The Magnitude of IMF B_y Influences the Magnetotail Response to Solar Wind Forcing

Abstract

Magnetosphere-ionosphere dynamics are determined by the solar wind and interplanetary magnetic field (IMF) forcing and the system's response to that forcing. Arguably, the most important quantity in the solar wind forcing is the rate of opening of magnetic flux, or the reconnetion rate, on the dayside magnetopause. For several decades, coupling functions have been used to quantify the upstream solar wind forcing. Typically, coupling functions are proportional to the product 𝐴𝐴 𝐴𝐴 𝛼𝛼 𝐵𝐵 𝛽𝛽 𝑇𝑇 sin 𝛾𝛾 (𝜃𝜃∕2) , where 𝐴𝐴 𝐴𝐴 is solar wind speed,is the IMF clock angle in the Geocentric Solar Magnetospheric (GSM) coordinate system. The exponents 𝐴𝐴 𝐴𝐴 , 𝐴𝐴 𝐴𝐴 and 𝐴𝐴 𝐴𝐴 are empirically determined, for example, by maximizing correlation between the coupling function and geomagnetic indices (Lockwood, Bently et al., 2019;Newell et al., 2007;Vasyliunas et al., 1982). Because geomagnetic disturbances on the ground are also much affected by how the magnetosphere and ionosphere responds to the upstream forcing, the interpretation of these coupling functions in terms of a dayside coupling efficiency is challenging. The coupling function presented by Milan et al. (2012) is slightly different in this regard, as it was specifically designed to predict the dayside reconnection rate in units of Weber per second, or Volt:where 𝐴𝐴 Λ = 3.3 ⋅ 10 5 m 2/3 s 1/3 . The coupling function coefficients 𝐴𝐴 𝐴𝐴𝐴 𝐴𝐴𝐴 𝐴𝐴 were in this case estimated by fitting the expansion of the open magnetic flux inside the polar cap as monitored by global Far Ultraviolet imaging of the aurora during intervals when tail reconnection was assumed to be negligible.

Geomagnetic Disturbances That Cause GICs: Investigating Their Interhemispheric Conjugacy and Control by IMF Orientation

“…Holappa et al. (2021b) noted that this By sign dependence is very strong in the winter hemisphere, but it is weak in the summer hemisphere, and is much stronger in the westward electrojet than in the eastward electrojet. Holappa and Buzulokova (2022) noted that the physical mechanisms of IMF By effects, which apply not only to auroral zone electron precipitation and ionospheric conductance but also to the fluxes of energetic magnetosphere protons and the growth rate of the ring current, are still not fully understood.…”

“…Holappa et al. (2021b) noted that many studies using ground magnetometers, beginning with Friis‐Christensen and Wilhjelm (1975) and using polar‐orbiting satellites (Friis‐Christensen et al., 2017; Smith et al., 2017), have shown that auroral electrojets in the northern hemisphere winter are stronger in both hemispheres for By > 0 than for By < 0, and that in NH summer the dependence on the By sign is reversed. Holappa et al.…”

“…A subsequent study by Holappa et al (2021a) found that the substorm onset latitude and the isotropic boundary latitude of energetic protons were both ∼1° lower during IMF |By| > 3 conditions than for smaller By, and that the substorm occurrence frequency was larger for small |By|. They suggested, consistent with the results of a resistive MHD study by Hesse and Birn (1990), that the magnetotail was more stable during conditions of large IMF |By|, requiring the magnetotail lobes and the polar cap to contain more flux to initiate a substorm compared to the situation when |By| was small.…”

Interplanetary Magnetic Field By Effects on the Strength and Latitude of Field‐Aligned Currents in Different Magnetic Local Time Sectors

“…Holappa et al. (2023) and Kubyshkina et al. (2023) adopted 1 hr averaged IMF components spanning from 1981 to 2019 to investigate the IMF B x effect on AL index.…”

Magnetic Perturbation Events (MPEs) that cause GICs: Investigating their Interhemispheric Conjugacy and Control by IMF Orientation

“…A subsequent study by Holappa et al (2021b) found that the substorm onset latitude and the isotropic boundary latitude of energetic protons were both ~1°l ower during IMF By > 3 conditions than for smaller By, and that the substorm occurrence frequency was larger for small By. They suggested, consistent with the results of a resistive MHD study by Hesse and Birn et al (1990), that the magnetotail was more stable during conditions of large IMF By, requiring the magnetotail lobes and the polar cap to contain more flux to initiate a substorm compared to the situation when By was small.…”