Effect of the Earth’s declination variation on characteristics of Jovian decametric radio emissions

¹ DIHPA, CGCE, Instituto Nacional de Pesquisas Espaciais (INPE), São José dos Campos, Brazil
e-mail: ezequiel.echer@inpe.br
² LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, Meudon, France
³ Observatoire de Radioastronomie de Nançay, Observatoire de Paris, Université PSL, CNRS, Univ. Orléans, Nançay, France
⁴ CCET, Departamento de Geofísica, Universidade Federal do Rio Grande do Norte (UFRN), Natal, Brazil
⁵ Aix Marseille Université, CNRS, CNES, LAM, Marseille, France

Received: 6 March 2024
Accepted: 17 July 2024

Abstract

Context. The variation in the Jovicentric sub-latitude (declination, D_E) of a radio observer of Jupiter has long been known to affect the observation of Jupiter’s decametric (DAM) radio emissions due to these emissions’ anisotropic nature (through cyclotron maser instability beaming cones centered on Jovian magnetic field lines). The effect of the D_E variation, however, is still not clearly understood. For ground-based observations of Jupiter, the D_E variation, from −4° to +4°, occurs concomitantly with the cyclic variation in the distance to Jupiter, R, and Jupiter’s elongation angle, γ, which also affect the emission observation. Those covariant effects must be removed, then, for an analysis of the pure effect of D_E.

Aims. The aim of this study is to investigate the pure effect of the Earth’s D_E variation on the maximum frequency, duration, average Io phase, and average longitude of Io-induced DAM emissions observed with the Nançay Decameter Array (NDA).

Methods. For this purpose, we selected from the NDA/Routine digital catalog Jovian DAM emissions with an intensity (distance-corrected) above or equal to 8.8 dB and a maximum frequency above or equal to 20 MHz (25 MHz) for southern (northern) emissions. Distinct maximum frequency thresholds were adopted because of the typical discrepancy in the emissions’ frequency due to the high amplitude anomaly in the Jovian magnetic field at Jupiter’s northern hemisphere. The selected emissions comprise a new “unbiased” catalog. After analyzing the tenuous variation in the characteristics of the unbiased set of Io-DAM emissions with D_E, we compared them with those of matching Io-DAM simulations obtained with the Exoplanetary and Planetary Radio Emissions Simulator (ExPRES).

Results. From both the NDA data and the ExPRES simulations, it is observed that the pure D_E effect on the Io-DAM emissions characteristics is minor, yet a clear proportionality of the maximum frequency and duration of the northern Io-DAM emissions with D_E is noticed.

Conclusions. The northern Io-DAM emissions seem to be more strongly affected by the D_E variation than the southern emissions. Additionally, ExPRES can predict Io-DAM emissions consistently, from which we conclude that the current understanding of emission generation and propagation is reasonable. This study may be extended for broader ranges of D_E, such as Juno’s.