W. R. Coley

Comparison of low-latitude ion and neutral zonal drifts using DE 2 data

We have used data from the ion drift meter and the wind and temperature spectrometer on the DE 2 spacecraft to make statistical comparisons of the zonal ion and neutral drifts at dip latitudes (DLAT) in the ±35° range over all local times. Fourier analysis indicates that the superrotation and the diurnal components of both flows are strongly peaked at the dip equator, with the superrotation term becoming negative for |DLAT| ≥20°. One interesting feature is the presence of a period (2200-0500 solar local time) in the 300-400 km altitude region near the dip equator where the ion drift is more strongly eastward than the neutral flow. This would seem to indicate the presence of an electric field source of greater strength than the F region dynamo elsewhere along the geomagnetic field line. Model calculations indicate that a possible mechanism for this source lies in the vertical shear in the zonal neutral wind in the 100-200 km altitude region.

Adaptive identification and characterization of polar ionization patches

Dynamics Explorer 2 (DE 2) spacecraft data are used to detect and characterize polar cap “ionization patches”, loosely defined as large-scale (>100 km) regions where the F region plasma density is significantly enhanced (≳ 100%) above the background level. These patches are generally believed to develop in or equatorward of the dayside cusp region and then drift in an antisunward direction over the polar cap. We have developed a flexible algorithm for the identification and characterization of these structures, as a function of scale-size and density enhancement, using data from the retarding potential analyzer, the ion drift meter, and the langmuir probe on board the DE 2 satellite. This algorithm was used to study the structure and evolution of ionization patches as they cross the polar cap. The results indicate that in the altitude region from 240 to 950 km ion density enhancements greater than a factor of 3 above the background level are relatively rare. Further, the ionization patches show a preferred horizontal scale size of 300–400 km. There exists a clear seasonal and universal time dependence to the occurrence frequency of patches with a northern hemisphere maximum centered on the winter solstice and the 1200–2000 UT interval.

Structure and occurrence of polar ionization patches

We use measurements from the retarding potential analyzer and ion drift meter on the DE 2 spacecraft to examine the density and velocity structure of 225 northern hemisphere polar ionization patches. An ionization patch is loosely defined as a large-scale (≧ 100 km) region where the F region plasma density is significantly (≧ 100%) above the background level. We examine the occurrence frequency of ionization patches as a function of interplanetary magnetic field (IMF), season, and UT. While for all values of Bz patches tend to occur mostly in the northern hemisphere winter, when Bz 0, the UT distribution is much more uniform. We also examine the plasma velocity structure inside and conjoint with patches as a function of IMF in order to determine their stability with time. A distinct IMF variation in the velocity structure of patches was found with a much lower level of velocity structure during southward IMF conditions. As patches convect across the polar cap, intermediate-scale (∼15 km) irregularities are found both on the edges of and inside a patch. The irregularity level is found to be only slightly higher on the trailing edge of a patch than the leading edge. Examination of the average density gradient on the leading and trailing edges of convecting patches shows that the leading edge tends to be steeper. This information is related to current theoretical mechanisms on the generation and evolution of patches.

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.

Seasonal and universal time distribution of patches in the northern and southern polar caps

Ion density measurements from the Dynamics Explorer 2 and Defense Meteorological Satellite Program F8 and F9 satellites are used to examine hemispherical differences in the occurrence patterns of polar ionization patches as a function of season, universal time (UT), and interplanetary magnetic field (IMF). When Bz<0, the greatest frequency of patch occurrence in the northern hemisphere is in the winter in the 1000–2200 UT range. This time corresponds to the interval when the northern magnetic pole (and hence the cusp) lies the farthermost toward the dayside. This fact is often used to explain the creation of patches in terms of the entrainment of dayside plasma into the cusp by high-latitude convection. In the southern hemisphere we see that the occurrence frequencies peak at over twice the northern hemisphere values in the same general UT region (1000–2300 UT). However, the southern hemisphere cusp is most dayward at approximately 0300 UT, a time of minimum patch formation. Examination of the relationship of the terminator to the polar cap boundaries in each hemisphere leads to a simple explanation in terms of the differing offset of the magnetic poles (and the ionospheric convection pattern) from the geographic poles.

A Comparison of in situ measurements of E→ and −V→×B→ from Dynamics Explorer 2

Dynamics Explorer-2 provided the first opportunity to make a direct comparison of in situ measurements of the high-latitude convection electric field by two distinctly different techniques. The vector electric field instrument (VEFI) used antennae to measure the intrinsic electric fields and the ion drift meter (IDM) and retarding potential analyzer (RPA) measured the ion drift velocity vector, from which the convection electric field can be deduced. The data from three orbits having large electric fields at high latitude are presented, one at high, one at medium, and one at low altitudes. The general agreement between the two measurements of electric field is very good, with typical differences at high latitudes of the order of a few millivolts per meter, but there are some regions where the particle fluxes are extremely large (e.g., the cusp) and the disagreement is worse, probably because of IDM difficulties. The auroral zone potential patterns derived from the two devices are in excellent agreement for two of the cases, but not in the third, where bad attitude data may be the problem. At low latitudes there are persistent differences in the measurements of a few millivolts per meter, though these differences are quite constant from orbit to orbit. This problem seems to arise from some shortcoming in the VEFI measurements. Overall, however, these measurements confirm the concept of “frozen-in” plasma that drifts with velocity E→×B→/B2 within the measurement errors of the two techniques.

