A new ionospheric storm scale based on TEC and <i>f_oF</i>₂ statistics

<jats:title>Abstract</jats:title><jats:p>In this paper, we propose the I‐scale, a new ionospheric storm scale for general users in various regions in the world. With the I‐scale, ionospheric storms can be classified at any season, local time, and location. Since the ionospheric condition largely depends on many factors such as solar irradiance, energy input from the magnetosphere, and lower atmospheric activity, it had been difficult to scale ionospheric storms, which are mainly caused by solar and geomagnetic activities. In this study, statistical analysis was carried out for total electron content (TEC) and <jats:italic>F</jats:italic><jats:sub>2</jats:sub> layer critical frequency (<jats:italic>f<jats:sub>o</jats:sub>F</jats:italic><jats:sub>2</jats:sub>) in Japan for 18 years from 1997 to 2014. Seasonal, local time, and latitudinal dependences of TEC and <jats:italic>f<jats:sub>o</jats:sub>F</jats:italic><jats:sub>2</jats:sub> variabilities are excluded by normalizing each percentage variation using their statistical standard deviations. The I‐scale is defined by setting thresholds to the normalized numbers to seven categories: I0, I<jats:sub>P</jats:sub>1, I<jats:sub>P</jats:sub>2, I<jats:sub>P</jats:sub>3, I<jats:sub>N</jats:sub>1, I<jats:sub>N</jats:sub>2, and I<jats:sub>N</jats:sub>3. I0 represents a quiet state, and I<jats:sub>P</jats:sub>1 (I<jats:sub>N</jats:sub>1), I<jats:sub>P</jats:sub>2 (I<jats:sub>N</jats:sub>2), and I<jats:sub>P</jats:sub>3 (I<jats:sub>N</jats:sub>3) represent moderate, strong, and severe positive (negative) storms, respectively. The proposed I‐scale can be used for other locations, such as polar and equatorial regions. It is considered that the proposed I‐scale can be a standardized scale to help the users to assess the impact of space weather on their systems.</jats:p>