July 15, 2011, 12:00 pm - 1:00 pm
Director's Seminar
Dr. Miho Hasegawa (NASA GSFC/Orau NPP), Incompressible Plasma Sheet
Thinning in the Magnetotail
Abstract: Thinning of the plasma sheet and the embedded current sheet is revealed as an increasing magnetic pressure, and such thinning is known as a typical harbinger of substorm onset in the magnetotail. One of the critical and unanswered question is how a global magnetospheric convection of flux transport leads to the evolution of the tail plasma sheet and finally form a weak field regions and a near-Earth neutral line at X=-10 to -30 Re. Erickson and Wolf [1980] investigated the adiabatic compression of a magnetotail flux tube convecting earthward and found that this will result in an unrealistically enhanced pressure in the near-Earth tail. This discrepancy has been known as the pressure balance inconsistency or entropy crisis. Using multi-spacecraft analysis of THEMIS, we investigate the flux tube evolution in the plasma sheet before substorm onsets and found that the evolution is characterized by incompressible changes of a flux tube with almost constant total pressure. We conclude that the increased lobe field strength is not the necessary and the primary cause for cross tail current sheet thinning but rather thinning can occur within the plasma sheet as a result of unknown internal processes. It should be emphasized that the presence of the incompressible thinning before substorm onset provides an explanation for the formation of the near-Earth neutral line and the development of the high beta regions as a result of the thinning.
Dr. Jeffrey Klenzing (NASA GSFC/ORAU NPP),
Plasma Density Enhancements in the Low-Latitude Ionosphere: Theory
and Observation
Abstract: Plasma density structures are frequently encountered in the nighttime low-latitude ionosphere by probes on the Communication/Navigation Outage Forecasting System (C/NOFS) satellite. Of particular interest to us here are plasma density enhancements, which are typically observed ±15◦ away from the magnetic equator. The low inclination of the C/NOFS satellite offers an unprecedented opportunity to examine these structures and their associated electric fields and plasma velocities, including their field-aligned components, along an east-west trajectory. Among other observations, the data reveal a clear asymmetry in the velocity structure within and around these density enhancements. Previous data has shown that the peak perturbation in drift velocity associated with a density enhancement occurs simultaneously both perpendicular and parallel to the magnetic field, while the results presented here show that the peak perturbation in parallel flow typically occurs 25-100 km to the east of the peak perpendicular perturbation. We discuss this and other aspects of the observations in relation to previous observations and models.
Dr. Fernando Simoes (NASA GSFC/Orau NPP), Overview of ELF Electric Field Measurements onboard C/NOFS: Schumann Resonance and Ionospheric Alfven Resonator Signatures
Abstract: The Communications/Navigation Outage Forecasting System (C/NOFS) satellite was launched in April 2008 and has been providing exceptional data to investigate electrodynamic processes of the ionospheric equatorial region. Among a wealthy of DC and AC electric and magnetic field and particle measurements, we present selected examples of Extremely Low Frequency (ELF) electric fields related to Schumann Resonance (SR) and Ionospheric Alfven Resonator (IAR) signatures. For both phenomena, this type of ELF signatures is observed from satellite borne instruments for the first time. In this talk we present a few results recorded during solar minimum activity and discuss implications for atmospheric electricity and tropospheric-ionospheric coupling mechanisms. We also briefly address SR and IAR capabilities for remote sensing applications and ionosphere dynamics monitoring.