Marc R. Hairston

Global Assimilation of Ionospheric Measurements (GAIM)

Abstract : Our primary goal is to construct a real-time data assimilation model for the ionosphere-plasmasphere system that will provide reliable specifications and forecasts. A secondary goal is to validate the model for a wide range of geophysical conditions, including different solar cycle, seasonal, storm, and substorm conditions.

The interaction of a magnetic cloud with the Earth - Ionospheric convection in the Northern and Southern Hemispheres for a wide range of quasi-steady interplanetary magnetic field conditions

This is the second of three papers which study a large interplanetary magnetic cloud, and its interaction with the earths magnetosphere. Here the authors study flows within the ionosphere during the passage of the magnetic cloud on Jan 13-15, 1988. This is the first study of ionospheric convections during prolonged periods of stable and different IMF orientations, which result from the stable, but spatially varying field structure within the magnetic cloud. Data from IMP-8 and DMSP-F8 are analyzed for this work. This observation gave information on ionospheric responses to greater than 10 hour period of northward and southward IMF, with a gradual change from one to the other. Issues studied included strengths of peak flows for north and south IMF; changes in cross polar cap potential with IMF B[sub z]; types and variations of convective patterns vs IMF; variations in size of the polar cap; etc.

Evolution of the global aurora during positive IMF Bz and varying IMF By conditions

The DE 1 imaging instrumentation provides a full view of the entire auroral oval every 12 min for several hours during each orbit. We examined five examples of global evolution of the aurora that occurred during the northern hemisphere winter of 1981-1982 when the z component of the interplanetary magnetic field was positive and the y component was changing sign. Evolution of an expanded auroral emission region into a theta aurora appears to require a change in the sign of By during northward interplanetary magnetic field (IMF). Theta aurora are formed both from expanded duskside emission regions (By changes from positive to negative) and dawnside emission regions (By changes from negative to positive), however the dawnside-originating and duskside-originating evolutions are not mirror images. The persistence of a theta aurora after its formation suggests that there may be no clear relationship between the theta aurora pattern and the instantaneous configuration of the IMF.

Transformation of high‐latitude ionospheric F region patches into blobs during the March 21, 1990, storm

Discrete F region electron density enhancements of a factor of 2 or more have been observed in the high-latitude ionosphere. These enhancements have been termed patches if they occur within the polar cap and blobs if they occur outside of the polar cap. It is important to understand the formation and evolution of these structures because they are associated with large phase and amplitude scintillation in transionospheric radio signals. Blobs are generally thought to result from the breakup of patches as they exit the polar cap; however, this process has not previously been observed. Detailed study of high-latitude ionospheric plasma transport is generally difficult because of the sparseness (spatial and temporal) of electron density and velocity observations. In this paper, we present electron density enhancements measured from the Qaanaaq Digisonde, the Millstone Hill incoherent scatter radar, and the DMSP F8 satellite during a 5-hour interval of the March 21, 1990, storm period and show definitively how a patch is transformed into a blob. We present a new trajectory analysis package that is capable of using ionospheric convection patterns to determine the motion of ionospheric plasma over a period of several hours. The new package uses convection patterns from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique to track the motion of observed patches from one site to another and thus determines where the measured electron density enhancements originated and where they went after being observed. The trajectory analysis also establishes that there is a direct connection between the enhancements observed by the different instruments at different locations. In this case, within ∼4 hours, plasma observed by a Digisonde near the pole is convected through 35° of latitude to the northeastern United States, where it is observed by the Millstone Hill radar, then roughly equal portions are transported westward to Alaska and eastward to Scandinavia where they are observed by the DMSP satellite. This study demonstrates that the changing convection pattern can significantly distort the patch shape and trajectory, and illustrates the high degree of mixing of ionospheric plasma by convection. The changing convection pattern leads to the simultaneous existence of a boundary blob and a subauroral blob which are both observed by the Millstone Hill radar. This work is very relevant to our future ability to specify and forecast ionospheric conditions at high latitudes. It represents a critical step from a merely qualitative ability to model the evolution of patches and blobs to a quantitative ability.

Analysis of the ionospheric cross polar cap potential drop using DMSP data during the National Space Weather Program study period

During the National Space Weather Program (NSWP) event study period of November 2–11, 1993 there were three operational DMSP meteorological satellites (F8, F10, and F11) in orbit, each carrying the Special Sensor for Ions, Electrons, and Scintillation (SSIES) plasma instrument package. Ion flow data from these instruments are used to determine the electrostatic potential drop across both the northern and southern polar caps. The magnitude and distribution of the potential are used to characterize the convection patterns present in the polar ionospheres. The results from all three satellites ate presented to show an overall observational history of the potential drop and the convection pattern during the study period. These observational parameters provide crucial inputs and checks to several ionospheric and magnetospheric models being used in this study. Evidence is presented of an unambiguous difference in the cross polar cap potential drop between the two hemispheres for an extended period of time.

