Storm time polar cap expansion: interplanetary magnetic field clock angle dependence

Abstract

Abstract. It is well known that the polar cap, delineated by the open–closed field line boundary (OCB), responds to changes in the interplanetary magnetic field (IMF). In general, the boundary moves equatorward when the IMF turns southward and contracts poleward when the IMF turns northward. However, observations of the OCB are spotty and limited in local time, making more detailed studies of its IMF dependence difficult. Here, we simulate five solar storm periods with the coupled model consisting of the Open Geospace General Circulation Model (OpenGGCM) coupled with the Coupled Thermosphere Ionosphere Model (CTIM) and the Rice Convection Model (RCM), i.e., the OpenGGCM-CTIM-RCM, to estimate the location and dynamics of the OCB. For these events, polar cap boundary location observations are also obtained from Defense Meteorological Satellite Program (DMSP) precipitation spectrograms and compared with the model output. There is a large scatter in the DMSP observations and in the model output. Although the model does not predict the OCB with high fidelity for every observation, it does reproduce the general trend as a function of IMF clock angle. On average, the model overestimates the latitude of the open–closed field line boundary by 1.61∘. Additional analysis of the simulated polar cap boundary dynamics across all local times shows that the MLT of the largest polar cap expansion closely correlates with the IMF clock angle, that the strongest correlation occurs when the IMF is southward, that during strong southward IMF the polar cap shifts sunward, and that the polar cap rapidly contracts at all local times when the IMF turns northward.

Electron Aurora and Polar Rain Dependencies on Solar Wind Parameters

“…In Figure 4, a clear dependence can be observed between the north‐south component of the IMF and the shapes of both the oval and the polar cap. The polar cap area is delimited by the open‐closed field line boundary (OCB), and the B z component of the IMF is often responsible for magnetic reconnections that impact the OCB's shape (Tulegenov et al., 2023). As demonstrated here, a large southward component of the IMF generally causes the boundary to move equatorward, while a northward component moves the boundary poleward (Tulegenov et al., 2023).…”

“…For the case of IMF B z > 0, it appears that an electron flux also increases with increasing B z perhaps for the same reason, but there could be other reasons as well. An increase in B z can reduce the polar cap size (open‐closed boundary moves to higher latitude) (Milan et al., 2004; Newell et al., 1997; Tulegenov et al., 2023). Moreover, an increase in IMF B z can also increase the occurrence of the polar cap arcs, which could be considered an extension of the auroral oval and which have higher fluxes than polar rain (Newell et al., 1997; Troshichev et al., 1988).…”

“…The polar cap area is delimited by the open-closed field line boundary (OCB), and the B z component of the IMF is often responsible for magnetic reconnections that impact the OCB's shape (Tulegenov et al, 2023). As demonstrated here, a large southward component of the IMF generally causes the boundary to move equatorward, while a northward component moves the boundary poleward (Tulegenov et al, 2023). Moreover, for positive B z , a boundary layer can form poleward of the cusp (Shi et al, 2013(Shi et al, , 2009, which can shift the poleward edge of the oval to higher latitudes.…”

Universal Time Variations in the Magnetosphere and the Effect of CME Arrival Time: Analysis of the February 2022 Event that Led to the Loss of Starlink Satellites

“…Note that subtle changes in the shape of the open polar cap are a key part of understanding the pattern of ionospheric convection, as predicted by the ECPC model (Cowley & Lockwood, 1992; Lockwood & Cowley, 2022; Lockwood et al., 1990; Lockwood, Lanchester, et al., 2006; Lockwood & Morley, 2004; Tulegenov et al., 2023), particularly in determining the pattern of flow following a burst in either the magnetopause or the tail reconnection voltages. However, the distortions to the boundary, and the flow patterns associated with them, propagate around the boundary (Morley & Lockwood, 2005).…”

Universal Time Effects on Substorm Growth Phases and Onsets

“…The factors f D and f N are the fractions of reconnection voltages (Φ D and Φ N , respectively) placed across the maximum diameter of the polar cap. These factors depend upon the shape of the polar cap and how it is changing: for the approximation of a polar cap that remains circular at all times f D = f N = 0.5 (Lockwood, 1993) but in general the polar cap boundary shape is always evolving (Tulegenov et al., 2023) and so the factors f D and f N are not constant.…”