Lawrence Sparks

Ionospheric total electron content perturbations monitored by the GPS global network during two northern hemisphere winter storms

The global evolution of two major ionospheric storms, occurring on November 4, 1993, and November 26, 1994, respectively, is studied using measurements of total electron content (TEC) obtained from a worldwide network of ground-based GPS receivers. The time-dependent features of ionospheric storms are identified using TEC difference maps based on the percent change of TEC during storm time relative to quiet time. The onset of each ionospheric storm is indicated by the appearance of auroral/subauroral TEC enhancements which occur within 1 hour of the beginning of the geomagnetic storm main phase. Significant TEC enhancements (> 100%) are observed in the winter northern hemisphere. The rate at which TEC enhancements appear is found to correlate with gradients in the Dst index. The large scale ionospheric structures identified during the storms are (1) nightside auroral/subauroral enhancements which surround the auroral oval, (2) dayside (around noon) high-latitude and middle-latitude enhancements associated with traveling ionospheric disturbances, and (3) conjugate latitudinal enhancements. For the November 1993 storm, a short positive phase (about 15 hours) is followed by a long negative phase (∼60 hours). In the November 1994 storm, we have identified the clear signature of a traveling ionospheric disturbance (TID) which propagated at a speed of ∼460 m/s from ∼60° N to ∼40° N. The motion of this disturbance appears to conserve angular momentum.

Global Ionospheric TEC Variations During January 10, 1997 Storm

The ionospheric storm evolution process was monitored during the January 10, 1997 magnetic cloud event, through measurements of the ionospheric total electron content (TEC) from 150 GPS stations. The first significant response of the ionospheric TEC to the geomagnetic storm was at 0300 UT as an auroral/subauroral enhancement around the Alaskan evening sector. This enhancement then extended to both noon and midnight. Around 0900 UT, the enhancement at noon broke from the subauroral band and moved to lower latitudes. This day side northern hemisphere enhancement also corresponded to a conjugate geomagnetic latitude enhancement in the southern hemisphere and lasted about 5 hours. At 1500 UT a large middle latitude enhancement appeared over Mexico and the southern US, and persisted until 2200 UT. The enhancement was probably caused by the equatorward neutral wind which pushed the plasma up. On the basis of this assumption, the kinetic energy of the neutral wind which caused the middle latitude enhancement is estimated as ∼4.1×l09 Joules. This is about 0.03% of solar wind energy impinging on the magnetosphere and about 3% of the energy deposited on polar cap ionosphere. After 2000 UT, a negative phase gradually became stronger (especially in the southern hemisphere), although the northern subauroral enhancement persisted one more day. The entire ionosphere gradually recovered to normal on January 12. Thus, large middle latitude enhancement, equatorward motion of the dayside enhancement (probably related to a TID), the persistence of the subauroral enhancement, and the conjugate features at both hemispheres are the main characteristics of this storm.

Hemispheric Daytime Ionospheric Response to Intense Solar Wind Forcing

We investigate the ionospheric response to events where the z-component of the interplanetary magnetic field, B 2 , becomes large and negative for several hours, associated with the largest geomagnetic storms over the prior solar maximum period (2000-2004). We compute the average vertical total electron content (TEC) in the broad region covering 1200-1600 local time and ±40 degrees geomagnetic latitude (dipole), using data from the global network of Global Positioning System (GPS) receivers. In several cases, we find approximately a two-fold increase in total electron content within 2-3 hours of the time when the southward-B solar wind impinged on the magnetopause. We also analyze daytime super-satellite TEC data from the GPS receiver on the CHAMP satellite orbiting at approximately 400 km altitude, and find that for the October 30, 2003 storm at mid-latitudes the TEC increase is nearly one order of magnitude relative to the TEC just prior to the B southward onset. The geomagnetic storm-time phenomenon of prompt penetration electric fields into the ionosphere from enhanced magnetospheric convection is the most likely cause of these TEC increases, at least for certain of the events, resulting in eastward directed electric fields at the equator. The resulting dayside vertical ExB drift of plasma to higher altitudes, while solar photons create more plasma at lower altitudes, results in a daytime super-fountain effect that rapidly changes the plasma structure of the entire dayside ionosphere. This phenomenon has major practical space weather implications.