Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Low-altitude ion heating, downflowing ions, and BBELF waves in the
  return current region   Heavy (O+) ion energization and field-aligned motion in and near the
ionosphere are still not well understood. Based on observations from the
CASSIOPE Enhanced Polar Outflow Probe (e-POP) at altitudes between 325 km and
730 km over one year, we present a statistical study (24 events) of ion heating
and its relation to field-aligned ion bulk flow velocity, low-frequency waves
and field-aligned currents (FACs). The ion temperature and field-aligned bulk
flow velocity are derived from 2-D ion velocity distribution functions measured
by the suprathermal electron imager (SEI) instrument. Consistent ion heating
and flow velocity characteristics are observed from both the SEI and the
rapid-scanning ion mass spectrometer (IRM) instruments. We find that transverse
O+ ion heating in the ionosphere can be intense (up to 4.5 eV), confined to
very narrow regions (~ 2 km across B), is more likely to occur in the downward
current region, and is associated with broadband extremely low frequency
(BBELF) waves. These waves are interpreted as linearly polarized perpendicular
to the magnetic field. The amount of ion heating cannot be explained by
frictional heating, and the correlation of ion heating with BBELF waves suggest
that significant wave-ion heating is occurring and even dominating at altitudes
as low as 350 km, a boundary that is lower than previously reported.
Surprisingly, the majority of these heating events (17 out 24) are associated
with core ion downflows rather than upflows. This may be explained by a
downward-pointing electric field in the low-altitude return current region.

25
The categories of this paper are: patt-sol
35
The categories of this paper are: mtrl-th
8
The categories of this paper are: hep-ex
2
The categories of this paper are: math
37
The categories of this paper are: supr-con
19
The categories of this paper are: eess
36
The categories of this paper are: cmp-lg
34
The categories of this paper are: chem-ph
27
The categories of this paper are: comp-gas
23
The categories of this paper are: solv-int
13
The categories of this paper are: quant-ph
16
The categories of this paper are: stat
32
The categories of this paper are: atom-ph
18
The categories of this paper are: econ
4
The categories of this paper are: cond-mat
1
The categories of this paper are: cs