Large‐scale, near‐field magnetic fields from external sources and the corresponding induced internal field

Abstract

Magsat data has been analyzed as a function of the Dst index to determine the first degree/order spherical harmonic description of the near‐earth external field and its corresponding induced field. The analysis was done separately for data from dawn and dusk with the following results: dusk external: q10 = 20.3 ‐ 0.68 Dst (nT); dusk internal: g10 = 29987.7 + 0.240q10 (nT); dawn external: q10= 18.62 ‐ 0.63 Dst (nT); dawn internal: g10 = 29992.3 + 0.287q10 (nT), where g10 and q10 are the degree 1, zero‐order, internal and external coefficients, respectively, in a spherical harmonic potential function describing the near‐earth magnetic field. Comparison with POGO data indicates that the constant term relating q10 and Dst has changed about 20 nT between 1970 and 1980, presumably due to an increase in the average ring current intensity. A local time variation of the external field persists even during very quiet magnetic conditions. Both a diurnal and 8‐hour period are present. A crude estimate of sq current in the ±45° geomagnetic latitude range is obtained for 1966–1970. The current strength, located in the ionosphere and induced in the earth, is typical of earlier determinations from surface data, although its maximum is displaced in local time from previous results.

Ring Current Energy Input and Decay

“…In (1), C I C is the correction due to induced currents in the Earth, taken to be 1.3 (Dessler and Parker, 1959;Langel and Estes, 1985); D MP is the perturbation from the Chapman-Ferraro currents on the magnetopause, taken to be 15.5 √ P sw (solar wind dynamic pressure in nPa) (Burton et al, 1975;Gonzalez et al, 1994), and D QRC is the contribution from the quiet time ring current (subtracted out as a baseline offset of Dst), taken to be −20 nT (Burton et al, 1975). The second panel of Figures 1-8 shows Dst (solid lines) and Dst * (dotted lines) for the 8 selected storms.…”

An annual proxy for the geomagnetic signal of magnetospheric currents on Earth based on observatory data from 1900–2010

“…However, none of these indices assesses the absolute baseline of the quite-time magnetospheric field. Based on Magsat vector data the quiet-time magnetospheric field level was first estimated as 20 nT by Langel & Estes (1985a) and Langel & Estes (1985b). This first approximation was then specified using 9 yr of CHAMP data that revealed a stable quiet-time field of 13 nT with an additional variable part of up to 15 nT depending on the solar cycle phase .…”

Azimuthally asymmetric ring current as a function of <i>D<sub>st</sub></i> and solar wind conditions

“…A symmetrical ring current of 1 MA magnitude flowing at a distance of 6 R E produces the disturbance B rc z (0)≈−16 nT in the Earth's center. The corresponding ground magnetic effect is DR=k B rc z (0)≈−21 nT, where k≈1.3 is due to induction currents in the Earth (Langel and Estes, 1985;Häkkinen et al, 2002). Note that the noon current presents a purely symmetrical part of the ring current, without contribution of the partial ring and cross-tail currents.…”