Low Latitude Geomagnetic Activity Using a Dst index Proxy

Geomagnetic storms are magnetic field variations caused by the interaction between strong solar wind conditions and the geodynamo. It has been known that these perturbations can cause problems in several modern infrastructure affecting power lines, communications, and GPS systems, among others.

The Disturbance Storm Time index (Dst index) measures the variation on the equatorial horizontal component of the geomagnetic field and characterizes the strength of the symmetric Ring Current (RC). The Dst index has been used to identify Geomagnetic Storms as the H component of the magnetic field decreases rapidly because of the RC enhancement during such events. The Dst index is calculated using the hourly values of the H component from 4 observatories; Kakioka (KAK), Japan; Hermanus (HER), South Africa; Honolulu (HON), Hawaii; and San Juan (SJG), Puerto Rico.

In the present study, we calculate a Dst proxy (Dstfuq) using data from an observatory near the equator (5° 24' 59.99" N), the Geomagnetic Observatory of Fuquene (FUQ), Colombia. We select 6 months (July 2004, May 2005, August 2008, October 2013, October 2014, and June 2015) with different geomagnetic activity; 2 severe storms (‑200 nT to ‑350 nT); 1 strong storm (‑100 nT to ‑200 nT); 1 moderate storm (‑50 nT to ‑100 nT); and 2 weak storms (‑30 nT to ‑50 nT or quiet).

To evaluate the reliability of our proxy we compare the Dstfuq with the Dst index, a latitude-dependent Dst index as well as a previously reported proxy from Ascension Island Observatory (ASC). During geomagnetic active months, all indices are well correlated (> 0.8), while quiet months are weakly correlated (0.5 to 0.7). On each month studied, we observe a fit between western Dst and Dstfuq proxies, suggesting a local time relation during the geomagnetic storms. On the other hand, during the quiet months, FUQ and ASC proxies present an irregular behavior compared to Dst, which may be due to its proximity to the magnetic equator.

We reproduce a high-quality Dst proxy that allows us to propose FUQ observations as an alternative to study the geomagnetic field that can be used to calculate proxies of geomagnetic indices taking advantage of its proximity to the equator. As future work, we plan to make a frequency analysis to understand the behavior during quiet time which may be related to the decrease of the correlation coefficient.