The Semiannual Variation in the Van-Allen probes data

POBLET FACUNDO; AZPILICUETA FRANCISCO

Buenos Aires

Workshop; COLAGE XI - 2018; 2018

COLAGE

p { margin-bottom: 0.25cm; direction: ltr; color: rgb(0, 0, 10); line-height: 120%; text-align: left; }p.western { font-family: "Liberation Serif",serif; font-size: 12pt; }p.cjk { font-family: "Tahoma"; font-size: 12pt; }p.ctl { font-family: "Lohit Devanagari"; font-size: 12pt; }a:link { }Thesemiannual variation is one of the most recurrent variations in theEarth?s magnetic activity and can be detected in many parameterse.g., geomagnetic storm annual distribution [Echer et al., 2011]. Itis characterized by maximums around the equinoxes and minimums nearthe solstices. Over the decades, several explanations have beenproposed, being the main ones the axial hypothesis, the equinoctialhypothesis and the Russell-McPherron effect (R-M) (Cortie, 1912;Bartels, 1932; McIntosh, 1959; Svalgaard, 1977; Russell andMcPherron, 1973). These three hypotheses consider the geometricalchanges along the year between the solar wind parameters and theEarth?s dipole as the main responsible of the changes in themagnetospheric energization that led to the semiannual behaviordetected in the surface of the Earth. However, the semiannualvariation has not been fully explored in the magnetospheric particlepopulations itself to the present days. Inthis work we investigate the semiannual pattern in the radiationbelts utilizing data from the Van Allen probes mission. The highlyelliptic and near equatorial orbit of the RBSPaand RBSPb probesallows the study of charged particle fluxes over a wide range of Lvalues. The DOY-L figures for the daily averaged fluxes at 2-6 MeVshow a 27-day pattern as the most noticeable characteristic in everyyear. When a 27-day running average is calculated, the semiannualmaximums come up but lagged in time with respect of the equinoxes.The maximums are centered around L = 4.TheDOY-UT figures of the controlling parameters for the three theoriesmentioned, i.e, the heliographic latitude of the Earth in the axialhypothesis, the angle between Earth-Sun line and the dipole axis ofthe Earth in the equinoctial hypothesis and the angle between Z axisin GSM coordinate system and Y axis in GSEQ system in the R-Mhypothesis are often utilized to associate a semiannual pattern in agiven parameter to a specific theory. A comparison with the DOY-UTfigure of the semiannual pattern found in the Van Allen belts couldserve in identifying which of three is the dominant mechanism behind.