Characteristics of the terrestrial field-aligned current system

Abstract

Abstract. We present the first ever comprehensive statistical study of the spatiotemporal characteristics of field-aligned currents in the terrestrial magnetosphere-ionosphere system using multi point measurements. We determine how the FAC density, variability and scale size are coupled. The three ST 5 satellites were in a pearls-on-a-string formation making measurements of the magnetic field with variable interspacecraft separations ranging from a few seconds to about 10 min. More than 4700 sets of satellite passes are analyzed using a robust correlation analysis aimed at determining the variability of the FAC system as a function of scale size and satellite spacing. We find significant differences between the FAC characteristics on the dayside and on the nightside in terms of dynamics of the current systems. On the dayside the FAC characteristics are found to be independent of IMF B z and geomagnetic activity while the nightside indicates increased variability during disturbed conditions. The boundary separating highly and poorly correlated FACs can be fitted by a linear line for satellite separations shorter than 60 s (dayside) and 160 s (nightside). We interpret this as the dayside and nightside magnetospheric reconfiguration times respectively. For times exceeding this the FAC characteristics are suggested to be controlled by the solar wind (dayside) and plasma sheet (nightside) dynamics. Finally, the characteristics of FAC system with scale sizes larger than ∼200 km (at ionospheric altitude) appear to be stable and repeatable on time scales of the order of a minute (i.e. comparable to the low-altitude orbiting satellite's traverse time across the auroral belt). In this sense, our results effectively validate the Iijima and Potemra (1978) assumption that on average the large-scale currents with scale sizes of the Region1 and Correspondence to: J. W. Gjerloev (jesper.gjerloev@ift.uib.no) Region2 are quasi-persistently significant in the transport of energy and momentum between the magnetosphere and the ionosphere.

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“…For example, Gjerloev et al [2011] concluded that large-scale field-aligned currents dominate energy transfer based on statistics of 3 month data from the three ST-5 spacecraft in a string-of-pearls constellation. For example, Gjerloev et al [2011] concluded that large-scale field-aligned currents dominate energy transfer based on statistics of 3 month data from the three ST-5 spacecraft in a string-of-pearls constellation.…”

“…For example, Gjerloev et al [2011] concluded that large-scale field-aligned currents dominate energy transfer based on statistics of 3 month data from the three ST-5 spacecraft in a string-of-pearls constellation. If events such as the one presented are common, analyses such as Gjerloev et al [2011] may be missing an entire regime of large-amplitude signal due to undersampling. Figure 19 (bottom) is smoothed with a 3 s running mean to approximate the effect of averaging at the 3 s spin-rate used in Gjerloev et al [2011].…”

“…This would partially restore the string-of-pearls configuration from the early Swarm mission, which has demonstrated utility for resolving temporal evolution of plasma structures [e.g., Spicher et al, 2015;Goodwin et al, 2015], and statistical characterization of the Earthˈs field-aligned current systems [Gjerloev et al, 2011]. Varying the cross-track separation of pairs a-b and c-d would allow the spatial scales of field-aligned currents to be studied by tracking the correlation in measured magnetic field perturbation as a function of cross-track separation.…”

“…(bottom) Same but smoothed with a 3 s running mean to approximate the spin-averaging used inGjerloev et al [2011]. Data from Swarm C is lagged by 12 s to account for along-track separation and angle of attack.…”

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“…This is consistent with the equatorward edge of the R1 current and a region in the ionosphere where R1 and R2 close. The FAC system has typical temporal variability scales of 40-60 s on the dayside and~90-140 s on the nightside (Gjerloev et al 2011). Thus, IT modeling based on models of FAC may not be able to capture the short-time and smallscale variability of the Joule heating patterns.…”