H. J. Opgenoorth

EISCAT observations of topside ionospheric ion outflows during auroral activity: Revisited

New EISCAT observations of large field-aligned bulk ion outflows from the topside ionosphere during auroral activity are presented. The ions (mainly O+) start their outflows from a variable altitude and may reach field-aligned outward velocities of up to 1500 m s−1 in the altitude region 900–1500 km. The observed ion fluxes are about a factor of 10 larger than previously observed reaching 2×1014 m−2 s−1, and in some cases is nonconstant with altitude. Two different types of ion outflows have been identified. The first type is related to periods of strong perpendicular electric fields, enhanced and anisotropic ion temperatures, and low electron densities below 300 km, indicating small amounts of auroral precipitation. The second type is related to auroral arcs and enhanced electron temperatures. The exact mechanism causing the ion outflows is still not yet understood, but additional mechanisms other than thermal expansion are required to explain the observations presented here.

Observations in the vicinity of substorm onset: Implications for the substorm process

Multi-instrument data sets from the ground and satellites at both low and high altitude have provided new results concerning substorm onset and its source region in the magnetosphere. Twenty-six out of 37 substorm onset events showed evidence of azimuthally spaced auroral forms (AAFs) prior to the explosive poleward motion associated with optical substorm onset. The azimuthal wavelengths associated with these onsets were found to range between 132 and 583 km with a mean value of 307±115 km. The occurrence rate increased with decreasing wavelength down to a cutoff wavelength near 130 km. AAFs can span 8 hours of local time prior to onset and generally propagate eastward in the morning sector. Onset itself is, however, more localized spanning only about 1 hour local time. The average location of the peak intensity for 80 onsets was 65.9±3.5 CGMlat, 22.9±1.2 Mlt, whereas the average location of the AAF onsets was at 63.8±3.3 CGMlat, 22.9±1.1 Mlt. AAF onsets occur during time periods when the solar wind pressure is relatively high. These low-latitude wavelike onsets appear as precursors in the form of long-period magnetic pulsations (Pc 5 band) and frequently occur on the equatorward portion of the double oval distribution. AAFs brighten in conjunction with substorm onset leading to the conclusion that they are a growth phase activity causally related to substorm onset. Precursor activity associated with these AAFs is also seen near geosynchronous orbit altitude and examples show the relationship between the various instrumental definitions of substorm onset. The implied mode number (30 to 135) derived from this work is inconsistent with cavity mode resonances but is consistent with a modified flute/ballooning instability which requires azimuthal pressure gradients. It is suggested that this instability exists in growth phase but that an additional factor exists in the premidnight sector which results in an explosive onset. The extended source region and the distance to the open-closed field line region constrain reconnection theory and local mechanisms for substorm onset. It is demonstrated that multiple onset substorms can exist for which localized dipolarizations and the Pi 2 occur simultaneously with tail stretching existing elsewhere. Further, the tail can be less stretched at geosynchronous orbit during the optical auroral onset than during the precursor pseudobreakups. These pseudobreakups can be initiated by auroral streamers which originate at the most poleward set of arc systems and drift to the more equatorward main UV oval. Observations are presented of these AAFs in conjunction with low- and high-altitude particle and magnetic field data. These place the activations at the interface between dipolar and taillike field lines probably near the peak in the cross-tail current. These onsets are put in the context of a new scenario for substorm morphology which employs individual modules which operate independently or couple together. This allows particular substorm events to be more accurately described and investigated.

