W. Jeffrey Hughes

The dependence of high-latitude PcS wave power on solar wind velocity and on the phase of high-speed solar wind streams

We have calculated the integrated ULF wave power in the Pc5 band at two stations, Kevo (part of the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer array in Scandinavia, at auroral zone latitudes), and Cape Dorset (part of the Magnetometer Array for Cusp and Cleft Studies (MACCS) in Arctic Canada, at cusp latitudes), and compared this power against the solar wind velocity for the last six months of 1993, a period characterized by two persistent high-speed solar wind streams. We find for both local noon at Cape Dorset, and for local morning at Kevo, the Pc5 band power (0.002 – 0.010 Hz) integrated over a six-hour period exhibits a clear power-law dependence on the solar wind velocity. At Cape Dorset we found power α Vsw4, with a correlation coefficient r = 0.73, and at Kevo we found power α Vsw6.5, with r = 0.74. Much of the remaining variation in Pc5 power is due to temporal patterns evident at both stations in response to recurrent high speed streams. Power was strongest at the leading edge of each high speed stream and subsequently decreased more quickly than Vsw. Our observations suggest that it is insufficient to make estimates of Pc5-range ULF wave power on the basis of Vsw alone: one must consider other physical factors, either intrinsic to the solar wind or related to its interaction with Earths magnetosphere. The Kelvin-Helmholtz instability is often considered to play a dominant role in this interaction, and the level of instability depends on both velocity and density. By means of a simple simulation using typical density and velocity values during the passage of a high speed stream, we were able to obtain good agreement with the temporal variations we observed. Finally, this study indicates that ground-based pulsation observations can provide reliable proxies of the initial passage of high speed solar wind streams past Earth.

On the formation and evolution of plasmoids: A survey of ISEE 3 geotail data

ISEE 3 magnetometer and electron plasma measurements from the 1983 Geotail Mission were surveyed to determine the magnetic and plasma properties of plasmoids and their evolution with distance downtail. Events were selected on the basis of a bipolar magnetic signature in either the geocentric solar magnetospheric Bz and/or By component; most had Bz bipolar signatures. We found 366 events consistent with this signature while ISEE 3 was in the plasma sheet. ISEE 3 observed plasmoids all along its trajectory whenever it was in the plasma sheet. Plasmoids are characterized by high-speed plasma flow. Plasmoid length was determined using both the magnetometer and the electron plasma velocity data. We found the average length of plasmoids is 16.7 ± 13.0 RE, significantly smaller than previous estimates. Many plasmoids have a well-defined magnetic core field, characterized by a field strength maximum at the center of the pass through the structure. Plasmoids appear to be relatively stable structures once their formation process is complete. The size, velocity, magnetic core strength, and Bz field amplitude of plasmoids do not depend on distance beyond 100 RE downtail. The average electron temperature inside plasmoids drops by a factor of 2 and the electron density increases by a factor of 2 as plasmoids propagate from near Earth distances (within 100 RE of the Earth) to the deep tail. We conclude that the stable size of the plasmoids, the density increase and the temperature decrease are consistent with a flux of cold electrons into the plasmoid. The strong correlation of interplanetary magnetic field By an hour before the event with the strength and direction of By observed inside plasmoids, the existence of events with the bipolar signature in both the By and Bz components, and the possible mass flux all are consistent with plasmoids being “open” magnetic structures.

Geomagnetic substorm association of plasmoids

The relationship of geomagnetic substorms and plasmoids is examined by determining the correlation of the 366 plasmoids identified by Moldwin and Hughes (1992) with ground auroral zone magnetograms and geosynchronous particle data signatures of substorm onsets. Over 84% of the plasmoid events occurred between 5 and 60 min after a substorm onset. We also find near one-to-one correlation between large isolated substorm signatures in the near-Earth region and signatures consistent with a passing plasmoid in the distant tail (i.e., a traveling compression region, or an actual plasmoid observation). However, there does not appear to be an absolute correspondence of every substorm onset to a plasmoid signature in the deep tail especially, for periods of prolonged disturbance that have multiple substorm insets. A correlation of inter-planetary magnetic field B. south with plasmoid observations was also found. The locations of the near- and far-Earth reconnection sites are estimated using the time of flight of the plasmoids from substorm onset to their observation at ISEE 3. The estimates of the near- and far-Earth reconnection sites are highly variable and range from 10 to 140 RE, 32 refs., 4 figs. 2 tabs.

