Evidence for particle injection as the cause of D(sub st) reduction during HILDCAA events

The Earth's magnetic field at the equator, as monitored by the D(sub st) index, can stay below its quiet day value for days. This can happen after storms resulting in a very slow recovery of the D(sub st) index, or it can happen in the absence of a storm. Such 'anomalous' behavior of the D(sub st) recovery is observed during times showing continuous auroral activity called High Intensity Long Duration Continuous AE Activity (HILDCAA). The D(sub st) is mostly attributed to the ring current (RC), but it also depends on other current systems in the magnetosphere such as magnetopause, tail and auroral currents. It is well established that the magnetic effect of the RC is proportional to the energy content of the charged particles generating it. It is thus of interest to determine if the reduction of the D(sub st) during HILDCAAs is due to an increased RC or/and can be accounted for by an increase or a relocation of other current systems. This paper considers the injection of electrons and protons into the auroral and subauroral zone for four cases exhibiting HILDCAA activity. The total energy flux of the ions into the midnight/evening quadrant gives a good estimate of the energy injection into the RC. This injection, if it occurs during the recovery phase of a storm, prolongs the final decay of the D(sub st) to quiet day values, and if it occurs during times without storms the injection can maintain D(sub st) at more or less constant negative values for days. The ion injection into the RC is sufficient to account for the reduced D(sub st) index during HILDCAAs. It is determined that the HILDCAA events are associated with a low-level injection of protons into the outer portion of the RC above L equal 4. The HILDCAA events are thus not due to plasma sheet current intensifications or earthward motion of this current. The prolonged low-level ion injection is associated with fluctuations in the B(sub z) component of the solar wind magnetic field giving rise to magnetic field line merging on the front side of the magnetosphere. The duration of the B(sub z) negative phases are not long enough to drive a magnetic storm.