the TPA. SuperDARN also found ionospheric flows typically associated with Tail Reconnection during IMF Northward Non-substorm Intervals (TRINNIs) on the nightside main auroral oval, before and during the TPA interval, indicating that nightside magnetic reconnection is an integral process to the formation of the nightside distorted TPA. During the TPA growth, SuperDARN also detected anti-sunward flows across the open–closed field line boundary on the dayside that indicate the occurrence of low-latitude dayside reconnection and ongoing Dungey cycle driving. This suggests that nightside distorted TPA can grow even in Dungey-cycle-driven plasma flow patterns.

Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 2. Response to solar wind power input and relationships with solar wind dynamic pressure and magnetospheric flux transport

This is the second in a series of papers that investigate the semi-annual, annual and Universal Time (UT) variations in the magnetosphere. We present a varied collection of empirical results that can be used to constrain theories and modelling of these variations. An initial study of two years’ data on transpolar voltage shows that there is a semi-annual variation in magnetospheric flux circulation; however, it is not as large in amplitude as that in geomagnetic activity, consistent with the latter showing a non-linear (quadratic) variation with transpolar voltage. We find that during the persistent minimum of the UT variation in geomagnetic activity, between about 2 and 10 UT, there is also a persistent decrease in observed transpolar voltage, which may be, in part, caused by a decrease in reconnection voltage in the nightside cross-tail current sheet. We study the response of geomagnetic activity to estimated power input into the magnetosphere using interplanetary data from 1995 onwards, an interval for which the data are relatively free of data gaps. We find no consistent variation in the response delay with time-of-year F and, using the optimum lag, we show that the patterns of variation in F-year spectrograms are very similar for geomagnetic activity and power input into the magnetosphere, both for average values and for the occurrence of large events. The Russell–McPherron (R–M) mechanism is shown to be the central driver of this behaviour. However, the (R–M) effect on power input into the magnetosphere is small and there is a non-linear amplification of the semi-annual variation in the geomagnetic response, such that a very small asymmetry in power input into the magnetosphere Pα between the “favourable” and “unfavourable” polarities of the IMF BY component generates a greatly amplified geomagnetic response. The analysis strongly indicates that this amplification is associated with solar wind dynamic pressure and its role in squeezing the near-Earth tail and so modulating the storage and release of energy extracted from the solar wind. In this paper, we show that the equinoctial pattern is found in the residuals of fits of Pα to the am index and that the amplitude of these equinoctial patterns in the am fit residuals increases linearly with solar wind dynamic pressure. Similarly, the UT variation in am is also found in these fit residuals and also increases in amplitude with solar wind dynamic pressure.

Ionospheric flows relating to transpolar arc formation

Transpolar arcs are large‐scale auroral features which are observed within the polar cap when the IMF has a northward component. One leading candidate formation mechanism proposes that they are formed by reconnection in the magnetotail some time after a period of dayside reconnection with a non‐zero IMF BY component which introduces a twist into the magnetotail. As a result of the twist, the mechanism predicts that the return flows of the newly closed magnetic field lines are asymmetric about midnight; their direction should depend upon the IMF BY component in the hours beforehand and should be opposite in the northern and southern hemispheres. In this paper, we use data from the SuperDARN network of high‐latitude ionospheric radars to examine whether such ionospheric flows are present before the formation of 33 transpolar arcs. We find that the flows are present and in a manner that is consistent with the reconnection mechanism for 76% of the events; in the remaining few, the discrepancy can be attributed either to an uncertainty in the formation time determined for the arc (due to previous polar cap activity in the same local time sector) or due to the geometry of the radars which observe the backscatter.

Effect of IMF BY on thermospheric composition at high and middle latitudes: 1. Numerical experiments