Observations of Earthward and tailward propagating flux rope plasmoids: Expanding the plasmoid model of geomagnetic substorms

A survey of IMP 8 magnetometer data for plasmoid signatures during magnetospheric intervals from 1981 through 1983 found 16 plasmoids and 37 traveling compression regions as well as two earthward propagating flux ropes and 19 south-north bipolar lobe signatures. The properties of these relatively near-Earth plasmoids, traveling compression regions, and earthward propagating flux ropes and a qualitative model for their formation are presented. The plasmoids have estimated sizes, durations, magnetic field signatures, downtail velocities, and substorm associations very similar to those of the plasmoids identified in ISEE 3 deep-tail observations. The occurrence frequency of these near-Earth plasma sheet plasmoids is significantly smaller than that of plasmoids found in the mid- and deep tail with ISEE 3. The earthward propagating flux ropes are characterized by a south-north bipolar turning in the GSM Bz component, are localized near the noon-midnight meridional plane, and are strongly correlated with interplanetary magnetic field Bz north and small isolated high latitude geomagnetic substorms. These events are also apparently very rare and/or spatially localized. We propose that these structures are “proto-plasmoids,” i.e., plasmoids for which near-Earth magnetic reconnection stopped before all the closed plasma sheet field lines were reconnected. The proto-plasmoids are then “trapped” inside closed magnetic field lines and propagate earthward owing to the effect of the distant X-lines earthward plasma flow. We suggest that the two different “types” of plasmoids are due to the different energy states of the magnetosphere during periods of southward and northward interplanetary magnetic field.

CRRES observation of Pi2 pulsations: Wave mode inside and outside the plasmasphere

The relationship between the plasma density and the mode of Pi2-band (40–150 s) waves in the magnetosphere is studied using measurements made from the Combined Release and Radiation Effects Satellite (CRRES). We focus on wave events that occurred from 1100 to 1500 UT on February 17, 1991, on a single CRRES orbit. During this interval the low-latitude ground station Kakioka (L = 1.2) observed a series of Pi2 pulsations within 2 hours of the local time of CRRES. When CRRES was in the plasmasphere, the poloidal components (Ey, Bx, Bz) at the spacecraft had high coherence with the Pi2 pulsations seen at Kakioka, where Ey, Bx, and Bz are, respectively, the azimuthal component of the electric field and the radial and compressional components of the magnetic field. This relationship lasted until the spacecraft completely moved into the plasmatrough. Waves were present in the plasmatrough, but they did not maintain high coherence with the ground Pi2. For a plasmaspheric event, Ey and Bz oscillated nearly in quadrature, in support of a radially standing wave mode. These results confirm previous AMPTE/CCE magnetic field studies that suggested that fast-mode waves in the inner magnetosphere are the source of low-latitude Pi2 pulsations. In addition, the CRRES results strongly suggest that the outer limit of the “inner magnetosphere” is the plasmapause. The observed wave properties are discussed in light of possible Pi2 mechanisms, including plasmaspheric cavity modes and braking of periodic bursty bulk flows.

Dynamical effects of geomagnetic storms and substorms in the middle-latitude ionosphere: An observational campaign

An observational campaign was conducted in October 1992 for ∼36 hours, at three high- to low-latitude sites near 75°W longitude (Sondre Stromfjord, Millstone Hill, and Arecibo). Vector plasma drift velocities are obtained using the incoherent scatter radar technique at each site. Neutral winds were measured using a Fabry-Perot interferometer, and 6300 A airglow structures were imaged at the midlatitude site. Electric fields and meridional winds for the period were perturbed when magnetic storms and substorms occurred on the day and night of the campaign. The penetration of magnetospheric electric field and the following interplays between ionospheric electrodynamics and thermospheric wind perturbations in the midlatitude ionosphere are assessed using the multidiagnostic measurements. Evidence for traveling atmospheric disturbances (TADs) and large-scale gravity waves induced by auroral heating effects upon the thermosphere is identified. Diffuse aurora and a stable aurora red (SAR) arc were observed from Millstone Hill during the night of the campaign. The SAR arc moved southward when there were westward electric field perturbations, indicating plasmasphere compression in the postmidnight sector under substorm conditions. The SAR arc location was used to infer the motion of the magnetospheric shielding layer past the Millstone Hill site. Ionospheric F region disturbances in hmF2, NmF2, and total electron content were driven by the observed dynamics, exhibiting a complex mix of wind and electric field perturbations. While standard model episodes of penetration and shielding/overshielding occurred during the daytime event, such unambiguous clarifications were far less obvious during the nighttime event. This is perhaps due to the prolonged period of moderate geomagnetic activity that served as the background conditions for the substorms that occurred during the campaign.

