Decrease In Total Electron Content Research Articles

Variations of total electron content during geomagnetic disturbances: A model/observation comparison

This paper studies the ionospheric response to the major geomagnetic storm of October 18–19, 1995, using the NCAR TIE‐GCM simulations and the global ionospheric maps (GIM) of total electron content (TEC) observations from the worldwide network of Global Positioning System (GPS) receivers. The TIE‐GCM results show a good agreement with the GPS‐GIM in terms of simulating storm‐time TEC disturbances over the polar regions. The model indicates that the increase of electron density in the high‐latitude E and lower F regions below 200 km is directly related to the magnetospheric energy input through auroral precipitation to the ionosphere, while the decrease of TEC is mainly due to the increase in O2 and N2 densities in the upper F region above 200 km. During the recovery phase, both the TIE‐GCM and GPS‐GIM reveal a distinct hemispheric asymmetry in the TEC integrated above |50°| magnetic latitude, with a 20% decrease in the southern (summer‐like) hemisphere and a 30% increase in the northern (winter‐like) hemisphere.

Equatorial plasma depletion precursor signatures and onset observed at 11° south of the magnetic equator

Coordinated radio and optical measurements of the structure and dynamics of the postsunset equatorial ionosphere were conducted on October 1, 1994, from Agua Verde, Chile (11.3°S magnetic latitude (MLat)). The measurements clearly show a north‐south aligned undulation or ripple on the bottomside of the F layer at 2000 LT, appearing as an eastward propagating decrease in the 630.0‐nm airglow, resembling a traveling ionospheric disturbance in the digital portable ionosonde measurements and causing a total electron content decrease in the Global Positioning System (GPS) satellite measurements. The initial development of this feature, toward the east and away from the magnetic equator, took place in an otherwise smooth, unstructured ionosphere. Spread F began to develop in the ionograms at 2020 LT, and, at this same time, local onset of satellite signal scintillation was detected using the multiple ray paths throughout the sky available from the GPS satellite constellation transmitting at L band frequencies. UHF scintillation measurements from Ancon, Peru, along the same magnetic field line, show that intense scintillation and ionospheric irregularities had developed over the magnetic equator almost 60 min prior to their development at 11°S MLat. The observations suggest that the east‐west electric field expected to be present within the earlier developed depletion and scintillation region at the magnetic equator mapped along magnetic field lines to lower altitudes and higher latitudes, resulting in an undulation or dome‐shaped structure, before evolving into a fully developed depletion (with associated ionospheric irregularities) all along the magnetic flux tube.

Ionospheric disturbances during low‐latitude auroral events and their association with magnetospheric processes

Ionospheric disturbances associated with low‐latitude auroral events that occurred during geomagnetic storms on October 21, 1989, and May 10, 1992, are investigated from measurements of the total electron content (TEC) by the U.S. Navy Navigation Satellite System (NNSS) and ionosonde data from the Japanese meridian chain. Features of the ionospheric disturbances are then associated with the progression of geomagnetic disturbances during the storms. After the onset of the main phase of each storm, anomalous TEC distributions characterized by depressed TEC distributions on the high‐latitude side (> 35° geographic latitude) and an enhanced equatorial ionization anomaly (EIA) on the low‐latitude side (<35° geographic latitude) were observed over Japan both by the NNSS and by the meridian chain of ionosonde stations. This enhancement of the EIA suggests the penetration of magnetospheric electric fields during the storms. Corresponding to the anomalous TEC decreases in the northern part of Japan, ionosonde stations in this region observed specific disturbances of ionospheric variation characterized by simultaneous decreases of ƒ0F2 and increases of h′F. These features of ionospheric variations on the high‐latitude side can be attributed to the upward escape of ionospheric plasma caused by the heating and evacuation mechanisms, which are induced by depletion of the plasmasphere and the resulting access of ring current particles to low latitudes. After the appearance of the characteristic ionospheric variations mentioned above, low‐latitude aurorae were observed at the maximum development stage of the Dst, associated with the recovery of midlatitude geomagnetic horizontal (H) components. Prior to the appearance of the low‐latitude aurorae, magnetic disturbances in the auroral region increased with the decrease of midlatitude H components, as a natural consequence of magnetic storms. However, they showed a recovery before the start of the low‐latitude aurorae, while the magnitude of the low‐latitude magnetic field continued to decrease. These observed features of the magnetic disturbances suggest that the position of the auroral oval shifted toward the equator before the appearance of the low‐latitude aurorae. Consequently, these observations are consistent with the plasmapause and auroral zone moving toward the equator before the onset of low‐latitude aurorae. We conclude that the convection electric field penetrating toward low latitudes causes the enhancement of the EIA, the shrinkage of the plasmasphere, and the penetration of high‐energy particles toward the low‐latitude region.

The behavior of total electron content and equivalent slab thickness during magnetic disturbances

The total electron content ( TEC) and equivalent slab thickness data reduced from observations of 137 MHz transmissions from the geo-stationary satellite AT-3 at a sub-ionospheric point near 50°N and 68°W were statistically investigated. We observed that: 1. (a) there exists a positive correlation between ΔTEC and K p max nearly all the time in winter. 2. (b) the coefficient of correlation is, in general, positive in the case of equivalent slab thickness during daytime, and negative after dark for all seasons: it changes from positive to negative during local sunrise, and 3. (c) the decrease of TEC with increasing disturbance is usually accompanied by an increase of the equivalent slab thickness in daytime hours in equinox and summer, but not in winter.