Interhemispheric asymmetry of the high-latitude ionospheric convection pattern

The assimilative mapping of ionospheric electrodynamics technique has been used to derive the large-scale high-latitude ionospheric convection patterns simultaneously in both northern and southern hemispheres during the period of January 27-29, 1992. When the interplanetary magnetic field (IMF) Bz component is negative, the convection patterns in the southern hemisphere are basically the mirror images of those in the northern hemisphere. The total cross-polar-cap potential drops in the two hemispheres are similar. When Bz is positive and |By| > Bz, the convection configurations are mainly determined by By and they may appear as normal “two-cell” patterns in both hemispheres much as one would expect under southward IMF conditions. However, there is a significant difference in the cross-polar-cap potential drop between the two hemispheres, with the potential drop in the southern (summer) hemisphere over 50% larger than that in the northern (winter) hemisphere. As the ratio of |By|/Bz decreases (less than one), the convection configuration in the two hemispheres may be significantly different, with reverse convection in the southern hemisphere and weak but disturbed convection in the northern hemisphere. By comparing the convection patterns with the corresponding spectrograms of precipitating particles, we interpret the convection patterns in terms of the concept of merging cells, lobe cells, and viscous cells. Estimates of the “merging cell” potential drops, that is, the potential ascribed to the opening of the dayside field lines, are usually comparable between the two hemispheres, as they should be. The “lobe cell” provides a potential between 8.5 and 26 k V and can differ greatly between hemispheres, as predicted. Lobe cells can be significant even for southward IMF, if |By| > |Bz|. To estimate the potential drop of the “viscous cells,” we assume that the low-latitude boundary layer is on closed field lines. We find that this potential drop varies from case to case, with a typical value of 10 kV. If the source of these cells is truly a viscous interaction at the flank of the magnetopause, the process is likely spatially and temporally varying rather than steady state.

Two substorm intensifications compared: Onset, expansion, and global consequences

We present observations of two sequential substorm onsets on May 15, 1996. The first event occurred during persistently negative IMF B-Z, whereas the second expansion followed a northward turning o ...

Understanding space weather to shield society : A global road map for 2015-2025 commissioned by COSPAR and ILWS

There is a growing appreciation that the environmental conditions that we call space weather impact the technological infrastructure that powers the coupled economies around the world. With that co ...

Ionospheric conductivities, electric fields and currents associated with auroral substorms measured by the EISCAT radar

Abstract E -region electron density profiles with high resolution in time and altitude (5 s and 2 km, respectively) measured by the EISCAT incoherent scatter radar are used to examine the conductivity changes during substorm growth, onset and expansion phases for seven substorms occurring in the local evening sector. The measurements are related to electric fields and neutral winds measured by the radar, to ground magnetometer and riometer records, and to optical features, including the westward-travelling surge and auroral bulge. Auroral features are identified using all-sky camera photographs and images from the Viking satellite. Conductances and electric fields in the zone of diffuse aurora corresponding to the westward substorm electrojet are found to be consistent with existing models. Conductances in the discrete auroral arcs marking the expanding edge of the substorm are found to be much higher, and electric fields rather lower, than previously assumed. The magnetic signatures of the discrete arcs are found to be best explained by Hall and Pedersen currents driven by a southward neutral wind, as is observed by the radar. The highest conductances observed, with Hall and Pedersen conductances reaching 120 and 48 S, respectively, are found to be associated with arcs appearing at the southern edge of activity in the vicinity of a westward-travelling surge.

A study of the dynamics of a discrete auroral arc

High resolution electric field and particle data, obtained by the S23L1 rocket crossing over a discrete prebreakup arc in January 1979, are studied in coordination with ground observations (Scandinavian Magnetometer Array—SMA, TV and all-sky cameras) in order to clarify the electrodynamics of the arc and its surroundings. Height-integrated conductivities have been calculated from the particle data, including the ionization effects of precipitating protons and assuming a steady state balance between ion production and recombination losses. High resolution optical information of arc location relative to the rocket permitted a check of the validity of this assumption for each flux tube passed by the rocket. Another check was provided by a comparison between calculated (equilibrium values) and observed electron densities along the rocket trajectory. A way to compensate for the finite precipitation time when calculating the electron densities is outlined. The height-integrated HalI-Pedersen conductivity ratio is typically 1.4 within the arc and about 1 at the arc edges, indicative of a relatively softer energy spectrum there. The height-integrated conductivities combined with the DC electric field measurements permitted calculation of the horizontal ionospheric current vectors (J⊥), Birkeland currents (from div J⊥) and energy dissipation through Joule heating (ΣpE2). An eastward current of typically 1 A m−1 was found to be concentrated mainly to the arc region and equatorward of it. A comparison has been made with the equivalent current system deduced from ground based magnetometer data (SMA) showing a generally good agreement with the rocket results. An intense Pedersen current peak (1.2 A m−1) was found at the southern arc edge. This edge constituted a division line between a very intense (> 10 μA m−1) and localized (~ 6 km) downward current sheet to the south, probably carried by upward flowing cold ionospheric electrons and a more extended upward current sheet (> 10 μA m−2) over the arc carried by measured precipitating electrons. Joule and particle heating across the arc were anticorrelated, consistent with the findings of Evans et al. (1977) with a total value of about 100mW m−2.

