Physics of auroral phenomena : proceedings of the 40th annual seminar, Apatity, 13-17 March, 2017 / [ed. board: N. V. Semenova, A. G. Yahnin]. - Апатиты : Издательство Кольского научного центра РАН, 2017. - 143 с. : ил., табл.

“Physics o f Auroral Phenomena”, Proc. XL Annual Seminar, Apatity, pp. 58-61, 2017 © Polar Geophysical Institute, 2017 Polar Geophysical Institute THEORY OF A RECEIVING ANTENNA APPLIED TO THE SPACECRAFT OBSERVATIONS OF QUASI-ELECTROSTATIC WHISTLER MODE WAVES E.A. Shirokov1, A.G. Demekhov1,2, Yu.V. Chugunov1,3’4, A.V. Larchenko2 1 Institute o f Applied Physics RAS, Nizhny Novgorod, Russia 2Polar Geophysical Institute, Apatity, Russia 3 Lobachevsky State University o fNizhny Novgorod, Nizhny Novgorod, Russia 4 Deceased 24 August 2016 e-mail: evshirok@gmail.com Abstract. In this paper, we develop the known theory of a quasi-static receiving antenna in a magnetoplasma and apply it to the calculation of the antenna effective length in case of reception of quasi-electrostatic whistler mode waves in plasmas. Also we analyze several typical chorus events detected by THEMIS spacecraft and show that the effective length of receiving antenna can be more than an order of magnitude greater than the geometric length. Introduction As it is known, the effective length of a receiving antenna in a magnetoplasma can significantly differ from its geometric length especially for the quasi-electrostatic whistler mode waves (see [Chugunov and Shirokov, 2016] and the references therein). Such waves propagate near the resonance cone. A wave packet composed o f them has a resonance structure, i.e., it is a superposition of plane monochromatic waves having a continuous and fairly wide spectrum o f wave numbers which are large compared to the inverse wave length A~elm of a parallel propagating electromagnetic whistler mode wave. For the resonance waves, the problem of calculating the complex amplitude E of the electric field from the voltage complex amplitude U induced on the receiving antenna terminals is nontrivial. We introduce the effective (or electrical) length /eff of a receiving antenna according to the formula U ^E leg cosT, ( 1 ) where Y is the angle between the antenna axis and vector Ё of the detected wave. Quantity /eff is determined by the reradiation efficiency o f the antenna [Balanis, 2016]. Importantly, it is not constant and can depend rather strongly on the mutual orientation of the electric field vector and the antenna. Some general relationships of the receiving antenna theory for magnetized plasmas were developed in the earlier works (see [Chugunov and Shirokov, 2016] and the references therein). In this paper, we apply this theory for emissions of natural origin, namely the very low frequency chorus in the Earth's magnetosphere. Typically, chorus emissions propagate quasi-parallel to the ambient magnetic field in their source region [Santolik et a l, 2014]. However, recent analysis of satellite data shows that chorus can also propagate in the quasi-electrostatic mode with wave normal angles в close to the resonance cone (в = 0m) [Agapitov et a l, 2014]. Expression for the Effective Length The effective length calculation is based on the reciprocity theorem [Chugunov and Shirokov, 2016] j/>(r,0Ф0 (r, t)dr = J/?0 (r, t)0 (r , t)df, (2) pi ant where the integrals are over the plasma (“pi”) and antenna (“ant”) volumes, Ф(г,г) is the scalar potential of the incident wave, p (r ,t) is the charge fluctuation in the plasma which induces voltage on the antenna terminals, and Ф0( г ,0 is the potential of a field due to the charge distribution p0(r,t) on the antenna. Applying spectral approach, one finally comes to the resulting expression for the effective length o f the antenna receiving a quasi-monochromatic wave with a carrier frequency co=coo [Shirokov et a l, 2017]: 64AL tec2я 0 0 °*d kdi// л/ e (£+ 1 Ч 1У» sin0rei | cos Y | +00 j k U2p tI(k)eiq(^ )Tokdk 0 58