Magnetohydrodynamic simulations of transient transpolar potential responses to solar wind density changes

Abstract

[1] Magnetohydrodynamic (MHD) simulations are used to examine the response of the transpolar potential (F TP ) to changes in the solar wind density during periods of constant solar wind electric field. For increases (decreases) in the solar wind density F TP responds immediately by increasing (decreasing) from the steady state values. In both cases the response of F TP is transient, decaying to near initial steady state values even when the density change persists. The magnitude of the F TP response is proportional to both the rate of erosion of the dayside magnetopause and the ionospheric Pedersen conductance. In our MHD simulations F TP is driven entirely by the dayside merging rate and is insensitive to changes in the nightside reconnection rate. The observed relationship between the modeled dayside merging rate and F TP is well characterized by an L-R circuit equation derived from integrating Faraday's Law around the Region 1 current loop. The inductive time constant for variations in the transpolar potential was found to be 6.5 (13) minutes for simulations using ionospheric Pedersen conductances of 6 (12) mhos. This corresponds in both cases to a magnetosphere-ionosphere inductance of 65 Henries. Observations of the transpolar potential derived using the assimilative mapping of ionospheric electrodynamics (AMIE) model are presented and shown to be consistent with the simulation results.

Balanced reconnection intervals: four case studies

“…Since both the solar wind velocity and the IMF B z remain steady during the pressure pulse, it is the increase in density that causes both the Alfvénic Mach number and the solar wind Beta to increase. This data is in agreement with Ober et al (2007) who show that an increase in the solar wind density of this type (with other parameters held constant) can cause the open flux in the polar cap to increase. However, Boudouridis et al (2003Boudouridis et al ( , 2004Boudouridis et al ( , 2005 have demonstrated that, when a pressure pulse interacts with the magnetosphere, the auroral will tend to move poleward.…”

“…The IMF B z for all of the events is less than −5 nT. This differs from previous studies that state that steady magnetospheric convection usually occurs when 0 nT>B z ≥−5 nT (Sergeev et al, 1996;O'Brien et al, 2002;Tanskanen et al, 2005). Event 1 has a B z close to −6 nT and Event 2 starts with a B z of −5 nT before it starts to turn northward.…”

“…Tanskanen et al (2005) used Geotail data to study loading and unloading phases along with SMCs. Their analysis supported O'Brien et al (2002) and Sergeev et al (1996), finding that SMCs are more likely to occur when 0 nT>IMF B z ≥−5 nT. Furthermore, approximately half of their 28 SMCs had bursty bulk flows (BBF) in the tail.…”

“…While this is beyond the scope of this study, it is recognized that this is quite important for identifying future BRI events. The 51 events that have been identified can be compared to other techniques used for determining SMCs/BRIs, such as those discussed by O'Brien et al (2002). Further, other techniques must be investigated, such as comparing the dayside and nightside electric fields produced from the AMIE technique or SuperDARN radars, to approximate the reconnection rates.…”

“…While these investigations illuminated many features of steady magnetospheric convection, there has been a lack of statistical analysis of SMCs. O'Brien et al (2002) presented a large scale statistical study of SMCs, but their selection criteria was only that AL(t)-AL(t-1 min)≤−25 nT. This led them to find SMCs that occured during weaker periods of geomagnetic activity.…”