[1] The Earth’s magnetosphere acts as a shield against highly energetic particles of cosmic and solar origin. On geological timescales, variations of the internal geomagnetic field can drastically alter the magnetospheric structure and dynamics. This paper deals with energetic particles in the paleomagnetosphere. Scaling relations for cutoff energies and differential particle fluxes during periods of reduced dipole moment are derived. Particular attention is paid to high rigidity galactic cosmic ray particles in the GV range and to lower rigidity solar energetic particles. We find that in the paleomagnetosphere of a strongly reduced dipole moment, solar protons of several tens of MeV may access the Earth’s atmosphere even at midlatitudes. Higher-order core-field components can widen the polar caps and open new particle-entry regions around the equator. Potential field modeling of the paleomagnetosphere and magnetohydrodynamic (MHD) simulations are utilized to study the impact of solar particles on the Earth’s atmosphere for more complex field configurations that are likely to occur during geomagnetic polarity transitions.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006JA012224

Energetic particles in the paleomagnetosphere: Reduced dipole configurations and quadrupolar contributions

[1] The strength of the Earth’s magnetic field changes over time. We use simulations with the Coupled Magnetosphere-Ionosphere-Thermosphere model to investigate how the magnetosphere, upper atmosphere, and solar quiet (Sq) geomagnetic variation respond as the geomagnetic dipole moment M varies between 2⋅10 22 and 10⋅10 22 Am 2 . We find that the magnetopause stand-off distance and the cross-polar cap potential increase, while the polar cap size decreases, with increasing M. Their dependence on M is stronger than predicted by previous studies. We also show for the first time that the shape of the magnetosphere starts to change for M ≤ 4⋅10 22 Am 2 . This may be due to enhanced magnetopause erosion and/or to strong changes in the ionospheric conductance, which affect the field-aligned currents and the magnetic fields they create in the magnetosphere,

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2012JA017555

The dependence of the coupled magnetosphere‐ionosphere‐thermosphere system on the Earth's magnetic dipole moment

[1] The Earth's magnetic field changes in orientation and strength over time. We study the response of the magnetosphere-ionosphere-thermosphere system to a 25% reduction in magnetic field intensity, using the coupled magnetosphere-ionosphere-thermosphere (CMIT) model. Simulations were performed with a dipole moment of 8 × 1022 A m2, close to the present-day value, and a dipole moment of 6 × 1022 A m2, both under the same solar wind conditions, intermediate solar activity (F10.7 = 150), and for March equinox and June solstice. The 25% reduction in field strength causes the magnetosphere to shrink and the polar cap to expand, in agreement with theory. The Pedersen and the Hall ionospheric conductances increase by 50%–60% and 60%–65%, respectively. This causes a ∼9%–12% decrease in electric potential and a ∼20% increase in field-aligned currents during equinox. Ion E × B drift velocities are enhanced by ∼10%–15%. The Joule heating also increases, by 13%–30%, depending on the season. Changes in the neutral temperature structure are caused partly by changes in Joule heating and partly by changes in the neutral wind. The neutral wind itself is also affected by changes in neutral temperature and by changes in ion velocities. The changes in the neutral wind, together with changes in the vertical component of the E × B drift, affect the height of the ionospheric F2 layer. Changes in electron density are related to changes in the O/N2 ratio. The global mean increase in neutral temperature causes the thermosphere to expand, resulting in a global mean uplift of the ionosphere. These effects are generally smaller during solstice.

/pdf/the-response-of-the-coupled-magnetosphere-ionosphere-1rfwxa88o3.pdf

The response of the coupled magnetosphere-ionosphere-thermosphere system to a 25% reduction in the dipole moment of the Earth's magnetic field

[1] We present a focused parameter study of solar wind–magnetosphere interaction for the young Sun and Earth, ∼3.5 Gyr ago, that relies on magnetohydrodynamic (MHD) simulations for both the solar wind and the magnetosphere. By simulating the quiescent young Sun and its wind we are able to propagate the MHD simulations up to Earth's magnetosphere and obtain a physically realistic solar forcing of it. We assess how sensitive the young solar wind is to changes in the coronal base density, sunspot placement and magnetic field strength, dipole magnetic field strength, and the Sun's rotation period. From this analysis we obtain a range of plausible solar wind conditions to which the paleomagnetosphere may have been subject. Scaling relationships from the literature suggest that a young Sun would have had a mass flux different from the present Sun. We evaluate how the mass flux changes with the aforementioned factors and determine the importance of this and several other key solar and magnetospheric variables with respect to their impact on the paleomagnetosphere. We vary the solar wind speed, density, interplanetary magnetic field strength, and orientation as well as Earth's dipole magnetic field strength and tilt in a number of steady state scenarios that are representative of young Sun-Earth interaction. This study is done as a first step of a more comprehensive effort toward understanding the implications of Sun-Earth interaction for planetary atmospheric evolution.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010JA016036

Modeling the Young Sun's Solar Wind and its Interaction with Earth's Paleomagnetosphere

[1] On geological timescales the Earth's dipole moment can vary between 0.1 and 2 times the present value. The weakest internal magnetic fields occur during geomagnetic polarity transitions when the Earth's internal dipole field reverses. Theoretically, the size of the paleomagnetosphere is expected to change in the function of the dipole moment according to a power law scaling relation. We carried out a series of numerical magnetohydrodynamic (MHD) simulations of axial dipolar paleomagnetospheres, gradually decreasing the relative dipole moment from 1 to 0.1, in order to test the validity of the theoretical scaling relations for different values of the north-south interplanetary magnetic field component (IMF B z ). We study the dipole moment dependence of the standoff distance, the flank distances, and the polar cap size, and derive power law scaling relations with significantly differing scaling exponents as compared to the theoretically expected ones. The extent of deviation from the theoretical scaling exponents is controlled by the magnitude of the southward B z . We quantify the B z -dependence of the size of the magnetosphere and validate our results with the Roelof-Sibeck bivariate function of magnetopause shape obtained from in-situ satellite measurements for the present-day magnetosphere. We conclude that the Roelof-Sibeck function cannot be applied for southward B z values stronger than 5 nT. A new B z -dependent dipole scaling relation is suggested for the magnetospheric scale size. Inserting our corrected scaling relation in the Hill model of magnetosphere-ionosphere coupling, a better fit can be obtained between simulated and theoretically calculated paleomagnetospheric transpolar potentials.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JA011531

B. Zieger

Papers

A parametric study of magnetosphere–ionosphere coupling in the paleomagnetosphere