The postsunset vertical plasma drift and its effects on the generation of equatorial plasma bubbles observed by the C/NOFS satellite

The prereversal enhancement (PRE) of the vertical plasma drift in the postsunset sector is an important factor that controls the generation of equatorial plasma bubbles. In this study, we use the measurements of the ion velocity meter on board the Communication/Navigation Outage Forecasting System satellite during 2008–2014 to identify the PRE and its effects on the occurrence of plasma bubbles. The seasonal and longitudinal distributions of the PRE are derived at different solar flux levels. Large PRE occurs at 240–360° longitudes in equinoctial months and December solstice, and small or downward PRE occurs around ±60° in June solstice. The seasonal and longitudinal distributions of large-amplitude equatorial spread F (ESF) (ΔN > 5 × 1010 m−3) are similar to that of the PRE, while the occurrence probability of ESF including smaller-amplitude perturbations (ΔN > 1 × 1010 m−3) can be quite high at any longitude in any season. A quantitative relationship between the PRE and the ESF occurrence probability is derived and well characterized by the cumulative distribution function of a continuous probability distribution. Such a distribution implies that the occurrence of ESF is a probability event. The ESF occurrence probability is small when the PRE is zero or downward and becomes larger than 80% when the PRE is greater than 40 m s−1. Both the ESF occurrence probability and the amplitude increase with the solar radio flux.

High-latitude ionospheric convection pattern during steady northward interplanetary magnetic field

The DMSP F8 satellites coverage of Earths polar regions provides horizontal ion drift velocities along the dawn-dusk meridian at approximately 835 km altitude in each hemisphere during the ∼100 min orbital period. We examine the ionospheric convection signatures observed by this spacecraft in the summer and winter hemispheres during periods when the interplanetary magnetic field (IMF) is directed northward for at least 45 min prior to the satellite entering the polar region and remains northward throughout the polar pass. These convection signatures can be readily categorized by the number of sunward and antisunward flow regions and by their potential distributions. Here we describe the most frequently identifiable and reproducible features of the convection pattern that exist during steady northward IMF conditions. In addition to IMF Bz, the influences on the convection pattern of the IMF Bz/|By| ratio, season, latitude, and solar wind velocity are all considered. The ratio Bz/|By| provides a first order organization of the signatures that occur on the dayside of the dawn-dusk meridian. Sunward flow at highest latitudes on the dayside of the dawn-dusk meridian is the dominant feature seen in the large-scale convection signature during steady northward IMF; however, sunward flow at highest latitudes does not imply the existence of a particular number of convection cells.

Three-dimensional ionospheric plasma circulation

Examination of the ion drift velocity vector measured on the DE2 spacecraft reveals the significance of ionospheric flows both perpendicular and parallel to the magnetic field at high latitudes. During periods of southward directed interplanetary magnetic field the familiar two-cell convection pattern perpendicular to the magnetic field is associated with field-aligned motion predominantly upward in the dayside auroral zone and cusp, and predominantly downward in the polar cap. Frictional heating by convection through the neutral gas and heating by energetic particle precipitation are believed to be responsible for the bulk of the upward flow with downward flows resulting from subsequent cooling of the plasma. Some of the upward flowing plasma is apparently given escape energy at altitudes above about 800 km. The average flow of ions across the entire high-latitude region at 400 km is outward and comparable to the energetic outflow observed at much higher altitudes by DE 1. 18 refs., 5 figs.

Global storm time auroral X-ray morphology and timing and comparison with UV measurements

The Polar Ionospheric X-ray Imaging Experiment (PIXIE) on NASAs Polar spacecraft provides the first global images of the auroral oval in X-rays and allows very accurate measurements of the timing of geomagnetic disturbances to a degree of temporal resolution not available from previous imagers due to its photon counting characteristics. On October 19, 1998, a magnetic cloud associated with a CME encountered the Earths magnetopause near 0500 UT, generating a magnetic storm that reached a minimum value in Dst of -139 nT. The z component of the interplanetary magnetic field (IMF) (B z ) remained remarkably steady for the first 10 hours of the storm as did the solar wind particle pressure. The PIXIE and UVI instruments on the Polar spacecraft were both imaging the auroral oval from 0800 to 1800 UT; six distinct impulsive auroral enhancements were observed by the imagers during this time period. Global imaging combined with geosynchronous particle observations allowed classification of the geomagnetic disturbances associated with the events. Only two of the events were classified as substorms; one was classified as a poleward boundary intensification, one was a convection bay, and one was a pseudobreakup. A sixth event occurred after a dramatic northward turning of the IMF at the end of the 10-hour B z south period but was very weak and transient. The effects of the northward turning were counteracted by a simultaneous increase in the B y component of the IMF. The first sign of significant substorm activity occurred over 8 hours after the cloud encountered the Earth and was not associated with any change in the solar wind magnetic field or particle pressure. The cross polar cap potential remained large (> 100 kV), and most of the X-ray emissions observed were associated with enhanced earthward convection caused by large cross-tail electric fields; 50% were collected from the 0000 - 0600 magnetic local time (MLT) sector.

Response of the ionospheric convection pattern to a rotation of the interplanetary magnetic field on January 14, 1988

Ionospheric convection signatures observed over the polar regions are provided by the DMSP F8 satellite. We consider five passes over the southern summer hemisphere during a time when the z component of the interplanetary magnetic field was stable and positive and the y component changed slowly from positive to negative. Large-scale regions of sunward flow are observed at very high latitudes consistent with a strong z component. When By and Bz are positive, but By is greater than Bz, strong evidence exists for dayside merging in a manner similar to that expected when Bz is negative. This signature is diminished as By decreases and becomes smaller than Bz resulting in a four-cell convection pattern displaced toward the sunward side of the dawn-dusk meridian. In this case the sign of By affects the relative sizes of the two highest-latitude cells. In the southern hemisphere the duskside high-latitude cell is dominant for By positive and the dawnside high-latitude cell is dominant for By negative. The relative importance of possible electric field sources in the low-latitude boundary layer, the dayside cusp, and the lobe all need to be considered to adequately explain the observed evolution of the convection pattern.