Peter Stauning

Ørsted satellite captures high‐precision geomagnetic field data

Space-based, high-precision magnetometry is essential for understanding a variety of phenomena ranging from secular variation of the Earths main field, through the signatures of crustal magnetism and the effects of plasma currents flowing externally to the Earth. Orsted, Denmarks first satellite, was launched on February 23, 1999 into a polar, low-Earth orbit to provide the first near-global set of high-precision geomagnetic observations since the Magsat mission of 1979–1980 (see Magsat Special Issue of Geophysical Research Letters., vol. 9, no. 4, pp. 239–379, 1982). With the new mapping of the Earths magnetic field, the International Geomagnetic Reference Field model (IGRF), a standard model used for navigation, prospecting, and other practical purposes, has been determined with improved precision for epoch 2000 [Olsen et al., 2000a; Mandea and Langlais, 2000]. The satellite has routinely provided high-precision vector data since August 1999, and the mission is continuing well beyond its nominal 14-month lifetime into 2001.

Ionospheric response to the interplanetary magnetic field southward turning: Fast onset and slow reconfiguration

[1] This paper presents a case study of ionospheric response to an interplanetary magnetic field (IMF) southward turning. It is based on a comprehensive set of observations, including a global network of ground magnetometers, global auroral images, and a SuperDARN HF radar. There is a clear evidence for a two-stage ionospheric response to the IMF southward turning, namely, fast initial onset and slow final reconfiguration. The fast onset is manifested by nearly simultaneous (within 2 min) rise of ground magnetic perturbations at all local times, corroborated by a sudden change in the direction of line-of-sight velocity near local midnight and by the simultaneous equatorward shift of the auroral oval. The slow reconfiguration is characterized by the different rising rate of magnetic perturbations with latitudes: faster at high latitude than at lower latitudes. Furthermore, a cross-correlation analysis of the magnetometer data shows that the maximum magnetic perturbation is reached first near local noon, and then spread toward the nightside, corresponding to a dayside-to-nightside propagation speed of ∼5 km/s along the auroral oval. Global ionospheric convection patterns are derived based on ground magnetometer data along with auroral conductances inferred from the Polar UV images, using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure. The AMIE patterns, especially the residual convection patterns, clearly show a globally coherent development of two-cell convection configuration following the IMF southward turning. While the foci of the convection patterns remain nearly steady, the convection flow does intensify with time and the cross-polar-cap potential drop increases. The overall changes as shown in the AMIE convection patterns therefore are fully consistent with the two-stage ionospheric response to the IMF southward turning.

Auroral signature of lobe reconnection

We report specific changes in the dayside auroral morphology in the winter hemisphere which occur in response to sharp transitions between northward and southward-directed interplanetary magnetic fields (IMF). In two case examples we show how a switch between large negative and large positive IMF BZ component was accompanied by a corresponding switch in the location of the 630.0 nm aurora: the cusp aurora situated at ≈ 74° MLAT disappeared and another form this time situated at ≈ 77–78° MLAT appeared simultaneously (within 1 min.). We suggest that the lower- and higher-latitude auroras correspond to injections of magnetosheath plasma associated with, respectively, magnetic reconnection at low and high magnetopause latitudes. They may be called cusp/LLBL and cusp/mantle auroras, respectively. According to this interpretation the cusp/mantle aurora thus corresponds to reconnection tailward of the cusp, the so-called lobe reconnection. The auroral signature is observed to last for a few tens of minutes, indicating that lobe reconnection can occur in a quasi-steady mode. During the 17 December 1992 case event sunward plasma convection in the polar cap was inferred from magnetometer records obtained during the period when the high-latitude aurora occurred.

Investigations of ionospheric radio wave absorption processes using imaging riometer techniques

Abstract The recent development of imaging riometer techniques has enabled a range of new, interesting observations of the complex dynamics of auroral and polar radio wave absorption events. These events mostly relate to the precipitation of energetic particles, creating enhanced ionization in the D-region. However, E-region heating by large electric fields and F-region electron density enhancements may also—at times—be responsible for observable absorption effects. Observations of ionospheric radio wave absorption processes using imaging riometer techniques may provide detailed characteristics of the spatial and temporal structures of small-scale disturbance events, velocity vectors for drifting features and frequency spectra for modulated events. This presentation will give a brief summary of imaging riometer techniques and a survey of existing and planned imaging riometer installations. Furthermore, the characteristics of frequently occurring absorption event types are summarized. In a companion paper imaging riometer observations are presented for some selected absorption events.

