A. T. Weatherwax

An overview of the early November 1993 geomagnetic storm

This paper describes the development of a major space storm during November 2-11, 1993. We discuss the history of the contributing high-speed stream, the powerful combination of solar wind transients and a corotating interaction region which initiated the storm, the high-speed flow which prolonged the storm and the near-Earth manifestations of the storm. The 8-day storm period was unusually long; the result of a high-speed stream (maximum speed 800 km/s) emanating from a distended coronal hole. Storm onset was accompanied by a compression of the entire dayside magnetopause to within geosynchronous Earth orbit (GEO). For nearly 12 hours the near-Earth environment was in a state of tumult. A super-dense plasma sheet was observed at GEO, and severe spacecraft charging was reported. The effects of electrons precipitating into the atmosphere penetrated into the stratosphere. Subauroral electron content varied by 100% and F layer heights oscillated by 200 km. Equatorial plasma irregularities extended in plumes to heights of 1400 km. Later, energetic particle fluxes at GEO recovered and rose by more than an order of magnitude. A satellite anomaly was reported during the interval of high energetic electron flux. Model results indicate an upper atmospheric temperature increase of 200°K within 24 hours of storm onset. Joule heating for the first 24 hours of the storm was more than 3 times that for typical active geomagnetic conditions. We estimate that total global ionospheric heating for the full storm interval was ∼190 PJ, with 30% of that generated within 24 hours of storm onset.

Characteristics of broadband ULF magnetic pulsations at conjugate cusp latitude stations

Although cusp latitude pulsation studies have for the most part focused on narrowband waves, analysis of magnetometer data from the Arctic has shown that the most common type of dayside long-period ULF wave activity at very high latitudes is broadband noise (Pi1-2), and that its occurrence and intensity is largely controlled by solar wind velocity [Engebretson et al., 1995]. However, the origin of temporal variations in the intensity of these waves is not understood. In order to further investigate these broadband waves and their origins, we present a similar data set from another season, data from a roughly conjugate site, and multi-instrument data. Comparison of conjugate station data revealed that there was a substantial fraction of days during which there was significant temporal disagreement between hemispheres, but the solar wind velocity still appears to control overall daily intensity in broadband power. The coincidence of increased riometer absorption from conjugate locations with strong broadband ULF wave power suggests that precipitating energetic particles are responsible for much of the broadband ULF noise, and further suggests that high solar wind velocity plays a role in precipitation of significant fluxes of energetic particles. Quantitative estimates based on riometer and photometer observations also indicate that modulated electron precipitation is sufficient to drive the broadband pulsations. We review possible source mechanisms for these broadband waves and the precipitating electrons associated with them. Finally, the clear temporal association between these waves and Pc5 waves on closed field lines may suggest a causal connection via modulations of a three-dimensional current system.

Discrete electrostatic eigenmodes associated with ionospheric density structure: Generation of auroral roar fine frequency structure

Radio emissions emanating from the Earth and other planets are often characterized by discrete frequency structures. For example, recent ground-based observations of auroral roar, an auroral radio emission which occurs near the second and third harmonic of the electron cyclotron frequency, show that it consists of fine frequency structure similar to that of auroral kilometric radiation and other planetary radio emissions. These auroral roar fine structures, sometimes as narrow as a few hertz, often occur in multiplets separated by the order of < 1 kHz which drift up and down in frequency. Theoretical and experimental efforts to explain the generation of auroral roar suggest that in the source region near the F region peak, the quasi-electrostatic Z mode (or upper-hybrid) waves are first excited, partly converted to free-space radio waves and subsequently observed on the ground. Using WKB-type calculations of the wave mechanics of upper-hybrid modes in a cylindrical field-aligned density structure, we show that discrete frequency eigenmodes are a natural consequence of such density structures. Discrete eigenmodes can exist within density enhancements but not within depletions. Cylindrical field-aligned structures the order of 100 m to several kilometers diameter result in eigenmodes spaced by a few hundred hertz as observed for auroral roar. Since structure of this scale size often occurs in the Earths auroral ionosphere at F region altitudes, it seems possible that the observed auroral roar fine structure results from this mechanism.