Magnetic storms and their effects on the thermosphere and ionosphere have been studied for many years, yet there are many aspects of the thermospheric and ionospheric responses that are not understood. The purpose of this paper is to show how the high‐latitude composition depends on the sign of the IMF BY component, using controlled simulations with a global first principles model. Because the high‐latitude convection and neutral wind systems are strongly controlled by the IMF BY component, it seems likely that the compositional response that is driven by high‐latitude forcing should also be sensitive to the BY component. To date, no first‐principles modeling has been performed to test the idea of IMF BY effects on composition. Numerical experiments using model simulations provide insight into this important scientific question, since the thermospheric compositional response to the convection patterns for different IMF BZ and BY can be studied in isolation in a model. In this paper we use a first‐principles model to determine the effect of the IMF BY component on the compositional response of the high‐latitude thermosphere. We show for the first time that a clockwise rotation of the potential pattern resulting from a change from BY‐negative to BY‐positive drives a corresponding rotation in the wind, neutral density, and composition distributions. BY control of thermospheric composition has been invoked in the literature to explain an apparent variability in the effectiveness of auroral activity in causing thermospheric storm effects at middle latitudes, as observed in global images of the far‐ultraviolet (FUV) OI 130.4‐nm emission from the DE‐1 auroral imager. However, the effect in the simulations presented here is opposite from that suggested by earlier work based on DE data, indicating another explanation must be sought for the DE results. These simulations are highly relevant for interpreting data being provided by more modern UV imaging instruments on the DMSP, TIMED, and IMAGE satellites.

Unified PCN and PCS indices: Method of calculation, physical sense, and dependence on the IMF azimuthal and northward components

The PC index estimates the polar cap magnetic activity generated by the solar wind and its embedded magnetic field. It shows a high degree of correlation with geophysical disturbance parameters characterizing the state of magnetosphere. After transition to 1‐min PC index values, significant differences were revealed between the PCN and PCS indices, derived on the basis of magnetic data from Thule and Vostok stations, respectively. Differences in values of the 1‐min PCN and PCS indices may give rise to discrepancy in results of various analyses and to quite erroneous physical conclusions. To eliminate any influence of the calculation technique on scientific results a unified method for derivation of the PC index was elaborated in the Arctic and Antarctic Research Institute (St. Petersburg) and Danish Meteorological Institute (Copenhagen), and new sets of the unified PCN and PCS indices were calculated. Under conditions of southward IMF the unified positive PCS and PCN indices are well consistent with one another in their value and behavior and linearly correlate with the solar wind merging electric field (MEF), irrespective of season. The IMF BY component is responsible for occurrence of the electric field MEF (and positive PC index) under conditions BZ > 0. Being proportional to MEF the PC index would be regarded as a measure of the interplanetary merging (geoeffective) electric field with the appropriate dimensionality of mV/m.

A Study on the Characteristics of the Flux Transfer Events Occourred on March 2, 2001

AbstractThree flux transfer events (FTEs) were detected in magnetosheath near the south cusp of the dawn side by Cluster II during the interval from 11:00 to 11:15UT on March 2nd, 2001. In this paper, we analyze the magnetic morphology and the plasma characteristics of these events using the magnetic field and plasma data observed by the four spacecrafts of Cluster II as well as the electric current density in the region Cluster II passed. The results show that the magnetic reconnection at the dayside magnetopause can occur near the cusp when the IMF BY component is dominant. The difference of p+B2/8τ (the sum of magnetic and plasma pressure) measured inside and outside the events may not be balanced with the tension of the helical magnetic field at the initial stage when a flux tube is forming, and it needs a growing time to reach an equilibrium. The size of the section of the FTEs flux tubes in L‐M plane is about 1.89RE. There are not only axis‐aligned current inside the flux tubes but also annular current, and the axis‐aligned current flows basically along the axis‐aligned magnetic field.

The influence of IMF By on the nature of the nightside high-latitude ionospheric flow during intervals of positive IMF Bz

Abstract. This paper further addresses the issue of nightside flow bursts which occur during intervals of northward but strongly BY-influenced IMF. Recent discussions of such bursts concerned intervals during which the IMF BY component was negative. The present study concerns an interval of BY-positive IMF which occurred on 20 March 2002 (01:00-12:00UT). During the interval BY increased steadily from ~2 to 12nT, whilst the BZ component decreased steadily from ~10 to 0nT. There was thus a ~6-h sub-interval during which the IMF clock angle remained between 30° and 60°, such that moderate dayside reconnection and open flux production was maintained. It is found that flow bursts of a similar size and speed to those observed under BY negative (~1000m s-1, spanning 2-3h of MLT in the midnight sector) also occur when BY is positive. However, the direction of east-west flow is reversed, indicating that they are driven by processes in the magnetosphere which are directly related to the orientation of the IMF. It is suggested that they are caused by a reconfiguration of an asymmetric tail resulting from prolonged dayside reconnection with a BY-dominated IMF. This is consistent with previous suggestions that they are associated with convective transport following reconnection in the more distant tail. Analysis of ground magnetic data, auroral images and geosynchronous particle data also show associated features, but indicate that the flow bursts are not directly associated with substorms.Key words. Ionosphere (plasma convection; ionospheremagnetosphere interactions) – Magnetospheric Physics (magnetotail)