On the threshold for triggering substorms

Abstract Many features of substorms are satisfactorily described by a phenomenological model in which the substorm onset is related to the formation of a neutral line within the plasma sheet close to the Earth. The substorm neutral line pinches off a portion of plasma sheet plasma and the substorm expansion phase is associated with the growth and tailward ejection of that plasma, called a plasmoid. This substorm model requires that the tail be stressed prior to the substorm onset and relates the development of tail stress to the input of energy from the solar wind, but the model does not specify the conditions required for the onset of the substorm expansion phase. In particular, the model does not account for the fact that the amount of tail stress released in different substorms is highly variable and that the intensity of global substorm-related signatures can differ greatly. Here we propose that the level of stress at which the substorm expansion starts is controlled by the tail field geometry and remark that the field line curvature required for the formation of a near-Earth neutral line is already present when the dipole is tilted towards or away from the Sun. Assuming that substorms are most readily initiated when the tail field geometry is favorable, we develop a new interpretation of the aspects of the annual and diurnal variation of the level of geomagnetic activity that are independent of the polarity of the interplanetary magnetic field. We attribute the variations to the “bent tail” (BT) effect. We believe that the BT effect provides a more reasonable interpretation of the observed modulations than does the previously-proposed Kelvin-Helmholtz mechanism. The BT effect leads to predictions regarding annual and diurnal signatures of substorm occurrence frequency and magnitude that can be tested.

Multi-satellite observations of plasmoids: IMP 8 and ISEE 3

An examination of IMP 8 and ISEE 3 magnetotail data during the 1983 Geotail Mission yielded one plasmoid event which was observed by both satellites and two other possible cases. These are the first multi-satellite observations of plasmoids. These observations provide a unique opportunity to examine how plasmoid characteristics change as plasmoids propagate downtail and they show that plasmoids are very stable structures.

Ionospheric convection response to changes of interplanetary magnetic field B z component during strong B y component

It is well established that ionospheric convection patterns are strongly controlled by the interplanetary magnetic field (IMF). There has been an increased interest in ionospheric convection response to IMF changes. Some studies found that ionospheric flows change first near noon and later near the dawn-dusk meridian plane, which is interpreted as propagation or expansion of newly generated convection cells in the cusp region. Other studies showed that the change in convection pattern in response to IMF reorientations is spatially fixed. In this paper, we investigate the ionospheric convection response to IMF B z changes during strong IMF B y . On March 23, 1995, B x was small, B y was strongly positive (7-1 nT), and the B z polarity changed several times after 1300 UT. The dayside ionospheric convection is dominated by a large clockwise convection cell. The cell focus (the eye of the convection pattern) is located in the prenoon sector for northward B z and in the postnoon sector for southward B z . It is found that the cell focus shifts from the prenoon sector to the postnoon sector following a southward B z turning and vice versa for a northward B z turning. However, the motion of the convection cell, or the largest change in the convection pattern, is limited roughly to the region between the previous cell focus and the new cell focus. Outside this region, the ionospheric flows could be greatly enhanced or weakened, while the convection pattern shape changes very little. When B y is strong enough, the B z reorientation causes changes in the flow intensity but not in the shape of the convection pattern. The results show the characteristics of ionospheric convection response during strong B y and suggest that the convection reconfiguration is not only determined by the changing B z but also significantly influenced by the stable and large B y .

Plasmoid observations in the distant plasma sheet boundary layer

Substorm associated large amplitude bipolar magnetic events occurred when ISEE 3 was in the distant geotails plasma sheet boundary layer (PSBL). The characteristics of these events, their substorm association and their possible source mechanisms are examined. We propose that these PSBL events are signatures of a passing plasmoid in the plasma sheet, analogous to the traveling compression region model in the geomagnetic lobes.