Relation between substorm characteristics and rapid temporal variations of the ground magnetic field

Abstract

Abstract. Auroral substorms are one of the major causes of large geomagnetically induced currents (GIC) in technological systems. This study deals with different phases of the auroral substorm concerning their severity from the GIC viewpoint. Our database consists of 833 substorms observed by the IMAGE magnetometer network in 1997 (around sunspot minimum) and 1999 (rising phase of the sunspot cycle), divided into two classes according to the D st index: non-storm (D st >-40 nT, 696 events) and storm-time ones (D st <-40 nT, 137 events). The key quantity concerning GIC is the time derivative of the horizontal magnetic field vector (dH /dt) whose largest values during substorms occur most probably at about 5 min after the onset at stations with CGM latitude less than 72 deg. When looking at the median time of the occurrence of the maximum dH /dt after the expansion onset, it increases as a function of latitude from about 15 min at CGM lat=56 deg to about 45 min at CGM lat=75 deg for non-storm substorms. For storm-time events, these times are about 5 min longer. Based on calculated ionospheric equivalent currents, large dH /dt occur mostly during the substorm onset when the amplitude of the westward electrojet increases rapidly.

Characteristics of variations in the ground magnetic field during substorms at mid latitudes

“…Since the peak time for substorm onset to occur is at 23:00 MLT, the list by Frey et al (2004) Figure 1 shows the locations of SAMNET stations and substorm onsets included in this study. The substorm onsets are divided into those that occur during storm time conditions (D st <−40 nT, 83 subtorms) and non-storm time conditions (D st >−40 nT, 470 substorms), the cut off value of −40 nT is consistent with Viljanen et al (2006). The D st used is the daily mean value of D st for the day at which the substorm occurs taken from NOAA National Geophysical Data Center (NGDC, 2009).…”

“…Studies of the characteristics of ground magnetic field variations have tended to focus on higher latitude areas such as Fennoscandia, by looking at the time derivative of the horizontal ground magnetic field, dH /dt (Viljanen et al, 2001). In particular, a study by Viljanen et al (2006) on the relationship between substorms and H , using the International Monitor for Auroral Geomagnetic Effects (IMAGE) network concludes that the largest values of dH /dt appear soon after the expansion phase begins, although many events have larger values at later times. The size of max|dH /dt| increases with increasing latitude equatorward of the auroral oval, poleward of which it decreases.…”

“…This is done by exploiting magnetometer data, including mid latitude magnetometer sites, to analyse the response of the ground magnetic field to substorms, however a method other than ground magnetometry is used to identify substorm onsets. This study also uses ground magnetometer measurements recorded at a higher sampling rate than the Viljanen et al (2006) study and use data from a different period in the solar cycle. The results of this study will aid GIC modelling in the UK's power transmission network.…”

“…It was also found that although H is greatly influenced by the electrojet, dH /dt is more susceptible to the effects of smaller scale ionospheric features. The events looked at by Viljanen et al (2006) occurred just after sunspot minimum and magnetometer data were exploited to detect the substorm onset and analyse H , between magnetic latitudes 56 • N and 75 • N.…”

Climatology of rapid geomagnetic variations at high latitudes over two solar cycles

“…However, the directional distributions of dH /dt are much more scattered than those of the simultaneous baseline subtracted horizontal variation field vector H . Another study restricted to 833 selected substorms in 1997 and 1999, before a solar maximum, showed a similar behaviour (Viljanen et al, 2006). Turnbull et al (2009) performed a corresponding investigation for 553 substorms at mid latitudes in 2000-2003 just after a solar maximum.…”