(ISR2) Low- and mid-latitude aeronomy and stormtime disturbance, Parts I and II (Final Report)

Conveners Part I:
Dave Hysell (daveh@geology.geo.cornell.edu),
Mike Sulzer (msulzer@naic.edu)

Conveners Part II:
Hyosub Kil (hyosub.kil@jhuapl.edu),
Jonathan Makela (jmakela@ssd5.nrl.navy.mil),
Mihail Codrescu (CU/CIRES, mihail.codrescu@noaa.gov)

2004, Wednesday 30 June 0930-1130 AM and 0100-0300 PM

PART I: Aeronomy research at the Upper Atmospheric Facilities

This workshop concerned aeronomy research at low and mid latitudes conducted at Arecibo and Jicamarca using the incoherent scatter radars and the adjoining instrument clusters. The workshop differed from those held in past years in two major respects. First, the conveners combined the two facilities into a single workshop and solicited presentations identifying research problems that can best be addressed using both facilities. Second, they held discussions pertaining to facility news and programmatics to a minimum, maintaining a focus on emerging problems in low latitude aeronomy and experiments designed to pursue them.

A number of joint investigations are already taking place at Jicamarca and Arecibo, including studies of the topside composition. Sixto Gonzalez pointed out that such studies have been underway at Arecibo for many years and have progressed to the point of validating physics-based models, which have only recently been able to reproduce the high helium ion abundances seen at midlatitudes at night during solar minimum. Fabiano Rodrigues presented new topside composition results from Jicamarca, where a topside observing program has just begun. All agreed that current empirical models of the topside are inadequate and potentially misleading (e.g. for TEC estimates). Another topic of common interest is meteor head echoes which are being studied at both facilities by Diego Janches, who pointed out the significance of being able to determine the bearing of micrometeorites at Jicamarca using interferometry in estimating total flux rates.

Some differences in the research programs at Arecibo and Jicamarca necessarily arise from differences in the capabilities of the radars. At Arecibo, observations of the photoelectron-enhanced plasma line make it possible to distinguish between molecular ions in the bottomside (Nestor Aponte) and to observe gravity-wave-induced plasma density fluctuations with very high spatial and temporal resolution (Frank Djuth). At Jicamarca, the ability to observe coherent backscatter from field-aligned irregularities facilitates investigations of gradient drift and Farley Buneman instabilities in the E region ( Esayas Shume, Meers Oppenheim) as well as of anomalous "150 kilometer" echoes from the valley region (Jorge Chau). Plasma instabilities and field aligned irregularities represent interesting physics in their own right but also permit certain radar diagnostics of ionospheric electric fields, plasma densities, and neutral winds that would otherwise be impossible at Jicamarca.

A surprising aspect of the workshop was the emergence of gravity waves as the dominant source of debate. Gravity waves are observed only indirectly through their influence on the plasma dynamics and structure. In addition to the Arecibo plasma line experiments, gravity waves are clearly evident in the vector plasma drift profiles measured routinely at Jicamarca, demonstrating their ability to polarize the plasma. Mike Nichols discussed a spread F event observed at both Arecibo and Jicamarca which was triggered by neutral forcing. Dave Fritts then outlined a new experiment which will look for evidence that gravity waves generated by convective storms in the troposphere actually seed equatorial spread F events over Brazil. Such a seeding mechanism, if it could be demonstrated, would represent an important space weather effect. Another space weather effect studied at Arecibo and Jicamarca is the equatorward propagation of storm-time TEC perturbations evident in data from the South American GPS chain as demonstrated by Cesar Valladares.

Optical instrumentation is an integral part of aeronomy research at the two low-latitude facilities. Michael Faivre presented Fabry Perot interferometer data showing clear signatures of the Midnight Temperature Maximum (MTM) observed over Arecibo and Jicamarca. Modelling this phenomenon has been challenging but provides unique insight into the tidal forcing that must be present in the lower thermosphere. Jonathan Friedman later discussed the collocation of sporadic layers and atomic layers observed over Arecibo. The assembled group looked forward to new optical instrumentation scheduled for deployment in Puerto Rico and South America.

Part II: Storm-time Ionospheric Disturbance in the Low Latitudes

The second part of the workshop focused on ionospheric disturbances in the low-latitude region during magnetic storms. The main drivers of the ionospheric disturbance in the low-latitude region are storm-time electric fields, neutral winds, and thermospheric composition disturbance (TCD). The goal of this workshop was to put together the observations and model simulations to distinguish their contributions to the ionospheric disturbance. The presentations in this workshop demonstrated the importance of storm-induced electric fields as drivers of ionospheric disturbances in the low-latitude region. They modify the location and strength of the EIA, create a deep equatorial ionization trough, suppress or promote the equatorial bubble activity depending on when they occur, and also affect the equatorward expansion of the TCD. The TCD is known as a primarily plasma depletion mechanism in the high- and middle-latitudes during storm time but its effect has been ignored in the low-latitude region. This workshop may be the first workshop that provided an opportunity for a full discussion of the TCD effect on the low-latitude ionosphere. The model simulations and observations provided clear evidence of the expansion of the TCD from the high latitudes to equatorial region and the equatorial plasma depletions by the TCD. This workshop stimulated the study of the TCD contribution to storm-time effects in the low-latitude ionosphere. The important questions raised in this workshop are:
  1. Could we distinguish the effects of the direct penetration and dynamo electric fields
  2. What are the suppression and promotion mechanisms of the bubble activity during storm time
  3. Could the TCD be the plasma depletion mechanism in the equatorial region?
This workshop suggested some clues to these questions and also motivated a new approach to the phenomena in the low-latitude ionosphere. The future challenges are to provide solid evidence that supports the proposed new ideas.

Below is the summary of the presentations with complete list of the speakers and topics:

Bela Fejer gave an excellent overview of the penetration and dynamo electric fields that are the main drivers of ionospheric disturbances during storm times. Jonathan Makela presented observations of the change in zonal plasma drift velocity and F-layer height during storm time from ground-based optical measurements and discussed the effects of the storm-time electric field on equatorial plasma bubble activity. Naomi Maruyama conducted Coupled Thermosphere-Ionosphere-Plasmasphere-Electrodynamics (CTIPe) model simulations and showed that the direct penetration electric field can modify the ionospheric dynamo by changing the conductivity and neutral wind. Sid Henderson investigated the morphology of the nighttime Equatorial Ionization Anomaly (EIA) by using OI 135.6-nm disk-scan data from TIMED/GUVI and showed that the strength of storm-time ionospheric disturbances is variable with season and longitude. Trevor Garner investigated the location and strength of the EIA during the storm of October 30-31, 2003 by using the Ionospheric Data Assimilation Three Dimensional (IDA3D) Algorithm and showed the formation of deeper equatorial ionization trough and greater separation of the EIA than predicted by climatological models. Geoff Crowley gave a tutorial on ionosphere-thermosphere coupling effects and also presented model simulation results that showed the suppression of the daytime oxygen density in the low-latitude region during big storms. Yongliang Zhang investigated thermospheric composition disturbance (TCD) by using optical measurements from the TIMED/GUVI and IMAGE/SI-13 instruments and showed the expansion of the TCD from the high latitudes to the equatorial region. Pallamraju Duggirala presented observations of large enhancements in OI 630.0-nm emissions during the daytime in Chile during the magnetic storm of November 6, 2001 and attributed this phenomenon to the enhancement of neutral density produced by the equatorward propagation of neutrals. Hyosub Kil investigated the formation of large equatorial plasma depletions during big magnetic storms and proposed that they are produced by the enhanced chemical plasma loss in the bubbles caused by the TCD.