Does Adding Solar Wind Poynting Flux Improve the Optimum Solar Wind‐Magnetosphere Coupling Function?

Abstract

We study the contribution of the solar wind Poynting flux Strue→sw to the total power input into the magnetosphere. The dominant power delivered by the solar wind is the kinetic energy flux of the particles, which is larger than Ssw by a factor of order MA2, where MA is the Alfvén Mach number. The currents trueJ→ flowing in the bow shock and magnetosheath and the electric field trueE→ of the solar wind give regions where trueJ0.25em→.trueE→<0, which are sources of Poynting flux, generated from the kinetic energy flux. For southward interplanetary magnetic field, trueE→ is duskward and the currents in the high‐latitude tail magnetopause are also sources of Poynting flux. We show transfer of kinetic energy into the magnetosphere is less efficient than direct entry of Strue→sw by a factor MA. Because MA is typically of order 10, this means that although the power density in the solar wind due to Strue→sw is typically only 1%, it is responsible for of order 10% of the energy input to the magnetosphere. To investigate the effect of this, we add a term to the solar wind‐magnetosphere energy coupling function that allows for Strue→sw and that increases the correlation with the geomagnetic am index for 1995–2017 (inclusive) from 0.908 to 0.924 for 1‐day averages and from 0.978 to 0.979 for annual means. The increase for means on daily or smaller timescales is a small improvement but is significant (at over the 3σ level), whereas the improvement for annual or Carrington rotation means is not significant.

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

“…In the growth phase of substorms, much of the energy that is extracted from the solar wind (P) is stored in the near-Earth tail as magnetic flux opened by reconnection in the dayside magnetopause is appended to the tail by the solar wind flow. This energy is subsequently released in substorm expansion phases as that open flux is rapidly re-closed by reconnection in the cross-tail current sheet (see discussion by, e.g., Lockwood, 2019). We know on timescales longer than the substorm cycle (such as the 3-hour time resolution of the am index or greater) that geomagnetic activity is highly correlated with energy input into the magnetosphere, P averaged over the relevant interval (Lockwood, 2019).…”

“…This energy is subsequently released in substorm expansion phases as that open flux is rapidly re-closed by reconnection in the cross-tail current sheet (see discussion by, e.g., Lockwood, 2019). We know on timescales longer than the substorm cycle (such as the 3-hour time resolution of the am index or greater) that geomagnetic activity is highly correlated with energy input into the magnetosphere, P averaged over the relevant interval (Lockwood, 2019). In addition, Appendix A of Paper 1 shows that am is highly correlated with both the average and the maximum of the AL and the SML auroral electrojet indices in the 3-hour window it is compiled over and therefore it is dominated by the substorm expansion-phase current wedge and electrojet.…”

“…This is the third in a series of papers investigating semi-annual, annual, and Universal Time (UT) variations in the magnetosphere. In the first paper of the series (Lockwood et al, 2019: hereafter Paper 1) it was shown that the Russell-McPherron (R-M) effect is indeed at the heart on the semi-annual variation of geomagnetic activity, as is taken to be the case in a great many publications since it was originally proposed by Russell and McPherron (1973), this seminal paper having been cited over 750 times in the literature at the time of writing. This effect is predicted by assuming that the interplanetary magnetic field (IMF) lies in the solar equatorial plane that is normal to the solar rotation axis (the XY plane of the Geocentric Solar EQuatorial reference frame, GSEQ, such that the north-south component of the IMF in GSEQ, [BZ]GSEQ, is zero).…”

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

“…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. This formula is given by equation 2of Paper 1 and the derivation has also recently been given, and expanded upon, by Lockwood (2019). Analysis by Finch & Lockwood (2007) showed that P a performed consistently better than a basket of other widely-used solar-wind/magnetosphere coupling functions on all averaging timescales tested (between 1 day and 1 year).…”

“…As also discussed in Paper 1, P a is based on the dominant energy flux in the solar wind, namely the bulk-flow kinetic energy flux of the particles, whereas some other coupling functions incorrectly use the Poynting flux in the solar wind, which is very small in comparison. The energy input to the magnetosphere across the magnetopause is in the form of Poynting flux, but most of that is generated from the bulk-flow solar wind kinetic energy by currents that flow in the bow shock and magnetosheath (Cowley, 1991;Lockwood, 2004Lockwood, , 2019Pulkkinen et al, 2016). Lockwood (2019) has recently added the minority solar wind Poynting flux to the Vasyliunas et al (1982) formulation for P a at the expense of adding a second free fit parameter.…”