D. Lummerzheim

Global energy deposition during the January 1997 magnetic cloud event

The passage of an interplanetary magnetic cloud at Earth on January 10–11, 1997, induced significant geomagnetic disturbances, with a maximum AE in excess of 2000 nT and a minimum Dst of about −85 nT. We use a comprehensive set of data collected from space-borne instruments and from ground-based facilities to estimate the energy deposition associated with the three major magnetospheric sinks during the event. It is found that averaged over the 2-day period, the total magnetospheric energy deposition rate is about 400 GW, with 190 GW going into Joule heating rate, 120 GW into ring current injection, and 90 GW into auroral precipitation. By comparison, the average solar wind electromagnetic energy transfer rate as represented by the e parameter is estimated to be 460 GW, and the average available solar wind kinetic power USW is about 11,000 GW. A good linear correlation is found between the AE index and various ionospheric parameters such as the cross-polar-cap potential drop, hemisphere-integrated Joule heating rate, and hemisphere-integrated auroral precipitation. In the northern hemisphere where the data coverage is extensive, the proportionality factor is 0.06 kV/nT between the potential drop and AE, 0.25 GW/nT between Joule heating rate and AE, and 0.13 GW/nT between auroral precipitation and AE. However, different studies have resulted in different proportionality factors. One should therefore be cautious when using empirical formulas to estimate the ionospheric energy deposition. There is an evident saturation of the cross-polar-cap potential drop for large AE (>1000 nT), but further studies are needed to confirm this.

Remote determination of auroral energy characteristics during substorm activity

Ultraviolet auroral images from the Ultraviolet Imager (UVI) onboard the POLAR satellite can be used as quantitative remote diagnostics of the auroral regions, yielding estimates of incident energy characteristics, compositional changes, and other higher order data products. Here incident energy estimates derived from UVI are compared with in situ measurements of the same parameters from an overflight by the DMSP F12 satellite coincident with the UVI image times during substorm activity occurring on May 19, 1996. This event was simultaneously observed by WIND, GEOTAIL, INTERBALL, DMSP and NOAA spacecraft as well as by POLAR.

Magnetospheric reconnection driven by solar wind pressure fronts

Abstract. Recent work has shown that solar wind dynamic pressure changes can have a dramatic effect on the particle precipitation in the high-latitude ionosphere. It has also been noted that the preexisting interplanetary magnetic field (IMF) orientation can significantly affect the resulting changes in the size, location, and intensity of the auroral oval. Here we focus on the effect of pressure pulses on the size of the auroral oval. We use particle precipitation data from up to four Defense Meteorological Satellite Program (DMSP) spacecraft and simultaneous POLAR Ultra-Violet Imager (UVI) images to examine three events of solar wind pressure fronts impacting the magnetosphere under two IMF orientations, IMF strongly southward and IMF Bz nearly zero before the pressure jump. We show that the amount of change in the oval and polar cap sizes and the local time extent of the change depends strongly on IMF conditions prior to the pressure enhancement. Under steady southward IMF, a remarkable poleward widening of the oval at all magnetic local times and shrinking of the polar cap are observed after the increase in solar wind pressure. When the IMF Bz is nearly zero before the pressure pulse, a poleward widening of the oval is observed mostly on the nightside while the dayside remains unchanged. We interpret these differences in terms of enhanced magnetospheric reconnection and convection induced by the pressure change. When the IMF is southward for a long time before the pressure jump, open magnetic flux is accumulated in the tail and strong convection exists in the magnetosphere. The compression results in a great enhancement of reconnection across the tail which, coupled with an increase of magnetospheric convection, leads to a dramatic poleward expansion of the oval at all MLTs (dayside and nightside). For near-zero IMF Bz before the pulse the open flux in the tail, available for closing through reconnection, is smaller. This, in combination with the weaker magnetospheric convection, leads to a more limited poleward expansion of the oval, mostly on the nightside. Key words. Magnetospheric physics (solar windmagnetosphere interactions; magnetospheric configuration and dynamics; auroral phenomena)

High time resolution study of the hemispheric power carried by energetic electrons into the ionosphere during the May 19/20,1996 auroral activity

