On the origins and timescales of geoeffective IMF

Abstract

Southward interplanetary magnetic field (IMF) in the geocentric solar magnetospheric (GSM) reference frame is the key element that controls the level of space weather disturbance in Earth's magnetosphere, ionosphere, and thermosphere. We discuss the relation of this geoeffective IMF component to the IMF in the geocentric solar ecliptic (GSE) frame, and using the almost continuous interplanetary data for 1996–2015 (inclusive), we show that large geomagnetic storms are always associated with strong southward, out‐of‐ecliptic field in the GSE frame: Dipole tilt effects, which cause the difference between the southward field in the GSM and GSE frames, generally make only a minor contribution to these strongest storms. The time‐of‐day/time‐of‐year response patterns of geomagnetic indices and the optimum solar wind coupling function are both influenced by the timescale of the index response. We also study the occurrence spectrum of large out‐of‐ecliptic field and show that for 1 h averages it is, surprisingly, almost identical in ICMEs (interplanetary coronal mass ejections), around CIRs/SIRs (corotating and stream interaction regions) and in the “quiet” solar wind (which is shown to be consistent with the effect of weak SIRs). However, differences emerge when the timescale over which the field remains southward is considered: for longer averaging timescales the spectrum is broader inside ICMEs, showing that these events generate longer intervals of strongly southward average IMF and consequently stronger geomagnetic storms. The behavior of out‐of‐ecliptic field with timescale is shown to be very similar to that of deviations from the predicted Parker spiral orientation, suggesting the two share common origins.

Interplanetary magnetic field properties and variability near Mercury's orbit

“…The probability of a change in B z polarity is also generally higher near perihelion than near aphelion. Lockwood et al [] (their Figure 14) show that in near‐Earth interplanetary space, the overall probability of a B z polarity change after 4 h is 0.83, which is lower than the average in all panels of Figure for such a lag. Hence, the difference between perihelion and aphelion found here is a trend that continues with increasing heliocentric distance to r = 1 AU.…”

Probabilistic Solar Wind and Geomagnetic Forecasting Using an Analogue Ensemble or “Similar Day” Approach

“…For B N , however, the RMSE for persistence is higher than climatology even at a lead time of two hours, demonstrating the short autocorrelation time in the B N time series and the inherent difficulty in B N prediction (e.g. Lockwood et al, 2016 and references therein).…”

“…For space-weather forecasting purposes, the radial solar-wind flow speed [V R ] and the component of the HMF normal to the solar-rotation plane [B N ] are the critical parameters, as they are the primary contributors to the dawn-to-dusk electric field that controls reconnection with the magnetospheric field (Dungey, 1961). Owing to the inclination of the Earth's magnetosphere and orbital plane to the solar-rotation plane, the HMF component along the solar-rotation direction [B T ] also leads to a magnetic field anti-parallel to the nose of the magnetosphere, in a manner that varies systematically with both day of year and time of day (Lockwood et al, 2016). The solar-wind density [N P ] also affects the compression of the magnetosphere and hence the efficiency of the magnetic coupling between the heliospheric and magnetospheric magnetic fields.…”

Historical space weather monitoring of prolonged aurora activities in Japan and in China

“…Lockwood et al . [] have recently shown that larger storms are caused by coronal mass ejections that generate more persistent southward interplanetary magnetic field. Further, multiple magnetic storms sometimes occur within several days, and the major driver of such multiple storms is also a series of coronal mass ejections [ Kataoka and Miyoshi , ].…”