EarthRef.org Reference Database (ERR) -- Campbell 2004

Magnetic indices provide valuable information for space weather evaluations. The recent literature shows many scientists still believing that a Dst-derived current circles the Earth like a ring of Saturn. Some actually use a subtraction of Dst to remove the magnetospheric contributions to surface fields. I have measured the storm time fields at paired locations to examine the ring current requirements of surface fields forming the Dst index. Also, I verified the daytime enhancement of the disturbance field at the magnetic dip equator. Two geomagnetic storms were chosen for their size, isolation, seasonal difference, and global data availability. For these storm periods, records from all available stations, within the latitude range of Dst contributors, were collected to choose those pairs of similar longitude. A daytime, dip equator, ionospheric enhancement of the storm fields was verified. Next, the 1/cos ($theta$) factor, used for the Dst derivation, was applied to the paired data to determine if that ring current adjustment was worthwhile. Results indicate that the local station fields behave more like those from other current sources, such as the ionosphere, and that the cosine factor, representing a magnetospheric ring current effect, typically fails. The Dst appellation equatorial ring current index is clearly a misnomer and should be avoided. Magnetic indices provide valuable information for space weather evaluations. The recent literature shows many scientists still believing that a Dst-derived current circles the Earth like a ring of Saturn. Some actually use a subtraction of Dst to remove the magnetospheric contributions to surface fields. I have measured the storm time fields at paired locations to examine the ring current requirements of surface fields forming the Dst index. Also, I verified the daytime enhancement of the disturbance field at the magnetic dip equator. Two geomagnetic storms were chosen for their size, isolation, seasonal difference, and global data availability. For these storm periods, records from all available stations, within the latitude range of Dst contributors, were collected to choose those pairs of similar longitude. A daytime, dip equator, ionospheric enhancement of the storm fields was verified. Next, the 1/cos ($theta$) factor, used for the Dst derivation, was applied to the paired data to determine if that ring current adjustment was worthwhile. Results indicate that the local station fields behave more like those from other current sources, such as the ionosphere, and that the cosine factor, representing a magnetospheric ring current effect, typically fails. The Dst appellation equatorial ring current index is clearly a misnomer and should be avoided.