M. Lester

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.

Convection and auroral response to a southward turning of the IMF: Polar UVI, CUTLASS, and IMAGE signatures of transient magnetic flux transfer at the magnetopause

We present the first spacecraft-borne imager observations of the auroral manifestation of transient magnetic flux transfer at the magnetopause. During an interval of interplanetary magnetic field Bz ≈ −10 nT, By ≈ 10 nT, and solar wind dynamic pressure and velocity Psw ≈ 5 nPa and vsw ≈ 650 km s−1, Polar Ultraviolet Imager (UVI) images show a sequence of events, each of which begins as a bifurcation of the main auroral oval in the 14 to 16 magnetic local time (MLT) sector which subsequently progresses antisunward (eastward) at 2 km s−1 toward the 19 MLT sector. The poleward portion of the bifurcation is interpreted as a poleward-moving auroral form (PMAF) as has previously been observed by ground-based optical instrumentation and identified as the auroral signature of flux transfer events. Ground-based measurements of the associated plasma drift, made with the Cooperative U.K. Twin Located Auroral Sounding System (CUTLASS) Finland HF radar, show poleward (1 km s−1) and westward (1 km s−1) convection flow, consistent with the By tension force, as well as poleward-moving regions of backscatter. International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers within the radar field of view observe poleward-progressing, 10 min period, X component magnetic deflections, which are consistent with the effect of Hall currents associated with the plasma flow. The combined radar and optical observations suggest that the PMAFs can be 3500 km or 7 hours of MLT in length. The antisunward motion of the bifurcation of the auroral oval is interpreted as an expansion of the reconnection X line along the flank of the magnetopause.

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 ...

Simultaneous observations of the cusp in optical, DMSP and HF radar data

A favourable conjunction of HF coherent radar backscatter, meridian scanning photometer data and an overflight of the DMSP F13 spacecraft has enabled the study of the ionospheric signature of the cusp with these three important techniques simultaneously. Strong HF backscatter power, poleward-moving red line auroral forms and latitude-dispersed ion precipitation features are all observed to be collocated. The precipitation of ions in the 0.1–2 keV energy range is found to be very closely associated with the production of the F region irregularities detected by the HF radar.

A comparative study of global ionospheric responses to intense magnetic storm conditions

We report on a study of three intense ionospheric storms that occurred in September 1989. Using Dst as a reference for storm onset and subsequent main and recovery phases, we analyze the observed worldwide responses of F region heights h m F 2 and densities N m F 2 as a function of universal and local times, latitudinal domains, and storm onset-times; and we compare the characteristics of all three storms. The following points are among the major findings: (1) The negative phase storm was the dominant characteristic, with the greatest intensity occurring in the regions which were in the nighttime hemisphere during the main phase; (2) at middle and low latitudes negative phase characteristics were observed first in the nighttime hemisphere and then corotated with the Earth into the dayside; (3) the most intense negative response occurred in the recovery phase; (4) observations of the negative phase characteristics supported thermospheric upwelling, increased mean molecular mass, and an associated enhancement in dissociative recombination as the principal cause-effect chain; but the observations suggest greater ion-neutral chemistry effects than accounted for in current models; (5) h m F 2 was observed to respond quickly to the storm onset (pointing to the importance of electric fields) with enhanced values in all latitudinal and local time domains; (6) positive storm characteristics were among the issues most difficult to reconcile with current descriptions of cause-effect relationships; and (7) the analysis of all storm phases and comparisons with several modeling efforts show that future advances in understanding require a more accurate accounting of the influences of magnetospherically-imposed and dynamo-driven electric fields, plasmaspheric fluxes, and vibrationally excited N 2 .

A comparison of a model for the theta aurora with observations from Polar, Wind, and SuperDARN

A model is presented according to which theta auroral arcs form after southward turnings of interplanetary magnetic field (IMF) and/or large variations in IMF By, following prolonged periods of northward IMF or very small Bz, with |By| ≳ |Bz|. The arcs start on the dawnside (duskside) of the auroral oval and drift duskward (dawnward) across the polar cap for positive (negative) By in the northern hemisphere and conversely in the southern hemisphere. After the theta aurora has formed, changes in IMF By or Bz readjust the merging configuration and continue the auroral pattern. The transpolar arcs are on closed magnetic field lines that bifurcate two open sections of the polar cap and map to the outer plasma sheet. Four theta auroral events were studied using data from the ISTP/GGS Polar and Wind spacecraft and the ground-based SuperDARN radars. Observations that are correctly predicted by our model include the following: (1) The formation and evolution of theta auroras observed by the visible imaging system are closely related to the IMF patterns measured by the Wind magnetic field investigation. (2) Both electrons and ions in the transpolar arc and poleward part of the night side auroral oval exhibit similar spectral characteristics, identified from the data acquired with Hydra and the comprehensive energetic particle and pitch angle distribution experiment. The low-energy electrons show counterstreaming distributions, consistent with their being on closed field lines that magnetically connect to the boundary plasma sheet in the magnetotail. (3) Ion composition measurements obtained from the toroidal imaging mass-angle spectrograph show cold plasma outflows from the ionosphere and hot, Isotropic magnetospheric ions in the two regions, also indicating transpolar arcs are on closed field lines. (4) Large scale polar cap convection inferred by SuperDARN observations is well correlated with IMF patterns. (5) Plasma convection in the transpolar arcs, inferred from the electric field instrument and the magnetic field investigation measurements, is sunward.