The ultraviolet imager (UVI) on board the POLAR satellite offers the opportunity to obtain high time resolution global auroral images. The spectral discrimination of the imager is sufficient to separate the auroral far ultraviolet emissions from the scattered sunlight, even when the entire auroral zone is sunlit. The energy flux of the precipitating electrons is derived from the surface brightness through the LBH-long filter. Global images which have the dayglow removed are spatially integrated to yield the total rate of energy input into the northern hemisphere. This parameter, the hemispheric power, has found much application in ionospheric modeling. It can also be derived from electron spectra measured along the track of the NOAA/TIROS satellites that are combined with average empirical auroral precipitation patterns. We show that the hemispheric power derived from the two-dimensional images represents a substantial improvement in the temporal variability of this parameter. We present an example for the period of 19/20 May 1996 by comparing the hemispheric power derived from NOAA/TIROS measurements with those derived from the UVI images.

Ionospheric conductances derived from DE-1 auroral images

Abstract The multispectral auroral images from the Dynamics Explorer satellite are used to construct maps of auroral electron energy deposition, mean energy, and ionospheric conductances. An auroral model is used to infer conductances from brightness ratios of different spectral emissions. We briefly describe this method and its limitations, and apply the analysis to eight pairs of auroral images obtained sequentially over a period of 1 h 30 min during an intense substorm on 22 November 1981. The derived conductance patterns exhibit large variability in time and considerable structure within the auroral oval.

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.

Detailed analysis of substorm observations using SuperDARN, UVI, ground‐based magnetometers, and all‐sky imagers

[1] A case study of a small-magnitude isolated substorm is presented. The substorm was observed by a variety of instruments including the Super Dual Auroral Radar Network (SuperDARN), the Polar Ultra Violet Imager (UVI), CANOPUS magnetometers, the Alaska chain magnetometers, the Poker Flat meridian-scanning photometer, and the Poker Flat all-sky imager. It was determined that the initial brightening was centered over the combined fields of view of the various instruments. Temporal and spatial relationships among plasma flows, auroral luminosity, and magnetometer perturbations are investigated. It is found that the initial substorm signature was observed in the plasma flows, followed by the auroral brightening, and finally followed by the magnetometer perturbation. Enhanced plasma flows were observed in a spatially confined region near the auroral oval for a period of ∼5 minutes prior to the brightening. After the brightness peaked, the plasma flow velocity decreased back to a preenhancement level.

Substorm convection patterns observed by the Super Dual Auroral Radar Network

A study of convection patterns during substorms observed by the Super Dual Auroral Radar Network (SuperDARN) radar network is presented. Specific attention is given to the growth phase and the time immediately following expansion onset. It is found that the growth phase is characterized by the enhancement of the velocity shear near midnight and its extension to low latitudes and to local times across the midnight meridian. The velocity shear was observed to diminish at expansion onset. In addition, the ionospheric flow velocity magnitude was observed to be enhanced for some period of time prior to expansion onset and to decrease at expansion onset. Possible explanations for the establishment of the midnight velocity shear during growth phase and its subsequent disappearance at expansion onset are presented. Also, arguments are presented regarding the decrease of ionospheric flow velocity at expansion onset.

Simultaneous Multispectral Imaging of the Discrete Aurora

Abstract A unique multispectral imager and an associated multispectral analysis framework are described which together constitute a new diagnostic tool for auroral research. By acquiring spatial and spectral data simultaneously, multispectral imaging allows one to exploit physical connections between auroral morphology and the auroral optical spectrum in a way that sequential spectral imaging cannot. The initial research focus is on imaging the transition in the incident energy spectrum during the formation of discrete arcs—that is, when the precipitating population is characterized by keV electrons. A technique is presented which uses two spectral bands (centered at 4278 and 7325 A) to extend the effective dynamic range of passive imaging to much lower energies.

Emission of OI(630 nm) in proton aurora

A red aurora occurred over southern Canada and central Maine on April 11, 1997, producing a brightness of OI(630 nm) of several Kilorayleighs, which lasted for several hours. Two passes of the Defense Meteorological Satellite Program (DMSP) F12 satellite occurred during this time, and optical data were obtained from four CEDAR Optical Tomographic Imaging Facility (COTIF) sites. The DMSP F12 particle spectrometers observed proton precipitation south of the electron aurora with energy fluxes of several mW m -2 . Tomographic inversion of the COTIF optical observations gives the altitude profile of emissions along a magnetic meridian. We combine all available data using an ionospheric auroral model. Our analysis shows that the model produces the observed auroral brightness from the proton precipitation alone.