Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 1. Geomagnetic data

Abstract

We study the semi-annual variation in geomagnetic activity, as detected in the geomagnetic indices am, aaH, AL, Dst and the four aσ indices derived for 6-hour MLT sectors (around noon, dawn, dusk and midnight). For each we compare the amplitude of the semi-annual variation, as a fraction of the overall mean, to that of the corresponding variation in power input to the magnetosphere, Pα, estimated from interplanetary observations. We demonstrate that the semi-annual variation is amplified in the geomagnetic data compared to that in Pα, by a factor that is different for each index. The largest amplification is for the Dst index (factor ~ 10) and the smallest is for the aσ index for the noon MLT sector (aσ-noon, factor ≈ 1.1). By sorting the data by the prevailing polarity of the Y-component (dawn-dusk) of the Interplanetary Magnetic Field (IMF) in the Geocentric Solar Equatorial (GSEQ) reference frame, we demonstrate that the Russell-McPherron (R-M) effect, in which a small southward IMF component in GSEQ is converted into geoeffective field by Earth’s dipole tilt, is a key factor for the semi-annual variations in both Pα and geomagnetic indices. However, the variability in the southward component in the IMF in the GSEQ frame causes more variability in power input to the magnetosphere Pα than does the R-M effect, by a factor of more than two. We show that for increasingly large geomagnetic disturbances, Pα delivered by events of large southward field in GSEQ (known to often be associated with coronal mass ejections) becomes the dominant driver and the R-M effect declines in importance and often acts to reduce geoeffectiveness for the most southward IMF in GSEQ: the semi-annual variation in large storms therefore suggests either preconditioning of the magnetosphere by average conditions or an additional effect at the equinoxes. We confirm that the very large R-M effect in the Dst index is because of a large effect at small and moderate activity levels and not in large storms. We discuss the implications of the observed “equinoctial” time-of-year (F) – Universal Time (UT) pattern of geomagnetic response, the waveform and phase of the semi-annual variations, the differences between the responses at the June and December solstices and the ratio of the amplitudes of the March and September equinox peaks. We also confirm that the UT variation in geomagnetic activity is a genuine global response. Later papers will analyse the origins and implications of the effects described.

Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 4. Polar Cap motions and origins of the Universal Time effect

“…Figure A1b shows that, in reality, a full range of h GSEQ values are seen with zero and 180°( respectively, purely northward and southward IMF in the GSEQ frame) being roughly half as common as h GSEQ = 90°. Appendix B of Paper 1 studied the implications of the fact that h GSEQ is not always 90° (Lockwood et al, 2020a).…”

“…Hence it is not just the half-wave rectification that is giving the good agreement to the semi-annual variation for the R-M effect demonstration, the simplifying assumption of [h] GSEQ = 90°is a vital component, and without it the semi-annual variation predicted is very small. The explanation of why this occurs for general [h] GSEQ was given in the Appendix B to Paper 1 (Lockwood et al, 2020a).…”

“…However, the geomagnetic field is not a symmetric, Earth-centred dipole (e.g., Koochak & Fraser-Smith, 2017) and this will cause UT variations even in global data and even when averaged over many years. The third mechanism discussed in relation to the semiannual variation is the "axial effect" (see review in Paper 1, Lockwood et al, 2020a) which depends on the variation of the heliographic latitude of Earth over the year because the ecliptic is inclined at about 7°with respect to the solar equator. This effect alters the probability of Earth intersecting faster solar wind and so can introduces a time-of-year variation but no UT effect because it does not invoke variations caused by the orientation of Earth's magnetic field.…”

“…We here also study for the first time, the hemispheric sub-indices that are averaged together to generate these global ar indices: this means that in addition to northern and southern hemisphere indices an and as (where am = (an + as)/2), we also employ: ar N (dawn), ar S (dawn) and ar(dawn) = {ar N (dawn)+ ar S (dawn)}/2); ar N (noon), ar S (noon) and ar(noon); ar N (dusk), ar S (dusk) and ar(dusk); and ar N (midn), ar S (midn) and ar (midn), for the 6-hour MLT sectors around 06hrs, 12hrs, 18hrs and 00hrs, respectively. The first paper in this series (Lockwood et al, 2020a) compared the semi-annual variations in the four ar indices to those in other geomagnetic indices and showed that they revealed a great many of the same characteristics as am. The amplification of the semi-annual variation, with respect to that in the estimated power input into the magnetosphere, P a , was shown to increase with distance away from noon, being minimal for the index for the 6-hour sector around magnetic noon, ar(noon), by a factor of near 2 for the ar(dawn) and ar(dusk) (and for the equivalent overall global index am) and by a factor of near 3 for ar(midnight).…”

Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 2. Response to solar wind power input and relationships with solar wind dynamic pressure and magnetospheric flux transport

“…Paper 1 in this series (Lockwood et al, 2020, hereafter "Paper 1") reviewed the semi-annual variation in geomagnetic activity, as seen in a number of indices, and highlighted a number of puzzles that require explanation. We study these in the present paper empirically using the power input into the magnetosphere, P a , deduced from interplanetary measurements using the formula that was originally derived theoretically by Vasyliunas et al (1982) from dimensional analysis.…”