Saturation of superstorms and finite compressibility of the magnetosphere: Results of the magnetogram inversion technique and global PPMLR‐MHD model

Abstract

We report on novel features of the saturation process of the polar cap magnetic flux and Poynting flux into the magnetosphere from the solar wind during three superstorms. In addition to the well‐known effect of the interplanetary electric (Esw) and southward magnetic (interplanetary magnetic field (IMF) Bz) fields, we found that the saturation depends also on the solar wind ram pressure Pd. By means of the magnetogram inversion technique and a global MHD numerical model Piecewise Parabolic Method with a Lagrangian Remap, we explore the dependence of the magnetopause standoff distance on ram pressure and the southward IMF. Unlike earlier studies, in the considered superstorms both Pd and Bz achieve extreme values. As a result, we show that the compression rate of the dayside magnetosphere decreases with increasing Pd and the southward Bz, approaching very small values for extreme Pd ≥ 15 nPa and Bz ≤ −40 nT. This dependence suggests that finite compressibility of the magnetosphere controls saturation of superstorms.

Dynamics for geomagnetic pulsations, field-aligned currents, and airglow at mid-latitudes within substorm activations during superstorms

“…The 2003 Nov 20 superstorm SSC caused by arrival of a powerful interplanetary magnetic cloud was recorded at 08:03 UT. An extended Dst-index minimum (with a peak value = -422 nТ) was observed within 18-21 UT (see reference in [Mishin et al, 2016]). In both superstorms, one observed several substorm activations [Huttunen et al, 2002;Baishev et al, 2008].…”

“…The description of the analyzed events from the ground-based and satellite geomagnetic and optical observations is presented in a number of papers [Huttunen et al, 2002;Mishin et al, 2010;Mikhalev et al, 2004;Ebihara et al, 2005;Hairston et al, 2005;Hori et al, 2006;Mishin et al, 2007;Solovyev et al, 2008;Karavaev et al, 2009;Valladares et al, 2015;Mishin et al, 2016;Mishin, Karavaev, 2017;Mishin et al, 2018].…”

Universal Time variations in the magnetosphere

“…To put these lobe flux changes in context we can compare with estimates of the polar cap flux F PC , which is equal to the tail lobe flux plus the relatively small open flux that threads the dayside magnetopause (Lockwood et al, 2021) in the literature vary between about 0.1 GWb and 1.2 GWb. The lowest estimate that we know of is 0.08 GWb during a northward-IMF "horse collar" aurora event (Wang et al, 2023) and it has been argued that F PC saturates at 1.2 GWb during major geomagnetic storms (Mishin and Karavaev, 2017). Kamide et al (1977), Boakes et al (2009) and Milan et al (2008) find that substorm onset becomes more likely when F PC is increased: Boakes et al (2009) found the probability of an onset to be negligible for F PC below 0.3 GWb but above this value increased linearly with F PC .…”

“…We take the flux in the tail lobe to be close to the polar cap flux F PC (i.e., we neglect the open flux threading the dayside magnetopause). There will certainly be other factors, but to illustrate the concept we here take the probability of an onset occurring to increase linearly with the average lobe flux F M from 0 at (and below) F M = 0.3 GWb and reach unity at F M = 1.2 GWb, which is postulated to be the largest value that F PC can attain (Mishin and Karavaev, 2017). Note that lowering the assumed initial lobe flux value of 0.1 GWb increases the average length of the growth phases but does not influence the distribution of onset UTs because the start UT values are randomly chosen: the same is true for increased nightside tail reconnection voltage Φ N that removes lobe flux.…”

Saturation of the magnetosphere during superstorms: new results from magnetogram inversion technique

“…Кроме того, в методе ТИМ рассчитывается поток вектора Пойнтинга как функция переменного маг-нитного потока . Подчеркнем, что мы используем получаемую ТИМ поверхность полярной шапки, а не предполагаемую, но не изме-ряемую на спутниках длину области пересоедине-ния l 0 на магнитопаузе, как это часто делается при расчете потока Пойнтинга ε методом, приведенном в [Perreault, Akasofu, 1978].…”