Observations of solar‐wind‐driven progression of interplanetary magnetic field BY‐related dayside ionospheric disturbances

Observations from August 2 and 3, 1991, of poleward progressing, dayside convection disturbances accompanied by geomagnetic perturbations and ionospheric radio wave absorption have been analyzed and compared to variations in the solar wind parameters as observed from the IMP 8 satellite. The convection disturbances appear to start at dayside cusp latitudes from where they progress antisunward to high latitudes. The reported observations have enabled calculations of the progression directions and velocities and precise estimates of the delays between solar wind variations as measured by the IMP 8 satellite and ionospheric convection changes as observed from an array of polar magnetic observatories. The progressing ionospheric disturbance events occur during intervals of southward interplanetary magnetic fields (negative interplanetary magnetic field (IMF) BZ component); they are found to be closely related to variations of the east-west component BY of the IMF. The close coupling between the solar wind and the polar ionosphere(s) is explained in an open magnetospheric model in which the geomagnetic field extending from a localized region of the dayside polar cap merges with the southward interplanetary field. Variations in the IMF BY component are reproduced in corresponding modulations of the east-west component of the plasma flow at the ionospheric foot points of the connecting “open” field lines. The perturbations of the plasma flow persist while the open field lines are convected with the ionospheric plasma across part of the dayside polar cap. The observed geomagnetic perturbations result from the combined effects of field-aligned currents and horizontal ionospheric currents, notably the convection-related Hall currents. The associated radio wave absorption events are explained as the result of E region electron heating by the horizontal electric fields associated with the convection enhancements.

Cusp/cleft auroral forms and activities in relation to ionospheric convection : Responses to specific changes in solar wind and interplanetary magnetic field conditions

This work is intended to be a first step toward a categorization of dayside auroral responses to various solar wind and interplanetary magnetic field (IMF) conditions and the corresponding states of plasma convection in the dayside magnetosphere. In this paper we relate cusp latitude auroral emissions observed during a 4-hour interval on December 17, 1992, to solar wind data supplemented by ground magnetograms. We focus on different types of variability associated with an interplanetary shock and with an IMF directional discontinuity. We further investigate the quasi-steady conditions related to periods of low activity versus periods of high activity, as well as periods of IMF By-related zonal convection in the cusp region versus periods of IMF BZ-related convection in the north-south direction. To these various conditions the dayside aurora responds differently. These observations are discussed in relation to entry of magnetosheath plasma into the magnetosphere associated with different modes of solar wind-magnetosphere coupling. Two main categories of persistent auroral forms in the cusp/cleft region are observed, a latitudinally wide (≥200 km) zone of weak 630.0-nm emission with no sharp boundaries extending to magnetic latitudes of ∼78°–79° during quiet intervals and another latitudinally narrow (∼100 km) zone of strong red line emission, located equatorward of 75° MLAT during disturbed periods. These forms occur in association with sunward and antisunward convection in the dayside polar cap, respectively. It is suggested that these two signatures of electron precipitation in the noon/near-noon sector reflect plasma entry by lobe reconnection and reconnection at low (subcusp) latitudes, respectively. The observed sudden transition from the former to the latter auroral condition may thus reflect a change of reconnection site. The associated brightening of the cusp/cleft aurora then corresponds to the larger efficiency of magnetosheath plasma entry at low magnetopause latitudes when the subcusp reconnection process is switched on, compared to the more limited plasma entry occurring at high latitude during lobe reconnection. The most intense and latitudinally narrow auroral forms at 630.0 nm are observed during intervals of strong IMF By-related zonal convection and a strong DPY mode of ground magnetic deflection. Information on the ionospheric convection pattern in the vicinity of the magnetic field separatrix is obtained for the different IMF orientations by combining the auroral and magnetic observations in the Arctic sectors of Greenland and Svalbard.

IMF By‐related cusp currents observed from the Ørsted satellite and from ground

Orsted is the first satellite to conduct high-precision magnetometer observations from low-altitude noon-midnight orbits passing through the polar cusp regions. Field-aligned currents (FAC) derived from Orsted magnetic field measurements have been combined with ionospheric current patterns inferred from ground-based magnetic observations to define the structure and location of cusp currents and their dependencies on interplanetary magnetic field (IMF) conditions. Example cases illustrate the close relation between IMF By-related FAC and horizontal ionospheric currents in the cusp region. Our statistical analysis defines for the noon region the variations in FAC latitude with IMF Bz. Comparisons with the statistical cusp location indicate that the more equatorward region of IMF By-related FAC is located on field lines closing at the dayside, while the more poleward FAC are on “open” field lines. High-energy electron measurements from the satellite confirm this result.