Radio-tomographic images of post-midnight Equatorial Plasma Depletions

For the first time, post-midnight Equatorial Plasma Depletions (EPDs) have been imaged in the longitude-altitude plane using radio-beacon tomography. High-resolution (~10 km × 10 km) electron-density reconstructions were created in the area between approximately -90° and -55° longitude near the geomagnetic equator. The reconstructions were created using Total Electron Content (TEC) measurements from an NRL receiver array sited in Peru and the MART inversion algorithm. TEC was derived from the 150 and 400 MHz signals transmitted by the CERTO beacon on the C/NOFS satellite. The MART algorithm requires an “initial guess” ionosphere, which was generated by using in-situ electron density data from the C/NOFS CINDI instrument and electron density profiles from an ionosonde operated by the University of Massachusetts at Lowell (UML). Each initial guess ionosphere was approximated below the F-peak by replicating an ionosonde profile over all longitudes within the imaging region; above the F-peak a Chapman function was fitted to the ionosonde F-peak density and the corresponding CINDI in-situ measurement at each longitude. In this study, EPDs spawned pre-midnight were imaged near dawn. Observed EPDs had widths of 100-1000 km, spacings of 300-900 km, and often appeared “pinched off” at the bottom. Well-developed EPDs appeared on an evening with a very small (4 m/s) Pre-Reversal-Enhancement (PRE), suggesting that postmidnight enhancements of the vertical plasma drift and/or seeding-induced uplifts (e.g. gravity waves) were responsible for driving the Rayleigh-Taylor Instability into the nonlinear regime on this night. On another night the Jicamarca incoherent scatter radar recorded postmidnight (~0230 LT) Eastward electric fields nearly twice as strong as the PRE fields seven hours earlier. These electric fields lifted the whole ionosphere, including embedded EPDs, over a longitude range ~14° wide. CINDI detected a dawn depletion in exactly the area where the reconstruction showed an uplifted EPD. Strong Equatorial Spread-F observed by the UML ionosonde during receiver observation times confirmed the presence of ionospheric irregularities.

Study of the Equatorial and Low‐Latitude Electrodynamic and Ionospheric Disturbances During the 22–23 June 2015 Geomagnetic Storm Using Ground‐Based and Spaceborne Techniques

Abstract We use a set of ground‐based instruments (Global Positioning System receivers, ionosondes, magnetometers) along with data of multiple satellite missions (Swarm, C/NOFS, DMSP, GUVI) to analyze the equatorial and low‐latitude electrodynamic and ionospheric disturbances caused by the geomagnetic storm of 22–23 June 2015, which is the second largest storm in the current solar cycle. Our results show that at the beginning of the storm, the equatorial electrojet (EEJ) and the equatorial zonal electric fields were largely impacted by the prompt penetration electric fields (PPEF). The PPEF were first directed eastward and caused significant ionospheric uplift and positive ionospheric storm on the dayside, and downward drift on the nightside. Furthermore, about 45 min after the storm commencement, the interplanetary magnetic field (IMF) Bz component turned northward, leading to the EEJ changing sign to westward, and to overall decrease of the vertical total electron content (VTEC) and electron density on the dayside. At the end of the main phase of the storm, and with the second long‐term IMF Bz southward turn, we observed several oscillations of the EEJ, which led us to conclude that at this stage of the storm, the disturbance dynamo effect was already in effect, competing with the PPEF and reducing it. Our analysis showed no significant upward or downward plasma motion during this period of time; however, the electron density and the VTEC drastically increased on the dayside (over the Asian region). We show that this second positive storm was largely influenced by the disturbed thermospheric conditions.

Using insitu satellite data to describe global scale variations in space weather

Quite frequently visible and uv imagery of the ionosphere and upper atmosphere is used to describe the global scale characteristics of ion density and composition. This invaluable data can be obtained from satellites at very high altitude, providing a complete global picture at modest scale sizes, or from low Earth orbit, where a more restricted view with higher spatial resolution is possible. Here we describe how in-situ data obtained from low Earth orbit can be visualized in a manner similar to optical emission data. With this approach global scale variations of key parameters like plasma temperature and ion velocity can be added to those of composition and density to reveal the evolution of the system in response to external drivers. During times of high magnetic activity the links between key parameters over large temporal and spatial scales can be easily visualized and cuts through the images reveal details that can be used in more quantitative descriptions.