High‐latitude ionospheric electrodynamics as determined by the assimilative mapping of ionospheric electrodynamics procedure for the conjunctive SUNDIAL/ATLAS 1/GEM period of March 28–29, 1992

During the conjunctive SUNDIAL/ATLAS 1/GEM campaign period of March 28–29, 1992, a set of comprehensive data has been collected both from space and from ground. The assimilative mapping of ionospheric electrodynamics (AMIE) procedure is used to derive the large-scale high-latitude ionospheric conductivity, convection, and other related quantities, by combining the various data sets. The period was characterized by several moderate substorm activities. Variations of different ionospheric electrodynamic fields are examined for one substorm interval. The cross-polar-cap potential drop, Joule heating, and field-aligned current are all enhanced during the expansion phase of substorms. The most dramatic changes of these fields are found to be associated with the development of the substorm electrojet in the post midnight region. Variations of global electrodynamic quantities for this 2-day period have revealed a good correlation with the auroral electrojet (AE) index. In this study we have calculated the AE index from ground magnetic perturbations observed by 63 stations located between 55° and 76° magnetic latitudes north and south, which is larger than the standard AE index by about 28% on the average over these 2 days. Different energy dissipation channels have also been estimated. On the average over the 2 days, the total globally integrated Joule heating rate is about 102 GW and the total globally integrated auroral energy precipitation rate is about 52 GW. Using an empirical formula, the ring current energy injection rate is estimated to be 125 GW for a decay time of 3.5 hours, and 85 GW for a decay time of 20 hours. We also find an energy-coupling efficiency of 3% between the solar wind and the magnetosphere for a southward interplanetary magnetic field (IMF) condition.

A comparison of midlatitude Pi 2 pulsations and geostationary orbit particle injections as substorm indicators

Both the injection of energetic particles at geostationary orbit and ground magnetic observations of Pi 2 wave activity are characteristic indicators of the onset of the substorm expansion phase. Occurrence statistics for the appearance of electron and proton particle injection at three geostationary spacecraft and for the detection of midlatitude magnetic Pi 2 pulsations in a 3-hour local time sector have been compiled from 240 hours of data. Throughout this interval a signature was detected on one or more of the instruments on average every 65 min. It is demonstrated that the detection of geostationary orbit particle injections and the detection of ground-based Pi 2 pulsations are correlated at a very high significance level, and that both appear to be effective substorm indicators. However, a small percentage of events (∼10% in each case) may be identified as a Pi 2 event but not as an injection event or viceversa, without any obvious explanation, such as the local time of the observing instrumentation. A number of possible explanations for the discrepancies between the two data sets are discussed.

The dayside auroral zone as a hard target for coherent HF radars

Observations from the CUTLASS Finland coherent HF radar on 23 February 1996 are employed to demonstrate that changes in propagation mode from 1/2F to 1 1/2F and back again, determined from elevation angle measurements, do not significantly alter the ranges over which ionospheric backscatter is observed. This indicates that the latitudinal extent of backscatter in the dayside auroral oval and cusp region correspond to the boundaries of geophysical processes, as opposed to limits in the illumination of the F region ionosphere by the radar. Hence, the HF radar technique is confirmed as an excellent diagnostic of the cusp and other dayside regions.

Coherent HF radar backscatter characteristics associated with auroral forms identified by incoherent radar techniques : A comparison of CUTLASS and EISCAT observations

Backscatter from decameter-wavelength field-aligned F region irregularities, as measured by the Cooperative UK Twin Located Auroral Sounding System (CUTLASS) Finland HF coherent radar, is compared with common volume plasma parameters and the electric field deduced by the European Incoherent Scatter (EISCAT) UHF incoherent radar system, for a 12 hour period from June 18 to June 19, 1996. During this interval we find an excellent agreement between irregularity Doppler velocity and bulk ion drift resolved along the CUTLASS beam. Backscatter is found to exist only in regions of nonzero electric field, as the E×B instability growth rate is dependent on E. Following a substorm expansion phase onset, backscatter largely disappears for a period of several hours, thought to be a consequence of nondeviative absorption of the HF radio wave in the D region or a quenching of the F region instability mechanism by enhanced E region Pedersen conductivity. Finally, the presence of auroral arcs within the scatter volume increases the intensity of backscatter returns and introduces a subsidiary peak, displaced from the preexisting peak, in the backscatter spectra; this subsidiary peak results in an increase in the apparent spectral width of the backscatter. We show how this allows the location of precipitation features within the field of view to be determined.