A multipoint determination of the propagation velocity of a sudden commencement across the polar ionosphere

We use magnetic field and riometer data from ground observatories in both the Arctic and Antarctic regions to characterize the high-latitude propagation of a sudden storm commencement (SC) that occurred at 0901 UT February 21, 1994. (1) High time resolution magnetic field data from both hemispheres indicate extremely rapid propagation of the initial part of the SC signal at high latitudes. An initial inflection point was observed in the data from dawn sector stations in both polar caps nearly simultaneously (Δt <2 s) but ∼3 s earlier in the southern hemisphere. (2) Data from the Magnetometer Array for Cusp and Cleft Studies (MACCS) in Arctic Canada, with stations from 0130 to 0600 magnetic local time, indicate dispersive propagation of the preliminary impulse (PI) at speeds decreasing from ∼150 to ∼50 km/s, directly away from a source near or slightly poleward of the cusp, rather than along the auroral oval. (3) Plots of two-dimensional, 20-s resolution equivalent convection vectors in the Northern Hemisphere reveal the imposition and rapid propagation and decay of a short-lived large-scale flow pattern opposite to the normal dawn sector flow, followed by an intensification of the original pattern. However, the perturbed flows were not dominated by the localized, tailward propagating vortices predicted in some models of SC events. In both hemispheres, observations of the PI are consistent with the temporary imposition of a field-aligned current pair antisymmetric about local noon in the polar ionosphere and with horizontal fast mode propagation of a pulse through the ionosphere. (4) Riometer signatures do not match the magnetic variations in either time or intensity, and they propagate with a much lower velocity (∼1 km/s). We infer that the initial magnetic signatures are generated by the arrival of Alfven waves and associated Birkeland currents at the near-cusp ionosphere, while the riometer signatures at these high latitudes are generated predominantly by interplanetary particles associated with the causative coronal mass ejection.

Magnetic impulse event: A detailed case study of extended ground and space observations

Analysis of conjugate data from extended magnetometer networks in northern and southern high latitudes is used to elucidate the initiation and the evolution of a magnetic impulse event (MIE) on June 6, 1997. In addition, data from all-sky imagers, imaging riometers, and Super Dual Auroral Radar Network radars in Antarctica are investigated to confirm the energy content, motion, and electrical current structure of the MIE. The MIE was accompanied by traveling convection vortices (TCVs) that began at ∼10 MLT and moved eastward (toward dusk) and slightly equatorward at 1-3 km/s across the noon meridian with north-south conjugacy. The MIE had upward field-aligned currents with soft electron precipitation that was located near the trailing edge of the Hall current loop. During the MIE interval the interplanetary magnetic field (IMF) was directed strongly outward from the Sun (B x = -5 nT), with a slightly positive (1-2 nT) B z , and a nearly zero By. Since abrupt solar wind pressure changes are unlikely under this IMF orientation (and none was, in fact, observed), classical mechanisms for MIE generation, such as a pressure pulse or dayside reconnection, are excluded. It is speculated that an abrupt IMF cone angle change from 60° to 20°, ∼30 min prior to the MIE onset, may have been an indirect trigger of this event via the interaction between the solar wind and the bow shock.

Global observations of substorm injection region evolution: 27 August 2001

We present riometer and in situ observations of a substorm electron injection on 27 August 2001. The event is seen at more than 20 separate locations (including ground stations and 6 satellites: Cluster, Polar, Chandra, and 3 Los Alamos National Laboratory (LANL) spacecraft). The injection is observed to be dispersionless at 12 of these locations. Combining these observations with information from the GOES-8 geosynchronous satellite we argue that the injection initiated near geosynchronous orbit and expanded poleward (tailward) and equatorward (earthward) afterward. Further, the injection began several minutes after the reconnection identified in the Cluster data, thus providing concrete evidence that, in at least some events, near-Earth reconnection has little if any ionospheric signature.

Evidence for a global disturbance with monochromatic pulsations and energetic electron bunching

We present data from a number of ground stations and satellites that reveal an example of a previously unreported type of global event. The event is characterized by the occurrence of monochromatic pulsations of widely varying frequencies in different regions of local times and L shells. The pulsations appear to be modulated with a ∼45 min periodicity. Simultaneously, energetic particle fluxes observed at geosynchronous orbit (with energies resulting in a drift period of ∼45 min) appear to become phase bunched and the pulsations are observed to occur coincident with minima in the particle bunching. Corresponding data from GOES 5 show an increase in the compressional component of the magnetic field with this period. Data from South Pole Station show fluctuations in the east–west component of the magnetic field at half this period, along with a weak auroral signature and riometer absorptions. We conclude that the data show the existence of a global disturbance with a compressional magnetic field signature, and we suggest that this compression induces a radial electric field, based on Faradays law. Phase bunching of energetic electrons (due to the induction electric field) and monochromatic pulsations are consequences of the global disturbance, although the mechanism responsible for exciting the pulsations is not clear.

