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\begin{document}

\title{Ion-ion cross-field instability of lower hybrid waves in the inner coma
of comet 67P}



\author[1]{Elias Odelstad}%
\author[2]{Tomas Karlsson}%
\author[1]{Anders I. Eriksson}%
\author[1]{Sofia Bergman}%
\author[1]{Gabriella Stenberg Wieser}%
\affil[1]{Swedish Institute of Space Physics}%
\affil[2]{KTH Royal Institute of Technology}%


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  \date{June 10, 2024}


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\begin{abstract}
We show that an ion-ion cross-field streaming instability between cold
newborn cometary ions and heated heavy ions that were picked up upstream
is likely a contributing source of observed lower hybrid (LH) waves in
the inner coma of comet 67P/Churyumov-Gerasimenko. Electric field
oscillations in the LH frequency range are common here, and have
previously been attributed mainly to the lower-hybrid drift instability
(LHDI), driven by gradients associated with observed local density
fluctuations. However, the observed wave activity is not confined to
such gradients, nor is it always strongest there. Thus, other
instabilities are likely needed as well to explain the observed wave
activity. Several previous works have shown the existence of multiple
populations of cometary ions in the inner coma of 67P, distinguished by
differences in mass, energy and/or flow direction. We here examine two
selected time intervals in October and November 2015, with substantial
wave activity in the lower hybrid frequency range, where we identify two
distinct cometary ion populations: a bulk population of locally
produced, predominantly radially outflowing ions, and a more tenuous
population picked up further upstream and accelerated back towards the
comet by the solar wind electric field. These two populations exhibit
strong relative drifts (\$\textbackslash{}sim\$20 km/s, or about 5 times
the pickup ion thermal velocity), and we perform an electrostatic
dispersion analysis showing that conditions should be favorable for
lower hybrid wave generation through the ion-ion cross-field
instability.%
\end{abstract}%



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