Interplanetary origin of geomagnetic activity in the declining phase of the solar cycle

Abstract

Interplanetary magnetic field (IMF) and plasma data are compared with ground-based geomagnetic Dst and AE indices to determine the causes of magnetic storms, substorms, and quiet during the descending phase of the solar cycle. In this paper we focus primarily on 1974 when the AE index is anomalously high (AE = 283 nT). This year is characterized by the presence of two long-lasting corotating streams associated with coronal holes. The corotating streams interact with the upstream low-velocity (300-350 km s-1), high-density heliospheric current sheet (HCS) plasma sheet, which leads to field compression and ~ 15-to 25-nT hourly average values. Although the B z component in this corotating interaction region (CIR) is often < -10 nT, typically the field directionality is highly variable, and large southward components have durations less than 3 hours. Thus the corotating stream/HCS plasma sheet interaction region can cause recurring moderate (-100 nT < Dst < -50 nT) to weak (-50 nT < Dst < -25 nT) storms, and sometimes no significant ring current activity at all (Dst > -25 nT). Storms of major (Dst < -100 nT) intensities were not associated with CIRs. Solar wind energy is transferred to the magnetosphere via magnetic reconnection during the weak and moderate storms. Because the B z component in the interaction region is typically highly fluctuating, the corresponding storm main phase profile is highly irregular. Reverse shocks are sometimes present at the sunward edge of the CIR. Because these events cause sharp decreases in field magnitude, they can be responsible for storm recovery phase onsets. The initial phases of these corotating stream-related storms are caused by the increased ram pressure associated with the HCS plasma sheet and the further density enhancement from the stream-stream compression. Although the solar wind speed is generally low in this region of space, the densities can be well over an order of magnitude higher than the average value, leading to significant positive Dst values. Since there are typically no forward shocks at 1 AU associated with the stream-stream interactions, the initial phases have gradual onsets. The most dramatic geomagnetic response to the corotating streams are chains of consecutive substorms caused by the southward components of large-amplitude Alfvtn waves within the body of the corotating streams. This auroral activity has been previously named high-intensity long-duration continuous AE activity (HILDCAAs). The substorm activity is generally most intense near the peak speed of the stream where the Alfvtn wave amplitudes are greatest, and it decreases with decreasing wave amplitudes and stream speed. Each of the 27-day recurring HILDCAA events can last 10 days or more, and the presence of two events per solar rotation is the cause of the exceptionally high AE average for 1974 (higher than 1979). HILDCAAs often occur during the recovery phase of magnetic storms, and the fresh (and sporadic) injection of substorm energy leads to unusually long storm recovery phases as noted...

Space Weather: Physics, Effects and Predictability

“…CIRs associated with compressed plasma and fields often produce larger dynamic pressure than the CMEs (Watari 1997). The increase of IMF fluctuations in CIRs is due to the presence of large amplitude Alfven waves within the body of the corotating streams (Tsurutani et al 1995a), which has been confirmed by Ulysses' observations (Tsurutani et al 1995b). Ulysses observations showed the tilted latitudinal structure of the CIRs; the tilt of the CIRs decreases with increasing distance from the Sun ).…”

Ionospheric Day-to-Day Variability Around the Whole Heliosphere Interval in 2008

“…The IMF component on the other hand oscillated with an amplitude of about 2 -4 nT. These are the typical signatures of high-speed streams and CIR events (see, e.g., Tsurutani et al, 1995Tsurutani et al, , 2006.…”

Relativistic electron flux dropouts in the outer radiation belt associated with corotating interaction regions

“…The intermittent southward IMF of CIR-driven geomagnetic activities is mainly carried by the Alfvenic fluctuation in high-speed streams after the stream interface [Tsurutani et al, 1995;Gonzalez et al, 1999;Miyoshi et al, 2013]. This southward IMF will be significantly enhanced or suppressed by Russell-McPherron effect, for the influence of Russell-McPherron effect on the amplitude of southward IMF is comparable with the Alfvenic fluctuation in high-speed streams [Miyoshi and Kataoka, 2008].…”