Cosmic noise absorption at South Pole Station during magnetic impulse events

We report the results of an examination of the records of cosmic radio noise absorption in the ionosphere during times of previously identified magnetic impulse events (MIEs) at South Pole (SP) Station, Antarctica. Approximately 80% of the 153 MIEs that occurred in the interval from January to December 1986 were accompanied by impulsive increases in riometer absorption; a much smaller fraction (∼4%) showed temporary decreases from a prevailing enhanced level. As is the case for high-latitude MIE events, generally, those associated with absorption exhibit a large morning peak at ∼1000 magnetic local time (MLT) and a smaller afternoon peak at ∼1400 MLT and occur most commonly at the equinoxes. Approximately 20% of the MIE events were not accompanied by an absorption response. These cases occurred preferentially in the afternoon hours. MIEs in the no response class may have occurred farther from SP Station, and thus out of the range of the riometer measurement, than those which show absorption. This is suggested by the distribution of their H component values which peaks at 40–70 nT, the lowest-amplitude range considered. The high percentage of absorption/MIE coincidences indicates that the MIE mechanism often leads to the precipitation of energetic electrons but that on rare occasions may result in the temporary reduction or cessation of preexisting precipitation.

Effect of Night Laboratories on Learning Objectives for a Nonmajor Astronomy Class.

We tested the effectiveness on learning of hands-on, night-time laboratories that challenged student misconceptions in a non-major introductory astronomy class at Rensselaer Polytechnic Institute. We present a new assessment examination used to assess learning in this study. We were able to increase learning, at the 8.0 sigma level, on one of the moon phase objectives that was addressed in a cloudy night activity. There is weak evidence of some improvement on a broader range of learning objectives. We show evidence that the overall achievement levels of the four sections of the class is correlated with the amount of clear whether the sections had for observing, even though the learning objectives were addressed primarily in activities that did not require clear skies. This last result should be confirmed with future studies. We describe our first attempt to cycle the students through different activity stations in an attempt to handle 18 students at a time in the laboratories, and lessons learned from this.

Rethinking the polar cap: Eccentric dipole structuring of ULF power at the highest corrected geomagnetic latitudes

The day-to-day evolution and statistical features of Pc3-Pc7 band ultralow frequency (ULF) power throughout the southern polar cap suggest that the corrected geomagnetic (CGM) coordinates do not adequately organize the observed hydromagnetic spatial structure. It is shown that that the local-time distribution of ULF power at sites along CGM latitudinal parallels exhibit fundamental differences and that the CGM latitude of a site in general is not indicative of the sites projection into the magnetosphere. Thus, ULF characteristics observed at a single site in the polar cap cannot be freely generalized to other sites of similar CGM latitude but separated in magnetic local time, and the inadequacy of CGM coordinates in the polar cap has implications for conjugacy/mapping studies in general. In seeking alternative, observationally motivated systems of “polar cap latitudes,” it is found that eccentric dipole (ED) coordinates have several strengths in organizing the hydromagnetic spatial structure in the polar cap region. ED latitudes appear to better classify the local-time ULF power in both magnitude and morphology and better differentiate the “deep polar cap” (where the ULF power is largely UT dependent and nearly free of local-time structure) from the “peripheral polar cap” (where near-magnetic noon pulsations dominate at lower and lower frequencies as one increases in ED latitude). Eccentric local time is shown to better align the local-time profiles in the magnetic east component over several PcX bands but worsen in the magnetic north component. It is suggested that a hybrid ED-CGM coordinate system might capture the strengths of both CGM and ED coordinates. It is shown that the local-time morphology of median ULF power at high-latitude sites is dominantly driven by where they project into the magnetosphere, which is best quantified by their proximity to the low-altitude cusp on the dayside (which is not necessarily quantified by a sites CGM latitude), and that variations in the local-time morphology at sites similar in ED latitude are due to both geographic local-time control (relative amplification or dampening by the diurnal variation in the local ionospheric conductivity) and geomagnetic coastal effects (enhanced power in a coastally mediated direction). Regardless of cause, it is emphasized that the application of CGM latitudes in the polar cap region is not entirely meaningful and likely should be dispensed with in favor of a scheme that is in better accord with the observed hydromagnetic spatial structure.