Scattering of electromagnetic waves from a plasma : enhanced ion acoustic fluctuations due to ion-ion two-stream instabilities

Observations by the ElSCAT and Millstone Hill radars of strongly enhanced, often asymetric, ion acoustic line spectra in the topside auroral ionosphere have been reported recently by a number of researchers. Such strongly enhanced ion acoustic line spectra are shown to arise naturally in a plasma unstable to the ion-ion two-stream instability. A linear theory of density fluctuations is used for the calculations, which should be applicable to weakly unstable non-magnetized plasmas near the onset threshold of the instability.

Electron energization in the topside auroral ionosphere: on the importance of ion-acoustic turbulence

Abstract EISCAT observations show enhancements of electron temperatures up to 8000 K in the topside ionosphere during active auroral conditions. These temperature enhancements correlate with electron density enhancements in the altitude region 170–230 km, which indicate that auroral electron precipitation in the 100–500 eV range is associated with the electron heating. Electron densities in other altitude intervals show no such correlation and thereby indicate that auroral particles outside the energy range 100–500 eV are not the major regular cause for this bulk electron heating. We also present observations of ion acoustic turbulence in connection with the bulk electron heating events. Such turbulence will give a greatly enhanced Joule heating in the presence of field-aligned currents because of the resulting anomalous resistivity by the turbulently fluctuating electric fields. This heating will occur in addition to the classical collisonal heating by precipitating particles, and is probably a major heat source for the topside ionospheric electrons. In fact, the precipitating electrons in the 100–500 eV range may themselves be runaway suprathermal electrons produced by the ion-acoustic turbulence. An ion-ion two-stream instability is suggested to be the cause of the observed enhanced ion-acoustic fluctuations.

Characteristics of ionospheric convection and field-aligned current in the dayside cusp region

The assimilative mapping of ionospheric electrodynamics (AMIE) technique has been used to estimate global distributions of high-latitude ionospheric convection and field-aligned current by combining data obtained nearly simultaneously both from ground and from space. Therefore, unlike the statistical patterns, the “snapshot” distributions derived by AMIE allow us to examine in more detail the distinctions between field-aligned current systems associated with separate magnetospheric processes, especially in the dayside cusp region. By comparing the field-aligned current and ionospheric convection patterns with the corresponding spectrograms of precipitating particles, the following signatures have been identified: (1) For the three cases studied, which all had an IMF with negative y and z components, the cusp precipitation was encountered by the DMSP satellites in the postnoon sector in the northern hemisphere and in the prenoon sector in the southern hemisphere. The equatorward part of the cusp in both hemispheres is in the sunward flow region and marks the beginning of the flow rotation from sunward to antisunward. (2) The pair of field-aligned currents near local noon, i.e., the cusp/mantle currents, are coincident with the cusp or mantle particle precipitation. In distinction, the field-aligned currents on the dawnside and duskside, i.e., the normal region 1 currents, are usually associated with the plasma sheet particle precipitation. Thus the cusp/mantle currents are generated on open field lines and the region 1 currents mainly on closed field lines. (3) Topologically, the cusp/mantle currents appear as an expansion of the region 1 currents from the dawnside and duskside and they overlap near local noon. When By is negative, in the northern hemisphere the downward field-aligned current is located poleward of the upward current; whereas in the southern hemisphere the upward current is located poleward of the downward current. (4) Under the assumption of quasi-steady state reconnection, the location of the separatrix in the ionosphere is estimated and the reconnection velocity is calculated to be between 400 and 550 m/s. The dayside separatrix lies equatorward of the dayside convection throat in the two cases examined.