• Analysis Of Methods For Solar Wind Propagation From Lagrangian Point L1 To Earth

      Jensen, Poul F.; Bristow, William; Newman, David; Nielsen, Hans; Otto, Antonius; Smith, Roger (2013)
      The Lagrangian point L1 is situated about 1.5 million kilometers sunwards from Earth and provides a unique orbiting point for satellites, placing them constantly upstream in the solar wind, allowing for prediction of solar wind conditions impacting Earth's magnetosphere. Short-term forecasting of geomagnetic activity requires extrapolation of solar wind data from L1 to Earth (typical propagation time around 1 hour), as does any research in interactions between the solar wind and the magnetosphere during intervals when no Earth-orbiting satellites are in the solar wind. To accurately predict propagation delays it is necessary to take the geometry of incoming solar wind structures into account. Estimating the orientation of solar wind structures currently has to be done using single satellite measurements, which will likely remain the case for another decade or more, making it important to optimize single satellite techniques for solar wind propagation. In this study a comprehensive analysis of 8 different single satellite propagation methods was performed, each involving several variable parameters. 4 of these used electric field calculations and had not previously been tested for solar wind propagation. Large amounts of data were propagated from a satellite near L1 to target satellites near Earth for comparison to measured data, using specific test scores to evaluate relative performance between methods and parameter values. Electric field methods worked well for continuous data but did not predict arrival time of discontinuities (abrupt transitions) as accurately as methods based on magnetic field data, one of which delivered the best results on all accounts. This method had also been found to give best results in a previous study, but optimal parameter values were significantly different with the larger data set used here. Propagating 6,926 discontinuities it was found that on average they arrive about 30 seconds later than predicted (about 1% of the propagation time). Barring a systematic error in velocity data or delay calculations the offset suggests an asymmetry in the geometry of solar wind structures. While this idea is physically plausible it was not unambiguously supported by the data.
    • Role Of Conductivity Spatial Structure In Determining The Locations Of Sprite Initiation

      Tavares, Fernanda De Sao Sabbas; Jeffries, Martin O. (2003)
      Sprites are transient optical signatures of mesospheric electrical breakdown in response to lightning discharges. Multiple sprites are often observed to occur simultaneously, laterally displaced from the underlying causative cloud-to-ground (CG) lightning discharge. The causes of this lateral displacement are presently not understood. This dissertation investigates the role of neutral density perturbations in determining the locations of sprite initiation. The work was performed in three interrelated studies. (1) A detailed statistical study of the temporal-spatial relationships between sprites and the associated CG was performed for July 22, 1996. The distribution of sprite offsets relative to the underlying lightning had a mean of ~40 km. The distribution of sprite onset delays following the parent lightning had a mean of ~20--30 ms, consistent with theoretical estimates for the electron avalanche-to-streamer transition in the mesosphere. (2) A follow-up study for the same observations was performed to investigate the relationship of the sprites to convective activity in the underlying thunderstorm, using GOES-8 infrared imagery of cloud-top temperatures. The sprite generating thunderstorm was a Mesoscale Convective System (MCS). The maximum sprite and -CG production of the system were simultaneously reached at the time of maximum contiguous cloud cover of the coldest region, corresponding to the period of greatest convective activity of the system. Thunderstorm convective activity is a potential source of gravity waves and mesospheric turbulence. (3) Computer simulations of the temporal-spatial evolution of lightning-induced electric fields in a turbulent upper atmosphere were performed. The modeled turbulence in the simulations spanned the amplitude range 10% to 40% of the ambient background neutral density, with characteristic scale sizes of 2 km and 5 km, respectively. The results indicate that neutral density spatial structure, similar to observed turbulence in the mesosphere, facilitates electrical breakdown in isolated regions of density depletions at sprite initiation altitudes. These spatially distributed breakdown regions provide the seed electrons necessary for sprite generation, and may account for the observed sprite offsets.