AE, D ST and their SuperMAG Counterparts : the effect of improved spatial resolution in geomagnetic indices

For decades, geomagnetic indices have been used extensively to parameterize space weather events, as input to various models and as space weather specifications. The auroral electrojet (AE) index and disturbance storm time index (DST) are two such indices that span multiple solar cycles and have been widely studied. The production of improved spatial coverage analogs to AE and DST is now possible using the SuperMAG collaboration of ground‐based magnetometers. SME is an electrojet index that shares methodology with AE. SMR is a ring current index that shares methodology with DST. As the number of magnetometer stations in the SuperMAG network increases over time, so does the spatial resolution of SME and SMR. Our statistical comparison between the established indices and their new SuperMAG counterparts finds that, for large excursions in geomagnetic activity, AE systematically underestimates SME for later cycles. The difference between distributions of recorded AE and SME values for a single solar maximum can be of the same order as changes in activity seen from one solar cycle to the next. We demonstrate that DST and SMR track each other but are subject to an approximate linear shift as a result of the procedure used to map stations to the magnetic equator. We explain the observed differences between AE and SME with the assistance of a simple model, based on the construction methodology of the electrojet indices. We show that in the case of AE and SME, it is not possible to simply translate between the two indices

Characteristics of the terrestrial field-aligned current system

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 inter-spacecraft 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 <I>B</I>_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 Region2 are quasi-persistently significant in the transport of energy and momentum between the magnetosphere and the ionosphere

On the Persistent Shape and Coherence of Pulsating Auroral Patches

The pulsating aurora covers a broad range of fluctuating shapes that are poorly characterized. The purpose of this paper is therefore to provide objective and quantitative measures of the extent to which pulsating auroral patches maintain their shape, drift and fluctuate in a coherent fashion. We present results from a careful analysis of pulsating auroral patches using all-sky cameras. We have identified four well-defined individual patches that we follow in the patch frame of reference. In this way we avoid the space-time ambiguity which complicates rocket and satellite measurements. We find that the shape of the patches is remarkably persistent with 85-100% of the patch being repeated for 4.5-8.5 min. Each of the three largest patches has a temporal correlation with a negative dependence on distance, and thus does not fluctuate in a coherent fashion. A time-delayed response within the patches indicates that the so-called streaming mode might explain the incoherency. The patches appear to drift differently from the SuperDARN-determined $\stackrel{\rightarrow}{E}$ X $\stackrel{\rightarrow}{B}$ convection velocity. However, in a nonrotating reference frame the patches drift with 230-287 m/s in a north eastward direction, which is what typically could be expected for the convection return flow

Characterising the ionospheric current pattern response to southward and northward IMF turnings with dynamical SuperMAG correlation networks

We characterize the response of the quiet time (no substorms or storms) large-scale ionospheric transient equivalent currents to north-south and south-north IMF turnings by using a dynamical network of ground-based magnetometers. Canonical correlation between all pairs of SuperMAG magnetometer stations in the Northern Hemisphere (magnetic latitude (MLAT) 50–82°) is used to establish the extent of near-simultaneous magnetic response between regions of magnetic local time-MLAT. Parameters and maps that describe spatial-temporal correlation are used to characterize the system and its response to the turnings aggregated over several hundred events. We find that regions that experience large increases in correlation post turning coincide with typical locations of a two-cell convection system and are influenced by the interplanetary magnetic field By. The time between the turnings reaching the magnetopause and a network response is found to be ∼8–10 min and correlation in the dayside occurs 2–8 min before that in the nightside

Effects of solar wind density on auroral electrojets and brightness under influence of substorms

Using the auroral electrojet indices and Polar Ultraviolet Imager auroral images, we examined two fortuitous events during which the solar wind density had clear enhancements while the other solar wind parameters were relatively constant. Two electrojet enhancements were found in each event. The first electrojet enhancement was likely to be related to a substorm in which an auroral bulge appeared at premidnight. The second electrojet enhancement was driven by the density enhancement in the solar wind. The auroral oval became wider in latitude and the auroral distribution became dispersed after the density enhancement arrived at the Earth. The total auroral power integrated over the entire nightside region from 50 to 80&amp;deg; MLAT, however, did not increase significantly in response to the density enhancement. Our interpretation is that the substorm that occurred prior to the solar wind density enhancement had drained out a significant portion of the stored energy in the magnetotail; therefore, less precipitation energy was deposited into the auroral ionosphere by the density enhancement

Network analysis of geomagnetic substorms using the SuperMAG database of ground based magnetometer stations

The overall morphology and dynamics of magnetospheric substorms is well established in terms of the observed qualitative auroral features seen in ground-based magnetometers. This paper focuses on the quantitative characterization of substorm dynamics captured by ground-based magnetometer stations. We present the first analysis of substorms using dynamical networks obtained from the full available set of ground-based magnetometer observations in the Northern Hemisphere. The stations are connected in the network when the correlation between the vector magnetometer time series from pairs of stations within a running time window exceeds a threshold. Dimensionless parameters can then be obtained that characterize the network and by extension, the spatiotemporal dynamics of the substorm under observation. We analyze four isolated substorm test cases as well as a steady magnetic convection (SMC) event and a day in which no substorms occur. These test case substorms are found to give a consistent characteristic network response at onset in terms of their spatial correlation. Such responses are differentiable from responses to the SMC event and nonsubstorm times. We present a method to optimize network parametrization with respect to the different individual station responses, the spatial inhomogeneity of stations in the Northern Hemisphere, and the choice of correlation window sizes. Our results suggest that dynamical network analysis has potential to quantitatively categorize substorms.Dimensionless parameters can then be obtained that characterise the network and by extension, the spatio-temporal dynamics of the substorm un der observation. We analyse four isolated substorm test cases as well as a steady magnetic convection (SMC) event and a day in which no substorms occur. These test case substorms are found to give a consistent characteristic net work response at onset in terms of their spatial correlation. Such responses are differentiable from responses to the SMC event and non-substorm times. We present a method to optimise network parametrisation with respect to the different individual station responses, the spatial inhomogeneity of stations in the northern hemisphere and the choice of correlation window sizes. Our results suggest that dynamical network analysis has potential to quantitatively categorise substorms