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Discussion papers | Copyright
https://doi.org/10.5194/nhess-2018-92
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Apr 2018

Research article | 12 Apr 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Natural Hazards and Earth System Sciences (NHESS).

Defining scale thresholds for geomagnetic storms through statistics

Judith Palacios, Antonio Guerrero, Consuelo Cid, Elena Saiz, and Yolanda Cerrato Judith Palacios et al.
  • Space Weather Group, Departamento de Física y Matemáticas, Universidad de Alcalá, Alcalá de Henares, Spain

Abstract. Geomagnetic storms, as part of the Sun-Earth relations, are continuously monitored with different indices and scales. These indices usually have some associated scale thresholds to quantify the severity or risk of geomagnetic disturbances. However, the most usual scale thresholds are arbitrarily chosen. In this work we aim to quantify the range of the thresholds through a new method. These new thresholds are based on statistical distribution fitting.

We used different geomagnetic indices, as Dst, SYM-H, and Kp, since they are relevant for space weather purposes. The first two indices have been discriminated between their negative values and the whole dataset. We considered two periods: a short-term one, comprising data from 1997 to 2012; and long-term ones, which are from 1957–2012 for Dst and 1932–2012 for Kp.

We look for the best fit for different statistical continuous distributions applied to these indices. The best fits and the data distribution functions yield to intersects that can be used to define thresholds. The best fit distribution functions are more coincidental between them when considering determined similar datasets, as non-central f-distribution for negative values, meaningful for geomagnetic disturbances; or non-central Student's-t, when the whole dataset is taken. The method yields different values for thresholds depending on the index. Thresholds for geomagnetic storms can be chosen by common values of SYM-H and Dst, as −75nT for moderate storms; −150nT for intense storms, and −330nT for extreme storms. For the case of Kp, the value equal to 5 may mark the departure from quiet time to stormy time.

The obtained values are close to those usually considered as thresholds for, typically, Dst and Kp; therefore the thresholds defined here may provide criteria to assess the vulnerability to geomagnetic activity on design or mitigation purposes.

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