Acceleration of protons and heavy ions to suprathermal energies during dipolarizations in the near-Earth magnetotail
<p>In this work we present an analysis of the dynamics of suprathermal ions of different masses (<span class="inline-formula">H<sup>+</sup></span>, <span class="inline-formula">He<sup>+</sup></span>, <span class="inlin...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-07-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/37/549/2019/angeo-37-549-2019.pdf |
| Summary: | <p>In this work we present an analysis of the dynamics of
suprathermal ions of different masses (<span class="inline-formula">H<sup>+</sup></span>, <span class="inline-formula">He<sup>+</sup></span>, <span class="inline-formula">O<sup>+</sup></span>) during
prolonged dipolarizations in the near-Earth magnetotail (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>X</mi><mo>></mo><mo>-</mo><mn mathvariant="normal">17</mn><msub><mi>R</mi><mi mathvariant="normal">E</mi></msub><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="60pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="f6796f1dfb62352512cf355219238a39"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="angeo-37-549-2019-ie00001.svg" width="60pt" height="13pt" src="angeo-37-549-2019-ie00001.png"/></svg:svg></span></span> according to Cluster/RAPID observations in 2001–2005. All
dipolarizations from our database were associated with fast flow braking
and consisted of multiple dipolarization fronts (DFs). We found
statistically that fluxes of suprathermal ions started to increase
<span class="inline-formula">∼1</span> min before the dipolarization onset and continued to grow
for <span class="inline-formula">∼1</span> min after the onset. The start of flux growth
coincided with the beginning of a decrease in the spectral index <span class="inline-formula"><i>γ</i></span>.
The decrease in <span class="inline-formula"><i>γ</i></span> was observed for protons for <span class="inline-formula">∼1</span> min after the dipolarization onset, and for <span class="inline-formula">He<sup>+</sup></span> and <span class="inline-formula">O<sup>+</sup></span> ions for <span class="inline-formula">∼3</span> and <span class="inline-formula">∼5</span> min after the onset
respectively. The negative variations of <span class="inline-formula"><i>γ</i></span> for <span class="inline-formula">O<sup>+</sup></span> ions were <span class="inline-formula">∼2.5</span> times larger than for light ions. This demonstrates more
efficient acceleration for heavy ions. The strong negative variations of
<span class="inline-formula"><i>γ</i></span> were observed in finite energy ranges for all ion components. This
indicates the possibility of nonadiabatic resonant acceleration of ions in
the course of their interaction with multiple DFs during dipolarizations.
Our analysis showed that some fraction of light ions can be accelerated up
to energies <span class="inline-formula">≥600</span> keV and some fraction of oxygen ions can be
accelerated up to <span class="inline-formula">∼1.2</span> MeV. Such strong energy gains cannot
be explained by acceleration at a single propagating DF and suggest the
possibility of multistage ion acceleration in the course of their
interaction with multiple DFs during the prolonged dipolarizations.</p> |
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| ISSN: | 0992-7689 1432-0576 |