Abstract
Space weather indices introduced for scientific purposes are commonly used to quantify operational nowcast of the geospace state during extreme space weather events. Some indices, such as the Disturbance storm time (Dst) index, have been applied to situations for which they are not originally intended. This raises a question about suitability as a space weather benchmark. In analysing historical records for different magnetometers at low- and mid-latitude, we find periods with longitudinal asymmetry in magnetic response that suggest important signals from individual magnetometers are being averaged out of the Dst record. This asymmetry develops as a double spike in the H-component: one negative in the observatories in the day sector and one positive in the observatories in the night sector. These spikes develop in short-time (about 2 h) and pose a potential hazardous effect for users affected by space weather. The results from historical events have been reinforced with the systematic study of magnetic records during extreme events (Dst ≤ −200 nT and AL ≤ −2000 nT) in the period 1998–2017 from six magnetic observatories at about 40° magnetic latitude. Moreover, we show that the largest asymmetries take place during the early main phase and are recorded in narrow local time sectors. An important outcome of these results is that space weather benchmarks should be based on local records instead of the commonly used global indices. This action improves two important aspects of space weather: the assessment of historical extreme events and that of the needs of users.
Highlights
The relevance of space weather as a natural hazard is undeniable as it can damage critical infrastructure and technology systems
Columns 4–7 in Table 4 display the values of Positive Residual Envelope (PRE) and Negative Residual Envelope (NRE) for that date and the Magnetic Local Time (MLT) of the observatories whose Robs,i coincides with PRE and NRE at the date in column (3)
These results indicate that these extreme events exhibit extreme longitudinal asymmetry and, as a first consequence, local records are crucial and cannot be disregarded when using the extreme events as benchmarks
Summary
The relevance of space weather as a natural hazard is undeniable as it can damage critical infrastructure and technology systems. Kawasaki et al (1973) noted the large growth of the longitudinally asymmetric disturbance (ASY) index following the storm sudden commencement (SSC) at 20:54 UT on 4 August, caused by an abrupt decrease of the H-component at stations in the late morning sector They indicated that similar unusual feature had been reported by Fukushima & Abe (1958) and Akasofu & Chapman (1960) during great magnetic storms, they suspected that the complexity of the low-latitude records might have been introduced by timing errors. In this paper we are interested in answering the questions raised above For this purpose our study is focused on the extreme longitudinal asymmetry, using the terminology by Love et al (2019b), or on the spike-like magnetic disturbances, using the terminology by Saiz et al (2016) searching, observed in low- to mid-latitude records of the H-component during extreme events.
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