Main patterns of the geomagnetic field: A case study using principal component analysis Virginia Klausner, 1,2,3, ∗ Odim Mendes, 3, † Margarete Oliveira Domingues, 3, ‡ and Andres Reinaldo Rodriguez Papa 3, § 1 Universidade do Vale do Paraíba - UNIVAP 12244-000 São José dos Campos, SP, Brazil 2 National Institute for Space Research - INPE 12227-010 São José dos Campos, SP, Brazil 3 National Observatory - ON 20921-400, Rio de Janeiro, RJ, Brazil The horizontal magnetic components observed by ground-based observatories belonging to the INTERMAG- NET network have been used to analyze the global pattern of the geomagnetic field variation. The approach is based on the Principal Component Analysis method applied to magnetograms from 2000 to 2005. Quiet and disturbed days were geomagnetically distinguished. The pattern of the geomagnetic field variation fluctuates bewilderingly over time. In this work, we are interested in determining the geomagnetic field variability pre- senting in each principal component. The results suggest that the oscillation patterns of the first, second and third components could be explained respectively by the first one, two and three terms of the geomagnetic field spherical harmonic expansion. The principal components provide a meaningful way to appraise the overall space-time decomposition of the geomagnetic field. I. INTRODUCTION The geomagnetic field varies with space and time in a non- trivial way. As typical behavior, ground based magnetic mea- surements show a repetitive diurnal variation that corresponds to the geomagnetically quiet days. However, there are many varieties of irregular variations that occur over time, character- izing a disturbed field. Among the magnetic disturbances we have geomagnetic storms, so called by analogy with weather, which are of great interest because their effects in several hu- man activities [1]. The intensity of the geomagnetic disturbance in each day is described by indices. There are different indices that can be used depending on the character and the latitude influences in focus. Kp, AE and Dst and their derivations are the most used geomagnetic indices. The Kp index is a number from 0 to 9 obtained as the mean value of the disturbance levels within 3-h interval observed at 13 subauroral magnetic observatories. The minutely AE index (sometimes 2.5 minute interval) is ob- tained by the H-component measured from the magnetic ob- servatories located at auroral zones. The index most used in low and mid-latitudes is the Dst index. It represents the vari- ations of the H-component due to changes of the ring current, see [2] for more details. Ref. [3] analyzed the day-to-day variation of the geomag- netic field by using principal component analysis (PCA) to better understand the oscillations of the daily profile. He has found that these oscillations associated with principal compo- nents (PCs) were originated by ionospheric currents. This work aims to analyze global patterns in the geomag- netic variations measured on the ground from 2000 to 2005. In other words, we will show the main variations of the ge- omagnetic field by using PCA to better understand the oscil- lations of the global profile. Also, we propose to associated these oscillations to the main characteristics of the geomag- ∗ virginia@univap.br † odim@dge.inpe.br ‡ mo.domingues@lac.inpe.br § papa@on.br netic field. Several observatories were chosen according to their geographical location in order to obtain a good repre- sentation of the magnetic behavior, and also, to examine how the magnetic effects during quiet and disturbed periods (geo- magnetic storm) can affect the global space and time config- uration features of the geomagnetic field. The observatory of Vassouras, Rio de Janeiro, Brazil, was included in our study to explore the influence of the SAMA (South Atlantic Mag- netic Anomaly), and also, for better understanding of magne- tospheric processes in this region. The organization of this work is as follows: in Section II, a short description of the physics related to magnetic variations is presented; in Section III, the methodology is described; in Section IV, the magnetic dataset is presented; in Section V, the results are shown and discussed; and in Section VI, the conclusions are established. II. THE PHYSICAL PHENOMENON Under quiet solar wind conditions, a quasi-stationary mag- netosphere current system is established surrounding the Earth. Primary electrical current systems are structured and linked preserving the well-known magnetic morphology [4]. Nevertheless, under disturbed solar plasma conditions, the magnetosphere is modified and those systems of currents are altered. A great contribution to the magnetic variations comes from geomagnetic storms. The primary causes of geomagnetic storms at Earth are strong dawn-to-dusk electric fields asso- ciated with the passage of southward directed interplanetary fields, passing the Earth for sufficiently long intervals of time (more than 3 hours) and with significant intensity, larger than 10 nT. The solar wind energy transfer mechanism is the mag- netic reconnection between the interplanetary magnetic field and the Earth’s magnetic field [2]. The magnetic field is af- fected significantly by variations of the solar wind ram pres- sure, which produces changes in the magnetopause current. As a result of this process, there is an increase of the hori- zontal magnetic field component at mid-to-low latitudes, the so-called storm sudden commencement (SSC) [5]. The char- acteristic signature of a magnetic storm is a depression in Physicæ 11, 2015 1