Solar Quiet Sq

The 100-km-high ionospheric current is given the name solar quiet-day variations (Sq). Such current is responsible for the recurring regular form of the daytime field variations seen at world observatories, typically 10 to 30 gammas for mid-latitudes. Seasonal changes in both the ionospheric conductivity, the wind system within the ionosphere, and the atmospheric expansion or contraction cause seasonal changes in the Sq current. As you might guess, there is also a lunar quiet-day ionospheric current, due to the lunar-tidal motion in the atmosphere (similar to lunar tides in our oceans). The lunar-caused ionospheric currents are typically less than 10% of the Sq field size.

Geographic latitude effects in Sq behavior are largely dependent on the Earth's main field and the Sun's elevation angle at the magnetic observatory. However, a special effect occurs in an equatorial band within approximately 6° north and south of the magnetic dip equator, where the main field is horizontal. This causes a special high equatorial ionospheric conductivity that concentrates daytime Sq dynamo currents; surface fields there can be as much as six times that of the nearby low-latitude values.

Studies in the early twentieth century have shown that Sq fields can be modeled by two global vortices of dynamo currents in the ionosphere that flow counter-clockwise in the Northern Hemisphere (Figure 3.18) and in the opposite direction in the Southern Hemisphere. The size of a local Sq field change in daytime depends on the ionospheric movement, the direction and strength of the Earth's main field, the electric conductivity of the lower ionosphere, and the location of the observing station with respect to the subsolar Sq vortex. Use Figure 1.21 to visualize the surface fields from this vortex.

Ionospheric Dynamo

FIGURE 3.18 ► The principal ionospheric current system, on the Sun side of the Earth, flows counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The currents are enhanced at the magnetic dip equator. All currents are more intense in the summer months than in winter.

FIGURE 3.18 ► The principal ionospheric current system, on the Sun side of the Earth, flows counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The currents are enhanced at the magnetic dip equator. All currents are more intense in the summer months than in winter.

Occasionally special disruptions modify the ionosphere sufficiently that the normal Sq currents are upset. Such events include:

1. decreased ionization in the narrow path of a solar eclipse,

2. increased ionization caused by an exposure of the day-side ionosphere to x-rays from a sudden disturbance on the Sun, and

3. jostling of the ionosphere by traveling pressure waves that arrive from volcanic eruptions, atmospheric atomic explosions, or sudden heating by auroral-related currents.

All these special effects are used by the upper-atmospheric physicists to determine and monitor the nature of distant source regions.

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