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

Abstract

This is the third in a series of papers that investigate the semi-annual, annual and Universal Time variations in the magnetosphere. In this paper we use the Lin et al. (2010) empirical model of magnetopause locations, along with the assumption of pressure equilibrium and the Newtonian approximation of magnetosheath pressure. We show that the equinoctial pattern arises in both the cross-tail current at the tail hinge point and in the total energy stored in the tail. The model allows us to study the effects of both dipole tilt and hemispheric asymmetries. As a test of the necessary assumptions made to enable this analysis, we also study simulations by the BATSRUS global MHD magnetosphere model. These also show that the reconnection rate in the tail is greatest when the dipole tilt is small but this only applies at low solar wind dynamic pressure, p SW , and does not on its own explain why the equinoctial effect increases in amplitude with increased p SW , as demonstrated by Paper 2. Instead, the effect is consistent with the dipole tilt effect on the energy stored in the tail around the reconnection X line. A key factor is that a smaller/larger fraction of the open polar cap flux threads the tail lobe in the hemisphere that is pointed toward/away from the Sun. The analysis using the empirical model uses approximations and so is not definitive; however, because the magnetopause locations in the two hemispheres were fitted separately in generating the model, it gives an unique insight into the effect of the very different offsets of the magnetic pole from the rotational pole in the two hemispheres. It is therefore significant that our analysis using the empirical model does predict a UT variation that is highly consistent with that found in both transpolar voltage data and in geomagnetic activity.

A Survey of 25 Years' Transpolar Voltage Data From the SuperDARN Radar Network and the Expanding‐Contracting Polar Cap Model

“…(2020), Lockwood, Owens, et al. (2020), and Lockwood et al. (2021) as being the effect of p SW in squeezing the near‐Earth tail.…”

“…The modeling analysis of Lockwood, Owens, et al. (2020) found the influence of p SW through both Φ N and the energy stored in the tail lobe. The results presented here show an effect of p SW on Φ PC , but that the effect is smaller than for am : this indicates that the effect of energy stored in the tail may be a larger factor for midlatitude range indices such as am .…”

“…Various observational studies suggest that increases in p SW cause enhanced general magnetospheric convection and field‐aligned current systems as well as enhanced geomagnetic activity (e.g., Boudouridis, Zesta, et al., 2008; Hubert, Milan, et al., 2006; Lee et al., 2004; Lukianova, 2003; Stauning & Troshichev, 2008). This phenomenon has been modeled using global MHD models of the magnetosphere as being caused by rises in both Φ D and Φ N (Connor et al., 2014; Lockwood, Owens, et al., 2020; Ober et al., 2006; Palmroth et al., 2004).…”

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

“…The factor P w (w) was modelled in Paper 3 (Lockwood et al, 2020c) using the asymmetric magnetopause location model of Lin et al (2010) by assuming that the tail is in equilibrium with a solar wind of dynamic pressure p SW . These authors modelled the magnetic shear DB across the cross tail current sheet for various values of the geoeffective southward component of the IMF [B Z ] GSM , and the solar wind dynamic pressure, p SW that are the inputs to the magnetopause model.…”

Linking Solar Minimum, Space Weather, and Night Sky Brightness

“…In some cases, after a maximum near local midnight, a decline in airglow brightness was followed by a substantial brightening several hours later. Insufficient data prevented us from determining if these features were the result of atmospheric airglow waves15 , energy being released from Earth's magnetospheric tail 16,17,18 , or an unknown physical process. Bottom Panel: The night airglow above the quiescent level, ΔMC-N(t) MC(t), for each site is plotted versus Julian Date (JD).…”