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

Abstract

This is the second in a series of papers that investigate the semi-annual, annual and Universal Time (UT) variations in the magnetosphere. We present a varied collection of empirical results that can be used to constrain theories and modelling of these variations. An initial study of two years’ data on transpolar voltage shows that there is a semi-annual variation in magnetospheric flux circulation; however, it is not as large in amplitude as that in geomagnetic activity, consistent with the latter showing a non-linear (quadratic) variation with transpolar voltage. We find that during the persistent minimum of the UT variation in geomagnetic activity, between about 2 and 10 UT, there is also a persistent decrease in observed transpolar voltage, which may be, in part, caused by a decrease in reconnection voltage in the nightside cross-tail current sheet. We study the response of geomagnetic activity to estimated power input into the magnetosphere using interplanetary data from 1995 onwards, an interval for which the data are relatively free of data gaps. We find no consistent variation in the response delay with time-of-year F and, using the optimum lag, we show that the patterns of variation in F-year spectrograms are very similar for geomagnetic activity and power input into the magnetosphere, both for average values and for the occurrence of large events. The Russell–McPherron (R–M) mechanism is shown to be the central driver of this behaviour. However, the (R–M) effect on power input into the magnetosphere is small and there is a non-linear amplification of the semi-annual variation in the geomagnetic response, such that a very small asymmetry in power input into the magnetosphere Pα between the “favourable” and “unfavourable” polarities of the IMF BY component generates a greatly amplified geomagnetic response. The analysis strongly indicates that this amplification is associated with solar wind dynamic pressure and its role in squeezing the near-Earth tail and so modulating the storage and release of energy extracted from the solar wind. In this paper, we show that the equinoctial pattern is found in the residuals of fits of Pα to the am index and that the amplitude of these equinoctial patterns in the am fit residuals increases linearly with solar wind dynamic pressure. Similarly, the UT variation in am is also found in these fit residuals and also increases in amplitude with solar wind dynamic pressure.

Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 3. Modelling

“…fraction of the open flux generated at a low-latitude magnetopause reconnection site is not appended to the tail lobe in the summer hemisphere as it is slower to evolve into the tail and so threads the dayside magnetopause. Hence, this effect not only explains the equinoctial pattern of geomagnetic activity but also why it is enhanced by increased solar wind dynamic pressure, as shown in Paper 2 (Lockwood et al 2020b).…”

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

“…We have compared our first-order model of the combined effects with observed mid-latitude range indices, using the , , and indices that are available for 61 years now. Lockwood et al (2020a;2020b) show that the largest geomagnetic events are not caused by the R-M mechanism but rather by events of strong southward IMF in the GSEQ frame: indeed, these authors show that the R-M effect actually reduces the geoeffectiveness for the most southward-pointing fields in GSEQ. These large events are mainly driven by field inside, and ahead of, coronal mass ejections and the impact of such an event on Earth must be completely random in and there is no evidence that the expectation that it is also random in is incorrect.…”

“…Figure 16 are the contours of 1 + 4 ( 2 ⁄ ) of 1.28 and 1.31 and it can be seen that agreement is very close. However, the colour pixels in Figure 16a and 16b highlight an important point made by Lockwood et al (2020b) namely that while the "favoured" equinox/ shows a marked enhancement in  ⁄ for a given polarity of [ ] the "unfavoured" equinox/ shows a decrease in  ⁄ that is almost as large. This is true for both polarities of [ ] and Lockwood et al (2020b) point out that the same thing is truebut, crucially, much less so for geomagnetic indices.…”

“…However, the colour pixels in Figure 16a and 16b highlight an important point made by Lockwood et al (2020b) namely that while the "favoured" equinox/ shows a marked enhancement in  ⁄ for a given polarity of [ ] the "unfavoured" equinox/ shows a decrease in  ⁄ that is almost as large. This is true for both polarities of [ ] and Lockwood et al (2020b) point out that the same thing is truebut, crucially, much less so for geomagnetic indices. Hence rather than it being the enhancement for the favoured equinox/ being the cause of the semi-annual variation (the traditional view of the R-M effect), it is the lack of a corresponding decrease for the unfavoured equinox/ that really generates the semi-annual variation.…”

“…where we compute  from the average of the interval in question from the regression equation given by equation (A4) in Appendix A of Lockwood et al (2020b).  = ( 6.6810 −5 ) 3 − ( 1.6610 −2 ) 2 + 1.89…”