Auroral activity associated with unsteady magnetospheric erosion : Observations on December 18, 1990

We report auroral observations made on December 18, 1990, when interplanetary conditions should lead to large-scale erosion of the dayside magnetosphere during a substorm growth phase. A long interval of strongly northward pointing interplanetary magnetic field (IMF) was succeeded by several hours of strongly southward pointing IMF. The interval of southward pointing IMF was punctuated by a number of IMF directional discontinuities during which the IMF north-south component, Bz changed polarity abruptly. The auroral responses, monitored at Ny Alesund (75° magnetic latitude) by meridian scanning photometers and all-sky cameras, were as follows: The interval of negative IMF Bz was characterized by a net equatorward migration of the equatorward boundary of the dayside cusp/cleft aurora, as expected from previous studies. On this occasion, however, we find that the latitudinal shift occurred in steps which consisted of an initial brightening of individual auroral events at ∼0.5° MLAT equatorward of the preexisting luminosity, followed by a steady poleward retreat lasting typically 4–5 min. The net effect over the first hour of IMF Bz < 0 conditions was to move the equatorward boundary toward the geomagnetic equator by ∼2.7° MLAT. The auroral data suggest that in this instance dayside magnetosphere erosion took place intermittently: bursts of reconnection (initial brightenings) are followed by a switch-off of the reconnection electric field (subsequent poleward retreat). The bursts of reconnection may be identified with flux transfer events or, equivalently, flux erosion events.

Reconnection‐associated auroral activity stimulated by two types of upstream dynamic pressure variations: Interplanetary magnetic field Bz ∼ 0, By ≪ 0 case

We present observations of transient auroral activity in the 1300–1600 MLT range at geomagnetic latitudes of 76° and interpret them in terms of modulations in the reconnection rate at the magnetopause which are, in turn, stimulated by the arrival at Earth of upstream dynamic pressure pulses. The period studied is 1000 - 1200 UT on December 17, 1992. The interplanetary magnetic field (IMF), monitored by IMP 8, was aligned in a Parker spiral direction with a strong westward component (IMF By ≪ 0) and a small or zero north-south component. The upstream dynamic pressure variations were of two types, reflecting two distinct origins: the unperturbed solar wind and the bow shock. The underlying reconnection interpretation, with the necessary magnetic shear across the magnetopause being provided by the strong westward component of the IMF, is substantiated by a study of ionospheric flows. A twin-cell pattern of standard polarity is evident in which strong asymmetries about noon are seen, as expected from the predominantly westward pointing IMF. Flow perturbations are superimposed on this general pattern. The auroral observations consist of a latitudinally narrow zone of persistent auroral emission, at the poleward edge of which auroral forms are seen moving eastward (tailward) at speeds of 1–2 km s−1. This intermittent activity, which is often detached from the persistent aurora, has a red line intensity (line of sight) > 1.5 kR, a broad intensity-versus-zenith angle profiles, and a weak green line intensity ( 20% increase relative to background levels) upstream dynamic pressure pulses. Smaller dynamic pressure changes do not seem to affect the aurora noticeably. Our interpretation is in line with some recent studies, namely, that an increase of dynamic pressure on the magnetopause enhances the reconnection rate there. Clearly, theoretical work is needed to understand this important, if indirect, effect of dynamic pressure on the magnetosphere.

High-latitude D- and E-region investigations using imaging riometer observations

Abstract The imaging riometer technique has proved a valuable tool for investigations of a variety of ionspheric and magnetospheric disturbances. To illustrate the potential of the new technique, this presentation will discuss the observations at cusp latitudes (approx. 75° inv. lat.) of PCA events, substorms and poleward progressing absorption features for which imaging riometer observations have provided important new information. For the PCA studies an exceptionally nice data set for the sunrise/sunset asymmetry is presented. It is argued that the asymmetry is so modest that temperature effects offer a simple explanation. For the substorm generation an augmented current wedge model is suggested on the basis of imaging riometer observations. Finally, imaging riometer observations of IMF-dependent poleward progressing absorption events are presented. This type of disturbance is considered the convecting ionospheric footprint of the B Y component of the interplanetary magnetic field. A typical example is examined.