Statistical survey of pitch angle anisotropy of relativistic electrons in the outer radiation belt and its variation with solar wind/geomagnetic activity

Introduction In this study, we use 7 years (2012–2019) of pitch angle resolved electron flux measurements from Van Allen Probe-B spacecraft to study the variation of near-equatorial pitch angle distributions (PADs) of outer radiation belt (L <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m1"><mml:mrow><mml:mo>≥</mml:mo></mml:mrow></mml:math> 3) relativistic electrons (E <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m2"><mml:mrow><mml:mo>≥</mml:mo></mml:mrow></mml:math> 0.5 MeV) with different levels of geomagnetic activity. Methods We calculate a pitch angle anisotropy index (PAI) to categorize the PADs into three types: pancake, PAI <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m3"><mml:mrow><mml:mo>≥</mml:mo></mml:mrow></mml:math> 1.05; butterfly, PAI <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m4"><mml:mrow><mml:mo>≤</mml:mo></mml:mrow></mml:math> 0.95; and flattop, 0.95 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m5"><mml:mrow><mml:mo>&lt;</mml:mo></mml:mrow></mml:math> PAI <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m6"><mml:mrow><mml:mo>&lt;</mml:mo></mml:mrow></mml:math> 1.05. Results and Discussion Our statistical results show that L shells <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m7"><mml:mrow><mml:mo>≥</mml:mo></mml:mrow></mml:math> 5 are dominated by pancake PADs on the dayside (9 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m8"><mml:mrow><mml:mo>&lt;</mml:mo></mml:mrow></mml:math> MLT <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m9"><mml:mrow><mml:mo>&lt;</mml:mo></mml:mrow></mml:math> 15), butterfly PADs on the nightside (21 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m10"><mml:mrow><mml:mo>&lt;</mml:mo></mml:mrow></mml:math> MLT <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m11"><mml:mrow><mml:mo>&lt;</mml:mo></mml:mrow></mml:math> 3), and flattop PADs in the dawn (3 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m12"><mml:mrow><mml:mo>&lt;</mml:mo></mml:mrow></mml:math> MLT <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m13"><mml:mrow><mml:mo>&lt;</mml:mo></mml:mrow></mml:math> 9) and dusk (15 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m14"><mml:mrow><mml:mo>&lt;</mml:mo></mml:mrow></mml:math> MLT <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m15"><mml:mrow><mml:mo>&lt;</mml:mo></mml:mrow></mml:math> 21) sectors, across almost all relativistic energies. In the inner L shells, the pancake and flattop PADs exhibit dependence on both L-shell and energy, with the occurrence rate increasing with decreasing L and increasing energy. For the butterfly PADs, we discovered a second population of low-L butterflies that are present at almost all local times. When the variation of electron PAI is compared with solar wind dynamic pressure <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m16"><mml:mrow><mml:msub><mml:mrow><mml:mtext>P</mml:mtext></mml:mrow><mml:mrow><mml:mi mathvariant="italic">dyn</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> and geomagnetic indices SYM-H and AL, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m17"><mml:mrow><mml:msub><mml:mrow><mml:mtext>P</mml:mtext></mml:mrow><mml:mrow><mml:mi mathvariant="italic">dyn</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> is found to be the dominant parameter in driving the outer radiation belt pitch angle anisotropy. During periods of enhanced <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m18"><mml:mrow><mml:msub><mml:mrow><mml:mtext>P</mml:mtext></mml:mrow><mml:mrow><mml:mi mathvariant="italic">dyn</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> , pancake PADs on the dayside become more <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m19"><mml:mrow><mml:mn>90</mml:mn><mml:mo>°</mml:mo></mml:mrow></mml:math> -peaked, butterfly PADs on the nightside exhibit enhanced flux dips around <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m20"><mml:mrow><mml:mn>90</mml:mn><mml:mo>°</mml:mo></mml:mrow></mml:math> pitch angle along with an enhanced azimuthal and radial extent, and flattop PADs turn into either pancake or butterfly PADs.