North-south asymmetry of the substorm intensity depending on the IMF B Y -component

The dependence of maximum variations of H-component negative bays at magnetic conjugate stations on a sign of the IMF B Y -component during the substorms of small and moderate intensity with |ΔH|max < 750 nT developed at B Z < 0 and |B Z / B Y | < 1 has been studied. By data from Kotzebue (Φ = 64.5°; Λ = 249.7°) and Macquarie Isl. (Φ = −64.5°; Λ = 247.8°) observatories in the eastern hemisphere and Leirvogur (Φ = 65.3°; Λ = 68.2°) and Syowa (Φ = −66.1°; Λ = 71.0°) stations in the western hemisphere it is shown that at B Y < 0 the absolute values of negative bays |ΔH|max are greater in the southern hemisphere and at B Y > 0, on the contrary, in the northern hemisphere.

Direct determination of IMF BY‐related cusp current systems, using SuperDARN radar and multiple ground magnetometer data: A link to theory on cusp current origin

We analyze an ionospheric “enhanced convection event” in the cusp on November 13, 1996, at 1900 UT, by using data of the SuperDARN radar, and of the IMAGE, Greenland, MACCS and CANOPUS magnetometer arrays; and from other magnetometer stations. The event occurs ∼20 minutes after a transition of the IMF BY component from positive to negative and an associated reconfiguration of the ionospheric electric potential pattern. The data allow an instantaneous two‐dimensional view of the event in terms of the ionospheric electric and ground magnetic field. The “method of characteristics” is used to obtain distributions of ionospheric conductances, actual ionospheric currents, and field‐aligned currents (FACs) for the interval under study. Our results show that the region of enhanced convection observed by the radar is associated with a region of low conductances, ranging only slightly above the UV conductance values. However, owing to the strongly enhanced electric field, it produces enhanced westward flowing Hall and southward flowing Pedersen currents. At the northern and southern borders of the enhanced convection region, sheets of downward and upward FACs are observed, respectively, with magnitudes of around 0.5 A/km2. The geometry of the current system resembles a DPY current system [Friis‐Christensen and Wilhjelm, 1975]. Using our results, we test alternative theories on cusp current origin that lead to different predictions of the relative location of the cusp current system with respect to the open‐closed field line boundary. The location of this boundary is inferred from DMSP F10 satellite data. The center of our resulting current system is located clearly poleward of the open‐closed field line boundary, thus favoring the idea of Lee et al. [1985] that the cusp FACs are caused by a rotational discontinuity of the magnetic field at the magnetopause. In contrast, the idea of Clauer and Banks [1986] that a mapping of the solar wind Ez component to the ionosphere is responsible for the cusp current system is not supported.

Location, spatial scale and motion of radio wave absorption in the cusp-latitude ionosphere observed by imaging riometers

Abstract Characteristic examples of the location, spatial scale and motion of radio wave absorption events in the cusp-latitude ionosphere are obtained from daytime observations on 18 September and 17 October, 1992, by imaging riometers in the Arctic region. One case observed near local magnetic noon at Ny-Alesund, Svalbard (invariant lat. = 76.1 °) displays small-scale absorption events of 100–200 km in extent superposed on large-scale absorption features extending at least 700 km in longitude toward the prenoon sector. Many of the small-scale absorption events show quasi-periodic variations with repetition periods of 3–5 min which correlate well with local magnetic variations. Short-lived, impulsive absorption events (1–3 min duration) found among the quasi-periodic variations corresponded to impulsive magnetic variations observed over a wide range of magnetometer stations. Some of these impulsive events showed northward or northeastward motions. This case is interpreted in terms of the variable precipitation of high-energy substorm electrons. Another characteristic case observed in the noon sector at cusp-latitudes is an event of slowly varying absorption intensities associated with magnetic bays in the cusp and polar cap regions during conditions of strongly negative IMF-By component (Bz≈0). An interesting feature of this event is the observed antisunward motion of the front-like absorption features extending over 700 km in longitude. From these characteristics the slowly varying absorption intensities are interpreted in terms of E-region ionospheric disturbances related to the east-west oriented DPY currents in the cusp and polar cap.

Simulations of the seasonal variations of the thermosphere and ionosphere using a coupled, three-dimensional, global model, including variations of the interplanetary magnetic field

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 thermospheric 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 thermospheric 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.